Full text of "The Quarterly journal of the Geological Society of London"

See other formats

oe Naa) en ete 
APY 1 ho deioheee baeepecg str A 
eC ete HPT sty nie 


ett HATE ek 


Oba bod red M bes 
LeeLee Moe 
14 Pay heed te ele Y 
eden Wed ‘J 


” eho 
aetod ny if 
Wiedi re kadl 


" 
are Ate it { iti ig aia f He 
; Yh pei 4 
: Tudeh r Ae sil aise 


Age fam 
ahh 8) ay el ay Beart tia > 
wip ey ahaiaba ae & 
} Ite moger? pee. 
i Saal fo) atogs 


ant 


Mis aaa 


> 
s53 
4 


suai 


Ait Feanane fey 
, " wet Hire 
ye He aig! eae 


4 RHA tans hae He afine we ‘} 
nail, ne itis ii Hi mM it 
ae mY | 
oP yr EET Ie ig 


dha 
vd sae with Teed 4,4, 


vane Neal ryi Ay aya ae 7 ay) el nt aah yal it " ot ae t hy My te ‘ait it pd 
ea raid Pint i ha ase aca ban aie! ia eh 
sana ean ae 


Hdeaeitar init a 4 an 
Ui Baty ts idee We hai 
raed Heed soi psa a bars oe i a. 


Hie ah * ent sth) Hey Ht) bait 
Hes wv Cie Le ane 
P LH be, 
when ve ttee 

aed 


te} eat Ve pigiaeseetiovar 
ay ite ayisi ig 


Pitts ver ith 


yen Mi oe 
py a i WS kesh 


rat ay ats pe N Hae AN a : 

mana aew Nok pei pea fy tba ste Magia aa 

¥ 434 i i 9 ’ t . 

i Vie Fela Ne {57h AN h {hiaeas pei tar its abit rf AP Paty! fae 

‘ Bis i Stig ne pees edad ays , iy aint eat Beh 4h sai avg 
4 in inde She : 

BA H ai nit Lhe ara 

Phila nce ne 
Lit 


as 


1 sha ¥ 
fe yes Lita vita Hs ee +e 
a tf Ne rel oly ood iste a! UH Hat 
aT rdahn al fe? ity ta 6 caihy 7) My Fah 
bY YM ivenda. Wpavins Bebagataby des Ht i iahiads 
bile faite FAL ad ‘th 
gant Ce Seta boked ' ve 


. 
aa Ra aerate 


(oie 


tinh 


s- as > 
Seiserstese: 
as 


Cb] hr} dik < " Way a Moe i Me 
rae fu sat ee ts : 
AP ifatet Piet Fe ree Nidigatn taf) hah t a 
inl ut i) Mele Hiatt ith tierididotea ht ‘ oe) 

eae dade - ay mu i} Bi ¥ Sopise MVD Js ‘ 
Pri i a SE ON bag { na Gene ai Levees 
: eS of OATS ett Boe ini ain Me a sets v 
uel Hide wae it Raniah th Hille : Os te + 


piv adn pats 


ee! olen betty 


Maye ay ra ra 
ab 5 eae wht iy Leb ee ait if 

jaleee koa ttasy 
‘ Ay ns Ra eee japt 
. rth w vel 

sant) AL BRt ia neta al eae HD 

‘ Ai eb ine ae sau ia) 
i} 


4 th 
Teh; Sei Has Feit t 
_ ine oo 


h ; liek pt 
eS NN atic lng 


Sah) Ye are Titi tM | PWIrET eS {Me ined ih wat 
u} aL te ih aah Hea Pe H 5 ee te oS { 
, gyi. ots $96 ae {se ee Be ee 1 
\y H tei ’ i Aer han antde a isu sn i ha 
tide daeriis tae, sent eit isnot Poa 
ME ee Pant en) 
14 


eis i 

ies rain) hd if Ht ie 
iff Hl Haq. a) yee i 
WAR 


uit rt it bi ae jane 
ee : 


+a) 


pa eit 4 Het 

Va4 : 
4 yay i, Minho i 
FAN iT Hitt 


Arann youve 
UEP vi Fy! 


Las 4 
Ahencnaee 
(aay dt 


wii 


yes 
ae sed 


hie ut jan ie 

ur ue 
Naa ane dake %4 hd wal iN 
Marat! enue l he pottily Lie He ald ia 
dain ( | 4 ie ; : 
i alah: Mh * al "an ai f 


Tain ray 
ix} )eykva't 


pce ae 
7B, 


a nS, 


—— a 


Set, ne, Say eee 


‘ 
4) a i 


(Mi 
‘7 
il. 


anita 


fi wy Hat 
pe 4H 
wae ; 


with! 


oe 


Ee 
Thal bya s ae Y 
eRe 


eee Sees ae 


ae 
ith 
waniit 

Ny 14: iy 
Hi ae i) 


= 
os rie es 


he TR id 


4a ane} He sight 


> [7 at ‘| fins 
dui U9 mare rhs 


My 
tide ! ne i ay Hal re 
An dat ite ae is ia edit) 


Sia 
a 
Sas eaeeetons 


x) i 

Pe tbs it 

ge Merl amy le ale 

( i linea iifo8s tata 
sat CEE AND Up hat 
he j 


Wy 

MYTR EY} \ i# i rT Pha | whi r ut 
biapadeda dah 222 a0 ii Ad Ht ‘ ‘ +S 
y rh ried Wis: Aoi ead ¢ hi aaa 


cage vate fe i 


Mf Hy With ' i ihiehih Py He 1) 
igh | a Hae pa oe Mike 


haa 
ey Hh Hols 4 


foe fad vt meat 


Hd 

ty) 
4 

bat itglitic ath ie 


4 

he +] Apettadiy toad jie 4 vi i, Bale i apt a 
nod bennide aloha erie tis te Hie My a am tt 
| PATTY Ya Wert at ahs 1 Pe Santa j ‘ Habre 
| Ric tty Pi ni ae iy a 


iva 
Hyer ag ss, At 4 4 
ahtad! m4 | Gain’ we th nit RH ah hy ete pi ‘t bya 
1) gas? 5 400) sha aeine i 
Ket 6 Fy sts Rah! Kai HN ha 
na iie 


{ ae thi 
" i 
fag! 


! ali 


i iy ae 


hie teh hs i} 


Wal ‘ 4 
10h oy VE St 
f iy ‘ er dyatse fi 
F Benepe eke er Eee ne " te 
( ) 4 } eh Meee BU en { | eid i 
ws ’ : 45 ih! Mai? Ny 4018 REE rb ee H Hilti 
i wt ¥ ' ih ‘ ih Ais eel y weal ie tatty a is Ost rere \s per) if ia 
ae | We) ey De be RG Oda ee pipe ha de ih 4 Ms ay cle tet ribet Wad tage! we Pein deus pe ede ul Hitt ily bt 
Ve eters “ag ? debe jy erent iff {ini teeth lirte! a4 1 ff ja beatae te 1 ‘ye they 
' pl qSencty bel ME pts HL Wbsenae Ya be Paraiba 4 foe poet speed ti ee i! LH 
Tey } tA " inn y Aifet plae | bea Ns shi gucrry Hy H telaliay itt i] 
A Whe) ' i TLE E pecan elt Waris * afdit th 
ivy Cee ee res ii ra Myeead N if 
an) a maejit \ Hews hare pepe ei ‘¢ ) WAR Shit dg i Hotire Y 
“4 ‘ : ALT idwatted Phila habs et i} regitt 
vee & ted rei he 4s ane ili 109 ie 
“gh ar ‘ if } i te halle be bite Ret f id Hi eae ii! 
u aia a t) ) doe (Ede PRG ne Hie tat Ai i Wee Mite 
VWiate na 10 Oe Tae Yeu be idle eleye in Mi tei pagal y i] 
wily War) 1 yt ia i ) wpe ltl pe ™ i iaiey fe de Wed Dan ia pals rt yee 
iL oN ae { ; tral ceh mend une vit nite 
é wie , eae ‘ Wo 5 Catan ‘ ' 
fu) tga a ' wt A ait was UA a Bay Gat ren realy yuna t pene Hat of 


hin ee yr —~_ wa ey th | Sas 7Vy", wr om 
net TT lth PANSisia Th 11] "NWHyc HV op NGA 


A SGnNe mee {f patty ey, mmreavery | 
Evoc Casa aon fill bref obs OT eee a 230 ant He asa nuuttu reel yv 


ECU \ \ v Whi ~ Tay tM Y ee Ming: | vA EEeyGS 
| | | | Wai 
cue Bece sSHHREREM | Waa | Winnett, 


TVs," why 
abel | Ty 
alWuven «6 Se Wwrde Yr woes Asa 
ub : ow ? bl bs c » ‘wie 
git Nt att} 
Fea LL) Uapareee ih 


ahwtee dinveute! 


: woo 
7 A@e ari ed “ MUTREERy 


| Phu eel : 


Ai } at ee 
it Trcrwweneeer” [lt Labeled Ty do 


arr nt allie 
yzeueeseeethy 


} «wv 


§ ¢ a - ad - 4 Peed 
; et et it oo Ay ' 


~~ \ 2 ° . {| Fe : | Pt : eee bes meee 
hy Wy. Ott. “4 
~ cee : STMT dt 44 WAL 
| uae Ty eatigueyeyce Bi j, 
$ Wy Ph iVevys” P05 bbb A | LL 
HST Ue Tyan ”  caaoactt tay PLUS {| |} 
Me Mee Lanett vet ee AAS TULL PaaS 
Viel Ne a4) Nee - = on tee re 
e sy wy wT ; ii Ze == : me ow! 
S ° ie Te op 
Mites TT iver seer acre 
" r™. wa uaanece, wre ce A. ‘wt Neri eM ve v 
Wyo sarreprtT Ti rrpeay awh oe wterl, Hy MeGiiired. b. tty 


Re Or 


. Wi vy wr G 
Facey 0” tly yy & 
eM, | gills Se aas d 
Neewvernyt Mt onahognntvtl | PN Ayy\, We LALA 
ieeeserenseey: Urn LS a rege Ty a A rene eed 
én yey uY x , . Vy 
| eit Halle i44 4 venetian: reff {\s Fieiecc, 
Fe vihal 1 CtOh db dal LT) 1 Nepabetarenatotadioh) | TENBUT De LL heed || | Mg MERE ULL th 
pewantmegce OU MMT ED Eee. WM aryl yt, NET LT htt 
TTL Mh hee LE Jo aT pak Ad | bk Dye nfs Shy LUT he 
; hte adel ST : IR | | i VOU mEET 
yhe's A hel st vn ey OF | ib bs “vo 
vovwneeteceayaecel sues Ueate aynennsttt ety Uosecse cose tctlMbaeme cen 
rey rewtlltgutus array tyeye y van dl riveweearyesV i pe Uy! 
= fe scerterpinvet inert! go ea. pubee eat! ak tt | “wp Peres: 
thay & Vp 
rsp: bg ew : AX 
vee OV vue > 


% . oO . : 7 f : 
pe ~~ GOO Lah ren oes ay ae So Wragg yor or 


et eae 
Pe CAE 


es 


5 


“i THE 


Or TUE 


GEOLOGICAL SOCTETY OF LONDON. 


EDITED BY 


THE PERMANENT SECRETARY OF THE GEOLOGICAL SOCIETY. 


Quod si cui mortalium cordi et cure sit non tantum inyentis herere, atque lis uti, sed ad ulteriora 
penetrare ; atque non disputando adversarium, sed opere naturam vincere ; denique non belle et probabiliter 
opinari, sed certo et ostensive scire ; tales, tanquam veri scientiarum filii, nobis (si videbitur) se adjungant 
—Novum Organum, Prefatio. 


VOLUME THE SEVENTY-EIGHTH, 


FoR 1922. 2 eee 
> BOGAGS 7% 


SE U6r2} Mysav' y 
LONDON : accel 
LONGMANS, GREEN, AND CO. 
PARIS: CHARLES KLINCKSIECK, 11 RUE DE LILLE. 
SOLD ALSO AT THE APARTMENTS OF THE SOCIETY, 


MCMXXII. 


“(st 
OF FT CRS. 


GEOLOGICAL SOCIETY OF LONDON. 


SR 


Elected | February 17th, 1922 


eristgint 
Prof. Albert Charles Seward, Sc.D., F.R.S., F.L.S. 
Wice-Prestvents, 


Prof. Edmund Johnston Garwood, M.A., 


Se.D., F.R.S. 
Richard Dixon Oldham, F.R.S. 


George Thurland Prior, 
F.R.S. 
Herbert Henry Thomas, M.A., Se.D. 


M.A... DiSe 


Secvetartes. 


Walter Campbell Smith, M.C., M.A. 


Foreign Secretarp. 


| James Archibald Douglas, M.A., B.Se 


Creasurer. 


Sir Archibald Geikie, O.M., K.C.B., D.C.L., | | Robert Stansfield Herries. M.A. 


LL.D., 8e.D., F.R.S. 


COUNCIL. 


Frederick Noel Ashcroft, M.A., F.C.S. 


Francis Arthur Bather, M.A., D.Sc., F.R.S. 
Prof. Perey George Hamnall Boswell, 


OBE: D:Se. 

Prof. William S. Boulton, 
Roy. Coll. Sci. 

Thomas Crosbee Cantrill, B.Sc. 

James Archibald Douglas, M.A., B.Se. 

John Smith Flett, O. BE., MA 
Dy Sea Mess R.S. 


D.Se., 


Prof. Edmund Johnston Garwood, M.A., 


Se.D., F.R.S. 


Sir Archibald Geikie,O.M., K.C.B., D.C.L., 


bes Sep. FERS, 
John Frederick Norman Green. B.A. 
Frederick Henry Hatch, O-B.E., Ph.D. 
Robert Stansfield Herries, M.A. 


Assoc, 


Prof. Owen Thomas Jones, M.A., D.Se. 

William Bernard Robinson King, O.B.E.. 
M.A. 

Richard Dixon Oldham, F.R.S. 


George Vhurland Prior, M.A., D.Se., 
F.R.S. 

Prof. Sidney Hugh Reynolds, M.A., Se.D. 

Prof. Albert Charles Seward, Se.D., 
RS. FSGS: 


Walter Campbell Smith, M.C., M.A. 

Sir Aubrey Strahan, K.B.E., 8¢e.D., LL.D., 
E.R.S. 

Herbert Henry Thomas, M.A., Se.D. 

Prof. William: Whitehead Watts, M.A., 
Se DS LigD shes: 

Henry Woods, M.A., F.R.S. 


Permanent Secretary. 
L. L. Belinfante, M.Se. 


Librarian. 
Arthur Greig. 


Clerk. 
M. St. John Hope. 


STANDING PUBLICATION COMMITTEE. 
Prof. A. C. Seward, President. 


Mr. W. Campbell Smith. 
Mr. J. A. Douglas. 


Prof. P. G. H. Boswell. 
Prof. W. 8. Boulton. 
Dr. J. S. Flett. 

Prof. E. J. Garwood. 


Mr. J. KF. N. Green. Prof. 


\ Secretaries. 


Dr. F. H. Hatch. 
Prof. O. T. Jones. 
Mr. R. D. Oldham. 
Dr. G. T. Prior. 

S. H. Reynolds. 


Mr. R. 8. Herries, Treasurer. 


Sir Aubrey Strahan. 
Dr. H. H. Thomas. 
Prof. W. W. Watts. 
Mr. H. Woods. 


TABLE OF CONTENTS. 


Page 
ANDREWS, CHARLES WiLLIAM. Description of a New Plesiosaur 
from the Weald Clay of Berwick, Sussex (Plates XLV & XV). 285 


Baiuey, Epwarp Batrerssy. The Structure of the South-West 
ichlands-of Scotland (Plate Div igen.s25 ieee... alee sess « by BD 


Buckman, 8. S. Jurassic Chronology: []—Preliminary Studies. 
Certain Jurassic Strata near Eypesmouth (Dorset): the 
Junction-Bed of Watton Cliff and Associated Rocks ........ 378 

Drxny, Frank. The Norite of Sierra Leone (Plates XVI-XIX).. 299 


ELLES, GERTRUDE LiniaAN. The Bala Country: its Structure and 
ioek-ouceesslom (Mlae tl). ey ar ae shwlaakhuc’s oles a’ wale olels bes 132 


Ho.itrum, Ricuwarp Eric (& A. C. Srewarp). Jurassic Plants 
MOTO elon Gate Mb) ve cus <a oe ne ehw nan Nae aval p= acd 3 201. 


JACKSON, JAMES FREDERICK. See BucKMAN,S.S. 


LAMPLUGH, GEORGE WiLiraAM. On the Junction of Gault and 
Lower Greensand near Leighton Buzzard (Bedfordshire) .... 1 


MitnER, Henry Brewer. The Nature and Origin of the Pliocene 
Deposits of the County of Cornwall, and their Bearing on the 
Pliocene Geography of the South-West of England.......... 348 


Prinete, Joun. See Buckman, S. S. 
RavDLeY, ERNEST GEORGE. See THomas, H. H. 


SEWARD, ALBERT CHARLES. On a Collection of Carboniferous _ 
lam USHrO Me een Um Ge lave Xe ieee eel, Se dk we oy koe ev ele 278 
I! 


— (&R.E.Hoxitrum). Jurassic Plants from Ceylon (Plate XII) 27 


SHANNON, WititIamM GrEoRGE Str. Joun. A Composite Sill at 
Newton bot, Devon (Plate: MD) yi ke nek oe le seco ee 261 


SmitH, BERNARD (& L. J. Witus). The Lower Paleozoic Rocks 
of the Llangollen District, with special reference to the 
Mectouresi(e lates MW) eet avo sne ace nsek. veaaveeeecs 176 


SPaTH, LEONARD FRANK. See BucKMAN,S. 5S. 


TEMPLEMAN, ANDREW. See Buckman, S. 8. 


1V TABLE OF CONTENTS, 
THomas, Herpert Henry. On Certain Xenolithic Tertiary Minor 
Intrusions in the Island of Mull, Argyllshire (Plates VI-X) .. 229 


Watts, FREDERICK STRETTON. The Carboniferous Limestone 
(Avonian) of Broadfield Down (Somerset) ...............+:. 22% 


Wiis, Leonarp Jounston (& B.Suiry). The Lower Paleozoic 
Rocks of the Llangollen District, with special reference to the 


Tectonics (Plates WI=V) ..< 0.55 22 Tee 176 
PROCEEDINGS. 

Proceedings of the Meetings ................... Or et 1, Lxix 
Amnualetveport © 2.6 6205 345 el) 2 eee ee ee ee vill 
Lists of Donors to the Library .......... his ial etme ae xl 
iistiol Foreign. Members... 24.1.2) 2200) XXII 
Hist-at- HN oreien Correspondents, ; 22h gi'h<ik deo bao eee xxiil 
List of Wollaston Medallists ...... See See ees aloe - XXIV 
Lisiiok Murchisen Medallists... £ «2/4 szSieuwtae. seen oe spoils XXKV 
istisar eLvell “Medallists, %..ker ae oc ~,. Bee. cae Oe ee ee Xa 
Lists of Bigsby and Prestwich Medallists ..... Spire tres nA XXVill 
Applications of the Barlow-Jameson Fund and Awards from 

the Damiel-Pideeon Fund: >. Jee si... a ns ae Nese stn XX1x 
imamcial epott atric Pace eet Takes ees eee ee “XXX 
Awards of the Medals and Proceeds of Funds .............. XXXVI 
Aumivetsary Address of the President). 2>-<.- =... oe ee xlv 


BrRoMEHEAD, CyRiIL Epwarp Nowriy. The Influence of 
Geolocyon the Historyiol London. 22... 22: .2'2. 245 ee at Vv 


CornwWALL, Ina EpmMunv. On Desmostylus Teeth from the 
Lower Miocene Sandstone of Southern Vancouver Island 


(iO) scone ate See oe Se tad ites by he A ee eee lxix 
Hormes, Sir Cuartes Joun. Leonardo da Vinci as a 
55310: (C1) GC AE Ss Ere ne ER AeA ge ans cane Gat itt Ixxi 


SewaARD, ALBERT CHARLES. Geological Notes on Western 
Greenland wes) $28. VER a oat atta Eek eR nate ee Ixxiil 


LIST OF THE FOSSILS FIGURED AND DESCRIBED 
IN THIS VOLUME. 


| 


; ] ; : 
Name of Species. | Formation. Locality. | Page 
BRACUIOvODA. 
ee aneaees e ae Hirnant Beds... Hirnant Valley .| | 163- GG 
Strophomena stluriana, fig. 10, | i 
Tetrarhynchia thorncombiensis, Pierce ibe | 
ANGIE OWE! “Sasi tiene See ses Tire Sos is eta yer eee | Beacon (Done). } 435 


PLUSIOSAURIA. 

Hurycleidus arcuatus, fig. B ..., 
megacephalus, fig. A...... 
Leptocleidus superstes, gen. et 
sp. nov.. pls. xiv & xv 


Lower Lias 


Lias 


i ee ae i aay 


eee nee 


Weald Clay 


FILICALES, etc. 


) 


Cladophlebis denticulata, pl. xil 


BCUCESH Ale llnee sees eon JURASSIC es .eee 
Teniopteris spatulata, pl. xii i 
Sphenopteris sp., pl. xiii ...... | Carboniferous... 
Lycopopiazs, 


Bothrodendron (2) sp., pl. xiii) } 


| | 


TOK CIO sai wacilen ses cau tensor Carboniferous... 
Lepidodendron Sm [ea eed J 
CoNIFERALES. 
Aracaurites cutchensis, pl. xii .| 
Brachyphylium mamillare,| 
Oy ae Wee UUTASS Guia. Sea. 


ple xin 


CC ii i ee ii i i eee ay 


eee ree 


| Street Somerset) 


293, 295 


PLANT& INCERT#H SEDIS. 


Desmophyllum sp., pl. Xii ...... | 
Innowinata, pl. xii 


eee een he cae 


| 3 294, 295 
...| Berwick (Sussex) 285-98 
ees 
| Ceylon cee | Th 
278 
Paracas (Peru). 279-80 
| Paracas (Peru) . | 281 
| | | 280-81 
| | ( 274 
| Ceylon aiataiaistenstalevelete 974-75 
| 2795 
Cemjlonin...<2s255 275 
| 1 OCETHT A eee eee 282 


EXPLANATION OF THE PLATES. 


PLATH 


FEOLOGICAL Map oF THE SourH-Westr Hicu anps, illus- 

i trating Mr. E. B. Bailey’s paper on the Structure | 
that area js eestcos. ke score o.oo eee 

GeEoLogicAL Map or tue Country Hast AnD SOUTH-EAST | 

aT; oF Bata Lake, illustrating Miss G. L. Elles’s paper on ! 

| the Structure and Rock-Succession of that area ...... J 


PLATES 


( Map oF THE AREA ROUND MoeEt-y-FAEN ; Norru-To-Sovrit | 
| SERIAL SECTIONS ACROSS THE LLANGOLLEN SYNCLINORIUM; | 
L-v 4 and GEOLoGIcaL Skercu-Map or THE CoUNTRY AROUND | 
|  LuLanGoLren, illustrating the paper by Dr. L. J. Wills 
; & My. Bernard Smith on the Tectonics of the Lower ! 
\. Palwozoie- Rocks of that tren \..-5. 24; -2-ccesee- eee ) 


MicroscoPe-Secrions illustrating Dr. H. H. Thomas's 
VI-X paper on certain Xenolithic Tertiary Minor Intrusions 
in the Island of Mull............ Leet ect co inane aoe J 


PLATE 


XI Mrcroscore-Sectioys illustrating Mr. W. G. Shannon’s } 
= paper on a Composite Sill at Newton Abbot ............ } 


{ T#NIOPTERIS, etc., illustrating the paper by Prof. A. C. 
XII Seward & Mr. R. E. Holttum on Jurassic Plants from 
[ Weylon 3. Bees s aot tecoe eet orn hae ee ee eee 


_ { Spuevorrertis, etc., illustrating Prof. A. C. Seward’s } 
XIil paper on a Collection of Carboniferous Plants from } 


GTO Seah owe ote haa ceeee neater icbmecne see Pea. 2 J 
PLATES 
XIV & XV LEPTOCLEIDUS SUPERSTES, gen. et sp. nov., illustrating 
s Dr. C. W. Andrews’s paper on that fossil ............ 


( View or THE Srerra-Leone Mountains; Contact of | 
| coARsE Norire witli NorMAL NoRITE; COARSE | 
XVI-XIX 4 Norire INVADING NORMAL Noritz; BrERBACHITE ‘ 
INVADING NORMAL Norite; and Microscore-Secrioxs | 
of these and other rocks, illustrating Dr. F. Dixey’s | 


| . . 
| paper on the Norite of Sierra Leone .................. ) 


PAGE 


82 


132 


176 


229 


285 


PROCESS-BLOCKS AND OTHER ILLUSTRATIVE FIGURES 


Bre. 1. 


ie) 


SY) 


6, 


Fies. 7 & 8. 


Fig. 9. 


Fics, 12 & 13. 


Fia. 14. 
iD. 


16. 


BESIDES THOSE IN THE PLATES. 


PAGE 
Sketch-map of the neighbourhood of Leighton Buzzard... 2 
Ground-plan of the Shenley-Hill sandpits ...............064 ) 
Profile section at the northern end of Harris’s pit, 

Sle alleny y Pte ae ayes cts an) wants Be ss snare wemnae dete s io nbc. wales oi a 
Combined section across Garside’s old pit at Shenley Hill 11 
Section at the north-eastern corner of the new (east) 

working of Garside’s pit, Shenley Hill .................. 14 
Re-excavated section in the western bank of the old pit, 

100 yards north of Sandpit Cottages .................066 15 
Sections im Nime-Acre Pit Shenley... ...4...2.0cesssseressasers 16 
Enlarged section of guttered iron-grit crags shown in the 

WEBherMpars Ob fie. Bi trina nants imma lsatienscnhieleesnas ee 18 
Part of section at the northern end of Miletree Pit, 

Shenley 4 grar.an vs ccimtan sake sone tee cegereeiiies oe incense uae. a 19 
Northern part of the section on the west side of Miletree- 

anti Bibs Shiomleyis.trnctt olan ik Caetaetten cians oxen, 20 
Combined sections through the Shenley-Hill pits and 

through the pits between Shenley Hill and Miletree- 
© ABRYBTO copia cnc bade 2asosereagsno: ine seonsanan6eia daese sOdSpDSEEAEeE 22 
Section at the northern end of Poplars Pit .................. 23 
Section at the southern end of the excavated platform 

abone Clamidees blGie as aectec emesis. aheaseacise thee ssvkoes end 26 


Section on the north-west side of the tramway-cutting 
into Chamberlain-Barn Pit 


ee a a) 


Vill 


Fie. 


Frias. 


PROCESS-BLOCKS AND OTHER ILLUSTRATIVE FIGURES, 


bo 


10. 


2&3. 


Section of the south-eastern corner of ‘Railway’ Pit, 
adjacent to Grovebury Brickworks ~..:./:.....:%.-«sanee 


Section in the trench at tle east side of Long Crendon 
Wand int) Pits rnc eacicsdda'wenls net sect osaeet se ot 


Map showing secondary folding of nappes ...............+6 
Clough’s criterion of south-eastward movement during 

‘ pre-anticlinal”’ times in Cowal. ......:..:. ....-. eee 
Map illustrating the metamorphism of nappes ......... mad 


Map illustrating the two great fo'ds of the Iltay Nappe . 


Diagrammatic section illustrating the geological contrasts 
north-west and south-east of Dalmally..................... 


Geological map of Loch Creran and Ardmucknish Bay... 


Geological map of Loch Etive, Pass of Brander, and 
Loch Awe...... stdkiut'aniat'ine valde eSemmanee aa sech asa 


Geological map of Glen Orehy and Ben Lui .............:. 
Section from Bala fo Moel-fryn ...c......2-.02c00c-4cos8eeeee 
NechOn up eb ryM- Cube a... eee eee las conse sees 
Diagrammatic sketch of Y-Garnedd Quarry-face ......... 
Vertical section of Long Quarry-face, Moel-fryn ......... 


Vertical sections showing the variation in the Gelli-grin 
CallcareoussAsh | (jp icuets dee au eae ta ta re 


Sketch-section showing the Hirnant Beds and their rela- 
tromatothe SUUtIAny Beek es <-e erence eee iia Sey on Ste ee 


Section from Creigiau Bychain to Bryn-pig ....... steer 


Diagrammatic sketches of Orthis hirnantensis and Stropho- 
MENA SUUTIANG ~iadss s2 8 oee2 see: so ASS ie ee 


Outline-map showing the chief tectonic features of the 
Uistangollen:distiriet: tae.7éc85. i228 Peedi 


Plan and section of the Bryn locality 2222: .2...--.9-4..s0o 


Section showing the variation and thickness in the Bala 


formation, Northern Berwyus 2: <.4-sc-0..2-.2. sae4e eee 
Geological map of Caer Drewyn ............ shar ssanees seem 
Geological map of the country near Llandegla ............ 


Sketch-map of the peninsulas of the Ross of Mull and 
Aardimeanael. Recs tenescenc sevaes CHORE AE ON ie snaineiene 


Sections of the xenolithic composite sill, Rudly a’ Chro- 
WUAUN gag Gaiea sa sane seaatieNvaigs Se ages sees Seameee fae Roane ee 


PaGE 


234 


PROCESS-BLOCKS AND OTHER ILLUSTRATIVE FIGURES. 1X 


Fig. 1. Sketch-map of Knowles Hill, Newton Abbot ............... On 
2. Diagrammatic section across Knowles Hill........ ......... 262 
Ey WDOOLROG CARON (OU SIU uis ra. ea c2chedes fesskde ene es porsaaeeeose 281 
Fics. A & B. Clavicular arch of Hurycletdus megacephalus and shoulder- 
SU CMO Ngee A NCUAUIUS tte aasciaaide avi enlace d vnc we weceiaciens'aids 299 
Fig: 1. Geological map of the Sierra-Leone Peninsula ............ 200 
2. Diagram expressing variable mineralogical composition 
ORAORING Usenet ee pa ana aaa at ote aiee cotinine Sac aedeae aatbecniedls 334 
1. Sketch-map of the South-West of England .................. 390 
Fics. 2-4. Kyanite from St. Erth, St. Agnes, and Seend............... 371 
Draar. letSketcheelevationvof Watton: CUM i... 0.c.desatieuncns.tvence 383 
2. Sketch eround-planof Watton Chil 2.22 .:..cscceceteacsss. 384 
+. View of fault in Watton Cliff, looking westwards ......... 385 
5. View of the same, looking eastwards  ...............sececeeee 386 
6. Lithology of the Junction-Bed [vertical sections] ......... 402 


7. Faunal comparision | Western Cliffs and Watton Cliff]... 404 


8. Comparative sections at Thorncombe Beacon and Watton 


Oats eceita sins penta: oo Toei au ce tat cntetes anrcancecise dellean 404 

9. Late Domerian denudation, showing the effect of the 
Weymouth Amtichinien sc acimnscseencoses-caemsncconcciiasweees 405 

“11. Presumed history of the White Bed of Burton Bradstock 
CW omsety ie Ses hac sonaace ot adeea cue aesicneabian eases ac pages 424-25 
We sOneol the Black Roeks (Worset)) scnc.osesecence sscessscusietrawe 430 


Dates of Issue of the Quarterly Journal for 1922. 


No. 309—March 31st, 1922. 

No. 310—July 4th, 1922. 

No. 311—September 28rd, 1922. 
No. 312— December 30th, 1922. 


) VOL, LX XVIII. b 


A ITN A! 


} 
4 4 e ig 7 ; ry , ¥ 
* + t 2 e. i eal 
Pree pal ee o - 
ie oe A a : . - 
* i =< P| i | 
“ ‘ 5 te: ~* . | 
LJ « sas 
+e eee - - ’ a * ys ee = * Ls 4 i 
4 * Pay =f srupts ees i] 
, | rei ire , + icy Cite h 
eet as ? : 3 I 2. Lt 
z 
eeeers ees es Cwregs oes . - aente . “* . 2 reece ° ona ' = - 
r - . : Cot 
: =) date tres | 
; P an in a at ee 
: z 
: } , Pn EE bre Ct gales tr 
‘ 
. er 7 = . ‘ - J 
. - wre . - ~— 
a ; ‘ fea ~) > 
my - ‘ ie Lie: re’. igo 
caret ‘ 
* . . Ce ee . * « + J ul Ue e 
ey J = ty 
PY Pie 1 
. « ot 2 é ved J 2 2 i 
' : : 5 s Si ba : Soa 
: 4 : 
eis yf = >t i > = . \ <n nay OD 
> 2 ? ¥ fie 
* 
SOvze ereorhrate Beer e gee 2s steer atene eee mete weer eee aee a - 
ty ta a= py iy eo ey has 
=) q ve 7. hal ae 
etwereereery Tere eer tere eee eT f ro Ps PD woe 
“ : eee ; ; Bla 
., 4 7 ee -* - 
p iz i Lda , aro Sate 
Heat} © 
¥ ot ? , - 
E war r. an tot Gea’ ast 
Se nh en tree taeeee ESAT ome AS | > ee é isu Pe 
f 5 = 
s 
in - 7) oo F s. 
rr , te de od int te: FS a, 
rk awe ie ites? eft: 3s oust 3 
CTT iat: tri ge A 
Soe jalt ariel. 
’ ~ 
Oy Gr Lie chips! “+ 
pape a ERE al = Spute 
‘ 
Geez Tek a -¢ ys = . 
Ie, c Sap co. — = 
Rash et Gen “9 Gee ee Nha apr 
;, q a . +4 
CA 4 
ee 
_ , i 
= a : > 
& 
; + . : wi P 
; * 
. - u 
= ; a a ty 
E 
P 
7 . 
* . 
,' - —_ 
a : : 
S 4 oe 
J “ 
» . 
- rs «, 


ms . val ay Te — th «@ 


LBD LLL LOLI OOOO AAR AA LAEA ALA EL LLLP DLP LDL 


Vol. LXXVIII. 
Part 1. 


THE 


OF THE 


GEOLOGICAL SOCIETY. 


EDITED BY 


THE PERMANENT SECRETARY. 


xh Sulliait i 
Marcu 8st, 1922. SOW 


LONDON : 
LONGMANS, GREEN, AND CO. 


QUARTERLY JOURNAL 


PARIS:—CHARLES KLINCKSIECK, 11 RUE DE LILLE, 
SOLD ALSO AT THE APARTMENTS OF THE SOCIETY, 


Price Seven Shillings and Sixpence, 


PPP DDI IDI I FSF I OI ae eee 7 


LIST OF THE OFFICERS 


AND COUNCIL OF THE 


GEOLOGICAL SOCIETY OF LONDON. 


AaAnRA 


SUEDE ‘February 1 7th, 1922, 


President. 
Prof. A. C. Seward, Sc.D., F.R.S., F.L.S. 


Gice- Presidents. 


Prof. Edmund Johnston Garwood, M.A.., 


Sc.D., F.R.S. 
Richard Dixon Oldham, F.R.S. 


George Thurland Prior, M.A., D.Sce., 
F.R.S. 
Herbert Henry Thomas, M.A., Se.D. 


Secretarics. 


Walter Campbell Smith, M.C., M.A. 


Foreign Secretary. 
Sir Archibald Geikie, O.M., K.C.B., D.C.L., 
LL.D., Sc.D., F.B.S. 


| James Archibald Douglas, M.A., B.Se. 


Creagurer. 
| Robert Stansfield Herries, M.A. 


COUNCIL. 


Frederick Noel Ashcroft, M.A., F.C.S. 


Francis Arthur Bather, M.A., D.Sc., F.R.S. 
Prot. Perey George Hamnall Boswell. 


O.B.E., D.Sc. 


Prof. William S. Boulton, D.Sc., Assoc. 


R.C.Se. 
Thomas Crosbee Cantrill, B.Sc. 
James Archibald Douglas, M.A., B.Sc. 


John Smith Flett, O.B.E., M.A., LL.D.,| 


D.Sc., M.B., F.R.S. 


Prof. Edmund Johnston Garwood, M.A..,| 


Se.D., F.R.S. 


Sir Archibald Geikie, O.M., K.C.B., D.C.L., 


LL.D., Se.D., F.R.S. 
John Frederick Norman Green, B.A. 
Frederick Henry Hatch, O.B.E., Ph.D. 
Robert Stansfield Herries, M.A. 


Prof. Owen Thomas Jones, M.A., D.Se. 

William Bernard Robinson King, O.B.E., 
M.A. 

Richard Dixon Oldham, F.R.S. 

George Thurland Prior, M.A., D.Se., 
F.R.S. 

Prof. Sidney Hugh Reynolds, M.A., Se.D. 

Prof. Albert Charles Seward, Sce.D., 
F.R.S., F.L.S. 

| Walter Campbell Smith, M.C., M.A. 

Sir Aubrey Strahan, K.B.E., Se.D., LL.D., 
F.R.S. 

| Herbert Henry Thomas, M.A., Se.D. 

Prof. William Whitehead Watts, M.A., 
Se.) hie Ds Bs: 


Henry Woods, M.A., F.R.S. 


Permanent Secretary. 
L. L. Belinfante, M.Sc. 


Lidrartan. 
Arthur Greig. 


Clerk, 
M. St. John Hope. 


STANDING PUBLICATION COMMITTEE. 
Prof. A. C. Seward, President. 


Mr. W. Campbell Smith. 
Mr. J. A. Douglas. 
Prof. P. G. H. Boswell. 
Prof. W. S. Boulton. 
Dr. J. S. Flett. 
Prof. E. J. Garwood. 
Mr. J. EF. N. Green. 


} Secretaries. 


Dr. F. H. Hatch. 
dare ia, O40 
Mr. R. D. Oldham. 
Dr7G. ESP rier, 

rok, Ss EL. 


Mr. R.S. Herries, Treasurer. 
Sir Aubrey Strahan. 


Jones. Dr. H. H. Thomas. 
Prof. W. W. Watts. 
Mr. H. Woods. 
Reynolds. 


Leone eee een ee a EEEEEEETEEEEIEEnEEEETESEEEEEEEREEEEEEIEEEEEEEEEREEEREEEEEEEEEIEEEE EERE) 


ORDINARY MEETINGS OF THE GEOLOGICAL SOCIETY 
TO BE HELD AT BURLINGTON HOUSE. 
Session 1921-1922. 


1922. 
= Weduesday, Apne s sian ssc baneice eeasueee 12 
= Way Shc. Lacon. ce ee eek eee 10*— 24 
Syne tect ee 14* 28 


[Business will commence ut 5.30 p.m. precisely. } 
The asterisks denote the dates on which the Council will meet. 


PROCEEDINGS 


OF THE 


GEOLOGICAL SOCIETY OF LONDON. 


a as iia 


ae , a 
see «4 4) ah as, big; ™ 
SESSION 1921522... 

at ai 


\ 


y& 
a AP 
t, (VY moe 


November 9th, 1921. 


Mr. R. D. OLtpHam, F.R.S., President, 
in the Chair. 


The List of Donations to the Library was read. 


The Names of certain Fellows of the Society were read out for 
the first time, in conformity with the Bye Laws, Sect. VI, Art. 5, 
in consequence of the Non-Payment of the arrears of their Annual 
Contributions. 


Tne PresrpENT made the following announcement :—‘ I have 
to report to the Society the Joss of two old and valued Fellows. 
The Eart oF Dvcte was elected in 1853, and, although not the 
most senior on our list, was the oldest Fellow with whom we had 
- maintained communication; throughout the long period of his 
Fellowship he has continuously shown interest in geology and 
helpfulness towards geologists. In Dr. Henry Woopwarp we 
lose a valued and familiar friend, and a very regular attendant 
at our meetings until quite recent years, when age and failing 
health prevented his presence. Elected in 1864, he had served for 
many years on the Council and in other offices of the Society, 
including the Presidency ; but his greatest service to geology was 
probably the founding of the ‘Geological Magazine,’ which he 
conducted continuously from its initiation until a couple of years 
ago. ‘The Council has already recorded its sympathy with his 
family in their bereavement, and I feel sure that you will wish to 
associate yourself with its action.’ 

The announvement was received by the Fellows present standing. 


VOL. LXXVIII. a 


ll PROCEEDINGS OF THE GEOLOGICAL society. { vol. Ixxviii, 


The following communications were read :— 


1. ‘The Igneous and Associated Rocks of Llanwrtyd (Brecon).’ 
By Laurence Dudley Stamp, D.8c., A.K.C., F.G.S., and Sidney 
William Wooldridge. 


2. ‘The Base of the Devonian, with especial reference to the 
Welsh Borderland. By Laurence Dudley Stamp, D.Sc., A.K.C., 
EGS: 


Downtonian fossils from Corvedale (Shropshire) were exhibited 
by W. Wickham King, F.G.S8. 


November 28rd, 1921. 


Mr. R. D. OLtpHam, F.R.S., President, 
in the Chair. 


- Andrew Menzies, Assoc. R.S.M., c/o The British Union Oil 


Company, Bridgetown (Barbados), was elected a Fellow of the 
Society. 


The List of Donations to the Library was read. 


The following communications were read :— 


1. ‘The Lower Carboniferous Rocks of West Cumberland.’ By 
Kenneth Wilson Earle, M.Sc., F.G.S. 


2. ‘A Composite Sill at Newton Abbot (Devon).’ By William 
George St. John Shannon, B.Sc., F.G.S. 


Prof. E. J. Garwoop exhibited the earliest recorded freshwater 
Gasteropod (Viviparus | Paludina]) from the local base of the 
Carboniferous rocks, near Horton-in-Ribblesdale (Yorkshire). 


December 7th, 1921. 
Mr. R. D. OtpHaAM, F.R.S., President, in the Chair. 


Geoffrey William Allen, B.A., B.Se., c/o H. Mewton, The 
Cliffe, Tutbury (Staffordshire); Violet Ruby Barge, B.A., 
29 Guilford Street, W.C.1,; Isaac Bond, B.Sc., Fairfield, Marl- 
borough (Wiltshire); Herbert Rosslyn Budgell, Headmaster of 
Carshalton College, Carshalton (Surrey); John Albert Child, 
B.Se., 60 Holland Park, W.11; Ritchie Brinley Davies, Bryn- 


part 1] PROCEEDINGS OF THE GEOLOGICAL SOCIETY. lil 


mawr House, Gilfach Goch, Bridgend (Glamorgan); Arthur 
Broughton Edge, Tudor House, Maidenhead; Daniel Griffiths, 
Granville House, Pontypool (Monmouthshire); the Rev. Harold 
Edward Grindley, M.A., Bosbury Vicarage, Ledbury (Hereford- 
shire); Wesley Hancock, B.Sc., Ashdene, Lower Ladywood, near 
Walsall (Staffordshire) ; Harold Hopkins, M.C., B.Sc., 16 Whitby 
Road, Fallowfield, Manchester; Robert Springett Mackilligin, 
Hurstbourne Tarrant, Andover (Hampshire); Jacob Thomas 
Morgan, Glasfryn, Nelson, Cardiff; Frank Mortimer Penney, 
176 Lower Addiscombe Road, Croydon; Frederick Anselm 
Redmond, B.Sc., Assistant Professor of Civil Hngineering in 
the University of Hongkong; Claude George Sara, Tabaquite 
(Trinidad); and the Rev. John Charles Thompson, 6 Bedford 
Avenue, Bexhill-on-Sea (Sussex), were elected Fellows of the 
Society. 


The List of Donations to the Library was read. 


The following communications were read :— 


1. ‘Jurassic Chronology: II.—Preliminary Studies. Certain 
Jurassic Strata near Eype’s Mouth (Dorset): the Junction-Bed of 
Watton Cliff and Associated Rocks.’ By 8.8. Buckman, F.G.S. 


2. ‘ Banded Precipitates of Vivianite in a Saskatchewan Fire- 
clay.’ Bv John Stansfield, B.A., F.G.S.1 


A small specimen of plattnerite (lead dioxide), a very rare 
mineral recently rediscovered at Leadhills (Lanarkshire), was 
exhibited by W. Campbell Smith, M.C., M.A., Sec.G.S. 


December 21st, 1921. 


Mr. R. D. OLrpuam, F.R.S., President, 
in the Chair. 


The List of Donations to the Library was read. 


The following communications were read :— 


1. ‘The Nature and Origin of the Phocene Deposits of the 
County of Cornwall, and their bearing on the Phocene Geography 
of the South-West of England.’ By Henry Brewer Milner, M.A., 
DLC., E.G. 


2. ‘The Phosphate Deposit of Ocean Island.’ By Launcelot 
Owen, A.R.S.M., A.R.C.S., F.G.S. 


1 Withdrawn by permission of the Council. 


a2 


iv PROCEEDINGS OF THE GEOLOGICAL socrery. [vol. lxxviu, 


Microscope-slides and lantern-slides were exhibited by Mr. 
H. B. Milner, in illustration of his paper; and rock-specimens and 
lantern-slides by Mr. L. Owen, in illustration of his paper. 


January 4th, 1922. 


Dr. G. T. Prior, F.R.S., Vice-President, 
in the Chair. 


The List of Donations to the Library was read. 


The following Fellows, nominated by the Council, were elected 
Auditors of the Society’s Accounts for the preceding year:— 
Ricuarp Mountrorp DEELEY, M.Inst.C.E., and Joun FREDERICK 
Norman GREEN, B.A. 


The appointment of Mr. N. E. Perrirr as Junior Assistant was 
announced and confirmed. 


The following communication was read :— 


‘Shales-with-Beef, a Sequence in the Lower Lias of the Dorset 
Coast.’ By William Dickson Lang, S8e.D., F.G.S., Leonard Frank 
Spath, D.Sc., F.G.S., and William Alfred Richardson, M.Sc., 
MGS. 


Specimens and lantern-slides were exhibited by Dr. W. D. Lang, 
Dr. L. F. Spath, and Mr. W. A. Richardson, in illustration of 
their paper. 


January 18th, 1922. 


Mr. R. D. OLpHam, F.R.S., President, 
in the Chair. 


Herbert Edward Bradshaw, A.R.S.M., 40 Eardley Crescent, 
Earls Court, 8.W.5 ; Major Edwin Massey Bull, 276 Holmesdale 
Road, South Norwood, S.E. 25; Harry Brenan Cronshaw, B.A., — 
Ph.D., A.R.S.M., The Briars, Penn, Wolverhampton; Frederick — 
Henry Dodd, 51 Shooters’ Hill Road, Blackheath, 8.E. 3; Harold 
Marriot Gell, A.M.Inst.C.E., 187 Northfield Avenue, West Ealing, © 
W.13; Sydney George Clift, B.Sc., 187 Willingham Street, 
Grimsby; Frank Gossling, B.Sc., F.C.S., 15 Birdhurst Road, 
Croydon; Timothy Hallissy, Senior Geologist, Geological Survey 
of Ireland, 89 Eaton Square, Terenure (County Dublin) ; Sydney 


part 1] PROCEEDINGS OF THE GEOLOGICAL SOCIETY. Vv 


Ewart Hollingworth, B.A., Geologist, H.M. Geological Survey, 
Flore, near Weedon (Northamptonshire); Arthur Lovatt, B.Sc., 
31 Park Road, Hendon, N.W.4; James Mitchell, B.Se., B.E., 

Professor of Geology & Mineralog oy in University College, Galw Tay ; 
John Isaac Platt, B.Sc., Assistant Lecturer in Geology, University 
College, Aberystwyth ; ‘Geor ge Robling, Llwyneelyn, Ponthenry, 
Llanelly ; Harry Hall Sands, “Milton Chambers, Nottingham; and 
Walter Perey Winter, B.Se., 20 Hurst Wood Road, Shipley (York- 
shire), were elected Fellows of the Society. 


The List of Donations to the Library was read. 


The following communications were read :— 


‘Jurassic Plants from Ceylon.’ By Prof. Albert Charles 
Seward, Se.D., F.R.S., F.G.S., and R. E. Holttum, B.A. 


2. ‘The Carboniferous Limestone (Avonian) of Broadfield 
Down (Somerset).’ By Frederick Stretton Wallis, M.Sc., F.G.S. 


February Ist, 1922. 


Mr. R. D. OtpHaAM, F.R.S., President, 
in the Chair. 


Arthur Bond, B.A., 31 Croxted Road, Dulwich, 8.E. 21; James 
Percy Tufnell Burchell, Fawke House, Sevenoaks (Kent); Francis 
Henry Arnold Hngleheart, B.A., The Priory, Stoke-by-Nayland, 
near Colchester; Kenneth Stuart Sandford, B.A., University 
College, Oxford; and James Alfred Steers, B.A., 2 Goldington 
Avenue, Bedford, were elected Fellows of the Society. 


The List of Donations to the Library was read. 


Mr. Cyrrin Epwarp Nowirt Bromennan, B.A., F.G.S., de- 
livered a leeture on the Influence of Geology on the 
History.of London. 

The 6-inch Geological Survey maps constructed by the Lecturer 
were exhibited, and some of the new features pointed out. The 
small streams now ‘buried’ are indicated on the maps, and the 
historical research involved in tracing them led to an appreciation 
of the connexion between the geology and topography on the one 
hand, and the original settlement and gradual growth of London 
on the other. ‘The reasons for the first selection of the site have 
been dealt with by several writers: below London the wide allu- 
vial marshes formed an impassable obstacle; traffic from the 
Continent came by the ports of Kent, and, if destined for the 


vl PROCEEDINGS OF THE GEOLOGICAL society. [vol. lxxviu, 


north or east of Britain, sought the lowest possible crossing of 
the Thames. This was near old London Bridge, where the low- 
level gravel on the south and the Middle Terrace deposits on the 
north approached close to the river-bank. A settlement was 
obviously required here, and the northern side was chosen as the 
higher ground. The gravels provided a dry healthy soil and an 
easily accessible water-supply ; they crowned twin hills separated 
by the deep valley of the Walbrook, bounded on the east by the 
low ground near the Tower and the Lea with its marshes, and on 
the west by the steep descent to the Fleet; the site was, therefore, 
easily defensible. The river-face of the hills was naturally more 
abrupt than it is now, owing to the reclamation of ground from 
the river; the most ancient embankment lay 60 feet north of the 
northern side of Thames Street. 

The first definite evidence of a permanent settlement was the 
reference in Tacitus. The early Roman encampment lay east of 
the Walbrook, and the brickearth on the west around St. Paul’s 
was worked. Later the city expanded, until the St. Paul’s lull 
was included, the wall being built in the second half of the 4th 
century. The great Roman road from Kent (Watling Street) 
avoided London, and utilized the next ford upstream—at West- 
minster—on its way to Verulamium and the north-west. The 
earliest Westminster was a Roman settlement beside the ford, 
built on a small island of gravel and sand between two mouths of 
the Tyburn. This settlement could not grow, as did London, 
since the area of the island. known to the Saxons as Thorney, was 
small. The road from London to the west joined the St. Alban’s 
road at Hyde Park Corner, running along the ‘ Strand,’ where the 
gravel came close to the river; a spring thrown out from this 
gravel by the London Clay was utilized for the Roman Bath in 
Strand Lane. 

Throughout Medizval times London was practically confined to 
the walled city, a defensible position being essential. The forests 
of the London-Clay belt on the north are indicated in Domesday 
Book and referred to by several writers, notably Fitzstephen, 
whose Chronicle also mentions many of the springs and wells 
and the marsh of Moorfields, produced largely by the damming 
of the Walbrook by the Wall. The same writer mentions that 
London and Westminster are ‘connected by a suburb.’ This 
was along the ‘Strand,’ and consisted first of great noblemen’s 
houses facing the river and a row of cottages along the north 
side of the road; this link grew northwards, at first slowly, 
but in the second half of the 17th century with great rapidity. 
By the end of that period the whole of the area covered by the 
Middle-Terrace Gravel was built over, but the northern margin of 
the gravel was also that of the town for 100 years, the Londen- 
Clay belt remaining unoccupied. 

The reason for this arrested development was that the gravel 
provided the water-supply. In early days the City was dependent 
on many wells sunk through the gravel, some of which were famous, 


part 1] PROCEEDINGS OF THE GEOLOGICAL SOCIETY. vil 


such as Clerkenwell, Holywell, and St. Clement’s. In the same 
way the outlying hamlets (for instance, Putney, Roehampton, 
Clapham, Brixton, Haling, Acton, Paddington, Kensington, 
Islington, etc.) started on the gravel, but later outgrew it, as 
pointed out by Prestwich in his Presidential Address of 1873. In 
the City the supply soon became inadequate, or as Stow says 
‘decayed,’ and sundry means were adopted to supplement it. The 
conduit system, bringing water in pipes from distant springs, began 
in 1236; London-Bridge Waterworks pumped water from the 
Thames by water-wheels from 1582 to 1817; the New River was 
constructed in 1618, and is still in use. It was not until the 
19th century that steam-pumps and iron pipes made it possible 
for the clay area to be occupied, thus linking together the various 
hamlets that are now the Metropolitan Boroughs. 

Some of the ways in which Geology atfects London to-day were 
briefly indicated, and the lecture was illustrated by a number of 
lantern-slides, reproduced mainly from old maps and prints. 


The cordial thanks of the Meeting were accorded to the 
Lecturer. 


ANNUAL GENERAL MEETING. 
February 17th, 1922. 


RicHarpD Dixon OrpHam, F.R.S., President, 
in the Chair. 


REPORT OF THE CoUNcIL FoR 1921. 


Durine the year under review 56 new Fellows were elected into 
the Society (24 less than in 1920). Of the Fellows elected in 
1921, 45 paid their Admission Fees before that end of that year, 
and, of the Fellows who had been elected in the previous year, 
26 paid their Admission Fees in 1921, making the total accession 
of new Fellows during the past year amount to 71 (7 more than 
in 1920). 

Allowing for the loss of 58 Fellows (17 resigned, 35 deceased, 
and 6 removed), but adding 1 Fellow re-instated, it will be seen 
that there is an increase of 14 in the number of Fellows (as com- 
pared with an increase of 21 in 1920). 

The total number of Fellows is, therefore, at present 1243, 
made up as follows: Compounders 198 (9 less than in 1920); 
Contributing Fellows 1036 (24 more than in 1920) ; and Non- 
Contributing Fellows 9 (1 less than in 1920). 

Turning now to the Lists of Foreign Members and Foreign 
Correspondents, the Council announces with regret the decease 
during the past year of Prof. A. G. Nathorst, Foreign Member. 
There are now three vacancies in the list of Foreign Members, and 
twelve vacancies in the list of Foreign Correspondents. 

The total Receipts from all ordinary sources of income amounted 
to £3894 5s. Sd., and the ordinary Expenditure of the year to 
£3964 12s. 9d. In addition, there was Special Expenditure on 
arrears of publication amounting to £897 17s. 6d., and there were 
Special Receipts amounting to £732 18s. 6d. from the sale of in- 
vestments and transfer from the Sorby and Hudleston Bequests 
and from the Prestwich and Barlow-Jameson Funds. 

In the estimates for the current year it has been possible to 
establish a balance between ordinary income and expenditure, and 
to provide an increased allotment for the Quarterly Journal. Of 
special expenditure there are still outstanding certain commitments, 
amounting to about £450, which may have to be met during the 
year by realization of part of the invested funds of the Society, 
or in some other manner, as may be decided by the Fellows at a 
Special General Meeting. 

All publications belonging to 1921 were issued and brought to 
account during the year. Six parts of the Quarterly Journal were 


part 1] ANNUAL REPORT. 1X 


published, constituting the remainder of Vol. LXXVI and the 
whole of the current volume for the year, Vol. LXXVII. The 
List of Geological Literature for 1913 (the compilation of which 
had been commenced by Mr. C. P. Chatwin during his tenure 
of the Librarianship, and completed after his retirement from 
that office) and that for 1920 (Authors & Titles) were also 
published within the year, and great progress has been made in 
the compilation of the List for the years 1915-1919. 

During the year the Council has given consideration to the 
Card Catalogue and List of Geological Literature. In view of 
the present financial position and commitments of the Society, it 
decided that the publication of the subject-index must be dis- 
continued for the present. The Council expresses its regret that 
this action, which it recognizes as seriously diminishing the value 
of the publication, should be necessary, and regards the resumption 
of the subject-index as a matter which must be kept in mind, 
when improvement in the financial conditions of the Society 
permits. In the altered conditions it became unnecessary to con- 
tinue the arrangement with Myr. C. D. Sherborn, by which he 
edited the Card Catalogue and incorporated the cards for both 
authors and subjects from year to year since he commenced the 
Catalogue in 1901. In terminating this arrangement the Council 
recorded its appreciation of Mr. Sherborn’s past services in the 
preparation of the Card Catalogue. 

The Apartments of this Society have been used for General and 
for Council Meetings during the past year by the Institution of 
Mining Engineers, the Institution of Mining & Metallurgy, the 
Institution of Water Engineers, the Society of Engineers, the 
Mineralogical Society, the Paleontographical Society, the Pre- 
historic Society of Hast Anglia, the Ray Society, the Conjoint 
Board of Scientific Societies, and the South-Eastern Union of 
Scientific Societies. 

Sir Aubrey Strahan and Prof. W. G. Fearnsides continued to 
act during the year 1921 as the Geological Society’s represen- 
tatives on the Conjoint Board of Scientific Societies, and Mr. R. D. 
Oldham was nominated to represent this Society on the Geophysical 
Committee of the Royal Astronomical Society. 

The Proceeds of the Daniel-Pidgeon Fund for 1921 were awarded 

to Mr. Frederic Stretton Wallis, of Bristol University and the 
Bristol Museum, who proposes to carry out researches on the Old 
Red Sandstone and the Carboniferous Limestone of the Bristol 
District ; and to Mr. Ralph Walter Segnit, of Oxford University, 
who proposes to carry outan investigation of the stratigraphical dis- 
tribution of the Cornbrash, based on a study of its faunal succession. 

Further, the following Awards of Medals and Funds have 
been made :— 

The Wollaston Medal to Dr. Alfred Harker, in recognition 
of his researches concerning the Mineral Structure of the Harth, 
especially in connexion with the natural history of igneous 
rocks. 


x PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [ vol. lxxviu, 


The Murchison Medal, together with a sum of Ten Guineas 
from the Murchison Geological Fund, to Dr. John William 
Evans, as an acknowledgment of the value of his researches, 
and in recognition of the manifold services that he has rendered 
to geological science. 

The Lyell Medal, together with a sum of Twenty-five Pounds 
from the Lyell Geological Fund, to Dr. Charles Davison, in 
recognition of the value of his seismological researches and their 
bearing on the science of geology. 

The Balance of the Proceeds of the Wollaston Donation Fund 
to Dr. Leonard Johnston Wills, as a mark of appreciation of his 
work on the older stratified rocks of the Midlands and the Welsh 
Borderland, and to encourage him in further endeavour. 

The Balance of the Proceeds of the Murchison Geological 
Fund to Mr. Herbert Bolton, in recognition of the value of 
his detailed researches in geology, particularly with regard to the 
insect-remains and faunal horizons of the Coal Measures of this 
country. 

A Moiety of the Balance of the Proceeds of the Lyell Geolo- 

gical Fund to Mr. Arthur Macconochie, in recognition of devoted 
service to the science of geology and the value of his work 
among the fossiliferous rocks of Scotland, more particularly the 
Cambrian of the North-West Highlands, and the Ordovician and 
Silurian of the Southern Uplands. 

A Moiety of the Balance of the Proceeds of the Lyell Geolo- 
gical Fund to Mr. David Tait, as a mark of appreciation of 
his valued contributions to Scottish geology, on stratigraphy, 
paleontology, and petrology, and especially in connexion with 
the Carboniferous rocks of the Lothians. 


REPORT OF THE LIBRARY COMMITTEE FOR 1921. 


The Donations received during the year 1921 number 56 
Volumes of separately-published Works, 389 Pamphlets, and 4 
detached Parts of Works, also 228 V olumes and 376 detached Parts 
of Serial Publications, 133 Volumes and 248 Parts of the Puble 
cations of Geological Surveys and other Public Bodies, and 10 
Volumes of Weekly Periodicals. 

As many as 59 sheets of Geological Maps were received during 
the year. 

The number of accessions by Donation amounts, therefore, to 
427 Volumes, 389 Pamphlets, and 628 detached Parts. 


part 1] ANNUAL REPORT. x1 


The Donors during the preceding year included 110 Government 
Departments and other Public Bodies, 130 Societies and Editors, 
and 112 Personal Donors. 

Considerable progress has been made with the resumption of 
exchanges with Societies and Institutions of former enemy 
countries. Most of the publications of such bodies, issued be- 
tween the years 1914 and 1919 inclusive, have now been received in 
the Library, and the exchange of their future publications has 
been arranged. 

The Author and Subject Slips for the year 1913 have now been 
incorporated with the Card Catalogue, and the Author Slips for 
the year 1920 are in process of insertion. 

The Purchases included 34 Volumes and 17 detached Parts of 
Works, and 6 Volumes and 18 detached Parts of Works published 
serially, and 4 Sheets of Geological Maps. 

The Expenditure incurred in connexion with the Library during 
1921 was as follows :— 


Sh Se 
OOksramdelertOdicals) sic Mew cea hc aek opens ccetankees es 62 19 9 
Bindinesand Map= Mounting... hjo..2 esses: sds oss ac 94 7 5 
SSITIACHVG Srna ry. Wetec High (ihre not, Me kes, Cier ct Mia ais <, 10) 0 


Motal wow  ESLOA i 2 


The appended Lists contain the Names of Government Depart- 
ments and other Public Bodies, Societies, Editors, and Personal 
Donors, from whom Donations to the Library have been received 
during the year under review :— 


I. GOVERNMENT DEPARTMENTS AND OTHER Pustic BopDIEs. 


Alsace-Lorraine.—Service de la Carte Géologique. Strasbourg. 

Alabama.—Geological Survey. Montgomery (Ala.). 

American Museum of Natural History. New York. 

Australia (South), etc. See South Australia, ete. 

Austria.—Geologische Reichsanstalt. Vienna. 

Barcelona.—Junta de Ciencias Naturals. 

Belgium.—Académie Royale des Sciences, des Lettres & Beaux-Arts de Belgique. 
Brussels. 

Bergens Museum. Bergen. 

Berlin.—Preussische Akademie der Wissenschaften. 

Bristol Museum & Art Gallery. 

British Columbia.—Ministry of Mines. Victoria (B.C.). 

British Guiana.—Lands & Mines Department. Georgetown. 

Brooklyn (N.Y.).—Museum of the Institute of Arts & Sciences. 

Buenos Aires.—Museo Nacional. 

California.—Academy of Sciences. San Francisco. 

——, University of. Berkeley (Cal.). 

Cambridge (Mass.).—American Academy of Arts & Sciences. 

—. Museum of Comparative Zoology in Harvard College. 


X11 


PROCEEDINGS OF THE GEOLOGICAL socrEeTy. [vol. lxxviu, 


Canada.—Geological & Natural History Survey. Ottawa. 
Department of Mines. 
Cape of Good Hope.—South African Museum. Cape Town. 


Connecticut.—State Geological & Natural History Survey. Hartford (Conn.). 


Cérdoba (Argentine Republic).—Academia Nacional de Ciencias. 

Denmark.—Geologiske Underségelse. Copenhagen. 

Kongelige Danske Videnskabernes Selskab. Copenhagen. 

Dublin.—Royal Irish Academy. 

Egypt.—Department of Public Works (Survey Department). Cairo. 

Federated Malay States—Government Geologist. Kuala Lumpur. 

Finland.—Finlands Geologiska Undersékning. Helsingfors. 

trance.—Ministeére de |’ Instruction Publique. Paris. 

Muséum d’Histoire Naturelle. Paris. 

Gold Coast.—Geological Survey. Accra. 

Great Britain.—Colonial Office. London. 

Geological Survey. London. 

——. Imperial Institute. London. 

——. Imperial Mineral Resources Bureau. London. 

Holland.—Departement van Kolonien. The Hague. 

Hungary.—Ungarische Geologische Anstalt (Magyar Foldtani 
Budapest. 

Ihnois.—Geologieal Survey. Urbana (I]l.). 

India.—Geological Survey. Calcutta. 

Trigonometrical Survey. Dehra Dun. 

Japan.—EKarthquake-Investigation Committee. ‘Tokio. 

Geological Survey. ‘Tokio. 

Kansas University. Lawrence. 

Kentucky.—Geological Survey. Frankfort (Ky.). 

La Plata, Museo de. 

Lausanne.— University of. ‘: 

Liége.—Collége des Bourgmestres & Echevins. 

Lisbon.—Academia das Sciencias. 

London.—Metropolitan Water Board. 

Madrid.—Real Academia de Ciencias Exactas, Fisicas & Naturales. 

Mexico.—Instituto Geolégico. Mexico City. 

Secretaria de Industria, Comercio & Trabajo. Mexico City. 

Milan.—Reale Istituto Lombardo di Scienze & Lettere. 

Minnesota.—School of Mines. Minneapolis. 

Munich.—Bayerische Akademie der Wissenschaften. 

Mysore.—Geological Department. Bangalore. 

Nancy.—Académie de Stanislas. 

New Jersey.—Department of Conservation. Trentham (N.J.). 

New South Wales.—Geological Survey. Sydney. 

New York State Museum. Albany (N.Y.). 

New Zealand.—Board of Science & Art. Wellington. 

Geological Survey. Wellington. 

Nigeria.—Geological Survey. 

Norway.—Geologiske Undersékelse. Christiania. 

Ohio.—Geological Survey. Columbus. 

Padua.—Reale Accademia delle Scienze. 

Paris.—Académie des Sciences. 

Peru.—Ministerio de Fomento. Lima. 


Tarsulat). 


Philippine Is.—Department of the Interior: Bureau of Science. Manila. 


Poland.—Service Géologique. Warsaw. 
Portici.—Reale Scuola di Agricoltura. 
Portugal.—Servico Geoldgico. Lisbon. 
Prussia. —Geologische Landesanstalt. Berlin. 
Quebec.—Department of Colonization, Mines, & Fisheries. 
Queensland.—Department of Mines. Brisbane. 
Geological Survey. Brisbane. 
Rhodesian Museum. Bulawayo. 

Rio de Janeiro.—Museu Nacional. 
Rome.—Reale Accademia dei Lincel. 
Rumania.—Academia Romana. Bucarest. 
Scotland.—Geological Survey. Edinburgh. 
Sierra Leone.—Geological Survey. Freetown. 
South Africa——Department of Mines. Pretoria. 


part 1] ANNUAL REPORT. xl 


South Australia, Agent-General for. London. 
Department of Mines. Adelaide. 

—. Geological Survey. Adelaide. 

Southern Rhodesia.—Geological Survey. Salisbury. 
Spain.—Instituto Geologico. Madrid. 
Stockholm.—Kongliga Svenska Vetenskaps Akademi. 
-Sweden.—Sveriges Geologiska Undersékning. Stockholm. 
Switzerland.—Geologische Kommission der Schweiz. Berne. 
Tasmania.—Secretary for Mines. Hobart. 

Geological Survey. Hobart. 

Tohoku.—Imperial University of Sendai. 

Tokio.—College of Science. 

Turin.—Reale Accademia delle Scienze. 
Uganda.—Geological Department. Entebbe. 

United States.—Geological Survey. Washington (D.C.). 
—. National Museum. Washington (D.C.). 

Victoria (Australia), Agent-General for. London. 

). Geological Survey. Melbourne. 
Vienna.—Akademie der Wissenschaften. 

—. Naturhistorisches Hofmuseumn:! 

Wales.—National Museum. Cardiff. 
Washington (D.C.).—Smithsonian Institution. 
Geophysical Laboratory. 

West Indies.—Imperial Agricultural Department. Bridgetown (Barbados). 
Western Australia.—Department of Mines. Perth. 
Geological Survey. Perth. 


eo | 


Il. Socterres anp Eprrors. 


Adelaide.—Royal Society of South Australia. 
Basel.—Naturforschende Gesellschaft. 

Belfast.—Natural History Society. 

Bergen.—‘ Naturen.’ 

Berlin.—Deutsche Geologische Gesellschaft. 

—. Institut ftir Meereskunde & Geographisches Institut. 
—-—. Zeitschrift fir Berg-, Hiitten-, und Salinenwesen. 
Berne.—Naturforschende Gesellschaft. 
Bonn.—Naturhistorischer Verein der preussischen Rheinlande. 
Bordeaux.—Société Linnéenne. 

Boston (Mass.).—American Academy of Arts & Sciences. 
Society of Natural History. 

Bristol Naturalists’ Society. 

Brussels.—Société Belge de Géologie. 

Société Royale Zoologique & Malacologique de Belgique. 
Bucarest.—Annales des Mines de Roumanie. 
Budapest.—F6ldtani Kézlony. 

Buenos Aires.—Sociedad Cientifica Argentina. 

Caen.—Société Linnéenne de Normandie. 

Calcutta——Asiatic Society of Bengal. 

Cambridge Philosophical Society. 

Cape Town.—Royal Society of South Africa. 

South African Association for the Advancement of Science. 
Cardiff—South Wales Institute of Engineers. 
Chambéry.—Société d’ Histoire Naturelle de Savoie. 
Chicago.—‘ Journal of Geology.’ 
Dorchester.—Dorset Natural History & Antiquarian Field-Club. 
Dorpat.—Naturforschende Gesellschaft. 

Dublin.— The Irish Naturalist.’ 

—. Royal Dublin Society. 

Edinburgh.—Royal Scottish Geographical Society. 

Royal Society. 

Falmouth.—Royal Cornwall Polytechnic Society. 


X1V 


PROCEEDINGS OF THE GEOLOGICAL Society. [ vol. lxxviii, 


Frankfurt am Main.—Senckenbergische Naturforschende Gesellschaft. 
Freiburg im Breisgau.—Na:urforschende Gesellschaft. 
Fribourg.—Société Fribourgeoise des Sciences Naturelles. 
Geneva.—Société de Physique & d’Histoire Naturelle. 
Giessen.—Ohberhessische Gesellschaft fiir Natur- und Heilkunde. 
Gloucester.—Cotteswold Naturalists’ Field-Club. 
Hague.—Société Hollandaise des Sciences. 
Halitax.—Yorkshire Geological Society. 
Halle——Leopoldinisch-Carolinische deutsche Akademie der Naturforscher. 
—. Zeitschrift fiir praktische Geologie. 
Helsingtors.—Société Finlandaise de Géographie. 
Johannesburg.—Geological Society of South Africa. 
K6nigsberg (Prussia).—Physikalisch-Ekonomische Gesellschaft. 
Lancaster (Pa.).—‘ Economic Geology.’ 

Lausanne.—Société Vaudoise des Sciences Naturelles. 
Leipzig.—Zeitschrift ftir Krystallographie. 

Liége.—Société Géologique de Belgique. 

Société Royale des Sciences de Liége. 
Lille.—Société Géologique du Nord. 

Lima.—NSociedad de GeogrAfia. 

Liverpool Geological Society. 

London.— The Atheneum.’ 

British Association for the Advancement of Science. 
Chemical Society. 

‘The Chemical News.’ 

‘The Colliery Guardian.’ 

‘The Geological Magazine.’ 

Geologists’ Association. 

Institution of Civil Engineers. 

Institution of Mining Engineers. 

Institution of Mining & Metallurgy. 

Institution of Water Engineers. 

Iron & Steel Institute. 

Linnean Society. 

‘The London, Edinburgh, & Dublin Philosophical Magazine.’ 
Mineralogical Society. 

‘The Mining Magazine.’ 

‘Nature.’ 

‘The Naturalist.’ 

Paleontographical Society. 

‘ The Quarry.’ 

Ray Society. 

Royal Agricultural Society. 

Royal Astronomical Society. 

Royal Geographical Society. 

Royal Meteorological Society. 

Royal Microscopical Society. 

Royal Photographic Society. 

Royal Society. 

Royal Society of Arts. 

Society of Engineers. 

South-Eastern Union of Scientific Societies. 
‘Water.’ 

Zoological Society. 

Manchester.— Literary & Philosophical Society. 

Melbourne (Victoria) —Australasian Institute of Mining & Metallurgy. 
Royal Society of Victoria. 

‘The Victorian Naturalist.’ 

Mexico.—Sociedad Cientifica ‘ Antonio Alzate.’ 
Milan.—Societa Italiana di Scienze Naturali. 
Modena.—Societa Sismologica Italiana. 


COU CUECEPGGUEEUIGEL TE Tara 


Newcastle-upon-Tyne.—North of England Institute of Mining & Mechanical 


Engineers. 

. University of Durham Philosophical Society. 
New Haven (Conn.).—Academy of Arts & Sciences. 
‘The American Journal of Science.’ 


part 1] ANNUAL REPORT. xv 


New York.—Academy of Sciences. 

American Institute of Mining Engineers. 
Northampton.—Northamptonshire Natural History Society. 
Ottawa.—Royal Society of Canada. 

Paris.—Annales des Mines, 

Société Géologique de France. 
Perth.—Perthshire Society of Natural Sciences. 
Philadelphia.—Academy of Natural Sciences. 

American Philosophical Society. 

Pisa.—Societa Toscana di Scienze Naturali. 
Plymouth.—Devonshire Association for the Advancement of Science. 
Rennes.—Société Scientifique & Médicale de ]’Ouest. 
Société Géologique & Minéralogique de Bretagne. 
Rome.—Societa Geologica Italiana. 

Rugby School Natural History Society. 

Santiago de Chile-—Sociedad Nacional de Mineria. 
Stockholm.—Geologiska Férening. 

Stratford.—Essex Field-Club. 
Stuttgart.—Verein fiir Naturkunde Wirttembergs. 
Sydney (N.S.W.).—Linnean Society of New South Wales. 
Royal Society of New South Wales. 
Toronto.—Royal Canadian Institute. 

Torquay Natural History Society. 

Upsala.—Geological Institution of the University. 
Vienna.—Geologische Gesellschaft. 

——. Berg- und Hiittenmannisches Jahrbuch. 
Zoologische- Botanische Gesellschaft. 

Washington (D.C.).—Academy of Sciences. 

Geological Society of America. 
Wiesbaden.—Nassauischer Verein fur Naturkunde. 
Worcester.—Naturalists’ Club. 

York.—Yorkshire Philosophical Society. 


Xvi PROCEEDINGS OF THE GEOLOGICAL society. { vol. Ixxviii, 


III. Personat Donors. 


Abelspies, J. F. C. 
Adolf, G. 

Aguilar y Santillan, R. 
Assmann, P. 


Barrell, J. 
Bekker, H. 

Bell, A. 

Bell, N. A. 
Bosworth, T. O. 
Briére, Y. 

Brown, J. Coggin. 
Burckhardt, C. 


Carus- Wilson, C. 
Chapman, F. 
Clarke, K. de C. 
Clarke, F. W. 
Cole, J. A. G. 
Crema, C. 


Dall OW Hi: 
Daly, R. A. 
Davies, David. 
Davisor, EK. H. 
Delhaye, F. 
Ditmas, F. I. L. 
Dixon, E. E. L. 
Dollfus, G. F. 
Dorlodot, H. de. 
Douvillé, H. 
Drygalski, E. von. 


Erdtmann, O. G. E. 


Fleury, E. 
Foshag, W. F. 


Gardiner, C. I. 
Green, J. F.N. 


Halle, T. G. 
Harmer, F. W. 
Haughton, S. H. 


Henderson, J. 
Hobson, B. 
Holmes, A. 
Holmquist, P. J. 
Howchin, W. 
Hume, G. 8. 


Jahn, A. 
Jones, T. A. 


King, W. W. 
Kunz, G. F. 


Launay, L. de. 
Leach, A. L. 
Leslie, T. N. 
Linck, G. 
Lugeon, M. 
Lull, B.S. 


McConnell, P. 
Marriott, R. A. 
Martel, E. A. 
Martin, E. A. 
Marty, P. 
Mennell, F. P. 
Mohr, H. 
Moir, J. R. 
Monestier, J. 
Morgan, P. G. 


Neaverson, E. 
Osborn, H. F. 


Parkinson, J. 
Penck, A. 
Penzer, N. M. 
Peragallo, M. 
Pilgrim, L. 
Plymen, G. H. 
Pruvost, P. 


Rastall, RK. H. 


Reed, F. R. C. 
Reid, Mrs. E. M. 
Renier, A. 
Reynolds, 8. H. 
Richardson, W. A. 
Roxo, M. G. de O. 


Sargent, H.C. 
Schlosser, M. 
Schuchert, C. 
Shannon, HK. V. 
Sheppard, T. 
Smith, S. 
Smith, W. Campbell. 
Spath, L. F. 
Spitaler, R. 
Stamp, L. D. 
Stevenson, S. 
Stuart, M. 
Sutton, J. R. 


Taber, S. 

Teilhard de Chardin, P. 
Termier, P. 

Thomson, G. M. 
Thorpe, M. C. 

Thorpe, M. R. 

Tilley, C. E. 

Torcelli, A. J. 

Troxell, E. L. 


Van Straelen, V. 


Walther, J. 
Washington, H.S. 
Watts, W. W. 
Wayland, E. J. 
Wentworth, C. K. 
Whitaker, W. 
Whitehead, H. 
Wieland, G. R. 
Woodward, C. J. 
Woolacott, D. 


part 1] ANNUAL REPORT. XVil 


COMPARATIVE STATEMENT OF THE NUMBER OF THE SOCIETY AT 
THE CLOSE OF THE YEARS 1920 anp 1921. 


Dec. 31st, 1920. Dec. 31st, 1921. 
Compounders .. HG oh DOT Na Ei iStee 198 
Contributing Fellows......... TOU Tah ee 1036 
Non-Contributing Fellows... LO a Shen. 5 
1229 1243 
moe Boreien: Members: .2.......... SADT Ee tees Bp 37 
Foreign Correspondents...... ZA eins See 28 


1288 1308 


Comparative Statement, explanatory of the Alterations in the 
Number of Hellows, For evgn Members, and Foreign Correspon- 
dents at the close of the Years 1920 and 1921. 


Number of Compounders, Contributing, and Non- 1229 
Contributing Fellows, December 31st, 1920... 


Add Fellows elected during the former set and a 26 
paid in 1921.. th 
Add Fellows elected and paid i Frat TIO ae 45 
Add Fellow reinstated ........... vos ae re 1 
301 
Weeuc: Compounders deceased .2..).4..-0:000...0 12 
Contributing Fellows deceased ............... 22 
Contributing’ Wellows resioned” 27. ............ 17 
Non-Contributing Fellow deceased ......... if 
Fellows removed in accordance with Sect. VI, 
amt .O, of the ye-Waws:.J......0c..sec.c 6 
— 58 
1243 
Number of Foreign Members and Foreign Cor- 59 
respondents, December 31st, 1920 ............... 
Deduct Foreign Member deceased and Foreign 9 
Correspondent resigned . 
Deduct Foreign Correspondents elected. Foreign G 
Members . VERO et at ea ARO rs f 
51 
Add Foreign Members and ee oe 14. 
dents elected | 
65 
— 65 
1308 


VOL. LXXVIII. ee b 


xvill PROCEEDINGS OF THE GEOLOGICAL society. | vol. lxxvii, 


DECEASED FELLOWS. 


Compounders (12). 


Evans, M. [elected in 1872]. Lucas, H. [el. 1864]. 
Feilden, Col. H. W. [el. 1875]. | Mallett, F. R. [el. 1868]. 
Fletcher, Sir Lazarus [el. 1879]. | Miall, L. C. [el. 1875]. 
Hawkshaw, J. C. [el. 1866]. | Nicholas, W. [el. 18741]. 


Love, J. fel. 1876}. | Ridewood, W. G. [el. 1895]. 
Lucas, Col. F. [el. 1883]. | Wrightson, Sir Thos. [ el. 1876]. 


Contributing HKellows (22). 


Clinch, G. [elected in 1899]. Lavanchy, F. M. [el. 1906]. 
Davies, T. W. [el. 1896]. Longstaif, G. B. [el. 1906]. 
Douglas, T. [el. 1873]. Maidwell, F. T. [el. 1919]. 
Ducie, Kar! of [el. 1853]. Molyneux, A. J. C. [el. 1897]. 
Fowler, G. [el. 1875]. Pearse, A. L. [el. 1894]. 
Galloway, T. L. [el. 1876]. Parke, G. H. [el. 1879]. 
Garforth, Sir William [el.1891]. | Reeve, J. [el. 1901]. 
Gwinnell, W. F. [el. 1889]. | Wadsworth, M. E. [el. 1889]. 
Harland, Rev. A. A. [el. 1878]. | Winwood, Rev. H. H. [el. 1864]. 
Hogben, G. [el. 1911]. | Woodward, H. [el. 1864]. 
Jenkins, D. M. [el. 1919]. | Zabel, C. F. [el. 1919]. 


Non-Contributing Fellow (1). 
Duckworth, H. [elected in 1858]. 


FELLows RESIGNED (17). 


Allworthy, 8S. W. | Homan, B. van. 
Barnett, W. J. | Hughes, W. E. 
Brown, E. G. | Mackenzie, G. L. 
Brown, H. T. | Peacock, A. 
Carpenter, S. W. | Pollen, Lt.-Col. C. H. 
Clark, J. E. : | Robson, V. E. 

Dodd, C. Satow, P. A. 
Edwards, W. Taylor, T. G. 

Fenner, C. 


FELLOWS REMOVED (6). 


Balcon, S. be ee. W. Rt. G. 
Bowen, D. Schofield, E 
Lubbock, M. | Wells, J 


part 1] 


ANNUAL REPORT. 


FELtows Evectep (71). 


Bain, A. D. N. 
Bisat, W. 8. 
Blanford, A. W. 
Brown, J. 
Budgell, H. R. 
Burling, L. D. 
Cave, C. J. P. 
Charles, A. J. 
Chilcott. B. G. 
Clark, Rev. L. K. 
Cornwall, I. E. 


Cunnington, E. B. H. 


Davies, D. J. 
Davies, R. B. 
Davies, 8. J. 

de Camps, E. B. E. 
Dines, H. G. 
Dixon, H. G. D. 
Duncan, J. S. 
Hdge, A. B. 
Erb, J. T. 
Glenday, V. 
Green, W. 
Griffiths, D. 
Grindley, H. E. 
Harrison, J. R. 
Hartley, J. J. 
Hatch, H. B. 
Haydon, R. H. 8. 
Henderson, F. Y. 
Henderson, T. 
Hogan, M. A. 
Hudson, R. G. S. 
Johnson, H. E. 
Jones, T. B. 
Kendrick, R. M. 


Le Grand, J. P. 
Lewis, H. P. 
Little, O. H. 
Littlehales, C. I. 
McCormick, J. 
McLintock, W. F. P. 
Merrett, EK. A. 
Mazzi, LL. F. 
Murray, E. F. O. 
Narke, G. G. 
Nuttall, W. L. F. 
enmey, have 
Perrott, B. 
l2Wlene, dia id, 
Prisk, A: 

Pooks Wis J. 
Raeburn, C. 
Rao, M. V. 

Rau Ss 5: 
Ridge, H. M. 
Furison,. Ji. ke. 
Robertson, 'T. 
Sadek, H. 
Segnit, R. W. 
Sikes, H. L. 
Stanworth, J. 
Thomas, R. H. 
Thompson, Rev. J. C. 
Trotter, F. M. 
Turner, H. W. 
Wadia, D. N. 
Watkins, F. L. 
Whitfield, T. W. 
Williams, R. R. 
Wilson, L. E. 


ForeIGN MremMBer DECEASED. 
Nathorst, Alfred Gabriel [elected in 1904]. 


ForREIGN CORRESPONDENT RESIGNED. 
Lehmann, Prof. Johann [elected in 1892]. 


KX PROCEEDINGS OF THE GEOLOGICAL Socrety. [ vol. Ixxviu, 


The following Personages were elected Foreign Members during 
the year 1921 :— 
Dr. Frank Wigglesworth Clarke, of Washington (D.C.). 
Prof. Emile Haug, of Paris. 
Prof. Maurice Lugeon, of Lausanne. 
Prof. Hans Schardt, of Ziirich. 
Dr. Jakob Johannes Sederholm, of Helsingfors. 
Dr. Henry Stephens Washington, of Washington (D.C.). 


The following Personages were elected Foreign Correspondents 

during the year 1921 :— 

Prof. Lucien Cayeux, of Paris. 

Dr. Maurice Cossmann, of Paris. 

Prof. Henry de Dorlodot, of Louvain. 

Dr. Henri Douvillé, of Paris. 

Prof. Louis Dupare, of Geneva. 

Prof. Johan Kizr, of Christiania. 

Prof. Waldemar Lindgren, of Boston (Mass.). 

Prof. John J. Stevenson, of New York City. 


After the Reports had been read, it was resolved :— 


That they be received and entered on the Minutes of the Meeting, 
and that such parts of them as the Council shall think fit be printed 
and circulated among the Fellows. 


It was afterwards resolved :— 


That the thanks of the Society be given to Mr. R. D. 
- Oldham, retiring from the office of President; to Mr. G. W. 
Lamplugh and Col. H. G. Lyons, retiring from the office of Vice- 
President (and also from the Council) ; to Dr. H. H. Thomas, 
retiring from the office of Secretary; and to Dr. J. V. Elsden, 
Prof. P. F. Kendall, and Lieut.-Col. Sir A. Henry McMahon, 
retiring from the Council. 


After the Bailoting-Glasses had been closed, and the Lists 
examined by the Serutineers, the following gentlemen were declared 
to have been duly elected as the Officers and Council for the 
ensuing year :— 


part 1] 


ANNUAL REPORT. 


XX1 


OFFICERS AND COUNCIL.—1922. 


PRESIDENT. 


Prof. A. C. Seward, 


Sc.D., F.R.S., F.LS. 


VICE-PRESIDENTS. 


Prof. Edmund Johnston Garwood, M.A., Se.D., F.R.S. 
Richard Dixon Oldham, F.R.S. 

George Thurland Prior, M.A., D.Se., F.R.S. 

Herbert Henry Thomas, M.A., Sc.D. 


SHCRETARIES. 


Walter Campbell Smith, M.C., M.A. 
James Archibald Douglas, M.A., B.Sc. 


FOREIGN SECRETARY. 
Sir Archibald Geikie, O.M., K.C.B., D.C.., LL.D., Sc.D., 
F.R.S. 


TREASURER. 
Robert Stansfield Herries, M.A. 


COUNCIL. 


Frederick NoelAsheroft,M.A.,F.C.S. 

Francis Arthur Bather, M.A., D.Sc., 
F.R.S. 

Prof. Perey George Hamnall Bos- 
well, O.B.E., D.Sc. 

Prof. William S. Boulton, D.Sc., 
Assoc.R.C.Se. 

Thomas Crosbee Cantrill, B.Sc. 

James Archibald Douglas, M.A., 
B.Se. 

John Smith Flett, O.B.E., M.A., 
LL.D., D.Sc., M.B., F.R.S. 

Prof. Edmund Johnston Garwood, 
M.A., Se.D., F.R.S. ; 

Sir Archibald Geikie, O.M., K.C.B., 
D.C.L., LL.D., Se.D., F.R.S. 

John Frederick Norman Green, B.A. 

Frederick Henry Hatch, O.B.E., 
Ph.D. 


Robert Stansfield Herries, M.A. 
Prof. Owen Thomas Jones, M.A., 
i ase: 
William Bernard Robinson King, 
I) O) Bon MEAS 
Richard Dixon Oldham, F.R.S. 
George Thurland Prior, M.A., D.Sc., 
HRS: 
Prof. Sidney Hugh Reynolds, M.A., 
Se.D. 
Prof. Albert Charles Seward, Se.D., 
Res. 2.8: 
Walter Campbell Smith, M.C., M.A. 
Sir Aubrey Strahan, K.B.E., Se.D., 
i ee ARES: 
Herbert Henry Thomas, M.A., Sc.D. 
Prof. William Whitehead Watts, 
M.A., Se.D., LL.D., F.R.S. 


Henry Woods, M.A., F.R.S. 


xxl PROCEEDINGS OF THE GEOLOGICAL society. [ vol. Ixxvui, 


LIST OF 
THE FOREIGN MEMBERS 


OF THE GEOLOGICAL SOCIETY OF LONDON, in 1921. 


Date of 
Election. 
Commendatore Prof. Giovanni Capellini, Bologna. 
1886. Prof. Gustav Tschermak, Vienna. 
1891. Prof. Charles Barrois, Lille. 
1893. Prof. Waldemar Christofer Brogger, Christiana. 
1894. Prof. Edward Salisbury Dana, New Haven, Conn. (U.S.A.). 
1896. Prof. Albert Heim, Ziirzch. 
1897. Dr. Hans Reusch, Christiania. 
1898. Dr. Charles Doolittle Walcott, Washington, D.C. (U.S.A.). 
1899. Prof. Emanuel Kayser, Munich. 
1899. M. Ernest Van den Broeck, Brussels. 
1900. M. Gustave F. Dollfus, Paris. 
1900. Prof. Paul von Groth, Munich. 
1901. Dr. Alexander Petrovich Karpinsky, Petrograd. 
1901. Prof. Antoine Frangois Alfred Lacroix, Paris. 
1908. Prof. Albrecht Penck, Berlin. 
1903. Prof. Anton Koch, Budapest. 
1904. Prof. Henry Fairfield Osborn, New York (U.S.A.). 
1905. Prof. Louis Dollo, Brussels. 
1907. Dr. Emil Ernst August Tietze, Vienna. 
1907. Commendatore Prof. Arturo Issel, Genoa. 
1908. Prof. Bundjiré K6t6, Tokyo. 
1909. Prof. Johan H. L. Vogt, Trondhjem. 
1911. Prof. Baron Gerard Jakob De Geer, Stockholm. 
1911. M. Emmanuel de Margerie, Strasbourg. 
1912. Prof. Marcellin Boule, Paris. 
1913. Prof. Johannes Walther, Halle an der Saale. 
1914, Prof. Friedrich Johann Becke, Vienna. 
1914. Prof. Thomas Chrowder Chamberlin, Chicago, Ill. (U.S.A.). 
1914, Prof. Franz Julius Loewinson-Lessing, Petrograd. 
1914, Prof. Alexis Petrovich Pavlow, Moscow. 
1914. Prof. William Berryman Scott, Princeton, N.J. (U.S.A.). 
1921. Dr, Frank Wigglesworth Clarke, Washington, D.C. (U.S.A.). 
1921. Prof. Emile Haug, Paris. 
1921. Prof. Maurice Lugeon, Lausanne. 
1921. Prof. Hans Schardt, Zrirech. 
1921. Dr. Jakob Johannes Sederholm, Helsingfor's. 
1921. Dr. Henry Stephens Washington, Washington, D.C. (U.S.A.). 


part 1] ANNUAL REPORT. XXI11 


LIST OF 
THE FOREIGN CORRESPONDENTS 


OF THE GEOLOGICAL SOCIETY OF LONDON, rn 1921. 


Date of 
Election. 


1889. Dr. Rogier Diederik Marius Verbeek, The Hague. 

1898. Dr. W. H. Dall, Washington, D.C. (U.S.A.). 

1899. Dr. Gerhard Holm, Stockholm. 

1899. Prof. Theodor Liebisch, Berlin. (Deceased.) 

1900. Prof. Federico Sacco, Turin. 

1902. Dr. Thorvaldr Thoroddsen, Copenhagen. 

1904. Dr. Erich Dagobert von Drygalski, Charlottenburg. 
1904. Prof. Giuseppe de Lorenzo, Naples. 

1904. The Hon. Frank Springer, Hast Las Vegas, New Mexico (U.S.A.). 
1906. Prof. John M. Clarke, Albany, N.Y. (U.S.A.). 

1906. Prof. William Morris Davis, Cambridge, Mass. (U.S.A.). 
1909. Dr. Daniel de Cortazar, Madrid. 

1911. Prof. Arvid Gustaf Hogbom, Upsala. 

1911. Prof. Charles Depéret, Lyons. 

1912. Dr. Whitman Cross, Washington, D.C. (U.S.A.). 

1912. Baron Francis Nopcsa, Vienna. 

1912. Prof. Karl Diener, Vienna. 

1912. Prof. Fusakichi Omori, Tokyo. 

1912. Prof. Ernst Heinrich Weinschenk, Munich. (Deceased.) 
1913. Dr. Per Johan Holmquist, Stockholm. 

1921. Prof. Lucien Cayeux, Paris. 

1921. Dr. Maurice Cossmann, Paris. 

1921. Prof. Henry de Dorlodot, Louvain. 

1921. Dr. Henry Douvillé, Paris. 

1921. Prof. Louis Duparc, Geneva. 

1921. Prof. Johan Kier, Christiania. 

1921. Prof. Waldemar Lindgren, Boston, Mass. (U.S.A.). 
1921. Prof. John J. Stevenson, New York City (U.S.A.). 


XXIV PROCEEDINGS OF THE GEOLOGICAL sOcIety. [| vol. Ixxviii, 


(Notre.—The Lists of Awards of Medals and Funds, up to the year 1907 
inclusive, are published in the ‘ History of the Geological Society.’ } 


AWARDS OF THE WOLLASTON MEDAL 
UNDER THE CONDITIONS OF THE ‘ DONATION FUND, 


ESTABLISHED BY 
WILLIAM HYDE WOLLASTON, M.D., F.R.S., F.GS., ere. 


To promote researches concerning the mineral structure of the Earth, and to 
enable the Council of the Geological Society to reward those individuals of any 
country by whom such researches may hereafter be made, —‘ such individual not 
being a Member of the Council.’ 


1908. Prof. Paul von Groth. ) 1916. Dr. A. P. Karpinsky. 
1909. Mr. Horace B. Woodward. | 1917. Prof. A. F. A. Lacroix. 
1910. Prof. William B. Scott. | 1918. Dr. Charles D. Waleott. 
1911. Prof. Waldemar C. Brégger. | 1919. Sir Aubrey Strahan. 
1912. Sir Lazarus Fletcher. | 1920. Prof. G. J. De Geer. 
1913. The Rev. Osmond Fisher. Dr. B. N. Peach. 

1914. Prof. John Edward Marr. 1921. ae John Horne. 


1915. Sir T. W. Edgeworth David. | 1922. Dr. Alfred Harker. 


AWARDS 
OF THE 


BALANCE OF THE PROCEEDS OF THE WCLLASTON 
‘DONATION FUND’? 


1908. Dr. Herbert Henry Thomas. | 1916. Mr. William Bourke Wright. 
1909. Mr. Arthur J. C. Molyneux. | 1917. Prof. Percy G. H. Boswell. 
1910. Mr. Edward B. Bailey. 1918. Mr. Albert Ernest Kitson. 
1911. Prof. Owen Thomas Jones. 1919. Dr. A. L. Du Toit. 

1912. Mr. Charles Irving Gardiner. | 1920. Mr. William B. R. King. 
1913. Mr. William Wickham King. | 1921. Dr. Thomas O. Bosworth. 
1914. Mr. R. Bullen Newton. 1922. Dr. Leonard J. Wills. 
1915. Mr. Charles Bertie Wedd. 


part 1] ANNUAL REPORT. XXV 


AWARDS OF THE MURCHISON MEDAL 


UNDER TH# CONDITIONS OF THE 


‘MURCHISON GEOLOGICAL FUND,’ 
ESTABLISHED UNDER THE WILL OF THE LATE 
SIR RODERICK IMPEY MURCHISON, Barr., F.R.S., F.G-S. 


“To be applied in every consecutive year, in such manner as the Council of the 
Society may deem most useful in advancing Geological Science, whether by 
granting sums of money to travellers in pursuit of knowledge, to authors of 
memoirs, or to persons actually employed in any enquiries bearing upon the 
science of Geology, or in rewarding any such travellers, authors, or other persons, 
and the Medal to be given to some person to whom such Council shall grant 
any sum of money or recompense in respect of Geological Science.’ 


1908. Prof. Albert CharlesSeward. | 1916. Dr. Robert Kidston. 
1909. Prof. Grenville A. J. Cole. 1917. Dr. George F’. Matthew. 


1910. Prof. Arthur P. Coleman. 1918. Mr. Joseph Burr Tyrrell. 
1911. Mr. Richard Hill Tiddeman. | 1919. Miss Gertrude L. Elles. 
1912. Prof. Louis Dollo. 1920. Dame E. M. R. Shakespear. 
1913. Mr. George Barrow. 1921. Mr. Edgar Sterling Cobbold. 
1914, Mr. William A. E. Ussher. 1922. Dr. John William Evans. 


1915. Prof. William W. Watts. 


AWARDS 
OF THE 


BALANCE OF THE PROCEEDS OF THE 
‘MURCHISON GEOLOGICAL FUND.’ 


1908. Miss Ethel Gertrude Skeat. | 1916. Mr. George Walter Tyrrell. 
1909. Dr. James Vincent Elsden. 1917. Dr. William Mackie. 

1910. Mr. John Walker Stather. 1918. Mr. Thomas Crook. 

1911. Mr. Edgar Sterling Cobbold. | 1919. Mrs. Eleanor Mary Reid. 
1912. Dr. Arthur Morley Davies. 1920. Dr. David Woolacott. 

1913. Mr. Ernest EK. L. Dixon. 1921. Dr. Albert Gilligan. 

1914, Mr.Frederick NairnHaward. | 1922. Mr. Herbert Bolton. 

1915, Mr. David Cledlyn Evans, 


XXV1 PROCEEDINGS OF THE GEOLOGICAL society. | vol. lxxviu, 


AWARDS OF THE LYELL MEDAL 


UNDER THE CONDITIONS OF THE 
‘LYELL GEOLOGICAL FUND, 
ESTABLISHED UNDER THE WILL AND CODICIL OF THE LATE 
STR CHARLES LYELL, Banrr., F.R.S., F.G:S. 


The Medal ‘to be cast in bronze and to be given annually’ (or from time to time) 
‘as a mark of honorary distinction and as an expression on the part of the 
governing body of the Society that the Medallist (who may be of any country 
or either sex) has deserved well of the Science,’—‘ not less than one third of the 
annual interest [of the fund] to accompany the Medal, the remaining interest 
to be given in one or more portions, at the discretion of the Council, for the 
encouragement of Geology or of any of the allied sciences by which they shall 
consider Geology to have been most materially advanced, either for travelling 
expenses or for a memoir or paper published, or in progress, and without 
reference to the sex or nationality of the author, or the languagein which any 
such memoir or paper may be written.’ 

There is a further provision for suspending the award for one year, and in 
such case for the awarding of a Medal to ‘each of two persons who have been 
jointly engaged in the same exploration in the same country, or perhaps on 
allied subjects in different countries, the. proportion of interest always not being 
less to each Medal than one third of the annual interest.’ 


1908. Mr. Richard Dixon Oldham. ; 1915. Prof. Edmund J. Garwood. 
1909. Prof. Percy Fry Kendall. | 1916. Dr. Charles W. Andrews. 
1910. Dr. Arthur Vaughan. | 1917. Dr. Wheelton Hind. 

Dr. Francis Arthur Bather. | 1918. Mr. Henry Woods. 

ios. | Dr. Arthur Walton Rowe. | 1919. Dr. William Fraser Hume. 
1912. Mr. Philip Lake. | 1920. Dr. Edward Greenly. 

19138. Mr. Sydney S. Buckman. —=§-_- 11921. M. E. de Margerie. 

1914. Mr. Charles S. Middlemiss, 1922. Dr. Charles Davison. 


part 1] ANNUAL REPORT. XXVIl 


AWARDS 
OF THE 


BALANCE OF THE PROCEEDS OF THE 
‘LYELL GEOLOGICAL FUND,’ 


1908. Prof. T. Franklin Sibly. ( 1916. Mr. Alfred S. Kennard. 
1908. Mr. H. J. Osborne White. 1917. Prof. A. Hubert Cox. 
1909. Mr. H. Brantwood Maufe. 1917. Mr. Tressilian C. Nicholas. 
1909. Mr. Robert G. Carruthers. 1918. Mr. Vincent Charles ling. 
1910. Dr. F. R. Cowper Reed. 1918. Mr. William Kingdon 
1910. Dr. Robert Broom. Spencer. 

1911. Prof. Charles Gilbert Cullis. | 1919, Mr. John Pringle. 

1912. Dr. Arthur R. Dwerryhouse. | 1919. Dr. Stanley Smith. 

1912. Mr. Robert Heron Rastall. 1920. Dr. John D. Falconer. 


1913. Myr. Llewellyn Treacher. 1920, Mr. Ernest 8. Pinfold. 
1914. The Rev. Walter Howchin. 1921. Dr. Herbert L. Hawkins. 
1914. Mr. John Postlethwaite. 1921. Mr. C. E. N. Bromehead. 
1915. Mr. John Parkinson. 1922. Mr. Arthur Macconochie. 
1915. Dr. Lewis Moysey. 1922. Mr. David Tait. 


1916. Mr. Martin A. C. Hinton. 


XXVill PROCEEDINGS OF THE GEOLOGICAL society. [ vol. Ixxviil 


AWARDS OF THE BIGSBY MEDAL, 


FOUNDED BY THE LATE 


De. J. J. BIGSBY? BIS. Ges: 


To be awarded biennially ‘as an acknowledgment of eminent services in any depart- 
ment of Geology, irrespective of the receiver’s country; but he must not be 
older than 45 years at his last birthday, thus probably not too old for further 
work, and not too young to have done much.’ 


1909. Dr. John Smith Flett. 1917. Mr. Robert G. Carruthers. 
1911. Prof. Othenio Abel. 1919. Sir Douglas Mawson. 
1913. Sir Thomas H. Holland. 1921. Dr. Lewis L. Fermor. 


1915. Dr. Henry Hubert Hayden. 


AWARDS OF THE PRESTWICH MEDAL, 


ESTABLISHED UNDER THE WILL OF THE LATE 


SIR JOSEPH PRESTWICH, F.B.S., F.G.S. 


To apply the accumulated annual proceeds ... at the end of every three years, in 
providing a Gold Medal of the value of Twenty Pounds, which, with the 
remainder of the proceeds, is to be awarded ...to the person or persons, either 
male or female, and either resident in England or abroad, who shall have done well 
for the advancement of the science of Geology ; or, from time to time to accumulate 
the annual proceeds for a period not exceeding six years, and apply the said 
accumulated annual proceeds to some cbject of special research bearing on 
Stratigraphical or Physical Geology, to be carried out by one single individual or 
by a Committee; or, failing these objects, to accumulate the annual proceeds for 
either three or six years, and devote such proceeds to such special purposes as 
may be decided.’ 


1909. Lady (John) Evans. 

1912. Library extension. 

1915. Prof. Emile Cartailhac. 

1918. Sir William Boyd Dawkins. 
1921. List of Geological Literature. 


part 1] ANNUAL REPORT. XK 


AWARDS OF THE PROCEEDS OF THE BARLOW- 
JAMESON FUND, 


ESTABLISHED UNDER THE WILL OF THE LATE 
Dr. H. C. BARLOW, F.G.S. 


‘ The perpetual interest to be applied every two or three years, as may be approved by 
the Council, to or for the advancement of Geological Science.’ 


1908. ‘Grey-Wether’ sarsens on | 1915. Mr, Joseph G. Hamling. 


Marlborough Downs. 1917. Mr. Henry Dewey. 
1911. Mr. John Frederick Norman | 1921. List of Geological Litera- 
Green. ture. 


(Mr. Bernard Smith. 


19138. ; 
) Mr. John Brooke Scrivenor. 


AWARDS OF THE PROCEEDS OF THE 
‘DANIEL-PIDGEON FUND,’ 


FOUNDED BY MRS. PIDGEON, IN ACCORDANCE WITH THE 
WILL OF THE LATE 


DANIEL PIDGEON, FE.G.S. 


‘An annual grant derivable from the interest on the Fund, to be used at the 
discretion of the Council, in whatever way may in their opinion best promote 
Geological Original Research, their Grantees being in all cases not more than 
twenty-eight years of age.’ 


1908. Mr. James A. Douelas. | 1916. Dr. John K. Charlesworth. 
1909. Dr. Alexander M. Finlayson. | 1917. Dr. Arthur Holmes. 

1910. Mr. Robert Boyle. 1918. Mr. James A. Butterfield. 
1911. Myr. Tressilian C. Nicholas. 1920. Miss M. EK. J. Chandler. 
1912. Mr. Otway H. Little. 1920. Mr. L. Dudley Stamp. 
1913. Mr. Roderick U. Sayce. 1921. Mr. Ralph W. Segnit. 
1914. Prof. Percy G. H. Boswell. | 1921. Mr. Frederick 8. Wallis. 


1915. Mr. E. Talbot Paris. 


XXX PROCEEDINGS OF THE GEOLOGICAL society. [ vol. lxxviul, 


Estimates for 
INCOME EXPECTED. 
2 Rs £ Siow 
ComposwiOus, «6. 94 eae eee eae 157 10° 0 
Ardmigsion—nees; 1902 py ie) 5S pee APO 20:30 
————._ 677 10 0 
Arrears of Annual Contributions ............ 150 0-0 
Annual ‘Contributions, 1922 0... 0. 2.scse ea ee 2100 0 O 
Annual Contributions in advance............ 502 0 20 
— 2320 0 0 
Quarterly Journal Subscriptions ............ 220 0 
Record of Geol. Lit. Subscriptions .......... 30 0 
a 250 0 0 
Sale of the Quarterly Journal, including Long- 

MANS PNCCOMMU,) 2. sts Can | oe te eee 400 0 0 
Salevoisother Publications: 7.2. :cra. cues 15 079 
Maccellancous Receipts |... ..2.5 seek Se 30 0 9 
Interest on Deposit-Account................ 20°05 
Dividends on £2500 India 3 per cent. Stock .. 75 O O 
Dividends on £300 London, Brighton, & South 

Coast Railway 5 per cent. Consolidated Pre- 

HERETICS TS UOGK 1 a5, aye teeter sce ccs meeuey states Malte Toe Loy 0.70 
Dividends on £2250 London & North-Western 

Railway 4 per cent. Preference Stock...... 90°07 0 
Dividends on £2800 London & South-Western 

Railway 4 per cent. Consolidated Preference 

EOC eos a be, ong Meee Pe ee eT ae 112 0 0 
Dividends on £2072 Midland Railway 23 per 

cent. Perpetual Preference Stock.......... ol AGS 0 
Dividends on £267 6s.7d. Natal 3 percent.Stock, 8 O O 

— 351 16 0 
Total Ordinary Income. . £3964 6 O 
Dee ee oat ae 445 14 0 


£4410 O O 


ee 
e 


part 1] FINANCIAL REPORT. XXX1 


the Year 1922. 


EXPENDITURE ESTIMATED. 


ase. BE TE Gls 
opts and. Maintenance MUNG ... 0.6.8 vee wet ee ee 2007 OPO 
House-Expenditure : 
ames sds: Hire-Insuvanee: ii leccsccaseses cee eines 2by 0.0 
Electric Lighting and Maintenance ............ 65 0 0 
COIS «an, PE IRS ate 9 ae hed cans an Ae Se eee 40 0 0 
TEEIS] LUN ao AR «sels act) Sa ee 60 0 O 
PATATUM EL CHOATE yl sles crs a cee i faicla Met ad oleie ae 25010) 
Washing and Sundry Expenses.................. 40> 700 
AP oa be MCE TENE SE heel cn cce anes ke nen deere te 30 0 0 
—— 315 0 0 
See AEP SPATLO MN AP OS MEUC oie ye sks easels eae ls age (erat wae Ores 1450 0 
Office-Expenditure : 
SSUTTSTTIGY Cen" Saas eye Meee Mine Uiban vee teres AVEO ARE crn 60% 0% .0 
MascellancousePrintine «eo keice sessed dacannniree as 130) 0: 0 
Postages and Sundry ee Schatten es LOO) O06 
List of Fellows ......... Ete eee tee OE Ore) 
390 0 0 
Grant to Conjoint Board of Scientific Societies............ 105 O20 
iibrary (Books and Binding) .............: LON Ok 0) 
(G@aralocue Cards) po ssn seen ele ee 5 20070) 0 
70 O10 
Publications : 
Quarterly Journal (Vol. lxxvii), including 
Commission on Sale ...... EME CORTES REA 950 0 0 
Postage on Journal, Addressing, ete. ......... fo. BORED 
Abstracts of Proceedings, including Postage. 250 0 0 
Record of Geological Literature for 1921 ... 150 0 0 
—— 1425 0 0 
Total Ordinary Expenditure. . ES W160)) V0) 
Special Expenditure. 
Compilation of Record of Geological Literature Cee 
EW rs ect pol ane SO TE ae re heared RK it. 450 O O 
£4410 O O 


ROBERT 8. HERRIES, Treasurer. 
January 27th, 1922. 


Income and Hxpenditure 


during the 


RECEIPTS. 
son ae eae ss SS eee 
To Balance in the hands of the Bankers at 
January Ist, 1921: 
Current Account ............... 289 «4 6 
,, Balance in the hands of the Clerk at 
January lst, W920 4 ie eee eee ie a 
——— 309 12 38 
F COMLPOSIEIONS %,,.:c10 =m po eae 157 10556 
,, Admission-Fees: 
ATTEATS Co nee ne ee £63. 16 7G 
CEPECH Ges eel eee 283 10 0 
—___—— 447 6 © 
,, Arrears of Annual Contributions ......... 131 ‘eae 
., Annual Contributions for 1921 :— 
Resident Fellows ...............20384 7 6 
,, Annual Contributions in advance ...... ro agen Oe eC} 
— 2121 7 6 
,, Publications : 
Sale of Quarterly Journal : 
»  Vols.ito lxxvi (less Commission 
SAOVaSe ORG. )a es eee ae 132 14 0 
» Vol, lxxvii (less Commission 
S36 "6s. 5d). oe 204 19 3 
—— = 337 138 3 
,, Quarterly Journal Subscriptions e 233 18 3 
,, Other Publications (less Commission) ..... o0 (alae 
eeMuscellaneous fueCCeIpts... :dk.\.0. Asete suet sane Ses 30 14 10 
,, Interest on Deposit 22 oe 
.. Dividends, as received :— 
£2500 India 3 per cent. Stock ............ io 04 0 
£300 London, Brighton, & South Coast 
Railway 5 per cent. Consolidated 
Preference Stock ................0. 10 10°. 0 
£2250 London & North-Western Railway 
4 per cent. Preference Stock...... 63. °0/ 50 
£2800 London & South-Western Railway 
4 per cent. Consolidated Prefer- 
ENCE NS LOK 4525 Sepa seen ook is 8.0 
£2072 Midland Railway 23 per cent. 
Perpetual Preference Stock ...... dO eee 
£267 6s. 7d. Natal 3 per cent. Stock...... 5, a 
£500 5 per cent. War Stock (1929-1947) Zo, Oe we 
-—— 293 15 6 
PERE OnIe— hax reCOVELet a cse> see aa ke Bee eatin 838 O10 
Special Receipts. 
To Transfer from Sorby & Hudleston Bequests. 171 15 0 
- a » the Prestwich Fund ............ G5 pls 76 
= e , the Barlow Jameson Fund ... 5G ASR 49 
—————_ 294 18 38 
,, Sale of £500 5 per cent. War Stock 1929-1947... 488 5 3 
£4936 16 5 


ae 


OO 


Year ended December 81st, 1921. 
PAYMENTS. 


By Maintenance Fund 

,, House-Expenditure : 
SANE SMR T eclo rn Mis ameter cnt Lae yen Re oa Oe 
Fire- and other Insurance ..........02..6.0.0 
Electric Lighting and Maintenance......... 
NE SMe cee tite ante last os Res uA a IS 
ite peer eRe eect Sak osteal op Miamec nek neds 
rnitnrevand: Repairs) ..ujicss.ssccse cence ss 
House-Repairs and Maintenance 
pXcamipre) Le aI: ies societies < kebiac gates Da 
Washing and Sundry Expenses............... 
Mearah Meee. ey h key aula Sach eck Gessun 


',, Salaries and Wages, etc. : 
Permanent Secretary 
LT OSC TET WT os AU A Me Cee, Ae eg A See SO 
CHSee es Na IS seagate Sse 
SuMIO Tr MASSIS EAT ac. ca ciccsvowisn ate Galstees ets esas 
House-Porter and, Wile: ..cocclecs.c0asowederes 
SE NOTE SO TINA CUM ar Psd catches ar was Sts ate ge cies 
Charwoman and Occasional Assistance ... 
Accountants’ Fee 
Extra Assistance 


i ee ce er rca) 


Pe 


Se 


,, Office-Expenditure : 
SSE RIOLUCT AA. Seas. eee ee enn een een ee 
Miscellaneous Printing ..5...... 5.0660 0+ssswees 
Postages and Sundry Expenses 


~~ 


, Library (Books and Binding, etc.)...... 
» Library-Catalogue: Compilation 
», Publications : 


Quarterly Journal, Vol. Ixxvii, Paper, 


Printing, and Ilustrations.................. 
Postage on Journal, Addressing, etc. 
Abstracts, including Postage 
List of Geological Literature 


i ee ny 


» Grant to the Conjoint Board of Scientific 


Societies (1920 & 1921) . 


Ce ee 


ist) 

io} 

ps 

Ne} 
WSOHMHMNOAKONS 


a 

ty 

ya 

ee — 
coornNnaooccoo 
eoqoocoo cocoon] 


289 4 


1429 0O 


eee eee eee are 


bo 
TING 
for) 
— 
or 
© & 0 00 


Special moeadiee 


By Quarterly Journal (parts 3 & 4 of Vol. lxxvi) 


= cGeolocical Eaterature, (90S +..20..5..5)..ass-205 
ss <5 e , 1915-19, Compilation 
(nia). ote ees 


623 12 2 
249 5 4 


By Balance in the hands of the Bankers 


a December dist, L921 2 os. 
,, Balance in the hands of the Clerk at 
Wecemaper ols, LOZ see snes es 


We have compared this Statement with 
the Books and Accounts presented to us, 
and find them to agree. 


J. FREDK. N. GREEN, 


R. M. DEELEY, 
January 27th, 1922 


VOL. LXXVITII. 


od 2 5 


£4936 16 


dl9 7 
164 17 
50 O 


1441 15 


20 10 


Sl ey 


6 


ono 


5 


Auditors. 
ROBERT S. HERRIES Treasurer. 


(67 


—— —— 


& G &6lé ) 6 6G SGlF 

Sole se! ‘S18 Laquiade(] YV SLoyUBg oy} 4v osouryeq “ 

0. SL ey ttt ** puny oy jo eouupeg oq) wouy epi | 0 £ Of See TOOLS so Mted 2@ WHTTOUuonte ly 
00-96) ee asiepelt eu) eoepammay” * ‘PO SL OLOGF UE poyoaur pung ey} wo spueprarq “ 
Oko Tia ee ae eee Se a eee ge eee GL GG ‘''''" 166L 4ST davnure ye stoyueg oq 1% souRleg OF, 
oy os) Me ‘SINAWAV Way Ig ‘SLAIMOWY] 


“EINTOODY LsOaL ~aNOW TVOIDOTOUL) TIMA'T , 


L 0 99F 4 0 99% 

BOO BL tit ititrsssrssssss netaaooer xB, ewO0dU 
Gs OaOG -e” TAGL “ITS Taq aoa] JB ScoyUBg O44 4 oourpL i RAO BEC anes eae TOONS eS + 
iP Fete LIMB Eas Pee | NUE TeelOh a ERE ‘yuo ted ¢ AvMyTVY U.19}89 AA -YI10 NT WY UOpUO’] PEElLF 
DONA 2 ea ee a a Sl] [Vpey oY} 07 prvmaw UL poJSeAUL puny oY} UO (XBT -eMODUT sseT) SpueprAt(y 
0 &G I ON OCG OTE6O000 08 06 a0 ee es ek! SOR etn Jo 4sog kg @ 0 96 see eee IZ6L 8] AIVNU (? 4B SLoy Ug eur qe gourleq OL 
Ds F ‘SLNANWAY i isaanee ‘SLAILONY 


“INDODDW ISAUT, (ANNO IVOIN0TOUD NOSIHOUNT , 


p L v9F p L POF 

Cee or Sra T 787 ‘SIE Iaqmieoeg] JU sieyxuvg oyj qv couvjeg “ |e @ ge “*** set sssersesenssss yoone queo ted e Aqan0Q 
Gecmmlelaroe ot ‘puny On gO) eouvled oY} UOT} prvary Hs arrysdue yy 6LO1F Ul po}seAUL puny 94} UO spusplAlyy ‘ 

() GO) WE a ee ea eae TRO OTtTO ISOC NAG Kum Cme CMI: ame mniaGin SI] Arena qu SLUR oY} 7B vouvleq oO], 
ier OF “SUNWIVAV J ie “See ‘SLAIMOM YY 


“INQOOODW Shay, “ANA NOILVNO(] NOLSVTTO AA , 


LE6L “ISIE saquasag: : spun y yowady puo spungy-jsns], fo quawuainyy 


“LNQOOOW sayy, 


Periss 9 SI 19g 
® © We cob sis shoo FWles arOak uate) ssc aea, 5 aha Meee ane “M9036 “41189 
0 SI ST °*"* TZ6L 481g tequiecog 4v StoyuVg oy} 4v sourjeq “ tad ¢ BIpUy CQZF Ul poysoAul pun, 943 uo Spueprarq “ 
ole) a eee Se GunGoae: sesoding [vlouex) 0} coysuery, NGG, iP Gg ove ec coc IG6L “FST Arenuey qe SLOYUV Ot} 4v oouvleg oy, 
P'S F ‘SINATNWAV OS Ge ‘SLdIGOaYy 
‘ENNOOOW ISAUy, “ANA Lsayy, Hormismg , 
Ee = 
O 6 8LF ORGr e275; 
r O Tie deeaaeee Chal feiag smote ‘i Liay-emey Bere: feuaeh Pietemcesiie qisodaq uo 4sereyuy “ 
p e 7 i Ns Te AG Ie ee over quconadse 
O T 02 °""" 1&6T S8Tg rsqutesag ae MOyUV oy} 78 sourreq ‘ VIPUT PL SS OSTF Ul poyseaur puny ay} uo Spuoprarq “ 
0 Q Q A ne a eee Sema Aer | G @) 7" TE6T “sy Avenue ye sroyueg oy} qv oourpes. of 
ha mas J ‘SINAWAVG i, 38 ee ‘SLIIGOUY 
“ENDODOV SAU, (ANAT FarTayy TVOIDOTONY) , 
IL SL 9OLé [Il SL 91 
(Cay ie EF eee ear rer ot creer Tot uiGaC CIC peleaoder xvy autoouy “ 
Cie Ore bald « cee sun's he ge Me YOIG “juso red e Yrpreg OLSF 
TILT ¥V °*"* TZ6T 48T¢ requies9q ye SLoyUr_ oy} 4v oourpeg “ UL po}soaur puny oy4 UO (xv p-autoouT Sso]) spueprayy 
0 Gili ral OO ONG OG Gta coo ish ee ts O 0-0" (0 U0 DeO-u t G5 ao [Bpesv jo 4soK) Aq IL ki OT soo ee ee "1261 4ST ATENUB ev SLoyULg eu} 4v ooUv[Vq OL 
Lama F ‘SINGWAV 4D oS + “SLI MOY 
“LNQOOOW ISaUy, “GNA Kasorg , 
@ % 909 € & 90F 
Z F fp ee Pore Nooen xu oamony (a4 
8 O16 TET Se Es Sale ee Ee a recess 
arnjzueqe(yT “yue0 ted ¢ Avmprery WIOYJAON JVOINH) QOPF 
9 @ G6 *""* T26T 48g tequiesag 42 STOyUR oy 4v sourvpeq “ UT peySoAul puny ey} wo (xey-emtoouy sgaq) spuepraiq “ 
G 6 9S) i  untopan, socodinyy [vloudsy) 09 Tajsuvay AG i|@ [T gq cttcees “TG6L ‘“IsT Auvnave ye SLOYURE 911} 4B oouLTLE OT, 
fe 5: aire ‘SINUNAV JT ps & ‘SLaIHOay 


, INA] NOSHWV P-MOTUVG 


‘AATANC ‘W 


Boal NOLO ‘NMC “Lf 
‘ga18v 0} WaT} puy pue ‘sn 0} payuosord syuNODDW puUL syoog oY} YITA JUOMIOIRIG sIyy poredutod oAvy o AA 


Ga PSGe 


——-- 


0 6 Z&I 
6 OL Olt 
‘8 F 


9 VI 984 


0 & OVE 


— — 


SB  Ga8il 
vy LI O& 
DS F 


"BC6L “YILE hannuny 
‘dainsvaty ‘SHTMUNAH S LUAAOuU 


cs: 


"'** TZ6T ISTE coqmoood 7B SloyUVg oy} 4v 9OUB|VE Mae Gg 
reeorereseseeses oy9k oy} culinp sjuomivg Aq|/gQ 0 O0&Z 
"SINGTINAV cL ge ee 


ooeevoeoereeenewneoe ee ee 


rrverseerees qtzodaq uo ysereqyuy “ 
rrreeeeees qunoooy sosoding [Bloor WLOI, Lojsuvty, OF, 
‘SLATCLOWY, 


"INO PONVNELNIV]L 


9 PL 98s 
i SEAL 
0 0 GI 
Wepcrennientcens apa nies Bane eto ormeorG * qunoooy Juetiny) Pate HOT 
> 1Z6L “asTe ToquLeIa(T 4B SloyUBE OY} 4B OOUBlBE us 
pee UTOOO NT sosoding [VloUex) 07 LoJSURAT, Ay OL OL Ff 
“SENANWAYV q 1's & 


“LSUNoA, ATUOG 


9 PL 98F 
r SLT 
0 O GL 
Piya ace: .4 atts ep PS PR aS ‘+ qunoooy quaring a 
SIZBL ISTE Loquisoeg qv saoyuvg oy} yv oourpeg “ 
Le tetoent oe st e PULOOD W sosodng [Blouoy) 0} LoysuBIT, Ag OL OL FF 
"SLNUWAV ‘m3 


ble: 0) 0; (6: verse) ec sate” (@. #0 c6) Or uel @w el ceeremetiin corr ear sa Cooma 8 


qisodecy uo yworequy “ 

poleaooed xBy, outoouT * 

* yoozg ‘yueo aed Fe vpeury QOOTS 

UL PoJsOAUL puny oy} UO (xy -eULOOUT ssoy) Spuspraly 

rreeseeerzer sty AlvNULE IV SIoyUB_ oy} 4V oouvpeg OT, 
‘SLALLO WY] 


floret. UM ree ry) (ti dh Oar 0 1) KPa OO TRO oeee 


‘*qtsodaqy uo ysotequy “* 


polaaooot XBT, oMODUT 
yo0jg “yuoo 10d. $e vpvurD QOOTS 
Ul poySOAUL PUNY OY} UO (XBJ-oMODU] Sse]) Spuoptat(y 
rreeressrcer ‘sy AIBnUBE yv SloxUV_ oy} 7B dOULBTVE OT, 
"‘SLAIMOMY : 


~ 
n 


n 
ron 


“LSaNoOUg NOLSATAN FL 
SAGNNH TIVIOUdS 


0 & IF 


(UL (Ok 


ors? Tee ISLS requrese([ qv SloyUBey OY 4v ooUBlRET Us 


ripteeeereseererses onremy fo | Q 
'SLNUNAY J ?P 
“INQOODY LSauy, 


GI 


(al 
‘Ss ¥ 


yoozg *yueo zed ¢ uorTyerodaoy [OIStAgT 

‘0G ‘ST BIOLF Ul poisoaut puny oy} wo spuoprayy “ 
reeeressrcar Gay Arenuse ye Sloyuvg oy} 7¥ soueleg OT, 
‘SLAILOMY, 


CANA NOWPAIG-IMINV(Y , 


part 1] FINANCIAL REPORT. 


Statement relating to the Society’s Property. 


December 31st, 1921. 


Balance in the Bankers’ hands, December 31st, 
ee Nerina 1g) cov eva Se wale Acasa Rese as 7A 8) 


Balance in the Clerk’s hands, December 31st, 1921 Oe 


Balance of the Maintenance Fund ............ 137.90 
Balance of the Sorby and Hudleston Funds .... 114 0 
Arrears of Annual Contributions .............. 


(Estimated to produce £1380 Os. Od.) 


Funded Property, at cost price :— 
2000 India 3 per cent. Stock ......60.+6. 2623 19 0 


£300 London, Brighton, & South Coast Rail- 
way 5 per cent. Consolidated Preference 


EIMOG Ge iiinieeh ooh enn hades p -Adn esis pe OUD Abas 
£2250 London & North-Western Railway 
4 per cent. Preference Stock ............ 2898 10 6 


£2800 London & South-Western Railway 
4 per cent. Consolidated Preference Stock. 8607 7 6 


£2072 Midland Railway 223 per cent. Per- 


Peepermal Preference Stock on... cus ccs raw 1850 19 
£267 6s. 7d. Natal 3 per cent. Stock ...... 250 0 0 
£2000 Canada 32 per cent. Stock (1930- 

Ped MO ee aetna et gsc sen RE ar shisia 4: nga Diao st 1982 11 0 


/ 


XXXV11 
£&s da 
{2 6 2 
1389 3 O 
226 6 O 
£439 15 2 


——_——— £13,716 2 9 


[Notrr.—The above amount does not include the value of the Library, Furniture, 
and stock of wnsold Publications. The value of the Funded Property of the 
Society, at the prices ruling at the close of business on December 31st, 1921, 


amounted to £7193 6s. 6d. ] 


ROBERT 8S. HERRIES, 7reasurer. 
January 27th, 1922. 


XXXVlll PROCEEDINGS OF THE GEOLOGICAL Socrety. [ vol. lxxviii, 


AWARD OF THE WOLLASTON MEDAL. 


In presenting the Wollaston Medal to Dr. Atrrep HaRrKker, 
F.R.S., the PRESIDENT addressed him as follows: — 


Dr. HarKEer,— 


From the first you have recognized, as you were among the first 
to realize, the necessity of combining field-work with laboratory 
research. Well qualified by your mathematical training to apply 
the laws of crystal optics and physical chemistry to petrological 
problems, your early work on the Bala volcanic series, the Shap 
granite, and Carrock Fell, established your position as a petrologist 
and field-geologist. Selected to the Geological Survey for the 
purpose of surveying portions of the Tertiary volcanic region of 
the West of Scotland, your ten years’ service produced two important 
memoirs, on the Tertiary igneous rocks of Skye and on the Small 
Isles of Inverness-shire. In addition to these, and many other 
records of observations, your work has always been characterized 
by a breadth of view and a recognition of fundamental principles. 
Applying the results of physical chemistry to the differentiation 
of rock-magmas, you have indicated a manner in which a separation 
of the alkaline and calcic portions might be brought about, and 
you were the first to draw attention to the correspondence of the 
distribution of these types with areas characterized by the Atlantic 
and Pacific types of coast-line. Your work on the ‘ Natural 
History of Igneous Rocks’ and your addresses to the British 
Association and to this Society have all been important contribu- 
tions to theoretical geology. For these reasons the Council has 
accorded to you the Wollaston Medal, in recognition of those 
manifold services, by which you have advanced our knowledge of 
the mineral structure of the Earth, both in the narrower and in the 
more extended meaning of the words. 


Dr. Harker replied in the following words :— 
Mr. PRESIDENT,— 


I wish to express my keen appreciation of the distinction which 
has been conferred upon me, and to record my thanks to the 
Council for their generous estimate of my merits. To so high an 
honour I have never ventured to aspire. Ifa perusal of the list of 


part 1] ANNIVERSARY MEETING—MURCHISON MEDAL. XXXIX 


former Wollaston Medallists left me with a lively sense of my 
own unworthiness, this feeling has at least been tempered by the 
kind congratulations of friends, and now by the graceful words 
with which you have accompanied the presentation. 

The occasion is one which invites retrospect, and the personal 
note will perhaps be pardonable. Looking back, I acknowledge 
that the lines have fallen to me in pleasant places. I should be 
ungrateful were I to forget the constant support extended to me 
by my College, or the debt which I owe to Prof. Hughes, who 
first turned my steps into the paths of Geology. Gratefully too, 
I recall the encouragement which I have received from this 
Society, and from the comradeship of colleagues at Cambridge and 
many friends in London and Edinburgh. Much of my active life 
has been passed in teaching. If such occupation limits in some 
measure the time that can be given to private work, it still 
brings notable compensations. It is good, I think, to be brought 
back often to first principles, and to be braced by contact with 
younger and more ardent minds. You, Sir, have emphasized the 
value of field-work in conjunction with laboratory research. It 
was by the good offices of Sir Archibald Geikie that I have been 
enabled to enjoy this advantage also, and to acquire some experience 
of systematic mapping amid the delightful scenes of the West 
Highlands. | Cys 

I wish I could believe that all these favours of fortune have 
been turned to the best account in the service of Geology, but I 
am conscious of many shortcomings. There remains the hope that 
some years of activity may still be left for me to justify, if I can, 
the honour now bestowed upon me. 


AWARD OF THE Murcutson MEDAL. 


The PresmpEntT then presented the Murchison Medal to Dr. 
Joun Wii1iaM Evans, F.R.S., addressing him as follows :-— 


Dr. Evans,— 


Taking up the study of geology, originally from pure love of the 
subject, you have had an extensive, varied, and fruitful experience. 
In Western and Southern India and in South America you have 
conducted and controlled important geological investigations, 


a PROCEEDINGS OF THE GEOLOGICAL society. [ vol. lxxvin, 


contributing largely by your own observations to, and by 
your writings to the dissemination of, the results; nor have you 
feared to attack the more abstruse and polemical problems of 
theoretical geology, of which you have been an eloquent illuminator 
and interesting exponent. To many, doubtless, this aspect of your 
activities looms largest, to the exclusion of the great amount of 
laborious and detailed work which you have devoted to observation 
in field and laboratory, and of the contributions which you have 
made to our knowledge of the origin of rocks, both igneous and 
sedimentary. It is this side of your activities which carried most 
weight with the Council in its decision to award to you the 
Murchison Medal of the Society, nor was it oblivious of the 
service which you have rendered to science, by the use of your 
influence, in initiating, extending, and guiding the conduct of geo- 
logical investigations in the colonies and dependencies of the 
British realm. Not in one way only have you earned that recog- 
nition which it is a pleasure to convey to you in the name of the 
Society. 


Dr. Evans replied in the following words :— 


Mr. PREsIDENT,— 


It is nearly a third of a century since Prof. Judd awarded to me 
a medal struck from the same die as that which I have just been 
so fortunate as to receive at your hands. The former was given 
as an encouragement to one who was commencing his geological 
career. This Medal, I trust, I can accept as an assurance that I 
have not entirely wasted the intervening years. If some may 
think that, in a life with many calls upon it, I have attempted too 
much and carried too little to completion, I would plead that I 
have learnt, as I could not otherwise have done, how vast are the 
problems which our science presents for solution, and have been 
enabled to assist the younger men, with whom I have come into 
contact in my College and Colonial Office work, to realize the 
wide field of research that lies open before them. 


part 1] ANNIVERSARY MEETING—LYELL MEDAL. xli 


AWARD OF THE LYELL MEDAL. 


In handing the Lyell Medal, awarded to Dr. CHaries Davison, 
to Prof. W. W. Warrs for transmission to the recipient, the 
PRESIDENT addressed him as follows :— 


Professor WatTrs,— 


The Council has awarded the Lyell Medal to Dr. Davison. Not 
unmindful of his contributions to observational geology, or of his 
contributions to the problems connected with the constitution and 
consolidation of the Earth, it has based this award mainly on his 
lifelong devotion to the study of earthquakes. Commencing ata 
time when he was almost alone, in this country, as a student of 
the subject, he has steadfastly accumulated a mass of detailed 
observations and critical discussion, which has very materially 
advanced our knowledge. His recognition of the duplex origin 
of certain earthquakes has been fruitful of result, and has been 
extended by others, until it is now a truism that the origin of an 
earthquake need not be from a single centre, but may be of a 
very complex and extended character. To a long list of published 
contributions to the study of earthquakes he has recently added, 
by the issue of a manual of that older branch of seismology, 
more especially connected with geology, in which the subject is 
handled in a lucid and philosophical manner. In recognition of 
these services the Council decided to award to him one of the 
medals at their disposal, and to select that which is associated 
with the memory of one who, in his time, did much to collect, 
disseminate, and extend the knowledge of earthquakes; and, in 
handing it to you for transmission to him, I may express a personai 
pleasure in the fact that almost the last act of my tenure of the 
Presidency should be the presentation of this award to one who 
has been known and valued, as co-operating in a line of research 
which has always had special interest to myself. 


Prof. W. W. Warts expressed his regret that Dr. Davison was 
prevented by illne:s from receiving the Medal in person. He 
pointed out the appropriateness of the award of the Lyell Medal 
to one who had done so much to prove the uniformity of geological 
causes at the present day, and whose method of research was 
essentially Lyellian in character. He asked permission to com- 
municate Dr. Davison’s acknowledgment in his own words :— 


‘It is with no little regret that I find myself unable to be 


xlil PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [ vol. xxviii, 


present at the Anniversary Meeting, and I am writing to ask if 
you would be so kind as to represent me when the Lyell Medal is 
presented, and on my behalf to thank the President and Council 
for their award. It is one that, for several reasons, I value very 
highly, especially because the Medal was founded by the writer of 
the remarkable chapters on earthquakes in the “ Principles of 
Geology,” and not less because it is presented by the author of the 
report on the great earthquake of 1897. I should like to take this. 
opportunity of saying that, whatever success my study of British 
earthquakes may have attained, is owing in great part to the help. 
and encouragement which I constantly received from Prof. Charles 
Lapworth and from my old teacher, Prof. Lebour.’ 


CHARLES DAVISON.’ 


AWARD FROM THE WOLLASTON DONATION FUND. 


The Prestpent then presented the Balance of the Proceeds 
of the Wollaston Donation Fund to Dr. Leonarp JoHNSTON 
Wits, M.A., addressing him as follows :— 


Dr. Wi1LLs,— 


Commencing your geological record with a study of the Lower 
Keuper rocks of Worcestershire, a study of the structure of the 
lower jaw of the Triassic Labyrinthodonts, and the discovery and 
description of the fossil plants at Bromsgrove, you have rendered 
more conspicuous service by your geological work on the Paleozoic 
rocks of North Wales and the Denbighshire coalfield, in the course 
of which you have developed important suggestions of tectonics, 
and added to our knowledge of the detailed stratigraphy of the 
region. Nor have more recent deposits been neglected, as is 
evidenced by your paper on late Glacial and post-Glacial changes in 
the Dee Valley, and on windworn pebbles in the high-level gravels 
of Bromsgrove. These studies you are still continuing, and we 
look forward to the early publication of some of your results. In 
recognition of the work done, and as an encouragement to future 
effort, the Council has awarded to you the balance of the Wollaston 
Fund, which I now hand to you in the name of the Society. 


part 1] ANNIVERSARY MEETING—MURCHISON FUND. xl 


AWARD FROM THE MurcuHIson GEOLOGICAL Funp. 


In presenting the Balance of the Proceeds of the Murchison 
Geological Fund to Herbert Bouton, M.Sc., the PRESIDENT 
addressed him in the following words :— 


Mr. Botroy,— 


The Council has awarded to you the balance of the Murchison 
Fund, in recognition of your contributions to our knowledge of the 
stratigraphy and fauna of the Carboniferous System and of your 
continued and successful conduct of one of the most progressive of 
our provincial museums. As an authority on the insects of the 
Carboniferous Period you are conspicuous. The results of your 
labours, in the rescue, preservation, and description of these relics 
of a long-bygone past, have proved of supreme interest in estab- 
lishing the great antiquity of some of the common insect-types of 
the present day. Added to this, your studies of the faunal strati- 
graphy of the Carboniferous rocks of the Bristol coalfield have 
been of value, which has led to their publication in our Quarterly 
Journal. For these reasons the Council has made this award to 
you, in recognition of the work which you have done in the past, 
and as an encouragement to others to emulate your example. 


AWARD FROM THE LYELL GEOLOGICAL FuND. 


The Prestpenr then handed the Balance of the Proceeds of 
the Lyell Geological Fund, awarded to Mr. Arruur Mac- 
CONOCHIE and to Mr. Davin Tarr, to Dr. J. S. Frerr for 
transmission to the recipients, addressing him as follows :— 


Dr. FLert,— 


The balance of the Lyell Fund has been awarded in two moieties 
to Mr. A. Macconochie and Mr. David Tait, in recognition of 
their joint and individual contributions to the advancement of 
geology. During his service as fossil-collector on the Geological 
Survey of Scotland, Mr. Macconochie was the discoverer of one of 
the most notable fossil localities in Scotland, where, at Glencart- 
holm in Dumfriesshire, he found, in Lower Carboniferous shales, a 
large number of new genera and species of plants, crustaceans, fishes, 


xliv PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [ vol. lxxviil, 


and land animals, and was the discoverer of the Olenellus fauna 
in the North-West Highlands, a discovery which caused the re- 
naming of two of the rock-groups of that region. Conjointly 
with Mr. Tait he collected a new fish-fauna from the Downtonian 
rocks of Lanarkshire, which, in the words of Dr. R. H. Traquair, 
opened out a new vista in the field of Palzozoie ichthyology. 
Mr. Tait, by his investigation of the Rhynie chert-band, was able to 
establish conclusively its true age as Old Red, and the investigation 
of his collections has thrown much light on the problems connected 
with this oldest land-fiora. I have referred briefly to the more 
outstanding of their individual contributions to knowledge, made 
during a long and devoted service, which has yielded a large body 
of material for detailed investigation by others. In recognition of 
this, I ask you to accept and transmit these awards to their 
respective recipients in the name of the Geological Society of 


~ TLLondon. 


part 1] ANNIVERSARY ADDRESS OF THE PRESIDENT. xlv 


THE ANNIVERSARY ADDRESS OF THE PRESIDENT. 
Ricuarp Dixon OrpHaM, F.R.S. 


By the death, on September 6th last, of Henry Woopwarp, we 
lose a Fellow of standing and distinction, who was for long a 
regular attendant, and familiar figure, at our meetings. Born at 
Norwich on November 24th, 1832, son of Samuel Woodward, well 
known in his day as an antiquary and geologist, he was a naturalist 
by disposition from the beginning. After a varied experience in 
his earliest years, he was appointed an assistant in the Geological 
Department of the British Museum in 1858, an appointment 
which he retained, with promotion to Keeper in 1880, until his. 
retirement in 1901. In 1864 he found a further outlet for his. 
energy in the foundation of the ‘ Geological Magazine,’ in con- 
junction with the late Prof. T. Rupert Jones, and was sole 
editor from the succeeding year until the end of 1918. In the 
November number of that Magazine will be found a notice of his 
published contributions to Paleontology and Geology, principally » 
dealing with fossil Crustacea; by these he will probably be best 
known to our successors, but those who worked with him are aware 
that, however valuable they may be, by no means the least service 
which he rendered was the opportunity given for the publication of 
the work of others, by the conduct and continuance of the 
Magazine founded in 1864. Hardly a geologist of any standing 
but has at one time or another found in it a channel of publi- 
cation, often in those early years when other avenues were still un- 
opened. Under his editorship the Magazine has always weleomed 
the expression of novel interpretations of observation, even when at 
variance with opinions commonly accepted, and in this way, no less 
than by numerous records of original investigations, the Magazine 
has served geology well in the past, as doubtless it will continue to. 
do in the future. 

In the same year, 1864, he was elected a Fellow of our Society, 
and in the course of his Fellowship gave twenty-seven years to. 
the Council, was three times Vice-President, and once President ; 
his services were further recognized by the award, in succession, 
of the Wollaston and Lyell Funds, and of the Murchison and 
Wollaston Medals. He was elected a Fellow of the Royal Society 
in 1873, received the Honorary degree of LL.D. from the 


xlvi PROCEEDINGS OF THE GEOLOGICAL society. [ vol. Ixxviii, 


University of St. Andrews in 1878, and had been President of 
the Paleontographical, Malacological, and Royal Microscopical 
Societies. 


Louis Compton Miatr, who became a Fellow of the Society 
in 1875, was born at Bradford in 1842, and died on February 21st, 
1921. He began his career as an elementary school-teacher, but 
was so much interested in natural science that he soon left his 
first profession for the curatorship of the Literary & Philosophical 
Society’s Museum at Bradford, and afterwards for the curatorship 
of the more important corresponding Museum at Leeds. In 1876 
he was elected Professor of Biology in the newly-founded York- 
shire College of Science, and he retained his Professorship in the 
University of Leeds, as it afterwards became, until his retirement 
from active work in 1907. In 1904-1905 he was also Fullerian 
Professor in the Royal Institution, London. Although an all- 
round naturalist. Miall was at first more especially interested in 
geology and paleontology, and he began original research with 
some experiments on the contortion of Carboniferous Limestone 
(Geol. Mag. 1869, p. 505). In 1869 he discovered in the Coal- 
Measures at Bradford a new Labyrinthodont, which was described 
as Pholiderpeton scutigerum by Huxley in the Quarterly Journal 
of the Geological Society for that year. Mliall wrote a geological 
appendix to this description, and, when he presented it to the 
Society, he made the acquaintance both of Huxley and of Lyell. 
His interest in the little-known group of Labyrinthodonts was 
thus aroused, and in 1873-74 he prepared important reports on 
these fossils, with many new observations and conclusions, for the 
British Association. In 1874 he also published a paper on 
Labyrinthodont remains from the Trias of Warwick in the Society’s 
Quarterly Journal. At the same time Maiall studied closely the 
Paleozoic Ganoid fishes, and he gave to the Society an account of 
the palate ef Ctenodus and the skull of Rhizodus. In 1884 he 
also contributed to the Quarterly Journal a valuable memoir on 
Megalichthys, which was afterwards published in an extended 
form by the Leeds Museum. In 1878 he began for the Palzonto- 
graphical Society a Monograph of the Sirenoid and Crossoptery- 
gian Ganoids, but did not proceed further than the introduction 
and a description of Ceratodus. From 1881 onwards Miall gave 
increasing attention to existing rather than to fossil animals, and 
che made important contributions to our knowledge of the structure 


part 1] ANNIVERSARY ADDRESS OF THE PRESIDENT, xlvii 


and life-history of insects. After his retirement he also wrote a 
most interesting history of the progress of zoology. He was 
President of Section D (Zoology) of the British Association at 
Toronto in 1897, and of Section L (Education) at Dublin in 1908. 
The Proceeds of the Wollaston Donation Fund were awarded to 
him by the Geological Society in 1875; he was elected a Fellow 
of the Royal Society in 1892, and received the degree of D.Sc. 
from Leeds University in 1904. He was an inspiring teacher and 
charming friend, and his memory will always be treasured by those 


who knew him. PALS. Wie] 


Dr. Marsuman Epwarp WapswortnH was born at Livermore 
Falls (Maine) in 1847, where he spent the early years of his life 
upon his father’s farm. He graduated at Bowdoin College in 
1869, and subsequently became a student of science in Harvard 
University, receiving the degree of Ph.D. from that institution in 
1879. From 1877 to 1887 he was closely associated with Prof. 
J. D. Whitney as assistant in the Geological Department. In 
1884 and 1885 he visited many Universities in England and on 
the Continent, familiarizing himself with their organization and 
studying modern petrographical methods under Prof. H. Rosen- 
busch. He first appears as a contributor to American geology 
and petrology in the late seventies of the last century. Between 
the years 1879 and 1885 many papers from his pen were pub- 
lished in the Proceedings of the Boston Natural History Society, 
in the Bulletins of the Museum of Comparative Zoology of 
Harvard College and elsewhere, among which the following may 
be specially mentioned :—‘ On the Origin of the Iron-Ores of the 
Marquette District’ (1880); ‘On the Trachyte of Marblehead 
Neck’ (1881); and ‘ Lithological Studies’ (1884), an important 
quarto memoir of 266 pages with eight chromolithographic plates 
illustrating the microscopic features of certain meteorites and 
rocks. Before 1884 the only important work on microscopical 
petrography, published in America, was Zirkel’s ‘ Report on the 
Rocks collected during the United States Geological Exploration 
of the Fortieth Parallel,’ which appeared in 1876. Wadsworth’s 
‘Lithological Studies’ is therefore the first work of its kind 
written by an American. In this memoir, after dealing with the 
classification of rocks, their origin, their mode of alteration, and 
their relation to meteorites, he gives detailed descriptions of the 
macroscopic and microscopic characters of siderolites, pallasites, 
and peridotites. 


xlvill PROCEEDINGS OF THE GEOLOGICAL society. | vol. lxxviu, 


In 1887 Dr. Wadsworth became President of the Michigan 
School of Mines, which was then in its infancy. He added to its 
equipment, organized courses of instruction, and made it in five 
years the greatest institution of its kind in the United States. 
From this time onwards he devoted himself mainly to adminis- 
trative and teaching work. In 1907 he was appointed Dean of the 
School of Mines in Pittsburgh University, retiring in 1912 under 
the age limit. His services were much appreciated by the Faculty, 
and at the time of his death, which occurred on April 21st, 1921, 
he was Dean Emeritus, Professor of Mining Geology, and Emeritus 
Curator of Geological and Mineralogical Collections in that Uni- 


versity. He was elected a Fellow of our Society in 1889. 
[J. J. Ee ae 


FrepEric RicHarpd Mauer, son of Robert Mallet, himself a 
distinguished member of our Society, joined the Geological Survey 
of India in February 1859. In his earlier years on the Survey he 
had a varied experience of field-work in the Himalayas, Central 
India, Assam, and Burma, but from 1876 until his retirement in 
1889 was almost continuously in charge of the Museum and 
Laboratory of the Department. He was the author of numerous 
published contributions to our knowledge of geology, of which the 
most important were probably his Memoir on the Vindhyan 
System and his masterly description of the dormant Volcano 
of Barren Island in the Bay of Bengal; but not less valuable 
than his published papers, though less apparent, was the large 
amount of careful work which he devoted to the maintenance 
of the Survey collections and to the assistance of other members 
of the staff. Possessed in an eminent degree of precision and 
neatness of method, a retiring disposition, covering a consistent 
application to and thoroughness in his work, and an unfailing 
courtesy and kindliness of demeanour, he was always ready to help 
his fellow-workers in their difficulties, and won the attachment and 
esteem of all who came in contact with him. After his retirement 
from the Geological Survey of India he published little, but con- 
tinued to devote himself to the pursuit of his favourite subject 
until advancing years deprived him of the power of continuing to 
work in his laboratory. He was elected into the Society in 1868, 
and passed away on June 24th of last year in the Slst year of 
his age. 


part 1] ANNIVERSARY ADDRESS OF THE PRESIDENT. xlix 


Rogert Logan Jack was a geologist of distinction, one of the 
pioneers of, and an extensive contributor to, Australian geology. 
Born in Ayrshire in 1845, he was educated at Edinburgh 
University, and joined the Geological Survey of Scotland in 1867. 
In 1877 he was appointed Government geologist for North Queens- 
land, and in 1879 for the whole of the Colony. He surveyed and 
reported on the Bowen-River coalfield, and, in 1879-80, led the 
first expedition to traverse the eastern portion of the Cape-York 
Peninsula. ‘This expedition gave him an opportunity of establish- 
ing his reputation as an intrepid and enterprising explorer ; in spite 
of hardship, difficult weather, lack of food, and hostility of the 
natives, by whom he was speared through the shoulder, he carried 
the exploration to a successful conclusion. His next important 
exploration was of the western portion of the colony, where he 
recognized the structural conditions, and predicted the occurrence 
of artesian water in the plains of the arid regions, of the interior ; 
this prediction led to successful boring, which has been extended 
until artesian wells are now numbered by thousands. In 1899 he 
resigned his appointment with the Queensland Government to 
undertake an exploration of the metalliferous deposits of Szechuan. 
Here adventure again dogged his footsteps, for he was caught and 
eut off from the coast by the Boxer rebellion, and had to make his 
escape westwards over the mountains to Burma. In 1901 he 
returned to London, and at the close of 1904 once more went back 
to Australia, where, in 1911, he was appointed Royal Commis- 
sioner on the Collie coalfield, and chairman of the Royal Commission 
to report on miners’ lung diseases. Dr. Jack’s published contri- 
butions to geology comprise 145 reports, during his service as 
Government geologist, as well as numerous books published inde- 
pendently. During the last three vears of his life he was engaged 
on a critical review and correlation of the explorations which had 
been carried out during the last three centuries in andiaround Cape- 
York Peninsula. He was elected a Fellow of our Society in 1870, 
and was a member of the Council in 1903-1904. He died at 
Sydney in the early part of November, 1921. 


By the death of the Haru or Duce, P.C,, F.R.S., which took 
place on October 28th, 1921, at the age of 94, the Geological 
Society lost a member of very long standing, who was elected in 
1853, and served on the Council as early as 1856-58. A man of 
varied activities and influence, having been a Member of the 


VOL. LXXVIII. d 


] PROCEEDINGS OF THE GEOLOGICAL Society. [ vol. lxxviil, 


House of Commons, Captain of the Yeomen of the Guard, Lord 
Warden of the Stannaries from 1888 to 1908, Lord Lieutenant 
of Gloucestershire and of the cities of Gloucester and Bristol from 
1857 to 1911, and President of Bristol College since 1860, he 
had throughout retained a keen and practical interest in geology, 
though he did not attempt original work. He had an extensive 
collection, chiefly of local fossils, and was always ready to offer 
hospitality and facilities for investigation to anyone studying the 
rocks of the Tortworth area. [S. H. R.] 


Joun CrARKE HawxsHaw was educated at Westminster 
School and Cambridge, where, apart from distinction in his 
academical studies, he became Captain of the C.U.B.C. On 
leaving Cambridge he adopted the profession of civil engineer, 
which he pursued with distinction, and was President of the 
Institution of Civil Engineers in 1902; during the late war he 
commanded the Railway Transport Staff Corps and was made 
Hon. Colonel for his services. Outside his profession he took a 
keen interest in natural history and geology, and, besides important 
papers on engineering subjects, contributed to the Journals of the 
Linnean, and of our own, Society. He was elected a Fellow of 
this Society in 1866, and served on the Council in 1879-83 and 
1S91—92. 


THomas Linpsay Ganntoway came of a family of mining 
engineers, his father and two brothers all having attained eminence 
in the profession in which he himself achieved distinction. In his 
younger days he studied at Glasgow University under Lord Kelvin, 
by whom he was selected to carry out the testing of the piano-wire 
method of making deep-sea soundings. On his return from the 
voyage to Brazil, in which these tests were successfully carried out, 
he devoted himself to the study and afterwards to the practice of 
mining, and was connected with the Campbeltown Colliery as 
manager, and afterwards as director, from 1881 till his retirement 
a few years ago. Of a scholarly disposition and philosophical turn 
of mind, he had wide interests outside his profession, and took a 
prominent part in the foundation of the Archeological Society of 
Kintyre. He was elected a Fellow of this Society in 1876. 


Sir Witi1aM EpwarpD GarFoRTH was a mining engineer of 
distinction, whose name will be principally remembered in con- | 


part 1] ANNIVERSARY ADDRESS OF THE PRESIDENT. h 


nexion with his discovery of the influence of an admixture of 
shale-dust in preventing explosions in collieries, a discovery which 
has had a far-reaching effect in diminishing the dangers of coal- 
mining, and has led to the compulsory treatment of passages in 
mines with stone-dust, as a preventive of the spreading of explo- 
sions. He took interest in the design of helmets, by means of 
which rescue-parties might penetrate into poisonous atmospheres, 
giving freely from his private funds to the research. He had been 
President of the Mining Association of Great Britain, and of the 
Institution of Mining Engineers, and was knighted in 1914. He 
was elected a Fellow of our Society in 1891. 


Henry Duckwortn, coming of a family which attained dis- 
tinction, one brother being the late Canon Duckworth, and another 
Sir Dyce Duckworth, Bart., F.R.C.P., was a Liverpool merchant 
who took a keen and active interest in geology. One of the 
founders, and the first President, of the Liverpool Geological 
Society, he contributed to the publications of that Society several 
papers, as the result of his observations and collections, on Perim 
Island in the Gulf of Cambay, in Egypt, Sicily, and the Somme 
Valley. He was elected a Fellow of this Society in 1858. 


Henry Wemyss FELDEN was born in 1888, son of Sir William 
Henry Feilden, second Baronet, of Feniscowles; he joined the 
Army, and served in the Indian Mutiny, the Chinese and South 
African wars, attaining the rank of Colonel, and receiving the C.B. 
for his military services. In 1875 he went out as naturalist with 
the British Polar Expedition, and travelled much on his own 
account, principally in the polar regions; to geology his principal 
contributions were his observation of the effects of ice-action in 
those northern regions, and especially his recognition of, and obser- 
vations on, the action of floating ice in abrasion and transport. 
He was elected a Fellow of this Society in 1875. 


GEORGE CLiIncH, born in 1860, was appointed to the library 
* of the British Museum on leaving school; in 1895 he was made 
Clerk to the Society of Antiquaries, and in 1910 Librarian. 
Interested from his boyhood in archeology and the collection of 
flint-implements, he was the author of numerous papers on pre- 
historic archeology, and on the topography and sculpture of his 
native county, Kent, two of which were published in our Quarterly 
Journal. He was elected a Fellow of this Society in 1899. 
d 2 


hii PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [ vol. lxxviil, 


The Rev. Joun More Gorpon, M.A., was born on April 13th, 
1849. He entered as a theological student at Balliol College, 
Oxford, and graduated in 1873, winning the Ellerton Essay Prize 
in that year, and the Denyer Johnson Scholarship in 1875, in 
which year he was ordained. He was for 31 years vicar of 
St. John the Evangelist, Redhill. Although he has published 
none of his work, he was a very keen student of petrology and 
mineralogy, and formed a very fine collection of minerals (the 
specimens from Swiss localities being particularly fine). His gifts 
of minerals have much enriched the collections, both at the 
Natural History Museum and in the Geological Department of 
the University of Aberdeen, in which University his grandfather 
was Professor of Ecclesiastical History. His interest in science 
was very wide. He was a Fellow of the Physical Society and_of 
the Royal Microscopical Society, a Member of the Mineralogica 
Society and of the Alpine Club. He was elected a Fellow of our 
own Society in 1888, and died at his London house on January 
18th, 1922. |W. Cosa 


FrEDERIC THoMAS MatpWELL was born at Gunnerside, in 
Swaledale, in 1872. He qualified as an instructor in handicraft, 
and, after acting as such in the Midlands, received in 1908 an 
appointment under the Education authority of Runcorn, where he 
rendered useful service to the town and was a member of the Free 
Library Committee. Devoting the greater part of his life to 
geology, he was indefatigable in observing and recording par- 
ticulars of sections, and gave special attention to the study of 
fossil footprints in the Trias. He was elected a Fellow of this 
Society in 1919. 


CuarLes Ferpinanp ZaBEL was a young man of great 
promise, who, after a brilliant course of studentship, was employed 
on various mining investigations. At the outbreak of the late war 
he was engaged on a survey of manganese-deposits in the Rhine 
valley, and, with many others, was interned at Ruhleben, where he 
distinguished himself by his exertions for the welfare of his fellow- 
prisoners, and ably maintained the honour of the country to which 
he owed allegiance. Always keen in his interest for geology and 
our Society, his books have been presented to our Library and have 
given us some useful additions. He was elected a Fellow in 1919, 
and died at Tete, in East Africa, on July 15th of last year. 


part 1] ANNIVERSARY ADDRESS OF THE PRESIDENT. lin 


Ernst Hetyricu Oskar Kastuirn WEINSCHENK was born on 
April 6th, 1865, at Esslingen in Wiirtemberg. He was first 
Privatdozent and afterwards for many years Professor of Petro- 
graphy in the University of Munich. He published numerous 
memoirs on mineralogy and petrology, but his most distinctive 
researches were on the subject of the mode of occurrence and 
genesis of graphite, in the course of which he contributed important 
additions to our knowledge of the deposits of Bavaria, Bohemia, 
and the Alps, as well as Ceylon. He was also especially interested 
in rocks and minerals rich in magnesia. He is best known, how- 
ever, for his text-books on petrology, which are clearly written and 
well illustrated. The ‘Introduction to the Petrological Micro- 
scope’ and the ‘ Rock-forming Minerals’ have been translated into 
English by R. W. Clark. He also published the ‘ Principles of 
Petrology’ in two volumes, a general and a special part. The 
former is a masterpiece of lucid exposition. The views expressed 
are sound and at the same time show considerable originality. 
The work has been translated into English by Johannsen. He 
was elected a Foreign Correspondent of the Society in 1912, and 
died in 1921. fee NV Ee 


Although he was never a Fellow of our Society, it would be 
impossible to pass without mention the name of one so well known 
to geologists as BensamMIN Harrison. Born in 1837, second son 
of a grocer at Ightham in Kent, a business which had been in the 
family for 150 years, and was afterwards inherited and carried on 
by him, he early developed a taste for geology and collecting 
fossils, and maintained his interest and zeal to the last. He 
first attracted attention by the identification of Roman remains 
at Ightham, and, shortly after, his attention was drawn to flint 
implements by the discovery of these relics of ancient man at 
Abbeville, and the controversy which arose therefrom. This led 
him to collect similar remains in his own district, of which he 
amassed a large collection, and made him known to men of eminence 
in that branch of knowledge. With Lord Avebury, Prof. Prestwich, 
and Sir John Evans he corresponded, and by them he was encouraged 
in his investigations. An indefatigable explorer of his district, of 
which he had an unrivalled knowledge, his memory will live mainly 
by his discovery of ‘eoliths,’ and the controversy which raged over 
the reality of the human origin of these alleged evidences of human 
activity in long-past ages. His services to Geology were recog- 
nized by the award of the Wollaston Fund in 1899. 


liv PROCEEDINGS OF THE GEOLOGICAL society. [ vol. lxxyin, 


For assistance in preparing these notices I have to express my 
obligation to Sir Jethro J. H. Teall, Dr. A. Smith Woodward, 
Prof. S. H. Reynolds, Mr. W. Campbell Smith, and many others 
who have been good enough to furnish information in reply to 
enquiries. 


After announcing the election of Officers, the retiring President 
said :— , 

Two years have now elapsed since you elected me to the office of 
President; they have been eventful years in the history of the 
Society. When I assumed office your Council was faced with a 
very serious state of affairs brought about by the war, expenses had 
greatly increased, while income remained but little altered, and it 
almost seemed that we should have to face the alternative of either 
abandoning our Library, or of suspending our Journal. It was 
necessary to overhaul the whole of the affairs of the Society, to 
impose certain additional charges on the Fellows and to restrict 
some of the benefits which they had been enjoying, before that 
equilibrium between income and expenditure could be established, 
without which our continued existence as a Society would be 
impossible. Asa result, we have been able to continue the two 
most important functions of the Society, the maintenance of a 
working library, and the publication of research, though we have 
been compelled to restrict our activities in other directions. Much 
has been done, yet the task is not complete; the axe has been 
wielded, I think sufficiently, but there is still work for the pruning 
hook on minor economies, insignificant separately but collectively 
appreciable. Of this your Officers are fully aware, and you may be 
confident that the matter will be borne in mind and attended to. 
It is not to be expected that all the action which your Council has 
taken during the last two years should receive unanimous approval ; 
but, taken as a whole, I think that we may justly be unashamed of 
the record: it is not the work of any one man, it has only been 
possible by the co-operation of the whole body of the Officers and 
Council, of the Fellows at large, and of the permanent staff. Inow 
hand over the care of the interests of the Society to a successor 
who will maintain its honour and dignity, and in relinquishing office 
I wish to render my thanks to all those who during the past two 
years have helped me in what has been an anxious and responsible, 
but far from thankless. task. 


part 1] ANNIVERSARY ADDRESS OF THE PRESIDENT. lv 


THE CAUSE AND CHARACTER OF HARTHQUAKES. 


The study of earthquakes having always been recognized as one 
of the departments of Geology, no excuse is needed for devoting 
my address, on this occasion, to a review of the present state of our 
knowledge of the cause and character of earthquakes, using that 
word in the restricted, and original, sense of the disturbance of 
the ground which is sensible to human feelings, which causes 
alarm and destruction, and is properly that seism of the ancient 
Greeks, from which our modern term seismology is derived. This 
explanation is necessary, for, of late years, seismology has been 
extended to the study of a phenomenon of different character, the 
long-distance records of disturbances, only to be detected by very 
sensitive instruments of special construction; in some cases these 
are clearly connected with great earthquakes—as the word is here 
used—and by inference have been presumed to be so in all cases, 
even when there is no independent evidence of the earthquake 
proper. The records, regarded as records of the progressive en- 
feeblement of the larger disturbance of the true earthquake, would 
represent the cryptoseism, or unfelt earthquake, and be correctly 
described in the observatory records as earthquakes. That they 
are correctly so described is indisputable, if the word is taken in its 
literal interpretation as a quaking, however feeble, of the earth ; 
but, if the implication is added that they have the same origin as 
the greater disturbance, the correctness of the description becomes 
doubtful. In presenting to you, some dozen years ago, the results 
of a study of the records of the Californian earthquake of 1906, I 
pointed out that, although the immediate origin of the earthquake 
proper might be traced to occurrences which took place in the 
outermost parts of the Earth’s crust, these were {but the secondary 
result of a deep-seated origin, or bathyseism, which gave rise, at 
the same time, to the disturbance which was recorded at long 
distances by suitable instruments. Later work and research has 
more and more confirmed both the correctness of this interpreta- 
tion and the conclusion that the proximate cause of great and 
destructive earthquakes is distinct from that of the long-distance 
records, although the two origins are connected with each other as 
effect and cause. 

In the present state of our ignorance of the nature of the 
bathyseism, it is difficult to give a clear and precise definition 


lvi PROCEEDINGS OF THE GEOLOGICAL society. [vol. xxvii, 


of the connexion between it and the earthquake proper: the 
subject is an interesting one, and a review of the evidence, to- 
gether with the deductions which can be drawn from it, would 
fill the time available; but it is not my intention to do more than 
to attempt, by analogy, to illustrate and explain the nature of 
the connexion of the bathyseism with its two independent results. 

Not many years have passed since, in the south-eastern corner 
of England, we heard what were known as the guns of Flanders ; 
and the description was correct. The sound—it was more a sensa- 
tion than a sound—which was heard in Kent and Sussex was 
undoubtedly produced by the report of great guns, by the explosion, 
that is, of the charge in the gun itself; but, had the explosion 
done no more than give rise to the sound-waves which travelled 
far in every direction, it would have little troubled the enemy. 
Simultaneously, however, with the production of the report, and 
by the same explosion, a projectile was sent flying through the 
air, and, after a trajectory of some miles, itself exploded, causing 
the damage which was the purpose of its despatch. The effect of 
this second explosion was severe but local, and at a short distance 
away neither sound nor shock was sensible. 

Here we have a very complete analogy, the explosion of the gun 
represents the bathyseism ; the report and sound-waves, travelling 
afar, correspond to the disturbance which, propagated through 
the substance of the Earth, gives rise to the long-distance records ; 
the explosion of the shell to those dislocations in the outer crust 
which produce the destructive earthquake; and the trajectory to 
the connexion, of which the character is as yet unknown, between 
the bathyseism and the surface-shock. 

If this interpretation be accepted, it becomes evident that the 
distant records represent something which is distinct from the 
earthquake, as originally understood, and that the study of them, 
with the deductions drawn from that study, have little or no 
bearing on the problems of geology, as we usually limit the scope 
of that science. It is otherwise with the earthquake proper; 
originating in, and affecting, the outermost crust of the Earth, it 
has long, and rightly, been regarded as one of the departments of 
geology, both as regards cause and character, and it is with this 
aspect of the subject alone that I shall deal. 


The character of earthquakes is known to an extent sufficient 
for my purpose; they are elastic waves, transmitted through the 


part 1] ANNIVERSARY ADDRESS OF THE PRESIDENT. lvil 


substance of the Earth, not, as was once supposed, merely waves of 
elastic compression, but of most complicated character, and in all 
but a small minority of cases nothing but this vibratory move- 
ment, the orchesis, can be recognized. Occasionally, however, 
and only in the case of some earthquakes of destructive violence, 
there is also a bodily and permanent displacement of the solid 
eround, and this mass, or molar, movement has been distinguished 
as the mochleusis of the earthquake, as distinct from the elastic 
displacement, accompanied by return to the original position, 
which constitutes the orchesis. Now, the elastic waves can only 
be initiated by some sudden impulse or disturbance, such as might 
be produced by the fracture of rock; and as, in those earthquakes 
where mochleusis can be recognized, there is usually evidence of 
sudden movement along some pre-existent fault-plane, or of rending 
and fissuring of the solid rock, faulting or fracturing has come to 
be regarded as the cause from which the vibratory disturbance 
originates. 

This conclusion is supported by the fact that the proximate 
origin of the shock can almost always be placed at a moderate 
depth from the surface. It is, unfortunately, impossible to give 
any precise figures, for none of the methods which have been 
suggested for determining the depth of the origin can be trusted, 
some because they depend on assumptions which the progress of 
knowledge has shown to be erroneous, others because they demand 
data which cannot be supplied with the requisite precision, if at 
all; but there is another way in which some idea of the depth of 
origin may be reached, based on the fact that there is almost 
invariably a well-defined area of maximum intensity of shock, 
surrounded by regions of diminishing intensity, as the distance 
from the central area increases. Since the violence of the dis- 
turbance will decrease with the increase of distance from the 
origin, it follows that the nearer the origin les to the surface, 
the more closely does the variation of surface-distance from the 
epicentre approximate to the variation in actual distance from the 
origin; hence it is evident that the rate of variation of intensity 
of the disturbance will give some notion of the depth of the origin. 
In this way, quite apart from any numerical estimates which have 
been made, it becomes clear that, excluding a small minority of 
earthquakes which will be referred to later, the origin hes at a 
very moderate depth below the surface, probably seldom over 10 
miles, and usually less. This places the origin within the limits of 


lvill PROCEEDINGS OF THE GEOLOGICAL Society. [ vol. lxxviui, 


the solid outer crust of the Earth, and in this region it is difficult 
to conceive of any cause, sufficient to originate the elastic wave- 
motion of the earthquake, other than the sudden fracture of the 
solid rock, where strain has outgrown the power of resistance. 

Apart from this general reasoning from observation, there are 
eases on record where considerable displacements of the ground 
have been measured by the comparison of careful and accurate 
surveys made before and after the earthquake. In three of these, 
the Cutch earthquake of 1819, the Sumatran of 1892, and the 
Californian of 1906, the largest movements took place close to 
the line of fracture, and in opposite directions on opposite sides of 
it, the displacements decreasing on either side’till a region was 
reached in which no change from the condition before the earth- 
quakes could be recognized. As this is precisely what would 
take place if a solid body, capable of elastic deformation, was 
strained until fracture took place, the conclusion is justifiable that 
such was in fact the origin of the dislocation and displacements. 

To this general rule there are some recognized exceptions. 
Quakings of the earth, identical in character with the true earth- 
quake, may be caused by natural rockfalls, or artificially by 
explosions; and there is the class of volcanic earthquakes, recognized 
as due to disturbances directly connected with volcanic activity, 
though it is probable that the majority of volcanic earthquakes 
are in reality originated by rock-fracture and, therefore, only 
indirectly the result of volcanic activity. ‘To these may, perhaps, 
be added earthquakes which are due to the direct trans- 
mission of elastic wave-motion from the bathyseism; but, when all 
these allowances are made, it must be admitted that, speaking 
generally, the immediate cause of earthquakes is the development 
of a state of strain in the outer rocky crust of the Earth, of such 
magnitude as to give rise to fracture, accompanied or not by 
displacement of the opposite sides. 


So far the conclusions, which may be drawn from observation 
as they have been briefly outlined, belong rather to the domain of 
physics than of geology ; but, when we go on to consider the cause 
to which the strain is to be attributed, and more especially the 
rate of growth, we are brought into contact with problems and 
deductions which are intimately connected with geology proper, 
and to which I propose to confine attention in the remainder of 
this address. As regards cause; this is usually attributed to what 
are known as the tectonic processes, a term which has never been 


part 1] ANNIVERSARY ADDRESS OF THE PRESIDENT. lix 


defined and is incapable of precise definition, but may approximately 
be described as the processes by which the folding and faulting 
of rocks were produced, and, in accordance with this attribution, 
the class of earthquakes, with which we are concerned, is referred 
to as tectonic. Therate of growth of strain has almost invariably 
been accepted as very slow, yet, when the subject is looked into, 
it will be found that there is really no evidence to support the 
acceptance; in part, it must be attributed to the general belef 
that all geological action is necessarily slow, and in part to the 
conclusion, forced on us in the latter days of the last century, that 
the Earth is a solid, inert, and highly-heated body, cooling slowly 
by radiation, with the subsidiary deduction that all deformation 
of the outer crust must be referred to contraction, consequent on 
that slow cooling. The latter of these reasons is now abandoned 
by those who forced it on us, and the former, though true in 
general, must not be treated as an unchangeable law, for there are 
many cases where a process, slow on the average, and as a rule, 
is occasionally subject to a temporary acceleration of rate. The 
evidence, too, which has been regarded as confirmatory of the slow 
growth of strain, may more properly be described as an interpre- 
tation of observed facts in accordance with an hypothesis. 

In the report on the Californian earthquake of 1906, for instance, 
the displacements caused by that earthquake, and an earlier one 
in 1868, are discussed, and the conclusion is drawn that they 
should be explained by a slow growth of strain, extending over a 
century or so, partly relieved by fracture in 1868, and again in 
1906. The argument is conclusive, in so far as it shows that the 
effects are consistent with the hypothesis ; but it was not noticed 
that they would be equally consistent with a condition of quiescence 
throughout the whole period, with the exception of two short 
intervals immediately preceding the two shocks, respectively. 
The same may be said of all the supposed evidence in favour of a 
slow growth of strain; the after-effects may satisfactorily establish 
the conclusion that the proximate cause is fracture resulting from 
excessive strain, but they can give no indication of the time 
occupied in preparation. The earthquake comes and passes, it 
leaves certain records behind it; but these records would be the 
same, whether the preparatory growth of strain was secular or 
instantaneous in duration. 


Yet the problem is not insoluble, for there is another line of 
attack, which has only become practicable within the last few 
years. If we regard the growth of strain as continuous, there will 


ex PROCEEDINGS OF THE GEOLOGICAL socrety. [ vol. lxxviii, 


be a certain increment which will lead to fracture, earthquake, 
and partial relief ; then with a further increment the process will 
be repeated, and so we reach the concept of a mean strain- 
interval for each shock, which may be regarded as constant, on 
the average, for any given region, provided that the average is 
taken over long enough a period. If, then, we divide the mean 
increment of strain in a unit period of time by the mean number 
of shocks occurring in the same period, we obtain a fraction which 
represents the mean stress-interval for each shock, a fraction which 
should remain constant in the region under consideration ; and from 
this 1t results that any variation in the rate of growth of strain 
must be accompanied by a corresponding variation in the frequency 
of earthquakes. We have, then, four quantities so inter-related 
with each other that, if three of them were known, the fourth can 
be determined. ‘Two of these, namely, the mean frequency and the 
variation from that mean in any chosen period, can be obtained 
from observation; and, if the variation from the mean rate of 
growth of strain is also known for the selected portion of the 
whole period, that mean rate, which is the object of search can be 
obtained by a simple rule-of-three sum.! 

The frequency of earthquakes is known to be subject to great 
variation from time to time, and this variation indicates a corre- 
sponding change in the rate of growth of strain. In the main, 
this change may, and probably must, be attributed to causes acting 
within the Earth, and directly related to the changes or processes 
by which the strain is produced; but it is evident that, if there 
were any external cause, which acted periodically and alternately 
in increase and decrease of the rate of growth of strain, and if it 
were possible to disentangle the variations due to this from those 
due to other causes, we should possess the means of framing a 
numerical estimate of the general rate of growth of strain. 

One such cause of periodic variation is to be found in the tide- 

1 The argument may be put in a different form, simpler and more easily 
intelligible to some. If S represent the mean increment of strain in a given 
period and N the mear number of earthquakes recorded in the same period, 
then S/N is the fraction representing the mean strain-interval corresponding 
to an earthquake. If the variation of the growth of strain in any particular 


period is represented by v and that of the number of earthquakes by d, we 


get the equation 
S+N = (S+v)+(N+d) 


where d=v/S 


whence we obtain the simple ratio 
a@:N i: v:s: 


part 1] ANNIVERSARY ADDRESS OF THE PRESIDENT. Ixi 


producing stresses set up by the sun and the moon. It is true 
that many attempts have been made at different times to detect 
some connexion between the frequency of earthquakes and the 
position of the moon, and that no such connexion has yet been 
established, but these attempts have all been based on very im- 
perfect records. In time it may, perhaps, be possible to apply to 
an earthquake record that method of harmonic analysis which has 
proved so fertile in the case of the ocean tides, but the day is long 
distant when a record of sufficient completeness will be available. 
Meanwhile, there are some simpler relations, of which a discussion 
is feasible, and the most promising of these seems to depend on 
the fact that the downward pressure is greatest at the time when 
the attracting body is on the horizon, and least when it is on the 
meridian. If, then, we divide an earthquake record into two 
groups, one containing all shocks which oceur within six hours 
before a meridian passage, and the other all that happened within 
six hours after, the first of the two groups will cover a period during 
which the downward pressure is, on the average, increasing, while 
the other will cover a period during which it is decreasing. As 
the amount of the change so introduced is known, with sufficient 
accuracy for the present purpose, and as it must, on the hypothesis 
being used, influence the frequency of earthquakes, it follows that 
we have here a method which should enable us to make an estimate 
of the rate of growth of the strain to which fracture is due. 
Although simple in principle, the method is difficult in appl- 
eation. To begin with, a record is required, of sufficient extent 
and continuity to give a trustworthy average, not merely of the 
general frequency, but also of the frequency in each of the two 
sections into which it is divided; and this means that the record 
must contain at least two thousand shocks and ought to contain 
double that number or more. Then it must be reasonably 
accurate as to times and complete as to occurrences, or at least 
must be fairly uniform in its incompleteness over the whole period 
investigated. There are not many records which fulfil these 
primary requirements, but there is another even more important. 
In all records there is a noticeable variation in frequency at 
different times of the day; moreover, the nature of this diurnal 
variation has been found to vary in different regions, but appears 
to be constant and characteristic in each region over the period of 
record. The cause of this periodicity may be reasonably attributed 
to some effect, meteorological or other, connected with the daily 
course of the sun; but its nature, no less than its variability, 


xii PROCEEDINGS OF THE GEOLOGICAL sociEeTy. [ vol. lxxviii, 


shows that it cannot be attributed to gravitational attraction. 
It is only, therefore, by a conversion of the record from 
solar to lunar times that the influences of these other effects 
can be eliminated, and the gravitational attraction of the moon 
be detected and estimated, and, for the satisfactory application 
of this method, it is necessary that the record should cover a 
complete lunar cycle, or a period of nineteen years. There are 
only two records extant, and available, which fulfil this requirement, 
and of these the Italian is not only the most complete and accurate, 
but is the only one to which the conversion into lunar times has 
been applied. 


A summary of the figures obtained has been published in our 
Quarterly Journal,! and from this we find that in the six hours 
preceding a meridian passage there were 3337 shocks, and in the 
six hours succeeding only 3270, giving a mean departure, from the 
general average for six lunar hours, of 33:5, or almost exactly 1 per 
cent. of the mean. So small a variation from perfect equality is well 
within the limit of what might reasonably be expected, if it were 
purely fortuitous and the stresses set up by the attraction of the 
moon had no effect whatever. This point will be returned to 
later on, but it will be useful to see what conclusions may be 
drawn if the variation is accepted as real, and due to the cause 
under consideration. The first of these is that the main stress, to 
which the strain is due, is of a compressive nature, consequent on 
an increase of downward pressure, or a removal of support from 
below. ‘The second is that the vertical component of the general 
increase of strain amounts to just 100 times the variation of the 
corresponding component of the gravitational stress set up by the 
moon. It has been established by mathematicians that the 
maximum upward stress set up by the moon, at the points on the 
surface of the Earth where it is in the zenith or nadir, amounts 
to 1/8,450,000 of the Earth’s force of gravity, and where it is on 
the horizon there is a downward stress of just one half of this: 
the total variation of downward pressure is, therefore, equivalent 
to a change of about 1/5,630,000 of that due to terrestrial gravi- 
tation, as between the points and times when the moon is on the 
horizon, or at the zenith or nadir. But the moon could never be 
directly overhead in any part of Italy, and a computation of the 
mean range of variation, over the whole period and the whole area 
concerned, reduces this fraction to about 1/9,400,000. As the 


1 Vol. lxxvii (1921) p. 2. 


part 1] ANNIVERSARY ADDRESS OF THE PRESIDENT. xi 


general increase of stress is 100 times this figure we get the result 
that the rate of increase of strain is equivalent to that which 
would be produced by an increase of downward pressure, or a 
corresponding reduction in the support of the crust, at the rate of 
1/94,000 of that due to gravity, in each period of six hours. 

It has long been established that the strength of the crust is far 
from being able to withstand the crushing strains which would be 
set up by removal of support from below, and the estimates, which 
have been independently made by different investigators, concur in 
putting the limit of the removal of support, which would result in 
crushing, where an area comparable with Italy is concerned, at not 
more than about 1/400 of the force of gravity. If this fraction 
is divided by that obtained in the previous paragraph we get the 
result that, starting from a condition of no strain, fracture would 
come about after an interval of 235 periods of six hours, or not 
quite 59 days. The calculation, therefore, indicates that the rate 
of growth of strain in Italy has been, on the average, such that 
the breaking point would be reached in about two months from 
start, with a wide variation on either side. Some other relations 
between the frequency of earthquakes and the diurnal variation 
of the tidal stresses might be, and have been, investigated; none 
of them seem so appropriate as that which has been detailed, and 
all give fairly confirmatory results, the longest period indicated, 
as required for reaching the breaking strain, being just about 
a year. 

It must not be supposed that value can be attached to the 
precise figures. As is invariably the case, in all calculations 
regarding physics of the Earth, many considerations are involved of 
a very uncertain nature; but the reasoning does show that the 
increase of strain must have taken place at such a rate that the 
breaking point was reached in a period measurable at most by 
months. They prove conclusively that the period could not have 
been of such length as to be measurable by years or decades, for, 
had this been the case, the disparity dealt with would have been 
much greater than that actually found. 

The same conclusion may be reached in another way. The 
stress-difference required to produce fracture in average hard rocks, 
as they are met with at the surface, is round about 1,000,600 
grammes per centimetre square, and, allowing for the greater 
strength at depth which is indicated by the experiments of Prof. 
Adams and the computations of Prof. Burrell, we may put the 
breaking strength of the Harth’s crust at about double of this, so 


lxiv PROCEEDINGS OF THE GEOLOGICAL society. [ vol. lxxviii, 


that, in order to reach this point in one year from starting, the 
strain would have to increase at the rate of about 1400 grammes per 
centimetre square in each quarter of a day. According to the late 
Sir George Darwin the stress-differences set up by the moon in the 
latitude of Italy would amount to about 20 grammes per centimetre 
square in an incompressible Earth, and in a compressible Earth with 
an incompressible crust—a condition much more akin to what we 
have reason to suppose is the reality—the stress-differences would 
be many times this figure!; but even the lower amount is nearly 
13 per cent. of the growth required to reach breaking point in one 
year, 1t would be close on 15 per cent. if the period is increased 
to ten years, and, with anything approaching this proportion, a 
periodicity would result, which could not have escaped detection 
before now. 

The figures, therefore, give us a lower limit of the rate of 
growth of strain, it must have been something faster than that 
needed to reach the breaking point in one year from starting, if the 
differences on which the argument is based are real. But are they 
real? The actual amount of difference, barely 1 per cent. of 
the mean, is so small that it may well be fortuitous, and the true 
interpretation may be that the gravitational stresses, and the 
stress-differences produced by them, have no effect whatever in 
determining the time of occurrence of an earthquake. If this be 
so, then the rate of growth of strain becomes infinite,and each 
earthquake becomes the result of a rapid development of strain, 
akin to an explosion in its suddenness. 

The truth may le anywhere and must le somewhere between 
these extremes, and so we reach the conclusion that there is no 
support for the commonly-accepted notion of a continuous, slow 
growth of strain, extending over years, decades, or even centuries, 
before the breaking point is reached. On the contrary, it appears 
that the cause of earthquakes isa rapid growth of strain. This 
strain cannot be developed without some deformation, but the 
magnitude of this has no relation to the frequency or magnitude of 
the earthquake; if change of form is slow and prolonged, relief 
may be provided by gradual yielding, if rapid, a very small amount 
of distortion may lead to fracture, and on the extent, form, and 
position of this fracture will depend the character of the resulting 
earthquake. 

This study of the growth of strain leads on to the question, 


1 Sir G. H. Darwin, Scientific Papers, vol. ii (1908) p. 502; and Phil. Trans. 
Roy. Soe. vol. elxxiii (1882) pp. 219 e seqq. 


part 1] ANNIVERSARY ADDRESS OF THE PRESIDENT. Ixv 


which is the really important one in its bearing on geology, of how 
the strain is produced. It can hardly be the result of those 
tectonic processes which result in folding, for these must necessarily 
be slow in their action; the change of form involved in the 
bending of solid rock from its original shape into complicated 
folds, without breach of continuity, can only have been a slow one, 
and, as we have seen, the deformation which produces earthquakes 
must be a rapid one. With faults the case is different; many 
earthquakes are known to have been accompanied by movement 
along pre-existing fault-planes, in others the origin evidently agrees 
in position with known faults, and in all of these the distribution of 
the intensity of disturbance is closely correlated with the faults, 
being greatest in proximity to them and decreasing as the distance 
becomes greater. So much is indisputable, yet, despite a general 
acceptance of the explanation, that the earthquake was a result 
of the same process as that which gave rise to the formation of 
the fault, it must be recognized that the proof is not logically 
complete, for it might be that the cause and processes which gave 
rise to the earthquake were wholly different from, and independent 
of, that which produced the fault, the only connexion being that 
the weakness, resulting from the fault-fracture, served to localize 
the yielding, and so controlled the distribution and intensity of the 
earthquake. Ina study of the Californian earthquake of 1906, 
where the greatest intensity of disturbance ranged along the line 
of the San Andreas fault, and was accompanied by considerable 
displacement and distortion of the surface along the fault-line, I 
was able to show that the ultimate cause of the earthquake was 
quite distinct from that which produced the fault, and that the 
fault was not the cause of the earthquake, nor the earthquake an 
incident in the formation of the fault. 

In support of the supposition that earthquakes are not produced 
by, or at any rate are not necessarily the product of, the tectonic 
processes which have given rise to the displacements in faults, may 
be instanced the fact that in some cases of minor earthquakes, 
where it has been possible to fix the position of the epicentre with 
a close approach to definiteness, it has been found that surface 
examination gives no indication of the presence of a fault. This, 
however, is not conclusive, for there might have been a deep- 
seated, incipient, fault which had not yet extended to the surface, 
and so could not be recognized by geological survey. 

“Much more weighty and suggestive evidence is to be derived 
VOL. LXXVIII. e 


lxvi PROCEEDINGS OF THE GEOLOGICAL SOCIETY. | vol. Ixxviti, 


from the great earthquakes which have been studied in detail. 
Reference has already been made to the Californian earthquake 
of 1906, and the conclusion drawn in that case is more fully 
exemplified by the Indian earthquake of 1897. Here there was no 
single leading fault and zone of maximum intensity of shock, buta 
complicated network of lines of extreme destructiveness, ramifying 
over an area not much different from that of England, and ex- 
tending right across a series of great tectonic features, across the 
great monocline of the southern face of the Assam range, across 
that range itself, across the alluvial plain of the Brahmaputra 
Valley, the great boundary-faults of the Himalayas, and probably 
even across the main axis of elevation of the range. 

A still more instructive instance is the Charleston earthquake 
of 1886. Here, in a region as devoid of any great structural 
feature, either of folding or faulting, and as little subject to earth- 
quakes, as could be found in our own country, there suddenly 
occurred a great earthquake, of destructive violence in the central 
area and felt over an area measuring about 1500 miles across. It 
was an earthquake of first-class magnitude, whether we regard the 
maximum violence of shock or the extent of area affected, yet 
there is nothing in the structure of the surface-rocks to suggest 
that its origin was due to any tectonic process, and equally 
nothing which could lead to its classification as voleanic; and, 
if we accept the conclusions of Col. Harbée, regarding the 
character and extent of-earthquake origins, the absence of any 
connexion, between the origin of the earthquake and the tectonies 
of the surface-rocks, becomes absolute, for, according to this 
interpretation, the origin becomes almost co-extensive with the 
seismic area, and the diminution of violence in the outer portions 
is not solely due to enfeeblement, resulting from the elastic propa- 
gation of the earthquake wave, but very largely to a diminution 
in magnitude of the originating impulse. 

Whether this explanation be accepted or not, it must be conceded 
that, as regards the two earthquakes particularly referred to, of 
Charleston in 1886 and India in 1897, Col. Harbée’s conclusions 
are not only supported by the particular facts on which they were 
based, but are in better accord with a number of peculiarities m 
the local variation of violence of the shock, and of other phenomena 
recorded, than is the current notion of a central foeus of compara- 
tively restricted dimensions. It accords also with those great 
earthquakes which, like the Calabrian earthquakes of the present 
century, had more than one centre of maximum intensity, con- 


nected by regions of less violence of shock. 


part 1] ANNIVERSARY ADDRESS OF THE PRESIDENT. Ixvil 


These are some of the considerations which have led me to believe 
that Col. Harboée’s interpretation is, in the main, well founded, and 
if it be true that earthquakes of great extent are due to systems of 
fracture, or analogous disturbance, ramifying over, and practically 
co-extensive with, the seismic areas of the earthquakes, that is, over 
areas of which the dimensions in any direction may be measured 
in hundreds of miles, it. becomes more than ever necessary to 
recognize that the earthquake origins cannot be the result of 
processes and displacements, recorded, and indicated, by the 
tectonics of the surface-rocks. The real and ultimate origin must 
be more deep-seated, and involve either a displacement of, or a 
change of volume in, the material underlying the outer crust. 


This is no occasion to enter into detail, and so I have merely 
indicated the general character of the studies which have gradually 
forced me to the conclusion that great earthquakes, and also to a 
great extent those lesser ones which are commonly classed as 
tectonic, do not owe their origin to the tectonics cf the outer 
erust, but to processes and changes which take place in the material 
below it. What these processes may be we cannot know, with the 
eertainty which comes from direct observation, for such knowledge 
as we think we have comes from inference, deduction, and, to 
some extent, simple assumption; but suggestions have been made 
which possess a considerable degree of probability. Among these, 
and especially apposite to present considerations, may be placed 
Dr. L. L. Fermoyr’s studies of the changes in mineral aggregation 
which may take place in the solidification of a magma; he has 
suggested that the determining factor in deciding the form in 
which the rock finally solidifies, is the inter-relation of pressure 
and temperature, and has shown that the change of volume, 
consequent on the change from one mode to another, may amount 
to over 20 per cent. in extreme cases. Mr. W. H. Goodchild 
has also studied the subject from another point of view. and 
suggested that some of the changes, especially the separation of 
metallic sulphides, take place with great, even explosive, rapidity. 

I may point out that we have, within our common everyday 
experience, familiar analogies to those changes which are presumed 
to take place in the material below the outer crust. Every time 
that we fire a gun, the impact of the hammer starts a change, by 
which the chemical elements, forming the material of the charge, 
pass from one mode of combination to another in which they 
occupy a vastly greater space, and in so doing give rise to the 
pressure by which the projectile, whether ball or shot, is propelled. 


Ixvill § PROCEEDINGS OF THE GEOLOGICAL socIEryY. [ vol. lxxviil. 


The familiar lecture experiments of supersaturated solutions, which 
remain quid until some disturbance, or the introduction of foreign 
matter, causes a rapid solidification, accompanied by a change of 
bulk, offer another analogy; and a third group of possible changes 
is represented by those allotropic alterations with which we are 
familiar, of which the alteration of aragonite into calcite may 
be quoted as an example. 

It is not improbable that, in the material beneath the outer 
crust, changes, more or less analogous to one or other of these 
types, are taking place, some slow and gradual, others more rapid 
and sudden, but all accompanied by a greater or less change of 
bulk, either of increase or decrease; and, if this be accepted, we find 
an explanation, not only of the forms and origin of earthquakes, 
but of many other phenomena, which are difficult of explanation on 
any hypothesis of contraction and compression alone. On the one 
hand, slow movements of elevation such as that of the northern 
Scandinavian region may be attributed to slow and gradual change 
involving the whole bulk of large masses, the lesser earthquakes 
may be due to more rapid changes in smaller portions, the greater 
to transformations involving a larger bulk of material, and possibly 
a more abrupt change of combination and density; while the 
greatest earthquakes, of first-class magnitude, result from similar 
changes involving a large bulk of material, the difference between 
the origin of small and great earthquakes being analogous to the 
difference in the effect of the explosion in a shot-gun, and that of 
the Vimy mines, or the recent havoc at Oppau. 

To elaborate these considerations forms no part of my purpose ; 
enough has been said to show that, even in our very fragmentary 
knowledge of what goes on within the substance of the Karth, we 
have means of explaining and interpreting the greater part of the 
facts known to us regarding the character of earthquakes. I shall, 
therefore, end my address by summing up the conclusions which 
have been put forward, as to origin and cause. These are, first, 
that earthquakes are not due to any slow acting process of secular 
duration, but to a rapid, possibly instantaneous, development of 
strain, a conclusion which I believe to be true of the greater part, 
at least, of those earthquakes usually classed as tectonic, and of 
all those of great magnitude ; and, secondly, that the development 
of strain is not the result of processes which have produced the 
tectonic structures recognized by surface observation, but to 
changes and displacements in the matter which lies below the 


eooled and solid outer crust. 


part 2] PROCEEDINGS OF THE GEOLOGICAL SOCIETY. Ixix 


February 22nd, 1922. 


Prof. A. C. Szwarp, Sc.D., F.R.S., President, 
in the Chair. 


Irene Helen Lowe, M.Sc., Egremont, Manorgate Road, Kingston 
Hill (Surrey), and Trevor Hughes Stonehouse, Lawnswood House, 
Hill Grove Crescent, Kidderminster, were elected Fellows of the 
Society. 


The List of Donations to the Library was read. 


The following communications were read :— 


1. ‘Description of a New Plesiosaur from the Weald Clay of 
Berwick (Sussex).’ By Charles William Andrews, B.A., D.Sc., 
E.R.S., F.G.S. 


2. ©The Carboniferous Rocks of the Deer-Lake District of 
Newfoundland.’ By Thomas Landell-Mills, F.G.S., Arthur Smith 
Woodward, LL.D., F.R.S., Pres.L.S8., F.G.S., and Albert Gilligan, 
D.Sce., B.Se., F.G.S. 


Specimens, microscope-slides, lantern-slides, and maps were 
exhibited in illustration of the above-mentioned papers. 


March 8th, 1922. 


Mr. R. D. OrpHam, F.R.S., Vice-President, 
in the Chair. 


Cecil Stevenson Garnett, 25 Crompton Street, Derby ; George 
Johnston, Keloil, Kelham, Newark (Nottinghamshire) ; William 
Russ, B.Sc., Assistant Geologist, Geological Survey of Northern 
Nigeria; and Cecil Edgar Tilley, B.Sc., A.I.C., Emmanuel College, 
Cambridge, were elected Fellows of the Society. 


The List of Donations to the Library was read. 


Dr. A. Smrra Woopwarp described certain photographs 
(natural size) of Desmostylus teeth from the Lower Miocene 
Sandstone of Southern Vancouver Island (B.C.) exhibited 
by Ira E. Cornwatt, F.G.S. 

The exhibitor wrote that these Desmostylus teeth are slightly 
different from any found in either California or Japan, as they 
show a well-developed cingulum. They may be from an older 
species than Desmostylus hesperus, as y hhas shown 
that the formation in which they »spsLower 

VOL, LXXVIII. 


Vas. al Mi years 


lxx PROCEEDINGS OF THE GEOLOGICAL society. { vol. lxxvuii, 


Miocene, while the formation in which Desmostylus remains have 
been found in California is Middle Miocene. 

One of the teeth shown in the photographs was found in 1916 
in the face of the sandstone-cliff west of Muir Creek, Sooke Bay, 
Southern Vancouver Island (B.C.). It was determined by the 
late Lawrence M. Lambe as the first right upper molar of 
Desmostylus hesperus Marsh, and is now in the British Columbia 
Provincial Museum at Victoria. The dimensions of this tooth 
are: Length=34 mm.; width=24 mm.; height of columns 
=17 mm.; diameter of the largest column =15 mm.; diameter 
of small column=10 mm. This tooth is considerably worn. 
The second tooth was found in the same locality last year by 
the Rev. Robert Connell. Its dimensions are: Length=47 mm.; 
width = 34 mm.; diameter of the largest column = 24 mm.; dia- 
meter of the smallest column = 17 mm. 


The following communication was read :— 


‘On the Geological Importance of the Primitive Reptilian Fauna 
in the Upper Cretaceous of Hungary.’ By Baron Francis Nopesa, 
For.Corresp.G.8. 


March 22nd, 1922. 


Prof. A. C. S—warp, Se.D., F.R.S., President, 
in the Chair. 


Leo Arthur Cotton, M.A., D.Sc., Assistant Professor of Geology 
in the University of Sydney (N.S.W.) ; Ivan Sidney Double, Lee- 
turer in Geology in the University of Liverpool, 10 Trinity Road, 
Bootle ; Edith Goodyear, B.Sc., Senior Assistant in the Geological 
Department of University College, London; Sidney Hall, B.Se., 
344. Brownhill Road, Catford, S.E.6; Isabel Elie Knaggs, 76 South 
Hill Park, N.W.3,; Cyrus Henry Perkins, B.A., Belcourt, St. Albans 
(Hertfordshire) ; Helen Marguerite Wood, M.Sc., 5 Hindes Road, 
Harrow-on-the-Hill ; and Leonard Langdale Wrathall, B.Sc., 
A.R.S.M., 37 Gwendwr Road, West Kensington, W.14, were 
elected Fellows of the Society. 


The List of Donations to the Library was read. 

The PRESIDENT read an appeal to collectors to purchase speci- 
mens of rocks and minerals on behalf of the Cornish Miners’ 
Relief Fund, and directed that the notice embodying the necessary 
particulars should be affixed to the board in the Society’s hall. 


The PRESIDENT announced that the Council had awarded the 


part 2] PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [xxi 


proceeds of the Daniel- Pidgeon Fund for 1922 to Herserr Price 
Lewis, B.A., Geological Department, University of Sheffield. 
The recipient proposes to investigate the structure of Caninoid 
Corals occurring in the Carboniferous Limestone of North Wales 
at higher horizons than their reputed range. 


Sir Cuarutes Joun Hoxumes, Director of the National Gallery, 
proceeded to deliver a lecture on ‘Leonardo da Vinci as a 
Geologist.’ The Lecturer began by referring to the growth in 
recent years of Leonardo’s reputation as a man of science. ‘This 
rapid growth led recently to a reaction, and it was now not 
infrequently stated that Leonardo’s scientific discoveries were in 
the nature of fortunate guess-work, and were neither proved nor 
accompanied by experimental research. In view of this attitude, 
the Lecturer felt that he could not present any statement of 
Leonardo’s discoveries to a scientific body, such as the Geological 
Society, except in the form of extracts from Leonardo’s own 
writings, which would enable them to judge for themselves 
whether his scientific reputation was firmly founded or not. 

Reading extracts from the translations made by Mr. McCurdy 
and Dr. Richter, the Lecturer pointed out how Leonardo was 
really the first to have a large and accurate conception of the 
causes underlying the physical configuration of the Earth. His 
studies of aqueous erosion, of the formation of alluvial plains, of 
the process of fossilization, and of the nature of stratification, led 
him to a logical conviction of the immensity of geological time, 
and were so far in advance of the dogmatic thought of his age, 
that they exposed Leonardo to the charge of atheism. There can 
be no doubt whatever, that if he had not confided these dis- 
coveries to the almost undecipherable script of his note-books, and 
kept them hidden there, he would have been one of the first and 
most notable of the martyrs of science. 

Caution thus compelled him to work in isolation, and to keep 
his results concealed: he had no scientific instruments, no corre- 
spondents to furnish him with observations on geological conditions 
elsewhere ; yet his grasp of the physical history of the portions of 
Italy which he had personally visited, was so sound, so firmly based 
on experiment and research, and so entirely in accordance with 
modern knowledge, that he must be considered the one great 
geological predecessor of Lyell. 

Since publication of his discoveries was impossible, Leonardo 
left a record of them in his paintings, as in the background of the 
‘Monna Lisa,’ the ‘Madonna & St. Anne,’ and in a less degree in 
our own ‘ Madonna of the Rocks’ in the National Gallery. Here 
we find pictures of the primeval world as he imagined it, when 
seas and lakes ran up to the foot of the mountains, to be slowly 
displaced and silted up by the detritus which the rain carried down 
from the summits. From this reconstruction the pictures derive 
that sense of action, apart from place or time, which has fascinated 


lxxil PROCEEDINGS OF THE GEOLOGICAL society. {[vol. lxxvi, 


generations who could not understand Leonardo’s meaning as we © 
can understand it now. 


After remarks by Mr. W. Wutraxer, Prof. W. W. Warts, 
Mr. W. Datsr, and the. PREestpENT, a cordial vote of. thanks was 
unanimously accorded to the Lecturer. 


April 12th, 1922. 


Prof. A. C. S—warp, Sc.D., F.R.S., President, and afterwards 
Dr. H. H. Tuomas, Vice-President, in the Chair. 


The List of Donations to the Library was read. 


The following communications were read :— 


‘Oligocene Mosquitoes in the British Museum, with a 
ne “of our present Knowledge concerning Fossil Culicidee.’ 
By F. W. Edwards, B.A. (Communicated by the Secretary. ) 


2. ‘Ona Collection of Carboniferous Plants from Peru.’ By 
Albert Charles Seward, Se.D., F.R.S., Pres.G.S. 


3. ‘The Geological History of the Genus Stratiotes: an Account 
of the Evolutionary Changes which have occurred within the 
Genus during the Tertiary and Quaternary Eras.’ By Miss 
Marjorie Elizabeth Jane Chandler. (Communicated by Mrs. E. M. 
Reid, B.Sc., F.L.S., F.G.S.) 


Specimens were exhibited by the Geological Department of the 
British Museum (Natural History), and specimens and lantern- 
slides by the Author, in illustration of Mr. F. W. Edwards’s - 
paper. 

Specimens and lantern-slides were also exhibited in illustration 
of Prof. A. C. Seward’s and Miss M. E. J. Chandler’s papers. 


May 10th, 1922. 


Prof. A. C. S—warb, Se.D., F.R.S., President, 
in the. Chair. 


William Ernest Victor Abraham, B.Sc., Nyounghla (Upper 
Burma); Ernest Batchelor, c/o King, King & Co., Bombay; 
Edmund Ernest Stockwell Brown, Wisteria House, Wisteria Road, 
Lee, S.E.13; George Tinline Button, 8 Marston Ferry Road, 


part 2] PROCEEDINGS OF THE GEOLOGICAL SOCIETY. Ixxii 


Oxford; Henry Day, M.Sc., c/o Bird & Co., Research Depart- 
ment, P.O. Box 46, Calcutta (India); John Kane, Glenside, 
Wattstown, Ynishir (Glamorgan); William Rushton Parker, M.A., 
M.D., F.L.8., Regent Palace Hotel, W.1; David Meredith Seares 
Watson, M.Sc., Jodrell Professor of Zoology & Comparative 
Anatomy in University College, London, 115 Greencroft Gardens, 
N.W.6; and Sidney William Wooldridge, B.Sc., Demonstrator 
in Geology in King’s College, London, 8 Bank Mansions, Herne 
Hill, 8.H. 24, were elected Fellows of the Society. 


The List of Donations to the Library was read. 


The following communications were read :— 


1. ‘The Lower Carboniferous Succession in the Settle District 
aud along the line of the Craven Faults.’ By Prof. Edmund 
Johnston Garwood, Se.D., F.R.S., V.P.G.S., and Miss Edith 
Goodyear, B.Sc., F.G.S. 


2. ‘The Miocene of Ceylon.’ By Edward James Wayland, 
Assoc.R.C.8., F.G.S., and Arthur Morley Davies, D.Sc., Assoc. 
R.C.S., F.G.S. 


Specimens and lantern-slides were exhibited in illustration 
respectively of the paper by Prof. Garwood & Miss Goodyear, 
and of that by Mr. Wayland & Dr. Morley Davies. 

A plaster cast of a model of .Peloneustes philarcus, modelled by 
the Rev. Henry Neville Hutchinson, M.A., F.G.S., & Mr. Edward 
Godwin, was exhibited, and 1t was announced that Mr. Hutchinson 
had presented the cast to the Society. 

A photograph (presented by Mrs. A. M. Blake) of a portrait 
painted by Eddis, of William Blake, Pres. Geol. Soc. 1815-16, was 
also exhibited. 


May 24th, 1922. 


Prof. A. C. SEwaRrD, Sc.D., F.R.S., President, and afterwards 
Dr. G. T. Prior, F.R.S., Vice-President, in the Chair. 


The List of Donations to the Library was read. 


The PrestpEntr then proceeded to deliver a lecture (illustrated 
by lantern-slides, microscope-slides, specimens of rocks, fossils, and 
plants) entitled ‘Geological Notes on Western Greenland.’ 
He remarked that Greenland isa ‘closed’ country ; the trade is a 
monopoly of the Danish Government, and no foreigners or Danes 
other than Government officials are allowed to go there without 
special permission. On June 18th, 1921, the Lecturer left Copen- 
hagen, accompanied by Mr. R. EH. Holttum, of St. John’s College, 


lxxiv PROCEEDINGS OF THE GEOLOGICAL society. [ vol. lxxvii, 


with the primary object of collecting fossil and recent plants 
on Disco Island and at other localities between lat. 69° N. and 
71° N. Godthaab was reached on June 28th, and Godhavn (Disco 
Island) on July 4th. Rather more than three weeks were passed 
at the Arctic Station at Godhavn with Mr. Porsild, the Director, 
who rendered invaluable service. The Arctic Station, which was 
planned and directed by Mr. Porsild, was afterwards taken over 
and subsidized by the Danish Government. In the course of two 
motor-boat excursions, a distance of over 600 miles was covered ; 
many localities were visited on the northern and north-eastern 
coasts of Disco Island, on the coast of Nugsuak Peninsula, also 
Hare Island, Upernivik Island, Ritenbenk, Sarkak, and Jakobshavn. 

Greenland is an island nearly 1700 miles long, with an average 
breadth of about 600 miles; approximately a hundred glaciers 
from the inland ice reach the sea, the largest of which (the 
Humboldt Glacier) ends in a cliff 60 miles broad. In the course 
of the lecture attention was called to the various forms of icebergs 
seen in Greenland waters, and to the views expressed by Mercanton 
on the origin of the various types. A brief account was given of 
some of the characteristic types of vegetation. A general account 
of the physical and geological features of Greenland as a whole 
was followed by a more detailed description of the Cretaceous and 
Tertiary sedimentary series of Disco Island and the Nugsuak 
Peninsula, and of the overlying and protecting basalts which in 
some places rest directly upon the old Archzan land-surface, to 
the exclusion of the sedimentary series. Special attention was 
directed to the nature of the sedimentary rocks (most of which 
are freshwater in origin), to the occurrence of raised beaches, to 
evidence of recent sinking of parts of the western coast, and 
to some of the more striking examples of dykes and sills in the 
Cretaceous and Tertiary sedimentary series. . 

No attempt was made to describe the paleobotanical results ; 
but allusion was made to some of the problems presented by the 
Cretaceous and Tertiary floras. 

A hearty vote of thanks was unanimously accorded to the 
Lecturer. 


June 14th, 1922. 


Dr. G. T. Prior, F.R.S., Vice-President, 
in the Chair. 


Meurig Thomas Adams, Croft House, 53 Morfa Street, Bridgend 
(Glamorgan); Cecil Thomas Barber, B.Sc., School House, Cook- 
hill, near Alcester; George Stanfield Blake, B.Sce., A.R.S.M., 
M.1.M.M., Imperial College of Science & Technology, S.W.7; 
John Henry Blizard, M.I.C.E., Roman Road, Bemerton, Salis- 
bury ; John McClelland Henderson, Ph.D., M.I.M.M., Apartado 
232, Maracaibo (Venezuela); Carl Archibald Phillips, P.O. Box 
459, Calcutta (India); Marie Carmichael Stopes, D.Se., Ph.D., 


part 2] PROCEEDINGS OF THE GEOLOGICAL SOCIETY. xxv 


F.L.S., Givons Grove, Leatherhead (Surrey) ; Wiliam Torrance, 
F.C.S., Grootfontein School of Agriculture, Middelburg (Cape 
Province) ; and Eustace Tanfield Vachell, Osmond House, 8 Cathe- 
dral Road, Cardiff, were elected Fellows of the Society. 


The List of Donations to the Library was read. 
The following communications were read :— 


1. ‘The Petrography of the Cretaceous and Tertiary Outliers of 
the West of England.’ By Prof. ee George Hamnall Boswell, 
O.B.E., D.Sc., D.L.C., F.G.S. 


2. ‘On some Rugose Corals from the Burindi Series (Lower 
Carboniferous) of New South Wales.’ By Prof. William Noel 
Benson, B.A., D.Sc., F.G.S., and Stanley Smith, M.A., D.S8c., 
F.GS. 


Lantern-slides and microscope-slides were exhibited in illus- 
tration of Prof. Boswell’s paper; and specimens of fossils were 
exhibited in illustration of the paper by Prof. Benson & Dr. 
Stanley Smith. 


June 28th, 1922. 


Prof. A. C. Szewarp, Se.D., F.R.S., President, and afterwards 
Mr. R. D. OLtpHam, F.R.S., Vice-President, in the Chair. 


The List of Donations to the Library was read. 


The Names of certain Fellows of the Society were read out for 
the first time, in conformity with the Bye-Laws, Sect. VI, Art. 5, 
in consequence of the non-payment of the arrears of their Annual 
Contributions. 


The following communications were read :— 


1. ‘The Petrology of the Metamorphosed Rocks of the Start 
Area (South Devon). By Cecil Edgar Tilley, B.Sc., A.I.C., 
E.GS. 


‘The Glaciation of the Counties of Gena Down, and parts 
a Armagh, Londonderry, Tyrone, Monaghan, and Louth in 
Treland.’ By Major Arthur Richard DSTENT ADESS, Ds DSe:, 
M.R.IA., F.G:S. 


Lantern-slides, microscope-slides, and rock-specimens were ex- 
hibited in illustration of Mr. C. H. Tilley’s paper, and lantern- 
slides were exhibited in illustration of Major A. R. Dwerryhouse’s 
paper. 

A giant Gasiropad (2?) from the sandstones in the Wadhurst 
Clay, Hastings, was exhibited by the Geological Department of 
the British Museum (Natural History). 


THE 


QUARTERLY JOURNAL 


OF 


THE GEOLOGICAL SOCIETY OF LONDON. 


Vou. LXXVITL 


FoR 1922. 


1. On the Junction of Gautt and LowER GREENSAND near 
LEIGHTON BuzzaRrpD (BEDFORDSHIRE). By GEonGE WILLIAM 
LampuveH, F.R.S., F.G.8. (Read May 25th, 1921.) 


CONTENTS, 
Page 

NEEL TOME G TOME nate: -ugihikinds on ced cokes as oA ei oes nee aah ENR 1 
Hee Meseription:, OF the Sections. ...0% -agenk secs nsua dhe wad ese. shiny aed ddetw hes 4, 
Hata eeontolosicals NObOS:. ic. sens gacsnwee se cnagitondan sis orse Pera cactce aes 44. 
IV. Classification, Structure, and Conditions of Deposition ............ 53 
V. Discussion of the ‘ Overturn’ Hypothesis .......................0008 68 
SRS ETI Te 8 acon ate ie cee Es thas ukisl concn Sab UST  Saieahiod tm oes 00 


Note.—Throughout this paper, where the names of fossils are printed in 
ordinary (Roman) type between single quotation-marks, this implies that the 
name is only an approximate or ‘field’ determination, and should not be 
regarded by the paleontologist as final. Where names are printed in the 
usual italics, the determination is expert or authoritative, and ought therefore 
to be of paleontological moment. 


I. IntTRODUCTION. 


EIGHTEEN years ago the late J. F. Walker and myself described 
to the Society! a remarkable fossiliferous limestone occurring 
immediately beneath the Gault at Shenley Hill near Leighton 


1 © On a Fossiliferous Band at the Top of the Lower Greensand near Leighton 
Buzzard’ Q. J. G.S. vol. lix (1903) pp. 234-65 & pls, xvi-xvill. 


Q.J.G.S. No. 309. B 


DOUBLE 
ARCHES 
POPLARS 
PIT 


3 


— 
Heath House 400 


- 


“Miletree Farm 


PE. PIT 


CHAMBERLAIN 
BARN PIT 


/ 
Sip ae 


Ze /Old Clay Pit 


Us (= 


Fig. 1. Sketch-Map 
of the Neighbourhood of 
Leighton Buzzard 
(adapted from the 6-inch 

Ordnance Survey map). 


Water Works 


[The numerals within circles refer to 
the numbers of the text-figures.] Brickyard a 


Scale 


tooo yards 


° aS , 
Ons WEBSTER S 


part 1] GAULT AND LOWER GREENSAND NEAR LEIGHTON. 3 


Buzzard. The bed differed, both in its lithological characters and 
in its fossil contents, from any other deposit then known at this 
horizon in England, and the problems which it presented demanded, 
and have received, further investigation. Walker continued 
assiduously to explore the paleontology of the bed, until his work 
was cut short suddenly by death in 1907. Partly by personal visits 
to the locality, but mainly through the agency of the quarrymen, 
he had been able by this time very largely to increase his collection 
ot the fossils, and had made progress in his study of the brachiopod- 
fauna, which was his particular interest. It was his intention 
ultimately to deal exhaustively with this element of the fauna, and, 
as a preliminary step, he had already prepared and sorted many 
thousands of specimens into their proper specific relationship, with 
provisional determinations where possible.!. The collections thus 
arranged by his own hand are, however, all that remains to us of 
the work that he did after the publication of our joint paper. 
Some information culled from them will be given in a later part of 
the present paper. 

While my late colleague continued to explore the paleontology 
of the Shenley bed, I devoted attention, as before, more particularly 
to its stratigraphical aspect. Visits in successive years constantly 
revealed new features of interest during the continued excavation 
of the Shenley sand-pits, and showed also that other pits in the dis- 
trict were approaching the junction of the Gault with the Lower 
Greensand. Eventually I succeeded in obtaining evidence, long- 
expected, for the presence of the Mammillatus Bed in sections 
south of Leighton,’ and afterwards in euttings within half a mile 
of Shenley Hill. 3 

The Leighton Sand trade received a great impetus as the 
result of the War, wherefrom the old pits have been energetically 
worked and several new ones opened. During last summer and 
autumn (1920) I was able to undertake a leisurely examination of 
all the sections, and I propose in this paper to combine the infor- 
mation thus obtained with the observations gathered at intervals 
since 1903, into a general review of the character of the Gault-and- 
Lower-Greensand junction in the district. This junction is of 
peculiar interest, and nowhere else in England has it been so 
extensively displayed in open sections. 


1 See reference to this work in ‘ Obituary: John Francis Walker’ Geol. 
Mag. 1907, p. 383. 

2 By the liberality of his widow and his son, Walker’s collection of 
brachiopoda in its entirety was presented to the British Museum (Natural 
History) at South Kensington, and the fossils other than brachiopoda were 
similarly presented to the Sedgwick Museum at Cambridge. My thanks are 
due to the authorities of these museums for the facilities afforded to me in 
examining the collections. 

3 Brief references to new features in the Shenley sections and to the dis- 
covery of the Mammillatus Bed south of Leighton are contained in my reports 
on two excursions of the Geologists’ Association, 1908 & 1915, Proc. Geol. 
Assoc. vol. xx, p. 475 & vol. xxvi, p. 310. See also my letter on ‘Gault & 
Lower Greensand near Leighton Buzzard’ in Geol. Mag. 1920, pp. 234-37. 

; B2 


4 MR. G, W. LAMPLUGH ON THE JUNCTION OF _ [ vol. lxxviii, 


The attractiveness of my subject has been enhanced by the 
recent publication of a paper by Dr. F. L. Kitchin & Mr. J. Pringle, 
in which it is argued that the strata above the Lower Greensand in 
the Shenley Hill sections have been inverted by Glacial agency ; 
also that there is an overlap of the Upper Gault upon the “Lower 
Greensand in this quarter! ; both of which suppositions can, I think, 
be shown to be incorrect. The discussion of these points will be 
dealt with separately in the concluding part of this paper, after I 
have described the sections of the new pits and the new features 
revealed in the pits previously described. 

The general geological structure of the country around 
Leighton Buzzard was shown in the paper of 1903 by a sketch- 
map reduced from the Geological Survey map (Q. J. G. S. vol. lix, 
fig. on p. 235), and for the present purpose requires no further 
illustration. 

As the original interest centred upon the pits under Shenley 
Fill, I will first record the additional information obtained from 
this place, and will then deal in turn with the new sections 
east, north-east, north-west, and south-west of Shenley. The 
position of the sections is marked on the outline-map (fig. 1, p. 2). 


II. DESCRIPTION OF THE SECTIONS. 


The Shenley Hill pits.—Shenley Hill is composed of Gault, 
based on Lower Greensand, and capped and protected by the 
remnant of a dissected plateau of Glacial Drift, chiefly Boulder 
Clay. The hill rises well above the 400-foot contour, an Ord- 
nance Trigonometrical Level of 457 feet being shown at less than 
400 yards from the sand-workings. The workings themselves 
are on the lower slope, with an average ground-level of about 
350 feet, the slope continuing south-eastwards into the broad 
Post-Glacial valley of the Clipstone Brook, where the 300-foot 
contour is reached at about half a mile south-east of the sand- 
pits. These are steep slopes for so ready a slipper as the Gault, 
and the consequences are apparent in most of the sections. 

Of the three contiguous pits mentioned in the previous paper,? 
which in 1902 were being pushed westwards into the steep slope of 
the hill, and in which the fossiliferous limestone was sporadically 
exposed, the middle one only—Harriss?—is now in operation. 
Chance’s on the north and Garside’s on the south were abandoned 
about 15 years ago, and the terminal sections in both are obliterated 
by downwash, vegetation, and spoil; their final position is indicated 
on the ground-plan (fig. 2, p. 5) which, if compared with the 


1 «On an Inverted Mass of Upper Cretaceous Strata near Leighton Buzzard. 
Bedfordshire; & on an Overlap of the Upper Gault in that Neighbourhood ~* 
Geol. Mag. 1920, pp. 1-15, 52-62, 100-138. 

2 For plan of the workings at that time, see fig. 2 of the paper. 

3° Through a misapprehension of the spoken name (Gregory Harris), the 
incorrect rendering ‘Rigby Harris’ was applied to this pit in the previous 
paper, 


part 1] GAULT AND LOWER GREENSAND NEAR LEIGHTON. 5) 


plan (fig. 2) in the previous paper, will give the measure of the 
later work. 

In regard to the general succession of the deposits and the com- 
position of the limestone-lenticles, our former descriptions have 
been found to hold good in all particulars, and need not be repeated. 
Minor changes were constantly observed in the few feet of variable 


Fig. 2.— Ground-plan of the Shenley Hill sandpits. 


pit 


Nine Acre pit 
@x& 


| ey: 
i ©) Si Cottages 
Harris's : a 
pe 2G) 
/ Ns 


@) Garside’s.. 
old pit \; 


Cee} 

2a 
ees 
/ 
of 


[Numerals printed within circles 


Scale 
o 30 100 200 300 4,00 


beds immediately beneath the Gault, and a new feature was pre- 
sented towards the western end of Garside’s pit by the incoming of 
a wedge of greensand between the Gault and the ironstone-breccia 
(see figs. 4 & 12, pp. 11 & 22), but otherwise the features of the 
sections remained the same throughout. The limestone was seen at 
intervals in all three pits, always in the form of impersistent tabular 
lenticles of variable size. It happened often on our visits that none 
of the rock was visible in place, while at other times it was well 


Miletree Farm (11) 


CSS) 3Miletrée 


. ei pit \ 


refer to the nunibers of the text figures.) 


500 yards 


ie eee Ee eee 


6 MR. G. W. LAMPLUGH ON THE JUNCTION OF  [vol. lxxvii, 


displayed. The larger masses occurred only under the low dome of 
unbroken iron-pan (C in fig. 8 of the previous paper) which rises 
from both sides towards Harris’s pit, declining gently northwards 
and southwards, as shown in fig. 12 (p. 22). In Chance’s pit on the 
north, the hmestone became much streaked and intermingled with 
glauconite and grit, and was usually more or less decomposed, so 
that its fossils, though readily disentangled, required sizing for 
their preservation ; and in places the rock passed into a soft ochreous 
caleareous paste in which the fossils were obliterated. A large 
number of the specimens in the Walker Collection were obtained 
from this pit. Similar conditions were also observed on the south 
as the working in Garside’s pit advanced westwards, the limestone 
decreasing in quantity until it was reduced to a few soft calcareous 
patches occurring as matrix amid the ironstone-breccia. 

It is noteworthy that the cleanest and whitest ‘silver sand,’ 
for which the pits are worked, has been found under the dome, 
where the iron-pan floors are unbroken; ferruginous discoloration 
sets in on both flanks where these bands become lenticular and 
brecciated, and where the firm thick tables of limestone are absent.! 

The clay-cover became steadily thicker as the workings were 
pushed westwards, and in this cover Harris’s pit, now cutting into 
the steeper clay-slope, has recently disclosed some new information 
respecting the Gault. From its slippery character the sections 
in the Gault are soon spoilt, and need to be seen under favour- 
able circumstances. Dr. Kitchin & Mr. Pringle appear to have 
found a clear section in this pit in October 1918, revealing about 
10 feet of Gault; but they mention that this was obliterated by an 
extensive slip early in 1919, which, however, enabled them to 
examine some higher Gault, estimated to be about 18 feet above 
the base?: by careful search they obtained some fossils, hitherto 
unrecorded, from the sparingly fossiliferous lower portion of the 
Gault, together with a few others from higher levels. Last year 
(1920) a Tichly fossiliferous band was exposed near the top of the 
section, and enabled me to collect material which throws light on 
the relations of the Upper and Lower Gault in the district. 

The section exposed in the northern part of Harris’s pit in the 
autumn of 1920 is shown in the following figure (fig. 3, p. 7), 
which may be compared with the section farther south in the same 
pit in 1902, given in fig. 3 of our previous paper. 


1 Many particulars respecting the Leighton Sands from the economic stand- 
point, including descriptions of the workings and the methods of treatment of 
the material, will be found in Mem. Geol. Surv. ‘ Special Reports on the Mineral 
Resources of Great Britain: vol. vi, Refractory Materials,’ &c., 2nd ed. (1920) 
pp. 180-83; and in Prof. P. G. H. Boswell’s ‘ British Resources of Sands 
suitable for Glass-making, &c.’ London, 1916, and ‘Supplementary Memoir 
on British Resources of Sands & Rocks used in Glass-manufacture, &c. 
London, 1917. 

2 Geol. Mag. 1920, p. 9. 


part 1] GAULT AND LOWER GREENSAND NEAR LEIGHTON. 7 


Fig. 3.—Profile section at the northern end of Harris's pit, 
Shenley Hill, August 1920. Surface (at breakaway), about 
385 feet O.D. 


i. 


10 feet a 


; Thickness in feet. 
Z. Brown clay-soil with flints, passing down into— 1 
ZY, Pale-grey marly Gault, with massive structure, disturbed and 
5 probably more or less sliding, with a few stones in the upper 
part, and some obscure bedding towards the bottom............... 5to6 
5a. Band of phosphatic nodules of all sizes up to 6 or 8 inches in 
diameter, containing many fossils, in very fossiliferous pale- 
grey clay; some of the nodules pale grey, others black and 
showing scars of adherent Plicatulze, polyzoa, etc. ; both kinds 
generally much corroded on the upper side, and the smaller 
nodules frequently embedded in a regrowth of concretionary 
phosphate (compound, nodules) 2. .c.hc502.(cncadse aes lose onpeeedes 
Ammonites abundant but mostly fragmentary, the majority 
belonging to the keeled ‘ rostratus’ group, but others akin to 
‘auritus, ‘splendens,’ etc. ‘Inoceramus sulcatus’ also 
abundant. (See further, p. 53.) 
AeGault.ooscured by Slips. hed -.0.ss.. cas satease te soe iem ve eee ade cont about 8 
[‘ At 8 to 10 feet above the base’ of the Gault Dr. Kitchin & 
Mr. Pringle found ‘a Hoplites of the tuberculatus group’ and 
others including ‘a large involute hoplitid of flattened discoid 
form’ apparently representing ‘one of the several series whose 
members are commonly united under the favourite collective 
name H.splendens.’ ‘ At this level and in the overlying part of 
the clay there occur large crushed examples of Inoceramus 
sulcatus Park.’ | 
-( (Upper Step). Excavation in slipping ground: material mainly 
the pale top clay, but mixed in places towards the bottom 
4 J with darker and more platy blue clay which yielded a fragment 
Ole AMMAIMOMIGes is PleMmM ens g.02 ces ose: ee weeaediec se de cdhice ete gedieow cee 3 
( (Lower Step). Gault in place, but with slide-planes in places, 
indicating movement under the slip; dark-blue (drying greyish- 


tole 


8 MR. G. W. LAMPLUGH ON THE JUNCTION OF  [Vvol. lxxvil, 


Thickness in feet. 

4. blue) platy clay, rather silty in texture, with green-mottled 
(cont.) markings here and there: small smooth brown-coated black 
phosphatic nodules, widely spaced, but with linear tendency : 
rusty joints often coated with small selenite-crystals, giving 
blocky fracture, due to shrinkage and weathering with decom- 
position of original pyrites. Streaky greenand crimson 3-inch 
layer in places at the base, resting on the iron-grit floor. Fossils 
rare, except ‘Belemnites minimus’ and crushed ‘ Inoceramus 
concentricus’; but there are some traces of crushed ammonites 
and other shells. [ Dr: Kitchin & Mr. Pringle record an ammonite 
of the auritus-group, supposed to be Hoplites catillus (J.de C. 
Sowerby), and “Nautilus deslongchampsianus d Orbigny, along 

with a few other fossils; see postea, pp. 51-52] .................. 4 
3. Iron-grit floor, 1 to 2 inches thick, with minor undulations, 
abraded on the knobs, and with a little coarse gritty sand pre- 
served in some of the hollows, capping ochreous grit and iron- 
stone-breccia with calcareous patches and ramifying veins and 
tables of iron-grit: as fully described in the previous paper, 

. 238.. ea es | Set t 2 to 23 

: Silty beds” : loamy. gr eensand, clay, ete. ; as described in the pre- 
vious paper (Bed F), but diminished in thickness and without 
the underlying lenticles of iron-grit, sandy Py ritous claystone, 


to 


etc. (Bed G) seen in 1902 . .... about 2 
1. ‘Silver sands’ Cee former paper), & ‘worked to a v depth, of 18 to 
20 feetiee seen : . Top only shown. 


All the other Sealey Hill sections have exhibited only the 
dark platy lower clays, usually with a ‘creep’ of amorphous 
clay above them, and this is the first section that has reached 
high enough to reveal the incoming of the Upper Gault fossils, 
‘Inoceramus sulcatus’ and the keeled ammonites, although these 
forms have long been known to occur at another pit near Heath 
House, 900 yards west of the Shenley Hill pits (see p. 28). 

The band of phosphatic nodules 5a is of particular interest, both 
from its structure and from its fossils. There can be little doubt 
that it is a prolongation of the nodule-bed seen by Jukes-Browne 
in 1884, in a brickyard-section north of Leighton Buzzard, from 
which he obtained numerous fossils, including eight species of 
ammonites (see p. 27), described as a ‘ mixture of Lower and Upper 
Gault species.’! The nodules are distributed rather sparsely and 
irregularly in a layer which could be traced horizontally for 12 or 
15 yards, in the top breakaway, before being hidden by the slip. 
The layer was unbroken, but showed some minor undulations pro- 
bably due to slipping, the pale marly clay above it being all more 
or less affected by ‘creep.’ It does not contain any extraneous 
coarse transported matter, differing in this respect entirely from 


1 «The Cretaceous Rocks of Britain—vol.i: The Gault & Upper Greensand 
of England’ Mem. Geol. Surv. 1900, p. 285. In our previous paper (op. cit. 
p. 245) I presumed that the brickyard was that, now disused, on the 
south-east side of the road from Leighton to Shenley Hill, 1100 yards 
south of the sand-pits; but I have since found that the place referred to was 
a working, now obliterated, near the Heath House sandpit (postea, p. 28). 

The nodule-band in Harris’s pit does not appear to have been exposed in 
1919 when Dr. Kitchin & Mr. Pringle examined the section, as they state 
(Geol. Mag. 1920, p. 61) that ‘No nodule-bed similar to that described by 
Jukes-Browne has been seen in any of the sections examined by us, and we 
are unable to test the value of his record.’ 


part 1] GAULT AND LOWER GREENSAND NEAR LEIGHTON. 9 


the coarse basement-beds of the Gault presently to be described. 
The ‘compound’ structure and general aspect of the nodules,! as 
well as their fossil contents, imply that they have been concentrated 
on the sea-floor during a rather long interval when normal sedui- 
mentation of the Gault clay was arrested, presumably by current- 
action. A similar phosphate-bed at about this horizon, formerly 
worked for ‘ coprolites,’ is described by Jukes-Browne as occurring 
at several places along the outcrop of the Gault in Bedfordshire 
and Buckinghamshire.? 

It is important to note that the band is also nearly on the same 
horizon as the nodular ‘Junction Bed, VIII’ of the Folkestone 
Gault, respecting which Price’s remarks are so apposite that I will 
recall them. He says :— 

‘I would suggest that these lines of nodules, mixed as they are with rolled 
fossils, occurring so plentifully throughout the deposit, mark the floor of the 
sea during a period, more or less vast, when great physical changes may have 
altered the course of the currents, and so borne away the sediment, to be 
redeposited in another direction ; or they may represent periods of upheaval 
or tranquillity, when the sea-bed was at rest. It may thus be argued that 
these nodule-beds, not exceeding 1 inch in thickness, are equivalent to a period 
of time far greater than was required for the deposition of several feet of clay.” 


The lowest part of the Gault has shown little or no change 
during the working-back of the pit, except that the impressions of 
crushed fossils sparingly present in it are rather less obscure than 
they were where the cover was thinner. ‘Traces of the decompo- 
sition of original pyrites are very apparent, and may partly explain 
the poor condition of the fossils. The small brown-coated nodules, 
an inch or two in diameter, which occur in this part of the clay are 
quite different in aspect from the nodules of the higher bed just 
described, and have not been concentrated or corroded. A fragment 
of decomposed pyritous wood, 4 or 5 inches long, associated with 
a line of the small nodules, had however been bored by marine 
organisms, and was partly encrusted with small oyster-like shell- 
sears. The fossils of this portion of the clay will be dealt with in 
the discussion of the supposed overturn (pp. 51, 78). 

Last autumn, no solid limestone was visible beneath the Gault 
in the portion of the pit illustrated. The ochreous ironstone- 
breccia (38) was, however, in one place very calcareous, and repre- 
sented the decomposed feather-edge of a tabular mass of limestone 


1 Incidentally the nodules supply proof that the band and its associated 
deposits are not turned upside down, as supposed by Dr. Kitchin & Mr. Pringle. 
Most of the larger nodules, and many of the smaller ones, are deeply corroded 
on their upper surface but comparatively fresh on the under side, this being 
particularly conspicuous when, as is frequently the case, the nodules are 
casts of portions of ammonites. The under surface often presents a beauti- 
fully-smooth bright cast of the shell, whereas on the upper surface the fossil 
is almost or quite indistinguishable. This condition of the larger fossils is 
well-known in many deposits of slow accumulation, as for instance in certain 
parts of the Chalk and other limestones (see Proc. Geol. Assoc. vol. xvii, 1904, 
pp. 287-89). 

2 «The Gault & Upper Greensand of England’ Mem. Geol. Surv. supra cit. 
pp. 277-78, 280-82. 
3 <The Gault’ by F. G. Hilton Price, London, 1879, p. 9. 


10 ‘MR. G. W. LAMPLUGH ON THE JUNCTION OF  [vol. Ixxvin, 


recently quarried away, as indicated in the figure. When I saw 
the section in the preceding spring the workmen were breaking 
through this table of limestone, which was 9 to 12 inches thick, 
and 10 or 12 feet long, lying everywhere in one plane immediately 
under the topmost layer of iron-grit. The rock was, as usual, 
full of fossils, and was similar in all respects to the lenticles seen 
and described 18 years ago. 

It has occasionally happened since that time that no hmestone 
has been met with in the pit for three or four years in succession, 
but every fresh patch exposed has been on exactly the same 
stratigraphical horizon, and has presented the same lithological 
peculiarities. 

The sharp line between the capping iron-grit band and the 
bottom of the Gault in the above section marks an interval of time 
and of non-deposition at this spot, on which fresh light was fortu- 
nately thrown by the workings in Garside’s pit immediately on 
the south, before they were suspended some 15 years ago. These 
workings will be described next. 


Garside’s old Shenley pit.—In 1902 the section in this 
pit was too near the outcrop of the base of the Gault to show a 
clear succession, there being only about 4 feet of disturbed and 
weathered clay overlying the capping of iron-grit and breccia 
above the ‘silver sand.’ But, when I revisited the locality in the 
following summer (1908), fresh sections had been cleared in the 
western part of the pit, and its north-and-south face revealed 
the gradual incoming of a wedge of calcareous greensand between 
the iron-grit floor and the Gault. The sections were practically a 
southward continuation of those opened up by Harris’s pit. The 
Basement beds sloped down to 20 to 30 feet below their level in 
the present exposure at Harris’s. The following details are from 
my note-book. 


Section at the west side of Garside’s old pit, August 15th, 1903. 
Surface, about 350 feet O.D. 
Thickness in feet. 


= Stiff grey-blue clay, with a few stones (probably mostly ‘creep’) ...... A 
Rather pale blue clay with ferruginous bands, somewhat disturbed. 6 
4 ‘TInoceramus concentricus ’” plentiful in one band. 
Darker blue and ferruginous brownish-blue clay, yellowish a T 
gritty in places at the base (4a). 


3b. Greensand, yellowish and rather clayey towards the top; clean, and ) 
dark sage-green below; full of worn and broken bits of shell | 
in the lower 6 inches, but hardly any fossils in the upper part. > 3 
‘Belemnites minimus,’ ‘ Inoceramus concentricus,’ small ‘ Ostrea,’ | 
‘Cidaris’ spines, teeth of fishes, etc. (See p. 49.) 

The bed thins out to about 9 inches as we go 7 yards northwards; 
and at the next working-place, a few yards farther north, has 
entirely died out; but at the base of the Gault there is a dark 
clay-band, full of small black pebbles (lydite, etc.), about 6 inches 
thick, resting on— 

3. Irregular floor of iron-grit, worn on the knobs, with breccia of 
ironstone, etc. in the hollows. 
2&1. Lower beds as in the preceding paper, not studied in detail. 


part 1] GAULT AND LOWER GREENSAND NEAR LEIGHTON. 1h 


Later in the same year (1903) the sections were examined by 
Mr. Walker, who sent me details which agreed with my own 
observations: In response to my request that he would search the 
greensand for fossils, he wrote : 


‘We [himself and his son] broke up a good many of the blocks {on the tip- 
heap | and also a lot of the section, but it was so damp that the fossils were 
softer than the matrix and they fell in pieces...... I could not see our bed 
[the limestone] pass into the greensand: I think it occurs underneath it; at 
the far end of the pit our bed is seen, but the fossils are very badly pre- 
served: we got a few out, but the space between was covered by the fluid 
clay which had run down. The fossil conglomerate-bed seems to thicken 
towards the end of the working.’ 


On mny next visit, in the summer of 1904, I found that some 
novel and highly interesting features had been revealed, as shown 
in the middle part of fig. 4. This figure is a combination of the 
three drawings made in my note-book in 1908, 1904, & 1906, 
and gives a somewhat shortened section across the pit from north- 
west to south-east. 


Fig. 4: s Garside’s old pit at Shenley 
Hill, from sketch-sections in 1903, 1904, 4 1906, described 


an the text. 
S.E. N.W. 
Ree ey Hides amet te Beevermhe Soh ot ae Gshiadusibs By. Laila wadvfatos hse use. Aattnctdles aonb INE, 


ee JB Beye Efi 7p hee é oe ot 


2 Scale? vertical 0 ie, ° 10 15 feet limestone 
horizontal slightly reduced 


Section of the south-eastern corner of Garside’s old pit, 
August 14th, 1904. Surface, about 350 feet O.D. 


Thickness in feet. 
ZY. Clay with a few flints, etc., passing down into disturbed clay, 3 
a passing into— 
4b. Rather pale-blue homogeneous Gault clay, crowded in places with 4 
‘Tnoceramus concentricus.’ 
4. Striped Gault clay, some bands dark and rather gritty (4a), 
others paler and some ferruginous: a few pale brown-coated 6 
nodules : fossils rare. 


12 MR. G. W. LAMPLUGH ON THE JUNCTION OF  [vol. lxxvi, 


Thickness in feet. 


3b. Loamy calcareous greensand, with lydites and small grey irregular | 
phosphatic nodules: fossils, chiefly towards the base and in | 
pockets at the margin of iron-grit crag—‘ Belemnites minimus,’ > 3 to 4 
*‘ Inoceramus concentricus,’ * Ostrea,’ Cirripedes, Serpule, ‘ Tere- | 
bratula,’ etc. (see p. 49). 


X. Protruding crag of hard purple iron-grit, worn smooth and bossy 
at the top and partly at the sides ; with hollows and pipes both 
at the top and the sides filled in with fossiliferous greensand, 
but the summit overwrapped by Gault clay: some adherent 
oyster-scars in sheltered parts: ironstone-breccia against the base 
on the west side. Crag about 5 feet broad and 8 to 4 feet high. 
A smaller detached lump of similar composition, about 7 yards 
farther west, is probably protruding from another crag behind 
the present section-face. 


3. Thin undulating pan or pans of iron-grit, associated with worn } 
ironstone-breccia containing soft calcareous patches indicative of | 
incipient or decomposed limestone-lenticles. The iron-grit pan r 1lto3 
fairly continuous, but sometimes duplicated, enclosing lenticles | 
of breccia, etc.: or with breccia in hollows above or below. J 


to 


Silty beds: well-stratified loamy sand (in part glauconitic), silt, 

thin clay-streaks, ferruginous layers with thin tabular concre- 

tionary ironstone, etc.; at the base, tabular ironstone-lenticles 

up to 3 inches thick. Se section in the previous bape fg. 3, 

D- 2382)" - 2.5 os esoseki ee bce bys ons 

a. aa coloured mer passing AEE: nr ee into Sitges aaa 
at the east side, orange-coloured and ‘ferruginous brown sand 
partly converted into jron-grit (1 x) down to 6 feet, with silver 
sand below, to the bottom of the pit ............ worked to about 1s 


The upstanding crag of iron-grit, which was the most singular 
feature of the section,! was photographed soon afterwards by 
Mr. Basil Schon, F.G.S., who kindly sent me prints (exhibited). 
Living at that time in Ireland, I had no opportunity to visit the 
section again until 1906, by which time no trace of the crag was 
left; but I was told by the workmen that it rose higher than I saw 
it, before they reached the end of it. In 1906, the section was as 
follows :— 


Section at the southern end of Garside’s old pit, 
August 11th, 1906. Surface, about 350 feet O.D. 


Thickness in feet. 
‘Top as before’ [not drawn | 
4. Gault, with well-marked bedding, dipping eastwards, brought out 
by ferruginous-stained bands. ‘Inoceramus concentricus’ very ( about 5d 
abundant, but hardly any other fossil seen. Line between ( examined. 
Gault (4a) and Greensand (3 b) not very well marked. 


3b. Dark loamy greensand, thinning rapidly eastwards _............... 5 to 2 
‘ Belemnites,’ ‘ Ostrea,’ and other fossils as before, but nothing and less. 
fresh. Sharp base. 


1 A brief account of the peculiarities of this section was given in my report 
of an ‘Excursion to Leighton Buzzard’ Proc. Geol. Assoc. vol. xx (1908) 
p. 475 


part 1] GAULT AND LOWER GREENSAND NEAR LEIGHTON, 13 


Thickness in feet. 
3. Ochreous conglomerate of partly-worn ironstone-fragments, grit- ) 
pebbles, etc.: in one place, capped by a ‘ table’ of hard iron-grit 
46 inches long and 5 inches thick; on a more or less continuous 
floor of generally coarse iron-grit, but passing down in one place p 1 to 13 
into coarse green pebbly sand or grit. Iron-pans above the | 
floor, mostly broken up, but with a féw veins later than breccia- 
tion. No limestone visible. Sharp base. 


2. Silty beds: loamy greenish sands, clays, etc., as before; on tabular 
FELTUSAMOMSKCOMCECUIONS) alae eee ahs ce atserse nan sce ica oeerses oigameienianin yea nen about 2 


1. Silver sand. 


That the bosses of iron-grit formed crags on the sea-floor is 
clear from all the circumstances, and their presence is probably 
responsible for the preservation of the surrounding greensand, by 
protecting it from the full sweep of the sea-currents. The relation 
of the ironstone of the crags to the underlying beds is not well 
shown, either in my note-book section or in Mr. Schon’s photo- 
graphs, but may be explained by recent sections in pits on the 
east, presently to be described. The masses were probably 
indurated bosses of the main lower sands, with the breccia, etc. 
banked around their undercut edges, as in the Nine Acre and 
Miletree pits (figs. 8, 9, & 10, pp. 16, 18, & 19). 

The southerly working of the pit ceased not long after my visit 
in 1906, for in March 1908 I found all the faces slipped and 
obliterated, and they have since been banked in by tip. 

I have entered fully into the particulars of my observations at 
this spot, because it is held by Dr. Kitchin & Mr. Pringle that 
the greensand beneath the Gault (which they never saw) must 
have been ‘ Upper Greensand’ brought into this position by Glacial 
inversion, an idea irreconcilable with the facts. 


The structure of Shenley Hill in a north-and-south line, nearly 
parallel to its axis, is shown in the reduced section (fig. 12, p. 22), 
in which the sections observed in the contiguous pits are combined. 

The great variability of the beds immediately below the base of 
the Gault in this region is exemplified to a still more striking 
degree in a group of practically continuous sand-workings stretch- 
ing eastwards from Shenley Hill, next to be described, as well as 
by others on the north and west. - 

The positions of the easterly workings are shown on the plan 
(fig. 2, p.5); most of them are still in operation, and the informa- 
tion now to be given embodies the evidence which they presented in 
the summer and autumn of last year (1920), supplemented in a few 
cases by observations of earlier date. ‘They have many features in 
common, along with points of individual peculiarity ; the following 
descriptions and figures of the more important portions are arranged, 
so far as possible, in an easterly sequence. Their relation to the 
section at Harris’s pit, above described, is shown in the combined 
section (fig. 18, p. 22). 


Fig. 5.—Section at the north-eastern corner of the new (East) 
working of Garside’s pit, Shenley Hill; 120 yards south 
of Sandpit Cottages. September 8th, 1920. Surface, about 
340 feet O.D. 


Scale:V ertical and Fivenaalt eo. 10 ee feet 
Thickness in feet. 
ZY. Soil passing down into gravelly loam, mainly of flints with a few) 
iron-grit fragments, quartzites, and other drift-stones and | 
boulders ; luoped and contorted with underlying clay, and with > 1to4 
slicken-planes in places :=Late-Glacial gravelly wash disturbed | 


by later ‘ trail’ movement. J 
ZY. Re-arranged pale-blue Gault clay, with an oceasional flint or drift-) 
4 stone in the upper part: much calcareous ‘race’ in places: | 


crumbly texture and massive structure with sharply-cut } about 2 
bright slicken-planes, probably due to ‘creep’: passing down | 
into— 

4 Disturbed Gault: brownish ferruginous clay, somewhat 
weathered, with a few small brown-coated phosphatic nodules ¢ 13 to 2} 
(black inside), and much white ‘ race.’ 

4a. Dark greyish-blue platy Gault; with streaks containing polished } 
grit-grains in the lower half; and mottled streaks of dirty | 
sreensand i in the lowest inch or two: im one place a lenticle of L 2to1 
eritty greensand, half an inch thick, at the base. No fale . 
seen, but all the clays are more or less weathered and aay 
by tree-roots. 

3. Loamy and ochreous iron-grit breccia, with some soft PEGE! 
patches, partly calcareous, partly phosphatic: and some tabular 
slabs of iron-pan, measuring up to 2 feet in length and about an 
inch thick. 

1. Orange-coloured and buff-coloured sand, rising to near the top 
farther south, and there passing into and capped by iron-grit. 


Section (weathered) at the} east side of a small old partly- 
overgrown pit, 100 yards east of Chanece’s pit (see plan, 
fig. 2,p.5), September 6th, 1920. Surface, about 350 feet O.D. 


Thickness in feet. 

Z. Clay soil, with a few flints and drift-pebbles... lto2 

4. Gault, poorly exposed, weathered and cramblr, “with roots of} 
recent vegetation down to 6 feet: darkish grey-blue and platy 
in the lower part, with a paler band about 1 foot thick ; bits of | L abont 10 
shell, but nothing identifiable found : a few small brown-coated | ace 
phosphatic nodules : not much ‘race’: red layer at the base, on 
pan of iron-grit. 

3. lron-grit and ochreous breccia, slightly calcareous in places : thin 
tabular iron-grit 1 to 2 inches thick at the top, nearly contin- 
uous, but broken in places, with worn and polished fragments up 
to 4 inches in diameter below, also small polished pebbles, ete. 

1. Ferruginous brown and yellow sand with ironstone, passing at the 
top into dark, liver-coloured, iron-grit rock, 6 to 9 inches thick: 

poorly exposelto 5 


part 1] GAULT AND LOWER GREENSAND NEAR LEIGHTON. 15 


Another old pit in the same field, 100 yards farther east, was 
being re-worked in 1903, and showed a somewhat different section 
from the above in its northern part, where the Basement beds (3) 
filled a hollow in the Sands. The section is now obscured, except 
at one place, where a small re-excavation, made recently for 
Dr. Kitchin & Mr. Pringle, shows the following succession. 
I understand that the so-called ‘catillus’ ammonite (see p. 52) 
was obtained from the Gault here, proving the identity of the lower 
part of the clay with the lower part of the clay in Harris’s pit 
(fig. 3, p. 7), and therefore including this section in the area sup- 
posed to be inverted. 


Fig. 6.—Re-excavated section in the western bank of the old pit, 
100 yards north of Sandpit Cottages, September 6th, 1920. 
Surface, about 350 feet O.D. 


Se N. 
thi Scal 
t 0 o caie. 
“ale - 10 feet 


Thickness in feet. 
Clay-slope, slipped and overgrown ; no section ................0..0.... about 9 


“8 Palish blue-grey Gault, perhaps slipped ...............-:.::eeeeesseeseeeeee’ about 2 


4, Dark greyish-blue and mottled platy Gault, with afew small brown- 
coated phosphatic nodules : ‘ Belemnites minimus,’ ‘ Invern 9 
concentricus,’ and traces of other shells very poorly preserved : 
passing down into— 

4a. Gritty clay, with ferruginous streaks and scattered lydites, be- } iva el 
coming more plentiful downwards : ee) 
passing into— 

3b. Dirty loam and partly decomposed loamy greensand, sprinkled ) 
with bits of polished grit and lydite (up to % inch in diameter), | 
and containing worn fragments of iron-grit (up to 8 inches) and Pseen to 13 
rough gritty phosphatic nodules, externally grey, internally black | 
(up to 6 inches in diameter, but mostly smaller). 


Base obscure, but Coarse Sand (1) occurs lower in the bank. 


The obscured south-eastern part of the above-described pit is 
practically conterminous with the western end of the workings in 
the big Nine Acre pit (see plan, fig. 2, p.5). The west-and-east face 
of the latter pit is rather more than 300 yards long; but the present 
workings are mainly at the eastern end, where the Sands rise nearly 
to the surface. A good section is still, however, exposed in one 
place near the western end (partly shown in fig. 7, p. 16) within 


tole 


16 MR. G. W. LAMPLUGH ON THE JUNCTION OF | vol. Ixxviul, 


50 yards of that last described. In the interval of 150 yards be- 
tween the section of fig. 7 and the beginning of the main easterly 
working, illustrated in fig. 8, the continuity and variability of 
the Basement beds are shown by small exposures which reveal 
features similar to those illustrated by the two sections here given. 

When I first examined the Nine Acre pit, in a more southerly 
working-face, the cover of Gault had not set in; but it began to 
be visible, along with the iron-grit breccia, at a few spots in 1906. 
The sections now described are better than any previously seen. 
The most significant features are the very uneven surface of the 
lower Sands with evidence of sharp erosion around the bosses of 
iron-grit, and the absence of the Silty beds (2 of figs. 3, 11, 14) 
beneath the Basement beds of the Gault. 


Fig. 7.—Section of the western end of Nine Acre pit, Shenley, 
September Sth, 1920. Surface, about 350 feet O.D. 
[Same explanation as fig. 8. ] 
W. E, 


Fence Top clay cut back and obscure: 
but clear section below working platform. 


ye 5 yards omitted 


Vertical -and_ horizontal, 


Fig. 8.—Section at Nine Acre pit, Shenley, 150 yards east of 
the last (fig. 7): showing the western part of the main 
working face, September 10th, 1920. 
Surface, about 350 feet O.D. 
WY. 
20 yards 
omitted 


0 5 10 feet 
Vertical: Horizontal slightly reduced 


part 1] GAULT AND LOWER GREENSAND NEAR LEIGHTON. 17 


Thickness in feet. 


Z. Clayey soil, with a few flints, etc., passing down into— 1 to 2 
ZY, Disturbed and weathered palish blue Gault, probably mostly 
4, ‘creep, with an occasional flint in the upper part: some ‘race’ : 2 to 4 
penetrated by tree-roots. 


4, Dark greyish-blue Gault, weathered, but showing bedding in the | 
deeper sections, with a weathered ferr uginous “brownish band, 
about 1 foot thick, in the upper part: “Belemnites minimus 
and obscure traces of shells: some brown-coated phosphatic’ 
nodules : ‘race’ in places: passing down into— 


| 
oe) Ol tos 
| 
J 

4a (in hollows only). Dark gritty clay, with polished lydites, etc. : 


(up to + inch in diameter), becoming plentiful dow nwards : 0 to2 


passing into— 


3b (in hollows only). Gritty greensand-loam, with mottled sandy | 
patches: contains a few rough gritty phosphatic nodules, ex- | 
ternally grey, and occasional worn slabs of iron-grit: patches of - 0 to 2 
calcareous induration: becomes mixed with ironstone-breccia | 


downwards. | 
. 


8. Ivonstone-breccia and undulating tabular iron-pan, the ‘ pan,’ an 
inch or two thick, more or Jess continuous over the rises between | 
the hollows, but broken up into short lenticles and subangular | 
partly-worn fragments in the hollows, and set in a matrix of > jto# 
ochreous loam with small patches of soft decomposed phos- | 
phatic limestone and with (in fig. 7 only) occasional rough | 
eritty phosphatic nodules. J 


partly calcareous, partly phosphatic; resting sharply with 


3a (in fig. 7). Dirty loamy greensand with concretionary induration, 
about 1 
an uneven base on— 


1. Sands, white, or only slightly lemon-stained, under the hollow in 
fig. 7, but elsewhere irregularly indurated at the top for a foot or 
two into bosses or tabular masses of hard liver-coloured iron-grit 
(1 x), sometimes a quartzite; the sands around and below these 
masses are usually stained brown, orange, or buff. 


The Gault in these sections is not thick enough to show any 
portion in an unweathered condition ; segregations of ‘race’ are 
present in it in places throughout, and tree-roots along the line of 
the field-fence penetrate it, and sometimes reach into the underlying 
breccia-series. The upper 8 or 4 feet is massive and structure- 
less, with an occasional flint or other stone in it, but does not 
present the aspect of a true boulder-clay, and is probably for the 
greater part ‘creep’-material from the adjacent slopes. Where the 
Gault is shallow, it is affected by ‘creep’ to its base; but in 
the deeper sections stratification makes its appearance in the lower 
part, and the lowest layers are charged with smooth bright grains 
of grit and lydite, this gritty clay (4a) bemg comparatively thick 
within the little basins, but thinning away on their slopes. The 
underlying ferruginous breccia and its associated eritty greensand- 
loams, variable in detail but well-characterized as a whole, have 
partly filled the hollows of a very uneven sea-floor on which the 
irregular ferricrete masses at the top of the Sands have formed up- 
standing crags like that seen in the old Garside’s pit (fig. 4, p. 11). 
The scouring away of the unconsolidated material from among the 
indurated tables has in some places left undercut gutters a foot 
or two deep between the crags, like those commonly seen on any 


Q. J. G.8. No. 309. C 


18 MR. G. W. LAMPLUGH ON THE JUNCTION OF  [Vvol. lxxviul, 


recent rocky shore or tidal reef in strata of unequal induration.} 
These features are illustrated in the following enlargement (fig. 9) 
of part of the preceding section. 


Fig. 9.—Knlarged section of the guttered tron-grit crags shown 
in the western part of fig. 8, p. 16. |Same explanation as 
for that figure. | 


Scale: Vertical and Horizontal 
Ot Bese ZY Giiecs 
= Le 


The soft calcareous patches, partly phosphatized, occurring at 
intervals among the ironstone-breccia in the Nine Acre sections 
are at the same horizon as the limestone-lenticles under Shenley 
Hill, and, though more decomposed, are like the lumps which 
occurred on the northern and southern skirts of the limestone- 
masses in the former workings of Chance’s and Garside’s pits. 
Traces of fossils were noticed in them in two or three cases, but in 
too poor a state for recognition (except in one instance as casts of 
brachiopods). If the pit should be pushed northwards under a 
thicker cover of Gault, it is probable that the section will yield 
better palzeontological material at this horizon. 

On the east, the Nine Acre workings reach the roadside near 
Miletree Farm, beyond which, on the other side of the road, another 
pit (‘ Miletree pit’) affords practically an eastern continuation of 
the section, while a separate excavation (‘Miletree Farm pit’) is 
worked northwards at right angles to this. I first saw and noted 
these sections in 1909, and have a few particulars gleaned on later 
visits. Work is still in progress in both pits, but large portions of 
the former exposures are now obliterated. The most striking feature 
is the reappearance of a thick wedge of the stratified Silty beds (2) 
above the Silver Sands in Miletree Farm pit (fig. 11, p. 20) and their 
tailing out southwards at the entrance to the pit. This feature 
has been described and figured diagrammatically by Dr. Kitchin 
& Mr. Pringle (op. cit. p. 57). The Silver Sands are uncensoli- 
dated beneath these Silty beds, but become indurated into bosses 


1 Examples might be cited of similar features at many other geological 
horizons, but the closest parallel in age and structure is presented by certain 
of the French ‘ Tourtias.’ The Paleozoic floor beneath the ‘ Tourtias’ is in 
some places guttered and scoured in almost exactly the same way as here: 
see H. Parent, ‘Sur lExistence du Gault entre les Ardennes & le Bas- 
Boulonnais’ Ann. Soc. Géol. Nord, vol. xxi (1893) figs. on pp. 207 & 212. 
See also pp. 56-57 of the present paper. 


part 1] GAULT AND LOWER GREENSAND NEAR LEIGHTON. 19 


of iron-grit in their upper part just where the Silts thin out, under 
the garden of Miletree Farm, so that the adjacent Miletree pit 
(fig. 10) has a section showing eroded crags and hollows, like 


Nine Acre pit. 


Fig. 10.— Part of section at the northern end of Miletree pit, 
Shenley ; September 13th, 1920. Surface, about 340 feet 
O.D.; with a gentle slope south-eastwards. 


W. E 


we FLL 


oe et 3 “obit 


sie ul pela ffs QE A at Lad 


on oh be 
222s Bae 7k eee eh gre yee 


ss ag Gonee ae Sanyo i ac 
ay) wwe ee = De TRAVIAN rat cag 


Gl a ob 
ste) 


he 
3a Vertical Scale Horizontal reduced 4 


Thickness in feet. 
ZY. Soil, passing down into ‘ trail’ of stony dark clay, full of flints and 
drift-stones, looped and pocketed among clay with few stones: 
passing downwards into— 


i 3 to 4 
ZY, Disturbed and rearranged pale- blue Gault, mostly’ or ees 2 to3 

| 

a 

| 


4 ‘creep’; much slickensides ; ; “race > abundant in some patches 


4, Weathered ferruginous brownish-blue Gault, with definite bed- 
ding: some ‘ race.’ 

4a. Dark greyish-blue Gault, with rusty planes, thickly sprinkled 
throughout with grit and lydites (up to $ inch in diameter), but 
most abundantly towards the base: also a few brown-coated 
nodules (up to 2 inches in diameter), internally black and some- 
times including grit grains: no fossils seen. 

3. Ivon-grit breccia, pockety, and uneven at the top and bottom, 
with a plentiful admixture of grit (up to 1 inch in diameter), A 
polished fragments of ironstone, and a few pale-grey gritty 2 
phosphatic nodules; not much tabular ‘iron-pan.’ 

3a. Very gritty dirty sand and loam, with grit-pebbles up to 14 inches 
in diameter and a few pale evitty phosphatic nodules: in up to 2 
hollows and pockety; uneven base. 

1. Coarse cross-bedded Sands indurated at the top on both sides of a 
hollow into irregular bosses of iron-frit (1x), quartzitic in places, 
up to 2 or 3 feet thick: the Sands mostly white under the 
hollow, but becoming stained lemon-, orange-, and coffee-coloured 
in the neighbourhood of the iron- orit sear toes +e . seen 12 to 15 


The Sands rise to the surface on both sides ber onal mie SeeHOn, 
within 20 or 30 yards. 


1 to 15 


\es 


Other sections similar in the main to the above are (September; 
1920) less clearly exposed on the south side of the pit; but at the 
entrance into it from the road, the ferruginous sands and iron-grit 
vise practically to the top (best seen in 1909), a continuance of 
the conditions visible on the opposite side of the road in Nine Acre 
pit. The Sands are also close to the top along the eastern side of 
the pit, partly owing to the downslope of the ground. 

c2 


20 MR. G. W. LAMPLUGH ON THE JUNCTION OF  [vol. lxxviul, 


Fig. 11.—Northern part of the section on the west side of Mile- 
tree Farm Pit, Shenley ; September 10th & 13th, 1920 
supplemented by previous observations, April 1914. 

Surface at the northern end, about 350 feet O.D., 


falling slightly southwards. 
S.S.W. N.N.E. 


ANE 
ax 


Wai my Guatinsin ze WM MK Casio: We empire NG 
we iN ey Ga Rea won 


ZN Pes Lead XC 
iS SSS = 


oO 5 10 feet e 
2 Sea 


oe ; 
Vertical Scale Horizontal reduced 


Thickness in feet. 
Z. Soil and wash; at the northern end, , clay ey with a few stones, } 


passing down imperceptibly into ae at the southern end, more . 1t03 


sandy, thicker, and containing some fragments from the iron- | 
stone-breccia. 


ZY, Disturbed and rearranged clay with an occasional flint or drift- 
4 pebble, mostly ‘ trail’ or Gault-creep ; massive and slickened ; in 
places, passing down into 4: in others, separated from 4 by a 

sharp slicken-plane. 

4, Dark greyish-blue Gault, disturbed at the top; darker, and with ) 
definite bedding below : weathered and full of ‘race’ in places: | 
‘Belemnites minimus,’ no other identifiable fossils seen. er Otod 

4a. Clay with small erit-grains, and a crimson streak in places; 2 to | 
3 inches thick. “y) 

(The Gault thickened eastwards, in a portion of the pit now 
obscured.) 

3. Ivon-grit breccia, ochreous, lumpy and irregular, streaked and ) 
mixed with gritty greensand-loam; soft pale calcareous con- 
cretionary patches with occasional traces of fossils; undu- 
lating and crinkled tabular ‘iron-pan’ at the top in places, an 
inch or so thick, sometimes also running among or under } 
breccia, with smooth-worn surface under the Gault: sporadic | 
pale gritty phosphatic nodules among the greensand-loam. 

* Janira Geta aueaes a was the only identifiable fossil obtained 
(in greensand). 5) 
3a. Mottled loamy calcareous greensand-grit with small pebbles ist 
0 to 


0to3 


Pe) 
co 
fo} 
to 


to 1 inch in diameter) aint occasional gritty phosphatic nodules, 3 
in hollows of the uneven eroded floor of (2). 

2. The Silty beds: well stratified silts and loamy sands: maximum 
thickness now visible as below; but all have disappeared 45 yards 


farther southwards, and the section is then as in Nine Acre pit. 


1 In 1914 I found a 6-inch boulder of pink granite among the stones from 
the top of the section, a point worth mentioning on account of the rarity of 
igneous rocks in the drift of the district. 


part 1] GAULT AND LOWER GREENSAND NEAR LEIGHTON. 21 


The Silty beds :— Thickness in feet. 
2c. Banded silty greyish-white sand, darker grey carbonaceous loam) 
and streaks of silty clay: the whole weathering buff and | 
loamy at the outcrop: some thin tabular claystone con- i | 
cretions with ferruginous crust, up to 1 foot in length; alsoa 
tew smaller sandy pyritous nodules, on definite bedding-planes. J 
2b. Streaky grey and buff loam and fine ash-coloured sand, with ) 
impersistent green streaks and streaks of coarse washed sand | 
up toan inch or two thick; some imperfect tabular ferruginous if r 12 
concretions along layers, and a few fine-sandy pyritous nodules, 
decomposing : clay ey streaks in the lower part. J 
2a. Ashy-grey carbonaceous silts, with clay-streaks and streaky fer- 
ruginous induration forming imperfect cakes of sandy iron- 
stone; with a 2- to 3-inch band of interlaminated wet carbon- -4 
aceous greyish-brown silt and greasy-feeling grey clay at the | 
base ; resting sharply and unconformably on— 


Silver Sands, cross-bedded and coarse in the upper part, finer below: seen a 18. 


wn 


Dr. Kitchin & Mr. Pringle mention the occurrence of ‘a few 
dark phosphatic nodules’ about halfway down in the stratified 
Silty beds (2b of the above section). and on the strength of this 
they regard part of the beds as ‘of tardefurcata-age’ (op. cit. 

yp. 57-58). Imade close search, both in this and in another section 
(Double Arches pit, p. 24) in which the series is well- developed, 
but failed to find in the Silty beds any phosphatic nodule of the 
kind described. The gritty phosphatic nodules in the Muiletree 
Farm pit are confined, so far as I have seen, to the gritty green- 
sand-loam and muddy grit associated with the ironstone-breccia 
(8 & 3a). The point is of consequence in the general interpre- 
tation of the sections (see p. 55). The Silts contain specks of 
vegetable matter in plenty, but have as yet yielded to me no 
identifiable fossil. 


The sections of the pits above described, from Harris’s pit on 
the west to Miletree pit on the east, are combined in the reduced 
section, fig. 13, p. 22, which is at right angles to the other reduced 
section, fig. 12, on the same page. It misses the Silty beds on the 
east side, and runs mainly along the belt of irregular iron-grit 
crags, except at the western end, where the craggy belt swings 
50 to 100 yards farther south. 


Sections north-east and north-west of Shenley Hill.— 
About half a mile north-east of the Shenley Hill pits there is 
another group of pits near some farm-buildings named ‘The 
Poplars’ on the 6-inch Ordnance map, where the road from Leighton 
joins the cross-road from Heath to Watling Street (see fig. 1, p. 2), 
the intervening tract being at present unbroken. One of these is an 
extensive re-working of an old pit, now known as ‘ Double Arches,’ 
opposite the junction of the roads, and the other two are new 
excavations a little farther eastwards. ‘The more easterly of these, 
which I shall call ‘ Poplars pit,’ reveals the base of the Gault 
(fig. 14, p. 23) ; the western part of ‘ Double Arches’ shows a good 
section in the Silty beds over the Silver Sands, but has not reached 
the Gault; the middle pit, at a lower level in the shallow valley 
which runs between the pits just named, is entirely in Sands and 


d1 ‘S14 
| 9919] 


acl 


TS Ae IO SR Se A I 


MLOTPOON OTL} JO PLO UtOgHOAL OTT] OF wupnorpuoedaod 
HT Ofte oy guq *(qg d's oy ‘ud-punoasd 008) ET 8] Ul WOTZOOS Of} JO UNOS Hpauds GOT gnoqu Aypungow sory gtd s staawpy "iLO N 
[‘UMOYS JOU OAV BUOTSTATPQUS LOTTO O14} fsornsy surpooord oy} UL BV POTOGUAI OB BPC out, | 


9a} 09 0S 
’ y 0 ana BOW We JANUOZILOY 
Ndedteatnnbebtntd (an)1) [PIILIA PUL PUUOZLO}( 
SUOIDOIS WO 404 2 ieley 
spurs OOL Oe ah Oe Ge Or de ig aie al 


pS nea ev enna Nasn saan cece EE CT EERE LGD TE EL EE a 


‘CJ'E saoge yy OOL JUL |- LUNE] 


preven = oe Q Saws 
| H 1 ' h Cee ret 
| ' i ‘ \ \ 
| ' 
payiwo | payyiweoe | payiwo pawiuio payiwo 
spavA OQ] ; spied (GI spavd OQ! spavA OO [ iid | spark OZ! 
6 F'O'SSIY ‘Lol 9°34 Ul paqiuoseq 


rd odio y et N) sid plo vid PIO 


‘suorss1mo hg N)pVpwoziMoy pauaploys pg ‘QQ0G) 8) Aap DI8 Jodaual guns 
Syspa Of psa WOLf UID AAtJ2TET, PUY D1 hap uays Waanpod sped ay) YhROLY) UOL/IAS PaUrquogj—"g | ‘6 


'CY'CE 2A0g"¥ “YY OOS aul (tune | 


UIIS JOU SBUIYIOM PIC 


-------}j a 


yitfal Gor] te) su emo (Ol gs] Ab ily 8 al id pjo s,apisavey 


‘2]DIS ([VANJVU) poanpat 0 WO + YplOW OF YpnOS woif ‘spd ppepp Napuoys ay? Ybnowy? Worpoes paurquoog—BL “Oli 


part 1] GAULT AND LOWER GREENSAND NEAR LEIGHTON. 23 


valley-wash, and will not here receive further notice. Poplars pit 
supplies the easternmost, and Double Arches pit the northernmost, 
of the sections dealt with in this paper. 


Fig. 14.—Section at the northern end of Poplars pit, 250 yards 
east-north-east of The Poplars and 1000 yards north-east 
of Miletree Farm; September 27th, 1920. 
Surface, on west side, 352 feet O.D., with a gentle slope 
south-south-eastwards. 


3 9 =| Aofeet 
SS eee 


Vertical Scale} Horizontal reduced 4 


Thickness in feet. 

ZY. Brown clayey soil, containing a few flints and drift-stones, 
passing down into structureless clay, probably ‘creep,’ with mt about 3 
occasional stone: passing down into— 

ZY, Disturbed and rearranged pale grey-blue Gault, mostly or wholly 

4, ‘creep, blocky and jointy, with crumbly structure and no clear 
bedding: some strong fresh slicken-planes: more or less ‘ race’ 
throughout, plentiful towards the base. 

This material cuts out the bedded Gault (4) at the west side 
of the section. 

4. Darker greyish-blue Gault, with platy structure and bedding, ) 
somewhat disturbed : the uppermost 6 inches paler, with brown | 
rusty weathering and much ‘race’; lower part, green-mottled in | 
places : ‘ Belemnites minimus’ and obscure traces of other fossils. | Ororl 

4a. Basal layer of gritty Gault, 2 to 3 inches thick at the eastern end, ° e 
but thins on the rising slope westwards: sprinkled with small 
pebbles, up to half an inch in diameter; red streak at the base 
where resting on iron-pan. 

3. Iron-grit breccia, ochreous; with tabular liver-coloured iron-pan )} 
1 to 13 inches thick, uneven and wrinkled, fairly continuous at 
the top and in irregular lenticles below ; associated with worn 
lumps and slabs of ironstone, lenticles of pebbly glauconitic 
loam, pale decomposed calcrete-patches with obscure traces of > #2 to 2 
fossils, etc., as in Nine Acre and Miletree pits. Upper surface 
of iron-pan worn and abraded, with patches of pebbly glauconitic 
grit in the hollows; and similar material, more coarsely pebbly, 
(3a) at the base of breccia in eroded hollows of underlying beds. } 

A smooth polished cuboidal block of chert, 3 x 3 x2 inches, 
embedded in the glauconitic grit, was found at +, but no other 
pebble more than an inch in diameter was seen. 

2. Evenly-bedded streaky and mottled ashy-grey silts and silty 
loams, with ferruginous streaks; towards the western end, a eon 
1- to 2-inch streak of rather dark glauconitic sand: uneven sur- ae 
faces of erosion both at the top and the bottom: resting sharply 
on— 

1. Silver sand, mostly white and of medium grain; with some thin 
tabular lenticles of imperfectly-developed concretionary iron- 
stone, passing in places into hard liver-coloured stone: worked 10 

A sump sunk to 10 feet below the floor of the pit shows similar, 
but rather finer, sand.with a few streaks, up to half an inch 
thick, of fine greasy-textured grey clay, which are wrinkled and 
twisted like the iron-pan in (3). 


15 to 2 


tol 


24 MR. G. W. LAMPLUGH ON THE JUNCTION OF  [Vvol. ]xxviu, 


| Postscripr.—Later excavations revealed, during the summer 
of 1921, a tapering lenticle of fossiliferous limestone of the Shenley 
type, enwrapped above and below by hard iron-pan, in the upper 
part of the iron-grit breccia (3) at the southern corner of the 
western face of the pit. The lenticle, at its maximum, was 34 feet 
long and 7 inches thick, of which the inner 4 inches was pinkish 
and iron-stained gritty limestone. The limestone was rendered 
soft and crumbly by weathering, like most of the smaller patches 
at Shenley, and the fossils were consequently in a poor state; I 
obtained the casts of several small Terebratule and Rhynchonellids, 
a small Avicula and portion of an Echinoid-spine—all forms 
common at Shenley. | 


Section at the north-western corner of Deuble Arches pit, 


500 yards north-west of The Poplars; September 27th, 1920. 

Surface, about 360 feet O.D., with a strong slope east- 

wards at right angles to the section. 

Thickness in feet. 
Z. Soil, passing down into structureless sandy loam (downcreep ace 3 
wash) with an occasional flint or other stone in the upper part. 
2. The Silty beds :-- 

ce. Interstratified dull dark-green and brownish loams, evenly ) »y 
bedded as a whole, but the individual layers wispy, wavy, and | 
inconstant: streaked with imperfect ferruginous concretions . Sap 
in thin tables along clayey bedding-planes, running in places { © 
into tabular clay-ironstone lenticles, best developed along the | 
base and in the upper part of (b). yp : 

b. Fine-grained very dark-green streaky loamy sand, traversed | 
by faint tubular markings; a thin sprinkling of coarse | Ls 
polished grit in the lower part, with rare particles up to # inch \ Hoses é 
in diameter ; a few wide-spaced hollow ironstone nodules more | ar 
or less elobular, in the lower part ; and an impersistent ae i 
of tabular flaky ironstone at the base. 

a. Buff-coloured and rusty ferruginous loams; streaked Bi 
brown and grey unctuous silt and clay, which predominate ona 
towards the base, and throw out water: base uneven, with mae 


small and large undulations. 3 
i. Rather coarse Silver Sands, with rusty buff and coffee-coloured 
streaking and staining at the top and towards the base.. on about 20 


Finer-grained sands of lower grade proved below. 


The full thickness of the Silty beds (2) has not yet been reached 
in the Double Arches pit, and may prove to be as great as at 
Miletree Farm (fig. 11, p. 20). Their quick reduction fo less than 
2 feet in the neighbouring Poplars Pit shows that here again, as in 
the Shenley group of sections, they have a wedge-like outline, which 
suggests that they fill hollows, partly original and partly of erosion, 
among the cross-bedded sand-banks of the coarse clean Silver 
Sands. They have, however, also been pared down, along with the 
coarse sands, at the close of Lower Cretaceous times. I saw no 
fossils, except small carbonaceous fragments, in these beds, and 
no phosphatic nodules. The section in the Silty beds is continued 
southwards, showing minor variation only, fur over 100 yards; also 
at right angles, eastwards from the northern end, for 20 or 30 
vards, until cut out by the slope of the little valley; but in other 
parts of the pit, the Silver Sands reach to the top. 


~ 


part 1] GAULT AND LOWER GREENSAND NEAR LEIGHTON. 2e 


Poplars pit provides a useful indication of the persistence of the 
peculiar Shenley conditions at the base of the Gault in a north- 
easterly direction, and Claridge’s pit, next to be described, shows 
that they extended northwards also. The Gault at present exposed 
in Poplars pit is too shallow and weathered to afford paleonto- 
logical data ; but its lithological characters agree with those of the 
beds in the same position at Nine Acre (figs. 7 & 8, p. 16) and Mile- 
tree (figs. 10 & 11, pp. 19 & 20). The iron-grit breccia, with its 
associated gritty glauconitic stuff, is also similar in all its essentials, 
and the patch of fossiliferous limestone links the breccia beyond 
question with that of Shenley Hill. The isolated chunk of chert, 
which I dug out of the gritty loam infilling a hollow immediately 
below the breccia, is rudely cuboidal, with well-rounded angles, and 
possesses all over a peculiarly smooth glossy greenish surface, with 
perfectly fresh, dove-grey, translucent rock under it. The only 
other pit in which I have found extraneous stones so large as this 
is at Chamberlain Barn, below the ‘Mammillatus beds’ (p. 31). 
Small bits of similar chert occur among the ‘lydites’ of the 
Sands; and the rock is almost certainly the same as that found 
rather plentifully in the gravelly Lower Greensand at Potton and 
other places.! 


West of Double Arches pit, the ground rises steadily for 
about 100 feet to a broad ridge which is due to a thick sheet of 
Glacial drift, and contracts southwards, terminating as a spur in 
Shenley Hill. The drift-sheet has been deeply trenched on both 
sides by Late-Glacial erosion, and the ground falls westwards from 
the ridge towards the valley of the Ouzel. On the west side most 
of the pits have been opened on the lower slopes, where the Sands 
are at the surface ; but in two or three cases they have been worked 
back into the covering deposits, and these only will be described. 
The northernmost is Claridge’s pit, a big working a quarter of a 
mile east of the high road between the villages of Heath and 


Reach, and nearly a mile due west of Double Arches. 


Claridge’s pit (fig. 15, p. 26) has broken into a sharp spur of 
upland drift, and shows a thick and varied capping of Glacial 
deposits which come down onto the Silver Sands in the northern ~ 
part of the section, but admit the feather-edge of the Gault, along 
with its basement-beds, in the southern part, which alone is figured. 
This is at present the best section exposed in the plateau-dritts of 
the district; but there is an old much-overgrown pit 500 yards 
farther north (just beyond the edge of the sketch-map, fig. 1, p. 2) 
which appears to have revealed a similar thickness and sequence of 
boulder-clays, with the addition of coarse morainic gravel at the 
base. In the pits on lower ground, the drift, having been eroded, 
is usually scanty or absent, or is of the redeposited gravelly type 


1 W. Keeping, ‘On the Included Pebbles of the Upper Neocomian Sands’ 
Geol. Mag. 1880, pp. 414-22; in which it is suggested that chert of this kind 
has been derived from Carboniferous strata. 


26 MR. G. W. LAMPLUGH ON THE JUNCTION OF  [Vvol. Ixxviui, 


and more or less influenced by ‘creep’ and ‘trail’ movements. 
The Glacial deposits present many features of interest, which, 
however, will be mentioned without discussion, as being beside our 
present purpose. 


Fig. 15.—WSection at the southern end of the excavated platform 
above Claridge’s pit; September 27th, 1920. Surface, at 
the northern end, about 410 feet O.D., rising a few feet 
higher north ne and failing sharply an ee ds. 


, ; , 5 Be < a 
ge a See saraera ENDS yeeaseaw Se 


Vertical Scale. Horizontal slightly reduced 


Thickness in feet. 
Z. Clayey soil and subsoil, with flints, etc.: hill-wash................ 4 to 2 


( c. Pale greyish Chalky Boulder Clay: massive ana blue- 
jointed : boulders, mainly chalk and large flints, with many 0 to 10 
Jurassic rocks, including fossiliferous limestones, sandy : 
limestones, claystones, etc., often well-glaciated. 


| b. Chalky eravel and sand streaked with loam: fairly well- 
yd bedded, with some cross-bedding: pebbles, mainly came 
and mostly small, averaging about 1 inch in diameter 
| a. Dark sandy creenish Boulder Clay, massive and jointed: ) 
with fewer and smaller stones than in Ye, mostly flint | 
| chips and glaciated bits of chalk; occasionally a boulder; + 0 to 4 
the matrix. appears to include much loamy Lower Green- | 


sand material: rests with a sharp junction on Gault clay. ) 


ao erey-blue Gault clay, showing a crumbly cr ae 
structure, but some traces of bedding: some smooth brown- 
coated phosphatic nodules, black inter nally (up to 3 inches 2 
in diameter): much ‘race’ in the upper part, and cael 
throughout : 

passing down into-- 

Darker Gault clay; brownish ferruginous weathering in a 
upper part;and with a gritty admixture in the lowest 2 or 

3 inches (4a): structure more or less disintegrated, with a iB to 4 
little ‘race’ to the bottom in places; but ‘fairly sta 
bedding. 

| No fossils seen in the Gault. 


(ese Saad 


| 
: 
ie 
| 


part 1] GAULT AND LOWER GREENSAND NEAR LEIGHTON. 27 


Thickness in feet. 
3. Tron-grit breccia, ochreous and gritty; with irregular wrinkled ) 
cakes of iron- -pan, abraded at the top; worn ironstone slabs | 
and fragments, mingled with mottled gritty glauconitic loam; - #2 to 2 
and soft pale calcareous patches with small pebbles: cuts | 
down in irregular hollows and pockets into the underlying beds. ) 


2. The Silty beds: ashy-grey silt and loam with clay-streaks, lto3 
and tabular ferruginous concretions towards the base. 


1. Silver Sands, cross-bedded: about 30 feet seen. 


Leaving the Glacial Drift out of account, the sequence above the 
Silver Sands in this section is the same as that of Poplars pit, and 
has the same correspondence with the Miletree and Nine Acre 
sections. The crushed aspect of the Gault may be due to Glacial 
agency, but is at least as likely to be a later effect of hill-drag 
upon the heavily-loaded soft clays when their margin was exposed 
through the post- Glacial erosion of the adjacent valley. The 
presence of ‘race,’ indicative of deep weathering, is, as usual, 
accompanied by an absence of fossils in the Gault clay. 


Heath House pit.—The next section southward requires full 
notice, because it figures prominently in the recent argument 
of Dr. Kitchin & Mr. Pringle, who refer to it as ‘No. 9 Pit.’ 
It is an obscure exposure in an old pit, formerly known as 
Heath House or Bushell’s pit, abandoned nearly 30 years ago, in 
the small plantation 250 yards east of Heath House, and half 
a mile due south of Claridge’s pit. At this place the Sands 
have been worked westwards beneath rising ground with a thick 
cover of Gault. Most of the section is hidden, and the pit partly 
filled, by clay-slips which now form grassy slopes; but the base of 
the Gault is still accessible in two or three spots on the south and 
east sides of the excavation, and the slipped ground itself affords 
some information. 

This is evidently the section ‘on the southern slope of . . . Heath 
Hill’ examined by Jukes-Browne in or about the year 1884, whose 
account of what he saw in the pit and in an adjacent brickyard is 
as follows! :— 


.. ‘the base of the Gault is shown in a sand-pit, where 14 feet of dark-grey 
clay with small patches of bright-red clay at the base rest directly on yellow 
sand with a well-marked plane of division. This occurrence of red clay is 
significant in connection with the age and-origin of the red marl and red chalk 
of Norfolk. Close by this pit is a brickyard which shows about 10 feet of 
bluish-grey clay with a seam of phosphatic nodules in the middle. This 
nodule bed appears to be a continuation of ... [one seen at Buckland near 
Aylesbury |... for it contains a similar mixture of Lower and Upper Gault 
species. The following is a list of the species found by myself’ :—[list of 24 
species, here condensed :—Ammonites :—beudanti, cristatus, interruptus, 
lautus, ochetonotus, rostratus, splendens, varicosus; Hamites intermedius ; 
Belemnites minimus & var. attenuatus; Dentalium decussatum.... Ostrea 
vesicularis ....Inoceramus concentricus & suleatus .... Nucula pectinata ; 
Plicatula pectinoides, etc. ].’ 


In referring to this account in our previous paper, I erroneously 
supposed that the brickyard was an old one still visible on the 


1 <The Gault & Upper Greensand’ Mem. Geol. Surry. 1900, p. 285. 


28 MR. G. W, LAMPLUGH ON THE JUNCTION OF  [vol. lxxviu, 


roadside between Leighton and Shenley Hill (see fig. 1, p. 2), as I 
could find no other in the neighbourhood ‘north of Leighton,’ and 
no other was marked on my 6-inch Ordnance map. But recently, 
on looking up an earlier edition of the same map (Sheet, Beds. 
28 N.E.), I notice that a ‘claypit and brickyard,’ of which there 
is now little or no trace, is shown just outside the fence enclosing 
the Heath House sandpit on the south, extending westwards from 
it. Jukes-Browne’s description fits these circumstances exactly, 
and what I wrongly presumed to be a rather vague localization 
is thus made quite clear. 

Some years later (1897) the pit was visited by the Geologists’ 
Association, when already in disuse, but with better exposures than 
now, and a short account of it was given by A. C. G. Cameron in 
his report on the excursion.! 

This was supplemented by some valuable notes by Dr. A. M. 
Davies in his paper on ‘The Base of the Gault in Eastern England,’? 
as follows :— 


‘As I understand that the pit will soon be closed over, it may be well to 
give a full list of fossils from it. These were in part collected by my wife on 
the visit of the Geologists’ Association, partly by myself on a later occasion. 
All except the belemnites are phosphatized. [List] Hoplites interrwptus,® 
H. tuberculatus, H. lautus(?) (a worn fragment), Schlenbachia “inflata, 
Belemnites minimus (abundant), Solarium, Dentaliwm, Inoceramas concen- 
tricus, I. sulcatus; fish-scales ; fish-coprolites.’ 

‘The mixture of Upper and Lower Gault fossils is striking, but has been 
already noted for this district by Mr. Jukes-Browne [ref. ‘‘ Handbook of 
Historical Geology, p. 412] .... I should, however, add that some thickness 
of Gault is exposed at Heath (at least 15 feet, speaking from memory), and 
that owing to the large extent of slip no fossils could be obtained actually in 
place, so that some may have come from a higher zone than others.’ 


In their account of the section, Dr. Kitchin & Mr. Pringle (op. 
cit. p. 60) regard the whole of the clay as Upper Gault, and claim 
that the sequence here proves the inversion of the Gault of Harris’s 
pit. But a comparison of the two sections, in the light of the 
information now brought together, will show that they agree in 
all essential particulars, except in the absence here of the Silty 
beds (2). Some of the nodules which I found on the upper slopes 
of the Heath House pit are ‘compound’ phosphates similar to the 
nodules in 5a at Shenley and including the same fossils, and are 
probably from the same nodule-bed as that of the adjacent brick- 
yard described by Jukes-Browne, as quoted above. 


‘ Proc. Geol. Assoc. vol. xv (1897) p. 184. 

2 Geol. Mag. 1899, pp. 160-61. 

3 Dr. A. M. Davies informs me that these determinations of the ammonites 
stand in need of correction. The naming of H. tuberculatus has been con- 
firmed, but the supposed H. interruptus is probably an Anahoplites of the 
‘splendens’ group, and the lawtus ? is an indeterminable specimen. 

I am indebted to Dr. Davies for the loan of the specimens; several are in 
the state of ‘compound’ phosphatic nodules, exactly like those of the nodule- 
bed near the top of Harris’s pit, Shenley Hill (see p. 9). 


part 1] GAULT AND LOWER GREENSAND NEAR LEIGHTON. 29 


The following observations, compiled from a sketch-section in 
my note-book, represent the present condition of the pit. 


Western side of Heath House pit; October 12th, 1920. 
Surface, at the highest part, 400 feet O.D. 


In top breakaway :— 


Pale-grey weathered Gault clay: little or no drift ............... 2 to 4 feet seen. 
On slopes: 


5. Slips of pale-grey Gault: thickness, displaced, or not seen ...... about 8 feet. 
Pale hard nodules weathered out, and hard fragments of 
keeled ammonites : ‘ Ammonites rostratus,’ ‘ Inoceramus 
sulcatus, ‘Dentalium,’ etc.: also on the lower slope, 
‘Ammonites cf. splendens’: probably mostly from the upper 
part of the section. 


Deciuon at the morthern end of the pit in a trench cut for 
the Geological Survey; March 30th, 1920. 


All the beds may be slightly slipped here. 


Clay, with ‘ Inoceramus sulcatus’ (absent in beds below). 
4, Darkish blue Gault, with ‘Inoceramus concentricus’ and allies: 
crushed imperfect ammonites: ‘Belemnites minimus’: ...... seen 4 to 5 feet. 


Exposed in a steep bank above a low crag of ironstone and 
ferruginous Sand at the southern end of the pit. 


4. Dark greyish weathered Gault, with a few small smooth brown- 
coated nodules: traces of pyrites: some ‘race’: ol be 
slightly displaced by slip... wes. Se aba . 2 to 3 feet seen. 


3. Iron-grit breccia, poorly exposed ; aehneous ath dabular pan 
1 to 2 inches thick and worn iron- erit fragments ya. about bs feet. 


1. Ferruginous coarse cross-bedded Sand; indurated at the top 
into massive iron-grit, forming a flat-topped crag ............... seen 5to 6 feet. 


Chamberlain Barn pit.—Rather over half a mile south of 
the last-mentioned pit, an instructive section has been lately 
exposed in the eastward extension of Chamberlain Barn pit, a big 
working on the northern outskirts of Leighton Buzzard, which has 
been gradually pushed back towards the western foot of Shenley 
Hill, and is now in places within 1000 yards of the nearest 
Shenley pit (see fig. 1, p. 2). All the western part of the pit was 
entirely in sand, but indications that the base of the Gault was 
not far distant on the east were observed in 1912, and the junction 
is now visible in the working-face for over 200 yards, while a still 
better section (fig. 16, p. 830) has been exposed in a cutting for a 
light railway leading into the pit from the eastern side. The 
section is of particular consequence, in proving the relation of 
the iron-grit breccia and the associated fossiliferous limestone 
of the Shenley sections with the fossiliferous beds of the zone 
of Ammonites mammillatus, previously recognized in certain pits 
south of Leighton. 


30 MR. G. W. LAMPLUGH ON THE JUNCTION OF  [vol. Ixxviu, 


Fig. 16.—Section on the north-west side of the tramway-cutting 
into Chamberlain Barn pit, Leighton Buzzard; October 
14th, 1920. Surface, about 315 feet O.D. 
W.N.W. E.S.E. 


ours = \ Fees a) 
Bye Za f Wy NN Be CLO J \\ Va Ne ial at hy ZN. ae Lg iA AY ie AMATO 


(AANA 


= SS 


5 10 feet 


Scale: Vertical 
Horizontal reduced 4 


Thickness in feet. 
Z. Soil, rather sandy, with a few flints, drift - pebbles, and 
weathered gritty nodules............ . about 


a. Weathered brow nish clay, Ww ine grit and ‘erackeedl phosphatic 
nodules: some ‘race,’ but not abundant: no bedding in the 
upper part; streaks of blue clay and gritty clay towards 
the base, with nodules, less gritty than those below. 

b. (v) Gritty brown unstratified loam, with polished grit and) 
lydites (up to 1 inch in diameter), some lumps of conglo- 
merate, and gritty grey-coated fossiliferous phosphatic 
nodules, black internally. ..... 6 Inches 

(iv) Bluish gritty clay, with similar nodules ...... 4 inches ! 

cin) Gritty loam, aSIADOVE 22 00). Gewese nas. eo aE MeS r 
(ii) Bluish gritty clay, as above .... 22. | 2anehes 

(i) Gritty loam, as above, with lumps of conglomerate and 

gritty phosphatic nodules, the latter less plentiful than 

m, the top leami(V)” ....4<:.20: 0 diviis | eee inehess) 


3. Rubbly iron-grit breccia; w fh GronStOn in worn slabs, ) 
fragments, ‘and flakes up to 15 inches long; some worn ! 
hematitic ‘ boxstones,’ hollow inside or containing a little | 
coarse sand; worn lumps of pebble-conglomerate; a few 
gritty phosphatic nodules; and rare quartzite and other r 
pebbles (up to 4 inches in diameter.) | 

Base uneven and pockety, with some patches of gritty loam 
(3a) in hollows below the breccia. 

1. Cross-bedded Sands; with reefs of cross-bedded iron-grit and) 
crimson-coated hematitic boxstones, some lined internally 
with quartz-crystals. At the top, immediately under the | 
breccia, a band (1b) of concretions of irregular shapes, + 8+ 
sandy externally, but often having hard horny pale-grey or 
mottled pinkish phosphatic cores, as at Grovebury (p. 33); | 
no fossils found in these, except sponge-like markings. 


1 to 2 


3 to 4 


tole 
us 
jo) 

Halo 


I 


The fossils from the phosphatic nodules in 3b include ‘ Ammon- 
ites mammillatus,’ ‘regularis’ (the most abundant), ‘ tardefur- 
catus,’ the small form called ‘beudanti,’ and others; many 
brachiopods, lamellibranchs and gasteropods; a few echinoderms ; 
some lobster-like crustaceans, etc. (p. 50). 

These well-characterized fossiliferous nodules oceur through all 
the beds exposed above the iren-grit breccia, as well as, more rarely ; 
in the breccia itself. They are widely -spaced and never clustered, 
but display a shght tendency towards a linear arrangement; and 


part 1] GAULT AND LOWER GREENSAND NEAR LEIGHTON. dl 


their grittiness appears to vary with the grittiness of their 
enveloping matrix, those in the more clayey upper layers being of 
smoother texture than those below. ‘he fossils are mostly in 
the form of casts, and are only present in recognizable shape in a 
small percentage of the nodules, though few are without some 
indication, more or less obscure, of former organic structure. It 
is possible that prolonged investigation might reveal some difference 
in the contained fossils from different levels, but only a few nodules 
are available at a time; most of my specimens of ‘ Ammonites 
beudanti’ were found in the upper part of 3b, and of ‘ Ammonites 
regularis’ in the middle part, but the numbers obtained are too 
small to warrant a positive statement. 

The breccia is more heterogeneous than the corresponding band 
in the Shenley Hill group of pits, and is less calcareous. Exclusive 
of those of iron-grit, it contains more large pebbles; among those 
which I collected are—one of veined felspathic grit (?), 84x38 x2 
inches ; two of quartzite, respectively 24 x 14 x linch and 13x14 
x 1inch; and another of hard flaggy sandstone of similar dimen- 
sions. Besides these pebbles, I noticed several worn subangular 
lumps of hard conglomerate! up to 6 or 8 inches in diameter, 
composed of smooth pea- or bean-sized pebbles, mostly quartz, set 
in a dense pale-buff or pinkish caleareous or phosphatic cement, 
evidently derived from the breaking-up of a local cake or lenticle 
of winnowed Lower Greensand pebbles which had become bound 
in a limy paste. A few smaller lumps of the same conglomerate 
were also found in the clayey grits above the breccia. Some frag- 
ments of the same rock occur in the breccia at the Miletree pit 
Gia; 10; p: 19). 

In the present (1920) working-face of the pit south of the 
tramway section, the rise of the Sands soon cuts out the overlying 
beds, leaving only a few nodules and hard fragments in the top- 
soil as a trace; but northwards the section continues, as in fig. 16, 
nearly to the end of the pit, at the fence adjacent to Leighton 
Farm. On reaching a shallow depression of the ground near the 
fence, the gritty clays are truncated by a wash of loamy drift 
with flints, etc., which has accumulated to a depth of 6 or 8 feet 
in a hollow, cut down into the Sands since Glacial times. 


The Grovebury pits.—Other pits showing the fossiliferous 
Mammillatus beds, but without the ironstone-breccia, lie along the 
southern or Grovebury outskirts of Leighton Buzzard, adjacent to 
the Dunstable Branch of the London & North-Western Railway. 
These sections are a mile.and a half south of Chamberlain Barn 
pit and 2 miles south-south-east of the Shenley Hill workings 
(see map, fig. 1, p. 2). Some small pits in the intervening tract, 
now abandoned, have shown only the Leighton Sands and gravelly 


1 At a casual glance the conglomerate looks not unlike some varieties of 
the ‘ Hertfordshire Puddingstone,’ but is differentiated from it sharply by the 
absence of flint-pebbles and by the character of the matrix. 


52 MR. G. W. LAMPLUGH ON THE JUNCTION OF  [ vol. Ixxvui, 


drift. A dismantled brickyard on the Stanbridge Road, near the 
Leighton Waterworks, was worked in the lower part of the Gault, 
and may at one time have revealed the junction, but showed no 
section when I examined it in 1912. The well-boring at the 
Waterworks, 150 yards farther east, is recorded! to have passed 
through:—Gravel, 10 feet; Blue Clay, 324 feet; Dead Sand, 94 feet; 
Blowing Red Sand & pebbles, 34 feet—continuing in the Lower 
Greensand to a depth of 144 feet. 

The range of workings at Grovebury, under different proprietors, 
extends east and west for over half a mile, and usually displays in 
one part or another magnificent exposures of the cross-bedded 
Leighton Sands. Though similar in general structure and size of 
grain, the sands of these sections differ from the Silver Sands north 
of Leighton in colour and mineralogical character, not having the 
same high silica value, and therefore not being adapted for the 
purposes requiring that quality. Many of their well-rounded 
highly-polished grains are coffee-coloured, or rusty, or nearly 
black,? and these are often arranged in streaks which bring out 
the current-bedding beautifully. They are also frequently mottled 
with tubular ‘ worm-markings,’ accentuated in the same way by 
the assortment of coloured grains; and some curious V-shaped 
structures in the mass are similarly outlined. The difference 
appears to be original, though it may possibly be due to some 
process of bleaching and ferric segregation in the Silver-Sand area, 
as suggested by Prof. P. G. H. Boswell.? It is noteworthy in this 
connexion that the development of hard iron-grit masses and iron- 
pan is rarely seen, except among the Silver Sands. 

Owing to the low level of the Grovebury pits, the saturation- 
limit or water-table of the Ouzel valley is reached in all the deeper 
excavations. This has led to the adoption of a method of quarrying 
the Sands by steam-dredgers or diggers afloat on the pool formed 
in the floor of the pit, the sands being thus extracted to 20 feet or 
more below water-level. 

It was mentioned in our paper of 1903 (op. czt. p. 239) that the 
junction of the Sands with the Gault had been nearly reached in 
the Grovebury pits; and the anticipation was realized in 1904, 
when I found that the Spinney pit (see map, fig. 1, p. 2), at that 
time the easternmost of the range—now worked out and filled with 
a deep water-pool, showed the following section :— 


1 ‘Water Supply of Bedfordshire & Northamptonshire’ Mem. Geol. Surv. 
1909, p. 54. 

2 I am indebted to Sir Jethro Teall for the following note on these coloured 
erains :—‘ The glossy brown grains ... are brown throughout, and when 
boiled in hydrochloric acid become nearly white. They are not reduced in 
size, and after treatment appear to consist of chalcedonic silica. The solution 
contains much ferric oxide’ (in litt., August 14th, 1920). 

3 *On British Resources of Sands suitable for Glass-making, &c.’ London. 
1916; p. 6G. 


part 1] GAULT AND LOWER GREENSAND NEAR LEIGHTON. 39 


Section at the south-western corner of Spinney pit, 
Grovebury; August 13th, 1904. Surface, 300 feet O.D. 


Thickness in feet. 
Z. Top soil, clayey, with flint and quartzite-pebbles, SS l 
down into— 
ZY. Disturbed and rearranged Gault clay, with an occasional 7 
4 bble. : ADS 2 
pe 
4. Stiff blue Gault, somewhat disturbed, with a few pale-coated 
phosphatic nodules, dark internally, mostly of ist about 3 
shape; passing down into— 
3b. Rusty and greenish glauconitic loam streaked with sandy and) 
clayey layers; sprinkled with polished quartz-grains, 
lydites, and hard flaky bits of ironstone, abundantly towards 
the base; scattered phosphatic nodules, rather plentiful, 
becoming a fairly continuous band in the lower part, and 
there very gritty and lydite-shot; also, near the base, an 
occasional iron-crusted boxstone with the outside polished 
and worn. One of the phosphatic nodules, close to the- 
base, held an imperfect cast of a large smooth sharp-keeled 
discoid ammonite; another, traces of a crustacean. J) 
?3a Rusty coarse sand, with large calcareous concretions, a = 
3 tol 


‘ about 3 
| 


or bits of ironstone, and traces of ferruginous boxstones ; 

1b. forms the top of the Sands, but lies parallel to the beds 
above, and across the current-bedding below. 

1. Coarse sharp Sand, rich coffee-coloured at the top, but grey 
below, with dark chocolate-coloured streaks bringing out 
magnificent cross-bedding and other structures. 


Similar sections were seen in 1906 and 1908 in the more easterly 
working of the pit; but the cover of Gault was then thinner, and 
no new features were observed. A year or two later the gritty 
phosphatic nodules began to show in fair numbers, in the detrital 
top-soil of the next pit on the west (‘ Railway’ or ‘ Firbank’s’), 
along a straight working-face of nearly 200 yards; and by 1912 
this section showed a foot or two of weathered gritty clay con- 
taining nodules of the same kind, beneath the soil. By repeated 
search I collected from the nodules a fair number of fossils in the 
form of casts; but they were fewer here than in the sections 
exposed later in Pratt’s pit (p. 85) and Chamberlain Barn pit 
(p. 30). The working-face has since been cut back for another 
50 yards or so, and showed in 1920 the subjoined section (fig. 17) 
in a bay at the south-eastern end, where the Gault is thickest. 


Fig. 17.— Section of the south-eastern corner of ‘ Railway’ pit 
adjacent to Grovebury Brickworks (adjoining the north- 
western corner of old Spinney pit) ; September 29th, 1920. 

Surface, 300 feet O.D. 


0 5 10 15 feet 
ed 


Scale’ Vertical and Horizontal 


Q.J.G.S. No. 809. — D 


= S 


34 MR. G. W. LAMPLUGH ON THE JUNCTION OF | vol. xxviii, 


Thickness in feet. 


Z. Dark clayey soil with flints, etc., passing down into— lto2 
ZY. Weathered and rearranged palish-grey Gault clay, ore 1to3 
4 ‘creep, with crumbly structure; penetrated by roots. 


4, ) Dark Gault, somewhat disturbed, with rusty weathering; )} 
4a. ; detinite bedding in the lower part: small grit-grains and | 
pale-coated phosphatic nodules mostly of tubular shape, > about 2 
both becoming larger and more abundant downwards: } 
passing down into— 
3b. Dirty, rather rusty, gritty loam streaked with very gritty dark ) 
clay in the upper part: small pebbles, up to 1 inch at 
diameter: gritty phosphatic nodules of irregular shapes 
(pale-coated, dark inside) occur all through; also an occa- > about 13 
sional worn fragment of iron-grit, and some streaks of 
crimson and liver-coloured concretionary iron-par up to 
+inchthick. The gritty nodules are sparingly fossiliferous. } 
?1b Discontinuous band of large calcareous concretions, sandy } 
or outside. but often having a dense phosphatic core, usually 
3a. _ pale buff or piukish, but occasionally nearly black: invested $ 3to1 
by a few inches of rusty (2 rewashed) sand, lying across, | 
and cutting irregular hollows in, the cross-bedded Sands. ~ 
1. Coarse sharp Leighton Sands with cross-bedded vara etc. 
as described above (p. 32) .............. ......... seen about 20. 


At the north-eastern end of the section figured the working- 
face bends at right angles, and continues north-westwards for over 
100 yards, showing the same capping beds, but more weathered, 
and diminished in the upper part by the slope of the ground. 


Grovebury Brickyard.—In the next field on the west, 
within a few yards of the south-western end of the last section, 
and divided from it only by a fence. a small brickyard has been 
worked intermittently in the Gault clay during the last 20 years, 
and has yielded some useful information as to the upward succession. 
Work has been resumed here recently (autumn of 1920), but the 
sections are at present poor. They were better, however, in 1909 
and 1912, and the following particulars are based mainly on my 
notes of those years, and on small collections made then and 
subsequently. 


Sections in Grovebury Brickyard; April 1912 &September 1920. 
Surface, 300 to 305 feet O.D. 
Thickness in feet. 
€.In a shallow pit between the sheds and the 
eastern fence; 1920:— 


Pale weathered greyish crumbly Gault clay ...... : up to 4 
on darker blue platy clay, with small smooth brown-coated 
phosphatic nodules; seen in trenches only. 

‘ Ammonites rostratus’ and * Inoceramus sulcatus’ obtained on 
a clay-heap from this digging. 


RB. In a pit, now waterfilled, south of C; 1912:— 


Band of palish blue clay containing small brown-black phos-) 
phatic nodules with fossils : ‘Ammonites auritus,’ ‘splendens,’ | 


ete. (see list, p. 52). > seen about 3 
Dark-blue platy clay with crushed and indeterminable fossils | 
only. 2 


A. In a sump in a deeper dry pit north of OC, 
close to the eastern fence; 1912:— 
Gault clay, slipped and obscure (including part of B)............... about 10 
Gritty clay, with gritty phosphatic nodules ... .. ) (same asin the 
Sand, with big sandy nodules in a band at the top § Sandpit, fig. 17). 


part 1] GAULT AND LOWER GREENSAND NEAR LEIGHTON. 30 


The total thickness of the beds above the Sands at this place, 
allowing for the dip, may be 17 or 18 feet. The clay-pits are 
practically on the crest of a low rise, the ground falling slightly 
to the south and east, and rather sharply to the west and north 
towards the Ouzel and a tributary valley. ‘There is no higher 
ground near, therefore we may assume that the top clay, although 
weathered and disarranged, is probably not far from its original 
position. The presence of the keeled ammonites and of the 
associated suleated Inoceramus indicates the incoming of the 
Upper Gault at the top of the section, and leaves a thickness for 
the Lower Gault of approximately the same order as at Shenley 
Hill (fig. 3, p. 7) and Heath House (p. 29). 


Several big sand-pits are being, or have been, worked west of 
Grovebury, on the slopes of the Ouzel valley, and two of these are 
near the top of the Sands; but none has yet reached Gault in 
place, and the sections, therefore, will not here be particularized. 
They have revealed good exposures of gravelly drift and contorted 
‘trail,’ with Boulder Clay in places,! directly overlying the Sands. 
The drifts deserve further study, and I hope to continue the 
investigation of them. 

In the opposite or eastward direction the main Grovebury 
sections terminate at the high road running south-eastwards from 
Leighton; but I found in 1912 that new ground had been broken 
on the farther or east side of the road, immediately north of the 
railway at ‘ Billington Crossing.’ The section, known as Pratt’s 
pit, was similar to that of the Spinney pit (p. 33), but afforded a 
better opportunity for obtaining fossils from the gritty phosphatic 
nodules. The pit was visited in 1915 by members of the Geologists’ 
Association under my guidance, and the first example of ‘ Ammo- 
nites mammillatus’ to be obtained from the bed was found on this 
occasion. The section at the eastern end of the pit, where the 
Gault was thickest, was figured and described in my report of 
the excursion (op. cit. p. 311). The text of the description is 
reproduced below (repetition of the figure being unnecessary). 
This part of the pit has since been worked out and filled in, and 
the present workings have reached northwards beyond the feather- 
edge of the Gault, but under an increasing cover of gravelly 
drift. 


Section at the eastern end of Pratt’s pit, Billington Crossing, 
Leighton Buzzard; 1912-1915. 
Thickness in feet. 


Z. Clayey soil, with a few stones ...... wae 1-2 
Y. Pockets of drift- gravel, CRE SINE “mainly ‘of flint and quartzite. 
pebbles... eee Rhee aa Oe On 
ZY. Tough dark- ince unfossiliferous Gault eas, sree ibeormtele 
4 and with streaks of ‘race’ in the upper part ; becoming gritty 
and including a few phosphatic nodules in the lower part (4a) : 2—3 


passing down into— 


1 For some notes on one of these sections, see Proc. Geol. Assoc. vol. xxvi 
(1915) p. 312. The position of another is shown in the sketch-map (fig. 1, 
p. 2) which falls, however, short of the other sites. 

D2 


56 - MR. G. W. LAMPLUGH ON THE JUNCTION OF _ [ vol. lxxviii, 


Thickness in feet. 

. The fossiliferous bed: gritty clay with small polished pebbles (up 

to inch in diameter), and streaked with gritty loam and rusty 

weathered glauconite ; small nodules (up to 6 inches in diameter) 

of dark gritty phosphate are scattered rather plentifully through 

the band, although with a few ferruginous nodules and worn 

fragments of ironstone . rere 

Nos. 3 and 4 become thicker towards the south. 


1. Coarse current-bedded greyish sand, with many dark coffee- 
coloured grains in streaks ; some indurated calcareous lumps (up 
to 1 foot in cee) occur (1b) ee below the sharp 
junction With Sb. 2......55.2.cessssases scores cneroseresecsees- SCOD! (Ola UIaReECem 


Among the ammonites which I obtained from the nodules in 
this section, Dr. F. L. Kitchin identified Leymeriella reqularis 
(Bruguiére) and L. tardefurcata (dOrbigny ). 

A further exposure of the same beds was open for a short time 
recently in ‘ Webster’s pit,’ on the east side of the road, 200 yards. 
south of the railway-crossing, on the site marked ‘ Brick Works’ 
in the 6-inch Ordnance map. The section corresponded to that 
seen in the adjacent Spinney pit; it was partly obscured when I 
visited it early in 1920, and has since been all but obliterated. It. 
was, however, seen clearly in May 1919 by Dr. Kitchin & 
Mr. Pringle, from whose published account (Geol. Mag. 1920, 
p. 52) the following details are taken :— 


Sectionin Webster's pit, May 1919 [fide Kitchin & Pringle]. 
Surface, 290 feet O.D. 
Feet. inches. 
(Z) Soules, 1 
(4) ‘ Greenish- e1ey “stiff lumpy. clay with a few smooth-skinned 
nodules. Numerous fragments of ammonites of the 


interruptus-ty pe occur mainly in the form of casts’............ abt. 4 
(4a) ( ‘ Brownish sandy clay with streaks of glauconite’ ............... 1to2 
“/ 0 © Thin band of dark-grey clay ’. atte 2to3 


(3b) ‘ Dark-brown well-bedded ae ‘lay w jebis numerous routded 
quartz-grains and pebbles. Pale-coated dark gritty phos- 
phatic nodules of irregular shape (up to 6 inches in diameter), 
their surfaces studded with Be grains, occur cae 


throughout’ meth » 1: Aibayaid site eee 
(1) ‘ Dirty Pomc w white fats se- ssbadded andl Sikh bands and 
masses of iron-grit in the lowest 5 feet,’ ete.. venooe sctcop) SEER 


The authors mention the following ammonites as having been 
obtained by them from the nodule-bed (8b) of the two pits, this 
and Pratt’s:—Desmoceras aft. beudanti (Brongniart), Douville- 
ceras mammillatum (Schlotheim), Leymeriella regularis (Bru- 
guiere), L. tardefurcata (dVOrbigny), and Sonneratia dutem- 
pleana (d’Orbigny). 


The Grovebury sections, as a whole, differ from those north and 
north-east of Leighton in the abserice of a definite band of iron- 
stone-breccia and pan, and in the fuller development of the gritty 
beds with phosphatic nodules at the base of the Gault. The 
Chamberlain Barn section (fig. 16, p. 30), with its thin and 
diffused breccia, affords the intermediate link; and the presence of a. 
few worn fragments of iron-grit, and of an occasional concretionary 


part 1] GAULT AND LOWER GREENSAND NEAR LEIGHTON. 37 


iron boxstone or streak of ‘pan’ in the gritty beds at Grovebury 
marks the tailing-out of the localized conditions which produced 
the iron-grit and its concomitants. ‘These conditions, as will be 
subsequently shown, were almost certainly the existence, in a 
current-swept sea, of a line of sand-banks, either awash or but 
slightly submerged, which were indurated in places and scoured into 
irregular reefs, with deeper water on the south. 


Brickyards in the Upper Gault.—Before leaving the 
Leighton sections, I will mention two brickyards in the upper 
part of the Gault, on the Stanbridge road, nearly 2 miles south- 
east of Leighton and rather over a mile east of the Grovebury pits, 
as reference will be made to them in the subsequent discussion, 
although they are not otherwise relevant to the subject of this 
paper. The pits, Faukner’s on the south side of the road (see 
map, fig. 1, p. 2), and Yirrell’s on the north,! are in pale silty 
marly clay of massive structure, unlike anything seen in the 
sections around Leighton. Fossils are very rare, and I have found 
nothing except impressions of ‘ Hamites’ and crushed indetermin- 
able shell-fragments. The absence of ‘ Belemnites minimus,’ so 
plentiful in all unweathered sections around Leighton, is particu- 
larly noteworthy. The pits are 20 to 25 feet deep, and were being 
worked in 1920, but the greater part of the cuttings were then 
slipped and obscure ; they were seen more clearly in 1912. 


Sections south-west of Leighton and beyond. 


Little is seen of the base of the Gault in the country east and 
west of the big sand-pits of the Leighton district, partly because 
of the prevalence of Glacial drift on the east and for 6 or 8 miles 
on the west, and partly because of the inherent weakness of the 
Gault, which rarely allows natural inland exposures. In three 
places, however, excavated sections have been observed which prove 
that the conditions characterizing the base of the Gault around 
Leighton continue, at intervals if not unbrokenly, for 17 miles or 
more south-westwards. [The first of these, discovered since this 
paper was read, 1s an exposure in a small sandpit in the village of 
Southcott, on the western outskirts of Leighton Buzzard, about a 
mile distant from the centre of the town.] The second was in a 
brickyard at Littleworth near Wing, about 3 miles south-west of 
Leighton ; the third, in an old brickyard and stone-pit at Long 
-Crendon in the Thame district, 17 miles south-west of Leighton. 
In the two last-mentioned the Basement beds have overlapped the 
Lower Cretaceous Sands, and rest directly on Upper Jurassic 
strata. 


[Southcott (Buckinghamshire).—In this village, on the 
Buckinghamshire side of the Ouzel valley, about a mile west of 
the edge of my sketch-map (fig. 1, p. 2), a small sandpit has 


1 Yirrell’s Pit lies just outside the eastern limit of the map. 


38 MR. G. W. LAMPLUGH ON THE JUNCTION OF [ vol. lxxviil, 


been worked now and again for local requirements in the bottom 
of a little post-Glacial valley which has here trenched the sheet of 
Glacial drift overspreading the upland ground of the neighbourhood. 
The pit is in a paddock between the water-mill and the mill-pond 
at the western end of the village. The following section was ex- 
posed during the summer of 1921 at the south side of the pit, in 
a bank held up by tree-roots; it was clear in the upper part, but 
obscured by talus below. This section touches the side of the 
little valley; a fresh digging 20 yards farther north showed a 
clean section, but here, being in the tloor of the valley, the Base- 
ment beds had been cut nearly through and replaced by gravelly 
wash and ‘ run-of-the-hill.’ 


Section at the southern end of the sandpit below Southcott Mill- 
pond, half a mile south-west of Leighton Railway Station. 
Height above O.D. about 310 feet. 


Thickness in feet. 
ZY. Clay soil, with a few flints and drift-pebbles, ee about 1 
down into— 
ZY, Dingy-blue Gault clay, with indications of ‘ creep’; con- 
4 tains a tew finger-shaped phosphatic nodules; posing about 1 
into— 
3b. Streaky gritty clayey ferruginous loam, with large and 
small gritty phosphatic nodules of Grovebury type in 2 to3 


the lower part, intermingled with and streaking into— 
3. Lenticle of soft gritty pinkish fossiliferous limestone ) 
of Shenley type, with two or three of the gritty phos- 
phatic nodules embedded in it ; becoming more pebbly 
and gritty towards the edge, and then ‘breaking into 
concretionary lumps and streaking out into 3b. 


| maximum 1} 
J 

3a. Ferr ginous pebbly loam, poorly exposed ....... ot to about 
4 


(at the southern 
end). 


8 feet, but now 
obscured, except 
at the top. 

to 5 feet of Sand seen 
in the northern 
part of the pit. 


1. Coarse ochrey Leighton Sands, with indurated ees 
and ‘ pan’ ; 


The limestone is soft and crumbly, as usual when not well-pro- 
tected, but is unmistakably of the ‘peculiar Shenley composition, 
showing also the same patchy concretionary structure and the 
clustering of the fossils. These were in a poor state, but rather 
numerous in one spot; I obtained several Terebratule, a Rhyncho- 
nellid, small Avicule, and ‘Janira quinquecostata,’ all of the 
commoner Shenley forms. 

The section is important in showing the association and inter- 
linking of this peculiar kind of fossiliferous limestone with the 
gritty loams containing gritty phosphatic nodules, exactly like 
the material of the Grovebury and Chamberlain Barn sections 
which has yielded the Mammillatus-fauna. I broke up the 
limited number of gritty nodules obtainable from the small ex- 
posure, and found traces of fossils about as frequently as in those 
of the big sections just mentioned; but the fragments which I 
succeeded in collecting are not determinable, unless one fairly good 
cast of a bivalve should prove to be so. | 


part 1] GAULT AND LOWER GREENSAND NEAR LEIGHTON. 39 


Littleworth Brickyard (Buckinghamshire).—This place 
is barely 2 miles south-west of the Southcott section. The brick- 
yard is situated 400 yards north of the fork at which the Little- 
worth road leaves the main road from Leighton, and is worked in 
the slopes of a small steep-sided valley (deeper than that at 
Southeott) cut by a rivulet through thick Glacial drift. The 
surface, at the top of the pit on the south side, is about 380 feet 
above O.D. The yard has been in operation intermittently for 
over 60 years, and has been recently reopened; but the old sections 
are mostly obliterated, and the present exposure (October 1920) 
does not reveal the base of the Gault. During the original 
Geological Survey of the district it was examined by A. H. Green, 
whose observations were incorporated and first published by 
A. J. Jukes-Browne in his general memoir on the Gault, from 
which the following passages are quoted.! 


“The late Prof. Green saw a clear section on the southern side of the 
excavations about 1860, and noted the succession as follows :— 


Feet. 
Ee Drihe. Sandlandypebbly Sand: =. ).k.o5.420sc.1seeace- on. dstedsseeaace cet 14 
Pale-blue laminated clay with whity-brown phosphatic 15 
TGC TIES eae Oi A Oe Pentair net a dh ter cia ane eee ae 


‘Gault. Yellow earthy concretionary limestone, with much ochre, | 
| pyrites, some carbonate of copper? and brown phos- ; iz to 2 
Geplatiesnodules’s 23 0.050.0.4 ann oreteme eta it Ses oon Ja et J 


‘Kimeridge. Stiff bluish-black clay with large septaria ............ 6 


‘When I visited the place in 1884 this section was obscured, but a cut on 
the north side showed Kimeridge Clay passing beneath Gault without any 
stone-bed, and only a thin parting of brown ferruginous matter. The Gault 
contained Ammonites interruptus, Am. lautus, and Bel. minimus. In the 
little stream, however, which runs through the yard I found blocks of the 
stone described by Prof. Green, a hard calcareous ironstone full of phosphatic 
nodules, and containing many small Terebratule, which were identified by 
Mr. Etheridge as Waldheimia tamarindus,*® a Lower Cretaceous form which 
however has been found occasionally in the Gault.’ 


When the section was re-examined many years later by Dr. A. 
Morley Davies, the base of the Gault was still visible in workings 
south of the stream, and its variable character was again demon- 
strated. The following are Dr. Davies’s notes on the exposure, 
published in his report of an excursion of the Geologists’ Associa- 
tion in 19014 :— 


1 <The Cretaceous Rocks of Britain—vol.i: The Gault & Upper Green- 
sand of England’ Mem. Geol. Surv. 1900, p. 278. 

2 Films of copper-stain occur occasionally on slabs of iron-pan in the iron- 
grit breccia of the pits around Leighton. 

3 Some of the brachiopods from Shenley are near to this species, and it is 
probable that, if the Littleworth specimens were still available for comparison, 
they would be found to represent one of the Shenley forms. 

4 Proc. Geol. Assoc. vol. xvii (1901) p. 140. In the same writer’s account 
of a later excursion (1914), ibid. vol. xxvi (1915) p. 92, when the base of the 
Gault was no longer visible, the particulars of the section above the Gault 
are stated :—‘ Coarse morainic gravels, 8 feet; Chalky Boulder-Clay, 23 feet ; 
Sands and finer gravels, about 10 feet; Boulder-Clay, 30 feet; Gault, 17 feet. 


40 MR. G. W. LAMPLUGH ON THE JUNCTION OF _ [vol. lxxvui, 


‘In the upper diggings there were exposed about 15 feet of Drift, chiefly 
gravel, but with intercalations of sand, and at one place of a boulder-clay. 
The materials of the gravel were chiefly flint, but Coal-Measure sandstone 
was also found. In the lower diggings the Gault was exposed—a light bluish 
clay with small brownish-white phosphatic nodules, and under this a remark- 
able basement bed in which black phosphatic nodules (one at least a cast of 
a lamellibranch) were imbedded in a bright bluish-green material, while here 
and there masses of red oxide of iron occurred. Immediately below this 
came black shaly Kimeridge Clay,! from which the workmen had collected 
fossils—chiefly reptilian bones, but including also a large clavellate Trigonia, 
and a stout Belemnite evidently Jurassic. In the Gault itself, B. minimus 
was found.’ 


I am indebted to Dr. Davies for the loan of specimens which he 
obtained from the basement-bed in 1901; one is of semi-indurated 
calcareous glauconitic sand, olive and dark green, streaked and 
mottled with ochreous material, very like the glauconitic loam 
associated with the iron-grit breccia and limestone at Shenley 
Hill; another is of glauconitic sand surrounding a worn fragment 
of black phosphatic stone; another, of similar sand with traces of a 
concretionary crust, probably once pyritous. The first specimen 
includes some small wisps of pinkish calcareous matter, and is 
doubtless the concomitant and variant of the limestone seen by 
A. H. Green. 

During my visits to the place in the autumn of 1920 the clay- 
pit showed a poor exposure of 10 to 15 feet of shattery dark-blue 
Gault containing small brown-coated nodules (black internally), 
surmounted by dark greenish boulder-clay like that of Claridge’s 
- pit (p. 26), with the higher drifts recorded by Dr. A. M. Davies 
not well seen, except the top bouldery gravel. I was informed by 
the proprietor that the ‘green band’ at the bottom of the Gault 
was 10 or 12 feet below the present floor. I found ‘ Belemnites 
minimus’ and ‘Inoceramus concentricus’ plentifully, but only 
some poor traces of other fossils, in the Gault. 


Long Crendon (Oxfordshire).—On the steep rising ground 
north of Long Crendon, 2 miles north-west of Thame, an irregular 
outlier of Gault, about a mile long and for the greater part less 
than a third of a mile wide, rests on an elevated platform of 
Purbeck and Portland rocks in which numerous stone-pits have 
been worked. The Jurassic strata have been described by many 
observers, from the time of Fitton onwards, and reference is made 
to the Gault in some of their descriptive sections; but the earliest 
observations (not however the earliest to be published) requiring 


1 On the old 1-inch Geological Survey map, Quarter-sheet 46 S.W., the 
Jurassic inlier along the valley west of Wing is lettered g’* and coloured as 
Portland Limestone, but the error is corrected on the later 6-inch MS. map, 
Bucks. xxiv, kept in the Library of the Geological Survey, this map showing 
the inlier as Kimmeridge Clay, with the lettering g. Last summer (1920) I 
saw a good exposure of dark-blue clay, with big septarian nodules of the 
* cement-stone’ type, at a watering-place for cattle in the brook south of 
Wing Park, and obscure exposures of similar clay in the banks at several 
spots above and below this place. 


part 1] GAULT AND LOWER GREENSAND NEAR LEIGHTON. Ay 


notice for the present purpose are those of Jukes-Browne,! made 
for the Geological Survey in 1885, and recorded in his memoir on 
the Gault. He saw at that time the following section in ‘a quarry 
worked for stone on the east side of the road,’ which can be 
identified as the long-abandoned working close to the old windmill, 
about a quarter of a mile north of the village. 


Section at Long Crendon; 1885. (A. J. Jukes- Browne.) 
Thickness in feet. 


(Sey soil passing down into tough grey clay, slightly 
‘Gault.’ / micaceous and showing layers of darker and lighter ere ; 


impressions of Inoceramus’ ........ 10 to 12 
(‘ Brown ferruginous sandstone with saci pebbles ah quartz 
aia ar | and ly dianite; in places are lumps of calcareous stone’. 1 to 13 
rconsand »~ ‘Thin layer of laminated grey and yellow clay; laminated 
<7 clays with large lenticular concretions of hoety pau 
\© “ironstone” ..2.: aa OE Reet Ree AY eke eR NLS 24 
‘Purbeck and Portland edeu seen tor? ee Sere eee totter nennatan tery were 16 


The interest of this record centres upon the ‘lumps of calcareous 
stone’ in the bed below the Gault, as, under Jukes-Browne’s in- 
structions, the section was visited in the same year by the Survey 
fossil-collector, Mr. J. Rhodes, who obtained a few fossils from 
the calcareous stone, which are preserved in the Survey collections 
at the Jermyn Street Museum.? These fossils, so far as they are 
determinable, all belong to common forms of the Shenley Hill 
limestone, and the matrix i in which they are preserved is identical 
with the common type of the Shenley Hill rock. The fossils are 
registered as from ‘ Brickyard and Stone Pit, 2 mile N.W. of 
Long Crendon Church.—Lower Greensand. F erruginous sandy 
clay and pebble-bed on Portland Oolite.’ The rock is a reddish, 
dense gritty limestone, with some ferruginous matter. The speci- 
mens have received the following identifications in the Survey 
Register :—‘ Terebratula depressa ? Lamarck [J. R. 1656]; 
Terebratula capillata @Archiac [J. R. 1658]; Terebrirostra lyra 
Sowerby [J. R. 1677]; Terebratella menardi Lamarck [J. R. 
1660]; Rhynchonella latissima Sowerby [J. R. 1678]; Tere- 
bratula [J. R. 1657, 1675]; Cucullea (fragment) [J. R. 1664] ; 
Serpula, Cidaris spine, and Polyzoon [J. R. 1667, 1680].’ (The 
numerals in square brackets are the Register Nos. ) 

The Cuculle@a of this list is Septifer lineatus (Sowerby), the 
commonest lamellibranch at Shenley. 

The section was re-examined some years later by Dr. A. Morley 


1 “The Cretaceous Rocks of Britain—vol.i: The Gault & Upper Greensand 
of England’ Mem. Geol. Surv. 1900, p. 277. 

2 I owe thanks to my friend, Mr. E. T. Newton, F.R.S., formerly in charge 
of the Survey collections, for Ihave written to call my attention to these 
fossils in 1902, soon after my discovery of the Shenley fossiliferous limestone ; 
but I was at the time resident in Ireland, and the matter escaped my memory 
until revived recently by the controversy respecting the Shenley deposit. 
So far as I am aware, the fossils have not been mentioned in any Survey 
publication, and are now recorded for the first time. 


4.2 MR. G. W. LAMPLUGH ON THE JUNCTION OF [ vol. lxxviii,, 


Davies, who published, in 1899, the following description of it in: 
his excellent paper, ‘Contributions to the Geology of the Thame: 
Valley ’ (Proc. Geol. Assoc. vol. xvi, p. 22). 


‘Section at Southern Windmill, Long Crendon.’ 
(A. M. Davies, 1899.) 


Feet. inches.. 


* Gault.’ ‘Clay with Inoceramus concentricus, Belemnites minimus, 
and foraminifera ........... 8 0 
(‘Sand, with aahiiles, of onan one iy dliee, “inal oncrone 
Gicnorarer | concretions ee CalGitey cit seavleehadcceoebacb anes Wee 
¢ 
TeGneande: >< ‘Green sandy clay . B\evida dear eee ha. Sen eueeee emer eL 


| olonstone™ saa. yen e. 

* Bluish clay, lack wb base’ pencrescsced ecco eee 
‘Purbeck.’ ‘ Limestone with clay-veins’ .. Sooskiichs ama 
With other details, down to Portland limestone. 


OD OD HD OD 


In a later paper Dr. Davies mentions that in the Gault here he- 
‘failed to find ammonites, though foraminifera were plentiful” 
(Geol. Mag. 1899, p. 161). 

I first visited the spot myself in June 1902; but by this time 
the deeper part of the section had become obscure, and I saw the- 
base of the Gault only where the cover was comparatively thin: 
and the beds much weathered. The following details are from: 
my sketch and section of the best exposure ! :— 


Long Crendon pit, in field north of South Windmill; 
June 22nd, 1902. 


Thickness in feet... 
Clay (weathered se : thicker on the east side of the pit, now over- 
SOMME ener ices SeenEZ 
Ferruginous clayey ue penile: fish i dite ane. mee “pevles 
(up | to $ inch in diameter) . as Een, 


In another exposure 15 yards farther east, ‘the band has thickened 
to 2 feet, is more sandy, and at the bottom is full of lydites in a 
clayey base. 
Grey and ferruginous clay, with a band, about midway, of large se 
clay-ironstone septaria with concentric coating: in the lower part a 
white streak occurs, with smal] organisms; and at the bottom a wedge > 2 to 25 
of grey clay with white fragments, like the jauey Purbeck ay 
Ob StOMe eer ecs cere 


Limestone: upper most 6 inches rokennt up ands Sicbbly Micolene ee seen to 4 


Like the previous observers, I regarded the gritty ‘ferruginous. 
clayey stuff’ as Lower Greensand, but was (and am still) inelined 
to assign the underlying grey clay with tabular clay-ironstone to. 
the Purbeck. 

This area is well outside the region of thick Glacial drift, which 
terminates Pe abruptly in steep-featured ground north of 
Aylesbury, 7 or 8 miles farther east. It is very generally veneered, 
however, with a surface-wash of flinty gravel or loam, containing 
a few quartzite and other stones along with the flints. 

The ferruginous breccia at the base of the Gault appears to- 


1 It is necessary to give all available particulars regarding this section, as: 
I believe that it is questioned whether the fossiliferous gritty limestone- 
lumps found by Mr. Rhodes occurred below the Gault. 


part 1] GAULT AND LOWER GREENSAND NEAR LEIGHTON. 4.3. 


have once extended over a wide area in the district, as I found 
traces of it in 1904 under the main outcrop of the Gault east of 
Thame, between Kingsey and Aston Sandford, in the then-un- 
finished cuttings for the Great Central Railway. At the north- 
western end of a cutting in Gault, I saw a poor exposure of 
ferruginous sandy loam full of bits of ironstone and lydites. The 
Gault was a dark-blue rather silty clay, with ‘Belemnites minimus” 
and small brown-coated phosphatic nodules, seen to a depth of 
8 feet. 

When it became evident that the fossiliferous material obtained 
by the Geological Survey in 1885 from Long Crendon was. 
identical, both lthologically and paleontologically, with the dis- 
puted Shenley rock, steps were taken by the Geological Survey to. 
re-investigate the section, now entirely obliterated by slips and 
overgrowth. The requisite permission having been obtained, a 
trench was dug, in the autumn of 1920, in the sliding clay-slope 
above the old stone-pit, and was carried across the base of the 
Gault and its accompanying beds. From my examination of 
the cutting soon after it was made, I obtained the information 
shown in the following figure. 


Fig. 18.—Section in the trench at the east side of Long Crendon 
Windmill pit ; September 24th, 1920. 


Surface at the top of the slope, about 400 feet O.D. 


wy Old 
10 feet 3 7S Ge ~~ _ \ Stone-pit 
0 ee ee SSS 
Scale: Vertical and horizontal * pan ae / eS < 
g . ose 4 Talus 


The trench, a foot and a half wide at the top, 24 feet at the lower part, was cut in 
steps, the upper part entirely in sliding Gault, but the lower part touching Gault 
in place, or only slightly displaced. All the beds were more or less affected by 
surface weathering. 

Thickness. 


Z. Brown, passing down into blue, clayey soil, with scattered 
flints (the largest seen, 4 inches in diameter), passing ¢ about 2 feet. 
down into— 


Rather pale blue-grey mottled clay (Gault), somewhat dis- 
turbed, with some shattered small brown-coated phos- 
Mivanenmadulest mee ores yiocdte ss nkas-oeblee esp aseemabout 2) fhe 


44. MR. G. W. LAMPLUGH ON THE JUNCTION OF  [ vol. xxvii, 


Thickness. 

( Sliding Gault and soil (trench 2 to 23 feet deep); obscure’ 8 to 9 feet. 

| (in2nd Step). Blue-grey Gault, probably in place, under 

| sliding Gault ......... ...... seen about 2 ft. 

(in Ist Step). Dark- blue Gault, probably i in n place (w ith 

4. < 3 to 4 feet slipped clay above) w ith soft 

| crushed decomposed traces of shells... .. seen abt. 4 ins. 

[ Yellowish-grey decomposed clay, probably 

i altered by recent weathering, with fairly 

'é shaepiDase.OMee 6 25 t2 Be ie eo ssg sd: 2 inches. 
3. (do.) Decomposed ochreous loam, streaky and ) 


patchy, with glauconitic dabs and streaks, 
and soft pale calcareous patches; rather 
sparingly mixed with polished grit-grains | 

and small flat worn bits of ironstone; 

also containing a nodular lump, 8 inches > about 1 foot. 
in diameter, of ferruginous pebbly mate- 

rial (lydites, up to 1 inch in diameter, 

and worn bits of ironstone), with decom- 

posed calcareous cement, mottled with 

glauconite; fairly sharp base, on— 


3a? (in sump). Rusty marly sandy rock, rather flaky; 
(possibly like soft sandy ironstone In parts; evi- = to 1 foot, 
Purbeck) dently altered by weathering. 


( (do.) Ochreous weathered rather sandy clay, ) 
with a l-inch streak of dark greenish- 
blue clay at the bottom. 
(do.) Septarian claystone nodule, blue intern- > 5 inches. 
ally, with a crimson and liver-coloured | 


(supposed ferruginous crust: tailing off into the 
Purbeck). | ochreous clay. J 
(do.) Rather pale buff marly clay, obscurely 
sho wmeaneeweb Peas) 2 2c. ek See seen to 6 ins. 


(Bottom of the sump filled in before my visit). 


The beds below the Gault were much weathered, but it will be 
seen that the section is identical, so far as it goes, with those 
previously recorded. The conglomeratic calcareous lump in (8) 
may be taken as a decomposed example of the ‘lumps of calcareous 
stone’ noted by Jukes-Browne in the fresh section. Only about 
8 square feet of the surface of the basement-bed was bared for 
examination in the recent trench. I understand that some traces 
of ammonites were found in the overlying Gault, but could not 
be preserved. It will be noticed that Jukes-Browne records only 
‘impressions of Jnoceramus,’ and Dr. A. M. Davies only Jnoce- 
ramus concentricus, Belemnites minimus, and foraminifera; so 
that the Gault here, even at its best, was but sparingly fossili- 
ferous, like the lower Gault at Shenley, and with the same kind of 
fauna. 


IIJ. Patrzonrotoeicat Notes. 


Noter.—In the short references within parentheses in the text of the sub- 
sequent pages (L. W.) denotes the Quarterly Journal paper of 1903 (vol. lix) 
by myself and Walker, and (K. P.) the Geological Magazine paper of 1920 
(vol. lvii) by Dr. Kitchin & Mr. Pringle. 

In attempting to deal with the paleontology of my subject, I 
am conscious how much has been lost to us by the death of my 
former fellow-worker, by whom, had he lived, the major part of 
the task would have been undertaken. As it is, I find myself 


part 1] GAULT AND LOWER GREENSAND NEAR LEIGHTON. 45. 


incapable, under the present specialized conditions of palzonto- 
logical science, of doing justice to the matter; and can hope only 
to convey such information as may indicate the general relationship. 
of the beds and assist the specialist in further investigation. 


Additional Fossils from the Shenley Limestone. 


I have already referred to the great growth of the Walker 
collection since our former paper was written, and to the pre- 
paratory work done on the brachiopoda by thei late possessor 
before his death. From an examination of this collection of 
brachiopoda, numbering very many thousands of specimens, now 
preserved in the Natural History Museum at South Kensington 
exactly in its original state,! I have been able to cull some infor- 
mation as to the trend of Walker’s later work which, even though 
provisional and unfinished, is likely to be of service to any future 
investigator. In all the lenticles of limestone which have been 
revealed since our former paper, the brachiopoda have been by far 
the most abundant element in the fauna, just as they were in 
those previously discovered. 


-Brachiopoda.—(i) In view of the controversy as to the age of the 
limestone, it is important to note that the absence of the common Upper 
Greensand forms, ‘ Terebratella pectita’ and the typical ‘Terebratula bi- 
plicata,’ commented on in our former communication (p. 245), still holds good 
in the vastly enlarged collection. 

(i) Determinations mentioned by Walker as having been based on scanty 
material are now much more fully represented: as, for example, Terebratula 
buplicata Sowerby, var. dutempleana d’Orbigny (formerly ‘a specimen,’ now 
over 20 specimens); Rhynchonella antidichotoma Bav. (formerly ‘one perfect,’ 
now at least 16 specimens, with others labelled Rh. latissima var. anti- 
dichotoma) ; Rhynchonella lineolata (?) var. mirabilis nobis (formerly ‘a 
remarkable specimen,’ now 4 specimens labelled simply Rh. mirabilis). Most. 
of the new Shenley species or varieties established by Walker are now very 
numerously represented in the collection. 

(iii) Some additional determinations have been indicated; when written. 
in ink, they were probably regarded as final; when in pencil, as provisional, 
and requiring further consideration. Among those in ink are——‘ Terebratula 
biplicata, var. ingens nobis’ (many specimens :—a large form differing from 
the var. gigantea nobis of our previous paper); ‘ Terebratula n. sp.’ (twice ;. 
two specimens, kept separate); ‘'Terebratula sella var.’ (a single specimen, 
possibly only a deformed individual of some other species; the collection 
includes many examples of deformity) ; ‘Terebratula obtusa Sowerby’ (one 
specimen, labelled ‘Greensand above Shenley bed. J..Webb,? 1904’); 
‘Terebratulina chrysalis Schlotheim ’ (6 specimens: Walker, however, must. 
have felt some doubt about the nomenclature, as he mentioned the occurrence 
of the form in a letter to me, dated September 17th, 1902, but added, after. 
the name, ‘ according to Schloenbach, who refers the Hssen (Tourtia) [ shells ] 
to this species’; and he did not include the name in his lst published in. 
1903: see also his remarks on ‘ Terebratulina’ in L. W., p. 253); ‘ Zeilleria. 


1 Walker sorted the specimens into glass-capped boxes, and wrote his 
identifications with localities, etc., on the under side, The containing: 
cabinets were presented along with the collection, which is thus kept intact. 

* The name of one of the quarrymen from. whom Walker obtained. 
specimens, 


46 MR. G. W. LAMPLUGH ON THE JUNCTION OF  [vol. Ixxviii, 


pseudojurensis var.’ (12 specimens); ‘Magas orthiformis (d’Archiac) var. 
spiriferoides’ (about 30 specimens, besides large numbers of the normal 
specific form and of assorted varieties; the species, according to Walker, 
chad not previously been found in Britain, and is rare in the Tourtia; see 
L. W., p. 255); ‘ Rhynchonella leightonensis var.’ (3 specimens); ‘ Rhyncho- 
nella cf. tripartita Pictet’ (4 specimens). 

The following (provisional) determinations are marked in pencil only. The 
Species, mostly belonging to the Tourtia, are generally represented by named 
specimens from abroad in Walker’s foreign collection, which he evidently used 
.as the primary basis for comparison. ‘ Terebratula cf. rcemeri’ d’Archiac, 
(many specimens) ; ‘ Terebratula robertoni’ d’Archiac, (many specimens: see 
remarks on this species in L. W., p. 252); ‘ Zeilleria dallasiana’ (2 speci- 
mens); ‘Magas subconcava’ (18 specimens) ; ‘ Kingena leptorhabdota’ (many 
Specimens); ‘Kingena psammos’ (about 30 specimens); ‘ Aulacothyris ’ 
(several boxes); ‘ Rhynchonella nuciformis var.’ (about 100 specimens: see 
remarks on the species in L. W., p. 259); ‘ Rhynchonella sigma’ (3 speci- 
mens: in a letter to me, dated January 30th, 1903, Walker wrote—‘ There 
is a curious new Rhynchonella, something like Rh. sigma; I shall propose the 
name Rh. sigmoides for it.’ He added:—‘ The bulk of the Rhynchonellas 
belong to the Rh. dimidiata group; the Continental geologists lump them 
into this species, but I think latissima should be kept separate.’ His later 
work appears to have strengthened this opinion: see (ii) above. 

A large proportion of the assorted brachiopoda in the collection, parti- 
-cularly of the Rhynchonellids, have no names on the boxes, or a generic name 
only, and I know that Walker regarded some of these as probably new. His 
-attitude in dealing with the material was expressed to me concisely as 
follows (in litt. January 16th, 1902) :— The difficulty is not to describe new 
species, but to prove that the old species are present in the bed.’ Walker 
-defined his conception of a species in a short paper in the Geological Magazine 
(pp. 15-17) in 1905, and from the point of view therein stated, he found that 
‘the specific centres of the Shenley fossils ‘where the individuals are most 
‘thickly clustered and most closely resemble each other’ rarely embraced the 
individuals which were nearest the form regarded as typical for the 
established species of other localities ; wherefore in most cases, if an already- 
-named species had been first described from the Shenley specimens, it would 
have had a somewhat different definition. The very numerous grouped but 
unnamed specimens in the collection will be found usually to represent 
~Walker’s ideas of the specific and varietal ‘centres’ calling for recognition. 


The list of fossils other than brachiopoda given in L. W., p. 268, 
was based on my own collection, since presented (excepting a few 
specimens) to the Geological Survey and now preserved at the 
Jermyn Street Museum. The non-brachiopodous material obtained 
later by Walker, now in the Sedgwick Museum at Cambridge, 
includes a large number of forms not already in the list; but only 
-some of these have as yet been identified, and it will not be possible 
‘to gain a comprehensive view of the fauna until the whole 
-eollection has been systematically worked through, and the fossils 
-compared with those from analogous deposits in France and 
Belgium (see p. 61). The collection, however, is scanty com- 
pared with that of the brachiopods, so that instead ef the species 
being represented by hundreds or scores of specimens, they rarely 
-count more than units, which is to be explained partly by original 
.sparseness, imperfect preservation, and difficulty of extraction 
(see L. W., p. 244), and partly, perhaps mainly, as the result of 
Walker having purposely selected the brachiopoda for his chief 
objective. As in the case of the brachiopoda, some of the forms 


part 1] GAULT AND LOWER GREENSAND NEAR LEIGHTON. A7 


appear to belong to species previously unknown in this country. 
‘The following notes are intended to give a general idea of the 
additional fauna, without pretence to paleontological accuracy, 
except in the case of certain specimens at Cambridge, duly indi- 
vated, which have been submitted to expert examination. 


Cephalopoda.—Although fossils of this Class have continued to be 
exceedingly rare, a few specimens have been obtained which are of high 
import. It was mentioned in the previous paper that the cast of portion of 
.an ammonite had been found, which, though not identifiable, showed affinity 
to ‘Ammonites milletianus.’ The Walker (Cambridge) Collection now con- 
tains four specimens, all showing the same general features, three of them 
poorly preserved, but one, the largest, a sharp ferruginous cast, # inch in 
diameter, in good condition and showing traces of nacreous lustre, which has 
‘been identified by Dr. Kitchin as Leymeriella regularis (Bruguiécre) ; and there 
is little doubt that the ammonite mentioned in our first description was of 
the same kinship. This species and its allies are the commonest forms in the 
gritty phosphatic nodules of the Basement beds at the Chamberlain Barn and 
Grovebury pits (pp. 30 & 36). Dr. Kitchin & Mr. Pringle speak of the lime- 
stone-specimens as ‘ derived from the tardefurcata bed’ (K. P., p. 102), but 
‘the state of the specimens runs counter to the hypothesis of derivation, 
which will be shown also on other grounds to be improbable (p. 55). Their 
condition is quite different from that of the small round waterworn fragment 
of a whorl referred to in L. W., p. 244, and now in the Cambridge collection. 

Another species of ammonite yielded by the limestone is represented by a 
‘single specimen which, in 1920, rewarded my search in recently-excavated 
material at Harris’s pit. The shell, about an inch in diameter, is somewhat 
crushed, but otherwise fairly well-preserved, and is a smooth discoid form, 
without ribs, agreeing well with the form common in the Mammillatus-beds, 
which has been named provisionally ‘Ammonites beudanti.’ This form occurs, 
along with ‘ Ammonites regularis,’ in fair numbers in the gritty phosphatic 
nodules of the Grovebury and Chamberlain Barn pits, where they are associated 
“with scanty examples of ‘ Ammonites mammillatus’ (pp. 30 & 36). 

With regard to the belemnites, no additions have been made to the two 
fragmentary specimens previously recorded, of which one is now at Cambridge 
-and the other in my own collection. The form, as stated before, is near to 
‘Belemnites minimus, although rather larger than the average of that 
‘species. I notice that a German investigator, in discussing recently the 
Lower Cretaceous sequence of North Germany, recognizes a form intermediate 
‘between ‘ Belemnites aff. strombecki’ and ‘B. minimus’ in the ‘ Zone of 
Hoplites regularis, and proposes for it the name Neohibolites minor Stolley.! 
It is likely that the Shenley belemnite belongs to this type. 

Gastropoda.—This Class is represented in the collection at Cambridge 
‘by many unnamed specimens, mostly in poor condition, but a few probably 
identifiable. They include examples of ‘ Pleurotomaria, and probably others, 
in addition to the genera already recorded. I recently collected a good cast 
of ‘ Scalaria’ from the limestone. 

Lamellibranchiata:—Most of the species previously recorded are 
represented in the collection at Cambridge by a few additional specimens, 
along with which there are several species new to the list. Some have not 
yet been identified; but the following determinations have been made by 
Mr. H. Woods :—Inoceramus concentricus Parkinson (3 specimens: the 
largest about # inch in length); Isoarca obesa (d’Orbigny) (5 specimens, 


1 E. Stolley, ‘ Beitrage zur Kenntnis der Cephalopoden der Norddeutschen 
‘Unteren Kreide: 1. Die Belemniten des Norddeutschen Gaults (Aptien & 
-Albien)’ Geol. & Paleont. Abhandl. n.s. vol. x, pt. 3, 1911; also ‘Die 
‘Gliederung der Norddeutschen Unteren Kreide’ Centralbl. fiir Min. 1908, 
~p. 246, 


4S MR. G. W. LAMPLUGH ON THE JUNCTION OF  [vol. Ixxviii, 


casts); Cyprimeria (Cyclorisma) resembling rotomagensis d Orbigny (6 speci- 
mens) ; Pecten (Camptonectes) curvatus Geinitz (3 specimens, small); Lima 
(Mantellum) resembling gaultina Woods (2 specimens) ; Lima (Plagiostoma) 
globosa J. de C. Sowerby (mentioned in our previous list as ‘ resembling’ this 
species ; identification now confirmed on the strength of several specimens) ; 
Pieria (Oxytoma) pectinata (Sowerby) (many specimens: probably—‘ Avicula 
sp. indet.’ of our previous list): also (genus only) Limatula and Nucula. 

Echinodermata.—tThe fossils of this Class in the collection at Cambridge 
have been recently examined by Prof. H. L. Hawkins, who has published two 
notes on the subject in the Geological Magazine, Feb. & Sept. 1921 (pp. 57-60 
& 420-26). The following are his identifications ; he says, ‘ Unfortunately, 
most of the Echinoids are very indifferently preserved, so that specifie deter- 
mination is often doubtful, and in some cases impossible.’ 

Catopygus columbarius Agassiz (2 specimens); Nucleolites lacunosus Gold- 
fuss (1 specimen); Pyrina aff. levis Agassiz (4 specimens) (all in our previous 
list). Cardiaster ?latissimus Agassiz (definitely so named in our previous 
list). Radioles of Cidaris bowerbanki Forbes. 

The above are species previously recorded ; the following are additions to 
our list:—‘Pyrina’ sp. nov. (? nov. gen.) (1 specimen); subsequently 
described and figured as Conulopyrina anomala Hawkins. Pyrina ef. 
inflata d Orbigny (1 specimen); P. ef. desmoulinsi d Archiace (2 specimens) ; 
Pyrina sp. (or spp.) (2 specimens); ‘ Echinospatagus’ aff. murchisonianus 
(Mantell) (1 specimen). Cardiasier cf. fossarius (Benett) (2 specimens). 
Internal moulds of—? Peliastes or ?Salenia (1 specimen); ? Pseudodiadema 
or ? Diplopodia (1 specimen) ; and ? Cardiaster or Holaster (2 specimens). 

Prof. Hawkins remarks :—‘* All the specimens of Pyrina are probably 
shape-variants of a single species; the forms attributed to P. levis beme 
immature.’ .. .‘* The outstanding feature is the relative abundance of Pyrina, 
about half of the specimens being referable to that group. This probably 
indicates littoral conditions’ (op. prius cit. p. 59). 

I shall have occasion to comment on this list in the concluding part of my 
paper (pp. 76—77). 

Crustacea.—The collection at Cambridge contains several casts of the 
carapace recognized by Mr. H. Woods as Cyphonotus incertus Bell. An 
example of the same form, named by Mr. E. T. Newton, is among the fossils 
of my own collection at Jermyn Street. 

Polyzoa.—Among the fossils at Cambridge are a few polyzoa, to some of 
which the following provisional names have been attached :—Repiomultisparsa 
cf. megalopora (Vine); Heteropora sp.; and Tremacystia sp. 

Crinoidea.—tThe fossils mentioned in L. W., p. 244, as ‘joints of a large 
round-stemmed crinoid, recalling . . . Bourgueticrinus ... have been examined 
by Dr. F. A. Bather and identified as Torynocrinus, a genus beginning in the 
Lower Cretaceous and not known above the Red Chalk ; the Shenley form is 
not referable to the species found in the Red Chalk of Hunstanton. Besides 
those in my own collection, there are several specimens at Cambridge. 


Fossils of the Greensand and Breccia associated with the 
Shenley Limestone. 


Excepting the traces of shells in the decomposed calcareous 
patches, mentioned on pp. 18 & 20, the irongrit-breccia and its 
accompanying gritty loam and sand are almost devoid of fossils, 
the only examples that I have found being a specimen of ‘ Janira 
quinquecostata’ and an imperfect ‘ Pecten’ in glauconitic loam at 
Miletree Farm pit (fig. 11, p. 20); a small round palatal fish-tooth, 
and a worn fragment of the whorl of a phosphatized ammonite, 
probably a pebble like that referred to on p. 47, at Nine Acre pit 
(fig. 7, p. 16); and two or three worn casts of ‘Terebratula’ at 


part 1] GAULT AND LOWER GREENSAND NEAR LEIGHTON. 49 


Poplars pit (fig. 14, p. 23). It is to be remembered, however, that, 
except under Shenley Hill, the breccia is never far below the 
present surface, and generally shows signs of recent weathering, 
as well as of an older alteration before the deposition of the Gault, 
so that caleareous fossils other than those embedded in the solid 
limestone-lenticles or in phosphatic nodules have had a poor chance 
of survival, even if originally present. 

The thick wedge of greensand which intervened between the 
Gault and the breccia in former workings at Garside’s pit, 
Shenley Hill, first exposed after our former paper was published (as 
described on pp. 10-13, fig. 4), was better provided with organic 
remains. It proved, however, a disappointing collecting-ground, 
owing to the patchiness of the fossils and their poor condition, 
aragonite shells (as in the limestone) occurring only as casts, and 
those of calcite being scaly and friable. 


The better part of my collection of these fossils is at the Jermyn Street 
Museum ; the remainder, along with a few collected by Walker, are still in 
my possession; the following rough list includes the material in both sets. 

‘Belemnites minimus’ and ‘var. attenuatus’ (the most abundant fossil) ; 
cast of a large Nautilus, resembling ‘N. radiatus’!; indeterminable fragment 
of a cast of ? ammonite ; ‘ Inoceramus concentricus’ (several casts, more or less 
crushed, chiefly of the broad coarsely-ribbed variety found in the overlying 
Gault, see p. 51); ‘ Anomia’; several small oysters, among which Dr. Kitchin 
& Mr. Pringle recognize (K.P., p. 6) ‘ well-developed valves of Ostrea vesi- 
cularis Lamarck, and O. canaliculata (J. Sowerby)’; ‘Terebratula,’ some 
crushed, others in waterworn fragments, rather numerous, but the only 
one found identifiable by Walker was the specimen of Terebratula obtusa 
Sowerby mentioned previously (p. 45); many fragments of ‘Serpula,’ among 
which Serpula antiquata J. de C. Sowerby has been recognized (K. P., 
p. 6); several detached valves of cirripedes, believed to be mostly referable 
to Pollicipes glaber F. A. Roemer and Pycnolepas rigidus (J. de C. Sowerby) 
(see K.P., p. 6); two small echinoderm-tests in very poor condition, sub- 
mitted to Prof. H. L. Hawkins, and reported to be probably ‘ Cardiaster ’ 
or ‘ Holaster,’ but beyond specific recognition. There are also two or three 
indeterminable casts of gastropods. 

Among the half-dozen small but fairly well-preserved fish-teeth, Mr. E. T. 
Newton has recognized Lamna appendiculata Agassiz. Scaphanorhynchus 
subulatus Agassiz, Sc. raphiodon (?) Agassiz, and Apuleodus (2). The three 
species named are recorded as occurring in the Mammillatus-beds of France, 
as well as in higher zones of the Gault.” 


Fossils of the Gritty Phosphatic Nodules (Mammillatus- 
bed). 


This fauna, discovered since the publication of our former paper, 
is probably the best development of the Mammillatus-fauna known 
in this country, but will require much specialized work, as well as 
further collecting, before it can be adequately discussed. It was 
briefly referred to in my report of the Geologists’ Association 


1 Approximate determination by Mr. E. T. Newton, F.B.S. 

2 A. J. Jukes-Browne, ‘ Gault & Upper Greensand,’ Mem. Geol. Surv. swpra 
cit. p. 379; and C. Jacob, ‘ Etudes Paléontologiques & Stratigraphiques sur la 
Partie Moyenne des Terrains Crétacés dans les Alpes Francaises, &c.’ Trav. 
Lab. Géol. Grenoble, vol. viii (1907) p. 311. 


Q. J.G.S. No. 309. E 


50 MR. G. W. LAMPLUGH ON THE JUNCTION OF  [vol. Ixxvill, 


excursion in 1915 (see reference on p. 35) and has been recently 
described in part by Dr. Kitchin & Mr. Pringle (op. c7t.), but is 
otherwise unrecorded. The following notes, based on my own 
collections (which I still retain), will serve as a rough indication. 
of the main elements of the fauna. The fossils, in nearly all cases, 
take the form of sharp casts in the dark gritty phosphatic rock, 
becoming obscure and worn on the pale exterior of the nodules. 
Owing to the tendency of the nodules to break with a prismatic 
fracture, it is generally a matter of chance, even when fossils are 
present, whether they will be disclosed or destroyed under the 
hammer, and the majority of the nodules appear to carry none. 

To distinguish the two principal localities, the letters G. = Grove- 
bury pits, B.=Chamberlain Barn pit, are used after mention of 
the specimens. 


Cephalopoda.—The ammonites recorded by Dr. Kitchin & Mr. Pringle 
are (1) Desmoceras aff. beudanti (Brongniart); (2) Dowvilleiceras mammillatum 
(Schlotheim) ; (3) Leymeriella regularis (Bruguiére) ; (4) Leymeriella tarde- 
furcata (d’Orbigny) ; and (5) Sonneratia dutempleana (d’Orbigny). Of these, 
1 & 3 are comparatively numerous and 2 & 4 comparatively rare at G. and B., 
mainly in small clusters of one kind. I have not myself obtained 5. I possess 
a well-marked fragment of ‘ Ammonites of interruptus-type’ in gritty-nodular 
matrix from G., probably from the more clayey upper part of the band. Some 
of the ammonite-fragments preserve traces of the nacreous lustre. 

Of belemnites I possess the cast of a phragmocone from G., rather large for 
‘B. minimus,’ but apparently belonging either to this or to an allied form (see 
remarks above on the limestone-specimens, p. 47). 

Gastropoda.—Casts of the shells of this Class are rather numerous, 
particularly at G., and often show good ornamentation, so that determination 
from mouldings should eventually be practicable. The following genera, 
with others, are probably represented among my specimens: ‘ Aporrhais,’ 
‘Natica,’ ‘ Pleurotomaria (?),’ ‘Scalaria,’ ‘Solarium,’ ‘ Trochus.’ 

Lamellibranchiata.—Also numerous as casts, but perhaps not often 
specifically identifiable. I have noted the following: ‘Inoceramus con- 
centricus, a rather small broadly-ribbed form, approaching ‘ salomoni,’ like 
that in the Shenley limestone (3 specimens, G.); ‘ Pecten orbicularis’ (several 
specimens, G., B.) and other pectens ; ‘ Janira quinquecostata’ (2 specimens, 
B.); ‘ Pteria pectinata ’ (G., B.) ; ‘ Nucula probably pectinata ’ (1 specimen, G.); 
an ornate ‘ Trigonia’ (2 specimens, B.); ‘Goniomya’ (1 specimen, B.); a small 
striated ‘Cardium’ (several, G., B.); ‘ Cyprimeria(?)’ (8 specimens, B.); 
‘Cucullea (?)’; ‘Panopzea (?)’; many remnants of a large shell, always 
crushed, ‘Inoceramus’ or ‘ Gervillia’ (G., B.). 

Brachiopoda.—tThe fossils of this Class are a subordinate element in the 
fauna of the nodules, and not predominant as in the limestone. They occur 
as casts, occasionally clustered but often singly, and are much more plentiful 
at B. than at G. The majority appear to belong to ‘ Terebratula,’ of forms 
near to ‘moutoniana’ and ‘ biplicata var. dutempleana’ (B., G.); another form 
may be ‘Zeilleria convexiformis’; and there are probably two species of 
’Kingena’ (?‘newtoni,’ B., and ‘lima’ G.). Only one ‘ Rhynchonella’ (G.) is 
in my collection; also a ‘ Lingula’ (B.). 

Echinodermata.—I submitted the three casts obtained to Prof. H. L. 
Hawkins, who found them too ill-preserved for identification, but considered, 
though without confidence, that two might be small ‘ Pseudodiadema’ (B., G.), 
and the third came nearer in proportion to ‘Cardiaster latissimus’ (G.) than 
to any other species (see p. 77). 

Crustacea.—The claws of a large lobster-like crustacean, rather well- 
preserved, together with three or four less promising specimens are in my 
collection from B. 


part 1] GAULT AND LOWER GREENSAND NEAR LEIGHTON. 51 


Among the unclassified remnants may be mentioned some unnamed polyzoa, 
teredo-borings, and small annelid (?) tubes, the latter sometimes riddling the 
nodules (B.) ; also some poorly preserved fragments of bone (? reptilian) (B.). 

I have found no fossils whatever in the gritty clay and loam surrounding 
the nodules. 


Fossils of the Gault. 


In the next part (§ IV) of this paper, reasons will be stated for 
concluding that, contrary to the supposition of Dr. Kitchin & 
Mr. Pringle, the Lower Gault is present throughout the district 
under discussion, though of less thickness than at Folkestone. 
The division between the Upper and the Lower Gault is marked 
approximately, as at Folkestone, by a band of phosphatic nodules 
which indicate a pause in deposition at the beginning of the Upper 
Gault period. In the sections described in § II the Upper Gault 
occurs certainly in place at Harris’s pit (fig. 8, p. 7) and Heath 
House pit; possibly also at Grovebury brickyard. It may have 
contributed to the top ‘creep’ in neighbouring sections at lower 
levels, but I have not seen any proof of this. 


Lower Gault.—-The lower beds contain few identifiable fossils, 
such shells as have been entombed being almost always crushed 
flat and, in the shallower sections, still further spoilt by weathering. 


‘Belemnites minimus and vars.,’ and ‘Inoceramus concentricus and allied 
forms,’ are always the most conspicuous, and often the only fossils of the belt ; 
and they distinguish it throughout the district, as also at Long Crendon 
(p. 42) and Littleworth (p. 40). The ‘Inoceramus’ shows much variation of 
form, sometimes attributable to crushing and slight shearing, but sometimes 
‘denoting original differences which may be of specific value; one variety is 
probably referable to Inoceramus anglicus Woods, and another, a wavy broad- 
ribbed form, is likely to be that mentioned by Jukes-Browne & Hill in their 
description of the Gault of West Norfolk,! as follows :—‘ Of the other Inocerami 
[at Muzzle], some seem certainly to be I. concentricus, and others resemble 
the larger and more compressed species which occurs frequently in the Lower 
Gault elsewhere, and may be identified with that known as I. Crippsii when 
found in the Red Chalk of Hunstanton.’ I have obtained the same range of 
forms in similar condition from the ‘Belemnites minimus marls’ below the 
Red Chalk at Speeton, where they were associated with a crushed ammonite 
doubtfully referred to ‘ Ammonites interruptus.’? 

It has been usual in stratigraphical work to ‘lump’ these forms under 
“Tnoceramus concentricus’ as a term of contradistinction to ‘I. sulcatus,’ 
which can be separately recognized, however crushed, and has not, to my 
knowledge, been found in these lower clays. 

In the recent deep section in Harris’s pit (fig. 3, p. 7), the lower clays have 
yielded a few other crushed fossils which come within the range of possible 
identification. Dr. Kitchin & Mr. Pringle (K.P., pp. 13, 14) record 
‘the presence of small impressions of the characteristically ornamented 
Nautilus deslongchampsianus d Orbigny’ and ammonites ‘ poorly preserved, 
in a compressed condition, often consisting of little more than brown rusty 


1 On the Lower Part of the Upper Cretaceous Series in West Suffolk & 
Norfolk’ Q.J.G.S. vol. xliii (1887) p. 572. 

2 “On the Subdivisions of the Speeton Clay’ Q. J.G.S. vol. xlv (1889 
p. 604; and ‘On the Speeton Series in Yorkshire & Lincolnshire’ ibid. 
vol. lii (1896), table facing p. 184. 


= ___ ons) 


52 MR. G. W. LAMPLUGH ON THE JUNCTION OF  [ vol. lxxviii, 


films on the bedding-planes; nevertheless, many show plamly the mam 
features. The commonest form is closely comparable with well-known 
specimens, ...[from other places mentioned] “variously named in museum 
collections as Hopliies auritus (J. Sowerby), H. catillus (J. de C. Sowerby) or 


H. auritus var. catillus’ (see further comments, p. 78). They also mention - 


the occurrence of Nucula pectinaia J. Sowerby, Pecten (Syncyclonema) 
orbicularis J. Sowerby. and ‘ Scalaria’ dupiniana d Orbigny. I have myself 
obtained specimens of these, except the last-named and the Nautilus. The 
ammonites are usually crushed in the horizonial plane!; but I found one 
specimen which has been crushed and spread edgewise; it appears to have 
been a ‘ Hoplites * form, with strong protuberances at the outer end of the ribs. 
T have also collected from the lower clays (2 to 4 feet above the base) the 
remains of a small fish; decomposed pyritous teredo-bored wood with traces 
of adherent ‘ Anomia (?)° and small oyster-like shells ; ‘ Natica (?)’: and the 
markings of many broken bits of shell. 

From the higher part of the Lower Gault under Shenley Hill I have not 
myself obtained any specimens im place,as this portion is so quickly obscured 
by slips that it is rarely accessible. It has been exammed however (wholiy 
or in part?) by Dr. Kitchm & Mr. Pringle, whose record relatimg to the 
fossils has already been quoted (fig. 3, description, pp. 7-8). Somewhere in 
this part, probably not far below the Upper Gault nodule-bed, the clays must 
contain some ammonites parily infilled with concretionary matter and thus 
preserved in an uncrushed condition, as I have collected several good frag- 
ments in this state, mostly of the ‘ Ammonites auritus’® and ‘ A. splendens* 
types, from spoil removed from the middle of the slips; and I have a specimen 
of the same kind (either ‘auritus’ or ‘lautus’) from Chance’s pit, obtained m 
1904 from a quarryman, and said to have been found im place. 

Fragments of the ‘ splendens” ammonite in the same condition may still be 
picked up on the clay-slopes of the old Heath House pit (p. 29). 

The other sections in the lower clays around Shenley Hill have yielded little 
beyond ‘ Belemnites minimus’ and ‘ Inoceramus conceniricus’; and it is note-~ 
worthy that the Littleworth brick-pit, from which Jukes-Browne records 
Lower Gauli ammonites (p. 39), has yielded to me nothing more than the 
same * Belemnites’” and ‘ Inoceramus’ in the present weathered exposure. 

From the lower clays of the Grovebury brickyard I have collected si 
various times the followimg fossils among others, mostly as fragments, but 
hard and uncrushed, the majority bemg from the band of small brown nodules 
described im the section (p. 34):—‘ Ammonites imterruptus’ (1 specimen); 
*A. auritus’ (3 specimens); ‘A. splendens” (5 specimens); ‘ Hamites* 
(3 specimens) ; ‘ Belemnites mmimus’; ‘ Dentalium decussatum’ (1 specimen) ; 
‘Natica’; ‘Nucula’: etc.; and from larger nodules probably occurrmg m 
the upper part of the section—* Ammonites rostratus” (1 specimen, good) ; 
* A. varicosus ° (2 specimens); and ‘Inoceramus sulcatus or subsulcatus.” 


Upper Gault.—Except the few specimens from Grovebury 
brickyard just mentioned, and a few representing the same species 
and in the same condition found on the weathered slopes of 
Heath House pit, my collection of the Upper Gault forms has 
been obtained entirely from the fossiliferous band with phosphatic 
nodules near the top of Harris's pit (fig. 3, p. 7). The following 


1 In the first sketch of the section in my notebook, dated June 2nd. 1902, 
made just after the discovery of the fos siliferous limestone, my description of 
the Gault reads:— Gault clay, Inoceramus concentricus, Ammonites imter- 
ruptus, &c., thickness: 5 to 8 feet. For some reason which I cannot now 
recall, but probably because I had not carried away specimens for reference, 
and was (quite rightly) doubtful of my field-determmation, I omitted 
reference to the ammonite in the paper published in 1903. The forms that I 
saw may have been like those now described. 


part L] GAULT AND LOWER GREENSAND NEAR LEIGHTON. 53 


rough determinations are poorly representative of the fauna, and 
the list could certainly be much extended :— 


‘Ammonites rostratus and vars.’ (many); ‘A. auritus and vars.’ (both in 
nodules and soft clay); ‘A. varicosus’ (few); ‘A. splendens and vars.’ ; 
‘A. near coelonotus’; ‘A. studeri’; ‘A. lautus?’; and others; ‘Hamites’; 
‘Belemnites minimus’ (many). ‘Inoceramus concentricus’ (both in nodules 
and soft); ‘I. subsuleatus’ (one specimen in black phosphate) ; ‘I. sulcatus’ 
(modules and soft); ‘Nucula pectinata’ (soft); ‘ Plicatula’ and ‘ Ostrea’ ; 
‘Dentalium’ (many); ‘Scalaria,’ ‘Solarium,’ and other univalves; ‘ Penta- 
cerinus’ (in patches). 


IV. CLASSIFICATION, STRUCTURE, AND CONDITIONS 
OF DEPOSITION. 


The sequence of beds exposed in the sections described may be tabulated 


in downward order as follows :— 


Thickness 
Formation. Composition. Locality. in feet. 
eee ees } (Z.) Soil, downwash and ‘ creep.’ All sections. 1 to 4 
Glacial. (Y.) { Boulder-clays with associated and later Absent in some 
gravels. places: best seen ; 
at Claridge’s LO to 20 


pit (fig. 15) and { 
Littleworth (p. | 
39) 


( (Upper Gault, lowest part only: pale blue-) Harris’s (fig. 3), ) 
grey marly clay, with numerous corroded | Heath House (p. | 0 to 6 

| (5)< phosphatic nodules (5a) at or near the base. f 27) and Grove- P 
Gault J bury brickyard ( (seen) 
(part of). } Sep) (po). y) 

| Lower Gault; dark grey-blue clays with 

(4) | small brown-coated nodules; generally tan sections. up to 18 
ie becoming gritty near the base (4a). 3) 
fo South of North of Leighton. South— 
Leighton. Similar material more Grovebury pits 


Gritty glauconitic coarsely gritty (8b) (fig. 17, p. 33) 
clay andloam(3b) associated with, and Intermediate— 
| with occasional inplacesthinningout Chamberlain 
worn fragmentsof upon or under (3a) a __ Barn pit (fig. 16) 
| ironstone;sparsely band of irongrit- North— 
Waco oat studded with fos- brecciawithferricrete All Shenley pits ; 
beds, tg siliferous gritty ‘pans’ and lenticles Poplars (fig. 14), 1 to 5 
L 


phosphatic nod- (8); andwithcalcrete Claridge’s (fig. 
ules. patches thickening 15), and Heath 
locally into lumps or House pits: also 

tables of fossiliferous West—at South- 

limestone. cout (ps 38); 

Littleworth (p. 

39), and Long 

Crendon (p. 40). J 


Uneven surface of erosion. 
(2) The Silty beds: regularly bedded ashy-grey Present in some ) 
silts, carbonaceous and ferruginous loam Shenley sections | 


and clay, and some sand. and northwards ; 70 to 12 
Plane of erosion. absent south- | 
Lower wards. 
Green- | (1) Leighton Sands: strongly  cross-bedded | 


Series almost purely siliceous ‘Silver Sand’ with 
(part of). local patches of ferruginous staining and | All the pits around 
induration (1x). South of Leighton: { Leighton. 
less pure, speckled, brownish or greyish | 
sands, rarely showing staining or indu- 


| 
sand < coarse sands. North of Leighton: 
| 
ie ration. 


54. MR. G. W. LAMPLUGH ON THE JUNCTION OF _ [ vol. Ixxvin, 


The general dip of the Cretaceous sequence is south-eastwards at 
a low angle; the plotting of continuous sections brings out, how- 
ever, some local anomalies due, not to tectonic structure, but to the 
mode of formation of the deposits. Thus, a long section drawn 
north-eastwards, on the general strike of the Gault,/ from Chamber- 
lain Barn pit through the Shenley Hill pits to Poplars pit, a 
distance of nearly 2 miles, shows a rise of 55 feet in the floor of 
the Basement beds in the first 1850 yards (between Chamberlain 
Barn and Harris’s, Shenley) and then a fall of about 20 feet in 
the further mile (between Harris’s and Poplars). If the initial 
rate of rise had been continued to the north-eastern end of the 
section, the base of the Gault would have been some 70 feet 
higher at Poplars than at Harris’s, instead of being 20 feet 
lower; so that the floor at this end of the section falls about 
90 feet below the plane of its southern part. Calculating by 
the same method in the reverse direction, from north-east to south- 
west, we find that the floor at Chamberlain Barn is about 70 feet 
lower than the plane prolonged from the northern half of the 
section. The gentle slopes of the low arch thus indicated are 
somewhat accentuated on both sides towards its crest, which is 
situated under Shenley Hill and has been cut across by the north- 
and-south range of sandpits. The top of the arch shows distinctly 
in the combined section of these pits on the natural scale (fig. 12, 
p- 22); but, owing to the vertical component being so small in 
comparison with the horizontal, it has been found impracticable to 
reproduce the full-length section from Chamberlain Barn to 
Poplars on a small scale, without such excessive vertical exag- 
geration as to destroy its utility.” 

It is clear that the Basement beds and Gault were laid down on 
an uneven surface, which may partly have reflected the original 
heaping-up of the Lower Cretaceous sand-banks, but appears to 
have owed still more of its irregularity to the erosive action of sea- 
currents during the early stages of the Upper Cretaceous trans- 
gression. The conditions, from late-Lower Cretaceous times 


1 The conjectural boundary of the Gault on the Old Series 1-inch Geological 
Survey map (Quarter-Sheet 46 N.W.) in the area north-east of Leighton has 
proved to be incorrect in many places, as was inevitable from the obscurity 
of the ground, the slipping of the clays, and the difficulty of distinguishing 
between Boulder Clay and Gault from surface-indications only. The Gault 
boundary near Heath is now known to lie at least half a mile farther west 
than shown. The district was partly re-surveyed on the 6-inch scale some 
30 years ago, and the boundary corrected so far as was then possible; the 
results have not been published, but are available for reference on manuscript 
6-inch maps in the Geological Survey Library at Jermyn Street. 

2 For the same reason, a still longer section, from the Grovebury pits to 
Poplars, intended to illustrate the upward curve of the Gault and the 
accompanying changes in the character of the Basement beds, has been found 
unadaptable for reproduction. Its purpose can be served by a combination of 
the small figured sections, in the following order (south-west to north-east) :— 
figs. 17, 16, 4, 3, 6, & 14. All these are to the same scale, and their relative 
position is shown on the sketch-map (fig. 1, p. 2). 


part 1] GAULT AND LOWER GREENSAND NEAR LEIGHTON. 55 


onwards until wellinto the Upper Cretaceous, were very favourable 
for the production of such currents, since we have evidence, both 
stratigraphical and paleontological, for the existence of a strait in 
this quarter, connecting a broad Anglo-French basin on the south 
with a broad Anglo-Germanic basin on the north.! 

It is common knowledge that borings in Hertfordshire and the 
Eastern Counties have proved the presence of an eroded Palzeozoic 
floor almost immediately below the Gault, indicating a near shore- 
line to the eastward; while immediately west of Leighton there is 
a double transgression and unconformity, first of the Lower Cre- 
taceous Sands onto the Oxford Clay, and then of the Gault across 
the Sands and likewise across the Jurassic sequence from the Oxford 
Clay upwards, proving an uplift and emergence of a shore-line in 
that quarter also; and the shallow-water current-bedded Leighton 
Sands, accumulated in the strait, have their main development 
between these borders. 


The Silty beds (2).—I do not propose in this communication to 
deal further with the Leighton Sands (1), but some discussion of 
the Siltv beds (2) is necessary, as these beds have been stated by 
Dr. Kitchin & Mr. Pringle to contain the gritty phosphatic 
nodules and to be the parent-bed from which these nodules have 
been washed into the Basement beds of the Shenley sections (K.P., 
pp- 95-8). They propose to divide the Silty beds seen at Miletree 
Farm (fig. 11) into two parts, describing the upper 8 feet as ‘the 
tardefurcata bed,’ though acknowledging that they failed to find 
any paleontological evidence for the correlation (see postea, p. 69). 
I am compelled to dissent from this interpretation, since, as pre- 
viously mentioned (p. 21), I have been unable to find a single 
nodule of the Basement-bed type in the Silty beds. The only 
nodules that they have yielded to my search are of the sandy 
ferruginous type (probably once pyritous), and of the tabular clay- 
stone type, both peculiar to these beds and quite different from the 
eritty phosphatic concretions. It is true that at Miletree Farm 
and elsewhere one can occasionally see, beneath the iron-grit breccia, 
small hollows of erosion scooped out of the Silty beds and filled in 
with the gritty glauconitic loam of the Basement beds, in which are 
embedded a few gritty phosphatic concretions, generally of small 
size; but these clearly belong to the Basement beds, and not to the 
Silts. The gritty loam of the Basement beds is the proper and 
original matrix for nodules of this kind; and the same association 
-of matrix and nodules is characteristic of the Mammillatus Beds of 
the North of France (p. 58). On the other hand, the silty car- 
bonaceous beds are of a type occurring in exactly the same relation- 


l Jukes-Browne postulated the existence, and traced the probable course of 
this ‘ narrow strait or channel, through which a strong current ran from the 
northern to the southern sea’ during the time precedent to the Gault, in 
‘The Building of the British Isles’ London, 2nd ed. (1892), p. 277 & pl. x; 
see also his memoir on ‘The Gault & Upper Greensand’ Mem. Geol. Surv. 
1900, p. 402. 


56 MR. G. W. LAMPLUGH ON THE JUNCTION OF  [vol. lxxvui, 


ship to the coarse sands at Aylesbury,! and among the Lower 
Greensands in other parts of England, as, for example, around 
Ash Wicken in Norfolk?; and such beds are generally, perhaps 
always, devoid of phosphatic concretions, rarely yielding any fossils 
except bits of plants. The composition and mode of occurrence of 
these deposits are suggestive of estuarine conditions produced by 
the local influx of land-waters. 

In the present instance, the deposits indicate a temporary 
cessation of the strong currents which had heaped the Leighton 
Sands into banks. The Silty beds rest sharply on an eroded and 
somewhat undulating surface of these sands, and appear, from their 
rapid variation in thickness, to have filled up the broader hollows 
between the banks. They are confined to an area lying north of 
a north-west and south-east line drawn from the northern end of 
the village of Heath? to the Shenley Hill pits; and, although 
their present termination along this line is irregular, and has to 
some extent been determined by the renewed erosion which preceded 
and accompanied the formation of the Basement beds, there are 
clear indications that the southward limit of their original basin of 
accumulation lay in this neighbourhood. This limit approximately 
coincides with the crest of the low arch of the Gault and its 
Basement beds above-described; and it is along this axis that all 
the beds undergo the changes in composition and structure which 
have lent so much interest to the exposures. 

The core of the belt is composed of cross-bedded ‘Silver Sand,’ 
which appears here to have been heaped up in a long bank or plexus 
of banks forming a shoal, with deeper water on both sides, the 
depression on the northern side being subsequently filled in by the 
Silty beds. 

Tt is all along the top of this old shoal that the patches of 
induration occur, by which the upper part of the Sands is converted 
in places into massive bosses of iron-grit and quartzite, this con- 
dition being prevalent in the pits ranging from Miletree (fig. 10) 
on the east, through Nine Acre (figs. 7, 8) and Garside’s (fig. 4) 
to the old Heath House pit (p. 29) on the west (see map, fig. 1, 
p-. 2). In discussing this peculiar induration in our previous 
paper (L.W., pp. 240-41), we showed that it must have been 
effected, in part at any rate, before the deposition of the Gault, as 
the breccia below the Gault was composed principally of waterworn 
fragments of this material, some encrusted with adherent oysters 
and serpule. The conclusion has been confirmed by all my later 


1 A. M. Davies, ‘ Contributions to the Geology of the Thame Valley,’ Proe. 
Geol. Assoc. vol. xvi (1899) p. 49. See also ‘Special Reports on the Mineral 
Resources of Great Britain, vol. vi, Refractory Materials, &c.’ Mem. Geol. 
Surv. 1911, p. 180. 

2 «Geology of the Borders of the Wash’ Mem. Geol. Sury. 1899, pp. 17-18. 

3 Besides the sections dealt with in § IJ, the Silty beds are seen to a 
depth of 6 or 7 feet at the top of a big sandpit (‘Stone Lane pit’) 700 yards 
north-north-east of Claridge’s pit (fig. 15) or 250 yards beyond the northern 
border of the sketch-map (fig. 1), at the turn of the road leading from Reach 
to Woburn; they form the capping of a deep section in the coarse Sands. 


part 1] GAULT AND LOWER GREENSAND NEAR LEIGHTON. 57 


observations, particularly by the new facts disclosed in Garside’s 
pit, as already described (pp. 10-183) and the similar features 
displayed in the Nine Acre and Miletree pits (pp. 16-19). It is 
significant that this massive induration should occur mainly in the 
neighbourhood of the boundary of the Silty beds; and it is at least 
probable that the mineralizing solutions may have been derived in 
the first instance from the mixed and unstable materials of these 
beds. In shallow-water deposits of this type, if a slight elevation 
ensued after their deposition, so that they were brought under the 
influence of weathering, the waters percolating from the surface 
would become charged, as they still often are, with mineral matter 
in solution, which might be redeposited in the cleaner beds below. 

But whatever may have been the cause of the induration, we can 
see from the sections that it had become effective at an early date; 
so that when, by another change of conditions, there was a sharp 
renewal of current-action on the shoal, and possibly wave-action 
also, during the stages ushering in the great marine transgression 
of Upper Cretaceous times, the old sand-banks yielded very 
unequally to the attack. The shoal, protected by its irregular 
capping of iron-grit, was scoured into crags and rocky ledges, which 
were undermined along their edges, and broken down into a breccia, 
but continued to persist in patches as a dissected reef, until, with 
the deepening of the water, the severity of the attack died down 
and passed gradually into the tranquil conditions required for the 
deposition of the Gault clays. In the sketch-map (fig. 1, p. 2) 
the cross-road from Heath past Shenley House to Clipstone runs 
nearly along the southern margin of the old reef. 

It has already been pointed out how closely this sequence of 
events agrees with that occurring during the same period in North- 
Eastern France, where the floors of Paleozoic and Mesozoic strata 
underlying the Upper Cretaceous rocks have been similarly scoured 
and guttered.1 The conditions are also repeated, on a modified 
scale, at the base of the Red Chalk along the western edge of the 
Yorkshire Wolds (see p. 64). 


The Basement beds (8).—The difficulties and misunder- 
standing which have arisen in respect of the correlation and classi- 
fication of the peculiar and variable beds immediately underlying 
the Gault clays in the sections described, have been caused mainly 
by the difference between the aspect of these beds in the Grovebury 
sections and in the Shenley and other sections about 2 miles farther 
north. This difference, however, is now bridged by the association 
of the two separate types of deposit in the intermediate section 
at Chamberlain Barn pit; and my original surmise as to the 
relationship of the types? is justified. The difference is mainly 
due to the persistence, for some time, of shoal-water and reef- 


1 See footnote and reference, ante, p. 18: also C. Barrois, ‘ Terrain Crétacé 
deg Ardennes, &c.’ Ann. Soe. Géol. Nord, vol. v (1878) pp. 278-80. 
2 Proc. Geol. Assoc. vol. xxvi (1915) p. 310. 


58 MR. G. W. LAMPLUGH ON THE JUNCTION OF _[ vol. xxvii, 


conditions on the north and of deeper water with strong and steady 
currents on the south; and it must be remembered that a distance- 
of 2 miles at right angles to the trend of a coast-line gives ample 
space for great variation in the character of marine sediments. 

About the beds of the Grovebury type, there is no question. In. 
their position, lithological characters, and fossil contents, they are 
recognizable with certainty as the equivalents of the ‘Mammil- 
latus Beds’ of English and French geologists. These beds are- 
everywhere notoriously variable, and their exceptionally wide- 
exposure in these sections affords av unusual opportunity for: 
studying them in detail. The beds are of about the same thick- 
ness as at Folkestone, but preserve, in their well-distributed 
nodules, a somewhat fuller, though still imperfect, record of- the 
life of the period, agreeing more nearly in this and other respects. 
with their equivalents i in the North of F rance, particularly in the 
region of the Argonne. At Folkestone, the fossiliferous phos- 
phatic nodules are almost entirely confined to one band, in which. 
they have been segregated into clusters,! and the accompany- 
ing glauconitic sandy “deposits contain scarcely any fossils. At 
Leighton, although again it is unusual to find ‘anything preserved 
in the elauconitic matrix, the phosphatic nodules stud the beds at 
all ie and contain a wider range of fossils. It is clear that the 
phosphatic concretions have been formed in a matrix like that 
which now surrounds them; but they often show traces of sub- 
marine corrosion and wear, indicating ‘that they have lain exposed 
at times on the sea-floor. They were formed quickly enough to. 
enclose in a fresh state the organic relics which lay on the sea- 
floor, nearly all of which have perished utterly where not thus en- 
closed. All the circumstances imply a lengthy period of very slow 
deposition, with bottom-currents strong enough at times to disturb. 
and winnow the sea- floor, but not strong enough to sweep away 
the nodules. 

The resemblance of these nodules to the coquins de sable 
occurring, sometimes segregated into a band and sometimes. 
scattered, in the Mammillatus Beds of the Argonne is remarkably 
close; as may be judged from the following extracts from the 
description of the French sections by Prof. C. Barrois ? :— 


‘Ces nodules, appelés coquins par les habitants du pays, ont la forme de- 
rognons arrondis, tuberculeux, ils sont compacts, noiratres, et dune couleur 
généralement plus foncée 4 Vintérieur qu’a la surface. ... ces nodules sont. 
rarement homogénes, ils sont pénétrés et souvent recouverts 4 la surface de- 
gros grains de quartz, de glauconie..... Les ouvriers distinguent parfaite- 
ment ces nodules quwils appellent coquins de sable, des nodules de la gaize. 
quils appellent coquins riches’ :— 


And it may be remarked here that at Leighton a corresponding 


1 See my notes on the Copt Point (Folkestone) section in Jukes-Browne’s. 
previously cited memoir on the Gault, &c. (Mem. Geol. Surv.) p. 73. 

2 “Sur le Terrain Crétacé des Ardennes & des Régions voisines’ Ann. Soc. 
Géol. Nord, vol. v (1878) p. 277. 


part 1] GAULT AND LOWER GREENSAND NEAR LEIGHTON. oD 


difference exists between the ‘Mammillatus’ nodules and those at 
the base of the Upper Gault. 

Prof. Barrois’s comments! on the manner of occurrence of the 
nodules are also very apposite. Referring to the segregated. 
nodules at the top of the Mammillatus Beds, he writes :— 

‘Les lits quils forment aujourd’hui dans ces sables ne sont pas dorigine: 
premiére ; ces nodules ont pris naissance isolément dans le dépét des sables, 
puis des affouillements locaux et sous-marins ont déchaussé en divers points 
ces nodules, ont enlevé le sable qui les empatait et laissé sur place sans les 
rouler les nodules et les fossiles trop lourds pour étre enlevés. En un mot, je 
crois les lits de nodules de phosphate de chaux (coquins de sable),. 
remaniés sur place, et indépendamment les uns des autres:’ [and, after 
figuring and describing examples, he adds] ‘on doit croire ici 4 un remanie-- 
ment sur place ; c’est-a-dire, 4l’enlévement des parties tenues argilo-sableuses. 
et 4 Vaccumulation conséquente des nodules plus lourds.’ 


The same conclusion with regard to the mode of accumulation 
of the Cretaceous phosphatic nodule-beds has been expressed by 
many English investigators. Yet, in spite of all, one often still 
finds that such nodules are postulated to be ‘derivative’ or ‘ derived 
pebbles’ as a matter of course, particularly if their fossil-contents. 
happen not to agree with paleontological preconceptions. The 
possibility of the nodules becoming derivative pebbles is obvious, 
though their material is not of a kind to sustain much hard wear ; 
but the weight of existing evidence demands that they should be 
assumed to be in place, or, at least, ‘remaniés sur place,’ unless 
the contrary can be proved; and not vice versa. 

It has long been recognized that phosphatic nodules are indica- 
tive of slow sedimentation (see ante, p. 9); and if there has 
been, in addition, a repeated washing-away of their matrix, with 
resultant concentration of the nodules, we may expect to find, as 
we generally do find, that the organic relics contained within 
narrow bounds represent a condensed fauna which elsewhere may 
tenanta great thickness of sediments of another type. The nodule- 
bearing Mammillatus Beds of the North of France are represented 
in North Germany by a fuller sequence which is capable of sub- 
division, and the later German investigators, on the strength of 
this, have thrown aside the French classification, and have adopted 
a new scheme of zones. In their general scheme our Upper and 
Lower Gault become ‘Upper Gault’; our Mammillatus Beds 
become the upper part of the ‘ Middle Gault,’ with a lower part not 
yet recognized in this country; and most of our Lower Greensand 


1 Ann. Soc. Géol. Nord, vol. v (1878) p. 278. 

2 As, for example, F. G. H. Price, ‘The Gault’ 1879, p. 9; and ‘ Probable 
Depth of the Gault Sea’ Proc. Geol. Assoc. vol. iv (1875) p. 269. A.J. Jukes- 
Browne, ‘The Cambridge Gault & Greensand’ Q. J. G. S. vol. xxxi (1875) 
p. 282; (with W. Hill) ‘The Lower Part of the Upper Cretaceous Series in 
West Suffolk, &e.’ Ibid, vol. xliii (1887) p. 572; and‘The Gault & Upper 
Greensand’ Mem. Geol. Surv. 1900, p. 428. G. W. Lamplugh,‘On.... the 
Speeton Clay’ Q. J. G.S. vol. xlv (1889) pp. 584, 588; ‘The Speeton Series 
in Yorkshire & Lincolnshire’ Ibid. vol. lii (1896) pp. 195-99; ‘ Lower 
Cretaceous Phosphatic Beds, &c.’ Geol. Mag. 1904, p. 551, and Rep. Brit. 
Assoc. 1904, p. 548. 


60 MR. G. W. LAMPLUGH ON THE JUNCTION OF | vol. Ixxviu, 


becomes ‘Lower Gault’!; while in the new zonal scheme the 
Mammillatus Zone is divided between an upper ‘Zone of Ley- 
meriella regularis’ anda lower ‘Zone of Leymeriella tardefurcata.’ 
Both zonal ammonites are common in the Mammillatus Beds of 
Northern France, and both have now been found also in the 
equivalent beds around Leighton Buzzard, L. regularis being 
indeed the most plentiful (or rather, least rare) ammonite of these 
beds. 

Ignoring the old term ‘Mammillatus Beds,’ Dr. Kitchin & 
Mr. Pringle have termed the Leighton deposits ‘the tardefurcata- 
beds,’ which seems to me to be at once unnecessary and unjusti- 
fiable, since this is the name of the lower German zone only, 
whereas both ammonites are present and, so far as we yet know, 
are inseparable, as in France. There is really no excuse for intro- 
ducing this complexity when the old term, with which we have 
become familiar, fits the case more conveniently. 

The discovery of these species at Leighton appears to be their 
first definite localization in England, though tardefurcata has 
been vaguely referred to as occurring in our country.’ 

The term ‘condensed beds’ might be usefully applied, in a com- 
parative sense, to deposits of this type?; and it is evident that 
such beds cannot be subjected to the same refinement of subdivision 
as the synchronous deposits of greater thickness and more regular 
sedimentation. As we shall see, not only the Mammiullatus Beds, 
but the whole of the Lower Gault is more or less ‘condensed’ in 
this region, as it is also in many parts of the North of France, with 
the natural result that the succession of zones traceable in it at 
Folkestone has been found inapplicable here.* 

The big coneretionary lumps which occur at the very top of the 


1 E. Stolley, ‘Die Gliederung der Norddeutschen Unteren Kreide’ Central- 
blatt fiir Min. &c. 1908, pp. 243-47 (note also his remarks on French and 
other nomenclature, pp. 247-50); and, later, E. Stolley ‘ Beitrage zur Kenntnis 
der Cephalopoden der Norddeutschen Unteren Kreide ...1. Die Belemniten 
des Norddeutschen Gaults (Aptiens & Albiens)’ Geol. Palzeont. Abhandl. 
n. s. vol. x (1911) p. 20; this classification is reproduced, with slight modifica- 
tions, by W. Kilian in ‘ Lethzea Geognostica,’ pt. 2, vol. iii, ‘ Kreide,’ sect. 1, 
‘ Unterkreide,’ 3te Lief. 1913, p. 327. 

2 C. Jacob (‘ Etude sur quelques Ammonites du Crétacé Moyen’ Mém. Soe. 
Géol. France, Palzont. No. 38, 1907), in his description of Leymeriella tarde- 
furcata, remarks: ‘ Cette espéce est généralement abondante dans toutes les 
localités ot Von trouve la zone de l’Albien qu'elle caractérise: Angleterre, 
Bassin de Paris, Allemagne, Jura, Alpes frangaises et suisses, etc.’ 

3 J. F. Blake would have included some of these ‘ condensed beds’ under 
his term ‘Aggregate Deposits’ (see his acute and suggestive paper ‘On 
Aggregate Deposits & their Relations to Zones ’ Geol. Mag. 1898, pp. 481-88) ; 
but he defines his term as essentially implying lateral transportation, ‘the 
etymology of the word connoting only the assemblage of materials that have 
been moved horizontally, like a flock of sheep, over the surface of the ground’ 
(p. 484). By the term ‘condensed deposit’ I wish to express the idea of the 
heavier materials on the sea-floor having been let down vertically without 
much lateral shift. The same idea is conveyed by Barrois’s term ‘remaniés 
sur place.’ 

+ A. J. Jukes-Browne, ‘ Gault, &c.’ Mem. Geol. Surv. supra cit. pp. 45, 69. 


part 1] GAULT AND LOWER GREENSAND NEAR LEIGHTON. 61 


Sands immediately below the Mammillatus beds at Grovebury 
(fig. 17), Chamberlain Barn (fig. 16), and in one or two places at 
Nine Acre are quite different in aspect from the fossiliferous 
nodules of the overlying beds, and have as yet yielded no fossils 
whatever, except some vague tubular markings. ‘They are generally 
composed internally of a dense smooth phosphate, pale buff, 
deepening centrally to dark grey or blackish, but often mottled 
with crimson patches, while outwardly they merge without definite 
boundary into their sandy matrix. ‘They may represent only the 
effect of phosphatizing solutions percolating into the sands from 
the overlying beds; but in some places, particularly in the section 
shown in fig. 17, they appear to form a distinct band, separable 
from the beds above and below. ‘They are only seen where the top 
of the Sands has been protected from weathering by a fairly thick 
cover of Basement beds, with which they are certainly i In some way 
connected. When they first made their appearance, some years. 
ago, in the Grovebury sections, I thought that they might prove 
to be the local equivalents of the Shenley limestone-lenticles, but 
in the absence of fossils no such proof is forthcoming. 

The ferruginous components, so conspicuous in the Basement 
beds farther north, are represented at Grovebury only by occasional 
waterworn flat fragments of iron-grit (the largest to come under 
my observation being 6 inches in “diameter), presumably derived 
from the shoal on fie north, and by a few small iron boxstones. 
and thin wisps of imperfect ferricrete. These are the final indi- 
eations of the fading-out of the breccia. 

The correlation of the Grovebury and Chamberlain Barn sections. 
is beyond question (figs. 17 & 16). The pronounced breccia-band in 
the latter marks the closer proximity of the shoal-reef, and proves 
its pre-existence and sharp erosion during an early stage in the 
accumulation of the nodule-beds. The large slabs of. iron-grit 
and other detrital material of the breccia are set in a matrix of 
eritty glauconitic loam indistinguishable from that of the over- 
lying beds, and phosphatic nodules occur occasionally in the breccia 
as well as above it. The patches of calerete and the iron box- 
stones are further indications of its approach to the condition of 
the irongrit-breccia of Shenley, and the only marked difference is 
the absence of unbroken tabular iron-pan. All the circumstances. 
point to the coarse material having travelled down the sloping 
floor of the sea from no great distance, and having come to rest in 
deeper water. Just as the Grovebury type of deposit has its: 
closest lithological analogue in the Mammillatus Beds of the 
Argonne, so the breccia-band of this and other sections has its 
nearest physical analogue in the ‘Tourtias’ of the Flanders. 
country, these ‘ Tourtias,’ as their investigators have long recog- 
nized, marking a condition of accumulation and not a fixed time. 

From the Chamberlain Barn section we pass to the old south- 
western working at Shenley (Garside’s pit, figs. 4 & 12, pp. 11 & 
22) about 1100 yards distant, and find here a cleaner calcareous. 
elauconitic sand, devoid of large nodules, with a strong ‘pan ’-bound. 


62 MR. G. W. LAMPLUGH ON THE JUNCTION OF  [vol. lxxviil, 


breccia at its base, banked around the crags and upon the slopes of 
the reef. It has been argued that the sand here is Upper Greensand 
in an inverted position, but every feature of the section tells against 
‘the supposition. ‘The peculiar breccia, so unlike any other deposit 
‘known in the district, is unquestionably linked with that of 
‘Chamberlain Barn, known to le at the bottom of fossiliferous 
Mammillatus beds; and the difference in the composition of the 
beds above it is no more than these inherently variable beds display 
‘from place to place in other sections. In this particular instance 
the reason for the difference is not far to seek, as the section shows 
that the sand was accumulated among the crags on the side of the 
veef, and presumably was thus sheltered from the full sweep of 
the currents. The calcareous particles mixed with the sand denote 
the wastage of the neighbouring shell-bank, probably already 
partly consolidated. It is possible that some of the sand at this 
‘spot may be equivalent in time to some of the lowest Gault clays 
-of the deeper-water areas, but it is certainly older than the clays 
of the immediate neighbourhood, and may be placed most con- 
veniently, as a whole, in tne Mammillatus Zone. 

In the Garside’s-pit section we reach the margin of the area in 
which the fossiliferous limestone occurs, and the beds of the Grove- 
bury type are wanting or found only in hollows and pockets. 
‘The description of the limestone and its concomitants in our 
previous paper has stood the test of all my later observations, and 
needs no modification in any essential. The conclusions as to its 
-stratigraphical position and mode of origin have been strengthened 
by the new evidence, and it will be shown (in § V) that the 
recently-urged paleontological argument for a hypothetical in- 
version of the beds in relation to the Gault has no foundation in 
fact. The discovery of ‘Ammonites regularis’ and ‘A. ef. beudanti’ 
in the limestone (p. 47) along with some other fossils, has afforded 
a paleontological link with the Grovebur y type of Basement beds, 
while its stratigraphical association with the same beds is now 
established by the new Chamberlain Barn section and that at 
‘Southcott. In our previous discussion we suggested that the 
‘Shenley fauna might be somewhat older than that of the Mammil- 


latus Bed of Folkestone. It is now apparent. however, that the bed 
-at Folkestone does not represent the whole Zone; and the Shenley 


limestone should fall within the Mammillatus Zone in the broader 
sense. J gave reason, in the same discussion, for holding that the 
English Mammillatus beds should be retained in the Lower 
‘Greensand, their original classification, on the grounds of priority 
‘and of stratigraphical convenience |; bute. as then stated, I regard 
‘these matters of conventional Biaccifedtion as of secondary con- 


1 Tt will be noticed that, in his account of the Long Crendon section (quoted . 


‘on p. 41), Jukes-Browne classed the thin ferruginous beds below the Gault 


as ‘ Lower Greensand,’ on stratigraphical grounds; but, if he had been aware 
that they contained fossils lmking them with the Mammillatus beds, he 
would have united them with the Gault as ‘Upper Cretaceous,’ since he 


_adhered to the French classification of the Mammillatus Zone. 


part 1] GAULT AND LOWER GREENSAND NEAR LEIGHTON. 63 


‘sequence if the actual sequence be not prejudiced by them. The 
French geologists always class the Mamimillatus Zone with the 
Gault as Albien, therefore Upper Cretaceous; the German authori- 
ties, as mentioned above, place the zone in the Middle Gault, and 
earry the whole of the Gault into the Lower Cretaceous. 

It is unfortunate that the opportunities for studying the fossili- 
ferous limestone have become very restricted, and are likely before 
long to be lost altogether. The rock is only preserved in an 
unweathered condition where the cover of Gault is thick, which 
implies costly working to reach the Silver Sand, particularly as 
the limestone and its associated irongrit-breccia and loamy beds 
have also to be removed as ‘spoil.’ The sporadic distribution of 
the lenticles adds a further element of uncertainty ; hence a visitor 
to the section can no longer count upon the probability of finding 
the bed exposed. On the other hand, the range of sections 
exhibiting the irongrit-breccia has been greatly enlarged since our 
former communication, and the opportunities for examining it are 
being constantly extended, while the recent appearance of a small 
lenticle of the limestone in the breccia at Poplars pit shows that 
any of these sections may make further disclosures of the rock. 

Sufficient information respecting the local peculiarities of the 
breccia and its continuity over the northern area has been given in 
the preceding sections and descriptions ; the occurrence of a similar 
band at the same horizon many miles away to the west, at Little- 
worth and Long Crendon, has also been described (pp. 89-44) ; and 
it remains only to discuss the general bearing of the facts. By 
Dr. Kitchin & Mr. Pringle the band has been termed ‘ Basal bed 
of the Upper Gault,’ on the supposition that there is a sharp 
overlap of the Upper Gault onto the Lower Greensand in the 
Shenley area; their opinion depends upon their interpretation of 
the Gault sections, with which I cannot agree, for reasons presently 
to be stated. The period of formation of the iron-grit ‘pans’ has 
also been brought into question by the same writers, who claim 
that those which enclose the limestone must be of post-Glacial 
date, a necessary corollary to their opinion that the limestone has 
been transported to its present position by Glacial agency. We 
will consider the evidence on this point first. 

Since the larger fragments in the breccia consist almost entirely 
of flat pieces of iron-grit, more or less waterworn, and occasionally 
encrusted with marine organisms (lL. W., p. 241), it is certain 
that some of this material was in existence in practically its - 
present condition when the breccia was formed. The fragments, 
like the still-continuous ‘ pans,’ show wide variation in texture and 
composition, ranging from an almost pure dense iron-ore (hematite) 
to a coarse grit with comparatively little iron; and the implication 
that the fragments have been derived from the breaking-down of 
pans similar to those still unbroken is borne out by sections (as, 
for example, figs. 7, 8, & 10) in which stages of the process of dis- 
ruption are visible, and by others (figs. 4, 8, 9, & 10) in which the 
contemporaneous existence of ironstone crags is proved. The 


64 MR. G. W. LAMPLUGH ON THE JUNCTION OF  [ vol. lxxviii, 


evidence of scouring and abrasion, to be noted on the upper surface 
of the top pan wherever it has remained unbroken, also shows that 
this layer was in existence as a hard bed before the covering strata 
were deposited. Most of these points were duly noted in our 
previous communication, and all have been confirmed by later 
observations. That there may have been further segregation of 
iron resulting from the weathering of the deposits in comparatively 
recent times also received early consideration (. W., p. 240), and 
more was then assigned to it than the later evidence indicates as 
probable. The main effect of recent weathering appears to have 
tended towards the decomposition of all except the more stable com- 
ponents of the breccia, and not to the induration of any part of it. 
Some of the patchy ferruginous discoloration of the highly porous 
Silver Sand probably marks the course of the iron-solutions per- 
colating from decomposing breccia in late times. 

It is remarkable that this condition of iron-concentration is 
exhibited in almost all the shallow-water deposits accumulated in 
this country towards the end of Lower Cretaceous times and during 
the early stages of the Upper Cretaceous transgression. Thus, in 
the Isle of Wight, we find at this horizon the Carstone or Fer- 
ruginous Sands!; in West Sussex, the crimson grit—a thin band 
of hard ferricrete?; in the Vale of Wardour and other parts of 
Wiltshire, gritty ferruginous beds?; in Oxfordshire, the Hurst- 
Hill and Boar’s-Hill Sands*; in Norfolk and Lincolnshire, the 
Carstone*®; in Yorkshire, at the western edge of the Chalk Wolds, 
patches of ferruginous sand, and, where these are absent, an iron- 
stone breccia in the attenuated Red Chalk. It has been observed 
in all these cases that the ferruginous beds hang in stratigraphical 
continuity with the overlying Upper Cretaceous rocks, but have 
usually a sharp and sometimes transgressive basement.’ Like the 
Tourtias of Flanders, they are evidently not strictly synchronous in 
separate districts, but mark a condition of the encroaching shallow 
sea, when the land was only in part submerged and still yielded 
much waste to the current-swept channels and straits which were 
gradually expanding around its remnants. In some cases the 
present aspect of the beds may be due to the decomposition of 
original glauconite long after their accumulation ; but often it can 


The following references might be greatly expanded, but will serve as 
general indications to the literature :— 

1 «Geology of the Isle of Wight’ Mem. Geol. Sury. 2nd ed. (1889) pp. 52-59. 

2 ‘Geology of the Country near Chichester’ Ibid. 1903, pp. 12, 16. 

3 *Cretaceous Rocks, &c. vol. i—Gault & Upper Greensand’ Ibid. 1900, 
p. 228; and ‘Geology of the Country South & East of Devizes’ Ibid. 1905, 

. 8-12. 

a ‘Geology of the Country around Oxford’ Ibid. 1908, pp. 75-78. 

> A. Strahan, ‘On the Lincolnshire Carstone’ Q. J. G.S. vol. xli (1886) 
pp. 486-92. 

6 «Geology of the Country North-East of York & South of Malton’ Mem. 
Geol. Surv. 1884, pp. 25-26; and ‘ Special Reports on Mineral Resources, &c. 
vol. xii: Iron Ores (contd. )—-Bedded Ores, &c’ Ibid. 1920, pp. 207, 208. 

7 A. Strahan, op. swpra cit. pp. 489-90, and ‘ Geology =o the Isle of Wight” 
op. supra cit. p. 53. 


part 1] GAULT AND LOWER GREENSAND NEAR LEIGHTON. 65 


be proved that their ferruginous condition was attained before they 
were covered by the newer sediments. In several instances it may 
be surmised with probability that the beds were originally glau- 
conitic, and that the decay of this constituent was brought about 
not long after their deposition, by some physical change by which 
the material was exposed to weathering agencies. It is likely 
enough that slight oscillations accompanied the general downward 
movement of the period, and that the shoals and sand-banks of the 
shallow sea may at times have been raised above low tide, if not 
above all tides.! The iron-pan and breccia above the Shenley shoal 
suggest conditions of the lateritic type, possibly in operation above 
or between tide-marks, or in very shallow water. I am not qualitied 
to speculate upon the chemical history of the processes; but an 
examination of the structures in the field shows clearly enough 
that the iron-pan has been produced, not directly as a precipitate 
or sediment, but by the replacement of some constituents of 
an existing deposit and the cementation of the rest. All the evi- 
dence goes to show that it was in practically its present condition 
before the clays of the Gault were laid down, and that, wherever 
it remained unbroken, it protected the beds beneath it from the 
scour of the currents which swept the strait until well into Gault 
times. 

During this heavy scour no sediment could lodge on the bare 
smooth ironclad surface of the reef; consequently we find that the 
glauconitic sands wedge out on its flanks (fig. 12, p. 22) and that 
the gritty glauconitic loams, etc., with their phosphatic nodules. 
are preserved only in the hollows and gullies by which the edge of 
the reef is broken, and in the tract on the south where the water 
was deeper. Fortunately, the lenticles of limestone on the top of 
the reef afford us a glimpse of its fauna at the beginning of these 
events, a fauna consisting for the greater part of forms adapted 
for rocky ground, such as the anchoring brachiopoda and the usual 
mollusca, echinoderms, and crustaceans of the ‘ reef-facies’; but, 
after this, the sequence is locally broken until the waters above 
the shoal became deep enough and still enough to allow a clay- 
mantle to be spread over it. 


The Gault Clays (4 & 5).—Having supposed that the Gault 
of the district, as a whole, had been already fairly well elucidated 
by the work of previous observers, particularly of Jukes-Browne & 
Hill,? I have not, until recently, devoted much attention to any 
except its lower portion. However, in consequence of the statement 


1 Many features of these Basement beds have their analogues in the 
ferruginous Dogger at the base of the Oolites in Yorkshire, which shows the 
same local variability on a bigger scale, the same association and changes 
between ferruginous and phosphatic-nodular beds, the same ‘ Tourtia’-like 
conditions in its relation to the underlying strata, and similar paleontological 
evidence for ‘ condensed’ deposition. The patchy local concentration of iron- 
ore in the Dogger, though on a much larger scale, is also comparable. 

2 «The Gault & Upper Greensand’ Mem. Geol. Surv. 1900, chaps. xix & xx. 


.J.G.S. No. 309. F 


66 MR. G. W. LAMPLUGH ON THE JUNCTION OF [vol. lxxviii, 


of Dr. Kitchin & Mr. Pringle that the Upper Gault alone is 
present in sections north of Leighton, and that the irongrit-breccia 
accompanies this division only, it has been necessary for me during 
the past year to re-examine as much of the Gault in the neighbour- 
hood as is accessible. The investigation has convinced me that 
the Lower Gault is continuous over the whole area, and is succeeded 
conformably by the Upper Gault, but with a long pause in the 
sedimentation, marked by a band of phosphatic nodules, at or near 
the base of the Upper Gault. 

Leaving controversial points for later discussion (pp. 70, 78), I 
will br iefly review the features of the Lower Gault as exhibited 
in the sections. Taking the incoming of ‘ Inoceramus sulcatus’ i 
association with ‘Ammonites rostratus’ as their upper boundary, 
the Lower clays (excluding the Mammillatus beds) appear in this 
neighbourhood rarely to exceed 15 feet in thickness, and may in 
places diminish to 10 feet, as compared with their 27 or 28 feet at 
Folkestone. This diminished thickness is maintained for a con- 
siderable distance southwards, judging from the description by 
Jukes-Browne of a boring at Slapton Lock, 3 miles south of 
Leighton, in which we read ! :-— 


‘It would appear that there is a nodule-bed about 10 feet from the base of 
the Gault; among the fossils preserved I identified the following, Ammonites 
interruptus, Am. cristatus, Am. rostratus, Am. varicosus (2), Inoceramus 
sulcatus, Inoc. concentricus, Inoc. tenuis (?)’ 


—a similar assemblage to that which the same investigator collected 
from the nodule-bed in the Heath brickyard (p. 27), and to that 
of the equivalent nodule-bed at Harris’s pit (p. 7). 

This reduction of the Lower Gault as we approach the shelving 
coast-line of the period has its exact counterpart in the North of 
France where, at Wissant, its thickness is only 16 feet (with 
2 feet of basement Mammillatus beds below, almost exactly as 
at Shenley) and still less in the country to the east, but expands to 
over 30 feet in the Argonne region, and to over 100 feet in parts 
of the departments of Marne, Haute Marne, and Aube; remaiming 
throughout its range, however, always very much thinner than the 
Upper Gault.” 

The sections afford a reasonable explanation for the thinness of 
the Lower Gault in the Leighton district. Everywhere, except on 


1 Op. supra cit. Mem. Geol. Sury. p. 279; the authors, however, did not 
recognize that the Lower Gault was comprised in the clays below the nodule- 
bed, and thought, with some misgivings, that it must include some higher 
clay or marl. In their conjectural classification of another boring, at Gubble- 
cote, 6 miles south of Leighton, they allow 145 feet to the Lower Gault; 
but I suspect that they should have confined it to the 22 feet of ‘dark 
brownish clay, becoming sandy and glauconitic below’ which has a ‘ layer of 
phosphatic nodules’ above it, and has beneath it, 3 feet of ‘ blue sandy clay 
with phosphatic nodules at the base’ (ibid. p. 282). 

2 C. Barrois ‘Sur le Gault dans le Bassin de Paris’ Ann. Soc. Géol. Nord, 
vol. ii (1875) pp. 1-61; and ‘ Terrain Crétacé des Ardennes’ Ibid. vol. v 
(1878) pp. 227-487; A. J. Jukes-Browne, ‘The Gault & Upper Greensand” 
Mem. Geol. Surv. 1900, chap. xxvii. 


part 1] GAULT AND LOWER GREENSAND NEAR LEIGHTON. 67 


the eminences capped by smooth iron-pan, the bottom layers of 
clay (4a), for several inches up, are sprinkled with coarse grit and 
small pebbles ranging up to the size of peas, which denote the 
continuance of current-action, less forceful indeed than during 
the accumulation of the underlying Basement bed, but yet strong 
enough to retard the sedimentation of clayey matter. Consequently 
these few inches of gritty clay, usually quite unfossiliferous, may 
represent a time-equivalent as long as, or longer than, that of the 
10-ft. ‘interruptus-bed’ (Price’s I) of Folkestone; and their 
absence on the knoll-tops may indicate the lapse of the same period 
without sedimentation. In the overlying gritless clays (4), the 
presence of strings of undisturbed brown-coated phosphatic nodules 
is still indicative of slow sedimentation, and the scantiness and 
crushed condition of the fossils point the same way, as most organic 
remains require quick burial for their preservation. So far as 
the scanty fossil-evidence goes, these gritless clays probably repre- 
sent, more or less imperfectly, Beds II to VI of Price’s Folkestone 
classification ; but it is hardly likely that all the Folkestone sub- 
divisions can be separately distinguished in this condensed 
sequence, just as it has been found impracticable in the similarly- 
condensed sequence of the North of France. As in France, too, 
‘Belemnites minimus’ is the mostabundant fossil of the beds, and 
is rare or absent in most of the Upper Gault. 

Besides this deficiency towards its base, the Lower Gault shows 
also an arrest of development at its top, which must have had a 
further effect in reducing its thickness, though at this horizon the 
Folkestone section has suffered similarly, as Price has impressively 
shown.! The phosphatic nodule-bed exposed near the top of 
ITarris’s pit (fig. 8, p. 7), and less clearly in evidence in other 
sections, has its equivalent, as already shown (p. 9), at approxi- 
mately the same horizon at Folkestone (Price’s Bed VIII, ‘the 
Junction-bed’), where identical conditions are implied. It¢ is likely 
enough, however, that these conditions were not absolutely syn- 
chronous in two spots so far apart, as their belt of impact probably 
moved from place to place under the influence of the progressive 
changes. The ‘compound’ character of the nodules and the way in 
which many have been abraded, and scarred by adherent organisms, 
while others are comparatively intact, afford proof of original slow 
deposition, of subsequent winnowing-away of the matrix, perhaps 
more than once, and of time for the regrowth of concretionary 
matter around the old nuclei. We are, in short, dealing with a 
‘condensed deposit’; and the assemblage of fossils found elsewhere 
at separate levels, first commented on by Jukes-Browne,? is thus 
to be accounted for. The mixture appears to be somewhat greater 
than at Folkestone, which may imply either an earlier beginning 
and longer persistence of the conditions, or the winnowing-away 


1 «The Gault’ op. supra cit. p. 9. 
2 «The Gault & Upper Greensand’ Mem. Geol. Surv. 1900, pp. 275, 278, 
235. 
F2 


68 MR. G. W. LAMPLUGH ON THE JUNCTION OF [ vol. lxxviu, 


of a greater thickness of clay. The fossils of the nodules include 
many that are characteristic of the lower part of the Upper Gault; 
therefore, as at Folkestone, it is a convenient ‘ Junction-bed’ for 
the subdivisions. 

It is particularly to be noted that no extraneous pebbles have 
been found in this nodule-bed. Indeed, its clay-matrix is not even 
gritty; and the condition and position of the ammonite-casts 
mentioned in the earlier part of this paper (footnote, p. 9), 
together with the very variable size and shape of the nodules, 
afford further proof that the sea-currents at this stage were too 
weak to shift any heavy material. 

The absence of pebbly matter and of any fragments of iron-grit 
marks the sharp difference between this bed and the Basement-bed of 
the Lower Gault. In no part of the district, nor indeed elsewhere 
in England, so far as J am aware, has any bed resembling the 
irongrit-breccia been found intercalated between the Upper and 
the Lower Gault. 

The higher part of the Upper Gault requires no discussion in 
this paper. So far as seen, it is quite different in aspect and fossils 
from the clays with which I have dealt (see p. 37). 


V. DiscussION OF THE ‘OVERTURN’ HyYPporHESIS. 


I may now, before concluding, enter into a particular consider- 
ation of the points of issue raised by Dr. Kitchin & Mr. Pringle, 
although most of these have, I think, been incidentally met and 
controverted by the foregoing descriptions. The supposition that 
the Gault, limestone, and glauconitic sand at Shenley Hill have been 
inverted by Glacial action is based mainly upon a paleontological 
argument, to explain the presence of some fossils in the limestone, 
that were not previously known to occur below the English Gault. 
The anomalous character of the fossil-assemblage in this respect 
was duly discussed and, I think, adequately explained in our 
previous paper; but our explanation has been set aside. In support 
of their paleontological argument, Dr. Kitchin & Mr. Pringle 
have brought forward some stratigraphical considerations, which I 
will deal with first. 


The stratigraphical argumeut.— Some fundamental 
points telling strongly against the inversion-theory were stated in 
my letter to the Geological Magazine (May 1920, pp. 234-37), 
close upon the publication of the hypothesis. The gist of these 
may be re-stated in brief as follows :— 


(1) The absence of any rock resembling the peculiar Shenley limestone, 
either in composition or in fossils, from any part of the outcrop above the 
Gault in the Chiltern escarpment, and the entirely different character of the 
known beds at this horizon. 

(2) The unweathered condition of the thin cakes of limestone and of the 
wedge of loose glauconitic sand, which are supposed to have lain at the 
surface before being overturned. 


part 1] GAULT AND LOWER GREENSAND NEAR LEIGHTON. 69 


(3) The absence of any trace of Drift material along the supposed dis- 
ruption-plane or in the overlying mass, and the lack of any structural 
indication of Glacial disturbance in the mass. Also the improbability that a 
slab of soft Gault clay, greensand, etc., not less than 15 acres in area and 
18 feet or more in thickness, could be turned over, pancake-fashion, and 
dropped back onto a flat bed, without disruption or entanglement with Glacial 
material. 

(4) The presence of a floor of unbroken iron-pan immediately above the 
limestone, with evidence that it was in the same position before the Gault 
was deposited (antea, p. 63). 


To these objections I may now add :— 


(5) The absence, so far as is known, of relics of similar limestone 
in the Boulder Clays of the district. 


(6) The presence of similar limestone, containing some similar 
fossils, in a bed immediately below the Gault at Long Crendon 
(p. 41); also at Southcott (p. 38) and Littleworth (p. 39). 

(7) The extreme improbability, in view of its general arrange- 
ment in the region,! that the Upper Gault around Shenley Hill 
was, as required by the inversion-hypothesis, ‘little more than 
40 feet in thickness’ (K.P., p. 108) when the Lower Chalk was 
deposited. 


Next, to consider some points of detail :— 


(8) I am unable, for reasons previously stated (p. 55), to 
accept the statements (1) that the upper part of Silty beds seen 
at Miletree Farm [and in other pits north of Shenley Hill] is 
equivalent to the so-called ‘ tardefurcata bed’ and contains gritty 
phosphatic nodules similar to those which elsewhere have yielded 
the Mammillatus-fauna, and (II) that nodules of this kind in 
the overlying beds have been ‘derived’ from it. The Silty series 
is quite different in composition, and was accumulated under 
different conditions, from the gritty glauconitic loams of Grove- 
bury and Chamberlain Barn in which, as in France also (p. 58), 
these nodules are always found and are reasonably held to have 
originated. No paleontological evidence is adduced for the pro- 
posed correlation ; indeed it is definitely stated— 


‘We could not break open a sufficient number of the nodules to enable us 
to obtain any of the characteristic ammonites, although a few other fossils 
[not specified ] were found’ (K. P. p. 55). 


The unnecessary difficulties raised by the proposed correlation 
affect the whole account given by Dr. Kitchin & Mr. Pringle 
of the Miletree-Farm and neighbouring sections. 


(9) The paleontological argument for the assumption that the 
Lower Gault is absent in the Shenley and neighbouring sections is 


1 Borings south of Leighton have proved the full thickness of the Gault 
to be about 230 to 250 feet, of which probably not more than 20 to 25 feet 
is Lower Gault (antea, footnote, p. 66). See A. J. Jukes-Browne, ‘ The Gault 
& Upper Greensand’ Mem. Geol. Surv. 1900, pp. 279, 284. 


70 MR. G. W. LAMPLUGH ON THE JUNCTION OF  [vol. Ixxvin, 


accompanied by arguments based on the lithology, preluded by the 
general statement— 

‘It is a matter of surprise to us that this clay [in Harris’s pit] has even 

been mistaken for Lower Gault’ (K.P., p. 13). 
This, however, merely expresses a personal impression, which is 
not likely to be shared by anyone who examines all the exposures of 
the Gault in the district, and notes the very different aspect of the 
Upper Gault in the undisputed sections farther south (see p. 37). 
The ‘true Lower Gault of the district,’ with which the lower clay 
of Harris’s pit is unfavourably compared (K.P., p. 13) on account 
of ‘the absence of sand and the rarity of glauconite,’ can only 
refer to the 4 or 5 feet of gritty clay above the Mammillatus beds 
in the Grovebury pits, since the clays of all the other sections 
which. they describe are assigned by Dr. Kitchin & Mr. Pringle 
to the ‘Upper Gault.’ But this peculiar gritty condition is 
confined to the lowest bands only of the Lower clays; it is present 
all round the Shenley reef in the lowest layers of the Gault-filled 
hollows, as I have shown, and is absent only on the bare top of the 
reef (p. 67). So far as I can judge, the Lower clays above these 
eritty layers are alike in all the exposures, with only such insigni- 
ficant differences as one expects to find in passing from place to 
place. 

In correlating the various sections of supposed ‘Upper Gault,’ 
Dr. Kitchin & Mr. Pringle lay great stress upon two lthological 
peculiarities as proofs of age :— 

(i) ‘conspicuous small white nodules (white throughout) is a point to be 
‘specially noted, since, owing to the rarity of fossils, the presence of these 
“nodules is helpful in identifying these beds [that is, ‘ Upper Gault Clay’ and 
‘Basal bed of Upper Gault’] in other sections’ (K.P., p. 57); and (ii) ‘im- 
‘perfectly bedded, crumbled, grey clay ....of a peculiar type... . made up 


‘ of innumerable small pellets, which are for the most part somewhat angular 
‘in shape. It might be termed a finely brecciated clay’ (K.P., p. 58). 


So far as I can discover, the ‘small white calcareous nodules’ (1) 
referred to are the knots of ‘race,’ produced by recent weathering 
and segregation of lime where the calcareous clays approach the 
surface. This ‘race’1s exhibited in varying quantity, usually at a 
depth of 2 to 5 feet, in the top clay of most of the Gault sections, 
without relation to the stratigraphical horizon; it was plentiful 
in the clay just above the Mammillatus bed in Pratt’s pit at 
Grovebury (p. 35), occurring also in other sections at this level 
(p. 30); it is most abundant where the beds have been affected 
by “creep.: 

The ‘brecciated clay’ (11) appears to be the superficially dis- 
turbed clay c of my sections) seen above the bedded Gault in 


most places where the clay-deposits come to the surface. This 
structure is the normal result of ‘surface-creep’ or ‘trail’ in 
breaking up and incorporating soft beds in the lower part of the 
‘creep’; wherefore its supposed origin as ‘the redeposited débris 
resulting from the denudation of some previously formed, well- 


part 1] GAULT AND LOWER GREENSAND NEAR LEIGHTON. 71 


consolidated clay’ (K.P., p. 58) may in one sense be allowed, but 
not in the sense implied by the authors, who consider that the 
structure is original and indicative of a definite horizon in the 
Gault. In such shallow sections as those in which the ‘race’ and 
the brecciated structure are seen, one could hardly expect to find 
soft calcareous clays of the Gault type in their original condition, 
particularly on sloping ground. 

The interpretation of the thin continuous band of Basement beds 
around Shenley as being ‘a remarkable collocation of heterochronous 
elements’ (K.P., p. 12), composed in part of post-Glacial iron-grit, 
in part of Glacial gravel, in part of Cenomanian limestone, in part 
of Upper Greensand, and in part of an Upper Gault Basement bed 
containing derivative Lower Greensand fossils, is so entirely 
dependent upon the paleontological argument that it requires 
further consideration only from the paleontological standpoint, 
which I shall now attempt. 


The paleontological argument.—As bearing on the 
general aspect of the fossil-evidence, we may recall that this is at 
least the third time in English geology that fossiliferous beds 
occurring below the Gault have been held, on paleontological 
grounds, to be newer than the Gault. First, the Faringdon Sponge- 
Gravels, notwithstanding previous opinion, now accepted, were 
supposed by T. Davidson,! on the evidence of their brachiopoda, to 
be possibly of Upper Greensand age; and by D. Sharpe,’ because 
of their polyzoa, to be newer even than the English Chalk, and 
probably ‘Danian.’ Particular stress was laid by both authors on 
comparisons with the Tourtias of the Continent; but, as Caleb 
Evans? pointed out, the argument might be legitimately reversed 
by questioning the reputed age of the Tourtias—themselves of 
uncertain antecedents. 

Again, some 380 years later, the presence of the Gault in part of 
West Norfolk was challenged by C. Reid & G. Sharman,* who 
argued that the calcareous clay in question was the Chalk Marl, 
mainly on the ground that— 

‘not a single characteristic Gault form occurred, but that there were 
several species which have not been recorded from below the Lower Chalk,’— 
the unquestionable Gault species in a phosphatic-nodule bed at the 
base of the clay being all regarded as ‘derivative.’ The challenge 
was answered effectively by A. J. Jukes-Browne & W. Hill,® who 


1 “Monogr. Brit. Cret. Brachiopoda’ pt. ii, p. 3, Pal. Soc. 1852. 

2 “On the Age of the Fossiliferous Sands & Gravels of Faringdon & its 
Neighbourhood’ Q. J. G. 8S. vol. x (1854) pp. 176-98. 

3 ‘Sketch of the Geology of Farringdon’ Geol. & Nat. Hist. Repertory, 
containing Proc. Geol. Assoc. No. 16, Aug. 1866, pp. 33-40. 

4 “On the so-called “Gault” of West Dereham, in Norfolk’ Geol. Mag. 
1886, pp. 55-59. 

5 “Note on the Gault & Chalk Marl of West Norfolk’ Geol. Mag. 1886, 
pp. 72-74; and ‘On the Lower Part of the Upper Cretaceous Series in West 
Suffolk & Norfolk’ Q. J. G. S. vol. xliii (1887) pp. 547-49 & 571-74. 


72 MR. G. W. LAMPLUGH ON THE JUNCTION OF  [Vvol. lxxviii, 


showed that the Gault forms were proper to the bed and that the 
clay was stratigraphically distinct from the Chalk Marl. The 
bearing of this discussion on the present case is close, not only by 
reason of the stratigraphical horizons involved, but also because 
most of the fossils dealt with in the argument are species found 
at Shenley, and now again brought into question: such as— 
‘Inoceramus concentricus’ with ‘crippsi’ and allies, ‘Ostrea 
vesicularis,’ ‘ Belemnites minimus’ and vars., ete. 


The limestone fauna.—The argument for the ‘Ceno- 
manian’ age of the lmestone is not based on a consideration of 
the fauna as a whole, but on certain species the occurrence of 
which is supposed to be impossible below the Gault; other species, 
to be regarded as neutral, are not discussed ; and those which tell 
distinctly against the hypothesis, if mentioned at all, are considered 
‘without doubt’ to be derivative. I think that it will bring the 
matter into better perspective if I summarize the evidence in each 
Class, and add particular comments on the species which have 
been used in the argument as they come under review. 


CEPHALOPODA.—Of the ammonites (six specimens in all), those recogniz- 
able belong to two forms proper to the Mammillatus Zone and not known to 
occur above it. The evidence against the supposed derivative origin of the 
identified specimens has been already stated (p. 47). The belemnites (two 
specimens) are probably a form of Neohibolites occurring in the same zone 
(p. 47). 

GASTROPODA (see p. 47).—No specific identifications have been made, and 
no argument raised. The assemblage will probably be found to present a 
‘Lower Gault’ facies, and to have its equivalents among the rich fauna of 
this Class recorded from the Mammillatus Beds of Northern France.! If there 
had been any recognizable ‘ Cenomanian’ forms among them, I think that they 
would have been mentioned in the discussion. 

LAMELLIBRANCHIATA (see p. 47).—Only four species received positive 
determination in our original list (L.W., p. 263), and all are species known 
to range downwards in the Lower Greensand, at least as low as the Hythe 
Beds, ard upwards into the Chalk. They happen to embrace the commonest 
fossils of their Class in the limestone. Of the six more or less doubtful 
determinations in our original list, three were species not previously known 
above the Lower Greensand, and the other three were known only in or above 
the Gault. One of the latter, Lima globosa, has since become a positive 
determination, and will be further commented on. 

Of the five or six new determinations (pp. 57-58), Inoceramus concentricus 
is important in strengthening the relationship of the bed with the overlying 
clays; while Pteria pectinata, an abundant fossil at Shenley, has a wide dis- 
tribution, ranging from the Hythe Beds up into the Chalk, but is commonest 
in the Lower Greensand.2 These two species are not referred to by Dr. 
Kitchin & Mr. Pringle, who quote the four following in support of their 
argument :— 

(i) Pecten cwrvatus, ‘ which occurs in the Upper Greensand of Great Haldon 
and in the Chloritic Marl’ (K.P., p. 5). Regarding this small pecten, Mr. H. 


1 See list in C. Barrois, ‘Terrain Crétacé des Ardennes,’ Ann. Soe. Géol. 
Nord, vol. v (1878), pp. 269-75. 

2 H. Woods, ‘Monogr. Cretac. Lamellibr.’ vol. ii, pt. 2, p. 59, Pal. Soe. 
1905. 


part 1]. GAULT AND LOWER GREENSAND NEAR LEIGHTON. 73 


Woods notes! that the species, for which English material is scanty, ‘ closely 
resembles P. striato-punctatus Romer, a shell of wide range in the Lower 
Cretaceous and doubtfully present in the Gault of Folkestone, where it is 
recorded by Price as occurring in the ‘Junction Bed’ VIII.2. The Shenley 
shell may be found to correspond to the Gault form. 

(ii) Cyprimeria rotomagensis, ‘a species of the basal Cenomanian of Wiltshire’ 
(K.P., p. 5), is, I believe, a somewhat doubtful determination. It is, at the 
best, a rather featureless shell, and is usually preserved as casts. ‘The genus 
has two species in the Lower Greensand of the Isle of Wight, one of which, 
CO. parva,is near to this form. The known range of the genus in the English 
Cretaceous rocks is from the Atherfield Beds to the Lower Chalk.* 

(iii) Isoarca obesa, ‘ which is not known elsewhere from below the Chloritic 
Marl’ (K.P., p. 5). This species is regarded as synonymous with Isocardia 
orbignyana A. dArchiac, occurring in the Flemish Tourtias.4 A. J. Jukes- 
Browne, in his General List of Chalk Fossils,> marks it as ‘found in 
Selbornian,’ but does not include it in the ‘Selbornian’ List in his previous 
volume,® where the only species mentioned is Isoarca agassizi P. & R. Pre- 
suming that the identification is unquestionable, the presence of the species 
at Shenley increases its hitherto known range. 

(iv) Lima globosa. Since the downward range of this fossil through the Gault 
is admitted, it seems beside the mark to note that ‘the type.... came from 
the Chloritic Marl’ (K.P., p. 6). The Lower Chalk is the limit of its upward 
range. The species is recorded by Price’ from near the bottom of the Lower 
Gault (Bed II) at Folkestone. Mr. Woods ® remarks that it ‘ closely resembles 
Lima albensis d’Orbigny,’ a species mentioned by Prof. C. Barrois ® as occurring 
in the Mammillatus Beds as well as in the Lower Gault of the North of France. 
There is nothing incongruous in the presence of Lima globosa in the Shenley 
limestone. 

Dr. Kitchin & Mr. Pringle bring also into the argument two species of 
small oyster, which I found in the wedge of greensand in Garside’s pit 
(pp. 10-12), believed by them to be inverted ‘ Upper Greensand.’ They mention 
them as ‘ well-developed valves of Ostrea vesicularis Lamarck, such as are 
‘found in the zone of Pecten asper Lamarck; also Ostrea canaliculata (J. 
‘ Sowerby), a species which occurs much more commonly above the Gault than 
‘below it’ (K.P., p. 6). But both species, believed by Woods to be allied,'® 
begin their long range in the Lower Greensand, O. canaliculata being found 
as low as the Hythe Beds, so that their presence below the Gault, whenever 
conditions favoured their growth, might be anticipated and certainly needs no 
abstruse explanation. 

On the other hand, the absence, so far as known, of the commonest 
oysters of the Upper Greensand, O. vesiculosa and Exogyra conica, is awkward 
to explain on the overturn-hypothesis. 

As for the limestone, if it were ‘ Cenomanian,’ we ought to find in it some 
of the commoner lamellibranchs of the period; as, for example, Pecten asper, 
P. beaveri, Lima aspera, L. elongata, Pholadomya decussata, etc.; all, as yet, 
missing. 


1 Tbid. vol. i. pt. 4, p. 161 (1902). 

2 <The Gault’ p. 56. 

3 H. Woods, op. cit. vol. ii, pt. 9 (1918), Tables, pp. 439, 447. 

4 Thid. vol. 1, pt. 1 (1899) p. 65. re 

> “The Cretaceous Rocks: vol. iii—The Upper Chalk’ Mem. Geol. Surv. 
1904, p. 477. 

6 «The Cretaceous Rocks: vol. i—The Gault, &c.’ Mem. Geol. Surv. 1900, 
p. 467. 

7 ‘The Gault’ p. 54. 

8 Op. supra cit. vol. ii, pt. 1 (1904) p. 17. 

9 «Terrain Crétacé des Ardennes’ Ann. Soc. Géol. Nord, vol. v (1878) p. 274. 
° Op. supra cit. vol. ii, pt. 9 (1913) pp. 360-78. 


74: MR. G. W. LAMPLUGH ON THE JUNCTION OF  [ vol. lxxviui, 


Brachiopoda.—Respecting this Class, it is first necessary for me to take 
exception to an implication by Dr. Kitchin & Mr. Pringle that the late 
J. F. Walker was swayed by stratigraphical considerations in naming these 
fossils. Their statement is—‘ They [that is, Lamplugh & Walker] met the 
‘ difficulty by explanations which we have always considered to be inadequate, 
‘ while they treated as so-called “‘ varieties” some of the species which appeared 
‘to occur so far below their usual horizon. Arguing from stratigraphical 
‘inferences, they claimed to demonstrate that these species in reality made a 
‘much earlier appearance and possessed a much longer vertical range than had 
‘previously been suspected. We need make no lengthy comment here on this 
‘manner of dealing with the paleontological aspects of the bed, ete. (K.P., 
Dp. 5.) 

But Walker’s attitude throughout, in working on the brachiopoda, was the 
reverse of that imputed to him. He recognized from the first that these 
fossils had their nearest analogues in those of the Tourtias, and he was ready 
to stretch the comparison to bring them within the limits of established 
species (L.W., p. 247). His previously quoted letter (p. 46) is sufficiently 
explicit in this matter, and others of his to me have the same tenour; as, for 
instance, ‘ The Zeilleria are very difficult. I don’t want to make new species 
unless obliged’ (letter, December 28th, 1902), and ‘I fear we shall have to 
make a new species of the Terebrirostra’ (October 13th, 1902, and a similar 
phrase with regard to the same form in a later letter). 

I know that especial importance has been assigned to the last-mentioned 
fossil, eventually described and figured by Walker as Terebrirostra lyra 
(Sowerby) var. incurvirostrum nobis, and I will therefore discuss it further in 
this connexion. T.lyra had long been prized asa rare and curious fossil 
both in this country and abroad, and the discovery of a shell of this type in 
unexpected abundance, along with another rarity, Terebratula capillata, in 
even greater abundance, in the Shenley limestone, provoked Walker’s imme- 
diate interest, and induced him to enter vigorously into the investigation of 
the bed. Apparently Terebrirostra had never before been represented by such 
richness of material from one place, exhibiting manifold growth- and varietal- 
phases, and this rendered comparison with the previous scanty material more 
difficult. There is a species, Terebratula arduennensis d’Orbigny, closely 
resembling lyra, occurring in the Mammillatus Beds of the Ardennes ! and in 
some of the Tourtias,? respecting which Walker wrote to me (January 16th, 
1903): ‘I want a Terebrirostra ardwennensis to compare with ours. I have 
written to [a foreign correspondent]; he is trying to get me one. But, 
apparently, the attempt was unsuccessful, as no example of the species is to 
be found in Walker’s rich collection of foreign brachiopoda now in the 
Natural History Museum ; and the critical comparison, much to be desired, is 
probably still lacking. 

It is, at any rate, certain that a Terebrirostra near to the Shenley and Long 
Crendon fossil has its horizon in the Basement-beds of the Gault in France; 
and among the five species of this aberrant genus recognized by A. d’Orbigny, 
one occurs in still older beds, namely, T. neocomiensis d’Orbigny, found in 
the Lower Cretaceous.? Consequently, there is nothing astonishing in the 
appearance of the fossil where we have now found it. 

The following analysis will serve to show the general aspect of the 
brachiopod-fauna of the limestone. 


1 ©. Barrois, ‘ Terrain Crétacé des Ardennes’ op. cit. p. 275. 

2 «Note sur.... Terebrirostra,-&c.’ Journ. Conchyliol. vol. ii (1851) p. 222. 
T. neocomiensis is figured by Zittel to illustrate the genus, in ‘ 'Text-Book of 
Paleontology ’ Eng. transl. London, vol. i (1900) p. 331. Another of A. d’Or- 
bigny’s five species, T. canaliculata, is from the Tourtia; but Davidson con- 
siders that this form is a true Terebratella, and not a Terebrirostra: see 
‘Monogr. Brit. Cret. Brachiopoda’ Pal. Soc. pt. ii (1852), footnote on p. 32. 


75 


part 1] GAULT AND LOWER GREENSAND NEAR LEIGHTON. 
Of the 28 species and varieties named in our previous paper (L.W., 
p. 262) :— 


5 had been found previously in England only in the Lower Greensand ; 
but two of these are recorded from Continental Tourtias, and other 
two from the Mammillatus Beds of Northern France. 

2 had been found as rare fossils in Lower Greensand and also in the 
lower part of the Upper Cretaceous of England. Both are Tourtia 
shells. One of them, Terebratula capillata, is the commonest fossil 
of the limestone. 

8 (or, including Terebrirostra lyra var., 9) fad been found only above the 
Lower Greensand, four occurring in the Gault or Red Chalk and the 
remaining at (gists horizons only. But one or two of the eight are 
now recognized as occurring in the Upper Aptian of the South of 
France.! The Tourtias are known to contain six of the eight. 

4 were new to England, but were previously known in one or other of 
the Tourtias. 

9 were described as new species or varieties. 


28 


In their argument Dr. Kitchin & Mr. Pringle mention five of the species by 
name (K.P., p. 4), but do not refer, except in general terms, to the other 23. 

Among the eighteen or twenty provisional determinations, in addition to 
the above, resulting from Walker’s later work (antea, p. 45), not more than 
two or three are species known i in this country ; but nearly all are Tourtia 
forms. 

So far from this, the most abundant and most closely studied element of 
the fauna, requiring the idea of an overturn, it appears to afford actual 
disproof of the hypothesis. 

Correlation with the Tourtias is useless for any narrow and critical demar- 
cation of age, since the Tourtias are known to occur at different horizons 
where actually intercalated in the Cretaceous sequence ; and, where they form 
the base of that sequence and rest directly on the much older rocks, they 
are ‘condensed’ deposits, probably covering a long period, and they then 
generally contain many fossils not known to occur in beds which lie above 
the Gault where it is actually present. 

The absence from Shenley of the common species of the Upper Greensand 
and Lower Chalk has already been commented on (p. 45). 

The hypothesis of the intermingling of two limestones of different horizons, 
one presumed to contain, besides its own fossils, others derivative from the 
Lower Greensand, raises many real difficulties in the place of an imaginary 
one; and has no basis, so far as the brachiopods are concerned. 

Terebratula capillata deserves a further word, as its history is typical of 
that of several of the commoner brachiopods of the limestone. The form 
occurs as a rarity in the Lower Greensand of Upware and in the Red Chalk. 
It is plentiful in several of the Tourtias, and is found also in the French 
Mammillatus Beds. Its profusion at Shenley, where it is ‘in full evolution,’ 
exhibiting a very wide range of varieties (L.W., p. 249), led Walker to propose 
that we should distinguish the limestone as ‘ the Zone of Terebratula capillata,’ 
which he regarded as the characteristic fossil of the bed (L.W., p. 250). 
The shifting sand-banks of the Lower Cretaceous strait afforded no oppor- 
tunity for the establishment or preservation of a ‘ reef-facies’ in the locality 


1 W. Kilian, ‘ Lethea Geognostica: II, Das Mesozoicum’ vol. iii, 1913. 
The species are Terebratula dutempleana d’Orbigny (p. 306), and (somewhat 
doubtful) Rhynchonella grasiana d’Orbigny (pp. 309, 361). Another species 
mentioned is Rhynchonella lineolata Phillips (pp. 307, 361), but this was 
already known as a Lower Cretaceous form in England. 


76 MR. G. W. LAMPLUGH ON THE JUNCTION OF  [vol. lxxviu, 


until the ferruginous induration of the Shenley shoal provided a suitable 
habitat. This was soon occupied in force by T. capillata and its congeners ; 
and they throve there until the oncoming of the Gault clay-conditions, which 
put an end to them in this place. The Gault is notoriously poor in brachio- 
pods, particularly the Lower Gault clays, which usually contain none. This 
species and the Terebrirostra, among others, evidently reached their acme at 
about the beginning of the Gault period, although they lingered for some 
time longer before extinction. 

ECHINODERMATA (p. 48).—The list of the Shenley fossils of this Class is 
suggestive of a horizon above the Gault, but the anomaly is not so tangible 
in reality as in print. The only material, except a few spines and other 
fragments in my own collection, is the Walker collection at Cambridge, 
containing about a score of ill-preserved tests and some spines, which are 
acknowledged to be generally too poor for confident determination,! and 
where confidence can be expressed, it is based on one, two, or at the most 
three, specimens. Our original list (L.W., p. 263) comprised four positive 
specific identifications ; one of these has been reduced by a ? and another by 
an aff. as the result of the recent expert re-examination. Without venturing 
to question the authoritative determinations, I will submit some comments 
on the species, which appear to be pertinent. 

The echinoid fauna of the Gault clays is scanty, and of a different facies 
from the shallow rock-reef fauna of Shenley. The English deposits, therefore, 
have hitherto provided us with little or no information as to the ancestry and 
progress of the echinoid ‘ reef-facies ’ during and just before the period of the 
clay-sediments. The Tourtias of the Continent, however, contain many relics 
of this facies ; and it is in them, and not in the English beds above the Gault, 
that comparisons with the Shenley forms should be sought and may, I think. 
be found. 

Catopygus columbarius (a single specimen), one of the smaller echinoderms, 
is (I believe) regarded as of prime consequence in the argument for the post- 
Gault age of the limestone. In the West of England it is a well-known fossil 
of the top beds of the Upper Greensand and the lowest part of the Lower 
Chalk ; but a very similar fossil occurs in the Lower Greensand, and is recorded 
as C. carinatus Goldfuss (supposed to be a synonym of C. columbarius) from 
the Hythe Beds of Hythe.? C. carinatus is also recorded from the lowest 
zone of the Albian of the South of France, from beds which we should class 
as Lower Greensand.* C. columbarius is a common fossil in some of the 
Tourtias,* and a form near to it is recorded from the ‘ Sables Verts de Denne- 
broecq’ occurring in association with Ammonites tardefurcatus and other 
fossils of the Mammillatus Beds.® The type, if not the actual species, appears, 
therefore, to have been in existence before the period of the Gault clays. 

Nucleolites lacunosus Goldfuss (1 specimen), another small echinoderm, ‘ one 


1 H. L. Hawkins, ‘ Note on a Collection of Echinoids from the Limestone- 
Lenticles in the Sand-Pits of Shenley Hill’ Geol. Mag. 1921, pp. 57-60. 

2 “Geology of the Weald’ Mem. Geol. Surv. 1875, p. 413. 

3 ©. Jacob, ‘ Etudes, &c. sur la Partie Moyenne des Terrains Crétacés, &e.’ 
(op. cit. antea, p. 49). 

4 A.d’Archiac, ‘Sur les Fossiles du Tourtia’ Mém. Soc. Géol. France, ser. 2, 
vol. ii (1874) pl. xiii, and G. Cotteau, ‘ Note sur les Echinides Crétacés de la 
Province du Hainault’ Bull. Soc. Géol. France, ser. 3, vol. ii (1874) p. 652. 

5 H. Parent, ‘ Sur l’Existence du Gault, &c.’ Ann. Soc. Géol. Nord, vol. xxi 
(1893) p. 218. The fossil is recorded as ‘ Catopygus cf. cylindricus Desor, with 
an appended note by M. Lambert: ‘ Cette espéce se rapproche surtout des 
C. carinatus Agassiz du Cénomanien et C. cylindricus Desor de |’ Albien,’ &c. 
A. J. Jukes-Browne in his ‘Gault’ memoir (op. supra cit. p. 477) places 
C. carinatus as synonymous with C. columbarius. C.cylindricus is a charac- 
teristic echinoderm of the Upper Aptian of the South of France (see W. Kilian, 
op. supra cit. pp. 309, 362). 


part 1] GAULT AND LOWER GREENSAND NEAR LEIGHTON. 77 


of the few confident identifications ’ (Hawkins), appears to have been found 
previously in England only in and just above the Upper Greensand; and 
Echinospatagus aff. murchisonianus (Mantell) (1 specimen) in the Upper 
Greensand only. Both genera have many representatives in the Lower 
Cretaceous. 

Cardiaster ? latissimus Agassiz (1 specimen) and C. ef. fossariws (Benett) 
(2 specimens) are the largest urchins that have been found at Shenley, 
measuring 13 to 2 inches in diameter. Prof. Hawkins states that his 
suggested identifications are ‘ given without any real confidence.’ The first- 
named species is a rarity in the Upper Greensand; the second-named is one 
of its commonest fossils ; but in North-Eastern France Holaster (= Cardiaster) 
latissimus is recorded by Jules Lambert as occurring occasionally in the 
upper part of the Aptian (of Grandpré) and more generally in the Gault;! and 
in the South of France it is recognized by Kilian (op. cit. pp. 309, 362) as an 
Aptian species. With respect to the Shenley forms, it is of importance to 
note that an echinoid of similar type occurs in the undoubted Mammillatus- 
beds at Grovebury. I collected there, from a gritty phosphatic nodule, an 
imperfect cast (see p. 50), and another specimen of the same kind was 
obtained by the Geological Survey ; Prof. Hawkins refers my specimen doubt- 
fully to C. aff. latissimus Agassiz (op. cit. p. 57) and notes that the Survey 
specimen, though not specifically determinable, agrees exactly with mine in 
form and matrix (op. cit. p. 60). From the same nodules I obtained the casts 
ot Pseudodiadema comparable with one of the unidentifiable limestone fossils 
(op. cit. p. 57). 

Pyrina ef. inflata @Orbigny (1 specimen); P. cf. desmoulinsi d’ Archiac 
(2 specimens); P. aff. levis Agassiz (8 specimens). These small urchins 
may, according to Prof. Hawkins (antea, p. 48), be shape-variants of a 
single species ; which presumably is the same as the P. desmoulinsi of some 
of the Tourtias. The genus as at present defined is fully represented in the 
Lower Cretaceous, so that its appearance under suitable conditions at the 
base of the Gault is not anomalous. It may be noted that Pyrina cylindrica 
A. de Grossouvre is recorded by C. Jacob (op. cit. p. 309) from the lowermost 
beds of the Albian of Southern France. 

Cidaris bowerbanki Forbes, is based, not on a test, but on three spines, and 
the determination can hardly be reckoned conclusive. 

Conulopyrina anomala Hawkins, being a new genus and species (Geol. Mag. 
1921, p. 420) has no present value for correlation. 

There is nothing among these echinoderms that seems to justify violent 
stratigraphical methods of explanation. The collection being poor, negative 
evidence has less weight here than in the case of the brachiopods ; but the 
absence of most of the commoner Upper Greensand and Lower Chalk species 
would be remarkable if the limestone were really newer than the Gault. 


The CRUSTACEAN Cyphonotus incertus Bell, mentioned by Dr. Kitchin 
& Mr. Pringle, represents a comparatively rare type of fossil, respecting which 
it must be unsafe to draw deductions from the scanty and imperfect material 
at present available. 


In arguing for the Upper Greensand age of the glauconitic sand of Gavr- 
side’s pit, the same writers mention Serpula antiquata J. de C. Sowerby as 
‘a common fossil in the Pecten-asper zone of Wiltshire’ (K.P., p. 6), but do 
not mention that it is also a fossil of the Lower Greensand and Red Chalk.” 
They also refer at some length to cirripede-remains from the same bed which 
were submitted to Mr. T. H. Withers, who ‘ believes that the more advanced 


1 «Recherches sur les Echinides de lYAptien de Grandpré’ Bull. Soc. Géol. 


France, ser. 3, vol. xx (1892) p. 89. 

2 “Geology of the Weald’ Mem. Geol. Surv. 1875, p. 13; ‘Gault & Upper 
Greensand’ ibid. 1900, p.476 ; and T. Wiltshire, ‘The Red Chalk of England’ 
Geol. Assoc. 1859, p. 15 & fig. 


78 MR. G. W. LAMPLUGH ON THE JUNCTION OF | vol. Ixxviii, 


evolutionary character shown by the valves from Shenley Hill indicate a 
later geological age’ [than the Gault]. The two species identified have a 
long range in the Gault, as well as later; and there may well be in this case 
some misapprehension of the supposed evolutionary characters. 


The Gault fauna.—The paleontological argument about the 
Gault centres around two points :— 


(1) The supposed inversion of the clays under Shenley Hill; and (ii) the 
supposed overlap of the Lower by the Upper Gault in other sections. 
On both points the evidence brought forward by Dr. Kitchin & 
Mr. Pringle in their paper is singularly weak. 


(1) To support their statement that the dark lower clays of the 
Harris’s-pit section (fig. 3) are newer than the overlying pale clays, 
they depend upon three fossils of the lower beds (K.P., p. 13) :— 
an Inoceramus ‘suggestive of I. crippsi’; ‘small impressions of 
the characteristically ornamented Nautilus deslongchampsianus 
VOrbigny’; and the compressed ammonite of the ‘ auritus ’-stock 
identified as Hoplites catillus (see antea, pp. 51-52). 

The Inoceramus I have already discussed; it is not the form 
defined as J. crippst by Mr. H. Woods in his recent monograph,! 
although it has sometimes carried that name, but is a form 
common in the lower part of the Gault (antea, p. 51). The 
Nautilus may or may not be correctly identified ; it is a small 
crushed impression, without shape, and hardly comparable with 
the figured type; and the species has not (so far as I am aware) 
been previously recorded from any part of the Gault clays in 
England. 

The weight of the argument has apparently to rest mainly upon 
the crushed ammonite, and it is unfortunate that the species should 
be one of considerable difficulty. It is discussed at some length by 
HK. T. Newton & A. J. Jukes-Browne in the ‘ Paleontological 
Appendix’ to the ‘Gault & Upper Greensand’ memoir (op. eit. 
pp. 443-45), who note, among other points, that ‘the flatness 
exhibited by so many specimens is evidently in most cases due to 
compression after embedment, and is not an original character.’ 
Jukes-Browne confined his determination to a form occurring in 
the Upper Greensand, and did not recognize the species as occurring 
in the Gault clays (op. cit. p. 458). Dr. Kitchin & Mr. Pringle 
identify their crushed Shenley specimens with the Upper Greensand 
form, and conclude that ‘they are members of an easily recognizable 
hoplitid group which have reached an evolutionary stage char- 
acterized by degeneration of the sculptural features.’ But the 
characters requisite in the specific determination are precisely those 
most likely to be simulated in specimens compressed to the condition 
of ‘films on the bedding-surfaces,’ as the Shenley examples are. 
The questionable Shenley ammonite belongs to a sub-genus ranging 


1 “Monogr. Brit. Cretaceous Lamellibranchia’ vol. ii, pt. 7, pp. 273-78, 
Pal. Soc. 1911. 


part 1] GAULT AND LOWER GREENSAND NEAR LEIGHTON. 79 


through the Lower and most of the Upper Gault, and it occurs 
where we should normally expect to find something of this type 
on the analogy of the Folkestone section, in which Price notes 
(‘The Gault,’ op. cit. p. 15) that his ‘ Bed II’ contains ‘a variety 
of Am. awritus, having long tubercles, which may perhaps be a 
distinct species.’ My tuberculate specimen crushed in the vertical 
plane (see p. 52), poor as it is, could not be called ‘ catillus,’ and 
indicates the presence of an undegenerate Hoplitid in the anti- 
cipated position. The determination of the catillus-species on 
such material must be open to doubt; and, whatever it may be, 
the fossil is not strong enough to bear the strain put upon it. 


In the foregoing notes I believe that I have dealt with every 
species relied upon by Dr. Kitchin & Mr. Pringle in their argu- 
ment. It only remains to be pointed out that the preceding 
descriptions have shown that the lower clays at Shenley Hull 
contain the usual abundance of ‘ Belemnites minimus’ and crushed 
‘Tnoceramus’ which characterizes the lowest part of the Gault 
clays all through this region; and that they have yielded none of 
the undoubted Upper Gault fossils which occur so plentifully in 
the overlying band of pale clay with phosphate-nodules. 

I conclude that the Shenley clays are not inverted, but in 
normal sequence. - 


(11) On the second point there is little more to be said than has 
been already implied. The fact that Lower Gault fossils were 
formerly obtained from the Heath House sections (p. 27) prac- 
tically answers the whole of the argument. Having assumed that 
the Lower Gault was absent there, Dr. Kitchin & Mr. Pringle 
throughout their paper have been led to apply the term ‘ Upper 
Gault’ not only to this section but to all the lowest clays of the 
sections around Shenley. The only reasons stated, so far as I can 
find, are :— 

‘We infer that the Gault in situ at Shenley Hill was originally some 40 feet 

‘thick at the most ; and we consider that the presence of the fauna of Bed IX 
‘of Folkestone such a short distance up in the series makes it certain that 
‘only the Upper Gault is represented here’ (K.P., p. 60)—[an argument with 
which I have already dealt, antea, p. 66 ; and, referring to the Miletree-Farm 
section | :—‘ We have nowhere seen any exclusively Lower Gault fossil in it’ 
CGP; p. 57): 
But the latter statement requires to be amplified by the addition 
of ‘nor any exclusively Upper Gault fossil,’ since it is mentioned 
that only Belemnites minimus was seen in the clay of this section, 
and only the same fossil and Inoceramus concentricus in the re- 
maining pit (Garside’s) in which the ‘ Upper Gault overlap’ 1s 
postulated. 

It is true that our knowledge of the full Gault sequence in 
Bedfordshire and Buckinghamshire is still very imperfect; but I 
think that it has been confused, and not advanced, by the proposed 
new interpretation of the Shenley sections. 


80 MR. G. W. LAMPLUGH ON THE JUNCTION OF [ vol. xxviii, 


VI. SumMMARY. 


The paper, a continuation of one by the Author and the late 
J. F. Walker published by the Society in 1903, describes about 
twenty sections exhibiting the base of the Gault in sandpits and 
other excavations around Leighton Buzzard and westwards at South- 
cott, Littleworth, and Long Crendon. 


(1) The variable ‘ Basement beds’ of the Gault are ‘condensed ’ 
deposits, strongly influenced by local conditions like the ‘ Tourtias’ 
of Flanders, and falling mainly within the ‘zone of Ammonites 
mamuillatus’ as recognized in Northern France (= zones of A. 
regularis and A. tardefurcatus of a later German classification). 


(2) The evidence bears out Jukes-Browne’s suggestion of the 
occurrence of a current-swept strait in this quarter during late 
Lower Cretaceous times, uniting northern and southern sea-basins. 
During the accumulation of the ‘ Basement beds,’ a shoal in this 
strait north of Leighton formed a reef capped by ferruginous ‘ pan ’ 
and breccia, with lenticular patches of shell-limestone preserving 
a fauna of ‘reef-facies,’ while the deeper water to the south 
gathered a stratum of gritty glauconitic loam and clay with 
fossiliferous phosphatic nodules of the French ‘ coquins de sable’ 
type. ‘The transitional stages are visible in the sections. 


(3) The dark clays above the ‘ Basement beds’ belong to the 
Lower Gault, here reduced to about half its thickness at Folkestone, 
the same reduction being exhibited also towards the opposite edge of 
the basin, in Northern France. These clays rest sharply on the 
ironstone ‘pans’ of the reef, but usually pass downwards by gritty 
intercalations into the glauconitic loams. Fossils other than 
‘Belemnites minimus’ and ‘Inoceramus concentricus’ are scarce 
and in a poor state, but are in agreement with the stratigraphical 
evidence. 


(4) The incoming of the Upper Gault, with keeled ammonites of 
the ‘rostratus’ group and ‘Inoceramus sulcatus,’ is indicated in three 
of the sections, of which, however, two are at present obscure. A 
band of corroded phosphatic nodules, like those of the ‘ Junction- 
Bed’ at Folkestone, occurs near the base of the division, and 
marks a pause in the sedimentation. This band has yielded many 
fossils. 


(5) The paleontology of the deposits is discussed, and is held to 
be in general agreement with that of the same succession in Northern 
France. 


(6) A recent suggestion that the beds at Shenley Hill may have 
been inverted by Glacial agency is fully considered, and shown to 
be untenable. 


[March 31st, 1922. } 


JAMES SWIFT & SON L™: 


Makers of the highest grade Microscopes 
and Accessories for Peirological Research 


Just out 


NEW ILLUSTRATED CATALOGUE 
OF 
PETROLOGICAL MICROSCOPES 
INCLUDING NEW MODELS 


A copy will be sent gratis on request. 


University Optical Works 
81 TOTTENHAM COURT ROAD, LONDON, W.1 


Publications to be had of the Geological Society, 
Burlington House. 


QUARTERLY JOURNAL (Vols. III to LXXVII, inclusive). Price to 
Fellows, £1 0s, 0d. each: £1 5s. Od. in cloth. [Postage 1s.] Separate parts (2,5, 
7, 9, 55, & 273 are out of print) 7s. 6d. each: price to Fellows 5s. [Postage 5d.] 


GENERAL INDEX TO THE FIRST FIFTY VOLUMES OF THE 
QUARTERLY JOURNAL (1845-1894). Part I (A-La). Part II (La-Z), 
Price 7s. 6d. each. To Fellows 5s. each. [Postage 6d. each. | 


GEOLOGICAL LITERATURE added to the Geological Society’s Library 
during the years ended December 31st, 1894-1913 and 1920. Price 5s. each. To 
Fellows, 3s. 9d. each. [Postage 7d.] (Nos. 8 & 14 are out of print.) 


THE HISTORY OF THE GEOLOGICAL SOCIETY OF LONDON, 
by H. B. Woopwarp, F.R.S. Price 3s. net. [Postage 1s.] 


THE CENTENARY OF THE GEOLOGICAL SOCIETY OF 
LONDON, recorded by W. W. Watts, F.R.S. Price 2s.. [Postage 5d. ] 


 HUTTON’S ‘THEORY OF THE EARTH,’ Vol. III, edited by Sir 
ARCHIBALD GuIkiz, D.C.L.,F.R.S Price 3s, 6d. To Fellows, 2s. [Postage 9d.] 


THE GEOLOGY OF NEW ZEALAND. Translated by Dr. C. F. 
FIscHER from the works of MM. HocustertTerR & PETERMANN. With an Atlas of 
Six Maps. Price 10s. net. [Postage 9d. ] 


CONTENTS. 


Pages 
Proceedings of the Geological Society, Session 1921-22, including the Pro- 


ceedings at the Annual General Meeting, the President's Anniversary 


Addrentt, GG, .2aso1 <iqes cums wsdacatzse es te ahaget ai eas me eee ys aaa i-lxviii 
PAPER READ. 
Page 
1, Mr. G. W. Lamplugh on the Junction of Gault and Lower Greensand near 
Leighton Buzzard... soi is 45000: ciue aceway sour cecte res sn0nee en: op gee een 1 


[The Editor of the Quarterly Journal is directed to make it known to the Public 
that the Authors alone are responsible for the facts and opinions 
contained in their respective Papers. | 


*,,* The Council request that all communications intended for publication by the 
Society shall be clearly and legibly written on one side of the paper only, with 
proper references, and in all respects in fit condition for being 
at once placed in the Printer’s hands. Unless this is done, it will be in 
the discretion of the Officers to return the communication to the Author for revision. 


The Library at the Apartments of the Society is open every Week Day from Ten 
o'clock until Five, except on Saturdays (when the hours are from 10 to 1), and 
except during the fortnight commencing on the first Monday in September, 
when the Library is closed for the purpose of cleaning, 


Fellows are reminded that they are not entitled to receive the 
Quarterly Journal regularly, unless a subscription at the rate of 
10s. per volume is paid in advance. Those Fellows who do not 
require the complete series can obtain separate parts at the price 
of 3s. each, if applied for within one year of the date of issue. 


P 
| 


| 


Vol. LXXVIIL. No. 310. 
Parr 2. 


( 


QUARTERLY JOURNAL 


OF THE 


GEOLOGICAL SOCIETY. 


EDITED BY 


(With Five Plates, illustrating Papers by Mr. FE: iley, 
Miss G. L. Elles, and Dr. L. J. Wills & Mr. B. string 


LONDON: 
LONGMANS, GREEN, AND CO. 


PARIS:—CHARLES KLINCKSIECK, 11 RUE DE LILLE. 
SOLD ALSO AT THE APARTMENTS OF THE SOCIETY. 


Price Seven Shillings and Sixpence. 


SP SE eee Oe’ 


BdbOt VE 2 th4dts VPA ststdO SAINT UOVUINYIL YP itits “oe 


GEOLOGICAL SOCIETY OF LONDON. — 


LARA RA RRA RRA 


Elected | February 17th, 1922. 


ween 


Presivent. 
Prof. A. C. Seward, Sc.D., F.R.S., F.L.S. 


Gice-Presidents. 
Prof. Edmund Johnston Garwood, M.A..,) | George Thurland Prior, M.A., D.Sce., 


Se.D., F.R.S. F.R.S. 
ceed Dixon Oldham, F.R.S. Herbert Henry Thomas, M.A., Se.D. 
Secretaries. 
Walter Campbell Smith, M.C., M.A. | James Archibald Douglas, M.A., B.Se. 
Foreiqu Secretary. Creasurer. 


Sir Archibald Geikie, O.M., K.C.B., D.C.L., | Robert Stansfield Herries, M.A. 
LL.D., Se.D., F.R.S. | 


COUNEIL., 
Frederick Noel Ashcroft, M.A., F.C.S. Prof. Owen Thomas Jones, M.A., D.Sc. 


Francis Arthur Bather, M.A., D.Sc., F.R.S.| William Bernard Robinson King, O.B.E., 


Prof. Percy George Hamnall Boswell, M.A. 


O.B.E., D.Sc. | Richard Dixon Oldham, F.R. S. 
Prof. William S. Boulton, D.Se., Assoc.) George Thurland Prior, M.A., D.Se., 
R.C.Se. F.R.S. 


Thomas Crosbee-Cantrill, B.Sc. 


John Smith Flett, O.B.E., M.A., LL.D.,| F.R.S., F.L.S. 
D.Se., M.B., F.R.S. Walter Campbell Smith, M.C., M.A. 


Prof. Edmund Johnston Garwood, M.A.,| Sir Aubrey Strahan, K.B.E., Se.D., LL.D., 


Se.D., F-R.S. F.R.S. 
Sir Archibald Geikie, O.M., K.C.B.,D.C.L.,| Herbert Henry Thomas, M.A., Se.D. 
LED: Se.Di F.R:S. Prof. William Whitehead Watts, M.A., 
John Frederick Norman Green, B.A. Se.D., LL.D., FERS. 


Frederick Henry Hatch, O.B.E., Ph.D. | Henry Woods, M.A., F.R.S. 
Robert Stansfield Herries, M.A. 


Permanent Pecretary. 
L. L. Belinfante, M. Se. 
Librarian. Clerk. 
Arthur Greig. M. St. John Hope. 


STANDING PUBLICATION COMMITTEE, 
Prof. A. C. Seward, President. 
Mr. W. Campbell Smith. 


Maa A banlie. Secretaries. Mr. R.S. Herries, Treasurer. 
Prof. P. G. H. Boswell. Dr. F. H. Hatch. Sir Aubrey Strahan. 
Prof. W. S. Boulton. Prof. O. T. Jones. Dr. H. H. Thomas. 
Dr. J. S. Flett. Mr. R. D. Oldham, Prof. W. W. Watts. 
Prof. E. J. Garwood. Dr. G. T. Prior. Mr. H. Woods. 
Mr. J. F. N. Green. Prof. S. H. Reynolds. 


ORDINARY MEETINGS OF THE GEOLOGICAL SOCIETY 
TO BE HELD AT BURLINGTON HOUSE. 


Session 1922-19993. 


1922. 
Wednesday, November ..............0.00005 8*—22 
= Weacewmber 3 ye eee 6 —20* 
1923 
Wednesday, fanuary - 25.2.3 5..e 10*—24* 
- (Anniversary, Friday, 
Feb. 16th) February...... 7*—28* 
We Sta 2 ool s Hg ran Onc re 14 —28* 
ae Aprile ey ote Ae se eee 18* 
be 1 Ey Re Pepe Pee I ee 2 —16* 
hiHe 68 oso ue eee ee» 6 —20* 


The asterisks Menote the dates on which the Council will meet. 


Prof. Sidney Hugh Reynolds, M.A., Se.D. 
James Archibald Douglas, M.A., B.Se.. | Prof. Albert -Charles Seward, Se.D., 


part 2] GAULT AND LOWER GREENSAND NEAR LEIGHTON. 81 


DIscuUSssION. 


Prof. H. L. Hawxktrys, speaking as an unrepentant paleontolo- 
gist, congratulated the Author on the lucid and almost convincing 
expression of his views. From a careful study of the Echinoid 
fauna of the ‘limestone lenticles,’ the speaker had come to the 
conclusion that a Cenomanian facies was definitely indicated; and 
this was the opinion of the majority of specialists who had 
examined other groups of fossils from those deposits. His view was 
that, had stratigraphers chanced to agree as to the Cenomanian 
age of the masses in question, no further interest (of a horizonal 
nature) would have attached to the fossils. As matters stood at 
present, there seemed to be a clear issue between stratigraphy and 
paleontology : to put back the time of appearance of a single species 
would be justifiable, but to treat practically an entire fauna in 
such a manner vitiated the principle of ‘time-indication’ by the 
evidence of fossils. Hence he hesitated to accept the Author’s 
conclusions, although, apart from the paleontological evidence, 
they appeared reasonable and even obvious. 

Mr. J. Prinexe regretted that no new facts had been brought 
forward, and said that he would like to defer his criticism until the 
paper was published. He would remark, however, that he thought 
that the interpretation put forward by Dr. F. L. Kitchin and him- 
self was the correct interpretation of the facts. 

The AvurHor, in reply, said that he had expected that the 
eritics of the previous work of himself and the late J. F. Walker 
would have embraced the opportunity to support their strictures. 
Since they had not done so, there was little scope for discussion. 

The paleontological argument for the supposed inversion was 
out of perspective, through his critics having insisted on par- 
ticular species and on their upward range, without having 
mentioned also their downward range and the presence of other 
species not favourable to their views. The echinoderms had beeri 
dealt with, along with all the other fossils brought into the argu- 
ment, in a portion of the paper which there had not been time 
to read. 

No paleontologist could suppose that his material, however 
plentiful, represented the final limits of our knowledge; and he 
must occasionally have to meet facts novel to his. experience. 
Purely paleontological methods of explaining the facts regarding 
the fossils were available, and could be applied, without recourse 
to an indefensible Glacial overturn. 


Onde 6-6) No. 310: - a 


82 MR. E. B. BAILEY ON THE STRUCTURE OF  [vol. Ixxviil, 


. 2. The StRucTURE of the Souru-WeEst H1eGHLanns of ScOTLAND. 
By Epwarp Barrrrsspy Baitzy, M.C., B.A., F.R.S.E., 
F.G.S. (Read March 28rd, 1921.) 


[PLatTE I—GzEoLoGicaL Map.] 


CONTENTS. 


Te eintrodte tions 2.45588. sco tae tence es Se ee 82 
Statement of the Problem. 
Clough’s Secret of the Highlands. 
Slides recognized. 
| Nappes and their Travels. 
Direction of Movement defined and discussed. 
Metamorphism. 
Corrigenda since 1910. 
: (1910) Recumbent Folds in the Schists of the Scottish 
Highlands. 
(1912) The Glen-Orchy Anticline (E. B. Bailey & M. Mac- 
gregor). 
(1913) The Loch-Awe Syncline. 
(1914) The Ballachulish Fold near the Head of Loch 
Creran. 
(1917 for 1916) The Islay Anticline. 


iM, ‘The "Three Great Structural Diyisions:...5.2..5:.c.s<o.-F eee 95 
The Loch-Awe Nappe. 
The Iltay Nappe. 
The Ballappel Foundation. 
Suggested Stratigraphical Correlations. 
III. Critical Discussion of the District reaching from Ard- 
mucknish to den Mang, o.oo tances ante he eee 108 
Appin and Ballachulish Folds. 
Contact of the Ballappel Foundation and the Iltay Nappe 
at Loch Creran. 
The Ballappel Foundation, north-west of the Thrust. 
The Thrust. 
Iltay Nappe, south-east of the Thrust. 
Outcrop of the Iltay Nappe north of the Loch-Awe Nappe. 
Ballappel Foundation and Iltay Nappe, north-west of the 
Loch-Awe Nappe. 
Loch-Awe Nappe. 
Ballappel Foundation and Iltay Nappe, north-east of the 
Loch-Awe Nappe. 
Ben-Lui Fold and the Thrust at the Base of the Loch-Awe 


_Nappe. 
VERE oneliston tye th iss cok 35. Hs vaec es aee ea e 125 
Appendix [— Bibliography ».212s4c045 95a eee 125 
Appendix 1——Locality-Imdex 22... :<:.95:2..as ee 127 


I. LyrRopucrion. 


Tue gradual development of structural interpretation in regard to 
the South-West Highlands of Scotland has much in common with 
the long-continued building of some great cathedral. The workers 
have been many, each with his own individuality, and each in- 
fluenced by the spirit of his time. In these circumstances, 


part 2] THE SOUTH-WEST HIGHLANDS OF SCOTLAND. 83 


preparatory demolition has been an almost inevitable feature of 
every attempt at improvement. But one is emboldened to the 
task by the thought that restoration can be carried out if occasion 
proves its need: a fairly representative literature enshrines the 
observations and opinions of the long line of researchers.! 


Statement of the Problem. 


As a preliminary to discussion, the following dogmatic statement 
is offered. The schists of the South-West Highlands of Scotland 
belong to three main structural divisions, which, in descending 
order, are (fig. 1, p. 84 & Pl. I) :— 

Loch-Awe Nappe, 


Iltay Nappe, 
Ballappel Foundation. 


The first title is derived from Loch Awe. The second is a hybrid 
of Islay and Loch Tay. The third is compounded from Balla- 
ehulish, Appin, and Loch Eilde. 

Each great division has its particular stratigraphical facies, 
although there are certain correspondences which suggest strati- 
graphical correlations from one to another. 

The Loch-Awe Nappe is relatively simple in structure. The 
Iltay Nappe includes two important recumbent folds—the Ben-Lui 
Fold, a syncline closing towards the north-west, and the Carrick- 
Castle Fold, an anticline closing towards the south-east (fig. 4, 
p. 102 &Pl.1). The Ballappel Foundation is a structural complex, 
with the Ballachulish and Appin Nappes among its component 
parts. During the development of these various structures, move- 
ment took place persistently towards the south-east. 

Little would be known of all the nappes and recumbent folds 
just mentioned, were they not bent and buckled by what we may 
term secondary folding. The secondary folds of the South- 
West Highlands are picked out in fig. 1. They include such 
conspicuous examples as the Cowal Anticline, Loch-Awe Syncline, 
Islay Anticline, ete. In regard toa large part of the region, it 
may be stated that the secondary movements were directed out- 
wards in both directions from the axis of the Loch-Awe Syncline. 
The Loch-Skerrols? Thrust of Islay belongs to the epoch of 
secondary movement which developed the Islay Anticline. It is 
probably a continuation of the Moine Thrust, so well known farther 
north. 

Two important tracts of the South-West Highlands are not 
dealt with in this paper :—- 

(1) The narrow belt of voleanic and fossiliferous rocks commonly styled 
the Highland Border-Rocks; and 

(2) The foreland of the Loch-Skerrols Thrust, as exposed in Islay and 
Colonsay. 


1 The dated references in the sequel are amplified in the Bibliography, 
App. I, p. 125. 
2 Scottish localities not shown in Pl. I are catalogued in the Locality- 
Index, App. II, p. 127. 
G2 


Fig. 1.—Secondary folding of nappes. 


EARLY NAPPES 


Nortrn- Wrest 


Fe om 


Movement 


E 

wis Loch Awe 
I 

eC on AF Iltay 


B 
8 8 8 Ballachulish 


STRUCTURES 


Movement Ourwarps Faem Locw Awe FAN-AxIS 


I a s + <2 
ee eee ee ee Gs me Axes y: os Py Se 


~ «fis pomites s 


5 


part 2] THE SOUTH-WEST HIGHLANDS OF SCOTLAND. 85 


It was in 1914 that I arrived at what are virtually my present 
conclusions. For some years previously my thoughts had ever 
turned to the difference characteristic of the two sides of the Loch- 
Awe Syncline, below the level of the Ardrishaig Phyllites (Pl. I).1 
It seemed fairly certain from the beginning that a thrust (or lag 
was in some way responsible; but it was not until shortly before 
the outbreak of war that I attained to anything approaching full 
illumination. In the hope that someone else might finish the 
work if I should not be able to do so, I summarized the leading 
facts and suggestions as an appendix to a paper on the Islay Anti- 
celine published, during my absence, by this Society. However, 
this proffered page-long appendix was refused, on the ground that 
it was incomplete. 


Clough’s Secret of the Highlands. 


I wish to take this opportunity of acknowledging that the 
interpretation of the South-West Highlands, sketched in the 
preceding section, is essentially a continuation of C. 'T. Clough’s 
interpretation of Cowal. To mention but one of his claims on 
our gratitude, Clough will always be remembered as having 
introduced a new element of technique into the investigation of 
Highland problems. Everybody, since the dawn of geology, has 
employed eroded anticlines and synclines in the elucidation of 
stratigraphical successions and variations in relatively undisturbed 
regions of low relief. Clough boldly extended their use to the 
investigation of the folded complexes of the Highland Schists, 
where close study of a comparatively late anticline or syncline may 
throw invaluable light on the scope and character of earlier folds 
and systems of folds. Long before his day the Cowal Anticline 
had been recognized as a conspicuous feature of South- West High- 
land geology (186la, p. 185). After very detailed consideration 
of this structure, Clough states his conclusions regarding it as 
follows (1897, p. 83) :— 


‘There is no doubt that this anticline is a true arch of an early foliation. 
Later foliations and other structures have been developed together with it 

. . .; but the most prominent foliation of the district, and an enormous 
amount of folding of the same age as this foliation, were already in existence 
before it, and were folded by it. 

‘Tt is clear then that “the anticline” cannot be a simple or exact anticline 
of bedding. It should rather be looked on as an anticline of the limbs and 
axes [Clough means axial planes] of the early folds which affect the 
bedding. To what extent it departs from being an anticline of bedding must 
depend on the amount of this early folding.’ 


He then proceeds to gauge the effect of the early folding by 
comparing the outcrops encountered on the two sides of the Cowal 
Anticline. The contrast is shown to be remarkable, wherefrom he 
derives the suggestion that 
‘there is a great folding of “ pre-anticline” age, which, roughly speaking, 
counterbalances the effect of “‘ the anticline ”’’ 


(1897, p. 86 & pl. x). An unfortunate vagueness of many of 
‘ See also 1922, Report A, par. 6. 


86 MR. E. B. BAILEY ON THE STRUCTURE OF [vol. Ixxvin, 


the lithological distinctions in the critical region prevents Clough 
from speaking ‘with much confidence’ (1897, p. 87) of this 
supposed important pre-anticlinal folding; but he is of opinion 
that in the neighbourhood of Carrick Castle the fold can be traced 
with fair certainty, as may be judged from his mapping between 
Loch Goil and Loch Eck (Sheet 37), and his explanatory section 
(1897, p. 204). 

We now know of several much clearer examples in the Highland 
Schists of secondary folds affecting earlier recumbent folds (fig. 1). 
Clough stands as the original interpreter of this great Secret of 
the Highlands—although, as he himself admits, the reality of the 
Carrick-Castle Fold cannot be regarded as beyond question. 

One word more in this connexion. It is undoubtedly true that 
secondary anticlines and synclines are of more importance to 
workers in the Scottish Highlands than to others who have 
really big mountains and deep valleys to assist them; but such 
secondary folds are by no means overshadowed, even among 
the greatest mountains of Hurope: a geologist on the shores 
of the Lake of Geneva may examine at his ease structural units 
higher than anything that erosion has spared on the summit 
of Mont Blane. 


Slides recognized. 


There was one important feature of Highland geology that 
Clough did not realize during his examination of Cowal, namely, 
the inconspicuousness and, at the same time, the abundance of 
fold-faults, or slides, as they are called (1897, p. 88). The slides 
of the Southern Highlands took place for the greater part under 
conditions leading to constructive metamorphism, and in conse- 
quence are much less marked by localized belts of sheared and 
mylonitic material than their fellows of the North-West. It is 
not that indications of intense movement are lacking in the 
vicinity of the slides. On the contrary, they are universally 
present, but not necessarily in a more pronounced degree than 
elsewhere in the neighbourhood.! 

In 1908, very clear evidence north of Loch Leven, in the Balla- 
chulish district, revealed to me a slide, the continuation of which, 
I felt certain, traversed Glen Etive, some 8 miles away, ata locality 
already mapped in the most minute detail by Clough himself 
(1909, p. 53). Naturally, Clough was incredulous that what he 
had missed on the spot could be seen from a distance. But he 
visited the exposures north of Loch Leven, and, after satisfying 
himself that they left no room for doubt, he assured me that the 
district would become an object of pilgrimage to an extent greater 
than the North-West Highlands had ever been, for the story that 
it had to tell was much more wonderful. 


1 See also 1922, Report A, par. 10. 


part 2] THE SOUTH-WEST HIGHLANDS OF SCOTLAND. 87 


Nappes and their Travels. 


Definition of a nappe.—In 1909 a little party of Scottish 
geologists was introduced to the wonders of the Pre-Alps under 
the able guidance of Dr. L. W. Collet, now Professor at Geneva, 
I well remember a comment by Mr. H. B. Maufe on this occasion : 


‘ Alpine geologists speak and think of nappes, while Scottish geologists have 
been accustomed to concentrate their attention upon thrust-planes.’ 


In the Southern Highlands, a combination of the two habits of 
thought has proved advantageous. It is convenient to bestow 
titles upon important structural masses, selecting for the honour a 
nappe, a fold-core, or an entire fold, as the case may be; it is also 
convenient to give names in certain cases to major slides, both 
thrusts and lags. 

On a previous occasion I have discussed the use of various 
tectonic terms (1916, p. 25); but nappe was not included, since 
I wished to present the main structural features of the district 
considered without reference to such abstruse matters as original 
order of succession and direction of movement. On the present 
occasion I am venturing to express an opinion on these difficult 
subjects, and accordingly offer a definition of the word nappe, as 
follows:—A nappe isa mass brought forward toa notable 
extent by recumbent anticlinal folding or by thrusting. 
In deciding upon the basal limit to be assigned to any particular 
nappe, one generally chooses some prominent thrust-plane ; failing 
this, one is entitled to select the axial plane of some recumbent 
anticline or syncline, according to local convenience. 


Direction of Movement defined and discussed. 


The definition of nappe given in the preceding section depends 
upon a proper understanding of the words brought forward. 
It may be of service to offer a few remarks in this connexion. 
Often the rocks of a district, considered as a whole, show evidence 
of marked horizontal compression at right angles to some parti- 
cular line of strike; while, considered in detail, they reveal a 
differential horizontal movement of their upper layers as compared 
with their lower. In sucha case, movement is said to have 
occurred in the direction in which the upper layers 
have travelled relatively to the lower. At the present 
time, this method of stating the facts is a well-rooted convention, 
associated with a dependent terminology including such familiar 
expressions as foreland and foredeep. 


How line of movement is recognized.—A few words 
may now be added regarding the criteria by which a geologist 
judges of the direction of movement. In the first place, the line 
of movement is at right angles to the strike of folds developed by 
the movement, or is established even more immediately by striation 
of thrust-planes. In this connexion onlv one comment is necessary. 


88 MR. E. B. BAILEY ON THE STRUCTURE OF [vol. ]xxviil, 


The strike of folds is very conspicuous in the case of small-scale 
steep folds, but is very elusive in the case of big-scale reeumbent 
folds. With the latter, all one can do is to determine the more or 
less sinuous! line, along which each particular fold closes upon 
some well-marked constituent member serving locally as its core. ° 
While recognizing the difficulty of the subject, I suggest that the 
greater folds of the Highlands seem to run roughly north-east and 
south-west in agreement with the lesser folds, by which they are 
so conspicuously accompanied and sometimes affected. Three 
examples pointing to conformity of strike will be noticed here :— 


(1) As already set forth, Clough accounts for the contrast on the two sides 
of the Cowal Anticline by suggesting that this anticline arches a 
sequence profoundly influenced by big-scale folding of earlier date 
(figs. 1 &4&Pl.1). As the difference referred to persists across 
the Highlands, it would seem that Clough’s early and late folds 
must have had very similar lines of strike. This is the more likely, 
since he recognizes countless small-scale ‘ pre-anticlinal’ folds 
conforming in strike to the line of the anticline, and associated 
with contemporaneously-developed stretching and rodding approxi- 
mately at right angles to the same line (1897, pp. 16, 17). 


(2) In like manner, I attribute the marked differences met with on the 
two sides of the Loch-Awe Syncline at levels below the outcrop of 
the Loch-Awe Nappe to early flat folding and thrusting,? which 
here again must have agreed fairly closely in strike with the rela- 
tively late folding that developed the syncline (figs. 1, 4, & Pl. I). 


(3) There is much in the gape of the Ballachulish Fold, as exposed along 
the north-western limb of the relatively late-formed Glen-Creran 
Syncline between Lairigmor and Loch Creran (1910a, pl. xiii), 
which does not reappear to the south-east either in Glen Coe or in 
the Windows of Etive. 


How direction along line of movement is recognized.— 
Once the line of movement is known, the next question is the 
actual direction along this line. Trustworthy vertical structures 
are rare in undisturbed rocks, else their deformation would serve 
as an invaluable index. Let us consider for a moment the section 
below the Moine Thrust, as exhibited at the base of the Stack of 
Glen Coul in Sutherland. An outcrop of Cambrian quartzite 
occurs there, resting uncontormably upon Lewisian Gneiss and dip- 
ping beneath the Moine Nappe with the same inclination as that 
which characterizes the intervening thrust-plane. In the quartzite 
are numerous annelid-tubes—the well-known ‘pipes,’ as they are 
called. In the undisturbed foreland these pipes are always at 
right angles to the bedding of the quartzite. In this particular 
exposure they have been sheared into a very oblique position, and 


1 Without entering into detail, 1 may point out that I suspect that the 
west-north-westward close of the Beinn-Udlaidh Fold (1912 b, pp. 168, 169) is 
a misleading local feature of-a fold which, viewed in its entirety, closes 
towards the south-east. 

2 See also 1922, Report A, par. 6. 


part 2] -THE SOUTH-WEST HIGHLANDS OF SCOTLAND. 89 


their upper parts have travelled markedly farther to the north- 
west than their lower. Charles Callaway clearly realized the 
significance of this phenomenon (1884 a, p. 221), as also did 
Dr. B. N. Peach & Dr. J. Horne, who met its counterpart in 
another locality (1884 6, p. 34; 1907 8, p. 481). 

But such weathercocks are seldom available. More often one 
has to rely upon the obvious relationship that isoclinal anticlines 
close in the direction of the movement of which they are a record. 
Where the folding has not a recumbent tendency, the application 
of this rule may present no special difficulty. As an example, one 
may cite the outward movement from the fan-axis of Loch Awe 
(P. Macnair, B. N. Peach, and others), with which may be 
grouped the south-eastward movement of the Cowal Anticline 
(Clough), and the north-westward movement of the Islay Anti- 
cline (Bailey). 

In the case of reeumbent folds, the recognition of the direction 
of movement is often more difficult, since the distinction of anti- 
clines and synclines may involve careful research. The general rule 
is that anticlines are characterized by cores of relatively old rocks, 
and vce versa. Basing my argument upon this rule, I venture to 
suggest that the recumbent fold of Ben Lui (PI. 1) is a svncline, 
which, closing as it does towards the north-west, indicates move- 
ment towards the south-east. There are, however, a few notable 
exceptions to the general rule just stated. In 1907, Sarasin & 
Collet (1907 c, pp. 586-89) explained why they no longer opposed 
the cumulative evidence of Dr. H. Schardt and Prof. M. Lugeon 
in regard to the northward movement of the Pre-Alps. ‘Their 
original difficulty had been the existence in the Zone des Cols of 
apparently anticlinal folds closing towards the south. They later 
realized that these folds had been involved in an exceptionally 
complex series of movements. I should myself describe them as 
secondary recumbent synclines developed in a sequence previously 
inverted. 

Fortunately, one is not restricted to age-relationship in the 
recognition of recumbent anticlines. Clough, for example, de- 
veloped a most ingenious method of attacking this difficult 
problem. He found that he could distinguish innumerable 
diminutive folds of ‘pre-anticlinal’ age in Cowal, and that, of 
these folds, those closing towards the south-east showed a great 
tendency to have their lower limbs attenuated and correspondingly 
lengthened (fig. 2, p. 90). On the ground that general experience 
teaches us that it is anticlines that preferentially exhibit attenua- | 
tion of their lower limbs, Clough identified these south-eastward 
closing folds as anticlines; from which, of course, he deduced a 
south-eastward direction for the originating movement. (Like 
certain other deformational indices, this one, interpreted em- 
pirically, survives inversion, as the reader will realize if he turns 
fig. 2 upside down. ) 

A difficulty in the way of applying Clough’s rule is the fact that 
in nature the tendency to attenuation is by no means restricted to 


90 MR. E. B. BAILEY ON THE STRUCTURE OF  [ vol. Ixxviii, 


one limb of an isoclinal fold.! Clough was fully conscious of 
this difficulty, so far as small-scale folding was concerned, but 
he was not deterred thereby from arriving at a positive conclusion 
in the particular case just considered. 

In the Ballachulish district I found the phenomenon of eddy- 
ing forward motion illustrated on a big scale: that is, I found 
important slides of two kinds, both thrusts and lags (1910a@). Of 
the two I should expect the thrusts, as a class, to be predominant, 
and on this account (among others) I have for many years inclined 
to the view that the Ballachulish and Fort-Wilham Slides are 
thrusts rather than lags (19100). If these two slides are thrusts, 
then here again the movement has been towards the south-east, 
as shown by the direction of close of the associated folds. 


Fig. 2.—Clough’s criterion of south-eastward movement during 
‘pre-anticlinal’ times in Cowal. 


Tt will be noticed that little or no attention has been directed in 
this discussion to the inclination of thrust-planes as a guide to the 
direction of movement. With the ‘double fold’ of the Glarus, 
and numberless other instances, in our minds, we are well advised 
to preserve a very cautious attitude in this matter. Where 
schuppenstruktur is clearly developed, as in the North-West 
Highlands so faithfully portrayed by Dr. B. N. Peach & Dr. J. 
Horne, the upward forward inclination of the individual scales, 
relative to the major thrusts above and below, affords a much 
more trustworthy indication. Schuppenstruktur is, moreover, 
a packing phenomenon appropriate to a compressed uninverted 
sequence. It differs in this respect from the attenuation-pheno- 
menon illustrated in fig. 2. One might hope, therefore, in a 
region of recumbent folds to find some analogue of schuppen- 
struktur predominant in one set of limbs and indications of 


1 Another difficulty is that attenuation is in part a solution effect, for 
Clough found it commonly more marked in regard to the quartz than to the 
mica of a deformed band (1897, p. 22). 


part 2| THE SOUTH-WEST HIGHLANDS OF SCOTLAND. 91 


attenuation in the other, and this, of course, would at once betray 
the direction of movement. It is disappointing, therefore, to find 
Clough quite definite in his asseition that fig. 2 represents the 
general state of affairs in both limbs of his Carrick-Castle 
Fold. (At the end of Chapter VII, 1897, p. 82, Clough, for quite 
other reasons, contemplates the inversion of the early folds and 
accompanying foliations of Cowal by the Carrick-Castle Fold. 
Such inversion, if superinduced upon a previous inversion, might 
well account for the absence of anything approaching schuppen- 
struktur. But Clough wholly neglects this suggestion of his 
elsewhere in his account, and I do not think that it should be 
regarded as an integral part of his interpretation. ) 

The intensive study of our North-West Highlands by Dr. 
Peach & Dr. Horne, and the contemporaneous investigation of the 
more ample exposures of Scandinavia by Térnebohm, alike 
emphasize another important aspect of the matter. On approach- 
ing a mountain-chain from the front, one commonly finds the 
original structures of the rocks of the foreland increasingly de- 
formed. Locally, the condition of the rocks of the South-West 
Highlands is difficult to reconcile with this familiar experience. 
I have already partly indicated the evidence of persistent south- 
eastward movement during the main period of nappe-formation. 
As indicated in Pl. I, the Loch-Awe Nappe is interpreted as a 
klippe, or outlier, resting on the Iltay Nappe. Naturally, one 
might expect less evident distortion of the rocks of the Iltay 
Nappe in Cowal than in Islay, but the reverse actually occurs. 
In Islay many outcrops of flagstone are known with ripple-marks 
still perfectly preserved; in Cowal such original details are lost 
sight of in the general deformation. As a tentative explanation, 
I suggest that local conditions, including temperature, may have 
been the determining factor in producing this anomaly. The 
Cowal region may have been more heated than the Islay region, 
and accordingly more inclined to react generously and diffusely to 
mechanical stimulus. 

There remains but one other important source of information 
that need be touched upon, and that is the facies of far-travelled 
nappes. Igneous, as contrasted with sedimentary, facies helped 
Hutton long ago to establish the subterranean origin of the lava- 
flows of Scotland. Alpine, as contrasted with Helvetian, facies 
helped Dr. Schardt in much more recent days to realize the 
southern origin of the great invading nappes of his native moun- 
tains. As in the Alps, so in the South-West Highlands, considera- 
tions of facies are of great importance. They do not, indeed, tell 
us from which direction the Loch-Awe Nappe travelled into its 
present position; but they do warn us against supposing this 
nappe to have spread in mushroom fashion from a root beneath. 
If the Loch-Awe Nappe originated as a mushroom, one would 
expect it to reproduce in itself the Islay facies; but such is not 
the case. 


92 MR. E. B. BAILEY ON THE STRUCTURE OF [| vol. lxxviii, 


Metamorphism. 


The metamorphism of the South-West Highlands is too wide 
and speculative a subject to be dealt with here in anything more 


Fig. 3.—Metamorphisi of nappes. 


<2) Mica Inconspicuous 
Mica Conspicuous 


Albite | Garnet 


[The nappes, in descending order, are lettered: L=Loch-Awe Nappe; I= 
Iltay Nappe; B=Ballachulish Nappe; A—Appin Nappe. The meta- 
morphic scale adopted refers only to the condition of normal grey 
pelitic sediment. | 


than the merest outline. The suggestions offered below will, it is 
hoped, be amplified on some future occasion :— 


(1) Fig. 3 furnishes a graphical statement of the metamorphism of normal 
grey pelitic (or semipelitic) sediments in the South-West Highlands. 


part 2] THE SOUTH-WEST HIGHLANDS OF SCOTLAND. 93 


(2) In such rocks increasing metamorphism leads from slates and phyllites 
through mica-schists into either garnetiferous mica-schists or albite-schists. 

(83) In rocks of a different composition (as, for instance, quartzite, lime- 
stone, or even carbonaceous or calcareous pelitic sediments) response to 
metamorphic influences may not agree very closely with what is shown in 
fig. 8. Thus the Ballachulish Quarries yield black roofing-slate, although 
situated on the margin of the garnetiferous zone of fig. 3 (1916, p. 202). 

(4) A vague agreement between the outcrops of successive metamorphic 
and structural zones, respectively, lends distinct support to a theory of 
metamorphism controlled by depth-temperature, such as was developed by 
Clough (1897, p. 91) to account for the relatively high metamorphism 
characteristic of the axial belt of the Cowal Anticline (see figs. 1 & 3). 

(5) The agreement noted above is so incomplete that no increase of 
metamorphism accompanies the emergence of nappes north-west of the 
Loch-Awe and Glen-Creran Synclines. This capriciousness suggests recourse 
to Mr. G. Barrow’s theory of metamorphism controlled by temperature 
dependent upon magmatic distribution (1912a). To me, there seems to 
emerge no reason against combining Clough’s and Barrow’s interpretations. 

(6) Fig. 3 shows sufficiently clearly that crystallization continued until 
the close, at least, of the main nappe-movements: there are no marked 
metamorphic inversions. 

(7) Clough did much to distinguish certain mineral developments of Cowal 
as of pre-anticlinal, or anticlinal, date, respectively (1897). I have now 
found good evidence of the development of albite in the albite-schists of 
Cowal during both these periods. As albite-development is due to specialized 
conditions of regional metamorphism (hydrothermal, according to H. H. 
Cunningham-Craig, 1904 a, p. 26), it looks as though the early and later 
movements classified in fig. 1 should be regarded as successive chapters in a 
long-continued history of mountain-building. Among the products of the 
later movements is the Loch-Skerrols Thrust, which I correlate with the 
Moine Thrust of the North-West Highlands. Accordingly, I am led to 
suspect a fairly close connexion between the south-eastward movement of the 
great nappes of the South-West Highlands and the north-westward move- 
ment of their better-known fellows of the North-West Highlands. Naturally, 
this opens the door for comparison of the Southern Highland nappes with 
those which will for ever be associated with Tornebohm’s name in Scandinavia. 
Moreover, if one is ready to look beyond the limits of Caledonian time, one is 
tempted to trace an analogy between the north-westward movement revealed 
in Islay and the back-movement of the root-region of the Alpine Chain 
towards the Plain of Lombardy. 


Corrigenda since 1910. 


From 1910 onwards the Geological Society has published a 
series of papers on South-West Highland tectonics, written by 
myself, and one describing the structure of the Glen Orchy district, 
in which I collaborated with Mr. M. Macgregor. All told, these 
papers cover more than half of the area dealt with on the present 
occasion. Their detailed descriptions and their beautifully 
reproduced maps are available to anyone who wishes to realize the 
full appeal of the interpretations here advanced. It is, therefore, 
important to assist the reader by enumerating at this juncture 
such modifications as have come in the wake of our steadily 
increasing knowledge. 

1910. Recumbent Folds in the Schists of the Scottish 


Highlands.—This paper deals with the recumbent folds of Ballachulish, 
Aonach Beag, and Appin, north and west of the granite-mass of Etive; their 


94 MR. E. B. BAILEY ON THE STRUCTURE OF _ [vol. lxxviil, 


attendant slides; and their liability to secondary folding. The reality of all 
these geological features is, I consider, unassailable by anyone acquainted with 
the field-evidence. At the same time, I should like to notice certain important 
modifications which I have been able to effect as a result of continued 
enquiry. For instance, the Hilde Flags of the Loch-EHilde Mor outcrop are 
no longer referred wholly to the upper limb of the Appin Fold (see 1916, 
figs. 9-10, and 19104, pl. xliii); while the limestone at Loch Dochard, east of 
the Etive Granite, is now definitely relegated to a lower structural position 
than the Appin Core. 

The two changes just mentioned, along with much else of importance, are 
due to later work by Mr. R. G. Carruthers. Whereas I had already ventured 
to correlate flags of two important outcrops, the one running past Loch Hilde 
Mor, the other through Fort William, I confess that I did not think that 
these outcrops had any underground connexion in the intervening country 
(1910 a, p. 616 & pl. xliii)—although I was well aware that my evidence in this 
matter was inconclusive. Mr. Carruthers was fortunate enough to find that 
a persistent south-westward pitch causes a remarkable zigzag approach of the 
Loch-Hilde and Fort-William outcrops in the district beyond that which I 
had examined, and I now agree with him that an underground connexion is 
highly probable. As an incident of the zigzag approach just mentioned, 
Mr. Carruthers noted folding of the Fort-William Slide. I am of opinion— 
but I admit Mr. Carruthers attaches little weight to my suggestion—that 
the Meall a’ Bhuirich Slide in its type-exposure is merely the folded con- 
tinuation of the Fort-William Slide. My original view that the Meall-a’- 
Bhuirich Slide reappears east of the Loch-Hilde outcrop (1910 a, h, pl. xlii) I 
frankly disavow. Pl. I shows the approach of the two outcrops of Hilde 
Flags, and, in so far as it is a continuation of my previously published 
mapping, it is mainly based upon Mr. Carruthers’s work !—except, of course, 
that I am alone responsible for the expressed suggestion that the Fort- 
William and Meall-a’-Bhuirich Slides are one and the same. The correction 
has two very pleasant features: looking back, one realizes anew that what 
has proved impossible to unravel in some particular region may be an easy 
matter to distinguish in an adjoining district; looking forward, one hopes 
that some of the remaining elements of doubt may in their turn be dispelled, 
sO soon as opportunity arises for further extension of the field of mapping. 

Mr. Carruthers’s other points need not detain us long, since I have no 
strong opinion in regard to them, and have modelled my present account on 
non-committal lines. They may be summarized as follows :— 

(1) Mr. Carruthers thinks that he can locally (that is, in the Kinlochleven 
district) distinguish three mica-schists and three quartzites, mtervening 
between the Ballachulish Limestone and the Hilde Flags—where previously I 
only had recognized one group of each, namely, the Leven Schists and the 
Glen-Coe Quartzite. I have discussed these two alternative interpretations 
elsewhere (1916, p. 62). Here, in order to avoid uncertainties, I treat the 
mica-schist and quartzite complex as one great stratigraphical group (PI. J). 
In regard to the structural relations, which this inclusive group bears to the 
Ballachulish Limestone, on the one hand, and the Hilde Flags, on the other, 
Mr. Carruthers and I are in very close agreement. I have myself dealt more 
particularly with the limestone side of the complex, while Mr. Carruthers has 
gathered much of the information regarding the behaviour of the flags. 

(2) Mr. Carruthers thinks that the rocks shown as belonging to the Sub- 
Eilde Complex in Pl. I are stratigraphically distinct from any of their fellows 
in the neighbourhood. I have already discussed this difficult question (1916, 
p. 62), and meanwhile do not intend to resuscitate it. In the present paper 
I merely use the outcrops of these rocks as indicating a lower structural 
level than is reached in the Loch-Hilde outcrop of the Hilde Flags, without 


1 For permission to publish this material I thank Mr. R. G. Carruthers, as 
well as the Director of H.M. Geological Survey. 


part 2] THE SOUTH-WEST HIGHLANDS OF SCOTLAND. 95 


expressing any opinion as to their stratigraphical equivalents. I am very sorry 
that Mr. Carruthers’s important contribution is not yet fully published. If it 
were, I am certain that it would be easy to criticize some of his statements— 
to judge from a preliminary notice (1913 a, p.51); but of its value as a whole 
there can be no question. 

It may be well to add that there was always an essential difference of 
status in regard to the 1910 interpretation of the folded district of Kinloch- 
leven (now admittedly wrong in important particulars), and that of (say) 
Ballachulish. The former was based on what seemed a mere sufficiency of 
evidence ; the latter upon the reiterated testimony of section after section. 
In the Kinlochleven district I am now of opinion that I made one cardinal 
mistake—I interpreted a syncline with steeply-inverted pitch as an anticline, 
—and the realization of this has left an insufficient foundation for a complete 
structural interpretation. No similar isolated accident could affect the 
reading of the Ballachulish Fold and Slide. 

As regards other corrections of the work covered by my 1910 paper I may 
refer to :—— 


(1) The elucidation of additional details concerning the Ballachulish 
Fold near the head of Loch Creran (see 1914 paper). 


(2) The much more important recognition of the extension of the Iltay 
Nappe to the shores of Loch Creran, where previously I had, with 
very imperfect knowledge of the ground, imagined that nothing but 
a local facies of part of the Leven Schists occurred. This point is 
dealt with in detail later on (p. 115). 


1912. The Glen-Orchy Anticline (EK. B. Bailey & M. Mac- 
gregor).—tThere is no correction to emphasize with regard to this paper. In 
order to avoid criticism, the quartzites, mica-schists, and limestone of the 
Beinn-Udlaidh and Loch-Dochard exposures are ascribed in the sequel to the 
Sub-Hilde Complex without definite stratigraphical correlation. 


1913. The Loch-Awe Syncline.—I no longer regard the St. Catherine’s 
Graphite-Schist and its immediately associated phyllites, or mica-schists, as 
in stratigraphical continuity with the Ardrishaig Phyllites (1913 6, p. 299). 
This involves, among other things, the separation from the Ardrishaig 
Phyllites of certain calcareous mica-schists, which are shown (1913), pl. xxxii) 
as extending northwards and eastwards from Meall nan Tighearn (around 
Ben Lui). The Erins Quartzite I still believe to be connected stratigraphi- 
cally with the Ardrishaig Phyllites (p. 96). 


1914. The Ballachulish Fold near the Head of Loch Creran.— 
This paper supplies certain corrections to its predecessor of 1910. 


1917 (for 1916). The Islay Anticline.—Nothing further has been 
published. 


Il. THe THREE GREAT STRUCTURAL DIVISIONS. 


A concise treatment will now be given of the stratigraphy and 
structure of the South-West Highlands. Discussion based upon 
detailed accounts of certain crucial sections is reserved for § III. 


The Loch-Awe Nappe. 


Constitution.—The structural independence of the Loch-Awe 
Nappe cannot be appreciated without extensive trespass into the 
country surrounding the nappe. Discussion of this cognate 
matter is, therefore, reserved for a later paragraph. Meanwhile, 


96 MR. E. B. BAILEY ON THE STRUCTURE OF _[ vol. Ixxviil, 


the reader is asked to excuse a rather dogmatic statement of the 
case. 

The outcrop of the nappe reaches south-westwards from the 
northern end of Loch Awe, until lost sight of under the sea. 
Its breadth is roughly 20 miles, and its position is clearly 
indicated in Pl. I. The stratigraphical succession within its 
limits is as follows :— | 


(Loch Avich, green Slates with grits, and (in the basal 


part) pillow-lavas. 
Loch-Awe | Tayvallich, black Slates and limestones (both often 
Group. pebbly), grits, conglomerates, and pillow-lavas. 


(Pebbly + The rock-fragments in the conglomerates are generally of 


character local origin ; but, in certain well-known occurrences, nord- 
recurrent.) markite and other foreign boulders are found. 
Crinan Grits and Quartzites with subordinate slates 
| and limestones. 
‘Shira Limestone, grey or cream-coloured, according to 
locality ; absent south of the Crinan Canal, and, elsewhere, 
Ardrishaig |» often interbedded with greenish-grey phyllites. 
Group. | Ardrishaig, soft, greenish-grey Phyllites, with cal- 
(Pebbly cha- + careous lenticles, occasional beds of buff-coloured or white 
racter very limestone, and a fair proportion of compact, fine-grained, 


restricted.) slightly-caleareous quartzite. 
; Erins fine-grained, slightly calcareous Quartzite, restricted 
|. to Lower Loch Fyne. 


Basic sills (epidiorite) are abundant everywhere, except in the central 
portion of the outcrop of the Loch-Avich Slates. 


The separation of the Loch-Awe and Ardrishaig Groups is due 
to Mr. J. B. Hill (1899, p. 473). The subdivision of the Loch- 
Awe Group (1913 4, p. 291) followed closely on Dr. B. N. Peach’s 
recognition of voleanic rocks in the Tayvallich Peninsula (1904 4, 
p- 68).1. The Erins Quartzite was early distinguished by Mr. Hill: 
he regarded it as a local facies of the lower part of the Ardrishaig 
Group. J am inclined to think that it is a separate entity, and 
that it owes its restricted occurrence to limitation by the thrust at 
the base of the Loch-Awe Nappe. 

At one time, I imagined (1913 6, p. 300 & pl. xxxi1) that the 
Erins Quartzite probably belonged, half to the Ardrishaig Group, 
and half to the Ben-Lui Group (p. 95). This working hypo- 
thesis was based upon a couple of rather insecure foundations :— 


(1) A tentative correlation, connecting certain graphitic phyllites at Stron- 
chullin, in the heart of the Erins Quartzite, south of Ardrishaig, with similar 
rocks occurring in bands at the north-western margin of the Ben-Lui Group, 
18 miles farther up Loch Fyne. 

(2) A tacit assumption that the Stronchullin outcrop does not mark the 


centre of a fold. 


Now that other evidence poimts to a complete structural 
separation of the Ardrishaig Phyllites (Loch-Awe Nappe) from 
the Ben-Lui Schists (Iltay Nappe), partition of the Erins Quartzite 
is no longer possible. 


1 See also 1922, Report A, par. 1. 


part 2] THE SOUTH-WEST HIGHLANDS OF SCOTLAND. oF 


Original order of deposition.—Two items of evidence 
indicate that the column on p. 96, with Loch-Avich Slates at the 
‘top and Erins Quartzite at the bottom, represents the original 
order of deposition of the rocks of the Loch-Awe Nappe. 


(1) In the Tayvallich Peninsula, the Tayvallich Slates, Limestones, and 
Lavas very clearly overlie the Crinan Quartzite. Where the volcanic escarp- 
ment reaches the sea, the second lava from the bottom has numerous pipe- 
amygdales springing from its base, while its upper portion is thoroughly 
slaggy. I have always agreed with Dr. Peach that this lava is ‘right way 
up, and growing experience has strengthened my belief. Anyone who has 
spent much of his life among lavas, must have met with many examples of 
pipe-amygdales rising from the base of a flow, or from the base of an 
individual band or lenticle within a flow, but never descending from the top. 

Dr. Peach has already published a horizontal section showing the position 
and relations of this invaluable Tayvallich Lava (1911, fig. 4, p. 69).1 If this 
flow is ‘ right way up’ (and who can doubt it?), then the field-relations show 
almost conclusively that the Tayvallich Slates and Limestones are of later 
date than the Crinan Quartzite.? 

(2) At Kilmory Bay, J. S. Grant Wilson found the Ardrishaig Phyllites dip- 
ping steeply beneath a conglomeratic grit forming the base of the Loch-Awe 
Group (1911, p. 64). The grit has the appearance of being ‘ right way up,’ 
for it includes a succession of seams of fine-grained conglomerate, all of them 
with well-defined bases and ill-defined tops. In this case, perhaps, it is wise 
to regard the evidence as suggestive rather than conclusive; but it gives 
valuable support to the testimony afforded by the pipe-amygdales mentioned 
in the previous paragraph. 


Structure.—Small-seale isoclinal folding is often an obvious 
feature of the geology of the Loch-Awe Nappe. The isoclinal 
folds and the concomitant cleavage are disposed with a marked 
tendency to fan-arrangement—steep or vertical along an axial 
belt, and inclined inwards on each side. Despite all this, there is a 
fine simplicity in the surface-distribution of the rocks. The Loch- 
Avich Slates have an outcrop in the neighbourhood of Loch Awe, 
and the other subdivisions follow round about it in the order given 
in the table. This simplicity betokens a general synclinal or anti- 
clinal structure for the district as a whole. Mr. Hill, who, as will 
be remembered, instituted the separation of the Loch-Awe and 
Ardrishaig Groups as two great stratigraphical units, always 
referred to the structure of the district as synclinal. I do not 
know what influenced Mr. Hill in this matter, but there are two 
very good reasons, which were open to anyone to make use of at the 
time when he wrote on the subject: the first is the upward structural 
succession encountered north-westwards from the flat central belt 
of the Cowal Anticline; and the second is the obvious emergence 
of the Ardrishaig Phyllites from beneath the outcrops of the Shira 
Limestone and Crinan Quartzite where these latter terminate 
northwards across the general strike of the folding. 

My own observations afford additional proofs of the existence of 


1T am not certain that the photograph (1911, pl. iv) was not taken, by 
mistake, of an overlying lava. 
2 See also 1922, Report A, par, 2, 


Q.J.G.8. No. 310. i 


98 MR. E. B. BAILEY ON THE STRUCTURE OF _[ vol. Ixxviii, 


the Loch-Awe Syncline. They show ascending structural succes- 
sions towards the Loch-Awe country in the adjoining districts of 
Islay and Glen Orchy (1917, 1912 }),! and also an upward structural 
succession from Crinan Quartzite through Tayvallich Slates into 
Loch-Avich Slates within the limits of the Loch-Awe Nappe (19184, 
pp. 292, 294; 1911, pp. 65-68).? 

There is nothing in what has been noted above that suggests 
bodily transport of the Loch-Awe Nappe. The obvious fan-folding 
may be taken as a typical example of movement outwards in two 
directions from a central axis. Such movement would not be 
expected to belong to a period of advance of the nappe as a whole ; 
it seems connected rather with the synclinal depression which 
affected the nappe after its mise en place. Anticipating much 
that has yet to be discussed, one may state that the Loch-Awe 
Nappe travelled into its present position from the north-west. 
The evidence relied upon is afforded by the distortion of the 
underlying rocks, leading, among other results, to the production 
of the recumbent syncline of Ben Lui (Pl. I & fig. 4, p. 102). 


The Itay Nappe. 


Constitution.—The following succession has been traced in 
that part of the South-West Highlands which is fashioned out of 
the Iltay Nappe (Pl. 1) :— 


Leny Grits with slates (or phyllites) of the Aberfoyle type. 

Aberfoyle (or Dunoon) grey, green-grey, buff-coloured, purple, and 
black Slates (or Phyllites) with numerous thin limestone- and 
grit-bands. 

Ben-Ledi (or Beinn- Bheula) Grits, grey phyllites (or mica-schists), 
and albite-schists (or gneisses), with locally important chlorite- 
epidote-schists (Green Beds). Garnets are common north-east of 
the albite-zone (fig. 3). Oligoclase-pebbles are often said to be 
characteristic of the Ben-Ledi Grits; my experience is that the 
common felspar-pebbles are albite and more or less _ perthitic 
orthoclase. . 

Green-Bed Group, in which chlorite-epidote-schist, often gritty, is 
strongly developed. Hornblende is common north of Cowal. 

Pitlochry (or Glen-Sluan) Schists and Grits. The schists 
earry albite in Cowal and garnet farther north (fig. 3). 

Loch-Tay light to dark grey crystalline Limestone (or marble), 
sometimes mottled with black calcite-crystals; in large part a 
calcareous schist or quartzite. Almost always invaded by epidiorite- 
sills. 

Ben-Lui Garnetiferous Mica-Schist with grits and minor Green 

Beds; graphite-schists, interbedded with phyllitic garnetiferous 
mica-schist, mark the edge against the Ben-Lawers Group in the 
St. Catherine’s district of Upper Loch Fyne. Limestone also occurs. 
In the district of Lower Loch Fyne garnets are but poorly developed. 

Ben-Lawers Calcareous Schists (or phyllites), with some 
limestone and fine-grained quartzite. The calcareous material is 
disseminated, and is often only clearly visible in quartzose lenticles. 

Easdale black graphitic Slates (or Schists), with subordinate bands 
of quartzite and black limestone. 


1 See also 1922, Rep. A, par. 4. 2 Tbid. Rep. A, par. 2. 


part 2] THE SOUTH-WEST HIGHLANDS OF SCOTLAND. 99 


Transition Group, including Black Slates and Quartzites. Con- 
glomerates occur in Scarba, Jura, and Islay, and grey phyllite 
(Port Ellen Phyllites) in Southern Jura and Islay. There are 
also limestone-bands, sometimes pebbly. 

Islay Quartzite in part pebbly, especially towards the margin of the 
Transition Group : locally, a flaggy dolomitic group occurs within the 
quartzite not far from the Portaskaig Conglomerate (Northern Islay) ; 
locally also, a flaggy semipelitic pebbly group develops not far from 
the Transition Group (Northern Jura, Scarba, etc.). 

Portaskaig Conglomerate, with nordmarkite and other boulders. 

Islay dark-grey Limestone, locally oolitic; this limestone and the 
Mull-of-Oa Phyllites are to some extent interbedded. 

Mull-of-Oa grey or greenish Phyllites with dark colour-striping ; 
thin cream-coloured sandy dolomites are common in some exposures. 

Maol-an-Fhithich fine-grained Quartzite. 


It must not be thought that the succession outlined here is to 
be met with as a whole in any one locality in the South-West 
Highlands. The constitution of the Iltay Nappe varies notably 
from place to place, according to the positions of the thrust-planes 
which serve as its boundaries above and below. The variation can 
be summarized as follows (see Pl. I) :— 


District. Partial Succession. 
(a) Islay Archipelago, north-west of | Maol-an-Fhithich Quartzite to Has- 
the Loch-Awe Nappe. dale Slates. 
(b) Kintyre and Cowal, south-east of | Ben-Lawers Schists to Leny Grits 
the Loch-Awe Nappe. (with a concealed underground 


continuation of the rocks of the 

Islay Archipelago). 

(c) Strip connecting (a) and (b) round | Islay Quartzite to Easdale Slates. 
the northern end of the Loch- 
Awe Nappe. 

(ad) North-eastward continuation of | Islay Quartzite to Leny Grits. 
(b) and (c) towards Loch Tay. 


The detailed evidence upon which I base my reading of the 
stratigraphy of the Islay Archipelago (a) has been recently 
explained in the Quarterly Journal of this Society (1917). 
It must be admitted, however, that Dr. Peach and Mr. Wilkinson 
previously put forward an interpretation differing in various im- 
portant particulars (1907 a). The statement of the Kintyre, Cowal, 
and Loch-Tay sequence (6 & d@) represents the verdict of my prede- 
cessors—Clough, Cunningham-Craig, Macnair, Hill, Kynaston, and 
Grant Wilson. The three last-named are more especially responsible 
for tracing that part of the succession which extends through the 
Calcareous Schist (Ben-Lawers Group) into the Black Schist and 
Pebbly Quartzite cropping out farther north. The geology of the 
intermediate belt (¢), complicated as it is by granitic intrusion, 
has not been adequately described as yet, and will accordingly be 
dealt with later on (p. 117). 

It will be readily understood, after what has just been stated, 
that definitely synthetic treatment is requisite to realize the full 


1 See also 1922, Report A, par. 8. 
. H2 


100 MR. E. B. BAILEY ON THE STRUCTURE OF  [vol. Ixxviu, 


sequence of the Iltay Nappe as developed in the South-West 
Highlands—that is, so long as one is restricted to the use of local 
evidence. I am delighted to say that it is possible to arrive at the 
result more simply along a quite independent line of enquiry. 

The same long stratigraphical sequence occurs in the South- 
Central Highlands, under conditions which render it relatively 
easy to read. Once read, its application to the South-West isa 
matter involving fairly obvious correlations. All this was realized 
many years ago by Mr. George Barrow (19136, p. 306; 1917, 
p- 160), whose sequence for the South-Central Highlands can be 
found on turning to Sir Archibald Geikie’s first Presidential 
Address to this Society (1891, Proc. p. 74). It is, however, only 
fair to remind the reader that in the Central Highlands, as in the 
West, grave differences of opinion exist as to the interpretation of 
local successions. The question at issue is whether the Perthshire 
(Islay) Quartzite is part of the general sequence, or whether it is 
an unconformable unit. Mr. Barrow adopted the former alterna- 
tive, and, after careful examination of the Pitlochry and Blair- 
Atholl districts, I have no doubt that he is justified by the nature 
of the evidence. As to the possibility of detailed correlation 
between the Central and the Western Highlands, there has long 
been sufficient excuse for optimism. Macculloch did well, when he 
wrote in 1819 of his discovery of a schistose conglomerate in the 
Garvellach isles, ‘ which,’ he said, ‘ will be seen hereafter to occur 
in Isla, and which I have also observed in Schihallien’ (1819, 
vol. 11, p. 159). This conglomerate is characterized by its abundant 
nordmarkite-boulders, its frequent close resemblance to tillite, and 
its well-defined stratigraphical associates. 


We may pass now to another important feature of the [tay 
Nappe. Clough directs special attention to the recurrence of 
‘green beds’ on widely different horizons in the Cowal succession 
(1897, p. 89). It is impossible to account for all these scattered 
outcrops by assigning them to a single group, reduplicated by 
folding—as a matter of fact, good examples occur on both sides of 
the Loch-Tay Limestone. Accordingly, Clough was impressed by 
the evidence which the repetition of this peculiar type of sediment 
affords of continuous deposition, from what he took to be the mar- 
ginal portion of the ‘Ardrishaig Phyllites,’ well nigh to the Dunoon 
boundary of the Ben-Ledi Grits and Schists. On quite other 
grounds I have had to transfer some of Clough’s ‘ Ardrishaig 
Phyllites’ to the Ben-Lawers Group (p. 121); and, in default of 
more precise local evidence, I have been guided in mapping the line 
of the separating thrust by the improbability of ‘green beds’ 
continuing in Cowal across the boundary of the Iltay Nappe. 
At the same time, I think that Clough rather exaggerates the 
peculiarities of the ‘ green beds.’ He compares them with the 
epidotie grits, ete., of the Lower Torridonian of Skye; he might 
have extended the comparison, I believe, to certain Ordovician 
sediments (for instance, the Tappins Group) figuring prominently 
in the Southern Uplands. 


part 2] THE SOUTH-WEST HIGHLANDS OF SCOTLAND. 101 


Original order of deposition.—Three reasons can be 
adduced for regarding the column on pp. 98-99 as arranged in 
descending order of age, with Leny Grits at the top and Maol-an- 
Fhithich Quartzite at the bottom :— 


(1) Beannan Dubh, a little hill halfway along the eastern coast of Islay, 
is composed of an outlier of typical Portaskaig Conglomerate lying flatly 
upon an extensive outcrop of Islay Limestone. Sandy cream-coloured dolo- 
mites are conspicuous as intercalations in the conglomeratic series, and appear 
to have suffered ‘ contemporaneous erosion,’ yielding fragments to the overlying 
interstratifications of conglomerate. One of these dolomite-bands, whiter 
than usual, can be traced for a couple of hundred yards. It has a regular 
base resting upon shale, and a highly irregular top with cavities choked by 
downward extensions from an overlying stratum of well-bedded gritty dolomite. 
Along the jagged junction there is often a foot or two of coarse breccia 
consisting of angular blocks of white dolomite enclosed in a brown gritty 
matrix (1917, p. 143). 

There seems no room for doubt, when one is faced with this exposure, that 
the Portaskaig Conglomerate is here ‘ right way up,’ and therefore of later 
date than the subjacent Islay Limestone. 

(2) A very suggestive sequence, from grey slate, to black slate, to con- 
glomerate—charged with black slate-fragments, to quartzite, is found at 
the southern extremity of Islay. I have described and figured the section 
(1917, p. 154), and need only repeat that this particular conglomerate appears 
to be younger than the associated black slate; and that such an inference, 
combined with my reading of the local stratigraphy, leads to the further 
conclusion that the Port-Hllen Phyllites (and therefore also the Hasdale Slates) 
are younger than the Islay Quartzite. 

(3) Clough’s evidence for repeated south-eastward movement in Cowal has 
already been discussed. (p. 89). It seems highly probable that his Carrick- 
Castle Fold (pp. 86 & 103) developed during some phase of these south-eastward 
directed disturbances. Accordingly, since the fold closes south-eastwards, it 
is natural to regard it as a recumbent anticline,! and to expect it to contain a 
core of relatively old rocks—in other words, to regard the Loch-Tay Limestone 
and its associates as older than the Ben-Ledi Grits and Aberfoyle Slates. It 
is only fair, however, to state that this apparently justifiable inference 
escaped Clough’s notice. He did, indeed, refuse tofollow the common custom 
of assuming that the reverse succession had been established ; but his position 
was entirely negative—in fact, he said that the schists of Cowal had afforded 
him no clue whereby to determine their age-relations (1897, p. 86). 


Passing reference has just been made to a prevalent opinion that 
the order of deposition is the exact reverse of that which is given 
above. I believe I am justified in saying that the sole foundation 
for this view is a widespread structural superposition of the Loch- 
Tay Limestone, Pitlochry Schists, and Green Beds upon the Ben- 
Ledi Grits and Schists in a tract of country reaching from Kintvre, 
north-eastwards past Loch Tay. When Sir Archibald Geikie, in 
his Presidential Address to the Geological Society, spoke of the 
Loch-Tay region (1891, Proc. p. 73), he said : 


‘It is difficult to resist the belief, though it would be premature to conclude, 
that this obvious and persistent order of succession really marks the original 
order of deposition.’ 


1 The Carrick-Castle Fold closes downwards in its type-exposure, and 
Clough calls it a ‘syncline.’ The apparent contradiction is one of terms, not 
of ideas. 


102 MR. E. B. BAILEY ON THE STRUCTURE OF  [vol. Ixxviui, 


His words are very true, for in the interval few of the geologists 
who worked under him have resisted the belief, although Clough 
in this, as in so many other directions, was a notable exception to 
the rule. Clough pointed to his Carrick-Castle Fold as a warning, 
since structural superposition in the two limbs of this fold naturally 
gives contradictory results if it be employed asan index of relative 
age (1897, p. 86). With increasing knowledge of the complexity 


Fig. 4.—The two great folds of the Iltay Nappe. 


‘Ben Lut Carricx Castte 
Fob Foup 


UPPER LIMB 
UPPER LIMB [| LOWER LIMB 


LOWER LIMB 


of Highland structure, it is not too much to say that the difficulty 
of resisting the temptation of arguing from superposition to 
relative age has steadily diminished. Clough’s criticism can be 
applied with great force, for instance, in the comparison of the 
Loch-Tay region and the Islay Archipelago. Here, again, the 
order of the superposition in the one case is the very reverse of 
what it is in the other (fig. 4). 


part 2] THE SOUTH-WEST HIGHLANDS OF SCOTLAND. 108 


Structure.—If for the moment the results of secondary folding 
are set aside, the main structural features of the Lltay Nappe can 
be stated in a few words (fig. 4). 

In its lower portion the sequence is predominantly normal. 
Thus, in Islay, the Islay Quartzite overlies the Portaskaig Con- 
glomerate and Islay Limestone (1917, pl. xii); and, in conformity 
with this, all the way from Luing to Dalmally the Easdale Slates 
persistently separate the Islay Quartzite from the overlying Loch- 
Awe Nappe (pp. 114, 118, & 119). 

At higher levels, reeumbent folding sets in, as proved by an 
extensive inversion of Loch-Tay Limestone, Pitlochry Schists, and 
Green Beds over the Ben-Ledi Grits. The flat-lying geology of 
the Loch-Tay district, mapped by J. 8. Grant Wilson (Sheet 46), 
has become too familiar to detain us; all that need be said is that 
the inversion of the Loch-Tay Limestone is revealed as holding 
good for some 15 miles measured across the strike. Far less 
famous, but equally instructive, 1s a set of exposures at Campbelton 
in Kintyre, where a restricted subsidence (perhaps of Tertiary date) 
has led to the preservation of a little coalfield in the axial regions 
of the Cowal Anticline. The Loch-Tay Limestone and overly’ ing 
schists are seen to advance their outcrops notably to the east as 
they come within the sphere of influence of the depression, whether 
their approach to it be followed from the north or from the south. 
It was a great pleasure to me, on visiting the ground in 1919, to 
find how accurately R. G. Symes, with Dr. Peach’s assistance, had 
traced the main exposures of limestone and. associated epidiorite 
(Sheet 12): it was also delightful to realize on the ground how 
clearly the Loch-Tay Limestone overlies the Ben-Ledi Grits, etc., 
which constitute the main part of the peninsula of Kintyre, both 
north and south of the depression. 

The normal sequence of Islay and the inverted sequence of 
Loch Tay are seen in conjunction on the slopes of Ben Lui. 
Together they supply the two limbs of a great recumbent fold 
(fig. 4)1; the normal sequence furnishes the lower limb, the 
inverted sequence the upper (p. 124). In such circumstances, as 
I have already pointed out, itis difficult to regard the Ben-Lui 
Fold as anything but a syncline, and since it can be seen to close 
towards the north-west, it has all the appearance of being a product 
of south-eastward movement (p. 89). 

At still higher levels, the Loch-Tay inversion gives place to a 
normal sequence once again, this time through the intervention of 
the Carrick-Castle Fold, which (as might be expected) closes 
south-eastwards (p. 86). It has already been pointed out that 
this fold is probably an anticline, because of Clough’s reading of 
the local evidence of south-eastward movement (p. 101) ; of course, 
if Clough’s evidence were not available, the same result would 
have been arrived at on my reading of the original order of 
deposition. 


! See also 1922, Report A, par. 5; Report B. 


104 MR. E. B. BAILEY ON THE STRUCTURE OF [vol. Ixxviii, 


The secondary folding of the Iltay Nappe has been mentioned 
on several occasions already (fig. 1). One main feature is the 
synclinal fan of Loch Awe followed in relay by that of Ben Lawers. 
South-east of this hes the Cowal Anticline overturned (north-east 
of Aberfoyle) south-eastwards. On the north-west lies the Islay 
Anticline, overturned north-westwards in connexion with the Loch- 
Skerrols Thrust. It is important to realize that the relationship 
of the Loch-Skerrols Thrust to the Islay Anticline shows almost 
certainly that this particular thrust is of rather late date in 
Southern Highland history ; and also that it originated during a 
phase of north-westward movement. There is additional and quite 
independent evidence for the inferred north-westward movement. 
The nature of the foundation which emerges from beneath the 
Loch-Skerrols Thrust is such as to suggest a correlation between 
this important dislocation and the Moine Thrust of Ross and 
Sutherland (1917, fig. 3, p. 138); and it is well known that the 
displacement along the Moine Thrust has been directed towards 
the north-west (pp. 88 & 90). 


The Ballappel Foundation. 


Constitution.—The Eilde Flags are one of the most im- 
portant stratigraphical units of the Ballappel Foundation. For 
reasons already explained (p. 94). certain rocks situated on a 
lower structural level than the Hilde Flags of the Loch-Eilde Mor 
eutcrop are classified in Pl. I as constituents of a Sub-Eilde 
Complex. Ina stratigraphical sense the rocks of the Sub-Eilde 
Complex are zncerte sedis. Their cover, however, can be arranged 


in sequence as follows (1910 a, 1912 6, 1914. 1916) :— 


Eilde Flags (commonly classed with the Moine Gneisses of the Central 
Highlands). 

Quartzite and Mica-Schist assemblage including two certain 
members, the Glen-Coe Quartzite and the Leven Schists (Mr. R. G. 
Carruthers argues for six instead of two subdivisions, see p. 94). The Glen- 
Coe Quartzite is fine-grained, except that near the head of Loch Creran 
and in Glen Strae it carries conspicuous pebbles. The Leven Schists are 
greenish-grey mica-schists (or phyllites) with a marked tendency to lamina- 
tion. They are often somewhat garnetiferous, and in the districts of Glen 
Spean and Glen Etive they are rich in pseudomorphs after actinolite. A 
feature of the group is the Banded Series, which connects the more pelitic 
portions with the Glen-Coe Quartzite; in this Banded Series, quartzose beds 
are very common, while black graphitic seams and calcareous lenticles are 
generally to be found. The Banded Series is, as a rule, subordinate in bulk ; 
but it assumes immense proportions about the head of Loch Creran, and 
thence north-eastwards towards Loch Etive. 

Ballachulish Limestone, of which there are two main subdivisions— 
(1) bands of cream-coloured limestone at the margin of the Leven Schists 
followed by pale-grey, more or less calcareous mica-schist, often highly 
quartzose, and (2) a dark-grey or black, relatively pure limestone at the 
margin of the Ballachulish Slates. 

Ballachulish black graphitic and pyritous Slates. 

Appin Striped Transition Series. 

Appin Pebbly Quartzite. 


part 2] THE SOUTH-WEST HIGHLANDS OF SCOTLAND. 105 


Appin, cream-coloured, pink or dark-striped, dolomitic Limestone; 
sometimes in two well-marked subdivisions separated by flaggy quartzite 
and phyllite. 

Appin grey Phyllites (or Mica-Schists), often with a large propor- 
tion of flaggy quartzite. 

Cuil-Bay black graphitic Slates. 


Original order of deposition.—The local evidence of 
aes. i 
relative age is much less conclusive in the Ballappel Foundation 
than in the preceding cases. I venture, however, to suggest that 
the order of statement in the previous section, with Hilde Flags 
at the top and Cuil-Bay Slates at the bottom, corresponds with 
the original order of deposition. My reasons are as follows :— 


(1) Stratigraphical correlation between the great structural subdivisions of 
the Highlands is at the present time tentative. Still, there is much to be 
said for the commonly accepted view that the Ballachulish Slates, Easdale 
Slates, and, in a broad sense, the Tayvallich Slates are on one and the same 
horizon (p. 106). If so, then, since the Easdale and Tayvallich Slates can 
both be shown to be younger than their associated pebbly quartzites, it 
follows that the Ballachulish Slates are younger than the Appin Quartzite. 

(2) Such a belief is in accord with the interpretation of the best-known 
slides of the district as thrusts (p. 90). 

(8) Such a belief leads also to the conclusion that the great recumbent 
folds of the Ballappel Foundation originated during south-eastward movement 
in conformity with the phenomenon that took place at higher levels, as 
evinced by the Ben-Lui and Carrick-Castle Folds. 


Structure.—The stratigraphical characteristics of the Bal- 
lappel Foundation are wonderfully helpful. The groups are, for 
the greater part, both distinctive and constant. ‘This renders 
mapping easy, and the reading of tectonic features correspondingly 
accurate and full. The more important results are as follows :— 


(1) Great recumbent folds are characteristic of the district. Two main 
examples, both of them closing south-eastwards, have been named the 
Ballachulish and Appin Folds respectively. 

(2) The development of these great folds has been accompanied by very 
extensive sliding”: in any particular fold, slides are not necessarily restricted 
to one hmb—they may occur in both; some of them are thrusts, some lags. 
The Ballachulish Slide, which occurs in the lower limb of the Ballachulish 
Fold, is interpreted by Mr. Macgregor and myself as having a displacement of 
more than 24 miles (1912 6, p. 174; the irreducible minimum is 9 miles, 1916, 
p. 83). 

(3) The recumbent folds and associated slides have been subjected to 
extensive secondary folding, often thousands of feet deep and isoclinal in 
character. A pronounced example is the Glen-Creran Syncline (fig. 1, p. 84), 
which may be regarded as a partial northward continuation of the Loch-Awe 
Syncline; the Glen-Creran Syncline is separated from the Ben-Lawers 
Syneline by a comparatively gentle anticline named after Glen Orchy (1912 b). 


Although slides of two kinds occur, it is reasonable to suppose 
that the dominant slides of the district are of the nature of thrusts, 
not lags. In my judgment the two most important slides are 
the Ballachulish and Fort-Wiliam Slides, which characterize the 


1 See also 1922, Report A, par. 5; Report B. 
2 Tbid. Report A, par. 5. 


106 MR. E. B. BAILEY ON THE STRUCTURE OF  [ vol. Ixxviil, 


lower limbs of the Ballachulish and Appin Folds respectively. 
I venture, therefore, to class these two slides as thrusts, which 
means, of course, that the Ballachulish and Appin Folds are anti- 
clines, and that their south-eastward close is an indication of 
south-eastward movement. 

The identification of the Ballachulish and Fort-William Slides 
as thrusts opens the way for the entrance of ‘nappe terminology.’ 
That. portion of the Ballappel Foundation which occurs above ‘the 
Ballachulish Thrust may be assigned to the Ballachulish 
Nappe, and is coloured accordingly in Pl. I; the next great 
mass, bounded above and below by the Ballachulish and Fort- 
William Thrusts respectively, may be spoken of as the Appin 
Nappe. 

A glance at Pl. I shows that the Ballachulish Nappe extends 
as a recognizable entity eastwards from Ballachulish to the limit 
of the district under consideration. Westwards between Loch 
Leven and Loch Creran it has been locally removed by trans- 
gression of the Iltay Nappe, which there rests directly upon the 
Appin Nappe. This latter is a very well-defined structural mass 
in the western part of the area, where it overlies a foundation 
consisting mainly of Hilde Flags (p. 94). South of the point 
where the Fort-William (Meal-a’-Bhuirich) Thrust is shown in 
Pl. I as losing itself in the Eilde Flags, the definiteness of the 
Appin Nappe fails. In this part of the district, the outcrops of 
the Sub-Eilde Complex serve as a valuable index to the general 
structure of the ground. 


Suggested Stratigraphical Correlations. 


My many years’ experience of the rocks of the three great 
structural subdivisions of the South-West Highlands has impressed 
me as much with their contrasts as with their resemblances; the 
Hilde-Flag and Glen-Coe Quartzite facies belong to the Ballappel 
Foundation ; the Ben-Ledi Grit facies to the Iltay Nappe; lavas 
and voleanic breccias are confined to the Loch-Awe Nappe. 

The most marked resemblance is the recurrence in each district 
of a thick series of sediments in which the sequence is black slate, 
transition series, pebbly quartzite, limestone. The agreement, so 
far as black slates! are concerned, is closest between the Ballachu- 
lish Slates of the Ballappel Foundation and the Easdale Slates 
of the Iltay Nappe, both of them the seats of an important 
roofing-slate industry. The comparison of the transition zone is, 
on the other hand, more telling in the case of the Iltay and Loch- 
Awe Nappes, where a conglomeratic tendency very commonly 
shows itself in this position as characteristic of a mixed assem- 
blage of black slate, black limestone, and quartzite. Some of the 
slate- and limestone-bands, as well as the quartzite, are charged 


1 It is an arguable point whether the Loch-Avich and Tayvallich Slates 
are not wholly included in the transition zone. 


part 2] THE SOUTH-WEST HIGHLANDS OF SCOTLAND. 107 


with pebbles of quartz and felspar, and where these are par- 
ticularly big they are often accompanied by rock-fragments. In 
the main, these latter are strictly local in origin (slate, limestone, 
quartzite, and, in the Loch-Awe Nappe, lava); but nordmarkite 
and other foreign boulders have been found by H. Kynaston 
ClL903>p2 ol) and Dr, Bb. IN? Beach (1911; p: 71) at two 
important localities in the Loch-Awe Nappe. 

Dr. J. 8. Flett (1911, p. 75) has shown that the foreign boulders 
of the Loch-na-Cille Conglomerate (Loch-Awe Nappe) agree 
in type with those occurring in the Portaskaig Conglomerate 
of the Iltay Nappe. This is a very significant fact, for it 
emphasizes what most Highland geologists consider a certainty, 
that much of the quartz and alkali-felspar of the Crinan Quartzite 
(Loch Awe) came from the same source as the similar material of 
the Islay Quartzite (Iltay). But the reader is warned against 
thinking that.the nordmarkite-boulders afford evidence for corre- 
lating the definitely voleanic conglomerate of Loch-na-Cille with 
the definitely non-voleanic conglomerate of Portaskaig. The two 
- are distinct, both in character and in associates. ‘The difference 
of character depends mainly on the circumstance that at least 90 
per cent. of the fragments included in the Loch-na-Cille Con- 
glomerate are of lavaform rocks unknown in the Portaskaig 
Conglomerate. The contrast of associates will be understood from 
the following statement :—The Crinan and Islay Quartzites, viewed 
broadly, are in each case susceptible of twofold division: one part 
is fine or relatively fine in texture, the other coarse. The Loch- 
na-Cille Conglomerate lies on the coarse side of the Crinan 
Quartzite, in a position marked by constant recurrence of con- 
spicuous quartz- and felspar-pebbles. The Portaskaig Conglo- 
merate, on the other hand, les on the fine-grained side of the Islay 
Quartzite. 

Without elaborating the subject, I offer below a table of corre- 
lation. It is important to remember that age-sequences have 
been fixed by very convincing evidence in the Iltay and Loch-Awe 
Nappes, and by suggestive relationships in the Ballappel Founda- 
tion. ‘This in itself adds to the weight of the proposed cor- 
relations :— 


Loch Awe. Iltay. Ballappel. 
Ben-Lui Schists. Leven Schists. 
Ben-Lawers Cale-Schists. Ballachulish Limestone. 
Easdale Slates. Ballachulish Slates. 
Tayvallich Slates, Lime- Transition Zone. Striped Series. 
stones, and Conglome- 
rates. 
Crinan Quartzite. Islay Quartzite. Appin Quartzite. 
Shira Limestone. Portaskaig Conglomerate Appin Limestone and 
and Islay Limestone. Phyllites, and Cuil- 


Bay Slates and (pos- 
sibly) Lismore Lime- 
stone. 

Ardrishaig Phyllites. Mull-of-Oa Phyllites. 

Erins Quartzite. Maol-an-Fhithich Quartzite. 


108 MR. E. B. BAILEY ON THE STRUCTURE OF _ [ vol. xxviii, 


The inter-nappe correlations suggested above are treated in this 
part as an end in themselves. They are not employed as links in 
the chain of evidence supporting the general structural interpreta- 
tion, except in so far as they add somewhat to the strength of the 
conclusion that the Ballappel succession is correctly stated in 
descending order of age on p. 104. I may add, perhaps, that a 
visit to Blair Atholl (Perthshire) in 1920 impressed me very 
strongly with the value of the correlations here suggested. One 
cannot well question that the Perthshire Quartzite as exposed in 
Ben-y-Glo belongs to the Islay Quartzite, and yet its associates 
remind me irresistibly of Appin Limestone and Phyllites (with a 
weak development of Portaskaig Conglomerate) followed by Cuil- 
Bay Slates and Islay Limestone. In the Ballachulish district the 
sequence ends with Cuil-Bay Slates—unless the tantalizingly 
isolated Lismore Limestone, restricted to island occurrences, be 
distinct from the Ballachulish Limestone with which it has been 
hitherto correlated.t 


Ill. Crirrcat Discussion oF THE DisrRIcT REACHING FROM 
ARDMUCKNISH TO Ben LUT. 


Having now clambered somewhat laboriously from the summit 
of the Loch-Awe Nappe across the Iltay Nappe right down to 
the Ballappel Foundation, the reader is in a position to turn 
round, as it were, and view with comprehension the outstanding 
features of the structural succession considered as a whole. Near 
Dalmally, the outcrops of the Loch-Awe Group and Hilde Flags 
approach unusually close, while whole assemblages, well known to 
intervene between these two farther north-west or south-east, as 
the case may be, are here entirely absent (fig. 5, p. 109). It is 
my present task to indicate how some of these discrepancies are 
attributable to recumbent folding? and others to thrusting, and 
in so doing to bring my long description toa close. Unfortunately, 
it will be impossible to avoid the introduction of considerable local 
detail, for much of the reasoning is based on hitherto-unrecorded 
field-observations. 


Appin and Ballachulish Folds. 


Since the Appin and Ballachulish Folds have been very fully 
described (1910 a; 19126; 1914; 1916), the following dogmatic 
statements regarding them must suffice :— 


(1) The two folds close quite clearly south-eastwards. 

(2) The groups enumerated in explanation of A & B, fig. 5, and with 
them the Ballachulish Limestone, do not continue far underground in the core 
of the Appin Fold. I once thought that a limestone exposed near Loch 
Dochard (12 miles south-east of Ballachulish) was probably the Ballachulish 
Limestone enclosed in the Appin Fold; but I now regard it as occupying a 
lower structural level (p. 94). 


1 Tt is coloured as Ballachulish Limestone in PI. I. 
2 See also 1922, Report A, par. 6. 


part 2 | THE SOUTH-WEST HIGHLANDS OF SCOTLAND. 109 


(8) Of the groups enumerated in explanation of A & B, the Cuil-Bay 
Slates are unrepresented in any exposure of the Ballachulish Fold; in the 
same fold the Appin Phyllites and Limestone do not extend into Glen Coe, and 
fail to reach more than about 2 miles east of the head of Loch Creran (1914, 
pl. xlv); the Appin Quartzite continues to Glen Coe, about a mile east of 
Ballachulish, and then stops; while near the head of Loch Creran it is found 
no farther east than the Appin Limestone; the Ballachulish Slates are still 
recognizable, some 10 feet thick, in Coire Mhorair, half-a-dozen miles east of 
Ballachulish: they are, however, absent in the Windows of Etive, and near 
the head of Loch Creran are little more persistent than the associated Appin 
Quartzite, Limestone, and Phyllites. 

(4) Admittedly, the limitations just outlined are rendered a little indefinite, 
owing to the part played by slides in the construction of the Appin and 
Ballachulish Folds. At the same time, I hold (and I do not think that anyone 
familiar with the evidence would disagree) that the Appin Quartzite and 


Fig. 5.—Diagrammatic section illustrating the geological 
contrasts north-west and south-east of Dalmally. 


N.W. SE. 


KINTYRE, 
APPIN DALMALL COWAL, & 
er eee LOCH TAY 
Locu Awe Group | 
Aroaisware PHYLLITES V 


Easpate Scares A it 
i O07 i 


Futoe frags 
Svus-Escve Compre 


[A & B=Cores of the Appin and Ballachulish Folds constituted of Cuil-Bay 
Slates, Appin Phyllites, Appin Limestone, Appin Quartzite, and Balla- 
chulish Slates. 

L=Core of the Ben-Lui Fold, constituted of Ben-Lawers Schists, Ben-Lui 
Schists, Loch-Tay Limestone, Pitlochry Schists, Green Beds, Ben-Ledi 
Grits and Schists, Aberfoyle Slates, and Leny Grits. ] 


Ballachulish Slates do not extend south-east of the Windows of Etive in 
either the Appin or the Ballachulish Fold. 

(5) The Ballachulish Nappe can be recognized with considerable confidence 
in the Dalmally district, where it has been interpreted as consisting of a 
great mass of Leven Schists overlying a tolerably constant remnant of 
Ballachulish Limestone. Mr. M. Macgregor and I have succeeded in tracing 
its outcrop from the south-eastern corner of the Etive Granite, right round 
the Glen-Orchy Anticline to Ben Doirean and beyond (1912 b, p. 172 & pl. x). 
The limestone rests, through the intervention of the Ballachulish Thrust, 
upon an attenuated representative of the Banded Series of the Leven Schists. 
The Banded Series in its turn rests on Glen-Coe Quartzite, very thick in the 
south and thin or absent in the north, and below this come Hilde Flags with 
local exposures of the Sub-Hilde Complex at Ben Udlaidh and (presumably) 
at Loch Dochard too. 


Wa 


& 
> 
~4 
© 
) 
= 
ra) 
~“ 


hog ysiuyonupsy puv wosayy yoorr : dou ynorhojoag—'9 “Siyq 


part 2] THE SOUTH-WEST HIGHLANDS OF SCOTLAND. iA 


Contact of the Ballappel Foundation and the 
Iltay Nappe at Loch Creran, 


Several workers have attacked the difficult problems of Loch 
Creran at one time or another in the past—Symes, Grant Wilson, 
Kynaston, Peach, and myself. Looking back at our record of 
partial success and partial failure, I feel that the difficulties as 
they at first presented themselves were insurmountable—that, in 
fact, we were confronted with a geological redoubt that could not 
be expected to capitulate until the whole surrounding country had 
been subjugated. In the summer of 1919 I renewed the assault, 
and remapped on the 6-inch scale the critical portion of the district 
covered by fig. 6. I then found that the present state of our 
knowledge of the South-West Highlands afforded a simple solution 
of much that had been previously inexplicable. 

The thrust shown as a thick black line in fig. 6 divides the 
district into two dissimilar portions. These will now be considered 
in some detail from north-west to south-east. First of all, it may 
be stated that the dip everywhere tends to be steep, while the 
more yielding rocks show very obvious corrugations. 


The Ballappel Foundation, north-west of the Thrust. 
—Three lithological belts have been mapped, succeeding one 
another from north-west to south-east as follows :— 


(1) Limestone, never fully exposed, owing to limitation on the north-west 
by sea or raised beach. Four good exposures occur: the northernmost on 
the shore west of Airds Hill; the next on the eastern shore of Airds Bay; the 
next on the western shore of Hriska; and the southernmost on the shore at 
Ardentinny. The northernmost exposure is the fullest, and measures some 
200 yards across the strike. It reads as follows from north-west to south- 
east :—Dark-grey highly-calcareous slates, with thin grey limestone-seams ; 
dark-grey slates ; sandy-grey limestone weathering brown, except that some 
of the purer bands weather grey ; thick, white, sandy, thinly-bedded limestone 
weathering brown; passage to grey phyllite, which at first has white limy 
streaks. 

(2) Phyllite, constituting a belt rather more than a mile wide, and well 
seen in coastal and inland exposures. The deposit is predominantly greenish- 
grey, and is in part laminated, in part homogeneous. As very subordinate 
characteristics of the belt, one may note the occasional occurrence of slightly 
calcareous bands, isolated layers of quartzite,and also seams of dark phyllite. 

(3) Limestone.—The best exposures from north to south are:—At the 
northern edge of fig. 6, beside a hill-track 500 yards south-south-east of 
Strathappin Farm—ochreous-weathering calcareous phyllites, with a band of 
sandy white limestone; 400 yards north-north-west of Ledgrianoch Farm 
and about 50 yards east of the high road—buff-weathering phyllitic limestone 
seen for 20 yards across the strike; beside the path east of Ledgrianoch— 
ochreous-weathering calcareous phyllites (and here I may say that these 
three very important exposures north of Loch Creran might very easily have 
been overlooked, and that we owe their discovery entirely to Grant Wilson). 
East of the road and west of Lochan Dubh—pale phyllitic limestone ; and, 
lastly, a series of exposures, a mile and a half long and 100 yards broad, 
leading to the coast at Camas an Fhais——very calcareous phyllites with white 
or yellowish limestone-beds. and (on the shore) a few feet of black slate and 
black limestone on the south-east side of the outcrop. 


112 MR. E. B. BAILEY ON THE STRUCTURE OF | vol. lxxviii, 


The claim that these three lthological belts belong to the 
Ballappel Foundation is easily vindicated. The limestone (1) is 
the southern continuation of the Ballachulish Limestone, which, in 
the upper limb of the Appin Fold, continues north-eastwards beyond 
the limits of fig. 6 for more than 80 miles. As has been already 
stated, the exposure of the limestone within the area covered by fig. 6 
is incomplete ; it is on this account that one cannot point to a purer 
black portion of the belt, but what is seen is thoroughly typical 
in character. Its associates on both sides—the gap in the section 
on the north-west side is not extensive—tell exactly the same tale. 
Of these associates the phyllite (2) on the south-east is manifestly 
the southern continuation of the western part of the type-outcrop 
of Leven Schists. It presents no noteworthy change of character, 
except a decrease in metamorphism (fig. 3), and this difference 
traced on the ground is found to make its appearance quite 
gradually. Finally, the limestone (8) must be regarded as a 
folded reappearance of the Ballachulish Limestone. It reproduces 
exactly the characters which are found in the limestone (1) with 
this addition that, in the southern coast-section (Camas an Fhais), 
it shows what may be regarded as a beginning of the pure black 
portion of the group. 


The Thrust.—The heavy black line of fig. 6 stands for a 
thrust. The local evidence points clearly to a dislocation of first- 
class importance, and this evidence I shall now consider under two 
headings :— 


(1) In the first place, the succession of the Ballappel Foundation, as exem- 
plified close at hand in the core of the Appin Fold or at the head of 
Loch Creran in the core of the Ballachulish Fold (1914; 1916, p. 51), 
would lead us to expect one of two things on crossing the south-eastern lime- 
stone belt: either a return to Leven Schists or, failing this, a continuance 
into the black part of the Ballachulish Limestone followed by Ballachulish 
Slates. Instead, one steps abruptly on to a quartzite, which in the south is 
of imposing dimensions. ‘ 

(2) Two exposures of the junction of the limestone with the unexpected 
quartzite can be closely examined. One occurs west of Lochan Dubh,} the 
other on the shore at Camas an Fhais. In both cases the ocular evidence of 
movement is very striking indeed. The junction is definitely transgressive, 
and the shearing is extreme. 


Tt will be noted that the evidence afforded by (1) is much more 
serviceable in many ways than that derived from (2): (1) impli- 
cates the whole line of contact as traced in fig. 6 for half-a-dozen 
miles; whereas (2), considered by itself, might be interpreted as 
of very local significance. That the dislocation, where well ex- 
posed, is definitely betrayed by an appearance of excessive shearing 
is due in large measure, I think, to the very low grade of meta- 
morphism of this particular district (fig. 3). 


Iltay Nappe south-east of the Thrust.—tThe series of 


1 See also 1922, Report A, par. 9. 


part 2] THE SOUTH-WEST HIGHLANDS OF SCOTLAND. 113 


rock-belts ushered in by the quartzite just mentioned present 
themselves in the following order :— 


(1) Quartzite, forming a continuous belt which is easily traced, about 
100 yards broad in its northern exposures, and 500 yards broad, where, in 
Garbh Ard, it finally goes out to sea. It is essentially fine-grained (in fact, 
pebbles are very rare indeed), thinly bedded, and white. 


(2) Greenish-grey Phyllite with dark seams. In the north the outcrop is 
600 yards wide, but is interrupted by four or five bands of quartzite, 
with sufficiently broad outcrops to be shown on a 6-inch map; they are 
probably folded repetitions of (1). Southwards the phyllite outcrop is un- 
broken, and is only 100 or 200 yards wide. A good exposure is afforded on 
the northern shore of Loch Creran, and another in a ridge, a little south of this 
loch, at Baracaldine Castle. The phyllite is last seen succeeding the Garbh- 
Ard Quartzite (1) south-west of Lochan Dubh. 


(3) Black Slates followed by a mixed assemblage of Black Slate, Quartzite, 
and subordinate Limestone. 


The succession grouped under (8) can be studied in shore- 
sections on the north side of Loch Creran (8a), or south of the 
loch eastwards from Rudha Garbh (340), or, again, at Selma on 
Ardmucknish Bay (8 ¢) :— 


(3a) North of Loch Creran, jet-black graphitic slates, rich in pyrites, give 
place south-eastwards to black slates varied by the incoming of quartzite- 
bands, some of them pebbly ; but, for about 1000 yards, these quartzite inter- 
calations are quite subordinate. Then follows a belt of predominant 
quartzite, forming a fairly prominent ridge. Where examined, the quartzite 
of the ridge proved to be well bedded and associated with thin partings of 
black slate. Pebbles were noted in some of its bands. Perhaps this quartzite 
has a_ discontinuous outcrop pitching up into the air without actually 
reaching the shore; but the exposures leave this in doubt. Beyond a gap 
in the coast-section (which appears to correspond to black slate with occa- 
sional pebbly beds seen a little inland) fairly definite quartzite is once again 
encountered, constituting the headland at the turn of the loch and also the 
north-eastward trending coast for a mile beyond. The quartzite in these 
excellent shore-exposures carries intercalations of dark and black slates, as 
well as of more or less sandy, pale-grey, dark-grey, and black limestones. All 
three rock-types (quartzite, slate, and limestone) are often seen to be pebbly 
with large grains of blue and white quartz and felspar, and one of the 
limestone-beds carries in addition small pellets of penecontemporaneous 
sediment. 

(3b) The exposures south of the loch, east from Rudha Garbh, are too 
similar to those just described on the opposite shore to deserve detailed 
description. At Rudha Garbh, black slate predominates, pyritous as usual, 
and associated with subordinate quartzite and black limestone. This belt is 
followed eastwards by a succession of outcrops in which the mastery lies 
sometimes with quartzite, sometimes with black slate; while dark limestone 
is in either case a subordinate, though characteristic, associate. The quartzite 
is generally fine in texture, but occasionally pebbly with grains of blue and 
white quartz and also felspar; and some of the dark slaty bands carry 
similar pebbles. This alternation continues until slaty-grey phyllite takes its 
place about 100 yards west of a fault introducing lavas of Old Red Sandstone 
age into the shore-section. 

(8c) The Selma exposures on Ardmucknish Bay recall those of Rudha 
Garbh, with its black slate, limestone, and quartzose beds; the section is 
chiefly noteworthy for a cleaved breccia, which Dr. J. 8. Flett has given good 
reason to believe may be a crush-conglomerate (1908 b, p. 58). 


Q.J.G.S. No. 310. 1 


114 MR. E. B. BAILEY ON THE STRUCTURE OF _[vol. lxxviu, 


The assemblage of interbedded black slate and quartzite just 
described is referred to the Iltay Nappe for two reasons :— 


(1) Character.—This mixed assemblage, except that it does not appear 
to contain a definitely conglomeratic horizon, almost exactly reproduces the 
Searba development of the Transition Group connecting the Islay Quartzite 
and Hasdale Slates. Only two other groups need be mentioned as possible 
rivals, namely, the Tayvallich Slates of the Loch-Awe Group (Loch-Awe 
Nappe), and the Striped Series connecting the Appin Quartzite with the 
Ballachulish Slates (Ballappel Foundation). Of the Tayvallich Slates it may 
be said that not only are they more conglomeratic than the Creran rocks, but 
also they are richer in limestone, while everywhere they are accom- 
panied by voleanie rocks. Of the Striped Series, that it presents a 
more definitely continuous interbanding of quartzite and slate; while its 
limestones, in my experience, never carry pebbles. 

It will be noticed that the correlation here advanced refers the Garbh-Ard 
Quartzite to the Islay position. Now, in Islay, Jura, and Scarba, and east- 
wards to Ben Vrackie in Perthshire, the part of the quartzite bordering the 
Transition Group is distinctly pebbly, whereas in Garbh Ard it is fine-grained. 
Possibly this betokens a local facies, or possibly it indicates a mechanical 
omission of the pebbly division from the Garbh-Ard outcrop. 

(2) Position.—Island and mainland exposures show that a belt of 
Easdale Slates of persistent character reaches north-north-eastwards from 
Luing, through Oban, to the mouth of Loch Etive. The slates are constantly 
emerging from under a cover of Old Red Sandstone sediments and lavas. 
Their larger outcrops are shown in Pl. I, while the position of several smaller 
inliers will be found in a text-figure of the forthcoming Geological Survey 
Memoir dealing with the 1-inch Map, Sheet 44. 

At Oban, where the Easdale-Slate belt is fully 5 miles wide, the sea hides the 
probable continuation of both the Transition Group and the Islay Quartzite. 
The first schists seen beyond the gap (in Mull, Lismore, and neighbouring islets) 
belong quite definitely to the Appin Fold of the Ballappel Foundation. Accord- 
ingly,itis betweenthe Hasdale Slates at the mouth of Loch 
Etive and the exposure of the Appin Foldin the western 
part of Ardmucknish that one would naturally expect the 
Transition Group and Islay Quartzite to reach the main- 
land. The absence of other members of the Islay succession, as, for 
instance, the Portaskaig Conglomerate and Islay Limestone, is readily attri- 
butable to the obvious dislocation which brings the Garbh-Ard Quartzite into 
conjunction with the Ballachulish Limestone (fig. 6, p. 110). 


The evidence of position forbids the reference of the Ard- 
mucknish-Creran rocks to the Tayvallich Group of the Loch- 
Awe Nappe. The Kilbride Inlier, east of Oban (PI. 1), almost 
certainly indicates a northward extension of the Craignish belt of 
Ardrishaig Phyllites, beneath Old Red Sandstone lavas, on its way 
to link up with the Pass-of-Brander exposures, where the same 
phyllitic group extends continuously from Loch Etive to Loch Awe, 
and so to Loch Fyne and Ardrishaig. Dr. B. N. Peach has de- 
scribed the Kilbride Inlier as probably in part Easdale Slates, in 
part Ardrishaig Phyllites (1908 4, p. 38). I could find nothing in 
it that did not seem definitely of Ardrishaig type—but such a 
difference of opinion is, of course, of quite secondary importance 
in this particular connexion. 

The evidence of position, if one were restricted to the Loch- 
Creran district, would, on the other hand, seem to favour a corre- 
lation of the tay Nappe with the Ballachulish Nappe, since little, 


part 2] THE SOUTH-WEST HIGHLANDS OF SCOTLAND. 115 


if anything, occurs between the Appin and Iltay Nappes in Ard- 
mucknish. The alternative (illustrated in fig. 5, p. 109) is that 
the Iltay Nappe has almost or quite pushed aside the Ballachulish 
Nappe in the Ardmucknish district; but the evidence for this in- 
terpretation is reserved for the following section, where it will be 
shown that black slate and pebbly quartzite of the LIltay Nappe 
have a south-eastward extension quite out of keeping with the 
recognized limitations of such rocks in the Ballachulish Nappe. 


In the foregoing argument no attention has been paid to a very 
confusing feature of Loch-Creran geology. An important fault, 
known as the Strath-Appin or Pass-of-Brander Fauit, crosses the 
loch at the eastern corner of fig: 6. Its downthrow is to the south- 
west, as shown by its effect on the distribution of the Old Red 
Sandstone lavas. Along its course, west of the loch, rock-exposures 
are altogether wanting in a belt upwards of half a mile wide. 
North of this obscure tract, they occur again in their usual pro- 
fusion, but they can be shown to belong almost entirely to the 
Leven-Schist Group of the Ballappel Foundation. The contrast 
of character is, however, not nearly so marked as might be desired. 
The Banded Series of the Leven Schists (p. 104) is very strongly 
developed beneath the Ballachulish Nappe between the Balla- 
chulish Granite and Loch Etive, and is for several miles well seen 
about the head of Loch Creran. A prevalent rock-type is a grey 
quartzose schist, passing gradually into non-pebbly impure 
quartzite, with repeated laminations of black seams. 

It is not surprising that at first this Banded Series was supposed 
to belong to the same stratigraphical group as the rocks south of 
the fault, now referred to the Transition Group of the [tay 
Succession. ‘The reasons which render such a correlation untenable 
at the present time are :— 


(1) The frequent pebbly character, the limestone-bands, and the genuine 
black slates of the Transition Group are wanting in the Banded Series—a 
contrast which impressed Clough strongly when in conversation he disagreed 
with the correlation of the two accepted by Peach, Horne, Grant Wilson, and 
myself, during a joint traverse in 1906. 

(2) At the time when we made our joint traverse, Grant Wilson was of 
opinion that the Banded Series at the head of Loch Creran was merely a local 
facies of the Ballachulish Slates. It therefore appeared that, to correlate the 
Transition and Banded Series of Loch Creran, was simply equivalent to corre- 
lating the Easdale and Ballachulish Slates—a probable enough correlation 
even to-day (p. 106). When later it was shown that the Ballachulish Slates 
continue with constant character to the head of Loch Creran, and that the 
Banded Series belong to the Leven Schists, the whole situation changed. A 
correlation of the Transition and Banded Series of Loch Creran now involves 
a correlation of Hasdale Slates and Leven Schists within the limits of a single 
nappe; and this, with its attendant stratigraphical consequences, seems 
incredible. 

(3) When we made our correlation, we thought that the outcrop of the 
Transition Series gave place in toto to that of the Banded Series. I am now 
in a position to show (next section) that the pebbly-quartzite and black-slate 
series preserves its individuality in an outcrop which escapes eastwards round 
the northern end of the Loch-Awe Syncline. 


12 


Fig. 7.—Geological map: Loch Ktive, Pass of Brander, and Loch Awe. 


Sic 
eS 


/ 


E 
y= a 
LL) 
A 
= 
<< 


| 


PASS of BR 


mit 
, i 


i 


Pe! 


(TORT 


3 | 
NN 
< 


> ( 


part 2] THE SOUTH-WEST HIGHLANDS OF SCOTLAND. TE 


One last point deserves notice before we turn eastwards from 
Loch Creran. Inspection of Pl. I shows a very limited dis- 
continuous outerop of the Iltay Nappe between the Strath-Appin 
Fault and the Ballachulish Granite. Narrow exposures of massive 
white quartzite (pebbly in the south with quartz and felspar, and 
fine-grained in the north) have been met with intermittently along 
one particular line of strike. They probably represent little 
klippen of the Garbh-Ard (Islay) Quartzite. They lie within the 
belt of Banded Series, if this term be used in its broadest sense ; 
but for a mile on the south-east side grey phyllite is the prevalent 
rock of the district. 


Outcrop of the Iltay Nappe north of the Loch-Awe Nappe. 


It is here proposed to trace the outcrop of the Iltay Nappe in 
its eastward course between Loch Etive and Loch Awe, and 
onwards past Dalmally into Perthshire! The only part of this 
description that is definitely new refers to the discontinuous 
portion of the outcrop included within the contact-aureole of the 
Htive Granite (fig. 7). 


Ballappel Foundation and Iltay Nappe, north-west 
of the Loch-Awe Nappe.—-North-east of the Pass-of- Brander, 
or Strath-Appin, Fault there is an imposing display of the Banded 
Series of the Leven Schists all the way from Loch Creran to within 
about a mile of Loch Etive. Grey phyllitic material (notably 
hornfelsed, and displaying pitted surfaces indicative of cordierite 
wherever it comes within a mile of the Etive Granite) is associated 
with perhaps preponderant fine-grained, grey, quartzose schists, 
often interlaminated with dark seams. 

From our detailed knowledge of the geology of the head of Loch 
Creran (1914; 1916, p. 51), the whole of this tract can be referred 
with confidence to that portion of the Ballappel Foundation which 
immediately underlies the Ballachulish Nappe. There is no very 
marked change until, east of the angle of the River Esragan, a 
massive pebbly quartzite figures prominently in the crags of the 
hillside, 2 miles in from the left-hand margin of fig. 7. The 
quartzite dips steeply, and its outcrop measures 300 yards across 
the strike. As seen in the crags it is but little sheared, and 
contains subangular pebbles, often half an inch long, consisting of 
quartz and subordinate felspar; some of the quartz is blue. 

Although the incoming of quartzite in bulk is abrupt, exami- 
nation reveals the presence of minor pebbly quartzite-bands in the 
banded quartzopelitic hornfels (pitted with cordierite) within a 
zone about 400 yards wide bordering the quartzite on its northern 
side. This belt of interbedded phyllite and pebbly quartzite I 
have seen at intervals at the margin of the pebbly quartzite far 
into Perthshire, where it is conspicuously displayed, for instance, 
in the Pass of Killiecrankie. The Perthshire sections which 


1 See also 1922, Report A, par. 8. 


118 MR. E. B. BAILEY ON THE STRUCTURE OF [| vol. lxxviii, 


I studied in 1920 seem to establish the identity of the Kullie- 
crankie Group with the flag group strongly developed in Northern 
Jura and Scarba (1917, p. 152) within the limits of the Islay 
Quartzite. 

On the southern side of the pebbly quartzite, interbedded 
quartzite and black slate-hornfels continue for some distance, until 
towards Loch Etive they give place to black slate. This latter, 
scarcely indurated at all, is seen at the bridge at Inveresragan, and 
in baked condition in the Blarcreen Burn. Between the Blarcreen 
Burn and Bonawe Ferry there are many roadside exposures of 
hornfelsed black slate with a considerable proportion of quartzose 
stripes. I first saw the Inveresragan exposures of black slate in 
1914 exactly where I hoped to find them from my knowledge of 
the Dalmally sections (to be described presently). In 1919 T was 
able to return to the subject, and was rewarded by the discovery of 
the pebbly quartzite. I have no doubt, on the score of character 
and position, that this pebbly-quartzite and black-slate assemblage 
represents the eastward continuation of some part of the Islay 
Quartzite, Transition Group, and Easdale Slates, as represented in 
the Loch-Creran and Oban districts. 


Loch-Awe Nappe.—In Eilean Duirinnis, immediately north of 
Bonawe Ferry, banded calcareous hornfels occurs at the edge of the 
Etive Granite. On the other side of the ferry, continuing for a dis- 
tance of 6 miles, similar calcareous hornfels constitutes the strip of 

country between the Pass-of-Brander Fault and the Etive Granite. 
The calcareous laminze are mainly represented by malacolite, 
colourless garnet, epidote, and tremolite. An.excellent petro- 
graphical account has been given by Sir Jethro Teall (1908 8, 
p- 141). Near the eastern limit of the outcrop, metamorphosed 
limestone is more than usually prominent. 

I have examined this Pass-of-Brander outcrop several times, and 
heartily concur with H. Kynaston’s reference of it to the Ardri- 
shaig Phyllites. The reasons for placing the Ardrishaig Phyllites 
as a whole in the Loch-Awe, rather than in the Iltay Nappe, will 
be discussed later (p. 121). 


Ballappel Foundation and Iltay Nappe, north-east 
of the Loch-Awe Nappe-—When Mr. Macgregor and I 
described the geology of the Glen-Orchy Anticline before this 
Society, we’traced a thick structural succession above the Ben- 
Udlaidh outcrop of the Sub-Eilde Complex (19126, p. 172). At 
that time we did not speak of the Ballachulish Slide as a thrust, 
and accordingly did not use the term nappe; otherwise, the 
ensuing sequence is little more than a repetition of our i 
conclusions :— 

Black pelitie schist. 

Pebbly quartzite. 

Interbedded grey pelitic schists and pebbly 
quartzite. 


ee 


part 2] THE SOUTH-WEST HIGHLANDS OF SCOTLAND. 119 


Thick grey pelitic schist—Leven Schists. 
Ballachulish Nappe ; Thin, fairly persistent limestone—remnant of 

Ballachulish Limestone. 

Ballachulish Thrust. 

Banded pelitic schists and quartzite—remnant of 
Leven Schists. 

Quartzite, locally very thick—Glen-Coe Quartzite. 

Gneissose flagstones—Hilde Flags. 


Of these rocks, everything below the interbedded pelitic schists 
and pebbly quartzite is referable to the Ballappel Foundation. 
The remainder is obviously the interrupted continuation of the 
similar association cropping out west of the Etive Granite, and 
already correlated (p. 118) with some portion of the Islay Quartzite, 
Transition Group, and LEasdale Slates. The agreement will 
perhaps be better appreciated after reading the following detailed 
description :— 


Near the south-east of the granite, one passes north-eastwards from the 
Ardrishaig-Phyllite outcrop on to a belt of black slate-hornfels about 
700 yards wide. Graphitic layers and streaks are abundant, and rusty 
weathering due to pyrites is characteristic. At the same time quartzose 
material is well represented, locally giving rise to a band of quartzite. 

Beyond the black slate, one encounters an equal breadth of quartzite, 
which, though somewhat impure, is everywhere markedly pebbly, and but 
little split by partings. Beyond this main quartzite, one meets with grey pelitic 
hornfels in which for 800 yards bands of pebbly quartzite continue as a minor 
feature. 


The black slate mentioned above as succeeding the Ardrishaig 
Phyllites is not so prominent in this section as it is west of the 
Etive Granite, but this defect is made up in the Dalmally exposures 
east of the Glen-Strae Fault, where a very big development of 
black schist with subordinate limestone occurs. Mr. J.B. Hill has 
rightly emphasized the manner in which these black schists are 
for the greater part interposed between the Ardrishaig Phyllites 
on the south, and the pebbly quartzite on the north (1908 6, 


_p. 28), although the latter has its outcrop broken to some 


extent by bands of black schist. Various conclusions follow from 
the evidence adduced in the present and preceding sections :— 


(1) A pebbly-quartzite and black-slate assemblage has been traced north- 
eastwards from the Islay Archipelago to Loch Creran and thence eastwards 
to Dalmally and the Ben-Doirean Range.! This, with the grey pelitic group 
carrying the interbedded pebbly quartzite, will be termed for brevity the 
Islay-Easdale Assemblage; its outcrop is clearly shown in PI. I. 

(2) On the inside of the curved outcrop of the Islay-Hasdale Assemblage 
lie the Ardrishaig Phyllites of the Loch-Awe Nappe; on the outside, a varied 
succession depending on locality. 

(3) There is abundant evidence that the Islay-Easdale Assemblage underlies 
the Loch-Awe Nappe and overlies the rock-groups appearing along its outer 
margin. This evidence may be considered under the headings 3 a-38 c.” 


1 See also 1922, Report A, par. 8. 
2 Ibid. compare (3 a) & (8c) with Report A, par. 4. 


120 MR. E. B. BAILEY ON THE STRUCTURE OF [vol. ]xxvill, 


(83a) The detailed downward succession from the centre of the Loch- 
Awe Syncline through the various subdivisions of the Loch-Awe Group 
to the Ardrishaig Phyllites has already been traced-(p. 97). The con- 
formity of outcrop of the Islay-Easdale Assemblage shows that this 
assemblage must in its turn underlie the Ardrishaig Phyllites. It has 
already been pointed out that the general east-and-west outcrop of the 
Ardrishaig Phyllites near the northern end of Loch Awe is due to pitch. 
The significance of the sympathetic east-and-west deflection of the Islay- 
Easdale outcrop is unmistakable. 

(3b) Local evidence (1917) shows quite as clearly that the Portaskaig 
Conglomerate, Islay Limestones, Mull-of-Oa Phyllites, and Maol-an- 
Fhithich Quartzite, as exposed in the Islay Anticline, structurally underlie 
the Islay-Easdale Assemblage here considered. 

(3c) Local evidence further shows that, in the Glen-Orchy Anticline, 
(pp. 118 & 124) a descending structural sequence can be traced which 
leads from the Islay-Easdale Assemblage right down to the Sub- Hilde 
Complex as exposed in the Beinn-Udlaidh Fold. 


(4) It follows, then, that the Islay-EHasdale Assemblage overlies that part 
of the Ballappel Foundation with which it comes into contact in the Loch- 
Creran district. 

(5) Accordingly, if for no other reason, the Islay Quartzite cannot be 
interpreted as a special facies of the Glen-Coe Quartzite in normal contact 
with the Banded Leven Schists of the Loch-Creran district. The Glen-Coe 
Quartzite, which is seen in normal contact with the Banded Leven Schists at 
the head of Loch Creran and in the Windows of Etive, could only reappear 
in the district of Lower Loch Creran by emergence from below the adjacent 
Leven Schists. 

(6) The Islay-Hasdale Assemblage is also, for two good independent 
reasons (6a & 6b), distinguishable from the Appin Quartzite and Hasdale 
Slates of the Ballachulish Fold :— 


(6a) The Islay-Easdale Assemblage has been traced into the Ben- 
Doirean Range 15 miles south-east of the Windows of Etive, whereas 
the Appin Quartzite and Ballachulish Slates of the Ballachulish Nappe 
do not reach so far south-eastwards as these Windows (p. 109). 

(6 b) The Islay-Hasdale Assemblage overlies what are, with very high 
probability, regarded as the Leven Schists of the Ballachulish Nappe in 
the whole of the Glen-Orchy district (pp. 118 & 119). 


(7) It must be admitted that the Islay-Hasdale Assemblage is transgressive 
in its relationships. Otherwise, one would not find it in Ardmucknish resting 
upon the Appin Nappe, where one would naturally expect to meet with the 
Ballachulish Nappe (this statement holds substantially, even though the 
eastern limestone of Ardmucknish be regarded as a remnant of the Balla- 
chulish Na»pe, as it well may be). 

(8) As the transgression obviously does not antedate the movement which 
gave rise to the Ballachulish Nappe, it seems necessary to regard it as 
mechanical, not stratigraphical, in origin. 

(9) The conspicuous shear-zone separating the Islay Quartzite and the 
Ballachulish Limestone in the Ardmucknish Peninsula has been discussed 
(p. 112). Another interesting case of special shearing near the base of the 
quartzite has been noticed in stream-sections entering Glen Orchy from the 
east. There are intrusions of basic igneous rock in quartzose schist 
(probably Killiecrankie Group) in this position, and the two have been 
sheared and rodded in a most remarkable manner, with a resultant inter- 
banding of films, in certain cases no thicker than tissue-paper. 

(10) The absence of the Portaskaig Conglomerate and Islay Limestone in 
the mainland exposures can be very readily attributed to the thrust invoked 
above on other grounds. The non-occurrence of these characteristic zones is, 
of course, merely local, since they reappear farther north-east in Schiehallion. 


part 2] THE SOUTH-WEST HIGHLANDS OF SCOTLAND. 121 


Ben-Lui Fold and the Thrust at the Base of the 
Loch-Awe Nappe.! 


Before discussing the Ben-Lui Fold, I wish to express my in- 
debtedness to Mr. J. B. Hill and to the late H. Kynaston for their 
mapping of that part of the district which, in fig. 8 (p. 122) hes 
south of the outcrop of the quartzite of the [tay Nappe. The more 
northerly portion of fig. 8 is taken, practically as it stands, from 
pl. x of the account given by Mr. Macgregor and myself (1912 0); 
but the southern two-thirds of the map are based, with trifling 
changes, on Hill & Kynaston’s work published in Sheets 45 & 46 of 
the Geological Survey l-inch map. At the same time, it should be 
clearly understood that I alone am responsible for two important 
matters of interpretation expressed in the southern part of fig. 8: 
one, the distinction of the Ben-Lawers Schist from the Ardrishaig 
Phyllites; the other, the recognition of the thrust-plane at the 
base of the Loch-Awe Nappe. 

In 1891, Mr. Hill traced the Ben-Lawers and Ardrishaig Groups 
into contact with one another in the neighbourhood of Ben Lui, 
and thus, it was thought, established their stratigraphical identity 
(1892 6, p. 385). The correlation is, of course, supported by a 
close general lithological resemblance of the two groups; but there 
has always been a difficulty: the Ben-Lawers-Ardrishaig Complex, 
considered as a unit, has three persistently different margins. The 
successions outwards from this complex are as follows :— 


(1) Hasdale Black Slates, leading on to Islay Quartzite ; the local evidence 
points to the slates as later than the quartzite (p. 101). 

(2) Crinan (Loch-Awe) Quartzite, approached through Shira Limestone, 
and followed by Tayvallich Black Slates, Limestones, and Lavas ; the local 
evidence points to the slates as later than the quartzite (p. 97). 

(3) Ben-Lui Garnetiferous Mica-Schist, succeeded by Loch-Tay Limestone ; 
the local evidence points to the limestone as later than the schist (p. 101). 


Exposures are far from continuous, but it seems certain that - 
each of the three successions holds true for more than 50 miles 
along curving lines of outcrop. Manifestly normal faulting cannot 
be the explanation. In fact, only two alternatives are feasible: 
the first is to interpret the successions (1) & (2) as in some sense 
equivalent; the second to admit that extensive fold-faulting (or 
sliding, to use the shorter term) has been the determining factor 
of the triple margin. Three independent reasons (A-C) for 
adopting the second of these two alternatives are outlined below :— 


(A) Succession (1) cannot be directly equivalent to Succession (2), since in 
(1) the rock-groups met with on leaving the margin of the Ardrishaig 
Phyllites are increasingly old, while in (2) they are increasingly young. This 
age-relation—coupled with the fact that the lithology of succession (8) is 
admittedly too dissimilar to be correlated with either (1) or (2)—obviously 
demands a slide. At the same time, age-relations are netoriously difficult to 
establish in unfossiliferous rocks, and therefore it is well that other evidence 
is available. 


1 See also 1922, Report A, pars. 5 & 7; Report B. 


Glen Orchy and Ben Lui. 


Fig. 8.— Geological map : 


we, 
a | 
ean 


ZDALMA 


part 2] THE SOUTH-WEST HIGHLANDS OF SCOTLAND. 123 


(B) When the problem was approached by the Geological Survey, it was 
with the preconception that the Ben-Lui Schists were older than the Ben 
Lawers-Ardrishaig Complex, which latter was readily accepted as a strati- 
graphical unit. Moreover, large-scale slides were not at that time recognized 
as a phenomenon of the Southern Highlands. Accordingly, both the suc- 
cessions (1) & (2) were interpreted as of later date than the Ben Lawers- 
Ardrishaig Complex, and, in a general sense, as equivalent the one to the 
other. As regards details, the Islay and Crinan Quartzites were correlated, 
and local erosion was invoked to account for the continuous absence of 
Easdale Slates at the junction of the Crinan Quartzite and the Ardrishaig 
Phyllites. An attempt was also made to minimize the importance of the 
contrast of (1) & (2) by pointing to the Tayvallich Slates as widespread 
relics of the Easdale Slates. It is true that the Tayvallich Slates do not 
occur at the Crinan-Ardrishaig junction; but in the initial stages of the 
enquiry it was easy to postulate underground continuations of Ardrishaig 
Phyllites forming unexposed cores to imaginary anticlines wherever black 
slate showed itself in the quartzite area; also, it must be admitted, Dr. B. N. 
Peach mistook Loch-Avich Slates (1913 b, p. 290) for Ardrishaig Phyllites. 
In course of time, it has become apparent both to Dr. Peach and to myself 
that the Crinan Quartzite structurally intervenes between the Ardrishaig 
Phyllites, below, and the Tayvallich Slates, above (1911, Chap. v; for 
Mr. Hill’s criticism of this change of front, see 1911, p. 61; 19135, p. 306). 
The progress of research has thus tended to emphasize the lithological 
dissimilarities of the successions (1) & (2). 

But the reader may well ask whether the magnitude of the folding does 
not in itself afford a sufficient explanation. As pointed out already, the 
Easdale-Islay succession completely underlies the Ardrishaig Phyllites, while 
the Crinan-Tayvallich succession completely overlies the same. Accordingly, 
a correlation of the Easdale-Islay and Crinan-Tayvallich successions involves 
a recumbent fold with a core of Ardrishaig Phyllites not less than 20 miles in 
length, and, with so large a fold, a marked difference of facies might well be 
expected in the lower and upper limbs respectively. It is, however, un- 
necessary to develop this anticipated criticism, since it will be shown in the 
succeeding paragraph that the particular large-scale fold here contemplated 
is flatly contradicted by the local evidence. 

(C) Fig. 8 illustrates the type-area for the union of the Ben-Lawers and 
Ardrishaig outcrops. It also shows clearly that the Ardrishaig Phyllites, 
where they extend westwards between the Islay-Hasdale and Loch-Awe 
Assemblages, cannot be interpreted as a 20-mile fold-core. So massive and 
extensive a fold would have a recognizable core of Ben-Lui Schists for 
some part of its course, and such assuredly does not exist. The weight of 
this negative evidence will be better appreciated on consideration of the 
Ben-Lui Fold close at hand. Here, Ben-Lawers Schists are folded into the 
- heart of the Hasdale Schists; but they only extend a couple of miles west of 
the termination of their associated Ben-Lui Schists. 


Enough has been said to show that the original interpretation 
of the district cannot support our present-day knowledge without 
extensive reconstruction. I shall now pass to the consideration of 
the new interpretation, which is the only one that I have been 
able to devise to take the place of the old.! 


1 [When in the summer of 1921 I revisited the ground with a small party 
of geological friends, I found myself criticized for not having explained in the 
text of my paper (here printed) how impressive a phenomenon the Ben-Lui 
Fold really is: I could only answer that, as I had in this instance added 
nothing material to the data collected by my predecessors, I thought brevity 
justified. At the same time, it is perhaps well to state that the Ben-Lui 
Fold is spectacular, according to South-West Highland standards—albeit 


124 MR. E. B. BAILEY ON THE STRUCTURE OF _ [ vol. Ixxviil, 


It will be readily understood that a small district does not afford 
opportunities for establishing a big structure. Accordingly, in 
reading this description, constant reference must be made from 
Hee tO. L. 

In the first place, although the superposition of Ben-Lawers 
Schists on Ben-Lui Schists in the upper limb of the Ben-Lui Fold 
is quite obvious in the immediate neighbourhood of Ben Lu, 
there is no local evidence to show that this order of superposition 
is of wide extent. ‘To realize the importance of the upper limb of 
the Ben-Lui Fold, one has to remember the clear evidence from 
Loch Tay to Campbelton of the superposition of the Ben-Lui 
Schists on Loch-Tay Limestone—a superposition which continues 
for about 15 miles across the strike (p. 103). In this matter I 
have merely confirmed the findings of my predecessors. 

Again, the superposition of Ben-Lui Schists on Ben-Lawers 
Schists in the lower limb of the Ben-Lui Fold is quite an obvious 
feature of the geology of the northern face of Ben Lui. The 
reality of the downward succession is enforced on anyone who 
descends the tectonic ladder connecting Ben Lui with Ben 
Udlaidh, where the Sub-Hilde Complex is so definitely exposed. 
The extent of country through which the lower limb of the Ben- 
Lui Fold has been traced will escape no one who recollects how 
Easdale Slates overlying Islay Quartzite have been followed from 
Ben Lui westwards round the outcrop of the Ardrishaig Phyllites 
into the Islay Archipelago. 


Prof. Collet several times complained that grassy slopes compelled him to use 
his legs as well as his eyes to realize this fact. It may be helpful to record 
some important features recognized during our joint traverse :— 

(1) Ardrishaig Phyllites form a prominent escarpment, Ra Chreag, where 
they overlie the Easdale Slates of the upper limb of the Ben-Lui Fold west 
of Allt Coire Lair (for place-names, see 1-inch map, Sheet 45). 

(2) The junction of the Ardrishaig Phyllites and Easdale Slates can be 
located to within a foot or so at the base of Ra Chreag, in a stream which, at 
the valley-bottom, passes Corryghoil. 

(3) The Easdale Slates below Ra Chreag are marked scenically by a grassy 
slope, where exposures are almost restricted to stream-courses. 

(4) The Ben-Lawers Schists immediately below the Easdale Slates crop 
out as a subdued escarpment. 

(5) Although the easternmost stream-exposure of the Hasdale Slates is 
afforded by Allt Coire Lair, the Ardrishaig and Ben-Lawers escarpments are 
distinguishable for some distance farther. In Meall nan Gabhar the Ardri- 
shaig escarpment seems to overreach its companion and come into direct 
contact with Ben-Lui Schists. 

(6) The appearance of Meall nan Gabhar is very suggestive of a thrust- 
mass, riding upon a thrust-plane at the base of the Ardrishaig Phyllites. 
The features of Meall nan Gabhar almost certainly indicate that the bedding 
within the Ardrishaig Phyllites is much more steeply inclined south-south- 
westwards than is the base of the Ardrishaig escarpment. Probably the 
high dip is due to some species of schuppenstruktur, and its direction, 
as Mr. Richey pointed out to me, lends support to the view that the Loch- 
Awe Nappe has travelled in a general south-eastward direction. From this 
interpretation, it would follow that some of the many minor complications of 
the Loch-Awe Nappe belong, as might be expected, to the primary category 
of movements.—H. B. B., January 1922.| 


part 2 | THE SOUTH-WEST HIGHLANDS OF SCOTLAND. 125 


The age-relations of the black slates and quartzites found above 
and below the Ardrishaig Phylltes no longer present a difficulty. 
From Dalmally westwards the large-scale movement has been 
accommodated by thrusting without inversion. The movement 
has been interpreted as having come from the north-west, because 
the Ben-Lui Fold, to judge from these same age-relationships, is a 
syncline closing in that direction. 


LV. CoNcLUSION. 


Perhaps I may be pardoned for returning once again to two 
elements of Highland tectonics which should never be lost sight 
of. The first is the comparison of two sides of large-scale non- 
recumbent folds wherever these are recognizable; the second is 
the critical enquiry into any case where what seems to be a single 
rock-group has three—or more—distinctive margins. The first 
will always be connected in my mind with Clough and the Cowal 
Anticline ; the second with Mr. H. B. Maufe, who, in the days 
before the recognition of slides in the Southern Highlands, was 
wont to insist that one of our main problems was the three-sided 
limestone of Ballachulish. 

The beacon which has lighted the way in the development of 
the views expressed in the present paper has been the asymmetry 
of the Loch-Awe Syncline—on the one side the rocks of the Islay 
Archipelago, on the other those of Cowal. Is it credible that 
this asymmetry is due to change of facies ? No,—the Islay rocks 
can be traced right under the Loch-Awe Syncline and away 
through Perthshire, not transformed into Cowal rocks, but in 
contact side by side.! As for the Cowal rocks, they too can also 
be followed for some distance beneath the Loch-Awe Syncline, but 
presently they are found to double back upon themselves, and thus 
to return whence they came.” 

While it was easy to see this much, at any rate in a vague and 
general manner, still there was a difficulty in obtaining anything 
approaching to clear vision. The long-accepted correlation of the 
Ardrishaig and Ben-Lawers Schists blocked the way. But this 
correlation involved a three-sided formation (p. 121). It had, 
therefore, to be considered very critically. It was found unstable, 
and with its fall hght gained access. 


APPENDIX I—BIBLIOGRAPHY. 


1819. J. Maccutnocu.—‘ A Description of the Western Islands of Scotland & 
the Isle of Man, &c.’ 

1840. J. MaccuLLocu.—Geological Map of Scotland, with Memoir (posthumous). 
1852. D. Suarp.— The Arrangement of the Foliation & Cleavage of the Rocks 
of the North of Scotland’ Phil. Trans. Roy. Soc. vol. exlii, p. 445. 

1858. J. Nrcou.—Geological Map of Scotland and Explanatory Note. 
1859. R. I. Murcutson.—‘ First Sketch of a New Geological Map of the North 
of Scotland’ Q. J.G.S. vol. xv, p. 419. 


1 See also 1922, Report A, par. 8. 
2 Ibid. Report A, par. 6. 


126 MR. E. B. BAILEY ON THE STRUCTURE OF _[ vol. Ixxviul, 


1861 (a). T. F. Jamreson.—‘ On the Structure of the South-West Highlands of 
Scotland’ Q.J.G.S. vol. xvii, p. 133. 

(6). R. I. Murcutson & A. GEIKIE.—‘ The Altered Rocks of the Western 
Islands of Scotland & the North-Western & Central Highlands’ Q.J.G.S. 
vol. xvu, p. 171. 

1863. J. Nicot.— The Geological Structure of the Southern Grampians’ 
Q.J.G.S. vol. xix, p. 180. 

1884 (a). C. Catnaway.— Notes on Progressive Metamorphism’ Geol. Mag. 
p. 218. 

(6). B. N. Peacu & J. Horne.—‘ Report on the Geology of the North-West 
of Sutherland’ Nature, vol. xxx1, p. 31. 
1891. A. GErkre.—Presidential Address, Q. J. G.S. vol. xlvii, Proc. p. 63. 
1892 (a). A. GEIKIE.—Geological Map of Scotland, revised 1910. 
(6). ‘ Annual Report for 1891’ Mem. Geol. Survey. 

1897. W. Gunn, C.T. Clovuen, & others.— The Geology of Cowal’ (Explanation 
of Parts of Sheets 29 & 37), Mem. Geol. Surv. Scot. (Petrological Notes 
by J. J. H. Teatt & F. H. Hatcn). 

1899. J. B. H11ii.—‘ The Progressive Metamorphism of some Dalradian Sediments 
in the Region of Loch Awe’ Q.J.G.S. vol. lv, p. 470. 

1903 (a). P. Macnatr.— The Building of the Grampians’ Trans. Roy. Phil. Soc. 
Glasgow, vol. xxxiv, p. 147. 

(6). W. Gunn.—‘ The Geology of North Arran’ (Explanation of Sheet 21), 
Mem. Geol. Surv. Scot. 

1904 (a). E. H. CunnrncHam-Crate.— Metamorphism in the Loch Lomond 
District ’ Q. J.G.S. vol. lx, p. 10. 

(6). ‘Summary of Progress for 1903’ Mem. Geol. Surv. 

1905. C. T. Clove, J. B. Hr11, & others.—‘ The Geology of Mid-Argyll’ (Ex- 
planation of Sheet 37), Mem. Geol. Surv. Scot. 

1906. P. Macnatr.— The Development of the Great Axial Lines of Folding in 
the Highland Schists’ Trans. Roy. Phil. Soc. Glasgow, vol. xxxvii, 
p. 129. 

1907 (a). S. B. Winxinson (with B. N. PEacu).—‘The Geology of Islay’ 
(Explanation of Sheets 19 & 27, and part of 20), Mem. Geol. Sury. Scot. 

(6). B. N. Peacu, J. Horne, & others.—‘ The Geological Structure of the 
North-West Highlands of Scotland’ Mem. Geol. Surv. Scot. 

[(c). C. Sarasin & L. W. Cottet.—‘ La Zone des Cols & la Géologie du 
Chamossaire’ Arch. Sci. Phys. & Nat. vol. xxiv, p. 586. ] 

1908 (a). P. Macnatr.— The Geology & Scenery of the Grampians.’ 

(6). H. Kynaston, J. B. Hrut, & others——‘ The Geology of the Country 
near Oban & Dalmally’ (Explanation of Sheet 45), Mem. Geol. Surv. 
Scot. 

1909 (a). B. N. Peacu, H. Kynaston, H. B. Mavure, & others—‘The Geology 
of the Seaboard of Mid-Argy 1? (Explanation of Sheet 36), Mem. Geol. 
Surv. Scot. 

(6). ‘Summary of Progress for 1908’ Mem. Geol. Surv. Scot. 

1910 (a). E. B. Batney.— Recumbent Folds in the Schists of the Scottish 
Highlands’ Q.J.G.S. vol. lxvi, p. 586. 

(6). KE. B. Barney.—‘ New Structures in the Argyllshire Highlands’ Trans. 
Edin. Geol. Soe. vol. ix, p. 363. 

(c). J. W. Grecory.—‘ Work for Glasgow Geologists—The Problems of the 
South-Western Highlands’ Trans. Geol. Soc. Glasgow, vol. xiv, p. 1. 

1911. B. N. Peacu, C. T. Ctoueu, E. B. Battey, & others —‘ The Geology of 
Knapdale, Jura, & North Kintyre’ (Explanation of Sheet 28), Mem. 
Geol. Surv. Scot. 

1912 (a). G. Barrow.—‘On the Geology of Lower Dee-Side & the Southern 
Highland Border’ Proc. Geol. Assoc. vol. xxiii, p. 274. 

(6). E. B. Battey & M. Macerecor.— The Glen-Orchy Anticline (Argyll- 
shire) > Q. J.G.S. vol. lxviil, p. 164. 

1913 (a). ‘Summary of Progress for 1912’ Mem. Geol. Surv. 

(6). E. B. BattEy.—* The Loch-Awe Syncline (Argyllshire)’ Q. J. G. S. 
vol. lxix, p. 280. 

1914. E. B. Barrzy.—‘The Ballachulish Fold near the Head of Loch Creran 
(Argyllshire)? Q. J.G.S. vol. Ixx, p. 321. 

1916. E. B. Barney, H. B. MauFe, & ce The Geology of Ben Nevis & 
Glen Coe, &e.’ (Explanation ‘of Sheet 53), Mem. Geol. Surv. Scot. 

1917 (for 1916). E. B. Bartey.— The Islay Anticline (Inner Hebrides)’ Q.J.G.S. 
vol. Ixxil, p. 182. 


Quart. Journ.Geol. Soc. Vol. LEAVITT, Pt. I. 
| 


Quart. Journ. Geol. Soc. Vol, LAXVMT PL I. 


BAILLAPPEL SESE TE INNE | / LOCH “AWE 
FOUNDATION NAPPE 


LACHUILISH 


a) 
= 
+a 


Bat 


Bilde Flags. 

Glen, Coe Qaartrite ( Pete.) 
Lever Schists. 
Ballachudish Limestone. 
Ballachitlish Slates. = 


Leny Grits. 


Tayvallich Slates. 
Gian Quartzite. | 
=| Shira Limestone. 
Pitlochry Schists Til 4 SS \Ardrisharg 
Appin (uartzxtte. Loch Tay Limestone. ae Phy ites. 
Appin Limestone. x Ben Lit. Schists. 6 S| Eris Quartile, 
Appin Phylittes. é Ben -Lawers Schrsts. 
Cail Bay STates : Easdale Stites. 


(perhaps Insmare Islay Quartztte. ais ‘ : Lavas abound in the | 
Limestone should ee Tayvallich Slates and tr 
follow here), te lower part of the 
Islay Limestone i x Loch Avich States. 


Wilt -of Oa Phy littes. 


Maol-anFhithich Sd A 
Quartxite. - F = HATO = 


Portashatg Conglomerate. 


SUB-EILDE COMPLEX. 


i ee, Mica Schists, & 


fat Loch Dochard) Lst. 


FORELAND of L. 
SKERROLS THRUST. 


Lewtsian Gnetss & 
supposed Torridonten 


Colonsay. \» 


_ Old Red Sandstone & 
Later Sediments & Lavas. 


ally 


gj 
dill 


—— 3 
| 


Kx x 


30 le x_| Granite 


AD 


Major Thrusts. 
CLS 


DIAGRAMMATIC SECTION 
TO SHOW SUPERPOSITION OF NAPPES. 


{9 |20 
[l2 


CC. — Carrick -Castle Fold. 


S.W.HIGHLANDS —24 | Ble = Borclasi Fold 


Gammbelion LOCH- AWE _ COWAL 
SYNCLINE ANTICLINE 


E.B.Bailey. 
1:633,600. 


20 miles 


- part 2] THE SOUTH-WEST HIGHLANDS OF SCOTLAND. 127 


1922. ‘The Structure of the South-West Highlands of Scotland. Two Reports 
on an Excursion, August 27th to September 2nd, 1921.’ Report A by 
E. M. Anperson, H. H. Reap, J. E. Ricuny, & W. B. Wrieut. 
Report B by L. W. Cornet. Geol. Mag. (in the press). 


In addition to the above, the Geological Survey has published Sheets 12, 19, 20, 
21, 27, 28, 29, 30, 35, 36, 37, 38, 45, 46, and 53 of the 1-inch (1: 63,360) Map of 
Scotland; also a special Map of Arran on the same scale; Sheet 13 (Islay) 
x inch to 1 mile; anda general map of Great Britain & Ireland on the scale of 
25 miles to the inch. 


AppEnpix I[]—Locatiry-InpEx. 


Scottish localities mentioned in the text, and at the same time 
not indicated in Pl. I, are enumerated alphabetically below. Their 
positions are stated by coordinates within the various numbered 
sheets of the Geological Survey 1 : 63,860 map of Scotland; many 
of these sheets are outlined on Pl. I :— 


For example, Airds Bay lies 2 miles west and 17 miles north of the south- 
western corner of Sheet 45: Airds Bay, 45, 2,17; Airds Hill, 45, 4, 17; Aonach 
Beag, 53, 20, 16; Appin, district including Appin Station, 53, 4, 0, and Port 
Appin, 45, 2,17; Ardentinny, 45, 1, 15; Ardmucknish Bay, 45, 2, 12; Argyllshire, 
stretching eastwards to Ben Lui, 46, 1, 6. 

Ballachulish, district with Ballachulish Village (East and West Laroch), 53, 13, 7, 
and Ballachulish Ferry, 53, 12,8; Baracaldine Castle, 45, 38,14; Beannan Dubh, 
27, 21, 2; Beinn Bheula, 37, 20, 6; Beinn Udlaidh, 46, 2, 10; Ben Vrackie, 
55, 19, 12; Ben y Glo (Beinn a’Ghlo), 64, 20, 0; Blair Athol, 55, 14, 13; Blarcreen 
Burn, crossed by road at 45, 8, 11; Bonawe Ferry, 45, 9, 9. 

Camas an Fhais (Camas Nathais), 45, 1, 12; Coire Mhorair, 53, 19,7; the Crinan 
Canal follows a fault south-eastwards from Crinan to Loch Fyne. 

Eilean Duirinnis, 45, 9,10; Erins, 29, 1,9; Hriska, 45, 2, 15. 

Garbh Ard, south-eastern headland of Ardmucknish, 45, 1, 12; Glen Creran, 
south-eastern valley in 53, mouth at 45, 9,17; Glen Etive, valley passing through 
the Windows of Etive, 53,18, 2; Glen Orchy, south-eastern valley through 46, 0, 10; 
Glen Shira, south-eastern valley, mouth at 37, 16,14; Glen Sluan, hamlet, 37, 15, 
7; Glen Spean, east-and-west valley through, 62, 23, 4; Glen Strae reaches Loch 
Awe, 47, 17, 7. 

Inveresragan, 45, 8, 11. 

Killiekrankie Pass, 55, 17, 11; Kilmory Bay, 28, 15, 9; Kinlochleven, 53, 20, 10. 

Lairigmore Pass, 53, 16,11; Ledgrianoch Farm, 45, 4,16; Leny Pass, 38, 21, 13; 
Loch Avich, 37, 5,16; Loch Creran, 45, 6,16; Loch Dochard, 45, 22, 15; Lochan 
Dubh, 45, 3, 14; Loch Eck, 37,18,0; Loch Goil, 37, 22, 6; Loch Lomond, 38, 10,0; 
Loch na Cille, 28, 14, 12; Loch Skerrols, 27, 17, 1; Loch Torridon, 81, 15, 16; 
Luing Island, 36, 17, 18. 

Maol an Fhithich, 19, 14, 8; Meall a’Bhuirich, 53, 24, 15; Meall nan Tighearn, 
45, 23, 4; Moine, 114 central. 

Perthshire, east of Ben Lui, 46, etc.; Pitlochry, 55, 18, 9; Portaskaig, 27, 22, 5. 

River Esragan, bend at 45, 8, 12; Ross-shire, 92, etc.; Rudha Garbh, 45, 4, 14. 

Schiehallion, 55, 4, 6; Selma, 45, 2,12; Shira (see Glen Shira); Skye, 70 & 71; 
Southern Uplands, south of line from 14, 0, 0 to 33. 19,15; Stack of Glen Coul, 
107, 23, 6; Strath Appin, east-north-eastern valley through, 45, 5,17; Strathappin 
Farm, 45, 5,17; Stronchullin, 29, 0, 12; Sutherland, 101, ete. 

Tappins, 8, 10, 15; Torridon (see Loch Torridon). 


EXPLANATION OF PLATE I. 


Generalized geological map of the South-West Highlands, based upon the 
l-inch map of Scotland of H.M. Geological Survey; revised and reduced 
by the Author. Scale: 10 miles=1 inch, or 1: 633,600. The numbered 
divisions correspond with the sheets of the 1-inch map. 


128 THE STRUCTURE OF THE [vol. Ixxviul, 


DISCUSSION. 


Mr. G. Barrow regretted that he had been unable to follow the 
Author in his repeated references to different districts. Having 
visited much of the area, he did not agree either with the Author’s 
view of the structure of the country or his nomenclature of the 
rocks. 'The existence of the ‘nappes’ shown on the map seemed 
most unlikely, as the speaker felt sure that individual beds could 
be traced across from one to the other. Judging from some of the 
lantern-slides exhibited, he suspected that the supposed ‘nappes’ 
were simply different aureoles of thermal alteration, similar to 
those mapped out in the South-Hastern Highlands, which were 
shown in detail in the pamphlet prepared by the speaker for the 
use of the Geologists’ Association, at the Dundee Meeting of the 
British Association. 

Dr. J. 8. Fuerr said that everyone interested in Highland 
geology admired the enthusiasm with which the Author had 
attacked the problems under discussion. It was a task of extreme 
difficulty to unravel the structure of the Southern Highlands, and 
no means of accomplishing that end could be neglected. Among 
others, recent geological work in the Alps has furnished new ideas, 
especially in regard to the features of ‘nappes,’ which were sure 
to be applied to the Scottish Highlands, and were likely to prove 
of cardinal importance. In his paper the Author had suggested 
that in Argyllshire three ‘nappes’ could be recognized. He had 
discarded the sequence hitherto adopted by Scottish geologists, 
and advanced a new correlation which was in harmony with his 
views of the structure. 

The speaker, while recognizing the attractive character of the 
Author’s hypotheses, did not feel convinced that the solution now 
offered was established on an incontrovertible basis. The vast 
movements postulated did not seem to have produced a corre- 
sponding effect on the outcrops. The rocks of Ballachulish and 
Loch Awe were very much those which would be expected to occur 
there if no ‘slides’ existed, or if the ‘slides’ were of small 
magnitude. The Ballachulish slide, for example, did not seem, in 
a large part of its course, to have shifted the outcrops to a notable 
extent. This might be because, as the Author suggested, the 
movement was very nearly parallel to the bedding-planes. But 
great movements could not be so confined, and we might expect, 
occasionally at any rate, to find that beds were brought into juxta- 
position which normally were widely separated. So far as the 
speaker had seen, the evidence both of the maps and of the natural 
exposures was not in favour of the large movements in which the 
Author believed. It was very desirable that a clear case of 
transport, with difference of facies in the exotic and the auto- 
chthonous beds and something resembling a visible thrust-plane 
should be brought to light. The Author’s slides were practically 
contined to the outcrops of the Central Highland Series (from the 


part 2] SOUTH-WEST HIGHLANDS OF SCOTLAND. 129 


Appin Limestone to the Leven Schists) and, so far as the evidence 
went, might be purely local adjustments due to the folding of that 
group. 

Of the Loch-Awe Nappe it was difficult to form a clear opinion. 
The presence of a volcanic group in that district did not necessarily 
imply that these rocks were not in place, as voleanic eruptions were 
often localized, and lavas had been found on what the speaker 
believed to be the same horizon, in Upper Banffshire. The 
‘ Boulder-Bed’ occurs in Islay, in Tayvallich, in Perthshire, in 
Aberdeenshire, and in Banffshire, always in association with a 
quartzite, limestone, and black or grey shales, and forms certainly 
one of the most useful landmarks in the correlation of Highland 
rocks. In that case the Loch-Awe Series could be traced from 
Islay to Portsoy and the Ballachulish Series from Ardmucknish (or 
perhaps Easdale) to the shores of the Moray Firth, and the varia- 
tion was by no means great. It was difficult to understand how in 
Loch Awe and Ballachulish, despite enormous displacements, the 
rocks were exactly of the type which occurred in similar positions 
evervwhere along the southern edge of the Moine Gneiss. 

The Argyllshire nappes were evidently of an entirely different 
type from their Swiss analogues. The Scottish nappes rested on 
no marked plane of disturbance ; they have no ‘roots’; their 
facies is that of the country in which they are found; their 
metamorphism is similar to that of the surrounding rocks ; they 
are not markedly transgressive ; and they seem to differ in no im- 
portant respects from autochthonous strata. At present, it was 
necessary to place them to a suspense account. Nothing would 
be more welcome to Scottish geologists than the proof that Alpine 
tectonics were repeated in typical development in the Southern 
Highlands, but much work had still to be done before that day 
arrived. 

Mr. H. H. Reap said that all workers in the Highlands were 
greatly indebted to the Author for his application of the wonderful 
results achieved by Alpine geologists to the solution of Highland 
problems, but the speaker considered that the major structures of 
the Highlands, owing to the metamorphosed condition of Scottish 
rocks, could never be demonstrated with even a small part of that 
definiteness which characterizes Alpine tectonics. 

It was unfortunate that there was so little correspondence 
between the metamorphic zones and the structural features of the 
eround described by the Author, and this, together with the 
absence of dislocation-metamorphism (not necessarily myloniza- 
tion), especially in the Loch-Awe region, was, in the speaker’s 
opinion, a somewhat serious objection to the Author’s inter- 
pretation. The Author’s suggestion that the presence of carbon 
in the Ballachulish Slates had prevented the formation of garnet 
did not apply to the garnetiferous, graphitic, and carbonaceous 
schists of Banffshire. 

The speaker would weleome some information as to where the 


Q.J.G.S. No. 310. K 


: 


130 THE STRUCTURE OF THE [vol. Ixxvin, 


Author considered the roots of his nappes to lie. The ‘ Iltay’ 
assemblage could be followed from Perthshire into Banffshire, 
where the speaker had worked for several years. Away to the 
north-west of the Banffshire rocks stretched Moine granulites for 
70 miles to the thrust-ground of the North-West Highlands. 
If the roots were in the thrust region, were they covered by the 
thrusts? The Moine region, the speaker held, carried nothing 
suitable as roots for the Dalradian assemblage. 

The Author suggested that the ‘ Ballappel Foundation’ was 
characterized by Hilde Flags and Glencoe Quartzite. But in 
Banffshire the speaker found rocks of ‘Iltay’ facies linked in- 
separably on the west with a quartzite and granulites of Glencoe 
and Hilde types. If the Banffshire Dalradian rocks were to be 
included in the ‘ Iltay’ nappe, then so musta large part of the 
Moine Series be similarly included. The phenomena of the Great- 
Glen Fault appeared to indicate to the speaker that the Dalradian 
rocks overlay the Moine Series. But the Author, in his tabie, 
showed the Hilde Flags at the top: the speaker disagreed with 
that sequence. Correlation from Banffshire to Perthshire could 
be made in some detail, and, if the Author’s table for the ‘ Iltay’ 
nappe were read from bottom to top, similar rock-types occurred 
in Banffshire from west to east. But correlation could be made, 
with equal chances of proving correct, from Banffshire into the 
‘ Ballappel Foundation.’ The Banffshire rocks, from west to east, 
were exactly like the Author’s ‘ Ballappel’ table read from top 
to bottom, and the speaker considered that this table should be 
reversed, so as to place the Hilde Flags at the bottom. That com- 
plicated still further the Ballachulish structures. 

The speaker believed in the reality of sliding or thrusting, but 
he considered that the existence of nappes in the South-West 
Highlands had not yet been demonstrated. 

Lorp CrirrorpD remarked that the Author’s evidence concerning 
the irregular folding of some of the strata in this part of Scotland 
appeared to support a theory that the speaker had long held, that 
folds and faults are mainly due to oceanic subsidences. Between 
a line drawn from the Firth of Lorne to the Moray Firth on the 
north, and a line drawn from Morecambe Bay to Sunderland on 
the south, there lies a tract of country that has been subject to 
more strain and contortion than almost any other portion of the 
world. -The folding of this part of Britain and the reversal of 
strata in the manner described by the Author are results that one 
would naturally expect. 

The AurHoR, in reply, said that Dr. Flett’s statement that 
Scottish geologists were searching the Highlands everywhere for 


analogues of Swiss structures might, perhaps, convey a wrong 


impression. The speaker, as a matter of fact, developed his 
interpretation of the Ballachulish district before reading the 
Alpine literature or visiting the Alpine exposures. It had been 
an added pleasure to find, on comparison of the two mountain- 


part 2] SOUTH-WEST HIGHLANDS OF SCOTLAND. 131 


chains in the field, how close the resemblances were. It was an 
asset to the new reading of the Highlands to have opponents of 
the standing of Dr. Horne and Dr. Flett, for it invited critical 
enquiry. The only regret that the speaker felt was that these two 
opponents had not investigated the field evidence. One claim at 
least might be made for the interpretations that the speaker had 
put before the Society: they did conform with exposed outcrops. 
On the other hand, while Dr. Flett had been speaking, it was often 
difficult to realize that he intended his remarks to apply to the 
South-West Highlands. 

The Author apologized for the lateness of the hour preventing 
him from answering in detail points raised by other speakers, 
and added that a small committee of geologists in Scotland had 
volunteered to investigate any criticism of the paper that might 
be tabled, so long as it dealt with mainland exposures. 


132 DR. G. L. ELLES ON THE BALA COUNTRY: [Vol. Ixxviii, 


3. The Bata Country: its STRUCTURE and RocK-SUCCESSION. 
By Gerrrupe Lintan Epes, M.B.E., D.Se., F.G.8. (Read 
March 9th, 1921.) 


[Puate II—Grotoaicat Map. | 


CONTENTS. 
Page 
Ti. Imbrodwetion ori oc c0 5. tee see peters te cae och aie ee eee 132 
I. ‘General Physical Heatumes: 27s aces 6 ccs Gee eee eee 135 
ENT.Che:-GeneralsSuccesston. 9...) 60. clea hod eee ee ees 
ING Detailsvot SpecialsSections-).5.s.0) eee eee eee 144 
Vi Structure ofthe; District we. eee eee ee ee nS i7/ 
Wi... Paleontology .2.\eAel iio en ee 
Vik Comparisoniwath otercAreas 2). eee eee eeeeee 166 
Wa. atime aS GS: ie nes «crim wattieicsai eevee ek Ga acre ee 169 


I. InrRopDUCTION. 


The district dealt with in the present paper hes all round Bala - 
Lake, though attention has been paid more particularly to the 
ground lying south-east and east of the lake, as affording an 
opportunity for the study of problems of special interest regarding 
the succession and structure of the area. 

The whole district has been classic ground since the days of 
Sedgewick, and its difficulties were vividly described later by Jukes ; 
it is just these difficulties that make the area so intensely 
interesting, and, largely owing to the more detailed mapping that 
can be carried out in these days, it is now possible to suggest a 
solution of some, at any rate, of the problems—more particularly 
those of a stratigraphical and structural nature. The palzeonto- 
logical problems are somewhat different, and will not be dealt with 
in detail at present. 


Previous Work. 


By far the most important contributions made to our general 
knowledge of the district are those of Sedgwick, Jukes, and 
tuddy, though Ramsay and, in later times, Lake have dealt with 
the question of the faulting as connected with the development 
of the valley in which the lake lies. 

Sedgwick made two important contributions to the study of 
the area. In the earlier paper on the ‘ Calcareous Slates & Lime- 
stones of Glyn Dyffws on the Holyhead Road, west of Corwen, & 
of Rhiwlas north-east of Bala,’! he gave what he believed to be 
the succession in the area, and illustrated it by sections made as 


1 Proc. Geol. Soc. vol. iv (1843) p. 252. 


part 2 | ITS STRUCTURE AND ROCK-SUCCESSION. 188 


early as 1832. This paper was later incorporated into a more 
general treatise! As he did not fully realize the complexities of 
the structure of the country, Sedgwick’s succession has naturally 
undergone some modification in later days, but the papers are 
noteworthy since he clearly understood that the faunas of the 
Rhiwlas and Bala Limestones were distinct; he drew attention, 
moreover, to the nature of the beds above the Bala Limestone, 
and indicated the true position and character of the Hirnant 
Limestone. 

J. de C. Sowerby & J. W. Salter, who contributed notes on the 
fossils, remarked on the paucity of brachiopods in the Rhiwlas 
Limestone as compared with their abundance in the Bala Lime- 
stone, mention being made of Orthis actonie as a highly charac- 
teristic form. They also record the distinctive peculiarities of the 
brachiopods of the Hirnant Limestone. 

In the paper published in 1852 2 on the ‘ Classification & Nomen- 
elature of the Lower Paleozoic Rocks of England & Wales,’ 
Sedewick notes the Coniston Limestone as the equivalent of the 
Bala Limestone, and subdivides the Bala Group of rocks into an 
Upper and Lower, the Upper Bala beginning with the Bala Lime- 
stone, including the Hirnant Limestone and Shelly Sandstone, and 
ending with the dark indurated Shales, passing in places into a bad 
pyritous roofing-slate. The position of the Rhiwlas Limestone is 
not indicated in this paper. 


The area was officially surveyed by J. B. Jukes, and the results 
of his work are given in the two editions of the Geological Survey 
Memoir on North Wales.? It is marvellous that Jukes was able 
to show so much on the 1-inch map, and it is clear from his notes 
that his general conclusions accorded more completely with those 
that I have reached than the 1-inch map would indicate, although 
my own conclusions have only been arrived at after laborious 
mapping on the 6-inch, and often on a still larger scale. Thus, in 
the end, Jukes was convinced that there was only one ash-bed 
in the lower part of the series east of Bala Lake, although he notes 
the possibility of the existence of more elsewhere, and records the 
occurrence in places of a definite ash-bed immediately below the 
limestone. My work has confirmed the existence of only one ash 
in the lower part of the series, not merely in the area east of 
the lake, but on the west side also, and again in the ground on 
the north so far as I have seen it. Jukes is also perfectly definite 
as to the nature of the Bala Limestone, and, from his remarks 
concerning the ash immediately below the limestone, it seems 
evident that he ineluded in the Bala Limestone not only the massive 
limestones, but the whole of the Calcareous Ash—in fact, he 


1 Q. J.G.S. vol. i (1845) p. 6. 

2 Ibid. vol. viii, p. 136. 

3 Mem. Geol. Surv. vol. iii (1866) chaps. xiii & xiv; vol. iii, 2nd. ed. (1881) 
chaps, xv & Xvi. : 


134 DR. G. L. ELLES ON THE BALA COUNTRY: [ vol. Ixxvil, 


states that the limestone is ashy in places, and also non-calcareous 
occasionally, ‘when it is not to be differentiated from the sand- 
stones.’ He does not, however, consider that the Bala Limestone _ 
is distinct from the Rhiwlas, being unappreciative of the value of 
the paleontological evidence. 

He records the existence and nature of the Hirnant Limestone 
at the head of the Hirnant Valley, and notes the position of beds 
ot the same age at other localities. 

His remarks on the structure of the country are highly sug- 
gestive: he notes the existence of the anticlinal line separating 
the synclinal lines of Bryn Pig and Creigiau Bychain,! notes the 
broken nature of the ash outcrops, and calls attention to the 
parallelism of certain lines of faulting. He also suggests that the 
lake les in a trough open to the south, of which the apex lies 
north of Moel Emoel at Pen y Bwlch-gwyn. He estimates the 
thickness of the beds between the ash and the limestone as ranging 
from 1200 to 1400 feet. 


In his admirable paper ‘ On the Upper Part of the Cambrian & 
Base of the Silurian in North Wales,’ Thomas Ruddy? de- 
scribes definite lines of section, and makes the earliest attempt at 
a paleontological classification of the beds, the fossils from each 
horizon being sedulously recorded and the actual succession seen 
at certain localities carefully described. The nature of the Hirmnant 
Beds, apart from the limestone, is noted, and attention is drawn to 
the change in character which they undergo when traced along 
their strike as also their relation to the overlying Tarannon Beds. 

Ruddy was an indefatigable collector, although his collections 
were not always made from beds zz situ, and his fossil lists have 
been of the greatest service to me, especially the summary given in 
his ‘ List of Caradoc or Bala Fossils found in the Neighbourhood 
of Bala, Corwen, & Glyn Ceiriog.’? 

Ruddy’s sections are generally easy to follow, with the exception 
of that across Gelli Grin, where the faulting of the beds appears 
to have escaped his notice, and therefore two ash-beds are noted 
instead of one. He accepts the official view as to the identity of 
the Bala and Rhiwlas Limestones, but considers the Hirnant Beds 
to be of Llandovery age. 


Su Andrew Ramsay * was the first to mention the continuity 
of the Dee-Valley and Bala-Lake fault, stating that it has an 
invariable downthrow to the north-west, so that part of the strata 
south and east of the lake are repeated by it and reappear on the 
west and north. 

He estimates the maximum amount of throw as 12,000 feet; 


1 The spelling of the place-names ee in this paper is that used in the 
6- — Ordnance Survey maps. 
2 Q. J. G.S8. vol. xxxv (1879) p. 200. 
3 Proc. Chester Soe. Nat. Hist. no. 3 (1885) p. 113. 
* Q.J.G.S. vol. ix (1853) p. 161. 


part 2] ITS STRUCLURE AND ROCK-SUCCESSION. 135 


but, later in the Geological Survey Memoir on North Wales (2nd 
ed. 1881), he gives 11,000 feet as a more probable figure, and 
states that the throw diminishes north-westwards in the region of 
the lake to 5000 feet. The angle of the fault, as drawn, shows a 
steep north-north-westward dip, but it is admitted that the angle 
is hypothetical. 


Philip Lake, in his paper on ‘ Bala Lake & the River-System 
of North Wales,’! deals with the faults in the region south-west 
of the lake, and considers that there are two faults traversing the 
lake, one parallel to the south-eastern shore (though some little 
distance within the water’s edge) and the other parallel to the 
north-western shore. 


In his paper on the ‘Composition & Structure of the Hirnant 
Limestone,’ L. W. Fulcher? points out that the grains in the 
rock are ellipsoidal in shape, measuring 1 to 38 mm. in their longest 
diameter, and they are more sparsely scattered in the crystalline 
matrix than in an ordinary oolite or pisolite; he also notices that 
the blackness of the grains, which is so marked a feature of the 
rock, is due to the presence of carbon in an amorphous form. 


Il. GENERAL PHystcaL FEATURES. 


The country is essentially a hilly one: on the south-east of the 
lake the ground rises steeply from lake-level (580 feet) to 1000 feet, 
the steepest part of this rise appearing to coincide with a well- 
marked line of fault; above the altitude of 1000 feet, however, 
the slopes are gentler and less regular up to the summit of Moel- 
fryn (1750 feet) ; beyond the summit the ground falls somewhat 
rapidly into the steep-sided Hirnant Valley, but rises again beyond 
to a height of about 2000 feet. On the north-west side of the 
lake the rise is also steep to the 900-foot contour-line, and is 
probably determined by another fault-line. Above this height 
there is a marked contrast with the opposite side of the lake, for 
there is a wide stretch of merely undulating moorland with scarcely 
any rise until the base of Arenig is reached, when the ground rises 
very steeply to the twin-peaked summit of Arenig Fawr 
(2800 feet), this being the most conspicuous height in the district. 
There is a far more gradual rise towards the hilly country lying on 
the north. The valleys are, as a rule, conspicuously deep and 
steep-sided, and are occupied by swiftly running streams, though, 
with the exception of Nant Rhyd Wen, they are still largely filled 
with Drift. Only one stream appears to show any features of 
interest, and that is the Hirnant: starting as a strike stream, it 
soon makes a bend at right.angles to its former course through 
Bwlch-yr-Hweh, although the existence of a well-defined channel 


1 Geol. Mag. 1900, pp. 204, 241. 
2 Ibid. 1892, pp. 114, &e. 


S.E. 


in the original direction at some 
height above its present course is - 
suggestive of capture by a stream 
with greater corrosive power, 
possibly due to the Bwlch-yr- 


2 Hwch Fault. The Hirnant con- 


Moel-fryn 


= 
= 
= cm - = 
2 2, © tinues to run as a strike stream 
a 33 again as far as Aber Hirnant, 
= oe where it makes another bend; 
_~ o = Oo 7 ’ 
eG c ie and the lower part of its course 
— 5S : 
= a= is clearly controlled by the strue- 
ae =o ture of the country. 
I : 28 
2 Es 
= Sas Il. Tre Genera 
= // see SUCCESSION. 


“ r. —— The rock-suecession here de- 
~ = 5 scribed comprises pre-eminently 
= g i 2 a series of shallow-water depo- 
= Ee = 6. sits: even the black Nant-hir 
S = oa = 

~ = = - Shales appear to be current- 
= 3 2.22 bedded, and to this we may per- 
"= = zm . - Poy 

S > =< = haps ascribe their unfossiliferous 
S z an ee : 

Bale ts, ~2£2 nature. The Sandy Mudstones 
ber E = =O contam bands and even whole 
- + 25 beds ripple-marked and to some 
> : <=O extent also current-bedded, while 
in the oolitic nature of the Gelh- 


5 
G: 
7 


Grin Limestone at Y Garnedd 
points in the same direction. 

The succession is interesting, 
mainly on account of the 
development of the ashes and 
limestones. The ash-beds, of 
which there are two in all, belong 


om Bala to Moel- 


Llangower displacement 


& S = to the Caradocian ; both die out 
= <z _ southwards, although the lower 
= 2 =3& is the more persistent, and both 
Ss 5 8 serve admirably to bring out the 
a 3 Shs structure of the country. The 
ca = S—+  Jimestones are remarkably im- 
e z =o = persistent, and occur more or 
pe a © =<£ less sporadically throughout the 
hy ="5s series; they have little or no 

3, / a | a value as stratigraphical units.! 

i 
if = = Ie Bes - : 

/ Some places mentioned in the fol- 
lowing pages lie outside the area of 
the accompanying map (Pl. Ii), but 
they may be found on the 1-meh 

P \ Ordnance Map and on the 1-imch Geo- 
. = f) ))) x logical Map, Sheets 74 N.W. & S.W. 
ase ££ 
| £8 / 
| Se aad 
42 8 A/ 


part 2] STRUCTURE, ETC. OF THE BALA COUNTRY. 137 
Graptolite 
Shelly Faunas. Zones. 
Zone of 
4 Monograptus 
4 | Cwm-yr-Aethnen Shales. <  crispus. 
E | Zone of 
zi . WM. sedgwicki. 
4 | Hirnant Grits and Mudstones, Orthis-hirnantensis 
Ss including local Hirnant fauna. 
Limestone. 
Foel-y-Ddinas Mudstones. Phacops-mucronatus 
Z fauna. 
S 
5 | Moel-fryn Sandstones. Unfossiliferous. 
S 
ei 
<| | Rhiwlas Limestone and Mud- Phillipsinella para- | Dicellograptus- 
stones. bola fauna, anceps Zone. 
Gelli-Grin Calcareous Ash, with | 
Gelli-Grin (Bala) Limestone, & Chasmops & Nicol- 
: Moel-fryn Limestone, ‘8  ella-actonie fauna. 
z Bryn-Pig Limestone, S 
ei Caerhafotty Limestone. iS 
Q > 
- Pont-y-Ceunant Ash. is 
: E Dent Re 
<> | Allt-Ddu Mudstones. S Asaphus-powisi & |) 38 
|S Heterorthis-alter- | SN 
Frondderw Ash. |S nata fauna. SS 
5 LSE 
Glyn-Gower Sandstones. > | S = 
IS) S'S 
JA 
4 
s 
| Nant-hir Shales and Derfel Climacograptus- 
Limestone. peltifer Lone. 
4 
4 


Resting upon the well-known Volcanic Series of the Arenig 
Mountains come the Dicranograptus Shales, in this area con- 
spicuously devoid of fossils except at the base and summit, and of 
somewhat different lithological character from those developed in 
In the Bala district they consist, for the 
greater part, of hard, black, gritty shales with sandy wisps, 
showing in many sections a str ucture strongly suggestive of current- 
bedding. Close to their base they contain the Derfel Limestone 
recorded by Prof. W. G. Fearnsides as the Orthis or Derfel Lime- 
the shales with which this limestone is interbedded are 


other Welsh areas. 


stone}; 


Dicranograptus Shales and Derfel Limestone. 


1 Q.J.G.S. vol. xi (1905) p. 627. 


138 DR. G. L. ELLES ON THE BALA COUNTRY: [vol. xxvii, 


rather softer than those at a higher horizon, and have yielded a few 
graptolites (see p. 144); but elsewhere no traces of any organism 
have been found, although the shales are magnificently displayed 
along the course of the Nant-hir flowing down from the slopes of 
Arenig across their strike. The shales appear to roll somewhat, 
and one marked fold brings up the voleanic rocks again; they may 
also be repeated to some extent by strike-faulting, although in the 
absence of fossils the magnitude of such repetition cannot be 
estimated : they are probably affected to some extent by tear-faults, 
but again the degree of this is difficult to detect in the absence of 
any clue as to the relative age of the beds, and owing to the fact 
that they are widely concealed beneath a tract of heathery moor- 
land. Towards their upper limit the shales again become some- 
what softer, but at this horizon beds of hard sandy mudstone 
begin to be intercalated among them; these gradually increase in 
number and importance, until they pass over into a definite series 
of fine massively-bedded sandstones containing an appreciable 
quantity of mica. 


Glyn-Gower Beds. 


This series is well exposed in many places, especially in Glyn 
Gower, probably by reason of its nature, and is distinguished by 
its hardness and widely-separated bedding-planes; the coarser beds 
gradually give place upwards to bands of finer texture, of the 
nature of sandy mudstones. These not only contain some shale- 
bands, but also a certain amount of calcareous material in the form 
of concretions of all sizes; the commonest of these concretions are 
about the size of a man’s head: in places, however, where they have 
been worked for lime (as on Bryniau Goleu) they are considerably 
larger. The shale-bands as a rule are somewhat cleaved, although 
the cleavage rarely is sufficiently well-developed to obliterate the 
bedding. It is in these sandy mudstones that the lowest ash- 
bed oceurs. Although the beds immediately above and below the 
ash are very much alike, yet, taking them as a whole, a marked 
difference may be noted in the lower beds both in general litho- 
logical characters and in their fossiliferous nature, so that they 
may be conveniently separated off as constituting a series by them- 
selves, the Glyn-Gower Beds. These deposits are but sparsely 
fossiliferous, and, although the fauna is everywhere meagre and 
lacks the variety of the higher beds, yet it is not wholly devoid of 
interest. In the black shales occurring in the sandy mudstones 
graptolites have been found (Or thogre aptus tr uncatus), and the 
bedding-planes of the massively-bedded sandy mudstones are occa- 
sionally covered with Plectambenites sericea—in fact, this fossil, 
together with Glyptocrinus basalis, is so much the commonest 
organism found that (from the paleontological point of view) the 
series might well be termed the Plectambonites-sericea 
Beds. 


part 2} ITS STRUCTURE AND ROCK-SUCCESSION. 139 


Frondderw Ash.? 


There is no hard-and-fast line to be drawn between the sandy 
mudstones and the ash, for wisps of ash at first show themselves 
in the sandy mudstones, then a thin band or two of definitely ashy 
material is commonly Pound and finally a bed of massive ash is 
seen, having a maxunum thickness of 12 feet; towards the top it 
behaves much in the same way, mudstone wisps making their 
appearance, and the whole passing into an ashy mudstone in which 
the ashy material gradually diminishes until it has completely dis- 
appeared. ‘The massive portion of this rock makes a conspicuous 
feature in the landscape wherever it occurs, and at close quarters 
is easily recognizable by its rough weathered surface, upon which 
the fragments are sometimes very conspicuous. It seems to be 
typically developed in the more northerly parts of the area, 
becoming thinner and less distinctive towards the south, where 1t 
is associated with a local development of limestone, and is split 
into two thin bands with mudstone between them. When last 
seen (Cefn Gwyn) it is merely an ashy mudstone quickly becoming 
indistinguishable from the surrounding sandy mudstones, which at 
this locality have yielded some interesting Ophiurids (Protaster 
saltert). 

This ash has been most extensively quarried throughout the 
district for walls and farm-buildings, so much so, that nearly all 
the exposures on the north-west side of the lake have been worked 
‘all along their strike, and the rock is only now visible at the very 
base of a small cliff of sandy mudstone, and in many eases is 
largely concealed by rubble. One of the most conspicuous of these 
outcrops lies north of Frondderw. Under the microscope this ash 
is seen to be of a very felspathic nature, being made up mainly of 
fragments of a coarsely vesicular felspathic Java, some of which 
are “devitrified, fragments of a coarsely crystalline rock suggestive 
of an intrusion, and a few pieces of an oolitic limestone. It is 
definitely less siliceous than the Pont-y-Ceunant Ash. 


Allt-Ddu Mudstones. 


The sandy mudstone series above the Frondderw Ash quickly 
takes on a different character from those below; while the beds 
immediately above the ash are very similar in their general 
character to the sandy mudstones below, and contain certain bands 
with concretions of a practically identical nature, softer beds come 
in with increasing rapidity. The beds quickly lose their pro- 
nounced sandiness, beds of this nature being confined to narrow 
partings only a few inches thick; the colour becomes dark bluish- 
grey instead of pale blue-grey, and the rocks are often iron-stained 
in irregular blebs: the whole series is intensely cleaved, so much 


1 T am indebted to Dr. R. H. Rastall for kindly confirming my examination 
of the slices of this and the Pont-y-Ceunant Ash. 


140 DR. G. L. ELLES ON THE BALA COUNTRY:  [vol. Ixxviu, 


so that the bedding is often quite obliterated on the weathered 
surface, in marked contrast to the prevalent clear stratification of 
the beds below the ash. The cleavage throughout the district 
is commonly north 10° east, and dips a little east of south. In the 
higher parts of this series some very fine-grained harder bands 
make their appearance: these, when weathered, exhibit a peculiar 
smoothed surface, and when glaciated may take on a very high 
polish ; their bedding-planes are commonly about 6 inches apart, 
and they are very easily recognized in the field. They are not of 
themselves very fossiliferous; but, since they resist cleavage to 
an extent comparable with that of the hard’ sandy mudstones of 
the Glyn-Gower Series, the fossiliferous surfaces of the softer 
_beds in which they are intercalated are often well preserved in a 
‘series of dip- slopes and scarps, forming a conspicuous and dis- 
tinctive feature in the landscape. 

An especially good series of the scarps and slopes is seen all 
along Allt-Ddu, from which these beds derive their name, though 
they are also well displayed at Bryn-bedwog, Bryn Porfa, Bryn- 
yr-aber, and on Cefn-ddwy-graig. 

In these Allt-Ddu Beds the fauna is both rich and varied, the 
bedding-surfaces being often crowded with numerous brachiopods, 
of which Heterorthis alternata and the variety retrorsistria,} 
Platystrophia biforata, and Dinorthis flabellulum are the most 
conspicuous ; while among the trilobites a big Asaphus of the 
powist type, Trinucleus gibbifr ons, and Calymene caractaci are 
characteristic, though less abundant. Heterorthis alternata and’ 
its variety retrorsistria are by far the most numerous fossils 
found, so that the beds may appropriately be termed the Heter- 
orthis alternata Beds,a name long ago given to them by 
Ruddy. 


Pont-y-Ceunant Ash. 


The Pont-y-Ceunant Ash succeeds the Allt-Ddu Beds, but its 
junction with these mudstones is characteristically very irregular, 
as may be well seen in the quarry at Y Garnedd, where the ash 
has been extensiv ely quarried: it has a thickness of about 25 feet 
in the northern part of the district, the only part of the area where 
it is at all really well-developed; southwards it becomes more 
completely merged in the Calcareous Ash, until in the neighbour- 
hood of Bryn- Pig only the lowest part (1 to 2 feet) can be 


1 Thomas Davidson first regarded this fossil as a variety of H. alternata, 
but subsequently, upon the evidence of specimens from Cerrig-y-druidion now 
in the Sedgwick Museum, he raised it to the rank of a distinct species 
(Appendix to ‘ Silurian Brachiopoda’). These specimens, however, in common 
with all from the same locality, have suffered severely from the deformation 
of the rocks in which they lie, and cannot be regarded as normal. Specimens 
of the same fossil from many other Welsh localities appear to be so very 
close in general structural details to H. alternata, apart from the more 
conspicuous bending-back of the surface-ribbing, that I regard Davidson’s 
original diagnosis as the more satisfactory. 


part 2] ITS STRUCTURE AND ROCK-SUCCESSION. 141 


separated as a definite ash, retaining its coarse character, while 
at Creigiau Bychain it does not exceed 6 inches in thickness, as 
distinct from the Calcareous Ash. It is very variable in character 
also, varying even in one quarry from a very coarse to an even- 
evained rock of fine texture containing mudstone patches; at 
times, also, there is very little ashy material beyond certain blacker 
lumps, which are, however, quite distinguishable from the mud- 
stone; seen under the microscope the ash is found to be largely 
composed of a great variety of highly silicified lavas, some of 
which are vesicular. 


Gelli-grin Calcareous Ash Series. 


The Caleareous Ash Series of Gelli-grin is the most interesting 
series in the district, on account of its variable nature. It seems 
to be throughout its thickness a potential limestone, and to have 
pure limestones developed within it at different horizons in different 
places; these are all of a more or less lenticular character, appear- 
ing and dying out fairly rapidly. ‘Thus, at Gelli-grin, above the 
Pont-y-Ceunant Ash, a thickness of about 40 feet of Calcareous 
Ash occurs with an ash-free mudstone-band near the middle, and 
above the Caleareous Ash is found the caleareous development so 
long regarded as the typical Bala Limestone, comprising 20 feet 
or so of massive and coneretionary limestones overlain by alter- 
nating limestone and mudstone bands; these appear to lie wholly 
above the Calcareous Ash, being at once succeeded by the 
Rhiwlas Beds, of a totally different nature. This development is 
continued on Bryn-cut, on the opposite side of the Hirnant valley, 
but it changes when traced southwards. In the stream-section 
between Gelli-grin and Moel-fryn there is no trace of any limestone 


whatever, although calcareous ashy mudstones seem to be well - 


developed; on Moel-fryn to the south the limestone reappears, but: 
in the middle of the Calcareous Ash approximately at the horizon 
of the mudstone-band of the Gelli-grin section, whilea considerable 
thickness of ashy mudstone at the top seems to represent the 
Gelli-grin Limestone. On Bryn Pig still farther south the 
development agrees in the main with that at Gelli-grin; but the 
limestone proper seems to extend rather higher up into the series, 
with a resultant greater thickness of crystalline limestone. On 
Creigiau Bychain there is no calcareous development at all, apart 
from the Calcareous Ash; but immediately south of Creigiau 
Bychain, in the stream at the foot of the hill, is another calcareous 
development—the Caerhafotty Limestone—at the very base of the 
Caleareous Ash. No further development of limestones of any 
thickness is found in the Calcareous Ash in the area here described, 
though a band about 2 feet thick of an ashy limestone has been 
worked for hme at Maes-meillion. 

In addition to these impersistent limestones, there is a definite 
variation in the Caleareous Ash itself, when traced from north to 


142 DR. G. L. ELLES ON THE BALA COUNTRY: [ vol. Ixxvii, 


south ; 1t is most conspicuously ashy in the north (Bryn-cut and 
Gelli-grin), and becomes less so towards the south, where ashy 
mudstones and calcareous mudstones take its place, although 
occasional intercalations of thin ashy bands among the mudstones 
seem to show that they really belong to the Caleareous Ash Series. 
When all the ashy material disappears, they are practically in- 
distinguishable lithologically from the underlying Allt - Ddu 
Mudstones. 

The Pont-y-Ceunant Ash at the base of this Caleareous Ash 
Series contains no fossils; but the remainder of the Series is richly 
fossiliferous, yielding many brachiopods and trilobites. Among 
the brachiopods Orthis (Nicolella) actonie, Triplecia spiri- 
feroides, and Plectambonites rhombica are the most conspicuous ; 
O. actonié occurs throughout, but is particularly characteristic of 
the limestone facies, while the > iplecita and Plectambonites 
abound in the more ashy beds. Among the trilobites Trinueleus 
gibbifrons and Calymene caractaci are common, and Pterygo- 
metopus jukesii is apparently characteristic of this horizon. 


Fig. 2.—Section up Bryn-cut. (Horizontal scale: 
6 inches=1 mile.) 


1=Allt-Ddu Mnudstones. 4—Gelli-Grin Limestone. 
2—Pont-y-Ceunant Ash. 5— Rhiwlas Mudstone. 
3—Gelli-Grin Caleareous Ash. 


The limestone facies of this Ashy Series usually comprises one 
or more bands of massive or coneretionary, dark grey-blue lime- 
stone, which is highly crystalline. It seems to oe very pure, and 
has invariably been worked for lime in the past, kilns being 
associated with practically every outerop. It sometimes contains 
small black phosphate-nodules, and at Y Garnedd is somewhat 
oolitie in character. It is usually a conspicuous feature in the 
landscape wherever it has been worked, since it shows up as a 
rock-wall in which the purer beds have a roughened outer surface ; 
but the more thinly bedded portions show a definite honeycombed 
appearance on their weathered faces: this honeycomb weathering 
is also characteristic of the more calcareous portions of the 
Caleareous Ash itself, though there is little resemblance between 
the two rocks when freshly fractured. 


part 2] ITS STRUCTURE AND ROCK-SUCCESSION. 143 


Rhiwlas Mudstone and Limestone. 


Following on the Gelli-grin Ashy Series come the Rhiwlas 
Beds, another mudstone series with impersistent limestones at its 
base. There is, however, a marked change in lithological character, 
whether limestone be developed or not: the characteristic beds are 
very fine pale-grey mudstones with some sandy mudstones, of so 
even a texture that they might be termed pasty; these are, as a 
rule, highly cleaved, and sometimes contain definite calcareous 
concretions. ‘The coarser micaceous sandy beds which lie above 
them are less cleaved, but are still pale grey in colour. ‘The base 
of these Rhiwlas Beds appears to be calcareous west of a north- 
and-south line running through Gelli-grin. There seems to be 
only one locality in the district where the Gelli-grin and Rhiwlas 
Beds are both calcareous, and this probably—in part, at any rate— 
accounts for the confusion that has existed in the past with regard 
to these beds, although they really differ conspicuously in fossil 
contents as well as in lithological character. The Rhiwlas Lime- 
stone is a pale-grey, minutely-lenticular, fragmentary limestone, 
and doubtless on this account is unsuitable for burning for lime: 
hence, unlike the limestones at the lower horizons, it has never 
been worked for that purpose. When weathered it presents much 
the same external appearance as the lower limestones, being charac- 
teristically honeyecombed. It commonly occurs also in ‘Wwall-like 
stretches varying from 10 to 15 feet in thickness, but never, so far 
as my knowledge goes, exceeding the latter thickness. 

The chaps genie nc Stes of “Phese Rhiwlas Beds is the same, 
whether the limestone be developed or not, and a noteworthy 
feature is the almost total disappearance of all the large brachiopods 
which form so essential a part of the fauna of the Gelli-grin Beds. 
Certain small brachiopods occur; but it is the trilobite fauna that 
is the most conspicuous element, and the trilobites also are very 
different from those found in the lower beds, the most typical 
forms being Phillipsinella parabola, Staur ocephalus murchisont, 
Cheirurus bimucronatus, Lichas laxatus, large Illenids, and 
Agnostus agnostiformis. There is a relatively greater number of 
individuals in the limestone, but the fauna is the same in the 
pasty mudstone; it gradually dies out in the overlying beds, and 
has completely disappeared by the time that the Moel-fryn Beds 
have developed in force. 


Moel-fryn Sandstones. 


These sandstones, which are first seen merely as bands in the 
Rhiwlas Mudstones, become eventually massive, and occupy a 
considerable area of ovound on Moel-fryn and on the hills north of 
the Hirnant valley beyond Aber Hirnant. They are a very mono- 
tonous and uninteresting set of beds, somewhat micaceous and of 
the same pale-grey colour throughout, without a trace of any 
fossils, so far as I have been able fa ecoren 


ae 


144 DR. G. L. ELLES ON THE BALA COUNTRY: [ vol. lxxviii, 


Foel-y-Ddinas Mudstones. 


Towards their upper limit the Moel-fryn Sandstones give place 
to rocks of more varied character: these are mudstones with large 
concretions, some fine-grained concretionary sandstones, and some 
bands of a peculiar rubbly rock, perhaps at one time calcareous, 
intercalated in the mudstone; their pale-grey colour is still cha- 
racteristic, but these beds have yielded a definite fauna of small 
brachiopods and big trilobites (Phacops mucronatus). 


Hirnant Beds. 


These deposits are notably different in character from the 
beds below them, showing in fact a return, as regards the mud- 
stones, to the darker blue-grey mudstone type of the Allt-Ddu 
Beds, but with a totally distinct fauna. The change takes place 
fairly rapidly, and, in the typical development as seen in the 
Hirnant valley near its head, the mudstones contain a remarkable 
pisolitie rock, the so-called Hirnant Limestone. This lime- 
stone has only a very local development, although the pisolitic 
mudstones associated with it are more widespread. These Hirnant 
Beds seem to change very rapidly along their strike; in place of 
the mudstones and the pisolitic limestone, a series of calcareous 
grits and purer quartzose grits occur, and this gritty character 
appears to become more pronounced northwards. 

The highest mudstones seem to show the incoming of deeper- 
water conditions, as fine slaty shales which form good slate-bands 
come in, and, gradually increasing in importance, finally supersede 
all the mudstones. Purely slaty rocks succeed, in which trial- 
shafts have been made. These slates fall into two groups, a 
lower blacker, harder, and somewhat less banded group, in which 
Upper Birkhill graptolites have been found; and an upper softer 
group, with some lighter bands, yielding typical Tarannon 
graptolites. 


LV. DETAILS OF SPECIAL SECTIONS.- 


Nant Aber Derfel. 


The nature of the Derfel Limestone and its relation to the 
Dicranograptus Shales have been described and mapped by Prof. 
W. G. Fearnsides, and, as he points out, the Derfel Limestone is 
best exposed in the gorge of the Nant Aber Derfel below the 
bridge carrying the old road from Arenig to Bala: here the shales 
above the limestone seem to be entirely without any trace of 
fossils ; but out on the moor, above the bridge, when the stream is 
low, the shales intervening between the limestone and the Volcanic 
Series may be seen. They have yielded the following graptolites : 
Dicellograptus sextans (Hall), Nemagraptus sp., Climacograptus 
schirenbergi Lapworth, and Cl. brevis Elles & Wood. It is quite 


part 2] ITS STRUCTURE AND ROCK-SUCCESSION. 145 


likely that the graptolites Dicranograptus rectus Hopkinson and 
Amplexograptus perexcavatus Lapworth, found by Prof. Fearn- 
sides in a loose block of shale on Mynydd Nodol, may have come 
from the same beds, since they, like the others, are characteristic 
fossils of the Upper Glenkiln Shales. The Derfel Limestone 
would, therefore, appear to be of Llandilian age; and, indeed, its 
fauna is a very remarkable one, since it is not of the type usually 
characteristic of the Welsh Llandilan, but approximates more 
closely to the fauna of the Scottish deposits of that age—more 
especially, perhaps, that of the Stinchar Limestone. The following 
are the more important forms :— 


Lichas laxatus M‘Coy. Orthis (Heterorthis) confinis(Salter?). 
Cybele verrucosa (Dalman). Skenidiwm lewist var. craigense 
Cybele rugosa (Portlock). Reed. 
Illenus cf. balclatchiensis Reed. Truplecia craigensis Reed. 
Acidaspis hysteriz Wyville Thomson. Clytonia andersoni Reed. 
Harpes flanagani Portlock. Clitambonites ascendens Pander ? 
Trinucleus sp. Leptena rhomboidalis Wilckens. 
Rafinesquina expansa var. 
Orthis (Platystrophia) biforata Rafinesquina subarachnoidea 
(Schlotheim),. Reed (?). 
Orthis (Nicolella) actonixe (Sowerby). Stropheodonta corrugatella (David- 
Orthis (Hebertella) crispa (M‘Coy). son). 
Orthis alata Sowerby. Plectambonites llandeiloensis(David- 
Orthis (Dalmanella) testudinarva son). : 
var. gracilis Reed. Plectambomites sericea var. semi- 
Orthis (Dalmanella) girvanensis rugata Reed. 
(Davidson). Numerous Bryozoa and a few Cri- 
Orthis playfawi Reed. noid-stems and Cystid-plates. 


Nant Rhyd Wen. 


Nant Rhyd Wen affords the best section in the Glyn-Gower 
Beds, up to their Junction with the Frondderw Ash. The massively- 
bedded Glyn-Gower Sandstones are seen close to the junction of 
the stream with the Afon Glyn dipping 30° south-eastwards, and 
these occupy the bed of the stream forabout 140 yards; then some 
soft shale-bands are occasionally seen alternating with the sand- 
stones: one of the lowest yielded traces of graptolites (Diplograptt), 
including Orthograptus truncatus Lapworth, and other bands 
yielded Zrinucleus of the concentricus type, Glyptocrinus 
basalis, and Beyrichia. 'These beds, which are now more sandy 
mudstones than sandstones, continue for another 300 yards until 
the stream makes a definite bend, and, instead of running north- 
westwards, turns nearly due north. At this point a low cliff on 
the right bank shows a section of some interest: at the top of the 
cliff the Frondderw Ash is seen, and at the bottom a good crystal- 
line limestone, a few feet of sandy mudstone separating the two. 
The limestone appears to be merely a lenticle, for it dies away 
rapidly northwards and southwards, and is never more than 
18 inches thick ; it is, however, richly fossiliferous, the following 


Q. J.G.S. No. 310. L 


146 DR. G. L. ELLES ON THE BALA COUNTRY: [Vol. lxxviil, 


forms occurring, and Orthis (Harknessella) vespertilio being 
especially abundant :— 


Orthis (Harknessella) vespertilio Calymene planimarginata Reed. 


(Sowerby). Homalonotus sp. 
Dalmanella testudinaria (Dalman). | Trinucleus of the concentricus type 
Dalmanella elegantula (Dalman). | (gibbifrons M‘Coy ?). 
Strophomena (Rafinesquina) erpansa | Trinucleus sp. nov. 

(Sowerby). | Enerinurus multisegmentatus (Port- 
Plectambonites sericea (Sowerby). lock). 
Asaphus powisi Salter. | Monticulipora fibrosa (M‘Coy). 
Calymene brevicapitata Portlock. | Glyptocrinus basalis M‘Coy. 


The Frondderw Ash then comes down to the stream, and strikes 
along it for about 150 yards: it is here thinning out rapidly, and 
seems to be really two thin ash-bands separated by mudstones. As 
it leaves the bed of the stream to run up the opposite hillside a 
second limestone lenticle is seen—this time, however, above the ash, 
and with a quite different fauna from the other ; in this gasteropods 
are especially abundant :— 


Lophospira gyrogonia (M‘Coy). Strophomena (Rafinesquina) ez- 
Bellerophon (Sinuites) bilobatus | pansa (Sowerby). 

(Sowerby). | Orthis of the calligramma type. 
Murchisonia ef. simplex M‘Coy. | Trinucleus of the concentricus type 
Monticulipora fibrosa M‘Coy. & sp. noy. 

Orthoceras vagans Salter. | Glyptocrinus basalis M‘Coy. 


This limestone lies at the base of the Allt-Ddu Mudstones. 


Y Garnedd. 


Though the beds at Y Garnedd are much broken, yet the suc- 
cession has features of especial interest, since some beds are better 
developed there than anywhere else in the district, and it is the 
locality where the Gelli-grin Limestone is definitely oolitic. Only 
the higher Caradocian Beds are seen ; north of the main Llander- 
fel road the Allt-Ddu Mudstones are well exposed, and exhibit the 
usual bedding-surfaces densely covered with fossils. The following 
have been recognized south-west of Pandy-isaf :— 


Orthis (Heterorthis) alternata Lingula ovata Sowerby. 
(Sowerby) & var. retrorsistria Asaphus powisi Salter. 
(Davidson). | Trinucleus sp. 
Orthis (Dalmanella) elegantula | Glyptocrinus basalis M‘Coy. 
(Dalman).  Beyrichia (Tetradella) complicata 
Strophomena (Rafinesquina) expansa — (M‘Coy). 
(Sowerby). _ Orthograptus truncatus Lapworth & 
Plectambonites sericea (Sowerby). | var. pauperatus Lapworth. 


The higher beds of these Allt-Ddu Mudstones are seen on the 
south side of the road, and with the Pont-y-Ceunant Ash and the 
Gelli-grin Caleareous Ash are sharply folded, good exposures that 
reveal the structure being visible on or close to the track running 
between the Llandertfel and Lake-Vyrnwy roads ; there is an almost 
continuous section of the Allt-Ddu Beds between the road and 


part 2] ITS STRUCTURE AND ROCK-SUCCESSION. 14:7 


Y Garnedd Quarry, and at one point on the track about 100 yards 
north-east of the quarry an interesting calcareous bed is seen full 
of Heterorthis alternata, yielding in addition :— 


Orthis (Heterorthis) alternata var. Leptxna rhomboidalis Wilckens. 


retrorsistria (Davidson). Strophomena (Rafinesquina)  ea- 
Orthis (Dinorthis)  flabellulum pansa (Sowerby). 

(Sowerby). Glyptocrinus basalis M‘Coy. 
Orthis (s.s.) calligramma Dalman. Beyrichia (Tetradella) complicata 
Plectambonites sericea (Sowerby). (M‘Coy). 
Plectambonites transversalis (Dal- 

man). 


This bed when traced round exhibits the folding very well. 

In Y"Garnedd Quarry the relation of the Pont-y-Ceunant Ash 
to the underlying beds is clearly seen, the junction being re- 
markably irregular and ‘ pockety.’ 


Fig. 3.—Y-Garnedd Quarry: diagr ammatic sketch wllustrating 
the ‘pockety’ nature of the gunction of the Pont-y-Ceunant 
Ash with the Alit-Ddu Mudstones. 


<----<<7/.-- 12, feet --------> 


The Pont-y-Ceunant Ash itself contains no fossils, but the Gelli- 
grin Caleareous Ash which overlies it is more or less fossiliferous 
throughout, particularly so in a band about 7 feet above the base. 
The following forms are all common :— 


Orthis (Nicolella) actonie (Sowerby). Plectambonites sericea (Sowerby). 
Orthis  (Platystropha)  biforata Plectambonites transversalis (Dal- 


(Schlotheim). man). 
Orthis (Dalmaneila)  elegantula Plectambonites rhombica (Davidson). 
(Dalman). Trinucleus concentricus Katon. 
Orthis (Plectorthis) plicata(Sowerby). | Trinwclews gibbifrons M‘Coy. 
Orthis (Dalmanella) testudinaria Calymene planimarginata Reed. 
(Dalman). Calymene caractact Salter. 
Orthis (Harknessella) vespertilio Beyrichia complicata Salter. 
(Sowerby). Monticulipora lycoperdon (Say). 
Triplecia (Cliftonia) spiriferoides Monticulipora fibrosa (M‘Coy). 
(M‘Coy). Cystid-plates. 
Leptena rhomboidalis Wilckens. Primitia. 


Stropheodonta corrugatella (Davidson).| Palezarca sp. 


L2 


148 DR. G. L. ELLES ON THE BALA COUNTRY: [vol. Ixxviul, 


On the east side of the track which runs between the Llanderfel 
and Lake-Vyrnwy roads east of Y Garnedd farmhouse, the trend of 
the beds is practically at right angles to that of those on the west 
side, against which they are obviously faulted. The Gelli-grin 
Limestone has here an extensive outcrop, since it appears to be 


just rolling over; it has, as usual, been quarried for lime, and has 


yielded the following fossils :— 


Orthis (Nicolella) actoniz (Sowerby). 

Orthis (Plectorthis) plicata 
(Sowerby). 

Orthis (Dalmanella) elegantula type. 

Orthis (Dalmanella)  testudinaria 


type. 


Triplecia (Cliftonia) spiriferoides 
(M‘Coy). 

Plectambonites sericea (Sowerby). 

Calymene caractaci Salter. 

Chasmops macroura (Sjogren). 

Monticulipora fibrosa (M‘Coy). 


Under the microscope the limestone is seen to be definitely 
oolitic : this character is, however, only visible in hand-specimens 
when they have been very much weathered. Underlying the 
limestone the Calcareous Ash may be detected; but it is ‘not well 
exposed, although the Pont-y-Ceunant Ash shows up beautifully 
on a good dip-slope with the Allt-Ddu Mudstones underneath it. 

The Gelli-grin Limestone is also seen again near an old limekiln 
on the north-east, nearer the river, with the lower beds beneath it 
a little farther north: the relation between these two sets of beds 
is rather obscure, but is suggestive of tear-faulting, with the tear 
running east and west. 

The limestone is also apparently faulted westwards against the 
Allt-Ddu Beds, which are overlain by the Pont-y-Ceunant Ash and 
the Calcareous Ash, all three forming part of a small syncline. 


Bryn-cut and Gelli- grin s 


Part of the succession is clearly seen on the slopes of Bryn-cut, 
although the outcrops of the different beds are shifted in places 
by a series of tear-faults. The lower part of the hill on the 
north side is occupied by the Allt-Ddu Mudstones, highly cleaved 
as usual, except in a few harder more sandy bands: these slightly 
harder beds make two well-defined little scarps of which the lower 
is the more conspicuous (fig. 2, p. 142). Fossils occur throughout, 
commonest forms being the following :— 


Orthis (Heterorthis) alternata Strophomena (Rafinesquina)  ez- 
(Sowerby) & var. retrorsistria pansa (Sowerby). 
(Davidson). Strophomena (R.) grandis (Sowerby). 
Orthis (Dinorthis) flabellulwm Trinucleus gibbifrons M*Coy. 
(Sowerby). Trinucleus concentricus Eaton. 


Plectambonites sericea (Sowerby). Asaphus powisi Salter. 


Calymene caractaci Salter. 


A short distance above the upper of the two scarps in the mud- 
stones, another feature is formed by the Pont-y-Ceunant Ash, here 
about 25 feet thick; and about 50 feet of rock separate ae ash 


part 2] ITS STRUCTURE AND ROCK-SUCCESSION. 149 


from the limestone outerop, which in places is practically vertical. 
The Gelli-grin Limestone is here particularly pure, and has yielded 
only a few fossils :— 


Orthis (Nicolella) actoniz (Sowerby). Christiania tenwicincta (M‘Coy). 


Orthis (Dalmanella) elegantula var. Hyattella portlockiana (Davidson). 
Orthis (Platystrophia)  biforata Lingula ovata Salter. 
(Schlotheim). Monticuliporids. 
Strophomena (Rafinesquina) ea- 
pansa var. 


The base of the limestone is a massive, blue-grey, crystalline 
rock; butit becomes more concretionary towards the top, and is 
immediately overlain by the pale-grey pasty Rhiwlas Mudstone, 
which close to Plas-Rhiwaedog contains :— 


Encrinurus sexcostatus Salter. | Trinucleus sp. 

Cybele verrucosa (Dalman). | Orthis (Dalmanella) elegantula var. 
Illenus davisi Salter. drummuckensis Reed ? 

Cheirurus sp. Orthis (Hebertella) crispa (M‘Coy). 


Lichas sp. | Cystid-plates. 
Stawrocephalus murchisoni Barrande. | Primitia. 


Most of the beds seen on Bryn-cut are also visible on the Lake- 
Vyrnwy road, with the.exception of the Gelli-grin Limestone, 
which has been faulted out; on the other side of the valley the 
rocks sweep up and round to the classic locality of Gelli-grin, 
where the Allt-Ddu Beds andall the members of the Gelli-grin Ash 
Series are well seen. Just as on Bryn-cut, small scarps mark the 
position of the harder beds; but here itis the Pont-y-Ceunant Ash 
that makes the most definite feature, forming a low wooded ridge, 
easily detected at the lower level on the north side of the hill 
where it has been shifted by a tear-fault. The flatter open ground 
above is occupied by the Calcareous Ash, which has a well-marked 
mudstone-band near the middle of its course, and above again a 
steep cliff-like rise marks the position of the famous Gelli-grin 
Limestone (Bala Limestone). In places this is a very massive and 
highly crystalline rock, but elsewhere it consists of large conere- 
tionary masses running together, considerable variation in this 
respect being noticeable in the different quarries along the outcrop. 
Upwards, however, both the more massive and the more concre- 
tionary types pass over into thin bands of small concretions, 
grading up into regularly-bedded, highly-caleareous mudstones with 
a few bands of limestone, the total thickness not exceeding 25 feet. 
The concretionary and massive types of the limestone weather to 
a somewhat rough texture, with a conspicuous rusty-red colour ; 
while the less massive upper beds show weathering of the honey- 
comb type. Immediately above, and continuing the sharp rise of 
the hill, are seen the pasty Rhiwlas Mudstones (fig. 5, p. 154). 
The most fossiliferous parts of the limestone appear to be in the 
quarry lying west of Gelli-grin farmhouse, and along the cliff- 
face west-south-west of the farm, this last being the exposure 
recorded by Jukes. The fossils noted are as follows :— 


150 DR. G. L. ELLES ON THE BALA COUNTRY:  [vol. lxxvill, 


Orthis  (Plectorthis) - plicata Lingula ovata Sowerby. 


(Sowerby). _ Pterygometopus jukest (Salter). 
Orthis  (Plesiomys) porcata | Cybele sp. 
(Sowerby). | Trinucleus concentricus Eaton. 


Orthis (Nicolella) actoniz (Sowerby). | Beyrichia (Tetradella) complicata 
Orthis (Dalmanella) elegantula var. (M‘Coy). 
Orthis (Dalmanella) testudinaria var. | Monticulipora fibrosa (M‘Coy). 


Orthis wnguis Sowerby. _ Monticulipora lycoperdon (Say). 
Leptena rhomboidalis Wilckens. | Monticulipora lens (M‘Coy). 
Plectambonites sericea (Sowerby). | Cystid-plates. 


Plectambonites rhombica (Davidson). 


A good section of the upper beds is displayed along an old cart- 
track leading to the moor: this track runs at first across the strike, 
and the Gelli-grin Caleareous Ash with the limestone above itis seen 
dipping at 30° along the track. Just where the track bends round 
so as to run at right angles to its former direction for a short dis- 
tance, it runs parallel with the junction of the limestone and the 
Rhiwlas Beds, which may be clearly seen on the south side resting 
upon the limestone; while higher beds are exposed still farther 
along the track after it has resumed its original trend. These 
Rhiwlas Mudstones contain :— 


Phillipsinella parabola Barrande. Plumutlites peachi (N. Etheridge). 
Lichas lawatus M‘Coy. Orthis (Hebertella) crispa (M‘Coy). 
Encrinurus sexcostatus Salter. Plectambonites quinquecostata 
Agnostus agnostiformis (M‘Coy). (M‘Coy). 

Remopleurides colbii Portlock. Stropheodonta corrugatella (David- 
Remopleurides sp. son). 

Staurocephalus murchisoni Barrande. Christiania tenuicincta (M‘Coy). 
Illenus davisi Salter. Skenidium sp. 

Trinucleus sp. Very small Ostracods. 

Cheirurus bimucronatus Murchison. 


Traced southwards the limestone is seen to bend sharply round 
slightly above a little fir-copse, where the main road divides into 
two, and this change in strike is very obvious in a good exposure 
close to the lower of the two roads. The Pont-y-Ceunant Ash also 
exhibits this folding, but part of its outcrop is faulted off. 

The course of the limestone is marked by dense vegetation, so 
that when, as the result of the tear-faulting, the ash is brought 
against the limestone there is little to distinguish the two in this 
respect; the feature due to the limestone dies away before Yspydd- 
adog is reached, and not a trace of it is to be found in the streams 
between Gelli-grin and the Moel-fryn quarries. 


Moel-fryn.' 


The sections seen in the Moel-fryn quarries are particularly 
interesting: the ashy material, as a whole, is very much less con- 
spicuous than at Gelli-grin ; no definite band of ash occurs at the 
base, but there is a correspondingly greater thickness of cleaved 
ashy mudstones and caleareous ashes, suggesting that the Pont-y- 


1 Probably Bryn-melyn of the old Geological Survey lists. 


part 2] 


Fig. 4.—Detail of Long Quarry- 


Face, Moel-fryn. 


(Vertical 


scale: I inch=42 feet.) 


|— | LL 
wy YY 
pnligayrye VA | y 


17, Dect eee 
f i — 7, 77 


— 


(11) 
WLLL LLL LLL LL 


Coarse ash. 


5 
Calcareous 
| ashy 
L 
mudstones 


(cleaved). 
J 


Impure 
limestone. 


Ashy mudstone. 


Limestone 
grading down 
into calcareous 


ashy mudstone. 


Limestone. 
Mudstone band. 


Ashy mudstone. 


Massive 
concretionary 
limestone 
(worked for 


lime). 


ITS STRUCTURE AND ROCK-SUCCESSION. 151 


Ceunant Ash has become 
merged in the Calcareous 
Ash Series, which is 
extremely fossiliferous 
throughout. The lme- 
stone, on the other hand, 
is well-developed, al- 
though it is at a dit- 
ferent horizon from the 
lmestone of Gelli-grin, 
being in the middle of 
the Caleareous Ash 
Series (fig. 6, p. 154). 
This limestone makes a 
definite feature as soon 
as it crops out, and is first 
seen just a httle below 
the moor wall; here it 
is much bent about, 
striking north-east and 
south-west, north and 
south, and east and west 
in rapid succession. A 
lower feature on the hill 
seems to mark the base 
of the Calcareous Ash 
Series, and the Allt- 
Ddu Mudstones are seen 
below, all these rocks 
being then abruptly cut 
off by the Moel-fryn 
displacement which runs 
across the whole country. 

The face of the long 
quarry shows anexcellent 
section (fig. 4) with the 
massive limestones at 


the bottom overlain by 


cleaved ashy mudstones, 
and at the top a distinct 
band of coarse ash, at 
least 1 foot thick. Above 
this band of ash there 
is about 20 feet of cal- 
careous ashy mudstone, 
and immediately above 
this, at the southern 


end of the exposure, are seen Rhiwlas Beds of the pasty mud- 


stone type, while a little farther out on the moor the higher 


152 DR. G. L. ELLES ON THE BALA COUNTRY: [vol. lxxviii, 
beds of the Rhiwlas Series may be noted. The fossils from the 
Calcareous Ash include the following :— 


Orthis (Harknessella) vespertilio Leptena rhomboidalis Wilckens. 


(Sowerby). | Plectambonites sericea (Sowerby). 
Orthis (Plectorthis) plicata | Plectambonites rhombica (Davidson). 

(Sowerby). | Plectambonites transversalis (Dal- 
Orthis (Plesiomys)  porcata | man). 

(Sowerby). | Strophomena deltoidea Conrad. 
Orthis (Dalmanella) elegantula var. | Trinucleus concentricus Eaton. 

parva Reed. Trinucleus gibbifrons M‘Coy. 
Orthis (Dalmanella) testudinaria | Calymene caractaci Salter. 

(Davidson). Pterygometopus jukesi (Salter). 
Orthis (s.s.) calligramma Davidson. Acaste apiculata (Salter). 
Orthis (Platystrophia)  biforata | Lichas lazatus M‘Coy. 

(Schlotheim). | Beyrichia complicata M‘Coy. 


Orthis (Nicolella) actoniz (Sowerby). Monticulipora lycoperdon (Say). 
Triplecia (Cliftonia) spiriferoides | Monticulipora fibrosa (M‘Coy). 
(M‘Coy). 


The fauna of the limestone appears to be much the same, 
although, on the whole, individuals are not so numerous. 


Bryn-pig.? 


The chief interest of the Bryn-pig exposures is the occurrence 
of the calcareous facies of both the Rhiwlas Beds and the Gelli- 
grin Caleareous Ash Series in the same section, proving clearly (if 
proof be needed) that on lithological grounds alone, apart from 
any paleontological considerations, it is possible to discriminate 
between the old so-called Bala Limestone and the Rhiwlas 
Limestone. 

The main exposure is that forming a more or less sheer cliff-face 
on the south-eastern flank of Bryn-pig, where the beds are seen 
forming part of a syncline obliquely faulted through its centre. 

The lower of the two limestones is here “especially well- 
developed, and is a beautiful, highly crystalline, dark blue-grey 
rock, which on the north side of the hill is much veined with both 
quartz and calcite, the presence of the quartz being due apparently 
to the proximity of a fault along which there has been much 
quartz mineralization. All along this north side of the hill the 
lower rock has been extensively quarried for lime ; about 1 foot of 
Caleareous Ash and 4 feet of pasty Rhiwlas Mudstone separate it 
from the lowest stratum of calcareous Rhiwlas Beds—that is, the 
Rhiwlas Limestone. This limestone, as developed here, is neither 
so crystalline in character nor so massive as in some of the more 
westerly exposures (Creigiau Bychain), but is still definitely a lime- 
stone; towards the top the calcareous bands are separated by bands 
of the pasty Rhiwlas Mudstone, which show up well on the 

weathered surface by virtue of their cleavage : if we reckon all these 
with the main limestone-band (about 6 feet thick), the calcareous 
development of the Rhiwlas Series may be estimated as having 
a thickness of 12 feet, and the occurrence of abundant small phos- 


1 Probably Bryn-bedwog of the old Geological Survey lists. 


part 2] ITS STRUCTURE AND ROCK-SUCCESSION. 153 


phate nodules in it is characteristic. The fossils from this locality 
include :— 


Phillipsinella parabola Barrande. | Orthis (Nicolella) actonize var. 
Agnostus agnostiformis (M‘Coy). asteroidea Reed. 

Encrinurus sexcostatus Salter. Christiania tenuicincta (M‘Coy). 
Staurocephalus murchisoni Barrande. Orthis (Hebertella) crispa (M‘Coy). 
Lichas sp. | Orthis elegantula, var. drummuck- 
Illenus davisi Salter. | ensis Reed. 

Cheirurus bimucronatus Murchison. Hemicosmites sp. 

Trinucleus sp. Echinospherites arachnoideus Salter. 


Above this are seen at least 20 feet of pasty Rhiwlas Mudstone, 
so that the faunal succession compares well with that observed at 
Gelli-grin. (See figs. 5 & 7, p. 154.) 

As we pass round the syncline to the west it becomes obvious 
that both limestones are abruptly cut off, and their place taken in 
the line of strike by the fossiliferous Caleareous Ash, the outcrop 
of the Gelli-grin Limestone being shifted into the line of the 
Rhiwlas Mudstones. Thus, the syncline, as seen so far, is affected 
by cross-faulting with a downthrow to the north, the result being 
that ort the south the Gelli-grin Limestone is practically folded on 
itself, the merest tongue of pasty Rhiwlas Mudstone occupying 
the centre of the fold; this limestone is not well exposed at the 
present time, but a double line of workings indicates its outcrop. 
Oddly enough, the outcrop of the limestone is almost continuous 
on the south-east side of the fold despite the fault, and both 
outcrops are truncated by the Moel-fryn displacement. The small 
dislocation east of the fold has the effect of diminishing the 
outcrop of the Caleareous Ash, and brings the Gelli-grin Lime- 
stone very close upon the Allt-Ddu Mudstones. 

Another small outcrop of limestone seems to belong to another 
fold, while a highly contorted outcrop lying south of this and 
east of the main outcrop is so affected by the great line of disturb- 
ance in its immediate neighbourhood that some of the associated 
mudstones are practically schists. At this locality, probably on 
account of the lenticular nature of the Gelli-grin Limestone, there 
is a rather greater thickness of rock separating the two limestones, 
a foot anda half of Calcareous Mudstones, 4 feet of Calcareous 
Ash, and 4 feet of pasty Rhiwlas Mudstone being clearly visible 
above the main limestone and below the calcareous development of 
the Rhiwlas (fig. 7, p. 154). At this locality the lower limestone is 
richly fossiliferous, and has yielded the following species :— 


Orthis (Nicolella) actonize (Sowerby). | Plectambonites sericea (Sowerby). 
Strophomena (Rafinesquina)  ex- | Calymene sp. 
pansa (Sowerby). | 


Creigiau Bychain. 
The group of synclines on Creigiau Bychain shows an excellent 
series of exposures of the calcareous development of the Rhiwlas 
Beds, with a maximum thickness of 12 feet ; occasionally the beds 


are calcareous right down to their junction with the Calecareous 
Ash, in other places there is a definite band of pasty mudstone 


seuojspnuyyl "Pd WIV souoj}spny UPC ITV 


souoyspnyy Op IV U My 


ysy jJueuness A yuog x 
suojspnyy is 
Aysy-snoeres[e) |, 


ysy snoorvote9] ysy stoereoyed | 


suo spnyy 
suojsouny uAI-[I0]/I 


ysy siovteoye 


. “ | euoyspnyy’ fp 
auoysowly sig usIg Aysy snoereoje)} 


ouo}spnyy snoe1vo[e_ ZEELLETTA 


YY YY 
OUuUOISPNYT sepMIyrAy/ YY, /} Wis 
Whi lili 


ysy Snooreoeg fs : 


: iy 
SSCS eae suojyspnyt sepany |7////7 Y 
My UY YW VON 


YY 


Yl jp. y 
/ y YY 

Vip / YY Y, 

MY Y PS fs 


4 


UAIJ-[9OJAT ur) M99 


Bid hig 
‘1 SA 9 SM G SLT 


(jaaf Gp=your JT + aqvos 1091440 4 ) 
‘YS Snoasva]vD Ursh-17;9-H AY} UWL WOUnHILWA AY? HULNOYS SU01Z998 1091}.00 4 


part 2] STRUCTURE, ETC. OF THE BALA COUNTRY. 155 


intervening between the two. The limestone shows up best on the 
scarp-face, and then forms a wall-like mass weathering in honey- 
comb fashion, and recalling in many respects the less massive part 
of the Gelli-grin Limestone as regards outward appearance. Com- 
pared with the lower limestone, however, the Rhiwlas is always, as 
already stated, paler in colour, less pure, minutely lenticular, and 
more fragmentary, characteristics which probably account for its 
rejection for burning to lime, while the Gelli-grin and other hme- 
stones of the Caleareous Ash Series have invariably been utilized in 
this respect. Highly caleareous mudstone-bands are intercalated 
in the Rhiwlas Limestone, and some of these, as also the purer 
more crystalline limestone-bands, contain phosphate-nodules which 
vary in size, the biggest found on Creigiau Bychain being about 
the size and shape of a blackbird’s egg. 

The greatest of the synclines presents a definitely ovoid outline, 
its axis trending practically north and south, and having a gentle 
southward pitch, the general shape of the fold being clearly indi- 
cated by the wall-hke outcrop of the Rhiwlas Limestone. A 
second minute fold occurs immediately north-east of this, in which 
the Rhiwlas Limestone is bent upor itself at the top of a little 
roll-over of the Caleareous Ash; farther north-westwards another 
almost complete syncline in the limestone is seen, and a fragment 
of the same rock indicates its connexion with another similar 
pitching fold still farther north-westwards. 

The Rhiwlas Limestone of the greatest syncline has yielded the 
following fossils :— 


Encrinurus sexcostatus Salter. Hyatella portlockiana (Davidson). 
Agnostus agnostiformis (M‘Coy). Plectambomtes transversalis (Dal- 
Illenus davisi Salter. man), 

Phacops sp. Echinospherites stellulifera Salter. 
Christiania tenuicincta (M‘Coy). Glyptocrinus basalis M‘Coy. 


While the Calcareous Ash immediately beneath is still more 
richly fossiliferous, yielding :— 


Calymene caractacz Salter. Plectambomtes rhombica (Davidson), 
Chasmops conicophthalmus (Boeck). Plectambomites sericea (Sowerby). 
Trinucleus gibbifrons M‘Coy. Plectambomites quinquecostata 
Trinucleus excentricus Haton. (M‘Coy). 
Homalonotus bisulcatus Salter. Leptena rhomboidalis Wilckens. 
Strophomena (Rafinesquina) expansa Glyptocrinus basalis M‘Coy. 
(Sowerby), Monticulipora fibrosa (M‘Coy). 
Orthis (Nicolella) actoniz (Sowerby). Holopella sp. 
Orthis (Dalmanella)  testudinaria Bellerophon (Oxydiscus) acutus 
var. (Sowerby). 
Triplecia (Cliftonia) spiriferoides 
(M‘Coy). 


Cwm-yr-Aethnen, Pen-y-Dallgwm, and Foel-y-Ddinas. 


These places together afford a good section of the highest beds 
of the Ashgillian and overlying Silurian rocks. 

The Moel-fryn Sandstones, as already described, are very 
monotonous pale-grey rocks, with some intercalations of mudstones 


156 DR. G. L. ELLES ON THE BALA COUNTRY:  [vol. lxxviul, 


towards the top; they form the steep-sided valley of the Hirnant 
below Bwlch-yr-Hwceh and the whole of the precipitous Dallgwm 

where they are seen dipping eastwards at increasingly higher 
angles. They occupy the greater part of Bwlch-yr- Hiveh itself ; 
but, at its south-eastern end, it is obvious from the change in 
the scenery that the rocks alter in character, and this alteration 
coincides with the beginning of the Foel-y-Ddinas Mudstones. 
These beds are, in the main, “highly- cleaved pale-grey mudstones, 
but comprise some of a rubbly concretionary nature, which weather 
in a very characteristic manner ; all dip eastwards at angles from 
60° to 70°, and appear to contain fossils throughout, although the 
fauna is not a rich one, the most noteworthy feature being the 
presence of Phacops mucronatus Brongniart, the remainder peing 
chiefly small brachiopods, such as Atrypa marginalis Dalman, 


Fig. 8.—Sketch looking northwards, showing the Hirnant Beds 
and their relation to the Silurian. (Horizontal scale: 
6 inches=1 mile.) 


Pen y Dallgwm Foel y Ddinas 


Bwich yr Hwch 
i 


! 
I 
I 
\ Dry Valley 


1—Foel-y-Ddinas Beds. 3=Hirnant Limestone. 5 &6=Graptolitic shales 


2—Hirnant Mudstones 4=Hirnant Mudstones yielding graptolites 
below MHirnant above Hirnant indicative of the 
Limestone. Limestone. zones of Mono- 


graptus sedgwickt 
and M. crispus. 


) Orthis (Dalmanella) elegantula (Dalman), Skenidium sp., and 
Platystrophia biforata var. fissicostata (Davidsen). These deposits 
are succeeded by the Hirnant Beds, dark-blue mudstones of much 
softer nature, which yield far more readily to weathering than the 
beds beneath them: they dip at a still higher angle, 70° to 80°, 
| being (as it were) plastered on the east side of Pen- y-Dallgwm. 
They seem to occupy the head of the Hirnant valley ( ‘Cwm 
| Hirnant), and the line of the old high-level valley east of Foel-y- 
Ddinas ; but a small tear separates the two. On Pen- y-Dallgwm, 
due west of the shepherd’s house in Cwm-yr-Aethnen, there is still 
to be seen the old classic section in which the famous pisolitie 
facies of the Hirnant Limestone occurs. This is exposed in an old 
quarry on the hillside, to which an any grass-track still leads from 
the main Lake-Vyrnwy road. The following succession can be 
made out there :— 


part 2] ITS STRUCTURE AND ROCK-SUCCESSION. 157 


Thickness in feet. 


(1) Fossiliferous blue-grey mudstone, full of fossils .................. 12 
(2) Pisolitic limestone in large concretionary masses measuring 
SCARCE TI cn tae as stereos: Seay | Bo 


(3) Coneretionary calcareous mudstone, almost a limestone in 
places, and with numerous scattered pisolitic grains; blue- 


grey when fresh, but weathering to a dirty cream-colour ... 10 
(4) Dark bluish mudstone, with a few scattered pisolitic grains, 
as Ouiiee WatOSSIlS) says. seaccticcte os: Ea eet any ecto 10 


[The total thickness of these rocks, as seen in the quarry, is 
from 25 to 30 feet. | 


The same characteristic fossils occur throughout :— 


Orthis hirnantensis M‘Coy. Strophomena siluriana Davidson. 
Orthis sagittyfera Davidson. Platystrophia biforata (Schlotheim). 
Dalmanella elegantula (Dalman). Monticulipora fibrosa (M‘Coy). 


The pisolitic limestone has been so well described by Fulcher 
that no further description need be given, although the pisolitic 
grains which he defines as ellipsoidal in form are only so in the 
direction of the cleavage. 

On the opposite side of the Hirnant several small slate-trials 
have been made; beneath the lowest of these mudstones similar 
to those last described are seen dipping into the hill at 75°, and 
these pass upwards into a similar rock but containing slaty bands. 
In all these the Hirnant fauna is to be found (fig. 8). The slate- 
bands increase steadily in importance, and without any break the 
Hirnant Beds pass over into a slate series to which I have given 
the name Cwm-yr-Aethnen Beds: these appear to be about 
350 feet thick. They are, however, capable of a twofold grouping, 
both on lithological and on paleontological grounds: the lower 
group (150 feet) consists of hard, fine, blue-grey slates with some 
banding, yielding Upper Birkhill - graptolites :— Monograptus 
sedgwicki (Portlock), Climacograptus scalaris (Hisinger), Ortho- 
graptus bellulus (‘Vornquist), Glyptograptus serratus Hlles & 
Wood, and Petalograptus sp., a fairly typical assemblage of the 
Zone of M. sedgwicki. These pass up into somewhat softer 
slates, more definitely banded in lighter tones than the lower group, 
and yielding well-preserved graptolites in the blacker bands :— 


Monograptus crispus (Barrande). _ Monograptus discus Tornquist. 
Monograptus turriculatus (Barrande). Monograptus becki Barrande. 
Monograptus priodon Brown. | Monograptus marri Perner. 


Monograptus nudus Lapworth. 


V. STRUCTURE OF THE DisTRICT. 


The structure of the district is somewhat complicated, although 
when the main principles underlying the general plan are under- 
stood it appears less complex than might be thought at first sight. 

The interpretation here given, which seems to accord with the 
facts now observed, necessitates a modification of the views hitherto 


158 DR. G. L. ELLES ON THE BALA COUNTRY:  [Vol. lxxviu, 


published regarding the nature of the Bala Fault; this line of 
disturbance has been held by Jukes, Ramsay, and others to be a 
normal fault with a maximum downthrow to the north-west of 

about 11,000 feet. The nature of 


ui ae this displacement appears to be dis- 
Z Ee tinctly more complex when viewed in 
Lh 33 the ight of completer knowledge, 
ae and instead of being continuous with 
A 
Vi = the Dee-Valley Fault in the manner 
f shown on the 1-inch Geological Sur- . 


Ay vey Map and accepted by Sir Aubrey 
Strahan,! it runs up into the country 
north of the lake; there it appears 
to have been profoundly affected by 
later post-Carboniferous movements. 
The evidence in the district as a 
whole clearly points, however, to its 
having been initiated as an im- 
portant structural line in connexion 
with the Caledonian System of 
folding. 

The Bala district forms part of 
the southern limit of the great S- 
shaped buckle of rocks round the Har- 
lech Dome, so beautifully brought 
out by the outcrop of the volcanic 
rocks”; the trend-lines are in the 
main those of the Caledonian System 
of folds modified by local cireum- 
stances, the modification in this case 
being primarily due to the presence 
of the Harlech Dome. 

The whole country may be re- 
garded as folded round and against 
the Harlech Dome by pressure acting 
from a general south-easterly diree- 
tion: thus, in the area here de- 
scribed, the primary folding appears 
to follow north-east and south-west 
trend-lines in those parts of the 
district north and south where the 
rocks escape more or less from the 
influence of the Harlech Dome; but 
in the intervening area where that 
influence is fully felt, the axes of 
the folds run approximately north and 
south. Nevertheless, it is the north- 


folding. 


Bryn Pig 
T Serer 
43 Synclinorium 


Tear-faults. 


til 


yn-pig, to illustrate the 


hes=1 mile.) 


, 
e 


4 ine 


Anticlinorium 


Frondderw Ash. 
Glyn-Gower Beds. 


2E 
5= Allt-Ddu Mudstones. 


ontal scale : 
6 
Hi 


~ 
~ 


(Hort 


Creigiau Bychain 


Synclinorium 


uo 


Wig. 9.—Section from Creigiau Bychain to Bi 


Rhiwlas Limestone & Mudstone. 


Bryn-Pig Limestone. 
Gelli-grin Caleareous Ash. 


= 
Lalit) 


3 
4, 


Afon Gl 


i 
a 
1&2 


W.S.W 


1 Pres. Address to Sect. C, Rep. Brit. Assoc. (Cambridge) 1904, p. 535. 
2 Geological Survey of England & Wales i-inch maps: New Edition, 
| Sheets 9 & 10. 


part 2] ITS STRUCTURE AND ROCK-SUCCESSION. 159 


east and south-west trend-lines of the volcanic masses of the Arans 
and Arenig Bach that appear to control the trend of the thrust- 
planes, the rocks above the voleanics being apparently thrust over 
the more resisting volcanic rocks underneath ; hence the major 
lines of displacement have a general north-east and south-west 
trend, and in the central area cross the lines of folding obliquely. 
The differential resistance to the thrusting force in different parts 
of the thrust-masses shows itself in the development of numerous 
tear-faults. The facts seem to indicate that the rocks must first 
have been thrown into a series of major folds with minor folds and 
ripples, the trend of which at the present time varies in different 
parts of the area between north-east and south-west and north and 
south, with a gentle southward pitch (fig. 9). Among these folds 
were developed an important anticline and syncline trending 
parallel to the line of the Central Wales Syncline; as pressure 
continued, the folds as a whole were packed with increasing close- 
ness against the Harlech Dome, and, the resistance of the volcanic 
belt becoming more pronounced, the anticline gave way, and its 
south-eastern hmb was driven over on to the syncline by means of 
a series of compressional faults, of which the Bala Fault is one. 
The existence of this synclinal fold is still indicated in the westward 
dips of the rocks along the north-western shore of the lake: as, 
for example, where the Frondderw Ash is brought up again half a 
mile south-west of Llan-y-cil, farther south 1 mile and 14 miles 
south-west of Llanwchllyn, and perhaps still more clearly north of 
the lake north-west of Moel-Emoel, where the apex of the fold, 
much torn by faults, is distinctly visible with the line of the Bala 
Fault lying to the east of it. Simultaneously with this squeezing- 
out of the anticline and the further packing or thrusting on to the 
Harlech Dome, owing to the unequal resistance of different portions 
of the thrust-masses there were developed the striking series of 
tear-faults which are so marked a feature of the district. 

That these really are tear-faults and not merely normal faults, as 
they were regarded by the earlier workers on this ground, is clear 
from the horizontal displacement of the beds affected, which is 
entirely irrespective of the amount of dip. 

The country appears to be affected by the following major lines 
of displacement (fig. 1, p. 186) :— 

(1) The Llyn-Tegid Line. 

(2) The Bala-Lake Line. 

(3) The Llangower Line. 

(4) The Cefn-ddwy-graig Line. 
(5) The Moel-fryn Line. 

(6) The Ffridd Defaid Line. 


_ There is almost certainly at least one other line to the north- 
west within the Dicranograptus Shales; but this unfortunately 
I have been unable to map, owing to the nature of the ground 
and the character of the rocks occupying it: for the black shales 
afford no evidence of their age, and the lines of break that can 
be seen in the section visible in the Nant-Hir cannot be traced for 


160 DR. G. L. ELLES ON THE BALA COUNTRY:  [vol. ]xxviu, 


any distance across the dense grass- or heather-covered moorland. 
The differential movements of the rocks, however, seem to necessi- 
tate the existence of such lines. 

The Llyn-Tegid Line was first noticed by P. Lake (Fault B)1 
as running along the north-western shore of the lake; it seems 
to emerge at the northern end of Bala Lake, and, acquiring a 
slightly more northerly trend, runs up to the Nant-Hafhesp valley, 
being then apparently bent round into one of the east-north-east 
and west-south-west lines of displacement of the Moel-Emoel 
area. Its chief effect is to cause concealment of portions of the 
higher beds, so far as the northern part of the district is concerned. 
It appears to be more affected by the topography than the Bala- 
Lake Fault and, therefore, is probably of lower inclination. 


The Bala-Lake Line is a more important displacement; it 
seems to run fairly near the south-eastern shore-line of the lake, 
probably just outside the Llangower peninsula, and, emerging at 
the northern end of the lake, runs up into the high ground on the 
north approximating to the line of the Nant Cwm-da. Every- 
where along it the rocks on its south-eastern side are driven over 
those on the north-west; in the extreme south the volcanic rocks 
forming Aran Ben Llyn and the beds beneath them are brought 
over the Dicranograptus Shales, while farther north the Caradocian 
beds are thrust over the Ashgillan in all the country north of the 
lake as far as Moel-Emoel. Its inclination is fairly high. 


The Llangower Displacement, with the subsidiary Cefn- 
ddwy-graig branch, is perhaps the most striking thrust-line of the 
district; for there has been considerable differential movement of 
the rocks above this Llangower thrust, and consequently a great 
development of tear-faults is associated with it, especially in the 
area between Llangower and Mynydd Cefn-ddwy-graig, where the 
structure of the country is clearly brought out by the outcrops of 
the Frondderw Ash. The tearing becomes definitely less with the 
development of the Cefn-ddwy-graig Displacement, which seems to 
have effected compensation ‘en bloc.’ The beds, however, above 
this line of thrust are very sharply faulted and folded near Pont- 
y-Ceunant, where the Llangower, Cefn-ddwy-graig, and Moel-fryn 
lines all come closer together; the rocks above the Cefn-ddwy-graig 
thrust and beneath that of Moel-fryn show a series of small sub- 
sidiary thrusts affecting the anticlinal lines of the small folds so 
as to bring about an effect of imbrication, and this, combined with 
tearing, has shattered the rocks to pieces. 

The chief effect of the Llangower thrust is to bring the lower 
beds of the Allt-Ddu Mudstones, close to their junction with the 
Frondderw Ash, over the higher Allt-Ddu Mudstones; while 
along the Cefn-ddwy-graig line the Glyn-Gower Sandstones, the 
Frondderw Ash, and the Lower Allt-Ddu Mudstones are all in turn 


1 Geol. Mag. 1900, p. 212. 


part 2] ITS STRUCTURE AND ROCK-SUCCESSION. 161 


brought over the higher Allt-Ddu Beds. The Llangower thrust, 
as seen in section near Llangower, has an inclination of about 40° 
south-eastwards, and close to the point where it branches off from 
this line the inclination of the Cefn-ddwy-graig thrust appears to 
be similar, but farther north its inclination becomes definitely 
lower and more affected by the topography. At Pont-y-Ceunant, 
despite the shattering of the rocks associated with it, the main 
effect is the concealment of some of the Allt-Ddu Mudstones. 


The Moel-fryn Displacement is a very extensive fault, 
bringing about displacement of the rocks along a lne which ex- 
tends completely across the area mapped ; it is responsible for much 
discontinuity in the beds of the Gelli-grin Calcareous Ash Series, 
these being often faulted out, together with parts of the Allt-Ddu 
Mudstones and the Rhiwlas Beds. The packing along this line is, 
however, decidedly less than along the Llangower displacement, 
and the inclination of the fault-plane seems to be at a lower 
angle ; tearing is not a characteristic feature of the rocks above it, 
except at the northern end of the district, where it is compara- 
tively slight as at Gelli-grin or on Bryn-cut. 


The Ffridd Defaid Line has been traced for only a short 
distance: it appears to cause concealment of some of the Lower 
Ashgillian Beds, and is very clearly indicated on the north side of 
the Hirnant valley; but on the south the country is at first heavily 
wooded and then merges into the open moor, where exposures are 
so few and far between that it is no longer possible to trace the 
displacement. 


There seems to be fairly definite evidence of the decrease in the 
importance of the displacements east of Bala Lake, and this is 
continued still farther eastwards, where along the line of the Hir- 
nant valley, at the source of that stream, the beds merely plunge 
down steeply without any break, the softer slates being apparently 
squeezed between the Denbigh Grits and Flags on the one hand 
and the hard Moel-fryn Sandstones on the other. All the major 
lines of displacement can be traced fairly easily across country, 
since they are generally marked by a feature in the landscape, this 
being, however, occasionally emphasized by the nature of the rocks. 
affected ; thus, the Llangower thrust is marked by a well-defined 
fault- scarp, which is more pronounced when the harder Glyn-Gower 
Sandstones are brought against the Allt-Ddu Mudstones. 

A smaller but clearly defined scarp also marks the line of the 
Cefn-ddwy-graig thrust ; on the other hand, the Moel-fryn line is 
more usually indicated by a depression, as it is on the pass 
separating the valleys of the Dwynant and the Afon Glyn. 


The tear-faults associated with the major lines of displacement 
deserve mention. ‘The most conspicuous are those associated with 
the Llangower thrust: of these, the Ty’n-y-mir and Ty’n-y-twll 
tears both effect a very noteworthy shift of the Frondderw Ash, 

Q. J.G.S. No. 310. M 


162 DR. G. L. ELLES ON THE BALA COUNTRY: | vol. lxxvin, 


while the Bryniau-goleu tear shifts the same bed when practically 
vertical for a distance of half a mile; the Beudy-Graienyn tear is 
the last of the big tears to bring about a noticeable horizontal 
movement of the beds. That all have probably been shifted to 
some extent is, nevertheless, suggested by the outcrop of the 
Frondderw Ash between the two main outcrops north-east of 
Cornelau farm; this can only be interpreted as implying a wholesale 
shortening of the line by differential movement north-westwards. 

The tear-faults associated with the Moel-fryn displacement are 
less important, although there is an interesting little group of them 
on Bryn-cut, north of the Hirnant valley, where the outcrops of 
the Gelli-grin Limestone and the Pont-y-Ceunant Ash are con- 
spicuously shifted. 

These tear-faults are also distinguished by definite topographical 
features, since they are nearly always marked by a gash limited by 
wall-like outcrops of the rock on each side, the width of the gash 
varying with the importance of the tear; they also are almost 
invariably accompanied by more or less horizontal slickensiding 
and by intense quartz mineralization. 


VI. PaALHZONTOLOGY. 


There are certain features of interest in the faunas found at 
different horizons in the Bala district. In the first place, the 
fauna of the Derfel Limestone is a remarkable one, for, from 
the list given on p. 145, it will be seen that in its general character 
the fauna is more closely related to that of the Scottish rather than 
to that of the Welsh Llandilian rocks. Brachiopods‘are far more 
conspicuous than is usual in the Welsh Llandilan, and there seems 
to be no trace whatever of the trilobites which are of common 
occurrence in the Welsh beds of that age, such as Trinwcleus 
jimbriatus, Ampyx nudus, Ogygia buchi, Barrandia radians, 
B. cordai, Calymene duplicata, and other forms, which even 
as near as Builth (Gwern-y-fed-fach) occur associated with the 
same graptolites as those that are found in Nant-Derfel. 

Anyone who has studied the faunas of the Lower Ordovician 
rocks of Scotland must have realized that they are essentially the 
forerunners of the Ashgillian fauna, and some of the forms found 
in the Nant-Derfel gorge appear to be identical with some occurring 
in the Ashgillian itself. The occurrence, therefore, of this type 
of fauna at this horizon in this area is of great paleontological 
significance. 

As respects the faunas of the Caradocian rocks, while the 
grouping here adopted is necessarily based mainly upon the 
assemblages found in the Bala country, the assemblages found in 
other parts of England and Wales have also been taken into 
consideration. 

With regard to the general aspect of the Caradocian fauna, 
it may be noted that it is essentially a trilobite—brachiopod fauna. 
It is very generally rich in brachiopods of large size, many of 


/ 


part 2] ITS STRUCTURE AND ROCK-SUCCESSION. 163 


which, however, are not peculiar to it, but are long-ranged forms ; 
though some, like Heterorthis alternata (Sowerby) and its variety 
retrorsistria (Davidson), appear to be characteristic Caradocian 
species. The trilobites, however, are more significant; Calymene 
planimarginata appears to. range throughout, as do also Homalo- 
notus bisulcatus and Trinucleus concentricus ; others, though 
_ they may perhaps be found eventually all through the Series, are 
at any rate more abundant in either the lower or the higher beds. 
Thus, the large Asaphus powist seems to be more particularly 
characteristic of the lower beds, as does also a form which agrees 
well with Calymene brevicapitata Portlock; on the other hand, 
species of Chasmops, Pterygometopus jukest, Calymene caractact, 
and <Acaste apiculata are undoubtedly more abundant in the 
higher beds. 

“Monticuliporids are abundant, similar species ranging not only 
through the Series, but above and below its limits. Glyptocrinus 
basalis is everywhere a common fossil, but Cystid-plates are more 
numerous in the higher beds, although they seem to attain a still 
greater development in the basal beds of the Ashgillian. 

Neither gastropods nor lamellibranchs, though locally present 

‘in fair abundance, form any important part of the fauna. 

The most satisfactory palzontological classification, therefore, 1s 
that which groups trilobites and brachiopods together : the former 
because they afford the surer guides to age, the Jatter because they 
constitute so great a part of the fauna asa whole. On this basis 
the Caradocian rocks, as developed in the Bala country, may be 
regarded as belonging to the Calymene-planimarginata fauna. 

‘Other fossils that range throughout in some abundance are :— 


Trinucleus concentricus Eaton, or a Orthis (Harknessella) vespertilio 
variety. (Sowerby). 
Homalonotus bisulcatws Salter, or a Orthis calligramma Davidson. 
variety. Orthis (Dinorthis) flabellulum 
Orthis (Platystrophia)  biforata | (Sowerby). 
(Schlotheim). Plectambonites sericea (Sowerby). 
Monticulipora fibrosa (M‘Coy). 


A further subdivision may be made into two sub-faunas : a lower 
one characterized by the abundant presence of Asaphus powisi and 
Heterorthis alternata, and an upper containing Chasmops and 
Orthis (Nicolella) actonie. The fossil assemblages characteristic 
of these two sub-faunas are recorded in the lists on pp. 170-71. 
The graptolites found are important, since they enable some definite 
comparison to be made as to the relative ages of the beds in the 
rocks of the “shelly ’ type and ‘ graptolitic ’ type respectively. 

The typical form of Orthograptus truncatus Lapworth has been 
found in the mudstones of the Glyn-Gower Beds (Pont-yr-Onen 
and Nant-Rhyd-Wen), aud, so far as our present knowledge goes, 

that fossil is highly characteristic of the zone of Dier anograptus 
clingant, although towards the top of that zone and in the 
succeeding zone of Pleur ‘ograptus linearis, its place is gradually: 
taken by “the longer and thinner variety pauperatus Lapworth. 
mM 2 


164: DR. G. L. ELLES ON THE BALA COUNTRY: [vol. lxxviii, 


These two forms occur together near the top of the Allt-Ddu 
Mudstones (south-west of Pandy-isaf), so that it appears extremely 
probable that the Glyn-Gower Beds, together with the Allt-Ddu 
Mudstones, may be taken to represent the shallow-water equivalents 
of the zone of Dicranograptus clingant. 


The general aspect of the Ashgilhan fauna, as developed in the 
Bala district, is widely different from that of the Caradocian ; 
though, unfortunately, ‘only the Rhiwlas Beds at the base of that 
series and the Foel-y-Ddinas Beds at the top contain fossils. The 
most noticeable feature in the Rhiwlas Beds is the almost complete 
disappearance of all the big brachiopods which are so numerous, 
as regards both individuals and species, at the lower horizon, those 
that remain being of small size only. The trilobites and Cystids 
form the greater part of the fauna, together with numerous small 
ostracods, but even a casual scrutiny will reveal the fact that these 
trilobites are of quite different type for the greater part from those 
forms in the Caradocian, although when the highest beds of the 
Caradocian are carefully searched a few of these forms may be 
found, showing that the new fauna came in gradually. Thus, 
species of Cheirurus, Lichas, Cybele, Encrinurus, Remopleurides, 
and Stawrocephalus are all highly characteristic of the lowest 
Ashgillian fauna; and, of these, specimens of Lichas laxatus, 
Cybele 7UGOSA, and Encrinurus sexcostatus are not infrequently 
found at the top of the Caradocian. Should conditions in any 
area have changed more gradually, a greater admixture of the two 
faunas might be looked for. 

Prof. J. E. Marr! has divided the Ashgillian into two, the 
Phillipsinella Beds below, and the Phacops-mucronatus Beds 
above, and in a limited sense his classification may be followed in 
the Bala area. The Rhiwlas Mudstone and Limestone belong 
unquestionably to the Ph7/lipsinella Beds. This Phillipsinella- 
parabola fauna is a very varied and highly characteristic one ; a 
list of the chief fossils found belonging to it is given on p. 172. 

The Foel-y-Ddinas Beds which appear to belong to the Phacops- 
mucronatus Beds, on the other hand, have a most meagre fauna, 
and it is probable that only a small portion of the beds containing 
that fauna are here represented (see list, p. 156). 

There seems to be a complete passage up from these Foel-y- 
Ddinas Beds, through the Hirnant Beds, to deposits of undoubtedly 
Silurian age. The Hirnant Beds contain—locally, at any rate— 
a distinct assemblage of brachiopods that may be regarded as con- 
stituting the Orthis-hirnantensis fauna (see list, p. 157). This 
fauna seems to have been included by Prof. Marr in his Ashgillian, 
as it occurs in the Ashgill Shales. 

Significant, also, is the occurrence of Glyptograptus persculptus 
in the higher beds of the Rhiwlas Mudstones (Garth Goch), this 
graptolite being usually characteristic of the zones of Dicello- 
graptus anceps or Cephalograptus acuminatus. 


1 Q.J.G.S. vol. Ixxi (1915-16) pp. 192 et seqq. 


part 2] ITS STRUCTURE AND ROCK-SUCCESSION. 165 


Notes on Strophomena siluriana Davidson and 
Orthis hirnantensis M‘Coy. 


In the Hirnant Beds of the typical locality there occur two 
brachiopods about which there seems to have been considerable 
confusion. The following notes are given as a contribution towards 
clearing up the differences and resemblances which exist between 
them. The shells are indistinguishable in fragments, for the 


Fig. 10.— Diagrammatic sketches illustrating differences in typt- 
cal forms of Orthis hirnantensis J/Coy and Strophomena 
siluriana Davidson. 


Cardinal Process 
i] 


Xers 


a=Characteristic appearance of brachial valve in O. hirnantensis. 
b & c=Characteristic appearance of brachial valve in two types (alate and 
non-alate) of Strophomena siluriana. 


ornamentation of the shell appears to be almost identical in both 
forms, although with a tendency to a greater coarseness in the 
form here referred to Strophomena. 


166 DR. G. L. ELLES ON THE BALA COUNTRY: [ vol. Ixxviii, 


The relative proportions of the two shells are markedly 
different :— 


In (a) the hinge-line is not the greatest width of the shell and the length 
is approximately the same as the width, which is greatest at the middle 
of the shell; the angles are obtuse, the pedicle-valve is definitely but 
gently convex, the brachial valve being nearly flat or very slightly 
convex, and showing a distinct but single cardinal process. 

The structure of the shell is very punctate in between the ribs, and 
the ribbing characteristically fasciculate. The transverse striations 
are very clear when the shell is well preserved. 

This seems to be the true Orthis hirnantensis M‘Coy. 


In (b) the shell is much wider than long, the hinge-line is always the 
greatest width of the shell, and though at times more pronouncedly 
‘winged’ than at others, the angles of the shell are always acute. 
The pedicle-valve is very slightly convex in the middle line, the 
brachial valve flat or slightly concave with a bifid cardinal process. 

Shell punctate and ribs fasciculate as in O. hirnantensis, but at 
times rather coarser than in that species. 
This appears to be Strophomena siluriana Davidson. 


VII. CoMPARISON WITH OTHER AREAS. 


It is not very easy to compare the shelly faunas of one area 
with those of another, since there is likely to be less uniformity in 
the general physical conditions than in the case of those controlling 
the distribution of the faunas of the deeper parts of the sea, and 
hence faunas will be likely to show local peculiarities. Correlations 
of this nature therefore can only be carried out on general lines. 

The Derfel Limestone contains a fauna which has not been, up 
to the present, recorded at that horizon from any other Welsh or 
English deposits; 1t does, however, show a noteworthy resemblance 
to the fauna of corresponding Scottish deposits of that age, as the 
list given on p. 169 clearly indicates. On the whole, it compares 
best with that of the Stinchar Limestone, though in some respects, 
especially as regards the trilobites, it is closer to that of the Bal- 
clatchie Beds ; it would appear therefore to contain a fauna of the 
Scottish rather than the Welsh type, which is both interesting and 
important on paleontological grounds. With regard to the Cara- 
docian rocks of the Bala area, they may perhaps be said to 
correspond in a general way to beds of similar age in the Glyn- 
Ceiriog area eel by Groom & Lake,! but aot certain rather 
str iking differences. The Teirw Beds of that area may be 
paralleled roughly with the Allt-Ddu Mudstones (see lists, pp. 146- 
47), and to some extent also the Bryn Beds have a fauna re- 
sembling that of the Gelli-grin Caleareous Ash; but a noteworthy 
difference in the fauna seems to be the absence in the Bryn Beds 
of Orthis (Nicolella) actonie, invariably highly characteristic of 
the beds at Bala (see lists, pp. 147-48). This fossil does, however, 
appear to be present in the higher shales when they are not faulted 


1 Q.J.G.S. vol. lxiy (1908) p. 546. 


-part 2] ITS STRUCTURE AND ROCK-SUCCESSION. 167 


off: so, perhaps, if the beds were completely seen, the difference 
might disappear. This seems all the more likely when the fauna 
of the Dolhir Beds is studied, because it is here obvious that we 
are dealing with a mixed fauna which naturally groups itself into 
two, that characteristic of the Upper Caradocian. containing both 
species of Chasmops, Calymene caractaci, and abundant Orthis 
(Nicolella) actonie with two large brachiopods; and that gene- 
rally regarded as belonging to the Ashgillian, which contains 
Acidaspis, species of Cheirurus and Remopleurides, and smali 
brachiopods. Therefore, the large brachiopods which so frequently 
disappear concurrently with the appearance of the Ashgillian fauna 
do not do so in this Glyn-Ceiriog area, remaining and occurring 
side by side with the smaller characteristic Ashgillian types— 
Orthis crispa, Strophomena corrugatella, and Bilobites biloba. 

Nevertheless, the incoming of new types in abundance, despite 
the absence of Phillipsinella parabola, justifies Groom & Lake 
in their contention that the beds are of Ashgillian age. The 
Caradocian beds of Bala perhaps find their nearest analogues in the 
Shropshire deposits of that age; as the lists given on p. 171 indi- 
cate, the fauna of the Soudley Sandstones and lower part of the 
Cheney-Longville Flags clearly belongs to the Asaphus-powisa 
and Orthis (Heteror this)- -alternata fauna, and unquestionably is 
closely related to that of the Allt-Ddu Mudstones. Moreover, the 
Harnage Shales which occur below the Soudley Sandstones contain 
Orthograptus truncatus Lapworth, so that they too afford confir- 
matory evidence of age as being referable also to the horizon of 
the zone of Dicranograptus clingani. Whether the still lower 
Hoar-Edge Beds should be paralleled with the Glyn-Gower Beds, 
or whether these last should be grouped with the Allt-Ddu Mud- 
stones and correlated with the higher beds, there is not sufficient 
evidence to show. 

The upper part of the Cheney-Longville Beds and Acton-Scott 
Beds, on the other hand, with their abundant Chasmops and Orthis 
(Nicolella) actonie, may readily be correlated with the Gelli-grin 
Caleareous Ash (see list, p. 170). 


Turning to the Lake District, I think fon is clear that a certain 
amount of parallelism exists between the beds of certain areas and 
those of Bala, although the parallelism must not be pressed too 
closely. The beds that Prof. Marr has described! as the Calymene- 
Beds beneath the Ashgillian Series, in the area west of Coniston 
Lake, are doubtless the faunal equivalents of the Gelli-grin Cal- 
careous Ash, and the occasional specimens of the higher fauna may 
be noted as parallel with similar occurrences in Wales. 

The Ashgillian rocks of the area are also comparable on palon- 
tological grounds with those at Bala; thus the Phillipsinella- 
Beds of Marr contain a fauna which bears a str iking general resem- 
blance to that of the Rhiwlas Limestone and Mudstones (see list, 


1 Q.J.G.S. vol. Ixxi (1915-16) p. 191. 


168 DR. G. L. ELLES ON THE BALA COUNTRY: [vol. lxxviul, 


p- 172). Phillipsinella parabola appears to be both generically 
and specifically a characteristic Ashgillian form, although many 
other trilobites more characteristic of the fauna have a much 
longer range in time. Here it may be noted that the general 
faunal lists of the beds belonging to this horizon are often rather 
misleading, for they do not bring out the degree of resemblance 
that undoubtedly exists, since species are apt to be different in 
widely separated areas and thus the strong generic resemblance is 
often obscured. The essential distinction between the Caradocian 
and the Ashgillian faunas as found over the greater part of Wales 
and the Lake District hes in the genera rather than in the 
species of trilobites; the species, however, are all-important in 
discriminating between the Ashgillian fauna proper and the early 
faunas of Ashgillian type, although indeed some forms are common 
throughout. 

Thus, two widely-separated areas, both containing abundant 
Phillipsinella parabola, and therefore presumably of Ashgillian 
age, may yield many Cheirurids, Lichads, and Remopleurids, and 
yet have no species of these genera in common. Consequently, 
the attempt to show the relationship that probably exists between 
the faunas of the Rhiwlas Limestone and Mudstones and those 
of the Phillipsinella Beds of the Cautley area, the area west of 
Coniston Lake, the Keighley Limestone, the Chair-of-Kildare 
Limestone, and the Sholeshook Limestone in South Wales cannot 
be regarded as satisfactory (p. 172): there is, in reality, a greater 
resemblance than these lists indicate. 

The Foel-y-Ddinas Beds of Bala represent part, at any rate, of 
Marr’s Phacops-mucronatus Beds, although the fauna is very 
meagrely represented (see p. 156). Marr’s Ashgill Shales, on the 
other hand, contain a fauna which recalls strikingly that of the 
Hirnant Beds, and, if the two belong really to the same horizon, the 
question of the upper limit of the Ashgillian Series seems to be 
involved. The section as seen at Cwm-Hirnant is, in my opinion, 
strongly suggestive of the Lower Llandovery age of the Hirnant 
Beds, as otherwise the whole of the Llandovery Series must be 
comprised within 150 feet of rock—not impossible in an area of 
deep-water deposition, but hardly likely in a district where shallow- 
water deposits occur so abundantly in such close proximity. 

The sections at Conway! would confirm this view. 


In conclusion, I would offer my grateful thanks to the many 
members (past and present) of the Cambridge University Sedgwick 
Club, who have helped me during the progress of this work, more 
particularly my pupils Miss H. Drew, Miss A. B. Dale, Miss 
E. W. Gardner, Mrs. J. Romanes, and Miss M. E. J. Chandler, 
and Mr. W. B. R. King and Mr. T. C. Nicholas. 

To Prof. Marr Iam much indebted for consultation in matters 
relating to the Ashgillian as a whole, and for permitting me to 
refer to his collections from the Lake District. 


1 GL, Elles, Q. J. G.S. vol. Ixv (1909) p. 169. 


part 2] 


VIII. Faunat Lists. 


ITS STRUCTURE AND ROCK-SUCCESSION. 


Derfel Limestone. 


c=common ; r—rare. 


| Lichas lawatus M‘Coy. MER 
Cybele verrucosa (Dalman). CEN RNA SNe eth ass 
Cybele rugosa (Portlock). CH teal Ea : 
Illenus balclatchiensis Reed. (Ca I ae ERE 
Acidaspis hysterix Wyville Thomson. Yo... 
Harpes flanagani Portlock. Ve SA A Rn rere EE 
Trinucleus sp. Cie tn prasauanse dec ers tee cat anttentene 
Orthis (Platystrophia) biforata (Schlotheim). c 
Orthis (Nicolella) actonie (Sowerby). AS That tak 
Orthis (Nicolella?) alata (Sowerby). cea ne pee 
Orthis (Hebertella) crispa (M‘Coy). Peer? 
Orthis (Dalmanella) testudinaria vay. aed lis 
Reed. ie Sass 
Orthis (Dalmanella) girvanensis (Davidson). ane 
Orthis playfairi Reed. c Sebi arse tea oe 
Orthis (Heterorthis) confinis Salter ? Fonte gest a 
Skenidium lewisi var. craigense Reed. Neha anate 
Triplecia craigensis Reed. Neues ett stn hcrcs 
Cliftonia andersoni Reed. Cpiias Magee igi ow aectico pepe 
Clitambonites cf. ascendens Pander. Wee sees tat ee 
Leptena rhomboidalis Wilckens. Chis veer 
Rafinesquina expansa vay. c oath Lace AR OD 
Rafinesquina subarachnoidea Reeds, dice wae 
Stropheodonta corrugatella (Davidson). Those 
Plectambonites llandeiloensis (Davidson). Coty cei 
Plectambonites sericea var. semirugata Reed.  ¢... 


Numerous Bryozoa, and a few Crinoid-stems 
and Cystid-plates. 


Scotland. | 
(———— 


Stinchar Limestone. 


+: 


++i tt++¢4¢44 


Balclatchie Beds. 


+ 


+ +4+4++4+ 


+++ 


TCome 


ea es 


| Welsh Llandilian. 


++ 


169 


Higher horizons in Wales. 


++ 


++ 


++ 


170 DR. G. L. ELLES ON THE BALA COUNTRY : 


[ vol. Ixxviil, 


Chasmops & Nicolella-actonie Fauna. 
Gelli-Grin Calcareous Ashes. 
C=very common; c=common; r=rare. 


Orthis (Nicolella) actonie (Sowerby) . sue 
Orthis (Platystrophia) biforata (Schlotheim) . scape 
Orthis s.s. calligramma Dalman_. 3 Bene 
Orthis (Hebertella) crispa (M‘Coy) _ ... 

Orthis (Dalmanella) elegantula eee 

Orthis (Dinorthis) flabellulum one tay 

Orthis (Plectorthis) plicata (Sowerby) . 

Orthis (Plesiomys) porcata (Sowerby) Hae eh he Ae 
Orthis (Dalmanella) testudinaria (Dalman) ote 
Orthis unguis Sowerby .. eee 
Orthis (Harknessella) vespertilio (Sowerby) .. dane 
Plectambonites quinquecostata (M‘Coy) er a: 
Plectambonites rhombica (Davidson).................. +4. 
Plectambonites sericea (Sowerby) . 

Plectambonites transversalis (Wahl)... sata. eae 
Triplecia (Cliftonia) spiriferoides (M‘Coy) Mere 
Leptena rhomboidalis Wilckens ...... 
Strophomena (Rafinesquina) expansa (Sowerby) . 
Stropheodonta TRS eee 

Lingula ovata Sowerby... ay 


Calymene caractaci Salter A Lae aeerisee men mee 
Calymene planimarginata Reed ..............:eeeceeeenees 
Chasmops conicophthalmus oe) 

Chasmops macrura (Sjogren) . 

Homalonotus bisulcatus Salter 

Illenus bowmanni Salter . 

Phacops (Acaste) apiculata alter) .. 
Pterygometopus jukesi (Salter) . 

Trinucleus concentricus Katon 

Trinucleus gibbifrons M‘Coy 


Beyrichia (Tetradella) complicata (M‘Coy) ......... 
Lindstroemia subduplicata (M‘Coy) .......6....cceeen eee 
Halysites catenularia Linneus ............ 0.606. 
Menticulipora lens (M‘Coy) .. 

Monticulipora lycoperdon Selle 

Monticulipora fibrosa (Me roo 

Ptilodictya ....... 

Conularia sowerbyi. M: ‘Coy .. cig ello. Sacra 
Glyptocrinus basalis M‘ Coy DA Raat 

Cystid-plates : 


Local assemblages of gasteropods and lamellibranchs. 

There are also certain ‘forerunners’ of the Ashgillian 

beds, of fairly common occurrence in different 

localities, though as a rule not abundant: such as 

- Encrinurus sexcostatus, Lichas laxatus, Cybele 
verrucosa, and Agnostus agnostiformis. 


Gelli-grin Ashes, 
Bala. 


BPROQRe AM ete QAQce Qeae DQ 


amMQanmeaeaacecnan 


@ Qe tt} QE is td i @ 


~ 
ioe 
oy Stee 
2 lee 
BS |Pac 
Zo \5a% 
2 Css 
Po | ee 
As 
ee: 
+ + 
+ + 
+ 
+ 
+ 
+ + 
+ 
<b 
+ + 
ig + 
+ 
+ + 
+ + 
+ 
ie + 
+ + 
Nab: 
+ + 
+ + 
+ + 
+ + 
+ + 
+ + 
hy + 
+ + 
+ 
+ 


Calymene Beds, 
Coniston. 


++ 


+++ 4+ 


+ ++ +444 ++ ++ 


$+ trot 


++ 


part 2] ITS STRUCTURE AND ROCK-SUCCESSION. 171 


Bala. | Shropshire. | 
rc | aaa. 
3 
jaa) 
Asaphus-powisi & Heterorthis-alternata Fauna. ; | 3 = 
1) ci S Me 
Glyn-Gower Sandstones and 3 ch | S| S = 
Allt-Ddu Mudstones. ee eate bee. 
| (go| 2/4) = 
S| eRe 8] -| G 
2S | Bel is |e 
C=very common; c=common ; r=rare. Seen be ey yee 5 
So/teH |Fla]4 
Orthis (Heterorthis) alternata (Sowerby) . UT Rah Nh a ike +] + 
var. retrorsistria (Davidson) sean sl Oven Pee 
Orthis (s.s.) calligramma Dalman ........ BO ree. oe 
Orthis (Dalmanella) elegantula (Dalman) v WaPo caly 8@e dle Ome lectern (ck ttt iat 
Orthis (Dinorthis) flabellulum Sow ae Beech leet eee pes vest allt 
Orthis (Plectorthis) plicata (Sowerby)... 2 ae C 
Orthis (Plesiomys) porcata (Sowerby)... Za cca leed ee ERG Seale. oe 
Orthis (Dalmanella) testudinaria (Dalman) . Same tee TE) ak Ox are es a ete aa a 
Orthis (Harknessella) vespertilio Coy Bae at (er | eC A a Ae ao ee | 
Plectambonites sericea (Sowerby) .. al ee ec A Omni Gem Mie ga acer eal eal | 
Plectambonites transversalis (Dalman)... nn? r 
Triplecia (Cliftonia) spiriferoides (M‘Coy) . See r +] + 
Leptena rhomboidalis Wilckens ....... eA eae 
‘| Strophomena (Rafinesquina) expansa (Sowerby) Set orca) eh tees lh a Abbr aaah 
Strophomena (Rafinesquina) pe es a) cere r wise, bouche leeets 
Lingula ovata Sowerby ; fossa gee 
Asaphus powisi Salter .......... CRO, st Wee ber isinars 
Calymene caractaci Salter ells 
Calymene brevicapitata Portlock — vey TOSI he aM Wea ed ef 
Calymene planimarginata Reed.. Cy 1 ects ei ahaa eet 
Encrinurus multisegmentatus | (Portlock) .. se 5G | 
MU UWCLEUS CONGENEPICWS LY PC oo. cu ook ond- ose ecceecacoc| © Cc +] Ft] +]+ 
Homalonotus bisulcatus (Salter) P Gy ais AM pace: 
| Tetradella complicata (M‘Coy) ... Crt’ Wee lee | ae Set 
Monticulipora lens (M‘Coy) .... ORs Onn a it ae 
Monticulipora fibrosa (M‘Coy) .. bey eral ate | iste |e: - lek 
Glyptocrinus basalis M‘Coy .... Cire see ae tae 
Ophiurids..... .. r | 
Bellerophon (Sinuites) Bilobatus (Sow erby) Taetieact S| aa 
Cyclonema crebristria (M‘Coy) ..... Toate + | + 
Lophospira gyrogonia (M‘Coy) . Fc se Re aL ek a ec | + 
Murchisonia cf. simplex (M‘Coy) . Brana Yr 


172 DR. G. L. ELLES ON THE BALA COUNTRY: 


Phillipsinella-parabola Fauna. 
Rhiwlas Limestone and Mudstones, 


— very common; c=common; r=rare. 


| Asaphus radiatus Salter 

| Ampyx tumidus Saiter ......... 

| Calymene blumenbachii Brongniart — 

| Cheirurus bimucronatus (Mur rehison) 
Cheirurus gelasinosus (Portlock)... 
Cheirurus juvenis (Salter) Sinise eens Eee 
Cheirurus octolobatus (M‘Coy)  .........-...46-+5 
Cybele loveni (Linnarsson) .............2.+0+02- +5 
Cybele rugosa (Portlock) .. 

Cybele verrucosa (Dalman) at! iat 
Dindymene cordai Etheridge & Nicholson 

| Encrinurus sexcostatus (Salter) .................- 


| 


| Encrinurus multisegmentatus (Portlock) aoe oF | 


| Tllenus bowmanni Salter ............22. 22.002 008-+ 
Illenus davisi Salter 
LTichas laxatus M‘Coy . 
Lichas cf. bulbiceps Reed... 


| Phillipsinella parabola Barrande ee 


Remopleurides colbii Portlock .. 

| Remopleurides radians Barrande 
| Spherexochus mirus Beyrich . ..........2.. +0 
Staurocephalus murechisoni Barrande Ee 
Trinucleus seticornis Hisinger ests 
| Trinucleus bucklandi Barrande ................-. 
| Plumulites peachi (Etheridge & eho 
| Caryocystites davisi (M‘Coy) ............-- 


| 
| 
| 
| 


| Caryocystites granulatus Forbes.............++--. 
| Echinospherites arachnoideus Forbes ......... 
| Hemicosmites rugatus Forbes ...:.....:.... 
| Spheronectes munitus Forbes .. 
| Spheronectes pyriformis Forbes. 
| Spheronectes punctatus Forbes ......-.....--.+.- 
Orthis (Eelobates) biloba (Linnarsson) .. ees ae 
Orthis s.s. calligramma var.. Re eee 
| Orthis (Heber tella) crispa (M: Coy)... 4: 
Orthis (Dalmanella) epic. var, drum- 
| muckensis Reed . 
| Orthis (Dalmanella) elegantula ¥ var. 
| Christiania tenuicincta (M‘Coy) 
| Hyattella portlockiana (Davidson) 
| Skenidium lewisi (Davidson) . 
| Plectambonites guinquecostata (MS Coy) 


Str opheodonta cor ceiea ee pan eg 


| Orthoceras vagans Salter .. 
| Holopea concinna M‘Coy .. 


Many small ostracods and a few larger forms | 
abundant. Also some ‘survivors’ from the | 


very rare: 
apiculata, Platystrophia biforata, and 
Nicolella actonie.. 


| 
| Chasmops Fauna, though these are usually 


i = Oe 


Agnostus agnostiformis (M‘ oO = EE Sed 


| N. Wales. 


Rhiwlas Lime- 


| 


namely, Chasmops sp., Acaste, 


} 
| 


Ney lab dat ter) dV QPS) loi) lr) ry (Puello) (PP eT) (ea) ) ol tt ler) x ey tc Ih (em 


and 


Mudstones. 


Dolhir Beds, 


stone 


HOAAAKQRA. 


o 
to] 


+++: 


Poe 


++ 


+++: 


++ 


Glyu Ceirio 


+i + wt 


Coniston. 


bebe bibs 


Lakes. 


+i 


+t -et +: 


Cautley. 


+ + 


[vol. xxviii, 


Keisley. 


++ +444: + 


++: 


Men Sate pl ha 


Chair of Kildave. 


Drummuck Beds, 


S. Scotland. 


- 


++: 


++ 


++ 


aye 


++: 


++ 


S. Wales, 


+ + +++ 


+++ + 


Sholeshook Lst. 


Te 


c. Vol. LAXVIT, Pl. 1. 


lourn. Geol. So 


fay 
a 
AOQNY 75 
2 ve, 


see oe oy 


S 


® 
Sa ee A, oe a e 
J 
ee 


aL = 
rail hens 


Ete 


a3 
1-1 


Vy 


garth) 


Perny 


jehiaye 


ne 


ij 


lany¢il 


HIRNANT LIMESTONE 
IMOEL-FRYN SANDSTONES. — 
RHIWLAS LIMESTONE. 
IPONT-Y-CEUNANT ASH. © + 
ALL DDU MUDSTONES 
& LIMESTONES C 
L 


HIRNANT BEDS, 
S]RHIWLAS MUDSTONES. — 


=| CALCAREOUS ASH. ~ 
+ |GLYN-GOWER 
+ -| SANDSTONES. 


= |GELLI-GRIN 


See 
pean eet r SC—! me—D 


INDEX 


HAnaAIAH) 


CE 


om] 


E.& S.E.oFr 


BALA LAKE. 


AP or COUNTRY 


M 


a 


GELLI-GRINE 
BRYN PIO LST, 
MOEL-FRIN LST | ier mr 
AM YO-WEN LST aS 
DERFEL LST bat 


CAE R HAFOTTY LST | ame 


— 


otf ae 


_ : St 


¥ w é 
: 
val Rem ages Ot 2 ay 
* - 
‘ « P 
7 “ 
- 
9 - 
*% 
* 
ow 
Fy 2 
s 
J 
* 
° 
° 
. 
™ * 
« 
ad . 
2 
’ 
' 


— 


Bar 


ee: 


rhe a8 pile ; 


0: 


part 2] ITS STRUCTURE AND ROCK-SUCCESSION. 173 


EXPLANATION OF PLATE II. 


Geological map of the country east and south-east of Bala Lake, on the 
scale of 4 inches to the mile, or 1:15,840. 


DISCUSSION. 


Prof. O. T. Jones congratulated the Author on the completion 
of an extremely interesting research upon a complicated district, 
one which, as was known from his writings, drove so accomplished 
a geologist as Jukes almost to despair. 

The paper raised many interesting questions, both of the nomen- 
clature and structure, and particularly furnished information as to 
the relation of the Llandeilo and Bala formations. The discovery 
of the Nemagraptus-gracilis fauna in the Dertel Limestone, asso- 
ciated with brachiopods similar to those in the Stinchar Limestone, 
was extremely interesting. 

In Sedewick’s tinal classification the base of the Bala was drawn 
at the base of the strata overlying the ‘ porphyries’ of Arenig. 
The relation of these ‘porphyries’ to the Llandeilo formation of 
South Wales was unknown. In South Wales it has been shown 
that the Nemagraptus fauna overlies directly everything that can 
be assigned by definition to the Llandeilo formation, and it was 
satisfactory, therefore, to find from the Author’s new evidence that 
the line proposed by Sedgwick for the base of the Bala in North 
Wales coincided with the line that was adopted by the Geological 
Survey in South Wales by direct reference to the Llandeilo forma- 
tion. It further became clear that the terms ‘ Llandeilo’ and 
‘Bala’ bore very little relation to Llandeilian and Caradocian. 

He did not quite understand the Author’s caution about regarding 
a fauna of Ashgillian type in South Wales and elsewhere as neces- 
sarily indicating Ashgillian age. The Rhiwlas Limestone of North 
Wales, with its Phillipsinella-parabola fauna, coincided precisely 
with the beds which were taken to mark the base of the Ashgillian 
in South Wales ; andthe numerous brachiopods from rocks of Ash- 
gillian age, which the speaker had examined from various localities 
in North Wales, agreed closely with those of the corresponding 
rocks in South Wales. | 

The position of the Hirnant Limestone apparently still remained 
unsatisfactory. The striped shales which succeeded it appeared to 
correlate with the Birkhill Shales farther south, where, however, 
they were thicker than at Hirnant, and it was also rather sur- 
prising that they yielded no graptolites, considering their extremely 
fossiliferous character farther south. 

He enquired whether there was a possibility that some part of 
the sequence was cut out by strike-faulting, in view of the existence 
of several faults of this type ranging towards the area from the 
south. He also asked whether there was any direct evidence of 
ereat lateral movements along the line of the Bala Fault itself, as 
seemed to be the case in the neighbourhood of Talyllyn, on what 
was apparently the continuation of the same fault. 


174 THE BALA COUNTRY: (vol. Ixxviii, 


Dr. A. WaDE said that perhaps some observations made by 
himself might help a little towards the solution of the problem of 
the Hirnant Limestone in the area dealt with in this valuable 
paper. On the eastern side of the Welshpool area the base of the 
Llandovery was, as a rule, a massive conglomerate, sometimes cal- 
careous and containing Pentamerus. On the north-western flank 
the conglomerate was still present, but tended to pass into finer 
sandstones, and in places thinned out altogether. It was, however, 
represented by large blocks of limestone, lenticular and non-con- 
tinuous, which seemed to contain great quantities of Orthzs and 
similar brachiopods, and practically no Pentamerus. The Llan- 
dovery here lay unconformably on the Upper Ordovician rocks, so 
that the position of the beds was fairly easily determined. Such 
changes apparently took place in these rocks in the area now 
described, and possibly such a change accounted for the difficulties 
in connexion with the Hirnant Limestone. 

Mr. A. K. WE ts stated that, as one of the younger workers in 
North Wales, he wished to add his congratulations and thanks to 
the Author for her very lucid account of the geology of what was 
well known to be one of the most difficult areas in that region. 
He was particularly interested in the discovery of a fossiliferous 
band in the series of monotonous mudstones that succeed the 
highest voleanic rocks in the Arenig district—a discovery that 
would go far towards proving the relative age of these lavas. The 
fauna seemed to be very similar to that found immediately above 
the volcanic rocks at Llanwrtyd Wells by the speaker’s colleague, 
Mr. L. D. Stamp. Here, however, the same graptolites were 
discovered in another shale-band within the volcanic rocks. The 
speaker, working in the Rhobell-Fawr district, a few miles west 
of Bala, had found similar shales containing} graptolites, which, 
though poorly preserved, would probably prove to be from the same 
horizon. 

Mr. C. B. Wepp wished to associate himself with earlier 
speakers in congratulating the Author on her completion and 
lucid presentation of a most valuable piece of work. He felt that 
the structural details were too complex to discuss offhand, and 
asked whether the Author could give further information as to the 
effect of the movements upon direction and hade of cleavage. 

Dr. C. A. Martey also congratulated the Author, and enquired 
whether she had found in the Bala area, at or near the horizon of 
the Derfel Limestone, any representative of the oolitic ironstone 
which occurred in the Glenkiln Beds of Anglesey and Carnarvon- 
shire. 

Prof. W. W. Warts congratulated the Author on her courage 
in tackling so difficult a district. He referred to the interesting 
point brought out in the paper that the Derfel Limestone yielded 
the ‘exotic’ fauna, which the Author had particularized in the 
Stinchar Limestone and elsewhere. He hoped that the time was 
coming when it would be possible to use only paleontological 


part 2] ITS STRUCTURE AND ROCK-SUCCESSION. 175 


time-horizons, and that the innumerable, and often cacophonous, 
local horizon-names would be dropped. 

The AurHor, in reply, thanked the Fellows for their generous 
reception of her paper. In answer to Prof. Jones, she stated that, 
with regard to the boundary between the Llandeilian and the Cara- 
docian, she based her position on the paleontological aspect of the 
question. She regarded the rocks of the ‘shelly’ facies of the 
Welsh Ordovician as belonging to different faunas—the Ogygia- 
selwynt fauna, the Placoparia fauna, the Ogygia-buchi fauna, 
the Calymene-planimarginata fauna, the Phillipsinella-parabola 
fauna, and the Phacops-mucronatus fauna; and, so long as beds 
contained the same Ogygia-buchi fauna as those of the Llandeilo 
area, she considered them Llandeilian in age. The beds with the 
Nemagraptus-gracilis fauna, and even slightly higher graptolitic 
horizons, were associated with the Ogygia-buchi fauna in the 
Builth area, and were, therefore, in her opinion, of Llandeilian 
age. 

“With regard to the calcareous development of the Gelli-grin 
Calcareous Ashes and the Rhiwlas Beds being usually separate as 
in South Wales, that was not the case invariably in the Bala 
country, since both occurred superposed at Bryn-pig. 

She had looked for evidence of faulting at Cwm Hirnant, but 
could find none; the beds were steeply inclined, yet along the old 
adits there seemed to be a perfectly continuous section from the 
Hirnant Beds to the shales with Mlonograptus crispus. 

There certainly was evidence in the area north-west of Bala Lake 
of movements similar to those on the south-eastern side; these 
appear to be related to a major thrust running at or near the 
junction of the Dicranograptus Shales with the voleanic rocks. 
She regarded the entire country as being thrust above the volcanic 
series as a whole. 

In answer to Mr. Wedd, she stated that, while the cleavage in 
the southern part of the area was inclined steeply east-south- 
eastwards, when the effects of the east-and-west movement made 
themselves felt in the northern part of the district, a second 
cleavage seemed to be developed, the exact direction of which she 
had not determined. 

She was in complete agreement with Prof. Watts in regard to 
the growth of all these local names, which, however, were certainly 
useful for local reference, but suggested the adoption of the 
paleontological classification of wide application for all other 
purposes. 


176 DR. WILLS AND MR. SMITH ON THE LOWER [vol. lxxviil, 


4. The LowrEr Patmozotc Rocks of the LuancoLtLEN District, 
with special reference to the TEctonics. By LEONARD 
Jounston Wiis, M.A., Ph.D., F.G.S., and BErnarpD 
Smitru, M.A., F.G.S. (Read January 19th, 1921.) 


[Puates ITI-V—Gzrouoeican Mars & SECTIONS. | 


CONTENTS. 
Page 
Te Mintroduehiany. es we shee sees be ond eee eee 176 
Li Eistorien).Summary-.4. 2.04929 sande cee. one 1g. 
JU. Stratioraphy 526. ee we ete eee ene ee 178 
DV). StRiGhUTeS. 23 esa) oleae, Sa es, Came ee 208 
V.. he: Pre-Carbomferous Bloor ).5....).chasa eae 222 
VI. Post-Devonian Modifications ..0c.... 0... cchiecss dens: 222 


I. InTRODUCTION. 


THE area dealt with in the following pages forms the western half 
of the 1l-inch Ordnance Survey Map, New Series, Sheet 121, with 
small contiguous strips in Sheets 120 & 187. It was mapped by 
us in the course of official work for the Geological Survey, in 
1912 & 1913. The memoir on Sheet 121 is in preparation ; but 
both it and the map which it accompanies have been greatly 
delayed by the War, ‘This applies even more emphatically to 
Sheet 137. In the projected memoir on Sheet 121, special 
attention will be devoted to the Wrexham-Ruabon Coalfield. 

In preparing the present paper, we have been influenced by a 
desire to publish in a concise form! an account of the older rocks 
which are more of theoretical than economic importance. We 
have attempted to describe the district as a geological unit, whereas 
the Survey Memoir must be primarily an explanation of the map 
and of the evidence on which it is based. 

We are, therefore, greatly indebted to the Director of H.M. 
Geological Survey for permission to publish the following account, 
which is divisible into two parts. In the first, we aim at stating 
new facts and important modifications of the hitherto accepted 
ideas of the stratigraphical sequence, without traversing in detail 
the work of previous investigators where their conclusions have 
been confirmed by us. In the second part, we give a fuller 
description of the tectonics than the space available in the official 
Memoir would allow. 

Our sincere thanks are due to Mr. C. B. Wedd and Mr. W. B. R. 
King for help and friendly criticism, and to Miss G. L. Elles, 


1 The arbitrary limits of the map-sheets are not those of a natural geological 
region, Accordingly, important parts of the sequence, because they happen 
to lie in one sheet, have to be omitted in the official description of the other. 


part 2] PALMHOZOIC ROCKS OF THE LLANGOLLEN DISTRICT. 177 


Prof. O. T. Jones, Mr. Philip Lake, and Mr. John Pringle for the 
determination of fossils—a difficult task with material so distorted 
by cleavage. To Dr. H. H. Thomas we are greatly indebted for 
his examination of some of the more difficult rock-types. 


Il. Hisrortcan SUMMARY. 


Pending the publication of the new Geological Survey map, 
upon which the map (Pl. V) is based, the reader is referred to the 
Old Series map, Sheets 74 N.E., N.W., & S.E., in which the major 
features of the structure are well displayed. This, it is believed, 
was largely the work of T. W. Aveline and J. B. Jukes. 

More recent research has been confined, with few exceptions, to 
the southern part of the area, that is, between the Dee Valley and 
the Berwyns. 

Our present knowledge of the succession is largely due to the 
work of Dr. T. T. Groom & Mr. P. Lake,! whose paper on Glyn 
Ceiriog contains a full bibliography. 

The zonal classification of the Salopian was described, in the 
case of the Wenlock by Miss G. L. Elles,? and in the case of the 
Lower Ludlow Series by Dame Ethel Shakespear (née Wood). 

The igneous rocks of the Berwyns have been redescribed in a 
posthumous paper by 'T. H. Cope.* 

North of the Dee the geology of the Lower Paleozoic rocks has 
been barely touched, although reference may be made to early 
work by the late Prof. T. McK. Hughes® and to the maps made 
by Sir Aubrey Strahan 6; also to his description of the area lying 
north of Llandegla. The work of G. H. Morton was confined to 
the Carboniferous rocks. 

During the recent survey, brief notices were published in the 
‘Summary of Progress’ of the Geological Survey for the years 
1911, 1912, & 1913. One of us has also given a sketch of the 
geology of the Llangollen district.7 


1 T. T. Groom & P. Lake, ‘The Llandovery & Associated Rocks of the 
Neighbourhood of Corwen’ Q. J.G.S. vol. xlix (1893) pp. 426-39; T. T. 
Groom & P. Lake,‘ The Bala & Llandovery Rocks of Glyn Ceiriog ’ ibid. 
vol. lxiv (1908) pp. 546-95; and P. Lake, ‘The Denbighshire Series of South 
Denbighshire’ ibid. vol. li (1895) pp. 9-22. 

* «The Zonal Classification of the Wenlock Shales of the Welsh Border- 
land’ Q.J.G.S. vol. lvi (1900) pp. 370-418. 

3 «The Lower Ludlow Formation & its Graptolite-Fauna ’ ibid. pp. 415-91. 

4 *On the Igneous & Pyroclastic Rocks of the Berwyn Hills’ Cope 
Memorial Vol. Proc. Liverpool Geol. Soc. 1915. 

PO dnGe So. vol, xxx (1877) ps 200. 

6 ‘The Geology of the Neighbourhoods of Flint, Mold, & Ruthin’ Mem. 
Geol. Surv. 1890, pp. 4-6. 

7 L. J. Wills, ‘The Geology of the Llangollen District’ Proc. Geol. Assoc. 
vol. xxxi (1920) pp. 1-15. 


Q.J.G.S. No. 310. N 


178 DR. WILLS AND MR. SMITH ON THE LOWER [ vol. Ixxviii, 


Ii]. SrratTiGRapHy. 
(A) General Considerations. 


The southern part of Denbighshire and the adjoining portion of 
Merionethshire, described in this communication, form a rather 
flat-topped upland, deeply dissected in the south-east by the Dee, 


Fig. 1.—Outline-map showing the chief tectonic features of 


the Llangollen district. 


feat 
> Q 
> os iG = 
= EE = ; 
5S ) =O) ; VY, 
= =e SH — = 
8 Zee = Fl landesiz: 
S) = 
Nantclwyd/+ = Ss 
——— | 
pMDANZS ESFAY. 7 
LAN 7 M.CRICOR DOME A 
Llanelidan mee ) | CORN: Y “BRAIN? 
i B = Mi ZDOMEZ Wy 
2 <i. Ap ff, 
N /, Yy yy ts ie 
Bryneglwys -* CSS 
| v = 
SYNCLINORIUM er <> C= 
< Ss 
Vv ae ow 
A LEE ae aM.MORFYDD 
“ S % ae COMPLEX LIMB = 
a = 25 "5 
L—"a\4 | LLANGOLLEN SYNCLINORIUM PS 
Sle ,Carrog < “Liantysilio’ ————— 
| = Sav ———— ES OS ——— 
Corwen 4 = ONS DINAS 
[ BRAN 
Za stangollen 
Ya Giff 
GEG 
if EY 
Om 


R. Dee 


ee 
==— a 
Silurian 
Yyy Ordovician 


Carboniferous 
and Trias 


a 
eis 


Mm. 


Pas 
fil 


Yyyy Yj) BERw UY ”“s“sss = 
Yh ee) Wen 4 YY = 
Uy Vj Ups “its Wf} Vii GF YY 

La ‘Seale of Miles Ly fy ae 
GEST) Se Gp =e 


the Ceiriog, and their tributaries. A depression runs north- 
eastwards from Corwen across the upland, along the line of the 
Bryneglwys Fault.1 The drainage from this flows in part to the 
Dee and in part to the Alyn. In the northern part of the area 


1 Sometimes known as part of the Bala Fault, see p. 219. 


part 2] PALEOZOIC ROCKS OF THE LLANGOLLEN District. 179 


the Vale of Clwyd forms a great sunken graben nosing south- 
wards into the upland. 

The chief structural units of the district are shown in the out- 
line-map (fig. 1), and the general arrangement of the formations 
in the map forming Pl. V. 


(B) Ordovician.—The Bala Formation. 


In the three anticlinal areas: namely, the Berwyns, Cyrn-y- 
Brain, and Mynydd-Cricor, rocks referable to the Ashgillian Series 
of the Bala Formation are found, while in the Berwyns the under- 
lying Caradocian Series also comes to the surface. 

The sequence of the complete Bala Formation in the Glyn- 
Ceiriog district at the eastern end, and in the Moel-Fferna district 
at the western end, of the Berwyn outerop is set out in Table J, 
pp. 180-81. The Ashgilian development of the Bala in the two 
northern anticlines is also shown. 

Where it has been found practicable, the names instituted by 
Dr. Groom & Mr. Lake (op. cit. 1908) have been adopted; but it 
was found, on mapping still farther west, that their ‘Graptolitic 
Slates ’ embraced two different formations of importance, to which 
the names ‘ Blaen-y-ewm Beds’ and ‘'Ty’n-y-twmpath Beds’ have 
been assigned. 


Caradocian Series. 


(1 & 2) Teirw Beds and Cwm-Clwyd Ash.—In considering 
the Caradocian Series in the Northern Berwyns, it is unnecessary 
to redescribe the Teirw Beds in the Glyn area, so fully are they 
dealt with by Dr. Groom & Mr. Lake; but farther west appears a 
thick ash-band which, on the Old Series map, was correlated with 
the Pandy Ash. It lies, however, in the middle of the Teirw Beds 
and may be called the Swch-Gorge Ash, since it is best exposed 
in the gorge of the Ceiriog near Sweh-cae-rhiw. 

The following is the sequence here :— 


| Pen-y-graig Ash, 

Bryn Beds. < Fossiliferous mudstones. 
| Pandy Ash, seen at the top of the gorge. 
( Mudstones and sandstones, with two thin ashes or agglo- 
| merates. 


Teirw Beds.{ Swceh-Gorge Ash in two bands, the lower containing rolled 
| pebbles of hard shale. 


| 
| Sandstones and sandy shales, with a thin ash-band. 
Cwm-elwyd Ash in two bands. 

The age of the Teirw Beds is in doubt, because they appear to 
contain a fauna intermediate between Llandeilian and Caradocian, 
a fact hinted at by Dr. Groom & Mr. Lake (op. ect. 1908, pp. 589— 
90). ‘Io the forms enumerated by them (bed. p. 559) we can 

N 2 


180 


TABLE I, 


The Bala Formation. 


Glyn-Ceiriog 
District. 


(8: Glyn Grit. 


(b) Massive, fine-grained, 
uncleaved grit. 


(a) Thinly-bedded fossili- 
ferous grit or sand- 
stone, locally inter- 
bedded with platy 
limestones. 


Dolhir Beds. 


(6) Very fossiliferous, 
cleaved, §micaceous 
shales, with some cal- 
careous bands, 


Chasmops macroura, 
Harpes dorani,Sphe- 
rocoryphe thomsoni, 
Phacops brongniarti, 
Trinucleus seticor- 
nis. 


ASHGILLIAN. 


fe Dolhir Lime- 
stone. 


(a) Limestone containing 
corals, interbedded 
with shales. 


Absent, ? by faulting. 


Moel-Fferna 
District. 


Corwen Grit. 


Massive, fine-grained, 
uncleaved grit. 


Dolhir Beds. 


(6) Cleaved shales, 
usually less  fossili- 
ferous and more 


sandy than at Glyn. 


As near Glyn, but with 
Meristina crassa 
near the top. 


Ty’n-y-twmpath 
Beds. 


(a) Compact, grey, 
cleaved shale, some- 
‘times speckled. Few 
fossils, except locally. 


A gnostus agnosti- 
formis, Trinucleus 
seticornis, Phillip- 
sinella parabola, 
Phacops  truncato- 
caudatus. 


Blaen-y-cwm 


Beds. 
Dark-blue or black 
graptolitic mud- 


stones, poorly cleaved. 
? Zone of Diplograp- 
tus pristis. 


DR. WILLS AND MR. SMITH ON THE LOWER _[vol. xxvii, 


Cyrn-y-Brain and 
Mynydd-Cricor. 


Plas-uchaf Grit. 


Massive ashy grit, and 
on Cricor platy grits 
and shales. 


Cyrn-y-Brain 
Beds.! 


(6) Tough, ashy, cleav- 

ed greywecke shales 
and sandstones, and 
one thin fossiliferous 
ash-band, on _ soft, 
micaceous, very fos- 
siliferous sandstone. 


Calymene senaria, 
Phacops brongniartt, 
Chasmops macroura, 
Meristina crassa, 
Orthis hirnantensis, 
Clitambonites ascen- 
dens. 


1 Not to be confused with the Gwern-y-brain Group in the Welshpool district, 


described by Dr. A. Wade, Q. J. G.S. vol. Ixvii (1911) p. 422. 


part 2] PAL®OZOIC ROCKS OF THE LLANGOLLEN DISTRICT. 


CARADOCIAN. 


CS Ce 
bo 


181 


TABLE I (cont.). 


Pen-y-graig Ash. 


Thin keratophyric ash, 
only found at in- 
| tervals. 


4. BrynBeds. 


Dark-blue slates and 
argillaceous, siliceous, 
and ashy sandstones, 
thin ashes and con- 
elomerates. 

Sills of  lime-bos- 
tonite in places. 


Homalonotus bisulca- 
tus, Trinucleus con- 
centricus, Orthis 
elegantula _ (large), 
Phacops apiculatus. 


ee 


Pandy Ash. 


phyre. 
Teirw Beds. 


(6) Rather unfossiliferous 
slates and thin, often 
uncleaved sandstones, 
with some thin bands 
of ash. 


sandstones 
with Glyptograptus 
teretiusculus and 
Calymene (2?) plani- 
marginata, Asaphus 
powisi, Trinucleus 
concentricus. 


(a) Massive 


1 Cwm-clwyd Ash. 


Pen-y-graig AshP 


Not observed west of 
Tomen-y-bwlch. 


Dark-blue slates. 


Pandy Ash. 


Agglomeratic, coarse and fine-grained kerato- 
‘phyric tuff, with occasional blocks of kerato- 
Very variable in thickness. 


Teirw Beds. 


Practically unfossili- 
ferous shales and 
sandstones, with a 
thick double band of 
ash incorporating 
pebbles of shale, in 


the middle of the 
series (Swch - Gorge 
Ash). 


Cwm-clwyd Ash. 


Massive and well-bedded, almost platy kerato- 
\e phyric ash, coarse or fine in grain. 


182 DR. WILLS AND MR. SMITH ON THE LOWER  [vol. lxxviil, 


add Calymene planimarginata (?) Reed, Orthis (Platystrophia) 
biforata Schlotheim, Orthis (Dalmanella) cf. testudinaria Dalman, 
Cryptograptus ef. schdfert Lapworth, and Diplograptus ( Glypto- 
graptus) teretiusculus (Hisinger). 

The graptolites were found by Mr. W. B. R. King in the lower 
part of the series near Pandy, and in Miss Elles’s opinion suggest 
a Llandeilian age—possibly even the zone of Diplograptus tere- 
tiusculus. On the other hand, the Plectambonites sericea from 
slightly higher beds is pronounced by Prof. O. 'T. Jones to be 
similar to the Soudley-Sandstone form of this species. 

Until further evidence of the exact age is forthcoming, the 
Cwm-elwyd Ash at the bottom of the Teirw Beds, in that it occurs 
on the south as well as on the north of the Berwyn Anticline, 
forms the most convenient base to the Bala Series in this district. 


(3) The Pandy Ash.—The Pandy Ash, although variable in 
thickness, forms a useful horizon for mapping. It contains frag- 
ments of keratophyre in its coarser parts, and forms the highest 
ash at the Falls of the Ceiriog, where it was formerly regarded as 
the ‘ Little Ash’ of Jukes (that is, perhaps, the Pen-y-graig Ash). 


(4&5) Bryn Bedsand Pen-y-graig Ash.—As pointed out 
by Dr. Groom & Mr. Lake, the Bryn Beds vary considerably in 
thickness and composition from east to west. From place to place, 
the rocks are penetrated by sills of lime-bostonite, the best-known 
example being the Coed-y-glyn Sill near Glyn-Ceiriog. The base 
of the Bryn Beds overlies the Pandy Ash. On the west of the 
Glyn valley, at least as far west as Tomen-y-bwlch (14 miles west- 
south-west of Nantyr), and possibly farther, under the cover of 
peat, their summit is at several places proved to be defined by the 
‘Pen-y-graig Ash’ of Groom & Lake. This is a thin, usually 
sheared, keratophyric ash, which is, perhaps, impersistent between 
the known limits of its outcrops. Throughout the western region, 
the Bryn Beds are rather uniformly dark-blue sandy mudstones or 
sandstones, often yielding abundant fossils. 

Kast of the Glyn Valley the upper part of the group is, in the 
main, similar in lithology, but is underlain by a series of shales 
containing numerous bands of felspathie sandstone, which increases 
the total thickness of the Bryn Beds very considerably (see 
Table II, p. 189, & fig. 3, p. 185). The Pen-y-graig Ash is, in our 
opinion, absent in this region. In adopting this view we differ from 
Dr. Groom & Mr. Lake, for they regarded an ash which occurs 
above a massive sandstone at Bryn (in Nant-lorwerth, south-east 
of Glyn-Ceiriog) as the Pen-y-graig Ash. The ash at Bryn, 
however, does not form the top of the Bryn Beds here, nor does it 
closely resemble the Pen-y-graig Ash. In our opinion, the 
following sequence can be established for the Bryn Beds near Bryn 
(see fig. 2, p. 184). It is of interest, as providing evidence of local 
contemporaneous movements. 


part 2] PALHOZOIC ROCKS OF THE LLANGOLLEN DIstRicT. 183 


Sequence in the Bryn Beds near Bryn. 
Thickness in feet. 


Go. Blue sandy mudstones wath fossuls) 4 ..ccccsetccs sasoecneecce ses about 70 
5. Subangular conglomerate, or breccia, in parts becoming a 

SHOUSV OSH EL OMV CRE SI 1a Npemnes a, Cat atte! Gi eee ane nie cee ea Ler ne 25 
4, Blue sandy mudstones containing Orthis elegantula ......... 23 to 3 
3. Coarse felspathic ash, calcareous in part, passing in its 


centre into an agglomerate of compact shale-fragments. 
The top of the ash contains O. elegantula, and the bottom 


locally is an extremely fine-grained tuff........................ 2s 
2. Massive felspathic sandstones with abundant fossils locally, 
and some thin bands of shale. (Bryn Sandstone.) ......... 30 
F. Fault, downthrow to the north-east. 
1. Cleaved shales, with thin bands of felspathic sandstone ... 350 to 400 
Pandy Ash. 


The conglomerate (5) has not been observed 7” situ elsewhere ; 
but blocks of a rock similar to the agglomerate in 3, built into 
a wall on the roadside, 200 yards south of Pant (half a mile west 
of Bryn), were probably quarried near there. 

The lower conglomerate contains pebbles of keratophyric ash, 
similar to the Pandy Ash, and also pebbles of a curious micaceous 
and very siliceous rock, which forms part of the Bryn Beds them- 
selves. It crops out about 470 yards south-east of Pant and also 
at Ty-nant (on the Oswestry roaa, half a mile south-east of Bryn). 
It was here mistaken for an outcrop of the Coed-y-glyn Sill by 
Groom & Lake; microscopic examination, however, shows it to be 
a clastic rock, very fine-grained in part, but clearly resembling an 
ash in its coarser portions. 

The presence of these pebbles in the conglomerate at Bryn seems 
to prove that the rocks near here were sufficiently uplifted during 
the formation of the Bryn Beds for erosion to reach even the 
Pandy Ash at the base of the group. Thus it appears that the 
upward tendency of the Berwyn Dome was in evidence even at 
that early date. 


(6) Blaen-y-ewm Beds.—These are black graptolitic mud- 
stones, in which the fossils are poorly preserved. They take their 
name from a farm near Nantyr, where they are well exposed. 

From their easternmost exposure near Gelli (13 miles south- 
west of Glyn-Ceiriog) they can be traced as far west as the 
mapping has been undertaken. In this region they overlie the 
Pen-y-graig Ash wherever it has been proved. The beds disappear 
east of Gelli, the Dolhir Beds resting directly upon the Bryn Beds 
(ics, p. 185). 

The zonal position of the Blaen-y-ewm Mudstones is still a 
matter of doubt, on account of the unreliable evidence afforded by 
the badly-distorted graptolites. Miss G. L. Elles, to whom the 
fossils were submitted, thought that the following forms were 
represented :—Dicellograptus sp., Dicranograptus tardiusculus 
Elles & Wood, Diplograptus (Orthograptus) calcaratus Lapworth 
var. acutus Elles & Wood, D. (O.) calcaratus, var. vulgatus 


Fig. 2.—Plan and section (A—B) of the Bryn locality. 


Z é 
Z, 
“O° 


| 


6 = Sandy fossiliferous shales 3 = Ash passing locally into agglo- 


(Bryn type). merate. 
5 = Angular conglomerate. 2= Bryn Sandstone, massive and 
4 = Sandy fossiliferous shale. fossiliferous. 


1 = Beds below the Bryn sandstone, 
consisting of shales and thin 
ashy sandstones. 


[ 185 }. 


‘Speq Miley, = T ‘spegq Iyjog = 2 


‘ysy Apurg =z ‘Ysy sivis-f-ueg = P ‘904s 
‘uAIg ‘speq wiM0-A-uovlq = c -OUILT PUv JIIH UATH poureis-oul = g 
ye yse oyy ‘VY YIM ‘speq udrg = ¢ ‘spog yyeduamy-A-u AT, = 9 ‘qIIH) UOMAIOD-UAT OAISSVP, = 6 


SSS 
490,77 008 009 00F 002 0 ~=- OTL “f 0 


OVO [VO4I0 A. OBO JVyIWOZ IOP] 


, ® = ~ © : 
a i) 7 wd a) uz Om wn tory VA 
5 == o op » @ 
@) id 2 QA BB — eo TU ° a=) 
» 3 ge tn =e aS ns a Q 3 
? mS eb eae s ae 
: ean oa 62 9 Q, 
Z Rs a) ®.0 e. 5) 
5 a” a! Uy fon 1) a 
5 cy ° 
is 5 
F} 


‘SUhNLAG UdaYJLONT “WOIQDWLOT VIVE ayy wu SSAUYIIY] UL U0IZVIWD 4Y¥ —'@ “BLT 


186 DR. WILLS AND MR. SMITH ON THE LOWER | vol. Ixxviii, 


Elles & Wood, D. (Mesograptus) multidens Elles & Wood, 
D. (Amplexograptus) perexcavatus (?) Lapworth, D. ( Glypto- 
graptus) teretiusculus (Hisinger), D. (G.) teretiusculus vay. 
euglyphus Lapworth. 

Dr. Groom & Mr. Lake further record Dicellograptus elegans 
Carruthers. This latter form suggests a horizon about the zone of 
Pleurograptus linearis, which would agree with the position of the 
rocks above the Bryn Beds that show a typical Caradocian fauna. 
The other identifications suggest a much lower horizon, possibly 
even in the Llandeilo, which goes counter to all the stratigraphical 
relations that can be shown in the case of the Blaen-y-cwm Beds. 
In view of this discrepancy, it is interesting to note that, at 
Pen-y-garnedd on the south side of the Berwyns, one of us, 
together with Mr. W. B. Rt. King, found well-preserved graptolites 
in black shales, occupying the same stratigraphical position.! The 
fossil assemblage occurring there has been claimed by Miss Elles 
as proving the presence of the Déiplograptus-pristis Zone of 
Sweden. hitherto unrecognized in this country. That zone js ap- 
proximately equivalent to the zone of Pleurograptus linearis. 
Therefore, until really well-preserved forms are forthcoming from 
the Blaen-y-cwm Beds, we are inclined to attach more importance 
to the evidence, stratigraphical and paleontological, which points to 
the Pleurograptus-linearis Zone, than to identifications, based on 
admittedly inadequate material, suggestive of a far lower horizon. 


Ashgillian Series south of the Llangollen Synclinorium. 


(7) Dolhir Beds.—The general lithological characters of the 
Dolhir Beds have already been indicated in Table I, p. 180, and the 
variation in thickness is shown in Table II, p. 189, and fig. 3, p. 185. 


(a) Lower or Ty’n-y-twmpath Beds.2—A group of grey 
slates, often with difficulty separable from the normal micaceous 
Dolhir Slates, comes in above the Blaen-y-ewm Beds, near Gelli, 
and thickens westwards, where it is frequently characterized by a 
speckly blotching of the rocks. It is usually far less fossiliferous 
than the Dolhir Beds proper, although locally it contains a rich 
fauna of trilobites. 

The upper and lower limits of this group merge into the Dolhir 
and Blaen-y-cwm Beds respectively, by insensible gradations, and 
it has not been possible to draw satisfactory boundaries to the 
formation on the map. It is also uncertain whether the Ty’n-y- 
twmpath Beds are the equivalent in the west of the Dolhir Lime- 
stone, which, with its associated shales, forms the base of the 
Dolhir Beds near Glyn-Ceiriog. 


1 «Summary of Progress for 1919’ Geol. Surv. 1920, pp. 4, 5. 

2 This group takes its name from a large farm, lying immediately west of 
the small homestead indicated as Bonc, about three-quarters of a mile east 
of Nantyr. Nant-Ty’n-twmpath, referred to in the sequel, is the valley that 
descends from the north to the farm. 


part 2] PALHOZOIC ROCKS OF THE LLANGOLLEN District. 187 


The distinctive Ashgillian fauna of this lowest member of the 
Dolhir Beds is set forth below :— 


c=common. i, P40 3}, 4. 5, 


—|——_— 


ee ———__—_ 


Mawosites feorosus GOlGIUSS: <4 (occacecses-2+<:) . X. | x 
Cf. Monticulipora lens M‘Coy ......... 22.0164 
Orbiculoidea perrugata M‘Coy Bee Meee 
Leptena rhomboidalis Wilckeus...............)  X ~ ah 
Orthis flabellulum J. de C. Sowerby ......... rie a2 c 
Orthis calligramma Dalman ....... Ret Cae xX 
Orthis calligramma, var. plicata J. de C. 
Sowerby ....... ain Bo c x 
Orthis (Dalmanella) elegantula Dalman . oa A x 
Orthis (Dalmanella) testudinaria Dalman.| ... Fi see 
Orthis (Heber ee vespertilio J. de C. 
Sowerby . re ; fa ahs ai ie x 
Plectambonites scissa Salter ............... |... ne Es, x x 
Plectambonites sp. ... ee Roe Se eS | Oe me | 
? Triplecia insularis Bichwald, oxeh Gh Wie x 
Agnostus agnostiformis McCoy ............... Ak Jot OX 4 a x 
Agnostus ct. agnostiformis aes sates [rom i: ve x 
Calymene senaria Conrad (Salter) .. Seatac x x 
Calymene cf. caractaci Salter . 
Chasmops sp. 
Cybele cf. verrucosa a Dalman eee 
<p PIPOTLOT RISA Oia PAA yagi te Naa a gn a eS oo Et x 
EES ENYIISD SUCRE SAN ots ENA ge Se a (Te to x 
Tuchas sp. .... eaten | cue ‘A Cc 
Phacops truncato-caudatus Portlock......... aie eG eater fe XE 
Phillipsinella parabola Barrande ............|«.. oy a xa c 
Spherocoryphe thomsoni (?) Reed ............) a c 
pinucleus cf. nicholsonit Reed ...............| Ae ae site x 
Trinucleus seticornis Hisinger ...............] X 
MEIN CULM SSDs ye Gila s Baca. acinus itu chose tek a8 ie pt xe 


KH 


A KAKA RK 


A 


A 


(1) Ty’n-y-celyn Farm, near Nantyr. 

(2) Ty’n-y-twmpath stream, 150 yards north of the farm. 

(3) Lane at Bone. 

(4) Old quarry, by the entrance-lodge to Plas Nantyr. 

(5) Headwaters of Nant-y-Lladron, 3 miles south by east of Corwen. 


(6) Upper or Dolhir (sensu stricto) Beds.—The remainder 
of the Dolhir Beds includes the rocks so designated by Dr. Groom 
& Mr. Lake near Glyn, and needs no further description so far as 
the eastern part of their outcrop is concerned. Westwards, how- 
ever, the group, as mapped by us, thickens (fig. 3, p. 185), and 
becomes more sandy, especially in its upper part, which embraces 
shaly sandstones that are almost certainly the equivalent of the 
lower part of the Glyn Grit of Glyn-Ceiriog. These beds are not 
separable in the field from the main part of the group, but their 
arenaceous nature provides a type of litholog y intermediate 
between the Dolhir Slates of Glyn and the gr eywacke slates and 
sandstones of the Cyrn-y-Brain Beds. It is accordingly interesting 
to find Meristina crassa occurring in them as a rarity, for this 
fossil may be said to be one of the most abundant and charac- 


teristic forms in the Cyrn-y-Brain Beds. 
The Ashgillian age of the Dolhir Beds was established by 


188 DR. WILLS AND MR. SMITH ON THE LOWER | vol. lxxviii, 


Groom & Lake. In addition to the forms recorded by them, we 


may note Harpes dorani: Portlock and Spherocoryphe thom- 
soni (7) Reed. 


(8) Glyn-Corwen Grit.—The Glyn Grit at Glyn is a platy 
sandstone passing down into a limestone, the two rocks being 
interbedded at their junction. The peculiarities and fauna of 
these rocks were described by Groom & Lake at some length. 
When traced westwards an upper massive grit is found to come in 
about a mile west of Glyn, and it is this upper grit which has 
been proved by us to be the continuation of the Corwen Grit. The 
disappearance of the platy grit westwards has just been noted. It 
is interesting, however, to find near the Ty’n-y-twmpath stream 
that the upper massive grit is sueceeded upwards by thin beds of 
grit interbedded with the Lower Valentian Slates, in the same 
manner that farther east the platy grit passes upwards into the 
slates, as described by Groom & Lake.” Near Corwen shaly bands 
in the grit resemble lithologically the Dolhir Beds. 

Thus it appears that we must treat the Corwen and Glyn Grits 
as parts of a single formation, conformable with the rocks above 
and below; but, if we judge from the fossil as well as from the 
lithological evidence, the formation is more closely related to the 
Ordovician than to the Silurian. Further reference to this is 
made on pp. 190 & 191 in describing their equivalent farther north. 


Relation of the Ashgillian to the Caradecian. 
(See Table IT., p. 189, & fig. 3, p. 185.) 


The relation of the Ashgillian to the Caradocian in the North 
Berwyns may now be reviewed in the light of the facts set out in 
the preceding pages. 

The hiatus shown by Dr. Groom & Mr. Lake to exist between 
the Dolhir Beds proper and the Caradocian Bryn Beds in the 
Glyn-Ceiriog region appears to be filled farther west by the 
Blaen-y-cym and Tyn-y-twmpath Beds, the former yielding 
graptolites probably indicative of an Upper Caradocian age, and 
the latter containing a typical assemblage of Ashgillian trilobites. 

The complete sequence appears to be developed at most places 
west of Gelli; but, east of that locality, the rapid diminution in 
thickness of the Dolhir Beds proper, that come to outcrop, and the 
absence of the Ty’n-y-twmpath * and the Blaen-y-cwm Beds, are 
best explained by faulting along a gently-dipping plane, a view 
put forward by Groom & Lake (see fig. 83, p. 185). This fault was 
described in detail as the Dolhir Fault, and was shown on 
their map as separating the Bryn and Dolhir Beds nearly as far 
west as Ty’n-y-twmpath. Our discovery of the Pen-y-Graig Ash 
west of Gelli and our present knowledge of the Blaen-y-cwm and 


1 Q.J.G.S. vol. lxiv (1908) pp. 572-73. . 2 Ibid. p. 576. 
3 The available paleontological evidence is insufficient to prove or disprove 
the identity in age of the Dolhir Limestone and the Ty’n-y-twmpath Beds. 


part 2] PALHOZOIC ROCKS OF THE LLANGOLLEN DISTRICT. 


189 


TasLE II].—APprprox1iMATE THICKNESSES IN FEET OF THE FORMATIONS FROM 
THE Bryn BEDS TO THE CORWEN GRIT, CALCULATED FROM THEIR OUTCROPS. 


=e a Ee: 
zB a acen o 
eo es = = 
on an aes 
[es 2,8 As gu noes ee 
S rc oR Z = 7, et oO 2 oo 
BEA | O% ia Se | ae aE 
Se ee ae Seu Bara a 
2 Ss se — §$ placa Ss > 5 = > 
oD ~— Pra lan (S) — nll os} el 
ay A H oS as) ao) oH a) 
( Upper or | 
Glyn massive grit 20 20? 20 10 10—0 a a a 
c Bee "2 Lower or | } |) 
? | platy grit 
and Glyn | 
| Limestone. | U | U 50 50+ 100—| 20 20 20 
Dolhir Beds (sensu | $1860 | +1500 | 1020 |) > 
stricto). > 1050 ( 1000 970 840 | 620 
Ty’n-y-twmpath Beds 300 180 150 We aie 20) U U U 
Blaen-y-cwm Beds ...... 120 90 100 50 a a a a 
Pen-y-graig Ash .. ...... a 10 20 4 10 a a? ang 
Bryn Beds 120° |, 160 210 210 360 400 500 | 560 
{[U=unrecognizable ; a=absent. See also diagrammatic section, fig. 3, p. 185. | 


Ty’n-y-twmpath Beds preclude us from accepting this wide range 
of the fault westwards, unless it passes to a lower horizon and cuts 
out the lower part of the Bryn Beds in this region, instead of the 
top, as suggested by Groom & Lake. The rapid westward thinning 
of the Bryn Beds might thus be accounted for; but there is no 
definite evidence for such a fault. 

From Gelli eastwards, however, we have drawn a fault similar 
to that adopted by Groom & Lake, separating the Bryn Beds 
from the Dolhir Beds proper, and cutting out the Blaen-y-cwm 
and Ty’n-y-twmpath Beds; but in our view the Pen-y-graig Ash 
is probably absent by non-deposition east of the Glyn Valley, and 
most of the increase in thickness of the Bryn Beds is due to 
lithological changes affecting especially their lower part. In order 
to produce the arrangement of the outcrops observed, the fault 
must be inclined at a lower angle than the beds, and is probably 
a thrust. 

The only other feasible explanation of the observed facts would 
involve an unconformity with overlap, or at least a marked non- 
sequence between the Ashgillian and the Caradocian beds east of 
Gelli. There is nothing, however, in the nature of the sediments 
composing the Ashgillian to suggest an unconformity. The gap 
in the sequence, also, is too extensive to be readily explained by a 


non-sequence. 
® 


190 DR. WILLS AND MR. SMITH ON THE LOWER [ vol. lxxviui, 


Ashgillian Series north of the Llangollen Synclinorium, 


Cyrn-y-Brain Beds and Plas-uchaf Grit.—The Ordo- 
vician inliers of Cyrn-y-Brain and Mynydd-Cricor are formed of 
greywacke-slates, sandstones, and grits, which can be correlated 
by their abundant fauna with the Dolhir Beds and the Glyn- 
Corwen Grit. Both inliers are anticlinal in structure, but the 
lowest beds exposed occur on Cyrn-y-Brain. ‘Those have been 
detected north of Plas-uchaf, at one place only, and consist of soft 
micaceous sandstones, slightly cleaved and full of fossils. Above 
them follows a great thickness (probably 1500 feet at least) of 
cleaved gritty greywacke-slates, which, especially towards the top, 
become sandstones or grits. The Plas-uchaf Grit, which forms 
the summit of the series, is a massive uncleaved rock, from 10 to 
20 feet thick on Cyrn-y-Brain, and quite comparable with the 
Corwen Grit. On Cyrn-y-Brain its highest part is composed of a 
tough platy sandstone with contorted lamination, about 3 feet 
thick, passing upwards with apparent conformity into a dark 
mudstone that weathers into a brown rottenstone. This may be 
taken as the base of the Valentian. On Mynydd-Cricor the thick- 
ness of the Plas-uchaf Grit varies greatly, and it appears to split 
and include bands of shale closely similar to the underlying cleaved 
greywacke-slates. 

Throughout the Cyrn-y-Brain Beds, brachiopods are locally 
plentiful. Of these, Weristina crassa is the most characteristic 
and abundant form, occurring from top to bottom of the series. 
It was on account of the abundance of this fossil that the late 
Prof. T. McKenny Hughes! assigned the Plas-uchaf Grit (and pre- 
sumably the underlying arenaceous beds) to the Llandovery Series. 
Our collections, however, prove that this brachiopod occurs here 
in association with undoubtedly Ordovician trilobites, such as 
Trinucleus and Chasmops macroura, the latter being of more 
frequent occurrence than Trinucleus. 

The list of fossils given below (p. 191) shows that the Cyrn-y- 
Brain Beds, together with the Plas-uchaf Grit, are the equivalent 
of the Dolhir Beds proper and the Glyn-Corwen Grit. 


Prof. O. T. Jones has pointed out to us that the types of 
Plectambonites sericea and of Strophomena antiguata found in 
the Cyrn-y-Brain Beds resemble those occurring in the Slade Beds 
of South Wales. The discovery of these forms, in shales inter- 
bedded with the Plas-uchaf Grit on Mynydd-Cricor, is useful 
evidence in favour of the Ordovician rather than of the Silurian 
age of that grit. 

Orthis vio pipes and Strophomena siluriana are most 
commonly found in, if not confined to, the upper part of the Cyrn- 
y-Brain Beds. Picea forms, in conjunction with Orthis sagittifera 
and O. (Platystrophia) biforata, var. fissicostata, point to a close 
comparison with the Hirnant Limestone and its associated rocks 


1 Q. J. G.S. vol. xxxiii (1877) p. 207. 


part 2] PALMHOZOIC ROCKS OF THE LLANGOLLEN DISTRICT. 


191 


which constitute the buffer formation between Ordovician and 
Silurian in the Bala country. In fact, Miss G. L. Elles! has 
expressed the opinion that the Hirnant Limestone is equivalent to 
the Conway-Castle Grits, the upper part of which she would regard 


as Llandovery in age. 


List oF FOSSILS FROM THE CYRN-Y-BRAIN BEDS. 


[Fossils found in the Dolhir Beds are marked D, those in the Glyn Grit G. 
C indicates fossils found on Cyrn-y-Brain, and M on Mynydd-Cricor. | 


D G Favosites (Monticulipora) fibro- 
sus Goldfuss. Common. C. 

D  Heliolites interstinctus Linneus. 
C. 

D G‘ Petraia’ elongata Phillips. C. 

D_ Petraia subduplicata M‘Coy. 
Common. C. 

D_ Petraia subduplicata, var. cre- 
nulata M‘Coy. C. 

DG Crinoid-remains. C. 


D_~ Cf. Caryocystites leitchi Forbes. 
C. 

D Echinospherites cf.  baltica 
Hichwald. C. ; 

D Phyllopora hisingerti M‘Coy. 
Common. C. 

Pinacopora grayi Nicholson & 

Etheridge. C. 

D G Ptilodictya acuta Hall. C. 


Atrypa marginalis Dalman. M. 
DG Clitambonites ascendens Pan- 
der. C. 
?Dinobolus sp. M. 
D G Leptzna rhomboidalis Wilckens. 
Common. OC,M. 
D = Inngula sp. C,M. 
Meristina crassa J. de C. Sow- 
erby. Very common. OC, M. 
D G Orthis actoniz J.de C. Sowerby. 
Cc: 
DG Orthis (Platystrophia) biforata 
Schlotheim. C, M. 
Orthis (P.) biforata, var. fissi- 
costata M‘Coy. M. 
Orthis (P.) spiriferoides M‘Coy. 
CP? M. 
DG O. calligramma (2?) Dalman. M. 
DG Orthis crispa M‘Coy. C. 
DG Orthis (Dalmanella)elegantula(?2) 


Dalman. ; 
D_ Oz. (D.) cf. testudinaria Dalman. 
D Orthis lirnantensis M‘Coy. 
Common. C. 


Orthis (Rhipidomella) ef. mul- 
lockiensis Davidson. C. 


DG Cf. Orthis (Plesiomys) porcata 
(M‘Coy). C. 

Orthis (P.) porcata, var. sladensis 
(Reed). C. 

Orthis (Heterorthis 2) sagvttifera 
(M‘Coy). C. 

Parastrophia divergens Hall & 
Clarke. C. 

Plectambonites papillosa Reed. 
M. 

Plectambonites cf. papillosa. C. 

DG Plectambonites sericea J. de C. 
Sowerby. C,M. 

Plectambonites sericea, var. 
rhombica (2?) M‘Coy. C,M. 

Rafinesquina expansa (?) J. de 
C. Sowerby. C. 

‘Rhynchonella’ sp. C. 

Strophomena antiquata J. de 
C. Sowerby. C. 

Strophomena siluriana David- 
son. Common. C. 

Triplecia insularis EHichwald. 
C 


? Zygospira sp. C. 
Murchisona ef. turrita 
locke€; 
Murchisonia sp. C. 
Tentaculites sp. C. 
Acidaspis sp. C. 
Calymene senaria Conrad (Sal- 
ter). Fairly common. C. 
Calymene cf. senaria. C. 
Chasmops macroura ‘Sjogren. 
Fairly common. C. 
Encrinurus multisegmentatus. 
Portlock. C. 
Encrinurus sp. C. 
Illenus bowmanni Salter. 
Illenus ef. bowmanni. C. 
Phacops brongniarti Portlock. 
C 


Port- 


C. 
aD) 


Phacops sp. C. 


Trinucleus sp. Rare. C. 


1 Q.J.G.S8. vol. lxv (1909) pp. 183-84. 


192 —s DR. WILLS AND MR. SMITH ON THE LOWER  [Vol. lxxviii, 


(C) Silurian: (a) The Valentian Formation. 


Following with apparent conformity on the grits that constitute 
the summit of the Ordovician, is a series of cleaved, usually 
unfossiliferous, pale grey-green mudstones and shales! comprising 
the Llandovery and Tarannon ‘pale slates’ of earlier writers. 
These can be conveniently grouped as the Valentian Formation, 
and may be subdivided as follows :— 


Upper Valentian or Tarannon (of Tarannon). 
Lower Valentian or Birkhillian, 


Dr. Groom & Mr. Lake studied these rocks first at Corwen,” and 
later at Glyn-Ceiriog,? where they gave the local names of ‘Ty- 
draw Slates’ to the Tarannon and ‘ Fron-Frys Slates’ to the 
Lower Valentian (their ‘ Llandovery’). We do not propose to adopt 
these names, because the sequence at Glyn-Ceiriog is very attenu- 
ated, and often shows signs of having been severely dislocated. It 
is probable that part of the Valentian is missing here, as a result 
of strike-faulting. 

The fullest development of the Valentian in the region under 
description is brought to the surface by the anticlines of Cyrn-y- 
Brain and Mynydd-Cricor. This is summarized in Table III, 
opposite. 

In the Cyrn-y-Brain area, the rocks are well exposed, but highly 
cleaved and much disturbed, and near Plas-uchaf, the eastern part 
of the World’s-End Fault cuts out much of the lower series, and 
in its western part much of the Tarannon. 

The paleontological evidence is very scattered. The chief data 
may now be given. 


Lower Valentian. 


The gritty grey-green mudstones with brachiopods are best seen 
in an old opening 130 yards north of Tai-newyddion, on the Ruthin 
road. Here were found :— 


Atrypa reticularis Linneus. Orthis (Hebertella) sp. 
ef. Meristina furcata J. de C. Plectambonites cf. dwplicata J. de C. 
Sowerby. Sowerby. 
Orthis (Bilobites) biloba Linneeus. Plectambonites cf. transversalis 
Orthis calligramma Dalman. Wahl. 
Orthis (Dalmanella) elegantula (2?) Plectambonites sp. nov. 
Dalman. Plectambonites sp. 
Orthis (Hebertella) protensa J. de Trilobite. 
C. Sowerby. 


This assemblage appears to represent a mixture of Lower and 
Upper Llandovery faunas; and, as elsewhere in the Llangollen 
district, is not very helpful for purposes of correlation. The rocks 


1 Where the rocks are referred to as shales or mudstones, the cleaved 
equivalent is implied. 
2 Q.J.G.S. vol. xlix (1893) p. 426. 3 Ibid. vol. lxiv (1908) pp. 553-54. 


"(qLug yeyou 
“S¥[q) JIS oatsseut pue A4vTg 


"9U10JSU9}4O1 OF SUL 


q -1ay}BoM ouoyspnut Apuss yorTq | 


‘spodorgoviq Moy B surppatd | 
asuoJspnut Ystudeis  yovduioy, 

[ ‘souv[d-aSvavayo 

ay} Wo soqtkd Jo syvais [}IM 
20084 |‘Soq¥{s pus sam0zspnut w9eI3-Aa.y 

| ‘spodormoraiq awios Suu, 
\_|-uoo seuojspnut ugets-fers £441.09 

(a ‘spodoryoviq maf v Surpyars 

pUv SNOddROIUL Mado ‘SaqBIS UALS 
209 AJ4LLS PUB SE4RIS pamno[oo-NOOIe TT 

"sojT[oydB.asg 

| {poarasoad - Sq100d yyim ‘ayers 
({AorS-y1ep pue auojspnut youlg 

‘aouanbeg-uoN 


— 
—_—_—_— | —— ——— 


OL 


RE Ue BOTHEe ILO AN, 
ay} Aq gno yno st saouenbas 
ay} JO YonyL ‘aoseyvoo Avut 
spueq patnojoo-uoorew oy yng 


‘QOLIstp udsvj-A-TOo| oy UL SY 


Aye 
-ULSLIO 


d069 


— ——__—_ —__. 


“SouoqjSpnut popuBd 


| 


‘VOUISIG FRYON-selg 
ee 


_———— |S 


—_— + 


‘ulvig-f-ulhsg 


"(ql JeYON-selq) JIG aAIsseyy 


“SOTLOJOTG OFITM ITM poryoods 94 j0 
OOP |jied aeMoy, ayy ‘souvd-acravelo 
0} ja} uo soqi{d jo syvaigs Surmoys 
ogg \javd 19ddn oy} ‘saqvis pus seu04s 

-pnut wudaid - Aad snoJezI[Issofu 


‘spodoryorrg 
‘sivas Apuvs Y}IM souojspnut 
weets-Laid A44IIs uo ‘au0z snwuvh 
-a46 4 “(UIT4) O}BTS OTGTTOIMV.LS Yare(] 


O€& 


*(qUesqe SOTUTJATUOS J) sNgNj,0AUN0d 

FO ‘euojspnut o1ipoqyders yovrg 
‘aouanbag-uoN 

00g ‘SoUOJSPNUL Usdts-AVIS TOV 

"snaSip "FT “Sa4v[s 1410 

“1198 OJUL padtaAuod puw ATAIOAVS 

paavalo atv souojspnul oyy Atle 

-UOISBIDQ) *UOWWOD seqno-seqIs gy 

‘SOUOJSpNUl pedi1ys JO spuvq [vuots 
“8000 YIM ‘seuOjspnul Teals-LaL 

‘souOJSpnU WaeIs 

-Kais Aq poyeredes spueq omy ut 
AT[ensn ‘souojspnutt pa.nopoo-uo001e pL 

OY SOMITE 

pus wayojs ‘gq ‘suvbaja sdoovyg 
"SYVIIJS UMOIG [JIM sauoyspnut fa. 

“suaprsnbup 

saqyouay pure sngnynuato “I1eA 

SNULAAWORA PT ‘*somoJSpnut painoy 
-00-9f0M puB saqvys ontq AraquItdg 


————— 


006 


09 


O€ 


O€ 


EE § 


‘JOLIZSTp wavy-A-[90 PL 
es, 


“Ssouojspnul pepueg | 


O 


“JlIs pue 
solo jspnut-ayovm fa. “UBIT[ISYSYV | 
| 
“euruvy Apues ‘sngpuunoDp | 
snidpvihoppyday ¢ 
YJIM saqvys usats-La1s 
“sN4sapout mee 
papyqgour uo snydho yr snydvrbosa yy § | eG Ue Mee eee 
| 10 
‘QUOJSpNuUt Yovyq uUttpy, Peace ate 
snydho “Tr | oe ae A 
“quasqy ‘smitpbaub yr | 
“quasq Wy "sngnjoauod "Fr _) 
‘vouanbeg-uoN 
"SNYD)NIVAANA ze 
‘snosip “TT w sndsiwo "yy | 
‘souojspnur pedis 
JO Spuvq [BUOIsBd00 YI WOUURIBY, 
‘souojspnul weeds - Aatx *sndswo "Ti jo 


‘sisuauogsarwb “Tyr 
“souojspnut 


pedis pus ued1s - Aauy |‘sesuawogsarwb “7 


"‘suaprysnbup saqv) 
-0199 ‘SnaDINUALI “IBA 
1 1 


| 
| 
: 


| 
| 


SNULLAUWOR "FT *Sozeys pu ‘sngvnuasa 
souoj}spnu pepurq 10 on{g snydnihouopy J 
“soJe[S ON[G pepusg | ‘2wosryounw “pO 


"LOdLIQ-ppAudk 


[vuo7 


“MOTFBOTFISSBL 


UOUUIE J, 
10 


UBIYUALBA 
1add q 


"YOO[UIM 


‘NOIMONITONAG NETIOONVIT HHL JO ACIS HLYON ‘+ GONGOOES NVILNATYA BHI—I]II PIAV 


OF 3G Ss No ek: 


194 DR. WILLS AND MR. SMITH ON THE LOWER _ [ vol. lxxviil, 


which yield this fauna le not far below the top of the Lower 
Valentian. 

It is, however, uncertain whether the upper limit of the Lower 
Valentian always occurs at the same horizon in these northern 
outcrops. The summit near Hafod-yr-Abad, for example, is formed 
by shales which 120 yards east of the house yielded the following 
graptolites, indicating the zone of Monograptus convolutus :— 


Climacograptus sealaris Hisinger. Monograptus argutus Lapworth. 

C. tornquistt Elles & Wood. M. convolutus Hisinger. 

Diplograptus (Orthograptus) bellulus M. decipiens Tornquist. 
Tornquist. M. leptotheca Lapworth. 


Diplograptus (Mesograptus) magnus | M. lobiferws M‘Coy. 
H. Lapworth. M. regularis Tornquist. 
Diplograptus (Glyptograptus) tama- M. sedgwicki ? Portlock. 
riscus Nicholson. M. undulatus Elles & Wood. 


These are well preserved. The outcrop of the soft shale in 
which they occur forms a depression which has been taken as a 
line of division between the Upper and the Lower Series. 

On the other hand, at the summit of the Lower Series on the 
moor road, 400 yards north-east of Bryn-yr-odyn, graptolites in a 
bad state of preservation were found, which suggest the zone of 
I. gregarius. But the reference of the rocks to that zone is not 
absolutely reliable. The list is as follows :— 


Climacograptus rectangularis (2) | Monograptus atavus Jones. 

M‘Coy. | M. gregarius Lapworth. 
C. tornquisti Elles & Wood. | M. revolutus Kurck., var. austerus 
Diplograptus (Mesograptus) magnus | Tornquist. 

H. Lapworth. | M. triangulatus Harkness. 


One other feature of the Lower Valentian on Cyrn-y-Brain must 
be noted: namely, the absence (except at one place) of the speckled 
variety of mudstone so characteristic of that formation in the 
other districts. 


Upper Valentian. 


In the Tarannon Series on Cyrn-y-Brain, the lowest beds are 
unfossiliferous, except near Llandegla, where at a point 600 yards 
west of Hafod-Bilston, Jlonogr aptus becki Barrande, JL. marri 
Perner, and JL. runcinatus Lapworth, suggest a horizon about the 
zone of DL. turriculatus. The presence of the Jf.-crispus Zone is 
well established by the following forms from the roadside 20 yards 
south-west of the barn at Ty-uchaf, a quarter of a mile south of 
Bryn-yr-odyn :-— 


Monograptus discus Tornquist, Monograptus nudus Lapworth. 
common. M. pandus Lapworth. 

M. griestonensis Nicol. M. planus Barrande. 

M. marri Perner. M. spiralis Geinitz. 


part 2] PALMOZOIC ROCKS OF THE LLANGOLLEN DisTRIctT. 195 


And the following from the farmyard at Hafod-yr-Abad :— 


Monograptus runcinatus Lapworth. Monograptus pandus Lapworth. 
M. nudus Lapworth. M. nodifer Tornquist. 
M., exiguwus Nicholson. M. priodon Bronn. 


M. turriculatus Barrande. 


This assemblage indicated a low position in the JL.-crispus 
Zone. 

The deposits overlying the J.-crispus Zone may represent the 
griestonensis beds, and are interesting on account of their occa- 
sional trilobite and brachiopod remains. In the steep lane heading 
from the old Ruthin road to Pentre-uchaf, half-a-mile south of 
Bryn-yr-Odyn, the following forms were collected :— 


Favosites sp. Phacops stokest Milne-Edwards. 
Orthis (Bilobites) biloba Linneus. Phacops sp. 
Phacops cf. elegans Sars & Boeck. 


The .-crenulatus Zone is characterized by its peculiar lithology 
(dark slates and mudstones ), which easily distinguishes it from the 
pale slates below, and the banded and laminated mudstones and 
slates of the overlying Wenlock. It occupies, as a result of gentle 
folding, a wide strip of country near the Moel-y-faen quarries. 

The following forms were found at a point 3800 yards north of 
Fron-adda on the side of the old Ruthin road :— 


Monograptus griestonensis Nicol. Retiolites (Gladiograptus) geimitz- 
M. vomerinus Nicholson, var. anus Barrande var. angustidens 
crenulatus Tornquist. Elles & Wood ; 


while, on the old tram-line 340 yards north-west of Fron-adda, an 
abundance of MM. vomerinus var. crenulatus is found. This out- 
crop appears to be part of a faulted inlier. 


The Mynydd-Cricor Area. 


In this district the Valentian, as a rule, dips outwards from 
Mynydd-Cricor in conformity with the anticlinal structure, but it 
is much affected by faults. Near Ffynnon-Tudur there is also 
considerable thrusting, and the general dip is reversed in the 
Tarannon Beds that override the Lower Valentian Series, so that 
Wenlock Beds he against the Ordovician on the west side of Cricor. 


Lower Valentian.—The Lower Valentian rocks of Mynydd- 
Cricor exhibit the typical speckled and pale grey-green aspect, and 
are rarely fossiliferous. The following forms were collected from 
Bryn-Rhedyn, 1 mile east by south of Cricor Farm :—Plectam- 
bonites scissa Salter and ? Strophomena ct. fletcheri Davidson. 

The summit of the Lower Series appears to be in the zone of 
M. cyphus, for the following forms occur, within a few feet of 
typical 'Tarannon mudstone in the ravine, half a mile south-east 

O2 


196 DR. WILLS AND MR. SMITH ON THE LOWER [ vol. Ixxviil, 


of Cricor Farm, and we can find no evidence of the junction being 
faulted :— 


Climacograpius rectangularis | Monograptus acinaces (?) Tornquist. 
M‘Coy. | Monograpius atavus (2) Jones. 
Dipl ograptus ( Glypiograpius) ta- | Monograpius cyphus Lapworth. 


mariscus Nicholson. 


Upper Valentian.—In the Upper, or Tarannon, Series, no 
fossils indicative of the turriculatus zone have come to light, but 
the rocks that may be referable to it are very unfossiliferous. 
The exposure yielding the above-mentioned A/_-cyphus Zone fossils 
is separated from an outcrop of the I/.-crispus Beds by the north- 
and-south Tan-y-graig Fault. Near the little reservoir here seven 
fossiliferous localities were discovered. in a thickness of some 
300 feet, in dark striped bands intercalated in the usual grey-green 
mudstone. The upper part may represent a portion of the zone of 
MM. griestonensis, but the following forms occurrmg near the 
reservoir indicate the Jf-crispus Zone :— 


Monograpius crispus Lapworth. Monograpius pandus Lapworth. 
M. discus Tornquist. M. priodon Bronn. 

M. griestonensis Nicol. M. runcinatus Lapworth. 

M. marri Perner. M. spiralis Geiniiz. 
M, nodijer Tornquist. 


The presence of the griestonensis beds, followed by the darker 
and often striped crenulatus passage-beds up into the Wenlock 
Series, has been established at several places im this area. Near 
Nant-uchaf, three-quarters of a mile south of Cricor Farm, we find 
Monograptus ci. griestonensits Nicol, M. marri Perner, and I. 
spiralis Geinitz, 210 yards to the north-east; also IZ priodon 
Bronn, WV. romerinus var. basilicus Lapworth, and var. crenulatus 
Tornquist, about 110 to 130 yards south of that farm. 

In the ravine half a mile north-north-east of Rhos-lydan the 
following forms, in Miss G. L. Elles’s opinion, probably indicate 
the zone of Monograptus griestonensis :— 


Monograpius crispus Lapworth. | Monograpius nudus Lapworth. 
M. discus Tornquisi. M. pandus Lapworth. 
WM. griestonensis Nicol. M. priodon Bronn. 


M. marri Perner. 


In Nant-y-garth the passage-beds (J/-crenulatus Zone) are 
admirably exposed about a third of a mile west-north-west of the 
6th milestone from Ruthin, and yielded Monograptus priodon 
Bronn, Af romerinus Nicholson, var. crenulatus Tornquist, and. 
Retiolites (Gladiograptus) geinitzianus Barrande, var. angustt- 
dens Elles & Wood. 


part 2] PALHOZOIC ROCKS OF THE LLANGOLLEN DISTRICT. 197 


Southern Limb of the Llangollen Synclinorium. 


On the south side of the Llangollen Synclinorium the Valentian 
succession 1s even more obscure than in the north. The whole 
group of rocks is much reduced in thickness, and is very unfossili- 
ferous. 


Corwen-Moel-Fferna Area. 


In the Corwen-Moel-Fferna district the following appears to Be 
the sequence :— 


Cyrtograptus- Banded slates. 
murchison Zone. 
? Monograptus- Dark, splintery, blue slate and 


Wenlock. 


crenulatus Zone. banded slate. 
? M.-crispus Grey-green sandstone and slates, 
Zone. with pebbles or nodules of 


or Tarannon. : : 
limestone in places. 


f 

i 

( 
Upper oe 

| M.-turriculatus Striped and grey-green slates. 

\ 

i 

| 

| 

{ 

| 

| 


Zone. 
M.-convolutws Dark mudstone yielding grapto- 
Zone. lites. 
Lower Valentian Grey-green gritty slates, in places 
or Birkhillian. speckled, and interbedded, es- 
pecially near the base, with 
KR thin sandstone-bands. 
Ashgillian. Corwen Grit. 


There is no apparent unconformity at the base of the Valentian. 
The convolutus zone was determined by Dr. Groom & Mr. Lake ! 
in Nant-Llechog. Near the Corwen slate-mine, and at the head- 
waters of the Ceiriog, forms were found by us indicating the 
turriculatus zone :— 


Climacograptus extremus H. 
Lapworth. 

Monograpius becki Barrande. 

M. dextrorsus Linnarsson, 

M. exiguus Nicholson. M. turriculatus Barrande. 

M. gemmatus Barrande. M. halli Barrande. 

M. priodon Bronn. x Retiolites gewmmtzanus Barrande. 


1—Head-waters of the Ceiriog, south side of Moel-Fferna. 
2—In stream 330 yards north-west of the Corwen slate-mine, near Carrog. 


Monograptus marri Perner. 
M. nudus Lapworth. 

M. pandus Lapworth. 

M. runcinatus Lapworth. 


KOK 187 
XXX KXe 


2 
x 
x 
x 
xX 
x 
x 


There is no great thickness of rock between the convolutus and 
the turriculatus zones, and no appearance of unconformity. 


The Glyn-Ceiriog Area. 


In this area Dr. Groom & Mr. Lake’s map and description of the 
Valentian succession have been confirmed in the main. ‘The con- 
formable upward succession from the Glyn Grit was proved by 
them, and can be studied in quarries and natural exposures near 


1 Q. J.G.S. vol. xlix (1893) pp. 432-33. 


198 DR. WILLS AND MR. SMITH ON THE LOWER [vol. lxxviu, 


the Nant-Ty’n-y-twmpath stream, in the neighbourhood of the 
Glyn-Ceiriog—Glyn-Dyfrdwy road. 
The following appears to be the sequence in this area :-— 


SECTION UP THE Nant TY N-Y-TWMPATH. 


Thickness in feet. 
Min oak Banded earthy-weathering slates. Cyr- 
tograptus-murchisoni Zone. 
( (5) Blue slates, nterbedded in the lower 
| part with grey mudstone of the 


Tarannonor Tarannon type. Monograpius- 


Upper Valentian ¢ ecrenulatus ZOne...05..20-. ee 10 
(Ty-draw Slates). | (4) Grey-green mudstone, with occasional 
| striped bands, cleaved imto thick 
rootmg-slates, .<)).. =... 285. -5--5 about 140 


((8) Pale gritty shale, with brachiopods. 6 
| (2) Speckled or mottled, rather gritty 
Lower Valentian | green. Slates. 22... ee 70 
(Fron Frys Slates). | (1) Platy sandstone with shale-bands, 
passing down into more thickly- 
\ bedded sandstone. .c.042.... <s-see 
Ashgillian. Glyn Grit. 


ho 


In the Lower Valentian no graptolites have been found, but in 
places brachiopods are fairly numerous. Although for the greater 
part these are Lower Llandovery forms, yet some, in Prof. O. T. 
Jones's opinion, are more typical of the Upper Llandovery. The 
following forms were identified by him. Fossils collected from a 
point 120 yards south-east of Cefn-isaf, near Glyn-Ceiriog, were :-— 


Camarotechia borealis Schlotheim. Orthis cf. rustica J. de C. Sowerby. 
Caiazyga (2). Orthis (Dalmanella) sp. 
Leptzna rhomboidalis Wilckens. cf. Orthotetes pecten Linnzus. 
Leptzna sp. nov. Pentamerus globosus J.de C. Sowerby. 
Meristina furcata J. de C. Sowerby. Plectambonites duplicata J. de C. 
Meristina subundata M*Coy. Sowerby. 
Meristina sp. Plectambonites scissa Salter. 
Orthis (Bilobites) biloba Linnzus. Plectambonites cf. transversalis 
Orthis (Hebertella) protensa J. de C. Wahl. 

Sowerby. . Acidaspis sp. 


In an old quarry on the right bank of the Nant-Ty’n-y-twmpath 
the following were discovered :— 


Catazyga (2). | Plectambonites scissa Salter. 
Meristina cf. subundata M*Coy. Plectambonttes cf. transversalis 
Orihis (Dalmanella) sp. Wahl 


The brachiopods enumerated above occur in the uppermost part 
of the Lower Valentian. 

In the Upper Valentian a few graptolites have been discovered. 

The erenulatus beds are recognizable here, as elsewhere, by 
their dark-blue colour, at the summit of the Tarannon pale slates. 
The following fossils were found at Nant-Fowe, a mile and a half 
south-east of Glyn-Ceiriog :—Monogi aptus romerinus Nicholson, 
and UL romerinus var. crenulatus Tornquist. 

The presence of the griestonensis zone is suggested by the 


part 2] PALMOZOIC ROCKS OF THE LLANGOLLEN DISTRICT. 199 


following forms from 450 yards north of Fron-Llwyd:—JL. grieston- 
ensis Nicol, M. marri (?) Perner, and MZ. priodon Bronn. 

From the data here brought forward it is clear that the ‘ Pale 
Slates’ of Glyn-Ceiriog are undoubtedly of Valentianage; yet itis 
obvious that we have but little precise information as to their 
zonal position. ‘Their remarkable thinness, coupled with the 
quartz-veining and disturbance of the Tarannon Slates, especially 
well seen near the Glyn slate-quarries, suggests that the strata 
observed at the outcrop do not represent the true development of 


the formation. 


Summary of the Valentian Sequence. 


It is now possible to give a conspectus of the Valentian formation 


in the whole district. 
The fullest development is seen in the northern outcrops ot 


Mynydd-Cricor and Cyrn-y-Brain, while, though thinner near 


TABLE IV. 
con Palys | . 
ap es Ga, |S 
ow =| . © 
- lag| ea] = 
4 = o S OF 
cb) ~ yA 1 
afr east el pe aes S) one Paes 
a=absent ; i=inferred ; p=proved. z eee | Reo ce 
Sm fe tee = 
— o a iS 
& S S a 
= Ss) 
ee crenulatus saersiew ttle p p » p 
: : SLONENSIS 2.0... 1 
Tarannon + a, rust ee Pp P P 
SCTUSPUS) crn. avo sceeeln- 1 p p p 
MISE MPT ICULALUS En od cnc:centetiane P I 1 


Thickness in feet ............ 150 | 300 | 900 
Non-sequence. 
(PU SOOM CHET ena otacar ection Ae a a a 
HUES CONVOUVMLUS, AAS naan ee p p a 
CS GMC TORUS na nere mn cin eet aay hee Re 1 p a 
opibtenelnallentay mt IE CaeUs ve. ns sucesss/ en cae ceeece|| LE 1 1 p 
| Orthograptus vesiculosus Mes . B 9 ; 
Mesograptus modestus : ne fe 
| Climacograptus acuminatus ...... i 1 1 1 
~ Mhickness) im feet +.05...60.08 ; ee 350? | 900? ae 


Corwen, the series is even there far more imposing than at Glyn- 
Ceiriog. 

Owing to the difficulty of collecting and identifying the fossils, 
to the monotonous lithology, and to the movements that the beds 
have undergone, it is not possible to dogmatize concerning the 
presence or absence of some parts of the series; but it does appear 
probable that the thinness noted at Glyn-Ceiriog may be in part 
due to faulting, though chiefly to attenuation of the sediments. 

Over the greater part of the district evidence seems to exist for 


7 


200 DR. WILLS AND MR. SMITH ON THE LOWER _[vol. lxxviil, 


the original deposition of all the usually accepted zones of the 
‘Tarannon Series. These have a peculiar lithology that in practice, 
as a rule, suffices to differentiate them from the lower series. The 
relationship of the Tarannon to the Lower Valentian or Birkhillian 
still remains obscure. It is probably in the nature of a non- 
sequence, for there are no signs of an unconformity in the type of 
sediment, yet the Monograptus-sedqwicki Zone has nowhere been 
observed, and the Tarannon slates appear to rest in places on the 
convolutus zone (Corwen and parts of Cyrn-y-Brain) and in places 
on that of IL. gregarius (parts of Cyrn-y-Brain) or of IZ. cyphus 
(Mynydd-Cricor). 

Table IV on p. 199 illustrates the development of the usually 
recognized Valentian zones in the various areas dealt with. 


(b) Salopian Formation—The Denbighshire Series. 


(1) General Observations. 


In the course of the present investigation some 70 square miles 
of the outcrop of the Wenlock and Ludlow Series have been 
mapped. The rocks comprise the ‘Denbighshire Series’ of previous 
workers. 
~ The Langollen-Corwen district has been dealt with by various 
workers from the days of J. E. Bowman!; but they have, for 
the greater part, confined their attention to the simple southern 
portion of the synclinorium. 

In 1895 Mr. P. Lake? put forward the following classification :— 


Dinas-Bran Beds. 

Slaty Beds, with Monograptus leintwardinensis. 

Gritty Beds. 

Flags, with Monograptus colonus, Cardiola interrwpta, Orthoceras 
prumevum, ete. 

Moel-Fferna Slates, with Monograptus flemingi, M. priodon. 

Pen-y-glog Grit. 

Pen-y-glog Slates, with Monograptus personatus, M. priodon, ete. 


Subject to modifications to which reference is made in the sequel, 
this classification, together with zonal information contained in the 
papers by Miss G. L. Elles and Dame Ethel Shakespear (7ée Wood), 
mentioned on p. 177, has formed the basis of our work. 

That the strata of the Denbighshire Series are of shallow-water 
origin is suggested by the great thickness of rocks of one litho- 
logical type, and by the presence of internal brecciation, cross- 
bedding, and contemporaneous erosion-phenomena. 

The series is probably between 4000 and 5000 feet thick, and is 
composed (with the exception of the Pen-y-glog Grit) of what 


1 Brit. Assoc. Rep. of Sections (Glasgow) 1840, p. 100; ibid. (Plymouth) 
1841, p. 59; and Trans. Manchester Geol. Soc. vol. i (1841) p. 194. 

2 Q.J.G.S. vol. li, pp. 9-22. This paper contains a bibliography complete 
up to 1895. 


part 2] PALHOZOIC ROCKS OF THE LLANGOLLEN District. 201 


were originally bands of laminated silt, alternating with bands of 
mud. The thickness of individual layers varies from perhaps 
1/10 inch to several inches, or occasionally a foot or more, but 
the monotonous alternation persists throughout. Except in the 
Pen-y-glog Slates, the mud-bands usually exceed the silt-bands in 
thickness, and there appears to be a fairly constant thickness-ratio 
of about 1 to 8 or 1 to 5, as between silt-band and mud-band as 
measured now in the indurated and cleaved condition. There is 
little doubt that the alternations represent seasonal variations of 
the transporting power of the rivers which supplied the sediment 
to the sea. 

The mud-bands have assumed a more or less perfect cleavage ; 
but the silt-bands are frequently devoid of it, although in many 
cases the cleavage passes through both. This factor, together with 
the original thickness of the bands, and the angle between the dip 
of cleavage and the bedding, produces an endless variety of 
appearance within somewhat monotonous limits. 

Apart from the Pen-y-glog Slates and Grit no subdivision of the 
eroup possesses any striking lithological characteristic by which it 
can be readily identified and traced. Nor do the fossils give much 
help, on account of their rarity and poor preservation. Hence 
there is considerable doubt about the accuracy of the lines 
representing the boundaries of the subdivisions of the Salopian 
shown in the map (Pl. V), which must be regarded as illustrating 
the structure, and as offering a solution (admittedly imperfect) to 
the riddle of the folding (see pp. 212-17), in the complex Llan- 
gollen Synclinorium. 

It appears probable that most of the accepted graptolite-zones of 
the Wenlock and Lower Ludlow (as developed in the Welsh 
Borderland, Shropshire, and elsewhere) are represented. The 
general sequence and the zones are indicated in Table V (p. 202). 


(2) Wenlock Series. 


The Wenlock is thin in comparison with the Lower Ludlow 
Series, and is relatively well known.! Its lowest member, com- 
prising mudstones and slates with the fauna of the Cyrtograptus- 
murchisont Zone, succeeds the Monograptus-crenulatus Beds of 
the Upper Valentian conformably. 

The lithology of this and the succeeding I.-réccartonensis Zone 
may be described as banded earthy-weathering slates,? the upper 
zone providing the roofing-slates of Glyn-Ceiriog, Moel-Fferna, 
Pen-y-glog (Corwen Slate-Mine), and Moel-y-faen. In the 
southern part of the district the roofing-slates are succeeded by 


1G. L. Elles, Q. J. G.S. vol. Ivi (1900) p. 397; P. Lake, ibid. vol. li (1895) 
pp. 9-22. 

2 They are of interest, on account of the problematical fossil Berwynia 
carruthersi (a plant of unknown affinities, now preserved as an anthracitic 
substance). 


"EHOB LOAN 
1 ¢ ‘OLLOJAPTU popu ‘HOPUTH popuu ‘ ‘wOPU[H popu sng yPopthy) 
HOPN[M pu \ at i " pov tt 1 91 ae i pl ; | é wh io ay Led - ad BS St = 
OUOPAPNUL POpUUe ‘(papruq) soy ‘(popruq) HoyayN 09 ‘(popunq) sopups SIRUPUO PA DOO EE 
‘poytpuope d 00 “Buyooyp BopseLettog Ay “Huyooyy Hops Aetiog oy gg] suypooyp Bopeedetogy snjpd niHowo py 
he} YOU KOULOZ WOLUIYH an eee ; . a . =e ee 
=f AQYLUA  popueiiiry "HO PUTY *Hopuye OL ‘sOyuye “nOdy) | ‘HOLLOR 
py “(nA Mp oudagy A098 | pepeq gana yom Ob poping SUL OM 0} pop und ULOT POM OLA Og YOO ta M 
5 JO qrOMetTPLOU) AYQtuO  puNyeugy eAYQtto “Page | gy eATPLUO "PU uey "PUOSBADUNED LD 
4 (ite) SOpee Lotto] + ad oa 
y HOTLOGN PELL AQ ]LLS a VUE) BOL Netlog 
) | PU KOPN poAuoypo ) 
8 ‘snOoOtUO,WO “OnE HO yuyH HOWUTS AQ ite On] "paubpuny 
Ll ( OOT d HOPMAN ONC pce] SULTON MOM Opp pu so pays wey yy SMPADMDOPAM, ) f ] 
& | PoyLpuopr Fou KOU? « ' ; ‘ ' " : ets - . +o Ss 
( OOL ( HOPUTH PU MAY AY “AIO (OOC, VIP] JO YAU) REDD PUL DE fb 
oF « OPUS AP MEL MOL pu HOUT TP MCP Oty Og UNO) 
ater) HOPETH PU MULT LLOOLT WUT OT HOPRH Pre MANY A] AMPMIJACT 
HULOLPOLOLOD (| “(Opty s) GOZLtotpequyy *(OpUTpA) HOZLLOT Se ups *(OpPMYH) TOZLLO TL uy S et ALK) 
OULOPHUOPJOL YL Q00T ) HOPE PU MAUL A HOUT PE WUT oy HOPUPH PUL MUTE | 6 LHOSS) EG * YI) 
WO PWTH pun la) — - _ 2 —| : ; ' . , WoT 
*HOWUTH aT ‘HON N [ MOL Puy 
, fee (pepo Appu piu (popdamao Apu pu (popu App “ANOd) OLpoury LO MOF] 
HOLLOJNPUEL pri f O06 ALLO PE HOZO) KOTO gH 006 “ULLOYULE HO) JO) HOtLO ys ALLOPUT Hay yO) SOTLO YH SHOTUDOS "TH eA JUU NY 
fworua “KAU Aputer e * PUA POUTiAne © OLY * PIN POUT = OUT] PUN POUL © oUtg SMAOSOUOND TY fb 
HOTLOJHPHUL Pu HOP NTH Bi HOTLOM PLU ES “ody poat 
WUTOLOLOUOD POPUUg 0081 AQ Lud Plt sOTLOgN PIG sip pA Powo fy rd 
a 
. F WOLLOG 
LOY ab & [00 1A] i . ; : : (Ot) ) ' 
UTM POAMOTOI Apis Pporenin Yip » MOTp Mt 
jo wu popuug 4 as aie OLUT[H POAvoy Pulls hee [nay Og ULE BUUIC] ) ath " 
4 i) vr) a) ‘ G 
"f AM] , HUOLMLATD 
Se or a LEM TLO A WOLY Ponty WOW TWLOTGQUOTSINH ULSD hee hl 
WA LE Oty yo rUNL! u ir tA ello of tu r Wide eyToo wuol aes 
/4eOM=YGtOU VOLY ST esl re UI Tr a | A powodoty 


HOI MMA DTANGC, Wy A eV, 


part 2] LOWER PALHOZOIC OF THE LLANGOLLEN DISTRICT. 203 


‘bastard slates’ which have yielded the following graptolites 
indicative of the zone of Cyrtograptus linnarssoni :— 


ie 2 ee 

Cyrtograptus linnarssom Lap- X | Monograptus flewilis Klles. aex 

worth. M. priodon Bronn. > dan¢ 
Cyrtograptus symmetricus Hlles. X M. vomerinus Nicholson. SEX 
Monograptus dubius Suess. x M. vomerinus var. basilicus X 
M. flemingi Salter. x Lapworth. 
M. flemingi var. primus Elles x 

& Wood. 


1=North side of the quarry, north of the upper bridge over the Nant 
Ty’n-y-twmpath. 
2—=Level in Moel-Fferna Slate-Quarry. 


It was the presence of this fauna at Moel-Fferna that led 
Mr. P. Lake to institute his ‘Moel-Fferna Slates’; but the 
present development of the mine clearly demonstrates that the 
‘bastard slates’ (with C. linnarssoni) le under, and not over, 
the Pen-y-glog Grit as he thought. 

Pen-y-glog Grit occurs only in the western part of the district. 
It is a tough, fairly massive, felspathic grit, often interbedded 
with hard blue shale. Miss G. L. Elles states that the grit overlies 
the IZ.-riccartonensis Beds at Pen-y-glog. It this be so, the base 
of the grit lies at a lower horizon there than at Moel-Fferna, a 
fact that may perhaps be related to its greater thickness at the 
former place, where it is perhaps 120 feet thick. It thins to 
30 feet at Moel-Fferna, and cannot be traced far to the east of 
this point. A similar eastward thinning occurs in its northern 
outcrop near Ffynnon-Tudur (north-west of Bryneglwys), an 
occurrence of the grit that was overlooked by the makers of the 
old geological map. 

In the Moel-Fferna district the grit 1s overlain by hard dark- 
blue slates, devoid of fossils, which in turn give place upwards to 
slates and silty bands of the Lower Ludlow type. But in Glyn- 
Ceiriog there is no trace of the grit, and a considerable thickness of 
banded, earthy-weathering, ‘bastard slates’ succeeds the roofing- 
slates; we only know that, near the Nant-Ty’n-y-twmpath, the 
lower part of these represents the /7nmarssoni zone (see above). 

It appears that the junction with the Lower Ludlow is possibly 
cut out in places by the Glyn-Ceiriog and Nant Ffrydd-isel 
Faults, but the evidence is meagre and difficult to interpret. 

In addition to the almost continuous outcrop on the south and 
west side of the Llangollen Synclinorium, the Wenlock deposits 
emerge at Caer-Drewyn Hill, north-east of Corwen (see p. 216), 
north and north-east of Bryneglwys, and on the south side of 
Cyrn-y-Brain. In the last-named area they have been extensively 
quarried in the past, on Moel-y-faen in particular. There is a 
general similarity of lithology here and in the southern outcrop ; 
but, on account of severe folding, and because the cleavage often 
crosses the bedding more or less at right angles, the sequence and 


204 DR. WILLS AND MR. SMITH ON THE LOWER  [ vol. Ixxviul, 


fossils are not so easily studied. The following appears to be the 
succession :— 


Thickness in feet. 
Slates and silty sandstones of Lower Ludlow type. 
Gritty blue slates, with occasional trilobites and Orihoceras > 100 
‘ Bastard’ banded slates, with a 20-foot bed, near the base, 
composed of brecciated blocks of lamimated shale set in 


a blue shaly matrix, the whole being well cleaved ...... ? 250 
Banded roofing-slates, with beds of spar........................ ? 60 
Banded mudstone, yielding fossils of the Cyrtograpius- 

miurchisont Zone... Si 8... ales eee es ea ee ? 15 to 20 


The brecciated bed mentioned above may be seen in an old 
quarry near Cae’r-hafod, and also near Ty-cerrig. The blocks of 
shale composing it are disposed in every direction, as revealed by 
the lamination ; but the normal cleavage goes through all, without 
reference to the orientation of the blocks. The band provides 
evidence of considerable consolidation of the silts and muds soon 
after their deposition—in fact, before the laying-down of the 
succeeding beds of shale. Brecciation within a bed the upper and 
lower surfaces of which are sub-parallel, and possess the same dip as 
the neighbouring rocks, seems to pomt to penecontemporaneous 
disturbance, perhaps by an earthquake, on similar lnes to those 
suggested by Prof. P. F. Kendall.! 

In the Bryneglwys area, the Wenlock deposits are blue and 
banded slates; but, as they have not been exploited, little can be 
made out of the detailed succession. The Cyrtograptus-murchisont 
Zone, however, has been detected at points near the Tarannon-Slate 
outcrops. The structure of this area is dealt with on p. 211. 


(3) Lower Ludlow Series. 


(a) The Ludlow Series in the Llangollen Synelinorium. 


(i) Glyn-Dyfrdwy Group.—lIn the southern part of the 
synclinorium there is no lithological feature and practically no 
palzontological evidence on which to draw the base of the Lower 
Ludlow Series. It is probable that the bottom part of the Glyn- 
Dyirdwy Group is absent through faulting in the Glyn-Ceiriog 
valley; and, if present farther west, it is poorly exposed on the 
moors. Near Carrog and Moel-Fferna the normal upward sequence 
from the Pen-y-glog Grit appears to be interrupted by a north- 
and-south fault along the Nant Ffridd-isel, which cuts out the 
basal part of the group. 

Could we but study this basal member adequately, it would 
probably be found to fall in the zone of Wonograptus culgaris 
which Dame Ethel Shakespear believed that she could recognize in 


1 Abs. Proc. Geol. Soc. 1918-19. p. 28; see also B. Smith, Geol. Mag. 1916, 
pp. 146-56. 


part 2] PALHOZOIC ROCKS OF THE LLANGOLLEN DISTRICT. 205 


the Nant-Arddau Valley (Nant-y-Pandy), south of the Dee-side 
Slab-quarries. Unfortunately, our collecting has not provided 
material enough to confirm or disprove the existence of this zone. 

On the northern flank of the synclinorium, there appears to be a 
complete upward sequence from the Wenlock Series into slates 
with thin silty sandstones, but there is no reliable paleontological 
evidence by which to distinguish the two formations. There is 
here, above the doubtful series, a well-marked slate-band (the 
Pentre-Dwfr Slates) the outcrop of which we have tried to 
represent on the map (Pl. V). The slates are not recognizable 
south of the Dee: they may, however, be cut out by the faulting 
mentioned above. As roofing-slates they have good weathering 
properties, but are too thick for modern requirements. The 
cleavage passes indiscriminately through mudstone and laminated 
silts in which uncleaved calcareous concretions are frequent. The 
slates pass upwards into an indefinite series of slates with thin 
silty sandstone-bands. 

The succeeding ‘Slab Horizon’ is traceable throughout the 
synelinorium with fairly constant lithological characters. The 
subdivision is essentially composed of rapidly alternating bands of 
cleaved mudstone and uncleaved laminated silt. When not broken 
up by close jointing, the rock splits readily between the cleaved and 
uncleaved layers into huge slabs of a. uniform thickness, and has 
formerly been extensively quarried for slate-slabs, stone-cisterns, 
flags, ete. As the graptolite fauna of this horizon has been shown 
by Dame Ethel Shakespear? to be that of the Monograptus- 
nilssont Zone, and, as other striking features of the fauna (the 
local abundance of ‘ Actinocrinus’ pulcher? and of ‘ Orthoceras 
primevum’) have long been known, it is unnecessary to go into 
further details. Our fossil-collecting confirms the reference of the 
slabs to the nz/ssonz zone. 

The uppermost subdivision of the Glyn-Dyfrdwy Group is 
composed of fairly thin-bedded shales and silty mudstones or sand- 
stones, occasionally micaceous, more or less perfectly cleaved. 
The sandstones are sometimes internally crumpled, and the shales 
frequently show concentric staining very similar to that seen in the 
Moughton Whetstone of the Austwick Valley (Yorkshire). The 
group as a whole is rather conspicuously unfossiliferous, but a few 
graptolites have been found on the hills south-west and south of 
Glyn-Dyfrdwy and elsewhere, which suggest that the rocks form 
part of the Monograptus-nilssoni Zone. 

There are fine exposures in this group on the hillside north 
of.the bridge over the Dee at Glyn-Dyfrdwy, and north-west of 
Dolywern near Glyn-Ceiriog. 

In the latter area the subdivisions of the Glyn-Dyfrdwy Group 


1 Flags of this rock, with the characteristic fossils, have been found in the 
Roman Camp near Capel-Curig. 

2 Q. J.G.S. vol. lvi (1900) p. 446. 

3D. W. Roberts, Trans. Edin. Geol. Soc. vol. i (1870) p. 329. 


206 DR. WILLS AND MR. SMITH ON THE LOWER  [ vol. lxxviii, 


appear to have the following thickness, although the lowest member 
is in part cut out by the Glyn-Ceiriog Fault :— 


Feet 
Uppermost eroup cis. an. see shee ee ee 600 
Slabeborizon 2.2.2 s22...daor sae oe een Cee Ree eee 270 
owestcroup: (part)..2.¢- sek os eee ce eee 240 P 


The thickness is not so easily measured in the disturbed northern 
part of the synclinorium, but may be approximately 1600 to 
1700 feet. 


(ii) Nant-y-Bache Group.—In this group the rocks are pre- 
dominantly uncleaved silty sandstones and roughly-cleaved sandy 
shales. They are best studied south of the Llangollen Fault, where 
they occupy most of the upland between the Dee and Ceiriog 
drainages, east of Glyn-Dyftrdwy. North of the fault, they are 
involved in the severe folding, and therefore, although well deve- 
loped, they are more difficult to investigate. The group appears 
to be approximately 900 feet thick. 

The base of the group has been taken to be the lower limit of 
somewhat calcareous and (asa rule) internally-crumpled! silty sand- 
stones, which form a bold escarpment from near Foel, past Glyn- 
Ceiriog and Dolywern, to the Ceiriog valley near Pont-fadog, where 
they cross to the eastern side and form the nameless hill 1330. 

Near Dolywern and Glyn-Ceiriog a calcareous conglomerate of 
rounded blue sandstone-pebbles, with a few shale-fragments and 
many small blackish pebbles, occurs in the lowest part of the 
group. It varies from 25 feet to about 1 inch in thickness. 
A search for fossils in the pebbles revealed only one Orthoceras, 
though their lithology is suggestive of the Teirw Beds. The signi- 
ficance of the conglomerate is not known, but it appears to be an 
indication of renewed uplift along the Berwyn axis (see p. 183). 

Associated with crumpled sandstones are fossiliferous layers, 
often crowded with MMonograptus chimera, M. dubius, and M. 
tumescens (?), while occasionally small brachiopods are very 
abundant in calcareous bands that weather to rottenstone. North 
of Dolywern, the following forms were found :—<dAtrypa reticu- 
laris Linneus, Dayia navicula J. de C. Sowerby, and Orthis 
(Dalmanella) sp. In slightly higher beds in the same area 
Oardiola (Slava) interrupta J. de C. Sowerby is fairly common. 

There are extensive exposures of the Nant-y-Bache Group in the 
stream from which it takes its name and in the other branches of 
the Cyflymen Brook, which descends towards Llangollen, although 
here no fossils have been found. The rocks are predominantly 
sandy ; but there is also some sandy, poorly cleaved shale. . 


1 We term the structure ‘internal crumpling,’ because it can be seen in 
many cases that a bed, say, a foot thick, preserves its upper and lower surfaces 
sub-parallel and seemingly undisturbed, while within there is folding and 
even overfolding. The more shaly bands interstratified with the sandstones 
are roughly cleaved. Less conspicuous internal crumpling occurs in the 
sandstones of the uppermost Glyn-Dyfrdwy Beds. 


part 2] PALMOZOIC ROCKS OF THE LLANGOLLEN DIsrRict. 207 


The uppermost beds of the group form the south side of the 
Penewern Valley. Here, about 200 yards south by east of Ty’n-y- 
celyn the following assemblage, possibly indicating the lower part 
of the Monograptus-scanicus Zone, was found :— 

Monograptus chimera Barrande. Monograptus dubius Suess. 


M. chimaera, var. salweyi Lapworth. M. varians Wood. 
M. colonus Barrande. 


These beds are some 800 to 400 feet below the lowest horizon 
at which Monograptus leintwardinensis has been found. 

On the north side of the Llangollen Fault the Nant-y-Bache 
Group is involved in the severe folding (Pl. V). 


Gii) Vivod Group (M.-leintwardinensis Beds).—The 
rocks composing this group are largely thinly-bedded flagey shales 
with silty, often laminated bands at frequent intervals. They are 
less well cleaved than any of the rocks so far dealt with, the result 
(it is believed) of their sandy nature. Fossils are rare in them, but 
M. leintwardinensis has been found at several places. The group 
is perhaps 1200 to 1800 feet thick. 

South of the Llangollen Fault the characteristic fossil was 
discovered at Craig-y-ddualt (south of Pengwern) and near 
Pant-Dafydd-goch, and probably all the high land between the 
Castle-Mills Fault and the Carboniferous of Chirk Castle is 
occupied by this group. 

North of the Llangollen Fault, the rocks underlying the Vivod 
Valley, Berwyn,! and part of Llangollen itself appear to belong 
to this subdivision. 

The lateral shift effected by the branches of the Llangollen 
Fault is well shown by the outcrops of the beds; but the exact 
distribution of the rocks east of Llangollen is very obscure. 


(6) Lower Ludlow Rocks outside the Llangollen 
Syneclinorium. 


The stratigraphy of the Lower Ludlow inside the Llangollen 
Synclinorium has been dealt with at some length, because the 
mutual relationship of the beds is clearer here than elsewhere in 
the area under description. There seems, however, little doubt 
that the rocks maintain the same general characters outside the 
synclinorium, and there.is paleontological evidence that the usual 
zones are present. Thus:—The JL.-leintwardinensis Zone, 
indicating synclinal cores, is found south of Moel-Truan, near 
Bryn-Tangor, 2§ miles west-south-west of Bryneglwys; near 
“Nant-Clywd, on the west of the Vale of Clwyd; and possibly on 
the eastern flank of the Clwydian range near Moel-yr-acre. The 
scanicus and tumescens zones oceur west of the Vale of Clwyd. 
The nzlssone zone covers a large area between the Bala and 


Lianelidan Faults. The Dinas-Bran Beds have not been identified 
with certainty. 


1 See HE. M. R. Wood (Shakespear), Q. J. G.S. vol. lvi (1900) p. 446. 


208 DR. WILLS AND MR. SMITH ON THE LOWER  [vol. Ixxviui, 


(ec) Intrusions in the Lower Ludlow Series. 


An olivine-dolerite (tholeite) occurs as small dykes in a few 
places in the Lower Ludlow rocks. Mr. Lake mentions one near 
Pen-y-vivod. Another outcrop is in the Afon- Ro, about 200 yards 
from its confluence with the Dee; but there is no proof that 
the two form parts of the same dyke. Although the rocks have 
not been found zn s7tu, there is evidence of a third occurrence on 
Craig-y-Rhos, a hill half a mile north of Glyn-Dyfrdwy. 


(4) ? Upper Ludlow Series. 


Dinas- Bran Group.—tThe rocks forming the conspicuous hill, 
crowned with the remains of the ancient fastness of Castell-Dinas- 
Bran, have long been regarded as the highest Silurian in the area. 
They are uncleaved, thinly-bedded, slightly caleareous, sandy shales, 
usually micaceous, and occasionally fossiliferous. 

Mr. Lake (Q. J.G.S. vol. li, 1895, p. 20) gives a list of fossils 
from here, several of which have been also collected during the 
present survey. The following new records may be added :— 


Chonetes cf. striatella Dalman. | Orthonota cf. amygdalina J. de C. 
Orthis ef. lunata J. de C. Sowerby. Sowerby. 
Orthis cf. orbicularis J.de C.Sowerby. | Plewrotomaria sp. 

Orthoceras ludense J. de C. Sowerby. 


Dayia navicula is the commonest fossil here, and has also been 
found at various other localities nearer Llangollen. Mr. Lake 
suggested that the fauna indicated an Upper Ludlow age. Since 
then Miss G. L. Elles & Miss I. L. Slater! have described in detail 
the Ludlow Series at Ludlow, giving an extensive list of fossils. 
According to these authors, Dayia navicula is not found in the 
Upper Ludlow. Its abundance in the Dinas- Bran Group suggests 
a correlation of that with the Mocktree or Dayza Shales; but, as 
these fall in the zone of JL. letntwardinensis, which at Llangollen 
is known to underlie the Dinas-Bran Beds, and as, with this 
exception, the fossils are, generally speaking, common to both 
Lower and Upper Series, we are inclined to agree with Mr. Lake 
in assigning the group to the Upper Ludlow. 


IV. SrructvurReEs. 


In the succeeding portion of this paper, we attempt to describe 
the structures that have resulted, in the main, from mountain- 
building movements in Devonian times, although in part from a 
continuation of the cycle of movements into later ages. But the 
later movements have merely modified, and not obliterated, the 
major tectonic features that were rough-hewn by the Caledonian 
folding. 


1 « The Highest Silurian Rocks of the Ludlow District’ Q. J. G. S. vol. lxii 
(1906) pp. 195-222. 


a 


part 2] PALHOZOIC ROCKS OF THE LLANGOLLEN DISTRIcT. 209 


The Lower Paleozoic rocks are folded on approximately east- 
and-west axes into a series of major and minor synclines and anti- 
clines, which are cut through by a few mighty master-faults and a 
multitude of smaller adjustment-faults. The earth-stresses were 
so intense that all save the most resistant rocks were severely 
cleaved. 

Practically all the folding was effected in Devonian or early 
Carboniferous times, while the major faults were also initiated 
then. Proof of these statements can be seen in the transgressive 
unconformity of the Dibunophyllum Zone of the Carboniferous 
Limestone, which strikes north and south at right angles to the 
earlier structures. All the Carboniferous is subject, on the one 
hand, to wide gentle warping that did not modify to any appreci- 
able degree the Devonian folds below; and, on the other hand, to 
influential faulting that produced considerable displacement of the 
fault-blocks already defined by the earlier movements. 

The problems of the tectonics of the region fall, therefore, 
naturally under two headings, namely:—(a@) Pre-Carboniferous, 
(6) Post-Carboniferous. This is true only in a general way, 
however; for the earth-movements went on intermittently 
throughout the Devonian, Carboniferous, Permian, and Triassic 
Periods, and possibly in even more recent times. In a paper dealing 
with the Lower Palseozoic rocks, a full account of the Devonian 
movements is appropriate and will be attempted here; while the 
post-Carboniferous modifications can only be touched upon. 

In the following description of the structures, our views have 
in part been influenced by the fact that one of us (B. S.) has 
collaborated with Mr. C. B. Wedd, of H.M. Geological Survey, 
in a study of the tectonics of North Wales. This study has led 
us to regard the structure of the Llangollen district as produced 
by torsion, set up by stresses originating in the Caledonian move- 
ments of Devonian times and continuing to act in later epochs. 
In this paper the data are set forth, without emphasizing their 
bearing on the torsional hypothesis. 


Pre-Carboniferous Movements. 


Viewed in the broadest outline, the area is embraced by the 
following essentially pre-Carboniferous structures (fig. 1, p. 178) :— 


Clwydian Anticline—the north-eastward extension of the Harlech Dome. 
Llanelidan Syncline—expressed by the Llanelidan Fault. 

Mynydd-Cricor Nodal! Anticline. 

Bryneglwys Synclinorium. 

The Central Wales or Llandderfel Syncline. 

Bryneglwys Fault. 

Cyrn-y-Brain Nodal Anticline. 

Llangollen Synclinorium with the Corwen Buttress. 


1 The anticlines defined as nodal bring to the surface Ordovician rocks, 
which, by their superior toughness, appear to have acted as knots, or nodal 
points, in the general earth-movement (see p. 212). 


Q.J.G,8. No, 310, e 


210 DR. WILLS AND MR. SMITH ON THE LOWER  [vol. lxxviii, 


Clwydian Anticline.—At the present time, the graben of 
the Vale of Clwyd, with its stupendous eastern boundary-fault, 
makes it difficult to appreciate the pre-Carboniferous structure. 
However, the Lower Ludlow rocks of the Clwydian range and of 
the country on the west of the Vale appear to form part of the 
extension north-eastwards of the great Harlech Dome. The axis 
of the anticline probably strikes east and west in this region, and 
appears to lie somewhere in the latitude of Moel-Famau, where 
Wenlock Beds are found. 

Although the initiation of the depression of the Vale of Clwyd 
probably took place in pre-Carboniferous times, it did not obliterate 
the effects of the folding. The rocks on each side of the Vale 
appear to have formed part of a local dome on the axis of the 
great Harlech upfold: for in the Nant-Clwyd area, west of the 
Vale, the folds pitch westwards, whereas in the Clwydian range 
they pitch eastwards. Further, in conformity with this general 
doming, we find in the Nant-Clwyd area that the lezntwardinensis- 
beds cover most of the ground, while they also probably occupy 
the easternmost part of the southern end of the Clwydian range. 


Llanelidan-Fault Syncline.—In view of the facts just 
related, it is held that in pre-Carboniferous times a synclinal 
tendency existed between the Clwydian anticline and the dome of 
Mynydd-Cricor. This tendency found its expression chiefly in the 
Llanelidan Fault, which strikes almost due east and west between 
the present trough of the Vale of Clwyd and the surviving half of 
the Cricor dome, and ultimately passes between the Millstone Grit 
of Moel-Garegog and the Ordovician of Cyrn-y-Brain. Con- 
siderable movement has taken place in post-Carboniferous ages 
along this fault; but it was one of the great master-faults in this 
region in Devonian times. The displacement produced by it has 
probably always been lateral rather than vertical, the block of 
country south of it travelling eastwards relative to that on the 
north. At the same time, it is probable that it conformed to the 
synclinal tendency by a downthrow northwards: for north of 
the Llanelidan Fault the Carboniferous Limestone rests upon the 
higher part of the Lower Ludlow, while, south of that fault, on 
Cyrn-y- Brain it hes upon Ordovician. It must also be poimted out 
that the Limestone crosses the Cyrn-y-Brain anticline by trans- 
gressive overlap, much of the lower part being absent. The post- 
Carboniferous movement along the Llanelidan Fault is, therefore, 
less than is suggested by the displacement of the outcrops (from a 
point immediately west of Minera to a mile and a half south-west 
of Llandegla). 


Mynydd-Cricor Nodal Anticline.—In the area between 
the Llanelidan and the Bryneglwys Faults the dominant structural 
feature is the periclinal dome of Mynydd-Cricor, the northern 
half of which is cut off by the Llanelidan Fault. 

The core of Ordovician slates and grits of Mynydd-Cricor is 


part 2] PALMOZOIC ROCKS OF THE LLANGOLLEN DisTRict. 211 


followed on the east and south by a broad stretch of Valentian 
Slates, dipping outwards, and broken by numerous faults. 

On the south the dip is mainly southwards, but traced eastwards 
the strike swings ronnd through north-east to north until it is 
almost north-north-west near the Llanelidan Fault. ‘The change 
in strike suggests the dragging-round of the beds by the westward 
displacement. of the block of country north of the Llanelidan 
Fault. 

The numerous faults affecting the area appear to be adjustment- 
faults ancillary to this anti-clockwise twist. They slice across the 
dome with strikes varying from almost north-west in the east to 
north in the west. They throw down sometimes one way, some- 
times the other. At the western end of the dome the arrangement 
of the strata is more complicated. A triangular strip of country 
is found there having the dip inwards towards the dome, from 
which it is separated by a fault. Asa result of this arrangement 
the Wenlock is brought to he against Ordovician a quarter of a 
mile west of Cricor Farm; while farther south, the Tarannon and 
the Wenlock Series are in juxtaposition. 

The rocks in this strip of country are affected on its south side 
by two curved thrust-faults, convex towards the south and south- 
west. The more southerly thrust forces Pen-y-glog Grit over the 
Ludlow Series; while the second, which may be termed the 
Ffynnon-Tudur Thrust, carries Lower and Upper Valentian over 
the Grit. 


Bryneglwys Synelinorium.—tThe general anticlinal struc- 
ture of the Cricor dome gradually merges southwards into a 
complex synclinorium, the axis of which runs east and west in the 
neighbourhood of Moel-Truan, 2 miles west by south of Bryn- 
eglwys. ‘The structure hereabouts is similar to that of the 
northern half of the Llangollen Synclinorium (see p. 213), and 
the two regions occupy analogous positions with regard to the 
Mynydd- Cricor and Cyrn- y-Brain domes respectively. The simi- 
larity of structure suggests that the two synclinoria are parts of 
one structure, subsequently displaced laterally by the Bryneglwys 
Fault. 


Cyrn-y-Brain Nodal Anticline.—The great anticlinal dome 
of Cyrn-y-Brain constitutes a node of Ordovician rocks similar to, 
though larger than, that of Mynydd-Cricor. Both nodes have 
similar effects on the rocks to the south of them. 

Cyrn-y-Brain mountain is covered to such an extent by peat and 
heather, except around its periphery, that little can be made of its 
internal structure ; but on its northern side there are patches of 
Valentian oe in the faulting, which show that, as a whole, 
the rocks are arranged as a periclinal dome. The large area 
covered by the Ashgillian Beds makes it almost certain that the 
anticline is complex in structure. The western end of the dome 
has been faulted out of sight by the Bryneglwys Fault or fault- 

PQ 


212 DR. WILLS AND MR. SMITH ON THE LOWER [ vol. Ixxviil, 
plexus ae joins the Llanelidan Fault at Llandegla (see map, 
fig. 5, p. 220). The faulting has locally produced considerable 
shearing, the rocks being almost converted into mica-schists. 

A broad outcrop of Lower Valentian rocks occurs on the 
southern side of Cyrn-y-Brain in which the rocks have fairly- 
uniform southward dips; but the structure may not be so simple 
as it seems, for strike-faulting may be easily overlooked in so 
monotonous a series. We know, in fact, that near Plas-uchaf the 
World’s-End Fault cuts outa large part of the Lower Valentian 
in the eastern part of its course, and passing westwards it elimi- 
nates much of the Tarannon Series. 

The Tarannon outcrop belongs more naturally to the complex 
northern half of the Llangollen Synclinorium, and will be dealt 
with under that head (see p. 213). It may be suggested, there- 
fore, that the core of Ordovician and Lower Valentian rocks acted 
as the head of a ram that was thrust into the syncline of newer 
strata, and produced the concertina-folding in them. 

As in the case of Mynydd-Cricor, so on “Cyrn-y-Brain, a number 
of transverse adjustment-faults (probably partaking of the nature 
of tear-faults) comply with the demands of torsional movement. 
These faults have little effect on the tough Ordovician greywackes, 
but become more conspicuous in the Valentian. Thei general 
direction is usually more or less north and south. 


Llangollen Synclinorium (see Pls. [V & V).—This struc- 
tural element may be defined as embracing the broad outcrop of 
Upper Silurian rocks southwards from Cyrn-y-Brain, across the 
Dee Valley, to the northern flank of the Berwyn anticline with its 
north-western buttress that projects towards Corwen. The syn- 
clinorium is obliquely truncated on its north-western side by the 
Bryneglwys Fault. 

The synclinorium as a whole pitches eastwards, and its axis 
strikes between 10° and 20° south of east roughly along the line 
of the Dee Valley.! North of this lies a complex limb in which 
concertina-folding has taken place, the axes of the minor folds 
having the same strike as that of the whole synclinorium. South 
of the axial line the structure is extremely simple, the rocks bemg 
gently folded into a single, shallow, pitching fold. The average 
angle of the pitch varies between 5° and 10° (Pl. IV). 

The two structural units contrasted in the last paragraph 
merge in reality more or less one into the other in the axial region; 
but a convenient boundary may be found in the Llangollen Fault, 


1 The structure will be more readily understood if reference be made to 
the map (Pl. V) and serial sections (Pl. IV). It must be pointed out again 
that, owing to the absence of easily traceable horizons in the monotonous 
Denbighshire Series, and to the complexity of the structure, both map and 
sections are to some extent generalized. The generalization is, however, 
based upon an enormous number of observations made during the re-survey 
of the area. Even so, the boundaries of the subdivisions suggest a degree of 
certainty that is unjustifiable, 


part 2] PALMOZOIC ROCKS OF THE LLANGOLLEN bisrricr. 218 


which, with a large downthrow northwards, strikes east 10° south 
roughly parallel to the axis. ‘Throughout the synclinorium the 
cleavage has a general strike of east 10° to 20° south. 


Before we enter upon the details of the folding, it will be well 
to describe the major faults. 

Parallel to the Llangollen Fault and about 2 miles south of it, 
runs another master-fault, the Glyn-Ceiriog Fault, which also 
throws down northwards. The parallelism of these two faults 
with the direction of folding and cleavage emphasizes their con- 
nexion with the stresses that caused both. It is thought that in 
both cases the initiation of the fault may have been connected 
with the bending-down of the Llangollen Basin, and that possibly 
they may be in the nature of ‘ undertow faults.’ But, at any rate 
in the Llangollen Fault, such movement appears to have been 
accompanied and succeeded by lateral movement westwards of the 
block on the northern side of the fault. 

This movement appears to have induced a series of adjustment- 
faults with a general northwestward trend springing from the 
master-faults. In some of these an apparent eastward downthrow 
may in reality represent a northward lateral displacement of the 
block on the east of the fault. Other of the adjustment-faults 
counteract the effect of the pitch of the synclinorium by throwing 
up eastwards, thus allowing its great length to be compatible with 
a steep pitch throughout. 

Of the adjustment-faults springing from the Llangollen Fault, 
the most important are the Aqueduct Fault (separating Carboni- 
ferous from Silurian north-east and north of Llangollen), the 
Llandynan Fault, and the Rhagatt-Hall Fault ; while the following 
are connected with the Glyn-Ceiriog Fault:—The Castle-Muills 
(near Chirk Castle), the Pont-fadog, the Nant-Fridd-isel (near 
Carrog), and the Bonwm (near Corwen) Faults, as well asa number 
of less conspicuous ones (see fig. 1 & Pl. V). 


The complex folding of the northern limb of the synclinorium 
may now be dealt with in somewhat greater detail. Abandoning 
the consistently southward dips observed on the flanks of Cyrn-y- 
Brain, the rocks, from the Tarannon slates upwards, become 
involved in a type of structure best described as ‘concertina- 
folding’ (see sections, Pl. IV). 

Upfold and downfold follow each other in rapid succession, 
with their axial planes nearly vertical and sometimes reversed. 
The packing of the folds has gone so far in places that an isoclinal 
structure has been produced. The height of the folds does not 
seem to be great, and sometimes the bottom of one fold and some- 
times the top of another are found at the surface, dissected for us 
by the deep valleys. 

As the axial area of the synclinorium is approached, flatter dips 
become the rule, although in places sharp buckling disturbs the 
gentler folds. This is well seen near the gorge of the Dee at 


214: DR. WILLS AND MR. SMITH ON THE LOWER  [ vol. Ixxvil, 


Berwyn, on a large scale; while a miniature reproduction of the 


sharp buckle superimposed on a gently pitching fold may be — 


studied on a few square yards of rock laid bare in the bed of the 
Dee when the river is low, at a point a quarter of a mile below 
the ‘ Chain Bridge.’ 

It is natural to expect a considerable amount of faulting and 
thrusting to accompany this concertina-folding; but, although 
there is doubtless far more strike-faulting than has been discovered, 
it is probable that the materials were extremely plastic at the time 
of the movement. In some cases it may be seen in quarry- 
sections that the beds can pass from a horizontal into a vertical 
position within a few yards by folding.1 Yet we feel that the 
sections in Pl. IV would more nearly conform to the truth if they 
showed more faulting in the steep limbs of the folds. The faults 
that are shown were either observed in the field, or else were 
inferred with considerable certainty from a as) of the strata 
mapped. 

A small area round Moel-y-faen may be chosen to illustrate the 
structure of the whole northern limb of the synclinorium. The 
map (Pl. III) shows that the surface-expression of the concertina- 
folding is, at first sight, puzzling in the field, owing to the 
singularly rapid changes in the amount and direction of dip that is 
observable. But, when carefully studied, the variability resolves 
itself thus :—Belts of country varying in width from 100 yards 
to about half-a-mile, and often with nearly parallel boundaries, run 
across country in a direction 10° to 20° south of east. In alter- 
native belts very steep dips (stippled on the map) and very gentle 
dips (plain) are found. Within the belts of steep dip the strike 
is consistently east and west, while in the belts of low dip the 
strike varies, but the dip usually has an easterly component. The 
former represent the steep flanks of the tightly-packed and 
sometimes isoclinal folds and overfolds, the latter their crests and 
troughs. In the belts of low dip, the easterly component is im- 
parted by the pitch (partly original and partly due to a general 
tilt of the whole region that finds expression in the eastward dip of 
the Carboniferous). In the steep flanks of the folds this eastward 
pitch is masked, although it accounts for the difference between 
the strike of the individual beds (east and west) and the run 
of the belt of high dip (east 10° to 20° south). 

It is perhaps unnecessary to describe in detail the map 
(Pl. ITI). It should be read in conjunction with the sections 
in Pl. IV (8rd, 4th, & 5th from the top). Nor is it proposed to 
describe the rest of the complex limb of the Llangollen Syn- 
clinorium, which is sufficiently illustrated by Pls. IV & V. with the 


1 The plasticity of the rocks as a whole has found expression in a variety 
of minor structures, such as slickensides and grooving along the bedding- 
planes, movement-surfaces between cleaved shale and almost uncleaved silty 
sandstone, internal contortion of sandstone-bands, the drawing-out of sand- 
stone-bands into sausage-like lenticles, etc. 


part 2] PALHOZOIC ROCKS OF THE LLANGOLLEN DISTRIcT. 215 


exception of the Caer-Drewyn area in the angle between the 
Bryneglwys and Llangollen Faults. 


Caer-Drewyn Hill forms part of an area shaped like an isosceles 
triangle, lying between the converging Bryneglwys and Llangollen 
Faults which form its two long sides, and the Rhagatt-Hall Fault, 
which forms its base (fig. 4, p. 216). It occupies a position 
analogous to that of the patch of Tarannon shale lying between 
the converging Llanelidan and Bryneglwys Faults, south-east of 
Llandegla (p. 219); but, owing to the different rocks and 
movements involved, the resulting structure is somewhat different. 

The rocks in the apex of the triangle and along the southern 
(or Llangollen Fault) side are concealed by drift and alluvium, the 
exposed rocks consisting of Valentian Slates and Pen-y-glog Slates 
and Grits. It seems probable that the rocks thrown down by the 
Bryneglwys Fault on its northern side also had a horizontal move- 
ment south-westwards; while those south of the Llangollen Fault 
(also throwing down north) travelled eastwards. The stresses 
involved squeezed the rocks in the triangle, and set up a movement 
of revolution in an anti-clockwise direction. The resistance of the 
tough Wenlock grits to the pressure apparently caused them to 
buckle into a syncline, and finally to break and to be overridden, 
except along the base of the triangle, by the underlying shales. 
The boundary between the grit and the shales is everywhere shown 
by the mapping to be a fault.! 

Simultaneously with the squeeze came the attempt at revolution 
which caused the rocks to fracture by transverse faults. The 
thrusting appears to have taken place to a greater or smaller 
degree in the strips between these faults, at some points bringing 
the Lower Wenlock Shales, at others different horizons of the 
Tarannon Shales, against the Grits. 


South of the Llangollen Fault, the adjustment-faults and the 
sumple pitching synclinal structure have already been noted; but 
the western termination of the fold is also of interest, and may 
now be described. 


Corwen Buttress.—The syncline passes westwards into a 
projection of the Berwyn anticlinorium that may be appropriately 
termed the Corwen Buttress. It occupies the high ground 
west of the Bonwm Fault and south-east of the Bryneglwys Fault, 
while the Llangollen Fault forms a convenient northern limit. It 
is an anticlinal spur separating the Central Wales (Llandderfel) 
Syncline from the Llangollen Synclinorium, 

The whole of the ground has not been thoroughly examined ; 
but it appears probable that it is occupied by shallow rippling 
folds which give a very broad outcrop to the Ashgillian and Lower 
Valentian beds that form it. The outcrop of the Corwen Grit on 


1 The interpretation of this fault as a thrust furnishes the only reasonable 
explanation of the geological map. 


MAP OF 


CAER-DREW YN bere ee 
N By B.SMITH Pa) ae CLAY os 


4 - s - Cikeurye 
ae 
% ay o aces saiaee 
43-5) YH I Peas | 1g. ov py 
Oey ee Ns es a . E 4 or ges 
GEES Pitre Nenee sale 
MPT Nooo Rose ees c= 
ae ee = 
| AF 
Ue oe 
eS ‘ 


EE ae eee 


“X~ BOULDER OLAY 


Alluvium 
eal Boulder Clay 


Ludlow Shales 


.| Wenlock Grit 


= 


Llandovery 
Shales & Grits 


Corwen Grit 


Hl 
es 
\N Patti Beas, 


7——_ dip of Cleavage 


Supposed : 
A Tie CAER Seale of Miles 
; B CORWEN 


at Supposed 
Bryneglwy: DREWYN | pirust BUTTRESS 


Fault H Ri 
: ; | River 
rits and Shales b  ayss ae 
G CEO LE 1 ! Liangollen Grit > 
Ao o ote x 2 
Te, BS HOUR ae 1 1 Fault Dolhir 
pdovery Beds IN 
re = 
eS 
: ' moe 


Section along the line A.B.C. 


part 2] LOWER PALHOZOIC OF THE LLANGOLLEN DISTRICT. 217 


the map (Pl. V) brings out this rippling structure which, as it 
approaches the Dee Valley, becomes very much complicated by 
faulting (fig. 4), as was shown many years ago by Dr. Groom & 
Mr. Lake.! Of the faults, the east-and-west faults are now seen to 
be parallel to the Llangollen Fault and probably part of it, while 
the others may be grouped with the north-west or north-and-south 
adjustment-faults.? 

The buttress doubtless operated to some extent as a resistant 
mass, comparable in its effects to the Cricor and Cyrn-y-Brain 
domes, and like them it has been truncated by the master-faults. 
where it merges into the adjoining synclinal structures. 


Berwyn Nodal Anticline.—The northern fringe only of 
the Berwyn Anticline falls within the scope of this paper. The 
region to be described may be said to be introduced by the Glyn- 
Ceiriog Fault to which reference has already been made (p. 218). 
The fault throws down northwards, and may also represent a west- 
ward movement on its north side; but its exact position and 
amount of displacement is not determinable with certainty. In 
the Moel-Fferna region it can be fixed fairly closely by reference 
to the easily recognizable grits and pale slates; but it is rather 
inferred than proved between the Moel-Fferna Slate-Mine and 
Glyn-Ceiriog, although from place to place signs of faulting have 
been noted. In the Glyn district it may have been touched in 
the Wynne Slate-Quarries, and its course down the valley to 
Liwyn-mawr is obviously implied by the abrupt termination of the 
Pen-y-glog Slates, the Valentian, and the Glyn Grit, on the south 
side, and the introduction of the Glyn-Dyfrdwy stage on the north 
side of the valley. This was not appreciated by Dr. Groom and 
Mr. Lake, and appears to nullify some of their arguments for the 
movements postulated for the Cae-mor Fault.? 

Near Dol-y-wern the Glyn-Ceiriog Fault probably divides into 
three branches, the westernmost of which is the Cae-mor Fault 
of Groom & Lake. It is not certain by any means which -of 
the three lines forms the true continuation of the Glyn-Ceiriog 
Fault towards the south-east. If it be the Cae-mor, there is an 
abrupt change in strike of the fault ; and it is more likely that the 
middle branch which runs into the greatly disturbed region west of 
Craignant was in pre-Carboniferous times the most important 
structural fault. On the other hand, the northernmost branch 
swings round into a fault-line of great importance in post-Carboni- 
ferous times, which passes across the coalfield at Chirk. The 
ground east and south-east of Dol-y-wern is complex, and ob- 
scured by Glacial drift. No attempt at finality can be claimed 
in the solution of the faulting hereabouts, and therefore we do not 


1 Q. J. G.S. vol. xlix (1893) pp. 426-39. 

2 The part of the map (fig. 4) neighbouring Corwen incorporates that of 
Groom & Lake with modifications in mapping by the present authors. 

3 Q. J.G.S. vol. lxiv (1908) pp. 585-87. They spell it ‘ Cae-mawy.’ 


218 DR. WILLS AND MR. SMITH ON THE LOWER  [vol. Ixxviu, 


propose to criticize or discuss the nature of the Cae-mor Fault as 
described by Dr. Groom & Mr. Lake, beyond claiming that it does 
not pass northwards of Llwyn-mawr, for the mapping of the rocks 
on the valley-sides north of Dol-y-wern precludes the possibility 
of this. 

South of the Glyn-Ceiriog Fault the same simple structure noted 
between it and the Llangollen Fault persists. The gentle north- 
ward dip steepens somewhat, it is true, and a greater number of 
adjustment-faults can be proved in the older rocks, which are 
possessed of more distinguishing qualities than is the Lower Ludlow 
Series. The faults range from almost north-and-south in the 
west to north-west and even west-north-west in the east. 

The simplicity of the structure appears to be disturbed in the 
eastern part of the area by the Dolhir strike-fault at the base of 
the Ashgillian. This fault has already been discussed on p. 189. 
It may be noted, however, that it has the same general strike as 
the Llangollen and Glyn-Ceiriog Faults, and is a thrust.’ 

Probably other strike-faults occur in this area, but none of any 
magnitude have been definitely proved. The thinness of the 
Valentian at the outcrop near Glyn-Ceiriog perhaps may be due to 
such a fault (p. 199). 


Caledonoid Structures. 


All the structures so far considered are related to folding on 
almost east-and-west axes. There are, however, two south-west 
and north-east (‘ Caledonoid,’ as the late Prof. Charles Lapworth 
would have called them) features of prime importance to the 
district. 


(2) Central Wales or Llandderfel Syncline.—The first 
may be called the ‘Llandderfel,’ or, better, the ‘Central Wales 
Syncline,’ for it is a continuation of the downfold to which Prof. 
O. T. Jones! gave the latter name farther south. The syncline 
lies for the most part outside the area under description. The 
Dee Valley follows it from Llandderfel to Corwen. Its influence 
is seen in the westward pitch of the western end of the Berwyn 
Dome. The downfold cannot be definitely traeed north-east of 
Corwen. Two explanations of its disappearance seem possible: 
either the fold has been bent on the fulcrum of the Corwen 
Buttress into an east-and-west structure (the Llangollen Syneli- 
norium); or the synclinal tendency has found expression in the 
westward downthrow of the Bryneglwys Fault between Corwen 
and Llandegla, and in the synclinal arrangement of the strata 
between Mynydd-Cricor and Cyrn-y-Brain (pp. 219, 220). 


(6) Bryneglwys Fault.—The second great ‘Caledonoid’ 


1¢The Geological Structure of Central Wales & the Adjoining Regions’ 
Q. J.G.S. vol. lxviii (1912) pp. 328-44, 


- aa 


part 2] PALHOZOIC ROCKS OF THE LLANGOLLEN DisTRicT. 219 


structure is the fault which ranges obliquely across the Central 
Wales Syncline from Llandderfel to Corwen, and thence (as shown 
in Pl. V) to Llandegla. The part lying within the district described 
in this paper has been named the Bryneglwys Fault, although, 
according to the Old Series maps, it is merely part of the great 
Bala Fault. Until this interpretation has been confirmed, a local 
name appears preferable. At Llandegla it joins the Llanelidan 
Fault, and the two pass as one across the northern side of 
Cyrn-y-Brain, and so into the Carboniferous country on the 
north-east. . 

The Bryneglwys Fault is in reality a plexus of subparallel 
fractures that enclose lenticular strips of various rocks. The 
nature of the fault can be best studied near Llandegla, and is 
illustrated in fig. 5 (p. 220). Here we find in the angle between 
the converging faults a triangular patch of Tarannon Shales. 
Probably the rocks north of the Llanelidan Fault moved west- 
wards, and those south of the Bryneglwys Fault eastwards, relative 
to the block in between. ‘The Tarannon Shales, squeezed between 
the two faults, have been tilted towards the Bryneglwys Fault, and 
thrust westwards over the lowest Wenlock zones, which either dip 
towards, or strike against, the junction. Farther west and south- 
west these Wenlock rocks form a (probably convex) syncline, the 
axis of which runs nearly due north and south, but is slightly 
curved so as to be convex westwards. Its axial plane is inclined 
eastwards. In the centre of the syncline Ludlow rocks crop out. 
This area seems to have been subjected to a clockwise movement 
which has twisted round the cleavage nearly 90° from the normal 
direction (see p. 221), so that it now strikes about due north and 
south and dips westwards. 

The triangular patch of Tarannon Shales, which we have just 
shown to be bounded by a thrust on the west, is faulted on the 
north by the Llanelidan Fault against a wedge of high-zone 
Carboniferous Limestone, which itself is separated by a secondary 
fracture on its north side from the Cefn-y-fedw Sandstone, south 
of Llandegla. 

The multiple nature of the Bryneglwys Fault-fracture is well 
seen on the south-eastern side of the triangle of Tarannon Shales. 
Here we find strips of Middle or ‘ White’ Limestone and of Cefn- 
y-fedw Cherts dropped down between the above-mentioned Tarannon 
and the Ashgillian of Cyrn-y-Brain. The main branch of the 
Bryneglwys Fault between the Limestone at Craig and the Ash- 
gillian is concealed by Boulder Clay, though indicated by a fine 
series of swallow-holes. 

The south-easternmost branch of the Bryneglwys Fault can be 
traced subparallel to the main fault across the north-western flank 
of Cyrn-y-Brain. It cuts off a corner south-east of Llandegla, 
and produces the shatter-belt along which the Hafod-dafalog 
stream has excavated its deep valley. 

For long distances south-westwards the line of the Bryneglwys 
Fault is masked by Glacial drift that obscures the flat-bottomed 
hollow in which it les. 


220 DR. WILLS AND MR. SMITH ON THE LOWER  [ vol. lxxviul, 


Speaking generally, the apparent aggregate throw of the fault- 
plexus is down to the north-west. Thus it truncates the Corwen 
Buttress, bringing Silurian and Carboniferous against the Ordo- 
vician ; it cuts across the Llangollen Synclinorium so that Jeznt- 
wardinensis beds which only crop out in the heart of the basin 


-BOU LDER CLAY 


AULt/ 


“asSkLAN 


== LID TZ YL YL, 
Z SY GL 


A 


- 


ae 
2 EE SS Alluvium, 
Se 
SZ. ~-5-= = and_ Peat 


LS SE = 40° Carboniferous 
LZ Limestone 


ae: ee ee Wenlock Y 

SIE "Shales. Yj 
° S6 ~~ a oof Hie — ~ 
ies Serr] WN 


\f/Pentreiisal Se 


- = =f XS ZA SS == Strike‘of Cleavage. 
re fe The small stroke indicates Cyrn-y-Brain moe 
LE the dip ofcleavage —~ Raise : as 
~ SS = ZS 
~ —_ — = = : 3 = 
Ss SCALE 
0 100 200 300 440 650 $so Yards 


Section along the line A.B.C. 


near Llangollen, reappear in the Bryneglwys area. Again, it 
shears across the western and northern sides of the Cyrn-y-Brain 
Dome, in such wise that Valentian and Carboniferous rocks le 
against the Ordovician. In the Carboniferous it produces a great 
displacement westwards on its northern side. 


part 2] PALMOZOIC ROCKS OF THE LLANGOLLEN DIsTRIcT. 221 


The exact nature of the movement which produced these apparent 
displacements is not quite clear. Where the Bryneglwys and 
Llanelidan Faults become one, there seems evidence of a southward 
crushing from the north, producing a sort of thrust, but not 
necessarily an overthrust; for, concurrently with this, there was 
relative movement of the block of country south of the fault in 
an eastward direction. A similar lateral movement doubtless 
occurred along the Bryneglwys Fault south-west of Llandegla. 

Despite its Caledonian trend, the Bryneglwys Fault owes its 
conspicuous displacement to post-Carboniterous movements ; and 
it is by no means easy to decide the function which it played in 
the Devonian dislocations. There is no doubt, however, that it 
was even then one of the master-faults of the region. 


Cleavage. 


The Devonian movements subjected all the Lower Paleozoic 
rocks of this district to forces that produced cleavage in them, in 
greater or less degree according to their nature and their position 
in relation to the folds. 

Over the greater part of the area, there is a marked agreement 
in the strike of the cleavage with the strike of the folds and 
buckling, so that a direction between east 10° and 20° south is 
normal. The dip is northwards and, except south of the Llan- 
gollen Fault where it is consistently about 30°, it may be said 
elsewhere to average 60° to 70°, although occasionally it is as low 
as 80° or as high as 85°. The northward dip suggests that the 
cleavage-forces (and therefore also the forces producing the folding) 
acted from north to south. The movement was, in our opinion, a 
superficial southward displacement relative to a more powerful and 
deeper-seated northward or northwestward Caledonian movement 
in areas not far removed from, or even underlying, this tract of 
country. The presence of a thorough, though low-dipping cleavage 
in the very gently folded rocks south of the Llangollen Fault and on 
the flanks of the Berwyn Anticline may, perhaps, be cited in favour 
of such an origin for the cleavage-forces. Be this as it may, the 
occurrence of good roofing-slates in beds in which the true dip 
is 10° to 15° and the cleavage-dip about 30°, both northwards, is 
remarkable and difficult to explain. Yet these conditions hold 
throughout the outcrop of the Pen-y-glog Slates from Carrog to 
Glyn-Ceiriog. 

Post-cleavage faulting has upset the general direction of cleav- 
age-strike and dip in a pronounced manner in the area between the 
Llanelidan and Bryneglwys Faults, eastwards from Llanelidan and 
Bryneglwys; in many cases the strike has been rotated from east- 
and-west to about north-and-south, so that the dip is now west- 
wards. In such cases the true dip and strike of the folded rocks 
have undergone a similar alteration, and it is obvious that the 
whole block defined by the faults has rotated, probably in post- 
Carboniferous times. This is well illustrated in fig. 5. 


222 DR. WILLS AND MR. SMITH ON THE LOWER [| vol. Ixxviii, 


The cleavage movements have produced a variety of minor 
structures of interest, which space forbids us to describe here. It 
may, however, be pointed out that these structures usually occur 
where hard and soft bands are interbedded. It is characteristic 
of this district that the effect of the pressure has usually been to 
break up the harder bands into phacoidal fragments, rather than 
to produce contortions of these bands as a whole. It is by the 
shuffling of the phacoidal fragments that the necessary adjustment 
appears to have been brought about. 


V. THE PRE-CARBONIFEROUS FLOOR. 


The folding, faulting, and cleavage so far described formed part 
of the Devonian mountain-building. Vigorous erosion accompanied 
and followed the elevation, and eventually produced a peneplain. 
But we can still see in the form of the floor upon which the 
Carboniferous rests, the influence of the major tectonic features on 
the then-surface relief ; for the anticlinal areas tended to stand out 
as hills, and the synclinal areas as depressions. On the line of the 
Llangollen Synclinorium the red Basement Beds occur, followed by 
the maximum thickness of Carboniferous Limestone, suggesting 
that here was a bay opening eastwards. The successive overlap of 
higher and higher beds against the anticlinal area of Cyrn-y-Brain 
can be clearly seen on the north (see p. 210), while southwards 
towards the Berwyn Dome the thinning of the Limestone is even 
more rapid. The steeper submarine contours on this side of the 
ancient bay may have been related to the Llangollen Fault. 

It has already been pointed out, in passing, that much of the 
movement along the Llanelidan Fault took place in pre-Carboni- — 
ferous times. ‘This may be argued from the fact that the Lime- 
stone lapping round the northern slope of the Cyrn-y-Brain and 
Cricor Domes transgresses rapidly from the Ordovician on to the 
higher part of the Lower Ludlow rocks. 

In a similar way the assumption of movement along the Bryn- 
eolwys Fault in pre-Carboniferous times helps to explain the 
occurrence of the Carboniferous Limestone at Corwen. Were it 
not for this possibility, the amount of movement required to 
introduce it there, by faulting, would be stupendous. 


VI. Post-DrEvontan MopIFICATIONS. 


The discussion of the post-Devonian changes is difficult, because 
most of the evidence lies beyond the limits of the Lower Palzeozoic 
outcrops, in the tectonics of the newer rocks on the east. 

All the post-Devonian movements appear to be explicable on the 
assumption of the continuous application of stresses, of varying 
intensity, which found expression in (a) widespread warping, in- 
fluenced by the major folds in the older series; and (4) renewed 
movement along already-established fault-lines. 

Although the folding or warping had little effect on the Lower 
Paleozoic substratum, the faulting that accompanied it produced 


Quart. Journ. Geol. Soc. Vol. LXXVII, Pl. II. 


© 100 200 300 400 500 Yards 


man ye 


ae yas oe 


A 7 


Cc. 


Leth 


\: 0 : \ aN (x /K NS 


}FAEN, ILLUSTRATING THE STRUCTURE OF 


THE 


HE LLANGOLLEN SYNCLINORIUM. 


Quart. Journ. Geol. Soc. Vol. LXXVIM, Pl, III. 


© 100 200 300 400 500 Yards _ 


iy 


Li 


SAAN 
A 


sh 


‘i 


s 33 2 a 
ig 23 ge Oeil n a oo 4 
bane) On oo Bis eae ae ers ins % m 2 go 
J BE DVD GSA, 8,5 bY = ro) ° Ens; 
o > oo ° o oT 4 %u0 a faa] o ss 2 = 20 
Ws AT Min sa ww Ontat mney oe = a 
(3) ie) as eo SHetTs wv 09) 3 = Tew 
As SH #: & 85 5nSS5q 38 si uw ic 358 
Q GY Gh © HOR aA SH = © 4 RiGee 
A} Se — 58 a 2 Ses 
AK 1 ys Hw o ‘ Chen 
=i Ae re eS UU SMM ells ae 
» é oo2 
mS] cv al a ie 3 S S's 
= ae [| a S ee 
a Ow 5° J yo 


Lee 


\ 


Zz 


OOS 


Marv or tue AREA ROUND MOEL-Y-FAEN, ILLUSTRATING THE STRUCTURE OF THEY 


1 
' 
t 
| 


Norraern Live or tHE LLANGOLLEN SYNCLINORIUM.— 


Quart. Journ. Geol. Soc. Vol. LEXVIL, PUIV. © 


H SERIAL SECTIONS 
ROSS THE 


SYNCLINORIUM 
Pe. WILLS. 
DRAWN AT EVERY MINUTE OF LATITUDE. 
GOLLEN, G.F.GLYN-CEIRIOG FAULTS. 
AL & VERTICAL SCALE. 

Yo (MILE 


! 


LLANGOLLEN 
L.F. 


5 ay aommey 


F|_<meeiamaneee re aie 


| SS - | SS 
a eee at 


LL. 

= FZ 
| 

BR nS A 


| RSS MZ, ‘ 
LF a eee |_O 
faa ai eat 


FFERNA 
# QUARRY 


= ae Ls jf Rea 


ee ee 
=e 


Quart. Journ. Geol. § 
NORTH-SOUTH SERIAL SECTIONS 
ACROSS THE 


LLANGOLLEN SYNCLINORIUM 


BY ob. od Wile eS 
THE VERTICAL LINES ARE DRAWN AT EVERY MINUTE OF LATITUDE. 
B.F. BRYNEGLWYS,L.F. LLANGOLLEN, G.F. GLYN-CEIRIOG FAULTS. 


HORIZONTAL & VERTICAL SCALE. 


Qo Ye IMILE 2) 
= 
1 E. 
63,360 : _— 
WS OR = LLANGOLLEN 


eae 


Spee Pare 
STE oe v 
tte, vitee ¥ VULV ETN Y NEN \| y oo 
Dy as 
7 Iw ese porno =e —e) 


ye | santana Wy, <S bd ae e or 
OOS AANA aa ha 7 ess 6 —o—=6 - as | A Se 
SOOO Wh vi ih ! eee = =— | mri i 
C Sa oe ree = 
Be wi, 


oF TERRA 
TV MO OO ON VEY iN 


GLYN-DY) FROWY 


LA ane 
| See 


INDEX. 

CARBONIFEROUS, 

I CARBONIFEROUS 
LIMESTONE. 


SY BASEMENT BEDS. 
SILURIAN, 


LUDLOW SERIES. 
=| SLAB BEDS. 


” 
a 
a 
> 
a 
€ 
PENTRE-DWER SLATES : 
3 


WENLOCK SERIES. 
PEN-Y-GLOG SLATES. 

Rae VALENTIAN SERIES. 

[es | 

Ly =e] ASHGILLIAN SERIES. 


<—TESs 


a 


ee 


BA sine coms a; 
é rf Ny 


bf 
y 


ey Saab es) Site M 


s 


DF te ggaa vice toe a si 
PE Me) REL Siar a, py asa ; SNS, 
VeLHNM ON hime hp 
alba Fivbigasin pining , : 


VL 


SICAL SKETCH—MAP 


Geol. Soc.Vol. LXAXVH, Pt .W 


OF THE 


___ Quart. Jou .Geot. Soc.Vol. LXXVII, Pl . V7 


VALE OF | GEOLOGICAL SKETCH—MAP 
OF THE 
e COUNTRY arounp LLANGOLLEN 
Ae) BY 
ee oo L.J-Wills and B. Smith. 
) 


ye INDEX 


3s 

N 

Ls) 
r 


WNantelyydh Sta, 


oe 
Ce 
Laine ro rauL 
—) 


3 
ar 


S 

® 

® 
— 


——= 
S 
: 
d 
& 
3 
: 
S 


<s 


"Ssayiggs MOTANT 8#IMOT 


THE SUBDIVISIONS OF THE 
WENLOCK & LUDLOW SERIES 
ARE NOT INDICATED NORTH 
OF THE BRYNEGLWYS FAULT. 


‘Sdzg AMGYIAG-NATI 


Gwyddelwern 


as] 
g 
s 
S 
: SS 
NOLLNSTWA “IOV 


Glyre-Corwer or i . 
Plas-uchaf Grit. 815 
Dothir & Cyrn-y-brain Beds _ |\t\O 
tneluding the Tyin-y-twmpath BY) |< 
Blaen -y-cwm Beds. 8 0 
Bryn and Teirw Beds 8 > 
irclut the Ashes. g 2 

Scale, 6560 = Tinch to Imile a 

0 7 2miles 


Nay Sete ideas ———— 
nr Oe ss 


peter See rte ce ae» hilar eae Halu Aeneid Ay aera | pn ee, beh Dpeie recmten etre 
ne van teaeres ‘ue teil i ee. eT 


Sdaceuka ee 
5 % teal 
7) 


part 2] PALMOZOIC ROCKS OF THE LLANGOLLEN DISTRICT. 228 


eveat changes in its level, and by lateral movement, great changes 
in the relative position of the blocks of country into which it was 
already divided. These changes are to some extent self-evident 
from the map. Without going into the detailed structure of the 
Carboniferous, we can make the following statements :— 


(1) All the master-faults (see Pl. V) moved again, aitd the 
fault-lines were propagated through the Carboniferous 
cover as major faults. | 

(2) The adjustment-faults which adjoin the Carboniferous can 
be shown to have moved, and by inference, therefore, all 
the adjustment-faults probably did so. 

(3) Many of the adjustment-faults are curved, both in the older 
rocks (see Pl. V) and in the Carboniferous, and their 
peculiarities can be best explained by the assumption that 
they result from the torsion of the region. 

(4) The down-sinking of the Vale-of-Clwyd graben along the 
Vale-of-Clwyd and Llanelidan Faults shows how a major 
synclinal tendency was expressed at this time by faulting 
rather than by folding. 


EXPLANATION OF PLATES III-V. 
PLate III. 


Map of area round Moel-y-faen, illustrating the structure of the northern 
limb of the Llangollen Synclinorium (see p. 214), on the approximate 
scale of 1 inch to 666 yards, or 1 : 24,000. 


PuatTE LV. 


North-to-south serial sections across the Llangollen Synclinorium, on the 
seale of 1 inch to the mile, or 1 : 63,360. (See pp. 212, 214.) 


PLATE V. 


Geological sketch-map of the country around Llangollen, on the scale of 
1 inch to the mile, or 1: 63,360. [The distinctive colour-bars have been 
accidentally omitted from the narrow strip of Corwen Grit, south of 
Bonwm. | 


DISCUSSION. 


The Secretary read the following letter received from Mr. 
BERNARD SMITH :— 


‘It should be pointed out that Dr. Wills had actually commenced to map 
the Ordovician and Valentian rocks of Cyrn-y-Brain, in his spare time, before 
it was decided to map the ground officially. He was therefore forestalled by 
the Geological Survey ; and it was my fortune to have allotted to me certain 
areas that he had planned to examine himself. Hence the collaboration that 
has given rise to this paper. The area covered by the latter, however, is 
much greater than Dr. Wills had contemplated mapping by himself, and the 
combined experience of two, if not three, surveyors—gained not only in this 
area, but in a more extensive field upon three sides of it—has been utilized. 

‘The structures now described may be taken as typical of what has 
happened in the district: they are, however, but a portion of a more wide- 
spread system in which a large part of North Wales is involved. It had 
been hoped that another paper written by Mr. C. B. Wedd and myself, would 
have been ready for communication to the Society at about this date, but that 


224 THE LOWER PALHOZOIC ROCKS [ vol. xxviii, 


paper has not yet been completely written. In it an attempt is made to co- 
ordinate the earth-movements and to explain their relations one to the other, 
both in place and in time. The present paper describes the results of some 
of these movements ; the future paper casts its net wider, and also attempts 
to give reasons for the movements. Those movements that are mentioned in 
Dr. Wills’s and my paper can be more clearly demonstrated in the larger area 
examined. 

‘If there is one point that requires emphasis at this stage it is this—that 
the cleavage-planes induced by the Caledonian directional movements, in the 
ground least disturbed by later faulting and torsional movement, dip invari- 
ably northwards. This points to a relatively greater northward movement in 
depth than at the surface.’ 

Mr. C. B. WEpD congratulated the Authors, with whom he had 
himself long been associated in North Wales, on their success 
in unravelling a complex and difficult piece of stratigraphy, and in 
producing a very fine map. Dr. Wills had referred to certain 
joint work on the tectonic structure of a larger district, by his 
colleague Mr. B. Smith and the present speaker, who regretted 
that that work was-not yet ready for presentation to the Society. 

The Authors had been handicapped in their account of the 
structure of the district by reluctance to use unpublished facts, 
of which they had knowledge, in regard to neighbouring ground 
outside their area. For example, the Minera fault-belt was 
essential to an understanding of the movements that had affected 
the Llangollen basin. That fault-belt, lying a little farther east, 
was a Se dislocation, extending with Tee convexity of 
curve from the combined Llanelidan aad Bryneglwys Fault in the 
north to the Llangollen Fault in the south. It showed how the 
Llangollen Syncline, with the adjacent anticline north of it, had 
been torn away from the eastern tract and slewed southwards by a 
rotary movement to which the Authors had alluded. That fault- 
belt, proved by mining in the southern part of its course, where 
such a movement should afford relief from pressure, consisted of a 
‘horst’-like arrangement of opposite throws, individually large, 
but with no appreciable net result in vertical displacement, the 
main movement having been chiefly in a horizontal direction. 

The speaker had called Dr. Wills’s attention to the significance 
of an apparent displacement of the trough of the Llangollen 
Syncline beneath the Carboniferous rocks by the agency of the 
Aqueduct Fault, as shown by that Author’s elaborate sections 
across the valley. 

Prof. O. T. Jones congratulated the Authors on the completion 
of an admirable piece of stratigraphical work in a region of great 
difficulty. He enquired whether it was still believed that there is a 
break in the succession between the Caradocian and the Ashgillian; 
if not, he would be inclined to suggest, in view of the developments 
farther south, that the Blaen-y-cewm Beds will probably be found 
to belong to the Dicellograptus-complanatus Zone or to the 
D.-anceps Zone—in other words, to the Upper Hartfell Group. 

The correlation of the shelly and graptolitic facies of the Bala 
was still far from satisfactory, and, in view of the undoubted 


part 2] OF THE LLANGOLLEN DISTRICT. 225 


Ashgillian affinities of the fauna in the succeeding beds, the 
determination of the precise horizon of the shales would probably 
throw some light upon the relation of the Ashgillian to the 
Hartfell Group. A curious feature of the Upper Ashgillian fauna 
was the association of Meristina crassa, formerly regarded as a 
distinctive Lower Llandovery fossil, with undoubted Ordovician 
trilobites, such as Chasmops. In Britain such an association was 
apparently found only in this district, and it might help towards 
the comparison of the strata near the Ordovician-Silurian boundary 
in Wales and in Norway where Prof. Johan Kier had united the 
Meristina-crassa Beds with the Ordovician. 

The brachiopods of the Valentian to which Dr. Wills had 
referred were unfortunately very poorly preserved, and did not 
afford as much help in the subdivision of the series as one would 
hke. 

The structure of the region was extremely interesting, and was 
quite unusual for Wales. It seemed probable that the deviation 
of the folds from the usual north-east and south-west into the 
east-and-west direction was accounted for by the region being 
beyond the protective influence of the rigid masses of North Wales, 
and that the rocks had therefore been wrenched out of their usual 
direction. It was not, therefore, surprising that the Authors had 
found evidence of considerable torsional structures and lateral 
movement in the district. 

The terms used in describing some of the structural elements 
appeared to have a theoretical implication which was not quite 
elear. Such were the ‘nodal anticline’ of Mynydd-Cricor and 
Cyrn-y-Brain and the ‘Corwen anticlinal buttress.’ He would 
like to know what was implied by the term ‘nodal anticline’ and 
what was its influence upon the structure. The expression 
‘buttress ’ appeared to imply that it had in some manner influenced 
the folding of the adjoining country ; whereas it seemed clear that 
this anticline was the result of the same folding movement as that 
which affected the rest of the country, but that the stresses in the 
eastern region are different from those on the west. 

Dr. T. T. Groom said that he had listened with great interest to 
Dr. Wills’s lucid description of the Authors’ work on the district. 
The structure, at any rate of the southern part of the area, with 
which alone he was familiar, while of much apparent simplicity, 
was in reality far from simple, and the Authors were to be con- 
gratulated on the achievement of a notable piece of work. The 
speaker was gratified by Dr. Wills’s generous appreciation of his 
joint work with Mr. Lake, and by the close agreement with this of 
the Authors’ independent researches. ‘There seemed, indeed, to be 
few points of difference. He would like to ask the Authors 
whether they had been able to prove definitely the identity of the 
Glyn and Corwen Grits, which Mr. Lake and himself had left an 
open question; and to have fuller evidence of a continuity of 
deposition between the Ordovician and the Silurian deposits, since 
some of the facts seemed to be rather in favour of a break. With 


Q. J.G.S. No. 310. Q 


226 LOWER PALEZOZOIC OF LLANGOLLEN DIsTRIcT. [vol. Ixxviii, 


regard to the Bala Series, he welcomed the introduction of a group 
of beds into the gap caused in the Ceiriog valley by the Dolhir 
Fault, which Mr. Lake and himself had only partly succeeded in 
filling. He lastly suggested the importance of recognizing phases 
in such movements as the Caledonian, and remarked that the 
evidence that had been given hardly sufficed to prove absolutely 
the ‘ Devonian’ age of the earlier folding and faulting, and did not 
seem to take into consideration the possibility that important 
movements may have taken place in late Silurian or early Car- 
boniferous times. 

Dr. Wits, in replying, expressed his gratitude for the kind way 
in which the Fellows present had received the paper—gratitude in 
which he knew Mr. Smith would join. Many points had been 
raised in the discussion, and he could only attempt to answer 
some of them. Miss Elles’s determination of the graptolites from 
the Blaen-y-Cwm Beds—which were very distorted—suggested a 
zone even lower than that of Pleurograptus linearis. She was, 
he believed, re-examining them in the light of better-preserved 
material from a similar stratigraphical horizon in the Southern 
Berwyns—material which appeared to indicate the Swedish zone 
of Diplograptus pristis. The Authors thought that in the 
western part of the Northern Berwyn outcrop, the sequence from 
Caradocian to Ashgillian was probably complete; but there was 
not enough paleontological evidence to prove this conclusively. 

The term ‘nodal anticline’ referred to the supposed function of 
the anticlinal cores of Ordovician, as hard knots relative to the 
less-resistant Silurian strata; the term ‘anticlinal buttress’ had 
been suggested for the projection of the Berwyn Anticline, which 
was necessitated by the sharp difference in strike in the Central- 
Wales Syncline and in the Llangollen Synclinorium, and by the 
pitch of the latter. 

The Authors had traced the Corwen Grit from Corwen to Glyn- 
Ceiriog, and found that near Glyn the massive uncleaved Corwen 
Grit overlies part of the Glyn Grit; but they regarded it as a 
lithological variation of the upper part of the Glyn Grit of Glyn- 
Ceiriog. 


part 2 | THE AVONIAN OF BROADFIELD DOWN. 227 


5. The CaRBONIFEROUS LIMESTONE (AvoNTAN) 0f BROADFIELD 
Down (Somerset). By FREDERICK StRETTON WALLIS, 
M.Se., F.G.S. (Read January 18th, 1922.) 


[ Abstract. | 


THe greatly denuded periclinal uplift of Broadfield Down is. 
mapped and described in terms of Vaughan’s system of zonal 
classification of the Avonian. All zones, with the exception of K, 
are present, and subzones Z, and D,, hitherto unrecorded from this. 
area, are proved to occur. 

A well-marked faunal assemblage (‘ Fossiliferous Level’), of no 
great vertical extent, is described from the top of S,, and is shown 
to constitute in this area a datum-line, useful in the field, for the 
determination of the junction between the S, and 8, subzones. 

Pustula elegans (M‘Coy) is here for the first time recorded 
from the S, subzone in the eastern part of the South-Western 
Province. | 

Both lthologically and paleontologically the area holds an inter- 
mediate position, and forms a link between the developments of 
the Bristol and the Mendip areas. Thus C, of Broadfield Down 
is composed of fossiliferous, massive, grey limestones similar to 
that of C, of the Mendip area, while the lower part of C, is directly 
comparable with the Laminosa Dolomites of the Bristol area. 

The non-oceurrence of igneous rocks at the head of Goblin 
Coombe, where normally they would be expected to occur, is 
explained by a system of thrust-faulting from the South. 


DISCUSSION. 


Prof. S. H. Reyvorps referred to the fact that, almost wherever 
detailed mapping of the Carboniferous Limestone of the Bristol 
district was undertaken, evidence of reversed faulting of pre-Triassic 
date was obtained. By considering certain other faults of Goblin 
Combe to be reversed, the Author had provided an adequate explana- 
tion of the non-occurrence of the igneous horizon at certain spots 
where evidence of its presence would have been expected. 

Mr. HK. EH. L. Drxon remarked that, thanks to the inspiring 
work of Vaughan and the energy of Prof. Reynolds, the Bristol 
school of geology, and other workers, a clear insight into the geo- 
graphy of Lower Carboniferous time was being rapidly gained. 
He was glad to see this important link between Bristol on the 
north and the Mendips on the south so thoroughly described. 
_ After asking for details of the lithological characters of Horizon y, 
he mentioned that the change in the facies of C, and C, as one 
goes south from Bristol could be paralleled in Gower and Pem- 
brokeshire. The thickening and also the ultimate disappearance 
of the Caninia Oolite were results of the southward deepening of 


228 THE AVONIAN OF BROADFIELD DOWN. __ [ vol. Ixxvil. 


the Lower Carboniferous sea. The disappearance of the lagoon- 
phase in C,, so well-developed in the Avon Section, was due to the 
same cause. He asked whether anything further had been learnt 
as to the presence of sandstones and grits in the D Zone. These 
became increasingly important, and entered at progressively lower 
horizons of the Upper Avonian north of Bristol, until in the Forest 
of Dean, as Principal T. F. Sibly had shown, and on Titterstone- 
Clee Hill, they constituted practically the whole of that formation . 
which, in consequence, had in those two districts the characters of 
Millstone Grit, and as such had been described. 'The northward 
increase of sand and pebbles indicated that the Bristol district 
lay in Upper Avonian times off the mouth of a southward flowing 
river. 

The AvurHor thanked the Fellows for their kind reception of his 
paper, and, in reply to Mr. Dixon, said that Horizon y was well 
defined in Broadfield Down, and consisted entirely of limestone of 
the ‘petit granit’ type. ‘The exposures in the D zone were scanty 
and poor, and included no arenaceous type of sediment such as 
occurred at Wickwar (12 miles north of Bristol), and to a less 
degree in the Avon section. 

The pre-Triassic landscape was being revealed by erosion in 
all the Carboniferous-Limestone uplifts of the Bristol district. 
Many of the old pre-Triassic valleys were still filled up with 
Dolomitic Conglomerate. 


(July 4th, 1922.] 


JAMES SWIFT & SON L®. 


MAKERS OF THE HIGHEST GRADE 
MICROSCOPES AND ACCESSORIES. 


Contractors to all Scientific Departments of 
H.M. Government. 


PETROLOGICAL 
MICROSCOPES 


‘*Primex,”’ ** Advanced Student,’’ ‘* Petros,”’ 
“Survey” and “Dick”? models for all classes 
of Petrological and Mineralogical Research, 


For specifications and prices see LIST OF NEW 
eescyanees MODELS (Petro 1922) post free. 


University Optical Works 
81 TOTTENHAM COURT ROAD, LONDON, W.1 


Publications to be had of the Geological Society, 
Burlington House. 


QUARTERLY JOURNAL (Vols. III to LXXVII, inclusive), Price to 
Fellows, £1 0s. 0d. each: £1 5s. 0d. in cloth. [Postage 1s.] Separate parts (2, 5, 
7, 9, 55, & 273 are out of print) 7s. 6d. each: price to Fellows 5s. [Postage 5d.] 


GENERAL INDEX TO THE FIRST FIFTY VOLUMES OF THE 
QUARTERLY JOURNAL (1845-1894). Part I (A-La). Part II (La~-Z), 
Price 7s. 6d. each. To Fellows 5s. each. [Postage 6d. each. ] 


GEOLOGICAL LITERATURE added to the Geological Society’s Library 
during the years ended December 31st, 1894-1913 and 1920, 1921. Price 5s. each. 
To Fellows, 3s. 9d. each. [Postage 7d.] (Nos. 8 & 14 are out of print.) 


THE HISTORY OF THE GEOLOGICAL SOCIETY OF LONDON, 
by H. B. Woopwarp, F.R.S. Price 3s, net. [Postage 1s. ] 


THE CENTENARY OF THE GEOLOGICAL SOCIETY OF 
LONDON, recorded by W. W. Watts, F.R.S. Price 2s. [Postage 5d.] 


HUTTON’S ‘THEORY OF THE EARTH,’ Vol. III, edited by Sir 
ARCHIBALD GEIKIE, D.C.L.,F.R.S Price 3s. 6d. To Fellows, 2s, [Postage 9d.] 


THE GEOLOGY OF NEW ZEALAND. Translated by Dr. C. F. 
FiscHER from the works of MM. HocustTeTTeR & PETERMANN. With an Atlas of 
Six Maps. Price 10s. net. [Postage 9d. ] 


CONTENTS. 


: Pages 
Proceedings of the Geological Society, Session 1921-22 ...............0c.c0008. lxix-lxxv 
PAPERS READ. 

Page 

2. Mr. E. B. Bailey on the Structure of the South-West Highlands of Scotland 
(Platie Dy vs... sss ate otic: atin: te dec Ces ee ee ee 

3. Miss G. L. Elles on the Bala Country: its Structure and Rock-Succession 
(Plate TE)" Pectin tec ce tees < sc deabig bis sone celt come aae i: oe ee 132 

4. Dr. L. J. Wills & Mr. B. Smith on the Lower Paleozoic Rocks of the 
Llangollen District (Plates TII-V) «2.0... cc.e cases mmaccsn ocsanscts thee 176 
5. Mr, F. S. Wallis on the Avonian of Broadfield Down [Abstract] ............ 227 


[The Editor of the Quarterly Journal is directed to make it known to the Public 
that the Authors alone are responsible for the facts and opinions 
contained in their respective Papers. | 


*,.* The Council request that all communications intended for publication by the 


Society shall be clearly and legibly written on one side of the paper only, with 
proper references, and in all respects in fit condition for being 
at once placed in the Printer’s hands. Unless this is done, it will be in 
the discretion of the Officers to return the communication to the Author for revision. 


The Library at the Apartments of the Society is open every Week Day from Ten 
o’clock until Five, except on Saturdays (when the hours are from 10 to 1), and 
except during the fortnight commencing on the first Monday 
in September, when the Library is closed for the purpose of 
cleaning. 


Fellows are reminded that they are not entitled to receive the 
Quarterly Journal regularly, unless a subscription at the rate of 
10s. per volume is paid in advance. Those Fellows who do not 
require the complete series can obtain separate parts at the price 
of 3s. each, if applied for within one year of the date of issue. 


Vol. LX XVIII. Wo. Sli. 
-Parrt 8. : 


THE 


~ QUARTERLY JOURNAL 


OF THE |. 


GEOLOGICAL SOCIETY 


Se 
oO cyl Ai iis 
fw = 
) Loo 
£ =? 
EDITED BY nl VV 109! 


a 


SEPTEMBER 23rd, 1922. 


: (With Ten Plates, illustrating Papers by Dr. H. H. Thomas, 
Mr. W.G. Shannon, Prof. A.C. Seward & Mr. R. E. Holttum, 
and Dr. C. W. Andrews. | 


LONDON : 
LONGMANS, GREEN, AND CO. 


PARIS :—CHARLES KLINCKSIECK, 11 RUE DE LILLE. 
SOLD ALSO AT THE APARTMENTS OF THE SOCIETY. 


Price Seven Shillings and. Sixpence. 


RAR A RRA AR RRR RRR RAR ARRAS ARRAS 


OP I I NON Oa aa rv verre 


-~ . 
- 2 Z 
4 it ) 
) te i iy . 
& oP 
ARARARAAAR AA ARRRARAARD ODRARAROAD DDD AAARAI SO NAC SNR DIN SPINOR DR PARADES, RRR ADD ene MRR RRR RRR RR = RRR A 
} 7 : @ > 
f 
{ i 
Af : 
> 


LIST OF THE OFFICERS AND COUNCIL OF THE 


GEOLOGICAL SOCIETY OF LONDON. 


RARRAYSH 


Elected | February 17th, 1922. 


Presivent. 
Prof. Albert Charles Seward, Sc.D., F.R.S., F.L.S. 


Gice-Presivents. 


Prof. Edmund Johnston Garwood, M.A..,) 
Se.D., F.R.S. 
Richard Dixon Oldham, F.R.S. 


George Thurland Prior, 
F.R.S. 
Herbert Henry Thomas, M.A., Se.D. 


M.A., D.Se., 


Secretaries. 


Walter Campbell Smith, M.C., M.A. 


Foreign Secretary. 
Sir Archibald Geikie, O.M., K.C. Pe aD .C hs 


LL.D., Se.D., F.R.S. | 


| James Archibald Douglas, M.A., B.Se. 


Creasurer. 
Robert Stansfield Herries, M.A. 


COUNEIL. 


Frederick Noel Asheroft, M.A., F.C.S. 

Francis Arthur Bather, M.A., D.Sc., F.R.S. 

Prof. Perey George Hamnall Boswell, 
0.3.1... DSe. 

Prof. William S. Boulton, D.Sc., 
Roy. Coll. Sci. 

Thomas Crosbee Cantrill, B.Sc. 

James Archibald Douglas, M.A., B.Sc. 

John Smith Flett, O.B.E., M.A., LL.D.,) 
D.Se., M.B., F.R.S. 

Prof. Edmund Johnston Garwood, M.A., 
Se.D., F.R.S. 

Sir Archibald Geikie, O.M., K.C.B., D.C.L..,| 
LE.D., Se-D., F-E.S. 

John Frederick Norman Green, B.A. 

Frederick Henry Hatch, O.B.E., Ph.D. 


Assoe. 


Robert Stansfield Herries, M.A. 


Prof. Owen Thomas Jones, M.A., D.Sc. 

William Bernard Robinson King, O.B.E., 
M.A. 

Richard Dixon Oldham, F.R.S. 

George Thurland Prior, M.A., D.Sce., 
F.R.S. 


| Prof. Sidney Hugh Reynolds, M.A., Se.D. 
| Prof. Albert Charles 


Seward, Sc.D., 
BRAS Balin: 


Walter Campbell Smith. M.C., M.A. 

Sir Aubrey Strahan, K.B.E., Se.D., LL.D., 
Eons: 

Herbert Henry Thomas, M.A., Se.D. 

Prof. William Whitehead Watts, M.A., 
Se.D.. LL.D., F.B.S. 

Henry Woods, M.A., F.R.S. 


Permanent Secretary. 
L. L. Belinfante, M.Sc. 


Pibrartan. 
Arthur Greig. 


Clerk. 
M. St. John Hope. 


STANDING PUBLICATION COMMITTEE. 
Prof. A. C. Seward, President. 


Mr. W. Campbell Smith 
Mr. J. A. Douglas. 
Prof. P. G. H. Boswell. 
Prof. W. S. Boulton. 
Dr. J. S. Flett. 
Prof. E. J. Garwood. 
Mr. J. F. N. Green. 


; \ Secretaries. 


Dr. F. H. Hatch. 
Prof. O. T. Jones. 
Mr. R. D. Oldham. 
Des Ge Eeerior: 
Prof. 8. H. Reynolds. 


Mr. R.S. Herries, Treasurer. 


Sir Aubrey Strahan. 
Dr. H. H. Thomas. 
Prof. W. W. Watts. 
Mr. H. Woods. 


ORDINARY MEETINGS OF THE GEOLOGICAL SOCIETY 
TO BE HELD AT BURLINGTON HOUSE. 
Session 1922-19238. 


1922. 
Wednesday; November —2.--.4:-.:.e-1 2-2 8*—22 
- December c2 oi ets aaa 6 —20* 
1923. 
Wednesday, Jangdary oe  c- tone at 10*—24* 
ee (Anniversary, Friday, 
Feb. 16th) Roane ei ie 7*—28* 
5; March 14. —28* 
a April 18* 
is May 2 — 16* 
; i TU Rene eee ae RE a 6 —20* 


The asterisks denote the dates on which the Council will meet. 


part 3] XENOLITHIC MINOR INTRUSIONS IN MULL. 229 


6. On CERTAIN XENOLITHIC TERTIARY Minor INTRUSIONS 77 
the IsuanD of Muti (ARGYLUSHIRE). By Herperr Henry 
Tuomas, M.A., Se.D., V.P.G.S.; with CHemicat ANALYSES 
by Ernest George. Raptey, F.C.S. (Read May 19th, 


1920.) 
[Puates VI-X.] 
CONTENTS. 
Page 
in Grociirc hoi mwentiny aoe meet Mend cnc ae taint ans hem ent 229 
II. The Intrusions: General Description, Distribution, and 
PERO Stal linge. Mah Rms inten apes KN Rl eu Nahe ca Sakic eo 231 
III. The Cognate Xenoliths (Hnclaves Homoeogénes)............ 238 
IV. The Accidental Xenoliths (Enclaves Enallogénes) ......... 239 
(ay The Sulteeous enol ns: yoiwwe as: acqeey eee see cee eels 239 
O)) The Alurnamour: Xenoli they oy. 0 on. scueeee esse sos ec « 240 
(i) General Description. 
(ii) The Minerals of the Xenoliths: their Mode of 
Formation and Mutual Relations. 
(iii) Probable Nature of the Unaltered Xenolithic 
Material. 
(iv) Relation of the Xenoliths to the Enclosing 
Rock. 
(v) The Conditions of Metamorphism. 
Ven SumMaTy, ANGMCOMGIMSTONS 202... <n cece cps ceWben is ode es aes twee on 255 


I. InTRODUCTION. 


In the years 1910 & 1911, the officers of the Scottish branch of 
H.M. Geological Survey, while engaged on a survey of that portion 
of the Island of Mull which lies south of Loch Scridain, were 
impressed by the great number of minor intrusions of a tholeiitic 
and andesitic character which penetrate the western part of the 
Tertiary lava-field. These rocks were described as being, for the 
greater part, of one type : namely, olivine-free dolerites or tholeiites, 
of which many have pitchstone centres, and in which a sheath- 
and-core structure is frequently developed.! 

Mr. HE. M. Anderson noted the occurrence within them of 
xenoliths of apparently fused or baked sandstone, and also recorded 
certain nodular masses of bytownite and some ferromagnesian 
mineral which he compared with similar masses, collected by the 
late C. T. Clough and described by Dr. J. 8. Flett, from a tholeiitic 
intrusion at Traigh Bhan an Sgoir? (Traigh Bhan na Seurra, 
Locality 11, fig. 1, p. 230). 


1 B. Lightfoot, ‘Summary of Progress for 1910’ Mem. Geol. Surv. 1911, 
p. 30; E. M. Anderson, ‘Summary of Progress for 1911’ Mem. Geol. Surv. 
1912, p. 34, 

2 «The Geology of Colonsay & Oronsay, with part of the Ross of Mull’ 
Mem. Geol. Sury. 1911, p. 92. 


Q.J.G.8. No. 311. R 


9IYySyuVvo 


8B 
S f uproar 


@° . uy 7.) 
° sey 


TINN 


uessoung, 
n 

s96q SUNN odds O 

epyoaeug3 ue 

Pber9y EON vaigaed fe 7 Oe 

OY. resus s-~B 
& Q ysiuysisyopuy 
SUBIIOL\S poo? 


joeybAlusd’S 


lo yUIPIPaQ yeu 
2] gy HIUYVNYINWGYY 


£) 


uUusaug 
auI09 


Dezcwns 


i uesoulL euseso/ 


WT 4 


((}xap ay} u} pajonb asp pup *$99)}]090] 
fo yp ay) uy uagia asou} O} puodsasso9 sjosownu ay 1) 


‘SUOISNAJUL SIY}[OUdX 
JO sat}t[eI0] yueyiodun suLtmoys 


‘HOVNVANCUV 8 TINAW J° SSOY 
ayy jo sejnsuiuad ay} jo deul-yo}24S *| “BI e| 


part 3] XENOLITHIC MINOR INTRUSIONS IN MULL. 231 


In 1912 Mr. Anderson discovered in the bed of a tributary of 
Abhuinn nan Torr (Locality 2) an exposure of a rock that con- 
tained abundant small plates of a deep-blue mineral. The rock 
was submitted to me for examination, and, after isolation, the 
blue mineral proved to be corundum of the sapphire-variety. 
Further work on this locality, and some excavation, yielded a 
series of most interesting specimens that proved clearly the abnormal 
and intensely xenolithic character of the rock in question; but the 
relation of the xenolithic mass to the surrounding rocks was unfor- 
tunately obscured by surface-accumulations. 

Later in the same year, Mr. G. V. Wilson & Mr. D. Tait! 
observed sapphire-bearing xenoliths among the beach-pebbles on the 
shore of Carsaig Bay, south-west of Carsaig village; following 
this clue, they detected the parent source of these xenoliths in a 
composite sill that cuts through the Jurassic sandstones and 
Tertiary lavas about a mile and a half south-west of Carsaig, and 
forms the low promontory of Rudh’ a’Chromain (Loeality 1). 

As the survey of the western portion of Mull progressed, other 
sills with sapphire-bearing xenoliths were encountered by EK. M. 
Anderson & C. T. Clough”: most of them lay, as before, in the 
peninsular region south of Loch Seridain ; but afew others occurred 
on the northern side of the Loch, in the neighbourhood of Tiroran. 

All the material collected by the officers of the Survey was 
handed to me for examination, and was supplemented by collections 
which I made when visiting the island in 1913 and 1914. 

The beauty of the xenoliths and the occurrence of such meta- 
morphic minerals as corundum, spinel, sillimanite, cordierite, and 
anorthite make these inclusions in themselves pre-eminently worthy 
of petrographical description; but, at the same time, an almost 
unparalleled opportunity offers itself for the study of the progressive 
metamorphic changes wrought in xenolithic material by an igneous 
magma of basic composition. 

The object of the present communication is, therefore, twofold : 
first, to describe in some detail the complex mineral assemblages 
that constitute the xenoliths of these tholeiitic intrusions; and, 
secondly, to discuss the relation of the individual minerals to each 
other and to the igneous magma that chemically and physically 
controlled their formation. 


Il. Tue Inrrvsions. 
General Description, Distribution, and Petrography. 


All the xenolithic intrusions that form the subject of this paper 
occur relatively close together in the south-western portion of the 
Tertiary lava-field, in that part of the Ross of Mull which lies 
between Carsaig and Pennyghael on the east, and Bunessan on the 
west. Also, to a less extent, they have been met with north of 
Loch Seridain, around Tiroran in the peninsula of Ardmeanach. 


1 «Summary of Progress for 1912’ Mem. Geol. Surv. 19138, pp. 48 & 66. 
2 ‘Summary of Progress for 1914’ Ibid. 1915, p. 34. 
R2 


232 DR. H. H. THOMAS ON XENOLITHIC [ vol. Ixxvin, 


List oF LOCALITIES. 


1. Rudh’ a’Chromain and Nuns’ Pass, Carsaig. 
2. Bed of small stream, tributary to Abhuinn nan Torr, 1100 yards north 
of the cairn on Mullach Glac an t’Sneachda. 
3. Streamlet on the northern side of a plantation, 1 furlong north of 
Feorlein Cottage, Carsaig. 
4. Old Road, 100 yards north-west of Feorlein Cottage, Carsaig. 
5. Close to Beach River, 500 yards above the junction with Abhuinn an 
Easa’ Mhoir, 
6. Coast, 1165 yards slightly north of west of Ormsaig. 
7. 2600 feet east-south-east, and 1850 feet south-east by south of 
Kilpatrick. 
8. 2560 feet north-north-east by north of the northern end of Eilean Ban. 
9. 1980 feet north of west of Ardchrishnish. 
10. 1150 feet east-north-east of Lochan a’Phuill. 
11. 55 and 85 yards north-east of the north-eastern end of Traigh Bhan an 
Sgoir. 
12. Shore, a quarter of a mile south-west from the mouth of Allt na Coille 
Moire. 
13 & 15. Allt a’Mhuchaidh, 100 yards above the bridge. 
14, 400 yards south of Seabank Villa. 
16. Abhuinn Bail’ a’Mhnilinn, 100 to 200 yards above the bridge. 
17. 700 yards south-west of Tiroran. 


To the foregoing list could be added many other localities within 
the same region, but those enumerated afford thoroughly repre- 
sentative examples, and will be found sufficient for investigators 
who wish to study these intrusions in the field. Further, it is from 
these localities that the bulk of the xenolithic material has been 
collected; and, in order to guide future workers, they have been 
indicated on the appended small-scale map (fig. 1, p. 280)! under 
their respective numbers. 


An excellent and fully exposed example of the xenolithic intru- 
sions is the composite sill that forms the low promontory of Rudh’ 
a’Chromain, west of Carsaig (Locality 1): here the components of 
the sill can be studied in detail, and their relative ages demonstrated. 
Further, the relation of the sill as a whole to the rocks into which - 
it has been intruded is particularly clear, and the occurrence is 
easily located. For these reasons this sill may be taken as the 
type with which to compare the other xenolithic intrusions, and its 
petrography will be described in some detail. 

It consists of an acid central member, 20 to 30 feet thick, ahicn 
is bounded on each side by lateral basie members of less, but 
unequal, thickness. It may be traced across the foreshore and the 
raised-beach platform to the cliff below Nuns’ Pass, where it is 
either faulted or rapidly cuts across the Tertiary lavas, to a position 
in the cliff about 100 feet above sea-level. 

On the west its basic lower portion is in contact with the 
Carsaig Sandstone (Jurassic); but on the east the upper basic 


1 A sketch-map showing the two original localities for sapphire-bearing 
xenoliths was reproduced in the ‘Summary of Progress for 1912’ Mem. Geol. 
Surv. 1913, p. 48. 


part 3] MINOR INTRUSIONS IN THE ISLAND OF MULL. 233 


margin is chilled against an earlier intrusion of bostonite, which 
separates it from the Carsaig Sandstone in that direction. 

The details of the intrusion are illustrated in the sketches 
(figs. 2 & 3, p. 284) made by Mr. D. Tait. 

The central portion of the sill is a pale-grey rock, finely crystal- 
line for the greater part, without any marked porphyritic con- 
stituents. Locally, it is distinctly hyaline in texture, and, hke 
many of the pitchstones of Mull, has developed the characteristic 
‘sheath-and-core ’ structure of these rocks.! 

Xenoliths occur sparingly throughout its mass, but perhaps are 
more prevalent in the somewhat less acid selvages that adjoin the 
basic members on each side. The xenoliths are mainly of sand- 
stone, and range up to 6 inches or so in longest diameter. Large 
xenoliths of both shale and sandstone occur in a more central 
position, and may measure several feet in diameter, with an average 
of about 2 feet. 

The basic marginal members of the sill clearly antedate the acid 
interior. ‘They differ somewhat one from the other in relative 
thickness, and in the number and character of the contained xeno- 
liths. In regard to texture, they appear to be fine-grained dark- 
grey to dark-brown rocks, exhibiting a more compact facies at 
their outer contacts with the Carsaig Sandstone and the bostonite 
respectively. 

The upper basic member is divisible into two zones. The outer 
zone, about 4 to 6 feet thick, contains abundant cognate xenoliths 
of gabbroid character. These xenoliths are of all sizes, from quite 
small dimensions to a foot or so in diameter. The inner zone of 
similar material, ranging from 24 to 5 feet in thickness, is densely 
crowded with accidental aluminous xenoliths of all shapes, which 
range in size from an inch to 4 feet in greatest dimension. The 
lower basic member of the sill is from 2 to 5 feet thick, and is 
characterized particularly by the size and abundance of its ‘cognate 
xenoliths and the relative rarity of accidental xenoliths. The 
concentration of cognate xenoliths in the lower portion of the 
Traigh-Bhan-na-Sgurra sheet is a point worthy of notice in this 
connexion.” 

The acid interior of the sill is, as a whole, a compact pale-grey 
to brownish-grey rock, which, towards its external margins, for a 
foot or two, darkens in colour, apparently becomes slightly more 
basic, and carries numerous xenoliths of sandstone. 'These more 
basic and relatively thin marginal portions are of the nature of 
inninmorite, a type of Mull pitchstone characterized by the 
presence of uniaxial augite and described in 1915 in the paper 
already cited by Mr. E. M. Anderson & Mr. H. G. Radley.? 


1H. M. Anderson & E. G. Radley, ‘The Pitchstones of Mull & their 
Genesis’ Q. J.G.S. vol. Ixxi (1915-16), p. 210. 

2 J. 8. Flett, ‘Geology of Colonsay & Oronsay, with part of the Ross of 
Mull’ Mem. Geol. Surv. 1911, p. 92. 

3 ‘The Pitchstones of Mull & their Genesis’ Q.J.G.S. vol. Ixxi (1915-16) 
p. 209; see also A. F. Hallimond, ‘ Optically Uniaxial Augite from Mull’ 
Min. Mag. vol. xvii (1914) p. 97. 


"aN 


HOISNAPUL LOT[LVO WB JO OY1OYSOG, =p ‘T[IS JO uotzod [rayuoo ploy =e 


“‘STJTTOUOX Y4TM “[[IS JO ULGLUUL OISUET = Z ‘(OIsSSUAN') OUOFSPUBY Srusauy = T 
| seals 4 
SA nl ‘iy i | ai / 7 ES oe (Wess os i 0 ohee co er 
ails pia S 
S\ VK C) aN 
mee # OD je a 
\8 GC D0 _ al = 
NY 0 Pea ee 
Sy ft oO Ne Da ues 
oy 4 
« OO 
oO! 4 
4 
yA oO 
o 0 
Za 
: : ; ‘M'S 
(‘spunk § pnogu=worpoas fo ypbuaT ) 
UIDUWOLY YY) DY Y Ply ‘QMOYSINOJ OY) WO 77PIS 90470] 0WAN ay) fo aunsodanp—"& “O1y] 


(spunk og jnogn=uorjvas fo yphuay) 
WIDUWOLY OY Ypnap “7108 apasodeuod aLyZYOWIL AY) S80LID WOLI9Y—'G ‘Ol 


part 3] XENOLITHIC MINOR INTRUSIONS IN MULL. 235 


Where vitreous, the rock consists of small porphyritic crystals 
of labradorite and less frequent uniaxial augite (usually pseudo- 
morphed in serpentine or calcite) in a brownish glass traversed in 
all directions by acicular crystals of augite and skeletal growths of 
magnetite. In portions of the rock that have undergone greater 
devitrification the ground-mass carries the same phenocrysts and 
acicular augite, but may be seen to contain alkali-felspar and 
quartz. The central and greater portion of the acid interior is a 
grey rock, locally vitreous and often showing the ‘sheath-and-core’ 
structure characteristic of many of the Mull pitchstones.! The 
acid portions vary from rhyolite to felsite, but are always very 
closely related to rocks classed with the extremely acid types of 
inninmorite. They consist of small porphyritic crystals of acid 
labradorite in a glassy base which, like that of the inninmorite, is 
traversed by blade-like crystals of a greenish augite. Hven in the 
glassy portions (Pl. VI, fig. 1) small devitrified areas are of 
frequent occurrence, and are composed of felspar-crystals, of less 
basic composition than the phenocrysts, which are often grouped in 
a radial or subvariolitic manner. 

When more completely devitrified (Pl. VI, fig. 2) the rock is 
lighter in colour; the base contains little glass, its place being 
taken by a feathery felspathic mass which is still traversed by the 
acicular crystals of augite, but holds (in addition to acid plagio- 
clase) some orthoclase and a little free quartz. 

The analysis of a felsite closely related to inninmorite, made by 
Mr. F. R. Ennos, of the Government Laboratory, and tabulated in 
col. 1 of the analyses on p. 236, although from another locality, 
may be taken to represent closely the composition of the acid 
interior of the Rudh’ a’Chromain sill. 

The normal stony basic portion (PI. VI, fig. 4) consists of some- 
what elongated irregularly-formed crystals of aluminous augite, 
which tend to assume a prismatic habit, and felspar, with which 
the augite often has subophitic relationship. 

The felspar is a basic labradorite zoned with plagioclase of com- 
position varying to that of oligoclase. These two minerals make 
the bulk of the rock, and together give rise either to a sub-variolitic 
or to an intersertal structure. The interstitial matter is devitrified, 
and consists of acid plagioclase and quartz. It contains a relatively 
small amount of magnetite, and abundant apatite in the form of 
slender needles. 

In the more vitreous portion (Pl. VI, fig. 8), in which cognate 
xenoliths appear to be the more prevalent, the rock has a definitely 
intersertal structure. 

The varieties met with in the basic margins are all closely 
related. Their composition is clearly on the border-line between 
that of basalt and augite-andesite; and this, combined with a 
frequently well-marked intersertal structure, warrants their inclu- 
sion among the tholeiites. In Mull it has been found that 55 per 


1 H, M. Anderson & E. G. Radley, op. swpra cit. pp. 210, 211. 


236 DR. H. H. THOMAS ON XENOLITHIC _[ vol. lxxviil, 


cent. of silica furnishes a very natural acid limit to the tholeiites. 
Beyond that one meets with the andesitic rocks already alluded to 
as leidleites and inninmorites, and also other types distinguished 
by the degree of crystallization. The Rhud’ a’Chromain tholeiitie 
margins have a very close analogy to leidleite, as shown by their 
tendency to acicular and variolitic structure, and this agrees with 
the silica percentage shown in Analysis II. In the Geological 
Survey Memoir on the ‘ Geology of Colonsay & Oronsay with ‘part 
of the Ross of Mull’ 1911, p. 92, the xenolithic sheet of Traigh 
Bhan na Sgurra is described by Dr. Flett as a dolerite of the 
olivine-free type, and is exactly comparable with the basic margins 
of the Rudh’ a’Chromain sill. 

The composition of the stony portion of the Rudh’ a’Chromain 
sill is recorded in Analysis II, by Mr. Radley. Compared with 
normal andesites the silica percentage is relatively low, and there 
are other divergences on the basalt side. 


ANALYSES OF XNENOLITHIC SILLS AND XENOLITHS. 


i; RE 549% IV. 

Per cent. Per cent. Per cent. Per cent. 

SO, 28868 wh eS 70°70 5. f 38°67 49°74 
TAO ee aber pl i ge ed 1°27 1-24 0°90 1:49 
PA ee eee 11°78 14°65 37°27 34°99 * 
Ras. ee A ee 1°32 3°62 2-78 153 
Pe) reese Set gee VA ee ed 3°45 6°32 5°07 0°34 
WS oe oe te 0-07 0°30 0-17 015 
CE C59) Ca Sees ere nt. fd. nt. fd. nt. fd. 
ee eee a ee — 0-04 — nt. fd. 
32) 0 en gtete SFOS ct ESE OMERS ES are 1°30 7°98 4-65 Oss 
1105 0 Ra oe en ree Re Ree iat 0°53 4°49 2-30 0°66 
[CS | eee Se, eee es 4-71 152 3°01 172 
(12 2 0 I Se Men Ret eeee nara Aa 2-48 2-54 3°63 3°76 
Gey Hs 2: Cas. nt. fd. trace nt. fd. trace 
H:0 at Ms! oe Sie 0°50 1°92 0°22 O61 
H20 above 105° C. ......... 1°14 0°94. 1°63 344 
Po eee Fn an ee 0°26 0°27 = 0°86 
FeSs Prose Sc sesbec ns Ser en Ssaanc a a 0°09 aad nt. fd. 
BY SE OO ee es T= O51 051 = 0-04 
Miokale? esis 2! 100°10 100°40 100°35 100°21 


I. Felsite related to inninmorite. Acid sill from an exposure south of Coire 
Buidhe, about half a mile north of Carsaig (S. 18464) ; ‘Summary of 
Progress for 1915’ Mem. Geol. Surv. 1916, p. 26 (Anal. F. R. Ennos). 


II. Tholeiite. Basic portion of composite intrusion, Rudh’ a’Chromain, west 
side of Carsaig Bay (S. 17170) ; ‘Summary of Progress for 1914’ Mem. 
Geol. Surv. 1915, pp. 56, 57 (Anal. E. G. Radley). 


III. Xenolith containing basic plagioclase, sillimanite, sapphire, and spinel 
(S. 16612), from the basic portion of the sill, Nuns’ Pass, Carsaig; 
‘Summary of Progress for 1913’ Mem. Geol. Sury. 1914, pp. 80, 81 - 
(Anal. E. G. Radley). 

IV. Sillimanite-buchite. Xenolith from the basic portion of a sill, Nuns’ Pass, 
Carsaig; *0°48 per cent. of the alumina is present as corundum, ; Summary 
of Progress for 1914’ Mem. Geol. Surv. 1915, pp. 56, 57 (Anal. E. G. 
Radley). 


The majority of the xenolithic sills are not markedly composite, 


part 3 | MINOR INTRUSIONS IN THE ISLAND OF MULL. 237 


although they often have a more or less vitreous centre (leidleite) 
with stony marginal portions. The xenoliths are generally con- 
centrated near the margins; and their containing matrix, where 
investigated, consists of tholeiite similar to the lateral members of 
the Rudh’ a’Chromain intrusion. 


The intrusion of Mullach Glac an t’Sneachda, in which sapphire 
was first detected, calls for special notice on account of its unusual 
character. It is unfortunately but indifferently exposed, and its 
relations to adjoining rocks are somewhat obscure. It appears as 
a dark-grey amygdaloidal rock of fine texture and igneous aspect. 
It is studded with small lustrous plates of sapphire, and packed 
with aluminous and siliceous xenoliths of all sizes. A mass of 
siliceous material 6 feet long exposed by the side of the burn is 
clearly a xenolith. Cognate xenoliths are unrepresented, except in 
the form of isolated and often broken crystals of dark-green 
hypersthene and less frequently of augite, indicating an early 
crystalline separation from a tholeiitic magma similar to that 
which gave rise to the cognate xenoliths of Rudh’ a’Chromain. 
The chief anomaly is the dark matrix, which, under the micro- 
scope, is seen not to be a true igneous rock, but to be a sillimanite- 
buchite (p. 240) modified by more or less complete admixture with 
igneous material. The apparent porphyritic crystals are all broken 
bytownite and anorthite, derived from highly crystalline aluminous 
xenoliths (p. 241), and, therefore, themselves of xenolithic nature. 

That the rock was intruded as a molten or semi-molten mass is 
proved by its numerous vesicular cavities, now filled with zeolites 
and other low-temperature minerals. 

The amount of xenolithic material carried by this intrusion is 
very considerable: for, apart from the abundant accidental xeno- 
liths held by the pseudo-igneous matrix, the matrix itself is largely 
composed of fused sedimentary material (sillimanite-buchite), 
modified to some extent and doubtless rendered more fluid by 
admixture with a tholeiitic magma. It will be seen in the sequel 
that such an intrusion would result from the squeezing-upwards of 
the viscous semi-fused material that formed the wall of the magma- 
basin (p. 250). 

A similar type of intrusion would appear to be represented in 
the kersantite of Michaelstein,! which forms a dyke of obviously 
mixed igneous and sedimentary material full of a variety of 
minerals foreign to the rock. 


Generally speaking, the tholeiitic intrusions have produced little 
thermal alteration of the rocks into which they have been ulti- 
mately injected, although the sill at Traigh Bhan na Sgurra is a 
notable exception. The intrusions are seldom of large dimensions, 
their type of crystallization indicates rapid consolidation, and we 


1M. Koch, ‘Die Kersantite des Unterharzes’ Jahrb. K. Preuss. Geol. 
Landesanst. 1886, p. 44. 


238 DR. H. H. THOMAS ON XENOLITHIC [ vol. lxxviil, 


can be fairly sure that their intrusion-temperature was not particu- 
larly high. The tholeiite of Rudh’ a’Chromain, however, at its 
junction with the Carsaig Sandstone, has for a few millimetres 
thermally altered the sedimentary rock. The action has been to 
produce an interstitial melt between the component grains of the 
sandstone, which has attacked and dissolved the quartz, with 
the subsequent crystallization of tridymite-fringes around the 
undissolved portions of the grains. The tridymite has reverted to 
quartz which, while being in optical continuity with the quartz 
of the original grain, retains the form of tridymite. This pheno- 
menon is much better displayed by the quartz-grains in the 
siliceous xenoliths, the metamorphism of which is of a more 
intense character (see p. 240). 


II. Tue Coenate Xenorirus (Encitaves Hommoceinss). 


Cognate xenoliths are a striking feature of the basic portions of 
the Rudh’ a’Chromain and Nuns’ Pass sill, and of the tholeiitic 
intrusions of several other localities under consideration. Such 
xenoliths were first described from the tholettes of this region by 
Dr. Flett, who recognized two types: one consisting of bytownite 
and pale-green augite, and the other of bytownite and a nearly 
colourless enstatite or bronzite.! 

In the Rudh’ a’Chromain sill they occur as dark coarsely-crys- 
talline patches, sharply marked off from the more finely-grained 
tholeute that envelops them. As was noted in the similar occur- 
rence of Traigh Bhan na Sgurra, they show a distinct tendency to 
congregate in the lower portion of the sill, suggesting a gravita- 
tional concentration. They consist commonly of bytownite and 
hypersthene. The hypersthene-crystals are often sharply iduio- 
morphic (Pl. VI, fig. 6) and have moulded upon them, or occa- 
sionally include, an optically positive basic plagioclase (basic 
labradorite or bytownite). 

Less frequently a dark-green augite takes the place of the 
rhombic pyroxene (PI. VI, fig. 5), and like it 1s usually idiomorphic 
towards the bytownite (16612a, 17173).2 In some eases olivine 
appears to have been present, but is now represented by serpen- 
tinous and calcitic pseudomorphs (17174). An orthorhombic and 
a monoclinic pyroxene are sometimes, though rarely, associated in 
the same xenolith. 

Hypersthene, presumably derived from cognate xenoliths, occurs 
in the intrusion of Mullach Glace an t’Sneachda (Locality 2), while 
coarse aggregates of bytownite and augite have been met with at 
Seabank Villa (Locality 14), where some of the augite-crystals 
are sharply idiomorphic (18530), and occasionally measure nearly 
2 inches in length. 


1 * Geology of Colonsay & Oronsay, with part of the Ross of Mull’ Mem. 
Geol. Surv. 1911, p. 92. 

2 These numerals in parentheses refer to the rock-slices in the collections 
of the Geological Survey. 


part 3] MINOR INTRUSIONS IN THE ISLAND OF MULL. 239 


IV. Tue Accrpentan XENOLITHS (ENCLAVES ENALLOGENES). 


The greater number of accidental xenoliths are carried by tho- 
leiitic sheets, or in the basic tholeiitic portions of the composite 
intrusions. They are, however, not entirely restricted to the more 
basic rocks, and may be found in less numbers and in a somewhat 
different state of metamorphism in the more acid portions of some 
of the sills, especially that of Rudh’ a’Chromain. 

They may be divided into two classes, according to their nature, 
which we may designate respectively as (a) siliceous and (6) alu- 
minous. The former class includes such xenoliths as arise from 
sandstones and granitic or gneissose rocks, while the latter class is 
exemplified by xenoliths that appear to have had their origin in 
aluminous shales of fairly constant composition. The aluminous 
xenoliths, besides occurring in greater quantity, also have greater 
average dimensions, and it is with these that we are chiefly 
concerned. 


(a) The Siliceous Xenoliths. 


These present no particularly new features, and the majority 
of the changes that they have undergone have been dealt with in 
a most complete manner by other authors, more particularly by 
Prof. A. F. A. Lacroix.) <A few points, however, are worthy of 
mention, especially those indicating the selective fusion of the 
felspathic constituents and the almost universal formation of tri- 
dymite around grains of undissolved quartz when the interstitial 
melt is acid. The separation of augite and rhombic pyroxene 
around undissolved quartz from a sufficiently basic melt, or from 
an interstitial melt modified by transfusion of basic igneous 
material, is too well known to call for further comment. 

Sandstones, granitoid rocks, and what presumably have been 
gneisses are all represented among the xenoliths, and all show a 
similar and relatively simple type of metamorphism. Sandstones 
are the most prevalent, being distinctly common in the intrusions 
of Localities 1 & 2. 

The Carsaig Sandstone, or some similar rock, has furnished a 
considerable number of xenoliths to the sills, and the nature of the 
metamorphism is identical with, but greater in degree than, that 
exhibited by the Carsaig Sandstone in contact with the intrusive 
tholeiite of Rudh’ a’Chromain. The metamorphism is usually 
unaccompanied by any transfusion of igneous material, and has 
been produced by purely thermal agencies. Felspathic constituents 
at the elevated temperature thus originated have given rise to an 
interstitial melt, which attacked the surfaces of the quartz-grains 
and thus became enriched with dissolved silica. Crystals of 
tridymite separated from the melt, and attaching themselves at 
all angles to the undissolved grains, formed fringes. The residual 
melt consolidated as glass. The tridymite-crystals have now 


' «Tes Enclaves des Roches Volcaniques’ Ann. Acad. Macon, vol. x, 1893. 


240 DR. H. H. THOMAS ON XENOLITHIC (vol. Ixxvill, 


reverted to quartz, and usually extinguish simultaneously with the 
quartz-grain that they fringe; but, recalling the identical struc- 
tures produced in fused ganisters and other siliceous refractories,! 
we can have no doubt as to their original character. 

Dr. J. S. Flett.2 some years ago, in a xenolith from Traigh 
Bhan na Sgurra, described fringes identical with those mentioned 
above, but doubtfully attributed to them a different origin. 

Of the recently collected material one of the best examples of 
such a siliceous xenolith comes from Feorlein Cottage, Carsaig 
(Locality 3), and is illustrated in Pl. VII, fig. 5. The quartz has 
suffered considerable solution by the interstitial melt; but, on 
cooling, large plates of tridymite have been deposited upon the 
undissolved quartz, and similar crystals have separated from the 
melt now represented by the glassy matrix. 

The amount of glass found im these siliceous xenoliths is variable, 
and appears to increase according to the amount of felspathic 
constituents in the original rock, also with transfusion of igneous 
material which would tend to lower the melting-pomt. The 
presence of igneous material usually betrays itself by the deeper 
colour of the interstitial glass, and by the separation of minute 
crystals of cordierite. A beautiful example of the ready fusion 
of the felspathic constituents and the formation of a copious inter- 
stitial melt has been furnished by a sandstone xenolith (20763) 
collected by Mr. E. M. Anderson from a tholeiite-sill near Bunessan. 
A sandstone xenolith metamorphosed by a tholeitie magma. 
and modified to some extent by transfusion of igneous material, 
is illustrated by an example from south-east of Gortein Driseach. 
between Localities 2 & 5 (Pl. VII, fig. 6). 


(6) The Aluminous Xenoliths. 
(1) General Description. 


The aluminous xenoliths present three distinct types, and may 
be grouped under three separate headings, accordmg to their 
structure and mineral constituents, and incidentally according to 
the manner and degree of modification induced by the igneous 
magma. The following types, which occur either im close depen- 
dent association or as separate units, can be readily recognized :— 


(1) The sillimanite- and cordierite-buchites. 
(2) The anorthite-corundum-spimel assemblage. 
(3) The cordierite-sillimanite-spinel assemblage. 


(1) The buchites(or basalt-jaspers of foreign writers) are 
composed essentially of glass with one or more characteristic 
crystalline phases, such as sillimanite, cordierite, etc. They are 


1 See H. H. Thomas, A. F. Hallimond, & E. G. Radley, ‘ Mimeral Resources 
of Great Britain” Mem. Geol. Surv. vol. xvi (1920) p. 60 & pl. iv. 

2 *Geology of Colonsay & Oronsay, with part of the Ross of Mull’ Mem. 
Geol. Surv. 1911. p. 95 & pl. vi, fig. 5. 


part 3] MINOR INTRUSIONS IN THE ISLAND OF MULL. 241 


produced by the simple fusion and subsequent consolidation of 
sedimentary material without or with admixture of magmatic 
matter supplied by the invading igneous rock. Such buchites 
have been described frequently, especially by Prof. A. Lacroix and 
Dr. Alfred Harker; and, later, by Dr. J. S. Flett, in association 
with the Tertiary dykes of the Oban and Dalmally district.+ 

The sillimanite-buchites, of which many examples have been 
collected from the various localities, are compact, vitreous, grey- 
blue or lilac-coloured rocks, consisting of glass in which the 
abundant minute needles of sillimanite are usually arranged in a 
parallel manner (Pl. VII, figs. 1 & 2). 

The cordierite-buchites are most often black, and cordierite is 
either the chief or the only crystalline constituent (Pl. VII, fig. 3). 

(2) What has been termed the anorthite-corundum-spinel 
assemblage is an almost holocrystalline mixture of these minerals, 
and is extremely prevalent in the intrusions under description. It 
is clearly of xenolithic nature, and is well illustrated by Pl. VIII, 
ne. 2. 

(3) The cordierite-sillimanite-spinel assemblage is less 
common than Group 2, and results presumably from somewhat 
special conditions that are discussed later. 


The relation and interdependence of the anorthite-corundum- 
spinel assemblage and the sillimanite-buchites are clearly indicated. 

In some of the more symmetrical xenolithic masses it can be 
demonstrated that the innermost portion is a dark-grey or dull 
lilac-coloured rock of vitreous character (sillimanite-buchite). This 
vitreous mass, followed towards its outer margin, is seen first to 
contain scattered felspar-crystals, often skeletal in form, and then, 
by the felspars becoming more frequent and better formed, to 
develop into a more or less completely crystalline felspathic zone. 
This zone, often several inches thick, and rich in corundum and 
spinel, normally separates the buchite from the enclosing tholeiite. 

The felspathic portion (anorthite-corundum-spinel assemblage) 
is roughly divisible into two—an inner sub-zone in which corundum 
in the form of sapphire is abundant and spinel subordinate, and an 
outer sub-zone in which the ratio of these two minerals one to the 
other is reversed. 

The line of demarcation between the holocrystalline portion of 
the xenolith and the surrounding igneous rock is usually quite 
sharp, and the igneous rock rarely shows any compositional 
variation that can be attributed to direct action of the foreign 
inclusion. 

In practically every xenolith of any size that has been examined 
a zonal arrangement can, in some measure, be made out, but not 
always of a symmetrical character (p. 249). Sometimes the inner 
vitreous portion (buchite) is reduced to a minimum, and at other 
times the outer crystalline zone is thin or of varying thickness. 

1 “Geology of the Country near Oban & Dalmally’ Mem. Geol. Surv. 1908, 


pp. 129-31; see also ‘ Geology of Colonsay & Oronsay, with part of the Ross 
of Mull’ Mem. Geol. Surv. 1911, pp. 98-95. 


242 DR. H. H. THOMAS ON XENOLITHIC [ vol. lxxviii, 


In small xenoliths the buchite may be entirely unrepresented, and 
the whole of the inclusion may then be composed of plagioclase 
with accompanying corundum and spinel showing a small amount 
of interstitial glass. 

I shall now briefly describe the chief minerals that enter into 
the composition of the aluminous xenoliths, and at the same time 
discuss in turn their probable mode of origin and relation to the 
other minerals with which they are associated. 


(ii) The Minerals of the Xenoliths: their Mode 
of Formation and Mutual Relations. 


Corundum.—The corundum of the xenoliths nearly always 
oceurs as deep-blue brilliant crystals of distinetly tabular habit. 
There are three modes of occurrence :— 

(1) As isolated crystals in the sillimanite-buchites ; (2) in close association 
with anorthite in the crystalline outer portions of the xenolithic masses 
(Pl. VIII, fig. 2); and (3) as well-formed crystals in a matrix of distinctly 
igneous nature, which adheres to, and penetrates, the crystalline outer zone 
of certain xenoliths (Pl. IX, fig. 1). 

All erystals have the same general habit; they are flattened 
parallel to the base {O0001!, and are usually rhombohedral or 
pyramidal in form. The prism of the second order {1120! is 
seldom met with, and the usual combination of forms is the base 
{0001} modified by the rhombohedra {1011} {3032}, or less 
commonly by the rhombohedra together with the pyramid {2243}. 

The faces are bright, and give excellent goniometric readings, 
by which the identity of the forms mentioned above has been 
established. The basal plane is stepped or deeply striated parallel 
to the trace of one or other of the rhombohedral faces, giving rise 
to well-marked equilateral triangular areas, one within the other, 
which are due to oscillation in regrowth of the base and rhombo- 
hedron. In size the crystals vary from a millimetre or so, 
measured across the basal plane, to at least a centimetre and a half 
in exceptional cases. Their thickness measured along the vertical 
axis is usually less than half the width of the basal plane. The 
acutely pyramidal habit of many varieties of corundum is entirely 
unrepresented. The tabular habit and general simplicity of forms 
appear to characterize corundum of xenolithic masses, and, what 
practically amounts to the same thing, that which has separated 
from basic igneous magmas on their becoming saturated with 
alumina through solution of aluminous material. 

Corundum such as we are considering has been fully and 
repeatedly described, and it will be sufficient to state that the 
crystals here mentioned find their closest analogue in the tabular 
erystals of Yogo Gulch (Montana). 

1 For detailed accounts of the occurrence of this mineral, see J. H. Pratt & 
J. V. Lewis, ‘Corundum & the Peridotites of Western North Carolina’ Mem. 
Geol. Surv. N. Carolina, vol. i (1905) pp. 225 et seqg.: L. V. Pirsson, Amer. 
Jour. Sci. ser. 4, vol. iv (1897) p. 421; also 20th Ann. Rep. U.S. Geol. Surv. 
1898-99, pt. i11 (1900) pp. 552, 5538. 


part 3] MINOR INTRUSIONS IN THE ISLAND OF MULL. 243 


The colour of the crystals is deep blue in thick sections, and very 
pale blue for the thickness of an ordinary rock-section. The pleo- 
chroism is generally quite distinct, if not strong :— 


w=pale blue, e=pale greenish-blue to sea-green. 


Optical anomalies are rarely met with, and twinning parallel to 
the fundamental rhombohedron {1011}, although present in some 
instances, is of rare occurrence. 

Inclusions are general, but almost invariably consist of brownish 
to colourless glass, and this feature appears to be more or less 
characteristic of the corundum of the metamorphic and non- 
plutonic igneous rocks. 

As regards origin, the corundum of the sillimanite-buchite 
represents the excess of alumina over that required for the forma- 
tion of sillimanite and melt, and was the first solid phase to 
separate out from the fused aluminous sediment. It rarely forms 
as much as 1 per cent. of the buchite, and is generally below 05. 
In the case of the corundum of the anorthite-corundum-spinel 
assemblage that appears to interpose itself normally between 
the sillimanite-buchite and the magma, we find that corundum 
is more plentiful towards the buchite: that is to say, towards 
the source of the alumina. It is invariably associated with 
anorthite, and in a manner that points conclusively to crystal- 
lization from a common solution of these two substances. In 
some instances the lack of crystalline form of the corundum 
in association with anorthite would suggest the simultaneous 
crystallization of these minerals in equilibrium with each other 
and with the melt, and the maintenance by transfusion of an 
anorthite-corundum eutectic condition. 

The later-formed corundum, which occurs associated with spinel 
and oligoclase in a matrix of more igneous character, has been 
formed by the separation of alumina from a partial magma 
enriched in alumina through the resorption of anorthite containing 
sillimanite (p. 244). A certain portion of the excess alumina has 
combined with the available magnesia and ferrous iron of the 
magma to form spinel (p. 248). Both corundum and spinel are 
often enclosed by a zone of anorthite-crystals (Pl. VIII, figs. 3 &4), 
a feature noted by Prof. Lacroix in xenoliths from the Cantal 
region. 


Sillimanite.—This mineral is a common product in meta- 
morphosed aluminous sediments; in the xenoliths under description 
it is, next to corundum, the earliest and most constant crystalline 
substance to separate from the melt produced by the fusion of the 
highly aluminous shales. It occurs mostly as minute needles about 
1 to 2 mm. long, which are either felted together or arranged in a 
parallel fluxional manner in a colourless or pale violet-tinted glass. 


1 «Tes Enclaves des Roches Volcaniques’ p. 190: ‘ On voit que lorsqu’un 
grand cristal de corindon ou de spinelle se produit, il est entouré d’une zone 
exclusivement feldspathique.’ 


244. DR. H. H. THOMAS ON XENOLITHIC (vol. lxxvu, 


To such an assemblage the name sillimanite-buchite is applied 
(p. 241). 

The mineral, even in the small needles, has a characteristic 
pleochroism of pale pink to colourless, similar to, but less intense 
than, that of the larger crystals described below. Inclusions are 
seldom present, and then consist solely of glass. 

In origin, the sillimanite is partly a product of direct vitrifica- 
tion and partly a primary phase separated, on fall of temperature, 
from the original aluminous melt. It furnishes the main source 
of the alumina present in the other aluminous minerals, such as 
anorthite, spinel, and cordierite, which aie of later appearance and 
are dependent for their formation on the transfusion of bases from 
the igneous magma.! 

Large crystals of sillimanite of a deep rose-pink colour occur in 
a dark glass, in association with cordierite and spinel (S. 18001a), 
Pl. VII, fig. 4. There is evidence that these crystals are due to 
the reheating of an original sillimanite-buchite, and that their size 
has thereby been increased. 

Their most striking feature is an intense pleochroism which is 
bright rose-pink parallel to c, the long axis of the crystals, and 
colourless parallel to a and b (S. 18001). So far as I am aware, a 
strong pink pleochroism, recalling that of the more highly-coloured 
varieties of andalusite, has seldom been met with in this mineral. 
The only record that I can trace of a pink sillimanite is that given 
by Des Cloizeaux and quoted by Hintze.? 

Crystallographicaily the mineral forms approximately rectangular 
prisms bounded by the face (110); these in cross-section provide 
diagonal directions of extinction, and show traces of the usual 
perfect cleavage parallel to (010). 


Felspar.—The chief and earliest-formed felspar of the acci- 
dental xenoliths occurs in the complex crystalline zone (anorthite- 
corundum-spinel assemblage) which separates the sillimanite- 
buchite from the igneous rock, and resulted from the crystalliza- 
tion of a hybrid melt of special composition due to the mutual 
influence of the tholeiite-magma and fused aluminous xenolithic 
material. The felspars occur as large crystals, often reaching 
several inches in length. Near the external margin of the crystal- 
line zone they are mutually interfering, but towards the buchite 
they become more distinctly separated by glassy matter and more 
noticeably idiomorphie. 

Their formation has, to a limited extent, been attended by a 
resorption of the sillimanite of the buchite; but the felspars are 


1 G. V. Wilson, ‘ Notes on the Formation of certain Rock-forming Minerals 
in & about Glass Furnaces’ Trans. Soc. Glass Technology, vol. ii (1918) 
Be ie 
‘ 2 A. Des Cloizeaux, ‘Manuel de Minéralogie’ vol. i (1862) p 179; see also 
C. Hintze, ‘Handbuch der Mineralogie’ vol. 11 (1897) p. 142: ‘ Zuweilen ist 
ein Pleochroismus wahrzunehmen: Des Cloizeaux beobachtete an Spaltungs- 
blattchen mit dem Dichroskop ein farbloses und ein schwach rosenrothes 


Bild.’ 


part 3] MINOR INTRUSIONS IN THE ISLAND OF MULL. 245 


usually crowded with undissolved needles of this mineral (Pl. IX, 
figs. 1-8, 5, & 6) which impart to the crystals a striking flesh-pink 
colour (p. 244). Occasionally, they are coloured a sky-blue by 
finely divided corundum (Pl. VIII, fig. 6). In composition they 
approximate closely to anorthite, and are thus more basic than the 
glomeroporphyritic felspars (bytownite) of the cognate xenoliths 
previously described (p. 238). Their formation was often accom- 
panied by the almost simultaneous separation of corundum and 
spinel, in a manner that suggests that these three phases were 
capable of existence in equilibrium one with the other and with the 
melt at the time of their formation (p. 252). 

The early-formed anorthite has most often undergone consider- 
able compositional modification as the conditions of equilibrium 
between xenohth and magia were disturbed. There has frequently 
been disruption of the coarsely crystalline zone and resorption of 
the anorthite and its contained sillimanite. This was followed by 
an additional crystallization on the remaining anorthite of an in- 
creasingly acid plagioclase, which ranges through labradorite to 
oligoclase. Such later-formed felspar is free from included silli- 
manite, and the excess of alumina furnished by its resorption has 
separated either as corundum (sapphire) or as spinel (Pl. VIII, 
figs. 1,2, & 5). Corundum, as might be expected, occurs more 
frequently nearer the source of alumina (buchite) ; while spinel 
crystallizes nearer the source of magnesia andiron (magma). The 
latest crystallization of felspar was of a rapid character, and ante- 
dated but little the intrusion of the modified xenoliths into their 
present position. It is usually of oligoclase composition, and may 
occur either asan outer zone to pre-existing more basic felspars, or 
as freshly grown individuals of skeletal form which have separated 
from the residual melt (Pl. X, figs. 4&5). With regard to the 
formation and resorption of the earlier felspars, it is interesting 
to note analogous processes going on in artificial melts. Mr. G. V. 
Wilson, writing on certain minerals formed in glass-furnaces, calls 
attention to the fact that felspars entering into the composition of 
an absorption-zone between the glass-melt and the furnace-brick 
(aluminous ) were full of sillimanite-needles, but that the sillimanite 
of the melt, as of the buchite, was undergoing resorption in order 
to supply alumina for the formation of felspar.! 


Cordierite.—Cordierite appears to result in the xenoliths, either 
from the complete solution of aluminous material by a tholeiite- 
magma, whereby a cordierite-buchite? or cordierite-sillimanite- 
buchite is formed; or from the reaction of the magma with the 


1 “Notes on the Formation of certain Rock-forming Minerals in & about 
Glass-Furnaces’ Trans. Soc. Glass Technology, vol. ii (1918) p. 177. 

2 For a description of- cordierite-buchites, see J. S. Flett, ‘ Geology of 
Colonsay, Oronsay, & part of the Ross of Mull’ Mem. Geol. Sury. 1911, 
pp. 94, 95; Prof. A. Lacroix, op. jam cit. p. 21, comments on the remarkable 
sharpness of the crystals and the triple twinning of the cordierite in rocks of 
this nature; see also J. J. H. Teall, ‘The Natural History of Cordierite & 
ee Proc. Geol. Assoc. vol. xvi (1899) p. 61, with Bibliography on 
p. 74. 

Q.J.G.8. No. 311. S 


246 DR. H. H. THOMAS ON XENOLITHIC [ vol. Ixxviu, 


still fluid matrix of a sillimanite-buchite which gives rise to 
cordierite and spinel, or cordierite and corundum, -according to 
the amount of available magmatic magnesia (see analyses, p. 236). 

In the former case of direct and complete absorption of aluminous 
material by the magma, the cordierites formed are either colourless, 
rectangular, single crystals with distinct outline, or small triple and 
complex twins. In magma contaminated with aluminous xenolithic 
matter, the percentage of alumina may vary within widely-separated 
limits, and there may be no excess of alumina beyond that necessary 
to form cordierite with the available magnesia and silica. The 
result, in such a case, is the formation of a cordierite-buchite that 
consists solely of cordierite and brown glass (Pl. VII, fig. 3). Such 
cordierite must be regarded as of pyrogenetic, rather than of meta- 
morphic, origin. When there is a greater proportion of xenolithic 
matter, sillimanite is the earlier product of crystallization, and this 
is followed by cordierite, giving rise to a cordierite-sillimanite- 
buchite similar to that figured in Pl. VII, fig.2. The formation 
of cordierite under such conditions has been dealt with and described 
by many authors. 

When the tholeiite-magma reacts upon an already-formed silli- 
manite-buchite, bringing about mutual modification by the trans- 
ference of alumina from the xenolith to the magma, and of magnesia 
from the magma to the xenolith, large crystals of cordierite grow 
outwards from the magmatic side into the buchite, keeping pace 
with the diffusion of magnesia. Much of the sillimanite of the 
buchite is generally resorbed, but such cordierite-crystals commonly 
enclose a considerable undissolved portion in the form of the usual 
slender needles. These needles are often to be seen passing across 
the boundary between cordierite-erystals and buchite (Pl. X, fig. 1). 

Material collected from Locality 6, 8500 feet north of west of 
Ormsaig, shows beautifully the result of interaction between silli- 
manite-buchite and magma, and is illustrated by figs. 2 &3 of Pl. X. 
The sillimanite-buchite may be seen to pass, in the direction of the 
igneous rock, into a sillimanite-cordierite-buchite of somewhat 
coarsely crystalline character, the relatively large size of the crystals 
being attributable to sustained elevated temperature. Adjoining 
the true sillimanite-buchite the cordierites are of large size, and, 
owing to their remoteness from the source of magnesia, contain no 
inclusions of spinel; but nearer the magma, where more magnesia 
and less silica were available, the cordierites are full of mirute 
brownish-green crystals of spinel (Pl. VII, fig. 4). 

As is occasionally the case with cordierite of a pyrogenetic 
nature, the mineral is colourless, exhibits no pleochroism, and in no 
instance shows pleochroic halos around inclusions such as are usual 
in the cordierite of the crystalline schists and metamorphic rocks. 
The formation of cordierite in the xenoliths appears in all cases to 
belong to a relatively late stage in their metamorphism, and to be 
the natural product of a magma that had become relatively richer 
in silica by the normal process of differentiation, acting on highly- 
aluminous material such as the sillimanite-buchite. The proportion 


part 3] MINOR INTRUSIONS IN THE ISLAND OF MULL. 247 


of the cordierite so formed to spinel is determined by the relative 
amounts of magnesia and silica available. If there be an excess of 
silica, cordierite would form alone; but, witha deficiency in silica, 
spinel would be its necessary companion.! 


Spinel.—The spinellids of the xenoliths, taken collectively, 
present a considerable variety of colour and, presumably, of compo- 
sition; but, considered in connexion with their respectively-associated 
minerals, they show a fair amount of consistency. The chief mode of 
occurrence of spinel is as a deep-green variety (hercynite-pleonaste), 
in intimate association with anorthite in the external crystalline 
zone of the xenoliths. The manner of association of these two 
minerals indicates contemporaneous crystal-growth, with a gradual 
convergence towards eutectic composition. Sucha condition appears 
to have been actually realized in some instances under special con- 
ditions, anda highly characteristic structure has resulted (Pl. VIII, 
fig.5). In this association the spinel is usually without well- 
defined crystalline boundaries, and its form is almost exactly similar 
to that assumed by olivine, when associated with anorthite in such 
rocks as the allivalites of Dr. A. Harker.’ 

It has presumably been formed through the solution of sillimanite 
and an excess of alumina of the sillimanite-buchite by a magma 
locally enriched with lime, ferrous iron, and, to a less extent, 
magnesia. An analysis of this early-formed leek-green spinel, 
separated from the anorthite-corundum-spinel assemblage of a 
xenolith from Locality 2, is given below, and indicates a variety 
lying between the magnesian pleonaste and the ferrous hercynite. 


ANALYSIS OF DARK-GREEN SPINEL FROM XENOLITH (ANAL. III, p. 236), 
Nouws’ Pass, py E. G. Raney. 


It ele 
Per cent. Per cent. 
SO by A, Ee ee O77 — 
ANI Oe. 8h SOA ane age 60°84, 61°70 
eb Ot ert fae eke ek 426 4°32 
REO Ma ea ates Fe Ae Oe 24°00 24°34, 
INO @ jie Oa i 8 re aria 0715 0°15 
CAON he ei. : te cate 0°36 0°36 
MGOW A otk e. LUenO 37. 9°50 
H>2O above 105° C.... 0°14 — 
Motalses 100°39 100°37 


The figures given in the second column are derived from those of the first by 
recalculation, after rejecting silica, titanic oxide, and water, which are regarded as 
non-essential constituents. See ‘Summary of Progress for 1913’ Mem. Geol. Surv. 
1914, pp. 80, 81. 


A very beautiful relation between spinel and anorthite may 


1 G, A. F. Molengraaff, ‘ Cordierit in einem Eruptivgestein aus Stidafrika ’ 
Neues Jahrb. vol. i (1894) p. 79; see also J. Morozewicz, ‘ Experimentelle 
Untersuchungen tiber die Bildung der Minerale im Magma’ Tscherm. Min. 
Petr. Mitt. vol. xviii (1898-99) pp. 56, 57. 

2 «The Natural History of Igneous Rocks’ 1909, fig. 49, p. 170. 

s 2 


248 DR. H. H. THOMAS ON XENOLITHIC [vol. lxxvin, 


be studied in specimens from Locality 16, figured in Pl. VIII, 
figs.3 & 4. Here spinel-crystals, separated from a glassy base, have 
acted as nuclei upon which have grown crystals of anorthite, in 
such a manner as completely to surround the spinel with an 
irregular but thin zone of this mineral. This relationship existing 
between spinel and anorthite is one of the most striking features of 
the xenoliths, and is easily explicable in the ight of work recently 
carried out on the ternary system anorthite-forsterite-silica.! 

Asa much darker variety (black, brown, or dark plum-colour), 
spinel builds regular well-shaped octahedra in the outer and 
most highly-modified portions of the xenoliths. There is good 
reason to believe, from the microscopic evidence, that these spinels 
result from a later interaction of magmatic and xenolithie material, 
whereby the already-formed anorthite that contained sillimanite has 
been resorbed by a magma relatively rich in magnesia; and that 
the excess of alumina so provided has been precipitated as spinel, 
accompanied by the separation of a sillimanite-free less basic 
plagiociase (oligoclase) and glass of magmatic nature. Sillimanite- 
bearing anorthite in all stages of dissolution, and the separation of. 
spinel together with the formation of less basic felspars, are features. 
illustrated by several examples in Plates IX & X. The sharply 
idiomorphie form of the spinel-crystals is well shown in Pl. X, 
fig.4 and Pl. VIII, fig. 2. A late conversion of spinel into yellow 
or green serpentine is not an uncommon feature of the xenoliths. 
The serpentine either forms a peripheral skin (Pl. IX, fig. 2), or 
completely replaces the crystal. 


= Probable Nature of the Unaltered 
Xenolithic Material. 


The aluminous xenoliths, although complex 1 in mineral contents, 
and varying considerably in composition in different parts of their 
mass, are clearly fragments of highly-aluminous shale that have 
suffered progressive metamorphism by a basic igneous magma. 

There is no doubt, from the character and composition of the 
sillimanite-buchite, that this material has been produced by the 
simple fusion of aluminous sediment, and that it is unmodified in 
any way by the transfusion of igneous matter. Its composition is 
represented by Analysis IV (p. 236), from which it will be seen 
that it 1s comparable to a highly aluminous fireclay, and that it is 
free from all bases such as would be present if contamination by an 
igneous magma had taken place. Microscopically, it consists of 
glass in which are embedded, often with clear fluidal arrangement, - 
minute but abundant prisms of sillimanite, an occasional crystal 
of corundum, and a very small quantity of magnetite or some other 
spinellid. The original rutile-needles of the sediment appear, in 
some instances, to be preserved. 

The analysis indicates that corundum, separately determined, 
constitutes about 0°5 per cent. of the whole, and _ sillimanite 


1 QO. Andersen, ‘The System Anorthite-Forsterite-Silica ’ Amer. Jour. Sci. 
ser. 4, vol. xxxix (1915) p. 407. 


part 3] MINOR INTRUSIONS IN THE ISLAND OF MULL. 249 


18 per cent., the remaining constituents entering into the compo- 
sition of the glass, which holds a fairly large amount of water in 
solution. Such rocks are well known from other localities, and are 
produced artificially every day in glass-pots and furnace-nozzles by 
the fusion of the aluminous material of which they are constructed. 

The uniformity observed in the aluminous xenolithic material 
makes it clear that we have only one type of aluminous sediment to 
consider, and that this was in all probability of wide distribution. 

One somewhat unusual feature brought out by Mr. Radley’s 
analysis 1s the excess of soda over potash. In most clays the 
reverse is the case, but exceptions are not unknown. 


(iv) Relation of the Xenoliths to the Enclosing Rock. 


A striking feature connected with the xenoliths is the extreme 
sharpness with which they are marked off from the enclosing rock, 
and the unmodified character of the igneous rock in contact with 
them. It is clear without additional evidence that the meta- 
morphism was not effected in place by the igneous magma now 
represented by the surrounding tholeiite, but was carried out at 
much greater depths and at a more elevated temperature than that 
appertaining to the rock at the time of its intrusion. 

The most convincing evidence of deep-seated metamorphism, 
however, is furnished by the fact that the enclosing rock often bears 
no relation to the various metamorphic zones of the xenoliths, and 
therefore cannot have produced them. It has been pointed out 
(p. 241) that these zones are occasionally concentric and roughly 
spherical in form, more often they occur as crescentic or straight 
parallel layers ending abruptly at the surface of a xenolith. It is 
abundantly clear, as we shall see, that these zones are the result of 
progressive metamorphism, and yet in the portion of the sill that 
we can study we find the enclosing rock crossing from one meta- 
morphic zone to another—from sillimanite-buchite to the anorthite- 
sapphire-spinel-layer—without any corresponding change in its 
texture or composition, or any diminution in the sharpness of the 
line of junction. No better evidence is needed to prove that the 
metamorphism of the xenoliths was practically complete before they 
were carried by the molten igneous rock into their present position. 

That the xenoliths were still plastic and partly molten at the 
time of their intrusion is shown by their often intensely vesicular 
character. ‘These vesicles occur in the now glassy portion of the 
xenoliths, and mark the escape of gas from the molten material on 
the intrusion of the xenolith from a region of high to a region of 
lower pressure. 

Prof. A. Lacroix has pointed out, from his study of xenoliths, 
that the mutual chemical changes produced by a magma on 
accidental inclusions and vzce versa are usually quite limited ; and, 
therefore, he rightly asserts that, in order to produce any con- 
siderable effect, a prolonged sojourn of the xenoliths within the 
molten magma is necessitated. 


1G. V. Wilson, op. jam cit. p. 200; A. H. Cox, ‘Notes on some South 
Staffordshire Fire-clays & their Behaviour on Ignition’ Geol. Mag. 1918, p. 59. 


250 DR. H. H. THOMAS ON XENOLITHIC [vol. xxvin, 


(v) The Conditions of Metamorphism. 


It is clear that the metamorphism of the aluminous sediment that 
gave rise to the xenoliths was effected before the intrusion of the 
sill. Further, it is evident that the xenoliths, as we now find them.,. 
owe their main characters to both physical and chemical action on 
the part of the molten igneous rock, the one producing simple 
melting without transfusion of material from the magma, the other 
bringing about a local enrichment of certain bases (lime, ferrous 
oxide, and magnesia), which resulted in the formation of anorthite, 
sapphire, and spinel. There is little doubt that the greater part of 
the metamorphism of the aluminous sediments was actually effected 
in the walls of the magma-basin, and that the xenoliths are mainly 
due to the disruption of the metamorphosed lining of the basin 
preliminary to the intrusion of the magma into its present higher 
position. Only in this way is it possible to account for the lack of 
symmetry of the metamorphic zones in the xenoliths and the trans- 
gression of the enclosing igneous rock across zones that clearly 
indicate different phases of metamorphism. A general idea of the 
process was many years ago clearly outlined by Sir Jethro Teall, 
who said :— 


‘The subterranean magmas act powerfully on their containing walls, and 
transform highly argillaceous sediments into crystalline rocks composed of 
cordierite, sillimanite, biotite, quartz, and sometimes spinelle and corundum. 
The rocks of the inner contact-zone become shattered, and the igneous magma. 
insinuates itself between the cracks, or may even permeate the mass. Portions 
of the metamorphic rock float off into the molten material, and travel with it 
through dykes and other channels to the surface.’ ! 


In the cases under description the first change effected in the 
aluminous sediment was simple fusion forming a viscous melt, from 
which the excess of alumina was early to separate as corundum and 
sillimanite. The former exists as small hexagonal plates, the latter 
as minute elongated prisms. The rapid separation of sillimanite 
rendered the fused layer still more viscous, and presumably enabled 
it, without application of external stress, to retain its position as a 
lining to the magma-basin. Simultaneously, however, the trans- 
fusion of bases, more particularly of lime, was proceeding from the 
magma into the aluminous melt that formed the matrix of the 
alr eady separated sillimanite. This transfusion produced a melt of. 
such composition that anorthite was capable of crystallization as a 
primary phase, and, owing to the partial resorption of sillimanite 
to furnish the necessary silica, there was a simultaneous precipitation 
of corundum. 

The anorthite thus formed enclosed the undissolved sillimanite- 
needles of the original melt (Pl. VIL). Similarly the transfusion 
of the bases ferrous iron and magnesia gave rise to spinels of 
various compositions, which occur occasionally intergrown with 
corundum, and either completely enclosed in anorthite or in 
approximate eutectic relationship to this mineral. 


1 <The Natural History of Cordierite & its Associates’ Proc. Geol. Assoc. 
vol, xvi (1899) p. 65. 


part 3] MINOR INTRUSIONS IN THE ISLAND OF MULL. 251 


The relations borne by the corundum, spinel, and anorthite one 
to the other indicate clearly that a melt was produced at the 
surface of the aluminous material from which these three minerals 
crystallized with more or less perfect equilibrium, but in which 
sillimanite was undergoing slow re-solution. The condition in- 
dicated is one of slow cooling. 

We are here dealing with the crystallization of a quaternary 
system subject to constant influx of material by diffusion; but, 
from the conditions controlling the separation of solid phases in 
the ternary systems anorthite-forsterite-silica and lme-alumina- 
silica, we are able to gather reasonable ideas as to the temperature- 
concentrations that operated in the case of the xenoliths. 

Let us consider the probable condition of the magima within its 
basin. We know from the cognate xenoliths present within the 
tholeiite, that the magma had commenced to crystallize before 
its upward intrusion, and that crystals of bytownite and pyroxene 
(both rhombic and monoclinic) were separating and being segre- 
gated under the influence of gravity. The separation of pyroxene 
and bytownite is what might be expected from a magma of the 
composition indicated by Analysis II, p. 236. The gabbroic or 
noritic matter so separated may be considered to have crystallized 
under conditions of more or less perfect equilibrium, without 
considerable under-cooling, and thus a temperature of initial crys- 
tallization between 1200° and 1300° C. is indicated.! 

We have a useful temperature-indicator in tridymite, the only 
phase of silica that existed as a metamorphic product, either in the 

xenoliths or in the Carsaig Sandstone in contact with the intruded 
sill. The absence of cristobalite and presence of tridymite in both 
instances would suggest that the temperature of the magma 
within its basin and at the time of its intrusion was lower than 
1470° C. : 

Except for the presence of tridymite in the siliceous xenoliths, 
we have little to guide us in an estimation of the initial tempera- 
ture of the magma within its basin; but from the strong evidence 
that fusion of the basin-walls proceeded more rapidly than the 
co-solution of the aluminous melt and magma, we may infer that 
the magmatic temperature was at any rate above that necessary to 
vitrify aluminous sediments. Such a temperature has been proved 
by experiment,” in the case of dry melts at atmospheric pressure, 
to be in the neighbourhood of 1500° C. The retention of much 
water, however, as well as the presence of small quantities of 
fluxes, would undoubtedly lower the temperature of vitrification 
considerably, probably below 1400° C. We are, therefore, fairly 
safe in regarding the metamorphism of the xenolithic material as 
commencing ata temperature above, but not far above, that which 
marked the separation of the cognate xenolithic material from the 


3G. A. Rankin, ‘The Ternary System CaOQ-Al,O,-SiO,’ Amer. Jour. 
Sei. ser. 4, vol. xxxix (1915) p. 1; O. Andersen, ‘The System Anorthite- 
Forsterite-Silica’ ibid. p. 407. 

2 A. H. Cox, Geol. Mag. 1918, p. 59. 


252 DR. H. H. THOMAS ON XENOLITHIC [ vol. lxxviu, 


magma, and probably continuing up to the time of the magma’s 
intrusion into higher levels. 

The vitrification of the aluminous ‘ bank’ was followed almost at 
once by the separation of corundum and sillimanite from the 
aluminous melt and, contemporaneously, a narrow diffusion-band 
of hybrid character was formed between the normal magma and the 
fused lining of the basin. Across the junction of sediment and 
magma the following zones would appear to have been present :— 

(a) Unfused aluminous sediment; (b) fused sediment from which silli- 
manite and some corundum has separated; (c) a zone of commingling of the 
aluminous melt with the magma; and (d) the magma in an unmodified 
condition. 

It is the hybrid zone (c) that gives to the xenoliths their most 
remarkable character. The liquid formed by the commingling of 
aluminous melt and magma was of such a composition that 
corundum, anorthite, and spinel were, relatively speaking, insoluble, 
and were early phases of crystallization, and that the sillimanite of 
zone (6) was unstable and tended to dissolve. 

Corundum and spinel have often crystallized together, and 
there is evidence of the attainment of eutectic relations between 
anorthite and spinel. Corundum was in most cases the primary 
phase, and was followed rapidly by the separation of anorthite and 
spinel, as the composition of the melt changed with continued 
crystallization. 

According to the work of Rankin, the temperature at which 
anorthite, corundum, and sillimanite can co-exist in equilibrium 
with the melt is represented by a quintuple point on the ternary 
diagram of the system CaO—A1,O,—Si0,, and corresponds to a tem- 
perature of 1512°C. In the xenoliths, however, this condition 
does not appear to have been attained, for sillimanite was evidently 
undergoing solution during the formation of the anorthite. The 
actual conditions of temperature and concentration, therefore, are 
more probably represented by some point on the boundary-curve 
between the corundum- and anorthite-fields and away from the 
quintuple point. The temperatures along this boundary-curve 
range from 1500° to 1880° C., generally lower than that of the 
quintuple point, and suggest that the separation of corundum and 
anorthite took place at a less elevated temperature. 

Confirmation of a lower temperature is also furnished by the 
mutual relations observed to exist between corundum and spinel. 
Their obviously contemporaneous and early separation from the 
melt would indicate some such conditions as those expressed by 
the boundary-curve between the corundum- and spinel-fields in 
the diagram for the ternary system anorthite-forsterite-silica, 
and covering a range of temperatures from 1450° to 13825° C. 
Again, we have the observed eutectic relation of anorthite to spinel. 
From the work of Olaf Andersen we learn that with relatively 
small percentages of magnesia, anorthite and spinel can co-exist in 
equilibrium with the melt at a temperature as low as 1440° C. 

All the evidence, therefore, so far as it can be co-ordinated, 


part 3] MINOR INTRUSIONS IN THE ISLAND OF MULL. 263 


points to a maximum temperature in the neighbourhood of 14.00° C. 
as that at which the formation of the xenolithic minerals was likely 
to have taken place. We must remember, however, that all the 
work that has been done on the various ternary systems has been 
carried out at atmospheric pressure and under anhydrous conditions. 
In the case of the xenoliths we are, in the first place, not dealing 
with pure ternary mixtures, but with melts containing a far greater 
number of components the presence of which will undoubtedly 
depress the freezing-points of the various solid phases. Operating 
in the same direction will be the large amount of water held in 
solution at the high pressure obtaining within the magma-basin. 
It is fairly certain, therefore, that the probable temperatures of 
separation of the various crystalline phases that occur in the xeno- 
liths, were lower than those judged to have existed in analogous 
but not identical anhydrous melts. How much lower the present 
state of our knowledge leaves an open question. 

So soon as crystallization commenced in the hybrid zone (¢) we 
must assume that transfusion of the components entering into the 
composition of the separating phases began, and was continued as 
fast, and for as long a period as the viscosity of the melt and 
magma would allow. That this diffusion accompanying crystal- 
lization was a factor of considerable importance will be seen at 
once on comparing the analyses of tholeite (11), hybrid zone (111), 
and buchite (IV), given on p. 236. From these it is obvious 
that the ultimate composition of the hybrid zone could not be 
reached by simple mixture of fused aluminous sediment with 
thole1te-magma, but only by the selective diffusion of their respec- 
tive component oxides. While Analysis III (p. 236) reptesents a 
particular phase of interaction between tholeiite and fused 
aluminous sediment, the reader is warned against regarding it 
as expressing the composition of a simple mixture. The normal 
erystallized zone consists of anorthite, corundum, and spinel, and 
clearly indicates diffusion of alumina from the sediment and of 
lime, and to a less extent of ferrous oxide and magnesia, from 
the tholeite. There has also been a relative concentration of 
alkalies. 

Anorthite being an early and dominant phase to separate from 
the melt represented by the hybrid zone, it follows that the magma 
in the immediate neighbourhood would be impoverished as regards 
lime and relatively enriched as regards magnesia, iron, and alkalies. 
‘The slower transfusion of magnesia makes itself evident in the 
precipitation of spinel, more particularly in that portion of the 
hybrid zone which hes nearest to the magma. The extent of such 
‘an impoverished zone would depend on the viscosity of the melt, 
and thereby the extent to which diffusion could operate. We have 
every reason to believe that this zone was not wide, and that, for 
all practical purposes, we may regard the bulk of the magma within 
the basin as unmodified by loss of bases or by the assimilation of 
‘sedimentary material. 


254: DR. H. H. THOMAS ON XENOLITHIC [ vol. Ixxviii,. 


We now come to another and obviously later phase of meta-- 
morphism that has been superinduced upon the buchite (6) and 
the crystalline zone (c). Disruption of the already crystallized 
anorthite-corundum-spinel assemblage of the hybrid zone, by 
stresses acting within the basin prior to the upward intrusion of the 
magma, allowed the locally-modified magma mentioned above to 
make fresh contacts with the buchite, and with the more aluminous 
portion of the anorthite-corundum-spinel zone adhering to the walls 
of the magma-hasin. This partial magma being relatively rich in 
magnesia, lron, and alkalies, the new contacts resulted in a fresh 
set of concentrations. Cordierite (p. 245) under the new con- 
ditions was one of the chief crystalline substances to separate out; 
while the rapid enrichment of the melt by alumina from the re- 
sorbed sillimanite, taken in conjunction with the excess of magnesia 
beyond that required for the formation of cordierite, produced an 
almost simultaneous precipitation of corundum and spinel. The 
melt, being of a more alkaline character than previously, caused 
the resorption of much of the early-formed anorthite, and only 
permitted the subsequent separation of a plagioclase of increased 
alkalinity (p. 245). 

Reviewing the temperature-conditions, as suggested by the erys-. 
talline phases of the accidental and cognate xenoliths, we are led 
to the conclusion that the temperature-interval during which the 
metamorphism of the aluminous material was accomplished was 
small, and probably lay between 14.00° and 1250°C. On the other 
hand, the time interval was great, for extremely slow cooling and 
no considerable change in the viscosity of the melt are indicated. 

Towards the end of this period, a limit only imposed by the 
upward intrusion of the magma, that magma had commenced to. 
crystallize on its own account, with the formation of the cognate 
xenoliths at a temperature that did not exceed 1800° ©., and was. 
probably still lower. At the time of upward intrusion the tem- 
perature was sufficient to keep molten the glassy matrix of the 
sillimanite-buchite, and to produce tridymite-frimges upon the 
quartz-grains of the Carsaig Sandstone. 

Experiments on the glassy matrix of the sillimanite-buchite 
prove that it melts at a temperature near that of fluorspar 
(1250° C.), so that we may suggest that the temperature of the 
magina at the time of its intrusion approximated to this value. 

It will be seen from the above that the probable range of tem- 
perature covering the metamorphic processes that operated within 
the basin, and the subsequent upward intrusion of the magma, is 
surprisingly small, possibly less than 200° C. 

At the time of upward intrusion all further transfusive action 
between magma and xenoliths would be checked by rapid cooling, 
and this would account for the sharp boundaries presented by the 
xenoliths to the surrounding tholeiite. 


part 3 | MINOR INTRUSIONS IN THE ISLAND OF MULL. 255: 


V. SUMMARY AND CONCLUSIONS. 


The western portion of Mull lying between Loch Scridain and 
Loch Buie is remarkable for a series of minor intrusions that are 
generally of a tholeiitic and andesitic character. Occasionally they 
are composite, and one of the best examples of such a sill is that 
which occurs at Rudh’ a’Chromain and at Nuns’ Pass west of 
Carsaig. They frequently contain abundant xenoliths of both 
accidental and cognate nature. ‘The cognate xenoliths are glomero- 
porphyritic masses of bytownite and hypersthene, or bytownite 
and augite, and are frequently congregated in the lower portions. 
of the intrusions. The accidental xenoliths are both of siliceous 
and aluminous types, the aluminous inclusions being generally by 
far the more prevalent and ranging up to several feet in diameter. 
The siliceous xenoliths show the usual type of alteration: quartz- 
grains have developed fringes of tridymite, and in some instances 
are surrounded by secondary augite. The aluminous xenoliths, 
with which this paper deals more particularly, are characterized by 
well-crystallized minerals. such as corundum (sapphire), spinel, 
sillimanite, cordierite, and anorthite. 

These aluminous xenoliths offer the clearest evidence of the 
modification of a more or less pure aluminous sediment by permea- 
tion of magmatic matter, more particularly by the diffusion of 
lime, ferrous iron, and magnesia. ‘The aluminous sediment, when 
simply altered by thermal agencies, takes the form of a sillimanite- 
buchite (sillimanite and glass), and this by the chemical action of 
the magma has most often been transformed, in whole or in part, 
into a coarsely-crystalline mass of anorthite that encloses silli- 
manite, corundum, and spinel. Further action by the magma in a 
modified form, in which it is relatively enriched by magnesia and 
alkalies, has resulted in the formation of cordierite arth a con- 
comitant separation of corundum, spinel, and an acid plagioclase. 

It is held from the evidence afforded by the xenoliths that the 
metamorphism was of a deep-seated character, and was produced 
by a tholeiitic magma acting upon the lining of its basin, this 
lining being constructed largely of aluminous sedimentary material. 
The deep-seated character of the metamorphism is supported by 
three facts :—(1) the xenoliths show practically no signs of modi- 
fying, or of modification by, the igneous rock now in contact with 
them; (2) during the process of intrusion into their present 
position they have suffered deformation, owing to their still plastic 
state; and (3) they have developed vesicular cavities in their 
contained glassy matter, consequent on the decrease of pressure by 
which dissolved volatile constituents were allowed to escape. The 
vesicular cavities have in many instances been filled with low- 
temperature minerals such as analcite, chalcedony, and calcite. 
The mutual relations of the various xenolithic minerals, interpreted 
in the light of recent work on fused mixtures of silicates and 
oxides, suggest that the temperature of the magma in its basin 
during the period in which the metamorphism was effected probably 


256 DR. H. H. THOMAS ON XENOLITHIC [vol. lxxviu, 


ranged from near 1400° to 1250° C., the lower temperature 
approximating to that at which the magma, together with its 
yenoliths, was intruded into its present position. 

There is practically no evidence such as would lead to the 
conclusion that the magma as a whole has been modified to 
any extent by assimilation of sedimentary material, despite the 
fact that certain intrusions consist almost entirely of xenolithic 
matter. 


EXPLANATION OF PLATES VI-X. 


[The numerals preceded by S are the registration-numbers of the respective 
rock-slices in the collections of the Geological Survey. | 


I am indebted to Mr. John Rhodes for the preparation of the photomicro- 
graphs which are reproduced in these plates. 


PLATE VI. 


Fig. 1. Glassy acid interior of a composite sill, Rudh’ a’Chromain. Rhyolite 
allied to inninmorite. Blade-like and acicular crystals of augite 
and small porphyritic crystals of acid labradorite in a pale 
lavender-brown glassy matrix. Local devitrification has given 
rise to patches of acid labradorite, showing white in the figure. 
S. 18486. 16:5. (See p. 235.) 

2, Acid interior of a composite sill, Rudh’ a’Chromain, more or less 
completely devitrified. The more complete devitrification has 
caused the separation of less basic plagioclase, together with some 
orthoclase and quartz. S. 18488. x 18. (See p. 235.) 

3. Tholeiite. Upper edge of the basic portion of a composite sill, 
Rudh’a’Chromain. Narrow lath-shaped crystals of labradorite and 
chloritized augite, in a brown glassy matrix charged with magnetite. 
S. 18480. x 18. (See p. 235.) 

4, Tholeiite. Basic stony portion of a composite sill, Rudh’ a’Chro- 
main, showing the characteristic structure assumed by these rocks. 
The constituents are labradorite and hypidiomorphic augite, with 
a relatively small amount of residual glass. S. 18490. xX 18. 
(See p. 235.) 

5. Cognate xenolith, in the lower basic portion of a composite sill, 
Rudh’ a’Chromain. The xenolith consists of glomeroporphyritic 
bytownite and greenish augite. The enveloping tholeiite is similar 
to that shown in fig. 4. S.17173. x 15:5. (See p. 238.) 

6. Cognate xenolith, in the lower basic portion of a composite sill, 
Rudh’ a’Chromain. The xenolith consists of bytownite and hyper- 
sthene. The hypersthene is sharply idiomorphic and intensely 
pleochroic. The felspars are idiomorphic, of large size, moulded 
on the hypersthene, and their composition is that of basic labra-_ 
dorite or bytownite. S. 16598. x18. (See p. 238.) 


Puate VII. 


Accidental Xenoliths. 


Fig. 1. Sillimanite-buchite, 3500 feet north of west of Ormsaig. Needles 
of pale-pink sillimanite, embedded in clear pale lavender-coloured 
glass. Distinct fluxion-structure is developed locally. The rock 
is slightly vesicular, the vesicles being filled with zeolites and 
calcite, and results from the simple fusion of an aluminous shale. 
S. 18005. x 36. (See p. 243.) 


part 3] MINOR INTRUSIONS IN THE ISLAND OF MULL. 257 


Fig. 2. Sillimanite-cordierite-buchite, 500 yards north-east of 


Fig. 


Abhuinn Bail’ a’Mhuilinn, 1000 yards north of west of the bridge 
over Allt a’Mhuchaidh. The rock contains a few large and many 
small crystals of anorthite, in a glassy matrix that is partly mag- 
matic and felspathic and partly a sillimanite-cordierite-buchite, as 
figured. The cordierite-crystals are particularly well-formed. The 
rock suggests the incomplete solution of aluminous shale in a 
tholeiitic magma. S. 18529. x 1381°5. (See p. 245.) 


. Cordierite-buchite, Allt a’Mhuchaidh, 1000 yards above the 


bridge. Composed of beautifully-formed crystals of cordierite, 
many showing complex twinning, in a matrix of brown glass. The 
rock is due to the local solution of aluminous xenolithic material 
by the tholeiitic magma. S.17997. x10. (Seep. 245.) 


. Sillimanite-cordierite-buchite, Ormsaig. Large well-formed 


rose-pink crystals of sillimanite, and crystals of cordierite crowded 
with minute brownish-green spinels, in a brown glassy matrix. 
The striking feature of the rock is the size and colour of the 
sillimanite - crystals: their large size suggests long - continued 
heating and relatively-slow crystallization. §. 1800la. x 11. 
(See p. 245.) 


. Siliceous xenolith of partly dissolved sandstone, Feorlein Cottage, 


Carsaig. The individual quartz-grains show beautiful tridymite- 
fringes, now reverted to quartz. Large plates of tridymite, similarly 
changed, occur in the glassy matrix. §. 20283. x 20. (See 
p. 240.) 


. Siliceous xenolith, south-east of Gortein Driseach. The quartz 


has undergone resorption by a melt of mixed character that has 
consolidated as a streaky glass. Tridymite-fringes occur around 
the quartz-grains, and cordierite is to a small extent developed in 
the matrix. The rock has been produced by the action of a 
tholeiitic magma on a quartzite. S. 16067. x 11. (See p. 239.) 


Puate VIII. 


. Small crystals of anorthite, showing evidence of rapid growth, forming 


in a glassy magmatic matrix. They are the result of reaction 
between the magma and an aluminous xenolith, and are accompanied 
by spinel. Allt a’Mhuchaidh. §S.17998a. x15. (See p. 244.) 


. Anorthite-sapphire-spinel assemblage. The normal crystal- 


line aggregate that appears to result from the initial action of the 
tholeiite-magma on an aluminous xenolith. S. 16611. x 12°5. 
(See p. 240.) 


. This shows the crystallization of sapphire and deep-green spinel in 


brown magmatic glass. Both minerals are separated from the 
glass by zones of anorthite-crystals. The manner in which the 
crystals of anorthite are planted upon the spinels is particularly 
striking ; see also fig. 4. Allt a’Mhuchaidh, 1000 yards above the 
bridge. S. 17999. x 7. (See p. 247.) 


. Another portion of the same section with higher magnification. Note 


the well-formed crystals of anorthite growing in brown glass, and 
planted on an elongated crystal of greenish-brown spinel. The 
spinel is possibly pseudomorphous after sillimanite. S. 17999. 
x 42. (See p. 247.) 


. Anorthite-spinel assemblage of an aluminous xenolith, showing 


green spinel and anorthite in eutectic relationship. The structure 
probably results from the reheating of the ordinary spinel-anorthite 
aggregate of an aluminous xenolith—a point emphasized by the 
occurrence of pseudomorphs after cordierite within the anorthite 
of another part of the slide. Old Road, 100 yards north-west of 
Feorlein Cottage, Carsaig. S. 20286. x 29. (See p. 247.) 


258 


Fig. 6. 


DR. H. H. THOMAS ON XENOLITHIC [ vol. Ixxviil, 


Aluminous xenolith, showing anorthite full of minute crystals 
of corundum and invaded by magma. The magma has resorbed 
part of the original felspar with its excess of alumina, bringing 
about the growth of a less basic felspar in optic continuity with 
that previously existing, and the precipitation of well - formed 
plum -coloured spinels. <A large crystal of spinel occupies the 
centre of the field. Rudh’ a’Chromain. 8.16605. x 12. (See 
p. 245.) 


PLATE IX. 


Fig. 1. Xenolith invaded by tholeiitic magma. The mass consists of anorthite 


Or 


full of sillimanite-needles which have been partly re-dissolved, with 
the subsequent growth of secondary felspar on the remaining 
anorthite. The later felspar is free from sillimanite, and consists 
partly of anorthite, partly of oligoclase, which forms the outer- 
most zone and sends skeleton growths out into the glassy matrix. 
The later formation of the felspar of more acid character has been 
accompanied by the precipitation of the excess of alumina as 
sapphire and spinel. The sapphires (of which several are shown 
in the figure) are surrounded by feathery growths of oligoclase. 
Rudh’ a’Chromain. §.17177. x 13°5. (See p. 245.) 


. Xenolith invaded by tholeiitic magma. The section shows anorthite 


full of sillimanite, and a sillimanite-buchite invaded by dark tho- 
leiitic matter (glass). Resorption of anorthite with its contained 
sillimanite has taken place, followed by the crystallization of large 
dark-green spinels (and cordierite enclosing spinel). A less basic 
felspar, following solution of the sillimanite-bearing anorthite, has 
grown outwards from the undissolved anorthite, and forms a 
noticeable zone between it and the magmatic glass. Shore a quarter 
of a mile from the mouth of Allt Coille Moire. 8S. 17405. »x 16. 
(See p. 247.) 


. Original anorthite of a xenolith full of sillimanite undergoing partial 


resorption by tholeiitic magma, with the separation of the excess 
of alumina as corundum and spinel, and the feathery crystallization 
of a less basic plagioclase (oligoclase) in the matrix. Rudh a’Chro- 
main. §.17178. xX 13°5. (See p. 248.) 


. Junction of normal tholeiite with sillimanite-buchite, showing a re- 


action-zone: this zone consists of basic plagioclase and cordierite. 
Cordierite occurs in relatively large crystals growing into the 
buchite, is enclosed in the anorthite, and forms a narrow band 
between the anorthite and the tholeiite. A few crystals of cord- 
ierite occur within the tholeiite itself near the junction. Old road 
100 yards north-west of Feorlein Cottage, Carsaig. §S. 20289. 
x 9. (See p. 245.) 


. Original sillimanite-bearing anorthite invaded and resorbed by magma, 


with the consequent crystallization of a sillimanite-free felspar of 
less basic composition on the undissolved nuclei. Rudh’ a’Chro- 
main §.16603. x 45. (See p. 245.) 


. An example of the breaking-up of the primary anorthite-sillimanite 


xenolith and resorption by magma. The excess of alumina fur- 
nished by the resorbed anorthite and sillimanite has been precipitated 
as a deep plum-coloured spinel and corundum. A regrowth of 
basic plagioclase edged with oligoclase has taken place on the un- 
dissolved felspar-fragments, and shows white in the figure. ‘The 
melt surrounding the large spinels finally consolidated as oligoclase, 
skeleton spinels, and glass. Nuns’ Pass, Carsaig. S. 20271. 
x 7. (See p. 254.) 


Quart. Journ. Geol. Soc. Vol Le ONOUUG 12, WIL, 


J. Rhodes photomicro. 


Composrre Stun (Rupw’ a’Crromaty); Fensire; THOLELITE; 
AND COGNATE XENOLITHS. 


; - 
lenient 
° 
5 
y ; 
ie ) 
—— 
— 
’ E 
7 6 
. 
} ~ 
ae » 7 . 
we > 9 x 
| . 
+ * ; 
=A 
- _ 
- = ; 
ad : ; : i 
= ¥ : : . 
- —_ : f : : : 
> $ : | 
- bs 
=> A é 
ae. ' 2 
= ~ ” < . s 
ae : | 
| : oh 
<- : Cl oat 
= ) 
P 
a] 
; e 
v , j 
: hi Pas 
- = 7 & s 
P f Pa a" ; 
; 7 . in eae . hn Fi tS 


- 


Quart. Journ. Geol. Soc. Wolk, IwOnuUG Bs Wa 


J. Rhodes photomicro. 


SILLIMANITE- AND CORDIERLTE-BUCHITES, AND SrnickoUS NENOLITHS. 


Quart. Journ. Geol. Soc. Vol. LXXVIII, Pl. VII. 


J. Rhodes photomicro. 


ANORTHITE-SAPPHIRE-SPINEL ASSEMBLAGES. 


= 


3 
4: 
’ 
= 
fi 
sie 
P= iP, 
, a 
Teeet* wl 
. a 


= 


Quart.Journ. Geol. Soc. Vig IAOOWIG, AG IG 


J. Rhodes photomicro. 


XENOLIFHS INVADED AND RESORBED BY 'THOLEIITIC MAGMA. 


- 


| 
| 


Quart. Journ. Geol. Soc. Vol. LXV, Plax. 


J. Rhodes photomicro. 


tric Maq@Mma. 


= 
a) 


Xenormmrus INVADED AND NRESORBED BY THOLI 


‘ 
} 


oo 


a el aT aaa eo 


eae 
‘ 


+ 


Se 


part 3] MINOR INTRUSIONS IN THE ISLAND OF MULL. 259 


PLATE X. 


Fig. 1. Sillimanite - buchite invaded by magma, with the resultant 
formation of large crystals of cordierite in a dark glassy matrix 
charged with acicular crystals of felspar, which range from 
labradorite to oligoclase in composition. The cordierite-crystals 
contain sillimanite-needles which are identical with those of the 
buchite, and are evidently the undissolved residue of the same. 
The sillimanite is pale pink in both cases. Shore 700 yards 
south-west of Tiroran. S. 18532. x18. (See p. 246.) 

9. Shows the formation of beautiful crystals of cordierite (partly pini- 
tized) as the result of the reaction between the igneous magma and 
» sillimanite-buchite. The cordierite-crystals are large and well- 
formed next to the buchite, but smaller and full of spinel nearer to 
the true igneous rock. Pink sillimanite occurs as in the above 
cases, and is obviously of earlier formation than the cordierite. 
Coast 3500 feet slightly north of west of Ormsaig. X 9. (See 
p. 246.) 

3. This rock consists of an aggregate of anorthite-crystals (large colour- 
less area) that contain small sapphires and a plum-coloured spinel. 
The anorthite has suffered resorption with the subsequent crystal- 
lization of labradorite upon its edges. Large cordierite-crystals, 
free from inclusions and often exhibiting beautiful twin-structure, 
have developed in the magmatic matrix. One such crystal of 
cordierite occupies the centre of the field. Slightly north of west 
of Ormsaig. S. 18003. x 17, crossed nicols. (See p. 246.) 

A. Brecciated anorthite-xenolith invaded by magma, with 
attendant resorption. The part of the section figured shows the 
result of the resorption of the original xenolith in the precipitation 
of large, well-shaped, reddish plum-coloured spinel, crystallization of 
feathery oligoclase-andesine, and the formation of large rectangular 
crystals of cordierite in a glassy matrix. North of Feorlein Cottage, 
Carsaig. S. 18493. x 25. (See p. 248.) 

5. Edge of a sillimanite-anorthite xenolith acted upon by magma, and 
showing the regrowth of basic felspar on the resorbed anorthite. 
Spinel and less basic lath-shaped plagioclase have formed in a 
glassy matrix. Note the circular form of an amygdale in the 
upper part of the figure. Rudh’ a’Chromain. §. 16601. x 165. 
(See p. 245.) 

6. This shows the brecciation of the primary sillimanite-bearing an- 
orthite, its partial resorption by magma, and the concomitant 
precipitation of a plum-coloured spinel and oligoclase in a glassy 
matrix. North of Feorlein Cottage, Carsaig. S. 18492. x 18. 
(See p. 254.) 


DISCUSSION. 


Sir Jeruro Teaun said that, so far as he knew, this was the 
most perfect case of its. kind that had ever been described. The 
paper was evidently of great interest and importance. 

Mr. G. Barrow congratulated the Author on the valuable 
results of his work on the inclusions of sedimentary material 
caught up in a voleanic magma at a great depth below the 
surface. The evidence produced by the Author seemed to indicate 
clearly that the mineralogical changes had been produced at a 
considerable depth. ‘The Author was fortunate in having to deal 
with a material that from its composition could only have been a 
pure shale originally. The distinctive minerals are the same as 


260 XENOLITHIC MINOR INTRUSIONS IN MULL. [vol. lxxvi, 


those characterizing the higher thermal zones of the Archean 
rocks, and occur continuously through belts of considerable 
breadth and of unknown length. The Author’s work thus sup- 
ports the view now generally entertained that the distinctive 
minerals in the Archzean rocks are due to deep-seated changes. 

Dr. J. W. Evans remarked on the number of important facts 
disclosed by the Author’s communication, and the interesting 
deductions which he drew from them. It was remarkable that an 
argillaceous rock should contain as much soda as that found in 
the buchite, and that the ratio of potash to soda should be 
increased by the metamorphism due to an igneous rock con- 
taining more soda than potash. He thought that the changes 
described might have taken place at a comparatively-moderate 
depth. 

The AvuTHOoR expressed his gratification at the reception ac- 
corded to his paper, and thanked the speakers, especially Sir Jethro 
Teall, for their kind remarks. As pointed out by Mr. Barrow, 
he considered himself extremely fortunate in having such beautiful 
material to work upon. In reply to Dr. Evans he admitted the 
somewhat unusual alkali-ratio presented by the buchite; but 
analyses of aluminous sediments show that an excess of soda 
over potash is occasionally met with, and he would rather regard 
this as an inherent property of the shale than as a result of 
metamorphism. With respect to the depth at which the meta- 
morphism of the xenoliths was accomplished, all that could be 
said was that it took place within the magma-basin and under 
a pressure sufficient to prevent the expulsion of any constituents 
in a gaseous form. 


part 3] A COMPOSITE SILL AT NEWTON ABBOT. 261 


7. A Composite Siut at Newron Azsgpor (Devon). By Witt1aM 
GEORGE St.JoHn SHannon, M.Sc., F.G.8. (Read November 
23rd, 1921.) 


[Prater XI. ] 
CONTENTS. Page 
Fe Mama O CUCU OM cs scr eae epee en Aye ace anche als oe Sys sloth ictese aes 261 
itiehieldhelatromay Suances eee ner ee cats cal aniee ch ele sa dane 262 
i Macroscopic Chataictorsiess spn ras cite acco eence asad ane 263 
TV raObrO OT Aap Vso, lk os eI oe tis Sat ae clo a 264 
Vig summary, and Conchisions asm neste neon ee eae 


I. InTRODUCTION. 
THE sill which is described in this paper forms the summit and 
part of the northern flank of Knowles Hill, Newton Abbot, 
6-inch Ordnance-Survey Map 109, 8.E. The hill is bounded on 
all sides except the western by the alluvium of the Teign and its 
tributary the Lemon; it forms a prominent hog-back lying in the 
fork between the two rivers. The base of the hill is formed of 
Upper Devonian slates, and these are continuous to the west, 


Fig. 1.—Sketch-map of Knowles Hill, Newton Abbot, on the 
scale of 6 inches to the mile, or 1: 10,560. 


Hl Picrite 
WL Mugearite 


Upper eS 


where, at a distance of about half a mile, another outcrop of 
dolerite occurs at Highweek Church. The occurrence of picrite 
was discovered by Busz, and a short description of the sill and of 
the presence of original quartz is given in the Geological Survey 
Memoir on the ‘Geology of Newton Abbot,’ No. 339, 1913. 


Oninces. No. sll. T 


262 MR. W. G. SHANNON ON A [ vol. Ixxviil, 


Il. Fretp RELATIONS. 


The summit of Knowles’ Hill has been quarried, and a smaller 

; quarry exists by the footpath on the 

5 north of the hill, bordering the Teign 

3 alluvium. The rocks described are 
mainly from the upper quarry. The 
northern slopes of the hill are in 
pasture, and the exposures are too 
weathered to admit of a petrographic 
description of the rocks; on the east, 
the geological lines have been drawn 
from very limited exposures, as the 
area has been completely built over. 
The sill is intrusive into Upper 
Devonian slates; its lower boundary 
is indicated by the fact that the floor 
of the quarry is in slates, the whole 
of the sill having been quarried away. 
The upper lmit is indicated by the 
occurrence of spotted slates, the 
alteration extending for a distance of 
6 to 9 inches from the contact; this 
clearly shows the intrusive nature of 
the dolerite. The total thickness 
estimated from these facts gives, at 
the southern wall of the quarry, only 
15 feet; within a distance of 30 
yards this has expanded to 30 feet, 
and the top is not seen, the sill on 
the north-eastern face becoming more 
laccolitic in habit. The notable 
differentiation occurs in the narrow 
portion. A thin cover of slates 
separates this upper quarry from a 
small outcrop on the south-east, and 
at the edge of the same field on the 
south-south-west picrite is visible 
beneath a thin cover of slates. It is 
probable that the top of the quarry 
is very little below the upper limit of 


25 feet 


Lemon Alluvium 


12 inches to the mile, or 1:5280. Vertical scale exaggerated, | 


Summit Quarry 
WV? 
U.D. Slates 


Kio. 2.— Diagrammatic section across Knowles Hill. 
8 | 


[ Horizontal seale : 


= the sill. 
E Some evidence can be obtained for 
< the existence of small thrust-planes 
ba or slight faulting :-— 
< o 
= x E (i) The slates on the floor of the quarry 
= - are not spotted. 


(ji) The slates on the south-west of the 
quarry abruptly change from purple to black, and are accompanied by a more 
compact rock which appears to have been a tuff, but has also undergone 


part 3] COMPOSITE SILL AT NEWTON ABBOT. 263 


contact-alteration ; in these rocks there were formerly found excellent large 
specimens of Trimerocephalus. The average dip on the south of the summit 
is 30° to 35° south-south-eastwards, indicated by the junction of the spotted 
slates, by the ‘ bostonite ’-vein, and by the inclination of the junction at the 
floor of the quarry. 


In the lower quarry the upper surface dips steeply westwards at 
60°, altering the overlying slates, in which Postdonomya venusta 
is found ; the lower surface is beneath the Teign alluvium. North 
of the summit the slope is steep, and it is possible that the dyke- 
feeder of the sill is here; but no satisfactory evidence could be 
obtained to decide this point. 


III. Macroscopic CHARACTERS. 


Upper Quarry.—Three rock-types are here in evidence—a 
dark picrite, ordinary dolerite weathering a rusty brown, and pale 
veins cutting the dolerite; these are much hghter in colour, and 
the ferromagnesian minerals are evidently present in but small 
quantity. 

(a) The picrite is dark green, with black spots of olivine, and 
the augite shows slight ‘schiller’ effect; the rock is hard and re- 
markably fresh. Its most characteristic feature is the ‘ xenolithic ’ 
structure and a certain amount of veining; the interspaces are 
occupied by a very decomposed brown material, weathering simi- 
larly to the ordinary dolerite. Sections were made, but showed 
only a structureless chloritic mass. The brecciated appearance 1s 
due to these xenoliths, and the occurrence is very similar to that 
described by Dr. A. Harker from the Skye peridotites; the later 
doleritic magma has partly resorbed the picrite, further proof of 
which will be found in the discussion of the xenoliths in the 
dolerite. 

A larger, greyish vein is formed of a decomposed clayey 
material with fibres of tremolite; this has also broken down, and a 
few fibres only can be extracted whole. The exact relations of 
this vein cannot be ascertained, as it disappears vertically down- 
wards ; its formation may be due to subsequent movement along a 
doleritic vein in connexion with the slight thrusting already 
mentioned or during the injection of the ‘ bostonite ’-vein ; 
tremolite is found in the picrite, but as an alteration-product of 
the augite. 

(6) Xenoliths in the dolerite.—The picrite thins out rapidly, 
and here the rock has been quarried for a few feet more nearly 
along the strike; but picrite is not visible. The dolerite weathers 
uniformly, except for some isolated patches from which the 
crust 1s easily removed, exposing a hard centre. At first sight, 
this appears to be due to spheroidal weathering, but the junction 
is fairly sharp, and microscopic examination confirms the presence 
of xenoliths; they appear to be restricted to a zone in close 
association with the salic vein. It will be seen later that these 

sa 


264 MR. W. G. SHANNON ON A [ vol. lxxviii, 


are cognate xenoliths obtained from the picrite, and have been 
partly absorbed and carried forward by the dolerite intrusion. 

(c) Vein of salic facies: ‘ bostonite.’—A conspicuous feature is 
the white vein dipping 35° south-south-eastwards, in the same 
direction as the jointing of the dolerite—its weathered surface is 
of a light buff colour. In the upper right-hand portion a much 
thinner band is just visible; this has weathered to a darker colour. 
The veins are strongly jointed perpendicularly to their surface, 
giving a partly columnar effect. _ The fresh rock is bluish grey and 
felsitic in appearance. There are no noticeable contact-effects, 
and the vein is sharply marked off from the surrounding rock; but 
the latter is so decomposed for over a foot below the surface that 
n) sections could be cut to determine the exact contact. That the 
vein is later than the dolerite is inferred from the columnar 
jointing and the sharpness of the contact. Both veins dip beneath 
the quarry-floor, and cannot be proved to cut the picrite. 

(d) Fine-grained modification of the dolerite—In the south- 
western portion of the quarry the wall is low, and a portion about 
a foot below the top is noticeably less weathered, of greener colour 
and finer grain. This merges imperceptibly into the ordinary 
dolerite. Within a few feet, on apparently the same horizon, the 
dolerite is coarse-grained. This fine-grained rock will be defined 
as the ‘mugearite ’ modification. 

(e) The ‘dolerite which forms the bulk of the intrusion calls for 
no detailed description, its chief characteristic being the facility 
with which it weathers to a rusty-brown ‘ wacke.’ 


(f) Lower Quarry.—The rock here is uniform and more 
compact than the dolerite from the upper quarry. It is of a 
predominant green colour, much veined by calcite. So far as I 
have examined it, the picrite or salic veins do not occur, the rock 
being dolerite throughout. The slates at the contact are altered 
to a depth of 2 or 3 inches only, as in many dyke occurrences. 


IV, PETROGRAPHY. 


(a) The picrites.—In hand-specimens these are dark-green 
rocks, with black patches of olivine and augite showing slight 
schillerization. In thin section the most notable constituent is 
olivine in subhedral crystals and smaller grains, the smaller olivines 
being partly enclosed in poikilitic augite- -plates. 

The olivine is optically positive, with an angle in air of about 70°, 
indicating that it belongs to the forsterite end of the series. Its 
alteration- products are characteristic of the olivines of plutonic 
rocks: at firsta heavy border of magnetite appears; this gradually 
extends into the interior, forming “magnetite dendroids, and the 
crystal becomes completely veined by magnetite ; later this is re- 
sorbed in the formation of serpentine and calcite. The augite is 
purplish, and is probably a titan-augite, as in the augite of the 
essexites; it gives extinction-angles up to 35° or 40°. The augite 


part 3] COMPOSITE SILL AT NEWTON ABBOT. 265 


alters to a pale hornblende giving extinctions of 15°, and low 
yellows of the first order: this is referable to tremolite; accom- 
panying this is a similar hornblende giving straight extinctions 
and more fibrous with rather less colour, probably anthophyllite. 
Another colourless augite occurs as subhedral crystals with straight 
extinction, accompanying the larger olivines; it is positive and 
without pleochroism, exhibiting the characters of an enstatite. 

The biotite occurs in strongly coloured red-brown laths, a few 
of which are partly bleached to a pale crimson-brown; it is biaxial 
with a small angle, and pleochroic in dark chestnut-brown. 

The felspar varies in amount, some of the rock approaching the 
peridotites; it gives extinction-angles in the few symmetrical 
sections observed up to 30°, indicating a basic labradorite. Much 
of it is strongly zoned, and some of the felspar ground is probably 
nearer anorthite. Apatite occurs in short blunt prisms. The 
iron-ore 1s always opaque, and the presence of chromite or any 
other spinel has not been established. Serpentine after olivine 
forms about 10 per cent. of the rock. 

A Rosiwal analysis of a section was undertaken, and is compared 
with an analysis by Busz:— 


I. IG 

SiO, 40°12 40°24, 
Fe 16°01 17°94 
MeOmi: LAM ek 23-69 22°04. 
(Ob OSs abel seat nearer 6°53 7°20 
Nias OMe sh .caercsetns 1:20 1°23 
K,0 0°53 0°50 

AKO) Et ect oe aie 0°37 = 

O 0:18 ps 
100°42 98°74 


Specific gravities = 2°96 and 2°97. 
I. Picrite, Knowles Hill, Newton Abbot. Busz. (Partial analysis.) } 
II. Do. do. do. do. do. Rosiwal analysis. 


(6) Xenoliths in the dolerite.—Slides from the outer parts 
of the xenoliths show nearly the normal character of the dolerite. 
The augite is subophitic; occasionally it may be subhedral, and is 
not quite fresh. Some small tracts of chlorite or serpentine are 
referable to olivine; the felspar is andesine, giving extinction-angles 
up to 20°. 

Slides from the central parts show a much greater proportion of 
ferromagnesian minerals. The augite is in part poikilitic to the 
smaller olivine-pseudomorphs; in general it is subophitic. In 
Pl. XI, fig. 1 is seen a large pseudomorph after olivine, mainly 
serpentine with some residual magnetite. A hornblende with very 
pale-green pleochroism is secondary after either augite or the 


1 Quoted by J. P. Iddings, ‘Igneous Rocks’ vol. ii (1913) p. 337. 


266 MR. W. G. SHANNON ON A [ vol. lxxviii, 


serpentine; it is nearer actinolite than tremolite. The specific 
gravity =2°85. 


(c) Bostonite-veins.—In hand-specimen these are bluish 
grey in the fresh rock and felspathic in appearance. In thin 
section the rhombs of plagioclase are conspicuous. This is of two 
kinds: one with albite-lamelle well developed, and the other with 
the peculiar streaky appearance of soda-orthoclase. All gradations 
can be followed, from coarse chequer-structure to the finest micro- 
cline; often there is a central chequer-crystal surrounded by 
chlorite, in turn surrounded by a newer growth of soda-orthoclase. 
The chequer-structure indicates an albite ; but a Becke determina- 
tion of the refractive index shows that this is higher than in 
albite, being slightly above that of the balsam of the slide. 

Extinctions are not easily measured, but appear to be nearly 
straight; probably the felspar is referable to potash-soda-oligo- 
clase-albite. A fair amount of orthoclase occurs in anhedral 
grains. 

Chlorite occurs between the crystals, as well as inclusions. 
Prehnite is an alteration-product of the felspar ; there are also a 
few small grains of epidote, and tremolite in outer portions. 

The rock approaches nearly to the salic variety of the albite- 
dolerite of Trusham as regards the felspars; it approaches the 
bostonites from the point of view of the scarcity of the ferro- 
magnesian minerals. The following Rosiwal analysis is compared 
with these rocks :— 


i if iG iE 

SiO, 53°59 52°00 58°47 
AGO re 17°80 18°06 16711 
FeO & Bes . fES 7°32 77d 
g = 2°44. 2°84 1°58 
CaO . th ty ee 1°34 4°59 0-94 
K30 ; 9°89 4-68 5°18 
Nao@ yee 5 i 378 4°34 
Motals\-.:-.. 96°81 93°27 94-37 


Specific gravity = 2°66, 
I. Bostonite-vein, Knowles Hill. Rosiwal analysis. 
II. Bostonite, Onston Ness, Orkneys.! 
III. Albite-diabase, felspathic variety, Trusham.2 E. G. Radley. 


In I, 46 per cent. albite was caleulated as Ab,An, 22 per cent., 
soda-orthoclase as 17 per cent. Na+ K, and 7-5 per cent. orthoclase, 
the felspar totalling 76-6 per cent. of the rock. An analysis was 
conducted on the same rock by means of a Harada-Brégger tube 
and the felspar separated by the Sonstadt solution ; the felspar and 
the heavy minerals were weighed, about 2 grammes of powder 

+ Quoted by Dr. F. H. Hatch, ‘ The Petrology of the Igneous Rocks *°1914, 
p. aoe 


‘Geology of the Country around Newton Abbot’ Mem. Geol. Surv. 1913, 
p. 62. (Both analyses are modified.) 


- 


part 3] COMPOSITE SILL AT NEWTON ABBOT. 267 


being used: this gave a reading of 72 per cent. for the felspar, 
agreeing fairly closely with the Rosiwal analysis. 


(d) Fine-grained dolerite (‘mugearite’ variation).— 
In thin section the plagioclase-laths give a trachytic aspect to the 
section, and the extinction is sensibly straight; the average length 
of these laths is about 155 mm. The plagioclase is not all of one 
kind ; in addition to these, more tabular felspars and some inter- 
stitial grains of unstriated felspar referable to orthoclase occur. 
In one case this could be established by a Becke test, giving a 
refractive index lower than that of balsam. The tabular crystals 
give extinction-angles up to 12° or 14°, and are evidently oligoclase- 
andesine, the refractive index being equal to that of quartz. The 
lath-felspars form about 35 per cent. of the rock; their refractive 
index is below that of quartz, and with the straight extinction this 
indicates oligoclase. In addition to these, a few rhombs are seen 
presenting the streaky appearance of soda-orthoclase. A fair 
amount of chlorite occurs, but no fresh augite; in some cases the 
chlorite may be after small olivine. The final product of crystal- 
lization is quartz moulding some of the felspar, and containing 
small elongated needles of apatite: hence this is primary. Larger 
quartz-grains occur, notably corroded, with inlets of the ground 
and with a border of granular chlorite; this is xenolithic quartz. 
Calcite is present in grains. The iron-ore appears to be always 
magnetite. 

The rock presents considerable affinities to the mugearites, fas 
defined by Dr. A. Harker. A partial Rosiwal analysis is compared 
with the percentages given by him ! :— 


) OF Per cent. II. Per cent. 

Oligoclase ............ 37°53 Oligoclasete.. OMS 
Oligoclase-andesine. 11°04 
Orthoclase...4..)..... SPP Orthoclasey ss. :.. 12°5 
OMAN GZ seri Sedans 0°81 
@hilonites. 20.065 | obi: Olivine, ete.......... 265 
Miaonetite casa... 9°38 A ATIGE Ay eee: 35 

Motalsinss 99°98 100°00 
Specitic gravity = 2°76. Specific gravity = 2°79. 


I. ‘ Mugearite’ variation of dolerite, Knowles Hill. Rosiwal analysis. 
II. Mugearite, Druim na Criche (Skye). 


The lower specific gravity is probably due to secondary altera- 
tion. The Knowles-Hill rock is more basic, and analcite was not 
found, although carefully searched for; the texture is also coarser. 


(e) Dolerite.—The sections of dolerite display no especial 
features beyond the occurrence of quartz, both original and xeno- 
lithic. They are not very fresh, even at some distance from the 
surface. The augite is usually ophitic to subophitic, but in some 


1 «The Tertiary Igneous Rocks of Skye’ Mem. Geol. Surv. 1904, p. 265, 


268 MR. W. G. SHANNON ON A | vol. Ixxviu, 


of the slides (N 20)! there are two generations, smaller crystals 
which are euhedral and showing diallage striation, and a later 
ophitic generation: in N 18 the augite is ophitic throughout. The 
plagioclase is of an acid variety, giving extinction-angles in sym- 
metrical sections up to 15°, with a refractive index higher than 
quartz, and is referable to andesine; in N15 this is acid, in 
N18 it becomes more basic. Orthoclase appears to be present in 
most of the slides—in N15 it forms about a third of the felspar, 
showing gradation towards a monzonite facies. 

Accessory minerals are quartz, iron-ores, and apatite. The 
bigger grains of quartz show corrosion-borders with granular 
epidote or chlorite, and deep inlets occupied by chlorite; the 
smaller grains are anhedral to the felspar, and contain prisms of 
apatite of the same size as in the normalrock. The amount of the 
quartz varies, In some cases amounting to at least 3 per cent. 
The iron-ore may be either ilmenite or pyrites, rarely magnetite. 
Secondary minerals are epidote in considerable quantity after 
felspar, and chlorite after augite. The texture is generally rather 
fine-grained, becoming coarser with the occurrence of euhedral 
augite. Specific gravity=2°81 and 2°84. 

A further variety of the dolerite occurs as a small outcrop on 
the flank of the picrite south-east of the field, adjacent to the 
quarry. ‘The sections cut vary considerably, but all contain some 
olivine. The latter is unevenly distributed; in N 21 olivine is 
the chief ferromagnesian mineral, in N22 augite is in much 
greater proportion. Enstatite generally accompanies the olivine 
when it occurs in considerable quantity. Part of the felspar is 
much decomposed; some undoubted andesine occurs, and is much 
fresher. The decomposed felspar shows no twin striation, and is 
considered to be orthoclase—in N 22 this forms about half the 
felspar. A fair amount of chlorite is present, and the iron-ore 
is ilmenite. Although the rock is much weathered, the olivine is 
notably fresh, and is not as much altered as the olivine in the 
picrite. Quartz forms the last product of crystallization, and 
contains apatite; further, the felspar is euhedral to it: hence this 
quartz is original. 

This variety of the dolerite is very restricted in distribution, 
occurring only in one part in immediate contact with the picrite. 
All other sections of dolerite, apart from the xenoliths, contain 
no olivine. This particular variety is a differentiation-facies of 
the picrite, in which the proportion of felspar is notably increased. 
But this explanation is not accepted for the rest of the olivine 
occurrence in the xenoliths, in view of the facts just stated. 


(f) Lower Quarry: dolerite.—The rocks here are com- 
pletely chloritized, the original mineral having been augite. 
The felspar proves to be andesine, where it is fresh enough for a 


1 The numerals N 20, ete. refer to the slides which I have presented to 
the Museum of Practical Geology, Jermyn Street, London, S.W. 1. 


Quart. Journ. Geol. Soc. Vol. LXXVIII, Pl. XI. 


W.G.S. photomicro, 


part 3] COMPOSITE SILL AT NEWION ABBOT. 269 


determination; but much of it is chloritized, with the development 
of glassy-clear secondary albite. Quartz occurs in considerable 
quantity, both original and xenolithic; ilmenite or pyrites re- 
presents the iron-ores; and secondary minerals are epidote and 
calcite, the latter forming veins. The specific gravity is 2°59, this 
low figure being due to the altered character of the rock. 


V. SUMMARY AND CONCLUSIONS. 


The sill described in the foregoing pages is intrusive in the 
Upper Devonian formation, the intrusive character being indicated 
by the spotting of the slates. It has been shown that differenti- 
ation has taken place, resulting in rocks which vary from picrites 
through dolerites to bostonite. The picrite forms the base of the 
intrusion, and has shght lateral extension: it has been partly re- 
sorbed by the dolerite, with the formation of basic xenoliths in the 
dolerite and of xenolithie structure in the picrite; the junction 
is well defined. The dolerite forms the main mass, and the latest 
products of the differentiation are the bostonite-veins. Mugearite 
occurs as a modification of the dolerite. Quartz is an original 
constituent, and has also been obtained from outside sources, 
perhaps from Lower Devonian grits beneath the slates. 

Ditferentiation has not taken place 7m sztw, as the junctions do 
not shade one into the other, but are always sharp—except in the 
case of the mugearite. 

The order of differentiation is of decreasing basicity, and hence 
probably represents a plutonic phase, although the scale of the 
intrusion is of the minor order. 

Stratification of the picrite and dolerite took place in the magma 
reservoir, parts of the picrite were carried forward by the dolerite, 
and the bostonite was of slightly later production, but earlier 
than the final cooling of the sill, as it shows no selvage. 

The albitic final phase of differentiation suggests a Culm age 
for the intrusion. 


EXPLANATION OF PLATE XI. 


Fig. 1. Xenolith in dolerite (N104). Summit Quarry, Knowles Hill 
(Newton Abbot). From the central part of the xenolith. Glomero- 
porphyritic aggregate of chlorite and serpentine after olivine ; augite- 
plates at the lower edge. The texture of the olivine is subhedral, as 
in the picrites. Ordinary light, x 30 diameters. (See p. 265.) 

2. Fine-grained dolerite, ‘mugearite variation’ (N14). Summit Quarry, 
Knowles Hill. Oligoclase-laths giving a trachytic aspect. On the 
right edge is a small orthoclase, north-west of this two small 
anhedral quartzes, while below is a small rhomb of soda-orthoclase. 
Crossed nicols, X 35 diameters. (See p. 267.) 

3. * Bostonite ’-vein (N 9). Summit Quarry, Knowles Hill. Rhombic 
sections of chequer oligoclase-albite, with inclusions of chlorite. 
Three crystals in the central upper part on the right show coarse 
chequer-structure ; to the left of these are two rhombs of soda- 
orthoclase. The central crystal shows albite-lamelle, nearly at 
extinction. Crossed nicols, x 30 diameters. (See p. 266.) 

4, Soda-syenite segregation in albite-diabase, West Quarry, Trusham 
Station. This shows sections of albite and soda-orthoclase, which 


270 A COMPOSITE SILL AT NEWTON aBBoT. _ [vol. lxxviii, 


may be compared with the crystals in fig. 3. Crossed nicols, 
xX 30 diameters. (See p. 266.) 

Fig. 5. Olivine-dolerite (N 22). Edge of field. Knowles Hill. Olivine- 
crystal in the centre, orthoclase on the left. The remainder of the 
slide consists of andesine, augite, and quartz. Ordinary light, x 30- 
diameters. (See p. 268.) 

6. Dolerite (N18). Summit Quarry, Knowles Hill. Augite subophitiec 

to andesine: quartz on theright. Theiron-ore is ilmenite. Crossed 
nicols, X 30 diameters. (See p. 267.) 


DIScUSSION. 


Dr. J. S. Frerr welcomed this paper as a contribution to the 
petrography of the greenstones of Cornwall and Devon. The 
diabases of that province not unfrequently showed a tendency to 
develop an ‘alkaline’ facies, especially in the abundance of 
felspars rich in soda; but, in the speaker’s experience, many of 
them had been extensively ‘ albitized,’ either during crystallization 
or shortly after consolidation. The abundance of albite was note- 
worthy also in the pillow-lavas and keratophyres, which he supposed 
were the effusive representatives of the same magma. Analcite 
was very rare in these rocks, and it seemed possible that albiti- 
zation occurred in place of analcitization when the post-voleanic 
solutions emanating from the magma were comparatively siliceous. 

Mr. L. Hawkes said that two types of quartz from the dolerite 
were described, and it was suggested that the larger individuals 
with corrosion-embayments had been derived from a sandstone. 
The speaker asked whether these crystals were scattered evenly 
throughout the rock, and whether they always occurred singly. 
From the origin indicated by the Author one would have expected 
to find irregularly distributed fragments, some built of more than 
one individual. The speaker had found big corroded quartz- 
grains quite commonly in basic rocks in Iceland, where no sand- 
stones were known, and had come to the conclusion that they had 
either crystallized from the basic magma, or had been incorporated 
from an acid one, in some way not yet satisfactorily explained. 

Mr. A. K. WEtts said that he had difficulty in agreemg with 
the use of certain rock-names for some of the specimens exhibited. 
In particular he would like to ask the Author whether his 
‘mugearite’ showed any of the structural and textural pecu- 
liarities characteristic of the type-rock as defined by Dr. A. Harker 
from Skye. As Harker’s ‘mugearite’ was a well-individualized 
type, it was perhaps undesirable to apply the term to a rock which 
appeared to resemble the type in one particular only—the compo- 
sition of the felspar. 

The AvuTHor expressed his thanks for the reception accorded to 
his paper. In reply to Mr. Hawkes, he stated that most of the 
quartz was primary, a few isolated and corroded crystals only 
being considered secondary. In reply to the criticism of nomen- 
clature by Mr. Wells, he said that ‘mugearite’ had been used as a 
generalized term for oligoclase-dolerite, and ‘bostonite’ for an acid 
differentiate of a basic magma. 


part 3] JURASSIC PLANTS FROM CEYLON. Dik 


8. Jurassic Prants from Cryton. By Prof. ALBERT CHARLES 
SEWARD, Sc.D., F.R.S., Pres.G.S., and Ricnuarp ERIc 
Hoirrum, B.A. (Read January 18th, 1922.) 


[Puate XII.] 


Tue plants described in this paper were collected by Mr. HE. J. 
Wayland, Government Geologist of Uganda, in the course of a 
mineral survey of Ceylon: they are the first fossil plants recorded 
from the island. Several small though well-defined impressions 
of plants were discovered at Tabbowain the North-West Province 
(lat. 8°S., long. 80° E.) in a pale-yellow shale resting ‘ directly on 
Archean rocks and folded in with them.’ In writing to Dr. A. 
Morley Davies, from whom we received the collection, Mr. Wayland 
speaks of the discovery of the plant-beds as the result of ‘the 
most arduous work’ that he has ever done :— 

‘dense jungle and lateritic earth masked exposures, and mosquitoes were 
dreadful. The monsoon was late, and one by one men fell sick, and the day 
came when all the men (and headman) were down. Then, after doctoring 
them to the best of my ability, I proceeded to carry on the work alone, 
cutting my own path through the jungle and doing everything myself.’ 

The results obtained by Mr. Wayland are of considerable 
meee from a phytogeographical and a stratigraphical point 
of view. 


PTERIDOPHYTA: Filicales. Fam.? Osmundacee. 


CLADOPHLEBIS REVERSA (Feistmantel) |=? TopirEs WILLIAM- 
SONI (Brongniart)]. (Pl. XII, figs. 18, 15 a, 15 6, & 16.) 


The three imperfect pinne referred to this species are character- 
ized by the relatively broad axis and the Cladophlebis type of 
venation. The venation is most clearly seen in fig. 15 6, which 
represents a crushed fragment with short and broadly rounded 
pinnules attached by the whole base. These and the more linear 
pinnules shown in fig. 16 agree very closely with examples of 
Todites Williamsoni figured from the Jurassic rocks of 
Yorkshire,t Graham Land? on the borders of Antarctica, and 
many other regions. ‘The smaller specimen shown in fig. 13 may 
belong to the same species, but the venation is invisible. In the 
absence of fertile pinnules we cannot with complete confidence 
refer the specimens to the Osmundacee, although, in view of the 
occurrence of fertile pinnules in the Graham-Land beds and their 
close agreement in habit with the Ceylon fragments, an Osmund- 
aceous affinity is probable. 


1 Seward (00) pl. xv, fig. i Numerals in parentheses refer to the Biblio- 
graphy, p. 276. 
? Halle (18) pl. iii, figs. 1-5. 


272 PROF. A. C. SEWARD AND MR. R.E. HOLTTUM  [Vol. lxxvin, 


We have no doubt of the specific identity of the Tabbowa fossils 
with those figured by Feistmantel! from the Jurassic rocks of the 
Madras coast as Pecopteris reversa, and we therefore adopt his 
yame in preference to Todites Williamsoni, substituting Clado- 
phlebis for Pecopteris. 


FILICALES INCERTA SEDIS. 


CLADOPHLEBIS DENTICULATA Brongniart. (Pl. XII, figs. lla, 
116, 12, 14a, & 146.) 


This designation is used in a comprehensive sense, as standing 
for a group of fern fronds which cannot (in the sterile condition) 
be assigned to well-defined species in the stricter sense.? The 
specimen shown in fig. 12, probably from near the distal end of a 
pinna, has falcate pinnules, while that reproduced in fig. 11 bears 
pinnules with more nearly parallel sides. The relatively broader 
pinnules seen in fig. 14 resemble in shape those of fig. 16; but the 
slender axis of the pinna in the smaller impression is a feature 
more suggestive of Cladophlebis denticulata than of Todites. In 
the three specimens, figs. 11, 12, & 14, the lamine are entire and 
the secondary veins showa single dichotomy (figs. 11d & 146). 
If larger specimens were available, it might be possible to differ- 
entiate between the fragmentary pinne; but we prefer to include 
them under the single group-name Cladophlebis denticulata, which 
comprises sterile fronds from Jurassic localities in almost all parts 
of the world. 

Except in their smaller size, the Ceylon specimens agree with 
those figured by Oldham as Pecopteris indica? and later by 
Feistmantel as Alethopteris indica from the Rajmahal Hills,* also 
with Alethopteris whitbiensis figured by Feistmantel from the 
Jabalpur Group.® Two small specimens, figured by Feistmantel 
from the Madras coast® as Alethopteris indica, appear to be 
identical with the Tabbowa impressions. A small specimen figured 
by Halle from Graham Land? as Cladophlebis sp. is similar to 
that in fig. 12, and his C. oblonga from the same locality is 
indistinguishable from our examples. It is, however, impossible 
strictly to differentiate between pinne of this general type, and, 
unless fertile pinnules are discovered, we consider that the better 
course is to adopt the comprehensive group-name Cladophlebis 
denticulata. 


1 Feistmantel (79) pl. i, fig. 5; pl. ii, figs. 1, 2, & 7. 
2 For synonymy, see Seward (00) p. 134. 

3 Oldham & Morris (68) pl. xxvii. 

4+ Feistmantel (77) pl. xlvi, figs. 3 & 4. 

> Feistmantel (777) pl. ii, figs. 2-7. 

6 Feistmantel (79) pl. i, fig. 1. 

7 Halle (18) pl. ii, fig. 4. 


part 3] ON JURASSIC PLANTS FROM CEYLON. 273 


FILICALES (?) or CYCADOPHYTA. 


TANIOPTERIS sPATULATA McClelland. (PI. XII, figs. 1-40, 5, 
6a&66,7,8a& 8b, 9, 10a, & 108.) 


Although it has been customary to regard Teniopteris as a 
genus of ferns, the evidence brought forward by Mr. Hamshaw 
Thomas,! in support of his contention that the European Jurassic 
species Teniopteris vittata is a Cycadean frond, leads one to 
suspect that some other sterile Tenzopteris leaves may also be 
Cycadean. The species 7. spatulata was founded by McClelland ? 
on leaves which he spoke of as abundant in the Jurassic plant-beds 
of the Rajmahal Hills in Bengal. He described them as ‘linear, 
2-3 ins. long, narrow at the base, becoming broader towards the 
apex, or sub-spatulate.’ McClelland’s figure of the type-specimen, 
as Zeiller? says, does not suggest accuracy in details. There is, 
however, no doubt of the identity of McClelland’s leaf with those 
subsequently figured by Oldham+* from the same locality as 
Stangerites spatulata and by Feistmantel® from the Godaveri 
district and the Madras coast as Angiopteridium spathulatum. 

Teniopteris leaves are the most abundant fossils in the Tabbowa 
Beds. The leaves are simple, entire, linear, 2°5 to at least 5 cm. 
long, 0°25 to 1-5 em. broad. Some of the fragments are probably 
from leaves longer than that shown in Pl. XII, fig. 1. The apex 
is acuminate or bluntly rounded, the lamina tapers very gradually 
towards the base, a strong midrib gives off lateral veins, almost at 
right angles, which are dichotomously branched, usually once close 
to the midrib or occasionally near the margin. There are 25 to 40 
veins per centimetre of lamina (figs. 40, 66, 86,&1006). The 
lamina of the fragment seen in fig. 5 appears to be lobed, but this 
may be accidental. The curved form shown in fig. 9 is probably 
an abnormality, and may be the result of injury. 

In 1860 M‘Coy § named some leaves from Victoria Teniopteris 
Daintreei, which (it is generally acknowledged) are identical with 
T. spatulata; but M‘Coy’s designation was afterwards employed by 
W. Carruthers for a larger and almost certainly a distinct species 
from Queensland. One of us, in an account of a collection of 
Jurassic plants from Victoria,’ used M‘Coy’s specific name, in 
place of the older designation of McClelland, without any adequate 
reason, for specimens which are indubitably identical with the 
Ceylon leaves. Dr. Walkom® has more recently figured 7. spatulata 


1 Thomas (15) p. 127. 

2 McClelland (50) pl. xvi, fig. 1. 

3 Zeiller (03) p. 76. 

4 Oldham & Morris (68) pl. vi, figs. 1-7. 

5 Feistmantel (77%) pl. i, figs. 6B & 7B; (79) pl. i, figs. 8-13 & pl. ii, figs. 3, 
5-6. 

6 M‘Coy (60) Proce. p. x. 

7 Seward (04) p. 168. In this paper a synonymy is given. 

8 Walkom (17) p. 30 & pl. v, fig. 2b. See also Dun (98) p. 390. 


274 PROF. A. C. SEWARD AND MR. R.E. HOLTTUM  [vol. lxxvui, 


from the Walloon Series of Queensland and from Jurassic rocks 
in New South Wales.! Arber has recorded TZ. Daintreez, this 
specitic name being used instead of ZL. spatulata, from several 
localities in New Zealand. Some leaves figured by Feistmantel 
from the Indwe River (Cape Province),? as 2. Daintree are 
indistinguishable from the Ceylon specimens. Etheridge * records, 
but unfortunately does not figure, 7. spatulata from Natal, in 
association with Glossopteris and Phyllotheca. In view of the 
fact that in India this species is confined to beds above those 
containing Glossopteris, it would be interesting to know whether 
Etheridge’s determination is correct. The association, in the 
Rhetic plant-beds of Tongking, of leaves named by Zeiller 
T. spatulata® and some members of the Gossopteris flora 
supports Etheridge’s record; but it is noteworthy that the 
Tongking leaves are characterized by a well-marked transverse 
folding of the lamina, which imparts to them a corrugated 
appearance unlike any other fronds of 7. spatulata. Although 
Zeiller’s specimens bear a close resemblance in form and venation 
to those from Tabbowa, we feel some doubt as to their specific 
identity. 


GYMNOSPERM A. Coniferales. Fam. Araucarinez. 
ARAUCARITES CUTCHENSIS Feistmantel. (Pl. XII, fig. 17.) 


The impertect cone-scale, 15 cm. long, represented in fig. 17 
shows the impression of a single median seed and the torn distal 
end of the scale, which was probably acute. Some of the smaller 
examples described by Feistmantel® from the Madras coast as 
A. cutchensis are very similar to the single cone-scale from Ceylon. 
This species is recorded also from Graham Land,’ and it does not 
differ in any very definite characters from certain Araucarian cone- 
scales from European, American, and Australian localities.® 


CONIFERALES INCERTA SEDIS. 
BRACHYPHYLLUM MAMILLARE Brongniart. (Pl. XII, fig. 19.) 


This fragment of a slender branched shoot, although too small 
to be determined with certainty, appears to be identical with the 
finer branches of larger examples figured by Feistmantel ? from the 
Jabalpur Group as Brachyphyllum mamillare, a widely distri- 
buted Jurassic conifer. The crowded appressed leaves, apparently 


1 Walkom (19) pl. viii, fig. 3. 

2 Arber (17) p. 46 & pl. vi. 

3 Feistmantel (89) pl. ii, fig. 11. 

4 Htheridge (01) p. 72. 

5 Zeiller (03) pl. xiii, figs. 6-12. 

6 Feistmantel (79) pl. xiv, figs. 6-9 & pl. xvi, fig. 15. 
7 Halle (18) pl. viii, figs. 3-10. 

8 Seward (19) p. 264. 

9 Feistmantel (77°) pl. xiii, fig. 1. 


part 3] ON JURASSIC PLANTS FROM CEYLON. 275 


disposed spirally, and the habit of the twig support a reference to 
Brachyphyllum. Some American Cretaceous examples of Brachy- 
phyllum have been assigned on anatomical grounds by Jeffrey to 
the Araucarinez, but it would be rash to assume this affinity for all 
the numerous impressions included in the genus. 


ELATOCLADUS PLANA (Feistmantel). (Pl. XII, fig. 20.) 


The piece of foliage shoot shown in fig. 20 bears two-ranked 
leaves spirally disposed, reaching a length of 3 cm. and barely 1 mm. 
broad. There is a well-marked midrib, and the lamina tapers 
evadually towards an acute apex; the lower margin is decurrent. 
Shoots of this type are figured by Feistmantel! from the Madras 
coast as Taxites planus. In view of the impossibility of 
determining the affinity of specimens such as these, we adopt the 
non-committal generic name Hlatocladus. 


PLANTA INCERTA SEDIS. 
DESMIOPHYLLUM sp. (Pl. XII, figs. 18a & 18 0.) 


This fragment of a linear leaf, 3°5 mm. broad, has seven parallel 
veins about 0°5 mm. apart. ‘The impression is not sufficiently well 
preserved to demonstrate the presence or absence of interstitial 
‘veins.’ It may be a piece of a Podozamites leaf, a leaf of 
Phenicopsis, or a fragment of a Cycadean leaflet. It agrees 
closely with some of the specimens from Indian Jurassic beds 
named by Feistmantel ? Podozamites lanceolatus, and it is also 
very similar to a small specimen figured by Feistmantel from the 
Madras coast as a piece of a Cycadean leaflet. In view of the 
impossibility of determining accurately the specimen, we employ 
the convenient designation Desmiophyllum.* It is, however, 
noteworthy that while Podozamztes is known from Indian beds, 
Phenicopsis has not been recorded from the Gondwana Series. 


CONCLUSION. 


The following is a list of our determinations :— 


Cladophlebis reversa (Feistmantel) 

[= Todites Williamsoni (Brongniart) }. 
Cladophlebis denticulata Brongniart. 
Teniopteris spatulata McClelland. 
Araucarites cutchensis Feistmantel. 
Brachyphyllum mamillare Brongniart. 
Elatocladus plana (Feistmantel). 
Desmiophyllum sp. 


Five of these species we believe to be identical with plants 
recorded by Feistmantel from the Madras coast, and in all 


1 Feistmantel (79) pl. xiii; pl. xiv, figs. 1, 2,4 & 5. 

2 Td. (77%) pl. iii, figs. 7-14 & pl. iv; (79) pl. ix, figs. 9 & 10. 
So ldeAo) plaax, fist 11, 

+ For the application of this name, see Seward (19) p. 70. 


276 PROF. A. C. SEWARD AND MR. R. E. HOLTTUM [Vol. lxxviu, 


probability Desmiophyllum sp. is identical with some of the 
imperfect leaves from the Indian beds. It is significant that the 
rocks in the Madras region containing Jurassic plants are said by 
Medlicott & Blanford ! to rest unconformably on gneissose strata, 
presumably of Archean age. From Trichinopoli, where these 
Madras beds are represented, the distance to Tabbowa in Ceylon is 
about 200 miles, and in both localities there appears to be the 
same relation to older igneous rocks. In our opinion, the fiora 
is unquestionably Jurassic in age, and probably dates from the 
Lower Oolite. Formerly, the Madras beds were included as the 
Kota Series with the Maleri Series; but recently Mr. G. de P. 
Cotter? has shown good reason for separating the Kota from the 
Maleri Series, and assigning 1t to a Lower Oolite and Upper 
Liassic horizon. The paleobotanical evidence is consistent with 
this conclusion. 


BIBLIOGRAPHY. 


ArRBER, E. A. N. (17) ‘ The Earlier Mesozoic Floras of New Zealand’ N.Z. Geol. 
Surv., Pal. Bull. 6, 1917. 
Cotter, G.de P. (17) ‘A Revised Classification of the Gondwana System’ Rec. 
Geol. Surv. India, vol. xlviii (1917) pt. 1, pp. 23-33. 
Don, W.S. (98) ‘Notes on the Australian Tzniopteridee’ Trans. Austr. Assoc. 
Advancem. Sci. 1898, p. 384. 
ETHERIDGE, R., Jun. (01) ‘ Notes on Fossil Plants from the St. Lucia Bay Coalfield, 
Enseleni River (Zululand)’ First Rep. Geol. Surv. Natal & Zululand 
(1901) p. 69. 
FEISTMANTEL, O. (77) ‘Jurassic (Liassic) Flora of the Rajmahal Group in 
the Rajmahal Hills’ Mem. Geol. Surv. India, Paleont. Indica, ser. ii, 
pt. 2, 1877. 
Do. (772) ‘Flora of the Jabalpur Group (Upper Gondwanas) in the Son- 
Narbada region’ Palzont. Indica, ser. xi, pt. 2, 1877 
Do. (77%) ‘Jurassic (Liassic) Flora of the Rajmahal Group from Golapili, 
near Ellore, South Godavari’ Paleont. Indica, ser. 11, pt. 3, 1877. 
Do. (79) ‘ Upper Gondwana Flora of the Outliers of the Madras Coast’ 
Palzont. Indica, ser. ii, pt. 4, 1879. 
Do. (89) ‘ Uebersichtliche Darstellung der Geologisch-Palaontologischen 
Verhaltnisse Siid-Afrikas’ pt. i, Abhandl. K6nigl. Bohmisch. Gesellsch. 
Wissensch. ser. 7, vol. iii (1889). 
Do. (90) ‘Geological & Paleontological Relations of the Coal & Plant- 
bearing Beds of Paleozoic & Mesozoic Age in Eastern Australia & 
Tasmania, &c.’ Mem. Geol. Surv. N.S.W., Paleontology, No. 3, 1890. 
Hatz, T.G. (13) ‘The Mesozoic Flora of Graham Land’ Wiss. Ergeb. Schwed. 
Siidpolar-Exped. 1901-1908, vol. iii, pt. 14, 1913. 
McCLetiann, J. (50) ‘Report of the Geological Survey of India for the Season of 
1848-49 ’ Calcutta, 1850. 
M‘Coy, F. (60) Trans. R. Soc. Victoria, vol. v (1860) Proc. p. x. 
Meputicort, H.B.,& W.T. Buayrorp. (93) *A Manual of the Geology of India ’ 
Calcutta, 1893. 
OtpHam, T.,& J. Morris. (63) ‘Fossil Flora of the Rajmahal Series in the 
"Rajmahal Hills’ Palzont. Indica, ser. 11. pt. 1, 1863. 
Sewarp, A. C. (00) ‘Catalogue of the Mesozoic Piants in the Department of 
Geology, British *Museum—Part law i The Jurassic Flora: I—The 
Yorkshire Coast’ London, 1900. 
Do. (04) ‘Ona Collection of Jurassic Plants from Victoria’ Rec. Geol. Surv. 
Victoria, vol. 1 (1904) pt. 3, p. 155. 
Do. (19) ‘Fossil Plants’ vol. iv. Cambridge, 1919. 


1 Medlicott & Blanford (93) p. 182. 
2. Cotter (17) p. 25. 


QuarT. JOURN.GEOL.Soc.VOL.LXXVIII, PL.XII. 


—* 


Mi 


[ 
ae: 


iil 


ZZ 


= 


T.A.BROCK DEL. 


TA NIOPTERIS (FIGS, 1-10); CLADOPHLEBIS (FIGS. 11-16); 
ARAUCARITES (FIG.17); DESMIOPHYLLUM (FIG.18); 
BRACHYPHYLLUM (FIG.19); ELATOCLADUS (FIG, 20); 


ih 
mrotneT 
are.) 


1 


part 3] ON JURASSIC PLANTS FROM CEYLON. 277 


Tuomas, H. HamsHaw. (15) ‘On Williamsoniella, a New Type of Benettitalean 
Flower’ Phil. Trans. Roy. Soc. ser. B, vol. cevii (1915) p. 118. 

Watxom, A. B. (17) ‘Mesozoic Floras of Queensland, Part I: The Flora of the 
Ipswich & Walloon Series: (c) Filicales’ Geol. Surv. Queensl. Publi- 
cation No. 257 (1917). 

Do. (19) ‘On a Collection of Jurassic Plants from Bexhill, near Lismore 

(N.S.W.)’ Proc. Linn. Soc. N.S.W. vol. xliv, pt. 1 (1919) p. 180. 

ZEILLER, R. (03) ‘ Flore Fossile des Gites de Charbon du Tonkin’ Etudes des Gites 
Minéraux de la France: Colonies frangaises, Paris, 1903. 


EXPLANATION OF PLATE XII. 


[All the figures are drawn of the natural size, except those with numbers 
followed by b. | 


Figs. 1-10b. Txmiopteris spatulata. 4b, 6b, 8b, 106~xX 3, 
lla, 116, 12, 14a, 146. Cladophlebis denticulata. 14b x3. 
13, 15a, 156,16. Cladophlebts reversa. 156 X2. 

Fig. 17. Araucarites cutchensis. 

Figs. 18a &18b. Desmiophyllum sp. 186 enlarged. 

Fig. 19. Brachyphytlum mamillare. 
20. EHlatocladus plana. 


DISCUSSION. 


Dr. A. Morty Davrns said that the discovery of these Jurassic 
plants was the outcome of the enthusiasm of Mr. Wayland, who 
had made the collection in circumstances of the greatest difficulty. 
He greatly regretted that Mr. Wayland, who was on his way to 
England, had not been able to be present to give an account of 
his field- work. 

The Presripent (Mr. R. D. OL~pHam) commented on the 
interest attaching to the discovery of these beds in Ceylon, as 
carrying on the deposits known in the Madras coastal region, and 
extending the length of the margin of the continent of Gond- 
wanaland which had been previously traced. 


Q.J.G.S. No. 811. U 


278 PROF, A. C. SEWARD ON A COLLECTION OF _ [vol. lxxviii, 


9. On a CoLLEcTION of CARBONIFEROUS Puants from PrEru.t 
By Prof. ALBER? CHARLES SEWARD, Sc.D., F.R.S., Pres.G.S. 
(Read April 12th, 1922.) 


[PLATE XIII. ] 


In the course of a geological expedition to Peru in 1911 Mr. J. A. 
Douglas collected some fossil plants from coal-bearing strata on 
the south side of the peninsula of Paracas, a few miles south of 
the port of Pisco (lat. 18° 45'8.). The plants, which are the 
property of the Geological Department of Oxford University, were 
handed to me for identification by Mr. Douglas, who also supplied 
information on the geology of the district. The specimens, though 
fragmentary and few in number, are worth recording, since this is 
the only known occurrence of fossiliferous Paleozoic rocks on the 
coast of Peru.? Moreover, from a phytogeographical point of view, 
any fossil plants from South America are worthy of attention. 

The coal occurs in a series of greenish sandstones and grey and . 
black carbonaceous shales, which have a north-easterly strike and 
dip about 25° south-eastwards. These are overlain unconformably 
on the neck of the peninsula by Tertiary sandstones and impure 
limestones. There is no definite stratigraphical evidence of the 
age of the coal-bearing beds, and the plants are therefore of special 
importance: in other districts the strata which are the chief source 
of Peruvian coal are Mesozoic in age. Mr. Douglas informed me 
that the rocks in question were described by Friedrich Fuchs,? who 
published a note on them in 1900 in the ‘ Boletin de Minas’ (Lima). 
Fuchs recorded the following species, and assigned the beds to the 
Upper Coal Measures :— 


Calamites Suckowi Brongniart. C. | Lepidodendron Sternbergii Brongniart. 

Sphenopteris Hartlebenit Dunker. 5 
W. Sigillaria tessellata Brongniart. C. 

Baiera pluripartita Schimper. W. | Stigmaria ficoides Brongniart. C. 


Four of these species (C) are well-known Northern Hemisphere 
Carboniferous plants and two (W) are Wealden species. It is 
obviously impossible to express an opinion on the nature of Fuchs’s 
specimens. A short account of the geology of the district is 
given by Mr. V. F. Marsters, who states that Fuchs mapped the 
whole area as Carboniferous, the plants being typical Carbeniferous 
species: the fact that two species are characteristic of European 


1 After this note had been sent to the Geological Society, a preliminary 
paper on ‘Carboniferous Plants from Peru’ by E. W. Berry, to be followed 
by a fully illustrated account, was published in the ‘American Journal of 
Science’ ser. 5, vol. 11 (1922) p. 189. Mr. Berry’s conclusion is that the 
Paracas plant-beds ‘ correspond to the Westphalian stage.’—[A.C.S.] 

2 Douglas (21) p. 250. Numerals in parentheses refer to the Bibliography, 

. 283. 
‘ 3 This paper has not been seen, either by Mr. Douglas or by myself. 


part 3] CARBONIFEROUS PLANTS FROM PERU. 279 


Wealden floras was not noticed. The account of the geology of 
the district by Marsters is as follows :— 


‘The geology of this district was studied by Don Federigo Fuchs, and 
according to his map the whole area is Carboniferous. The area termed 
Cerro de la Mina is occupied by Carboniferous strata, as the fossil flora 
indicates; the plants are typical species of the Carboniferous Period. The 
best exposure of the beds is to be seen on the coast between the house of the 
Director of the Exploration Company and the Punta de Huaca. This shows 
thick beds of sandstone with thin and fissile shales, the coal being associated 
with the shales; local folding, attenuation, and pinching-out of the seams are 
common phenomena. The dip of the strata varies in direction from 8. 40° E. 
to S. 45° E., at angles between 16° and 27°. If we examine also the north- 
western boundary of the Cerro de la Mina on the south-eastern slope of the 
little valley of the Arquillo, we observe the same lithological conditions as in 
the former region. The Carboniferous strata are clearly in contact with rocks 
which are, without any doubt, Tertiary. In many places the shales are very 
carbonaceous, containing at times thin lenticles of pure coal. The dip of 
the beds varies from 16° to 20°; the whole formation is inclined towards the 


south-east, 
‘After having visited this region, Senor Bravo states that west of the 


Cerro de la Mina Carboniferous sediments exist in the neighbourhood of the 
Punta de Lechuza.’ ! 


To Mr. Douglas I am also indebted for the following extract 
from a paper by Prof. C. I. Lisson (18) :-— 

‘In the year 1900 Prof. Fuchs published the discovery of a Carboniferous 
flora situated on the peninsula of Paracas. Although a revision of the de- 


terminations adopted by Fuchs is necessary, nevertheless it is a proved fact 
that the plants show decisively the presence of a Carboniferous deposit.’ 


Here again is no reference to the fact that Fuchs’s list included 
some typical Wealden plants. : 

_In view of the inclusion of two Wealden species in the list of 
plants given by Fuchs, it is noteworthy that the occurrence of a 
Wealden flora was recorded in Peru in 1907 and 1910.? 

A photograph of the coal-bearing beds on the Pacific coast is 
published in a book by R. Enock*? ou the Andes and the Amazon : 
this author speaks of the coal-deposits of Peru as one of the 
country’s most valuable assets. These carbonaceous deposits are 
of Mesozoic, and not of Paleozoic age. 

So far as I am aware, no Upper Carboniferous flora, in which are 
not included members of the G'lossopteris flora, has been described 
from South America. 


Description of the Specimens. 


SPHENOPTERIS sp. (Pl. XIII, figs. 1-3.) 


The collection includes several fragments of pinne, pieces of 
rachis, and pinnules of a fern-like plant, although these are un- 
fortunately not sufficiently well preserved to be identified with 


1 Marsters (09) pp. 40-41. This extract is taken from a translation kindly 
made for me by Mr. Douglas. 
2 Neumann (07); Zeiller (10). 
3 Enock (07) p. 207. 
u2 


280 PROF. A. C. SEWARD ON A COLLECTION OF _ [vol. lxxviii, 


certainty. The best specimen is shown in Pl. XIII, fig. 1: the 
branched axis is longitudinally striated and smooth; the pinnules 
are more or less deltoid, deeply dissected, and the ultimate segments 
are obtuse or truncate. A smaller piece of pinnule is reproduced 
in Pl. XIII, fig. 2. The broader piece of rachis seen in Pl. XIII, 
fig. 3, if found without any associated pinnules, might be mistaken 
for a portion of a Cordaites leaf. 

Fig. 1 recalls Sphenopteris furcata Brongniart, an Upper Car- 
boniferous species by some authors referred to Diplotmema or 
Palmatopteris; but in the Peruvian pinnules the segments are 
obtuse, and not acute as in Sphenopteris furcata. Comparison 
may also be made with Hremopteris missouriensis Lesquereux, as 
figured by D. White! from the Lower Coal Measures of Missouri, 
in which the ultimate segments are obtuse or truncate. Specific 
determination is probably impossible, and the occurrence of ferns 
or pteridosperms, in both Upper and Lower Carboniferous strata, 
with pinnules very similar to those shown in figs. 1 & 2, precludes 
any definite conclusion as to the question of geological age. 
Several Lower Carboniferous species have been figured, in which 
the pinnules bear a very close resemblance to the Peruvian 
specimens; the deeply-dissected form of the lamina suggests com- 
parison with pinnules of species of Rhodea and Sphenopteridium.? 


LEPIDODENDRON sp. (Pl. XIII, figs. 4-6.) 


This specimen, part of which is shown in fig. 4, 1s an impression 
on a carbonaceous sandstone of a piece of a stem or branch 16°5 cm. 
long and 3 em. wide, with several attached and indistinctly pre- 
served leaves spreading over the rock at the edges. The fossil is 
doubtless part of a Lepidodendroid plant; the surface does not 
reveal at all clearly the form of the leaf-cushions, except at the 
lower end. An examination of the carbonized surface shows im- 
pressions of linear leaves pressed against the stem. Fig. 5 
represents a single leaf-cushion, with a leaf-scar and the impression 
of part of a leaf. The specimen seen in fig. 6 shows more clearly 
the form of the leaves, each of which has a midrib. 

These specimens unfortunately do not enable one to determine 
with any confidence the precise age of the plant-beds, as Lepido- 
dendroid plants generally resembling that from Peru occur in both 
Upper and Lower Carboniferous strata. 


SIGILLARIA sp., or LEPIDODENDRON sp. (PI. XIII, figs. 7 & 8.) 


The two specimens reproduced in figs. 7 & 8 are pieces of a stem 
having contiguous leaf-cushions which bear leaf-scars agreeing both 
with some types of Sigillaria, as, for example, Sigillaria Brardi 
Brongniart, and with certain species of Lepidodendron. On the 
upper part of several leaf-cushions there is a small circular scar, 


1 White (99) pl. v, figs. 1-3.a & pp. 6-19. 
2 See Nathorst (20) pl. i, figs. 11-13. 


part 3] CARBONIFEROUS PLANTS FROM PERU. 281 


presumably a ligule-pit. No vascular-bundle scars or parichnos- 
scars can be detected. Stg@llaria Brardi, although mainly a 
Northern Hemisphere Upper Carboniferous species, is recorded 
also from South Africa! and Brazil.2 Some of the numerous 
specimens figured by E. Weiss as Szgzllaria mutans Weiss ® closely 
resemble the small pieces shown in figs. 7 & 8. On the other hand, 
comparison may equally well be made with species of Lepido- 
dendron from Lower as well as from Upper Carboniferous beds. 


BorHRODENDRON (?) sp. (Pl. XIII, fig. 9, and text-figure.) 


Although it is by no means certain that the specimens shown in 
Pl. XIII, fig. 9 and in the text-figure belong to the same species, 
they may be considered together and regarded as possibly specifically 
identical, or at least very closely allied. The flattened piece of stem 

seen in fig. 9 is characterized 

Bothrodendron (?) sp. (Natural size.) gran ies eesti 
prominent leaf-scars. Most 
of the surface is bereft of 
any carbonaceous film, and 
does not exhibit the original 
surface - features; but, on 
the right-hand side, a thin 
carbonaceous layer probably 
represents the actual surface, 
and on magnification the 
outlines of cells are clearly 
visible. The leaf-scars are 
prominent, transversely 
elongated, and rhomboidal. 
ie On the partly-decorticated 
Sch ede surface there are discon- 

tinuous longitudinal ridges 

and an irregular transverse 

wrinkling, but on the carbonized film no wrinkling is seen. There 
is no indication of any leaf-cushion, no ligular pit, and only a very 
faint suggestion in a few of the scars of a median vascular scar. 
The leaf-scars shown in the text-figure are rather more rounded, 
a see as slightly concave areas (not projecting as in Pl. XIII, 

e 9). 

In the small and widely-separated leaf-scars these fragments 
agree with Bothrodendron, Pinakodendron, and Asolanus. The 
forni of the leaf-scar and the absence of a leaf-cushion are features 
more suggestive of Bothrodendron. The last-named genus extends 
from the Upper Devonian to the Upper Carboniferous. 


! Seward (97) p. 326. 
® "White (08) p. 458. 
* Weiss (93) pp. 84 et seqq. 


282 PROF. A. C. SEWARD ON A COLLECTION OF | vol. lxxvii, 


PLANTA INCERTH sepDIs. (Pl. XIII, fig. 10.) 


On one piece of sandstone there are faint impressions of crowded 
branched filaments, some of which are shown in fig. 10. Their 
occurrence in a fairly-dense mass reminds us of ‘the branched 
leaves of Dicranophyllum Richiri Renier ! from the Coal Measures 
of Belgium; but they are more probably portions of pinnules of a 
fern-like plant, such as some of the Lower Carboniferous species 
referred to Rhodea or Sphenopteridium. Generic determination is 
hardly possible. - 


Conclusion. 


Mr. Douglas, in his recent paper on ‘Geological Sections through 
the Andes of Peru & Bolivia,’ referred to the importance “of 
obtaining accurate determinations of the plants which he collected 
from the Peruvian strata.2 After a careful examination of the 
material, I submitted most of the specimens to Dr. Kidston,? who 
kindly expressed his opinion thereon. My first impression led me 
to regard the Peruvian plants as Upper Carboniferous in age, and 
this conclusion, if correct, would raise a question of considerable 
interest, in view of the fact that no member of the Glossopteris 
flora is included in Mr. Douglas’s collection. Dr. Kidston, however, 
is inclined to regard the paleobotanical evidence as more favourable 
to a Lower Carboniferous horizon. ‘A re-examination of the 
specimens in the light of his suggestions caused me to modify my 
conclusions. ‘The plants are too imperfect to serve as trustworthy 
guides: the rocks may belong to the upper part of the Car- 
boniferous formation or, on the whole more probably, to the Lower 
Carboniferous. Lower Carboniferous plants similar to Northern 
Hemisphere types have previously been recorded from South 
America; but, so far as I am aware, no Upper Carboniferous flora 
is known from South America composed exclusively of Northern 
Hemisphere types, with no admixture of members of the Glosso- 
pteris flora. The inclusion of two Wealden species in the list 
given by Fuchs must be attributed to incorrect determinations ; 
none of the specimens collected by Mr. Douglas can reasonably be 
referred to a Mesozoic flora. Further research is greatly to be 
desired, since the available data are inadequate as a basis for any 
positive statement. 


My thanks are due to Mr. R. E. Holttum, of St. John’s College, 
Cambridge, for assistance given to me in the preparation of this 
paper. 


1 Renier (10) pl. exvii. 

2 Douglas (21) p. 250. 

3 I am greatly indebted to my friend, Dr. Robert Kidston, F.RB.S., for his 
helpful criticisms and suggestions. 


QUART.JOURN.GEOL.Soc.VOL.LXXVIII, PL.XIII. 


T.A.BROCK DEL. 


SPHENOPTERIS; LEPIDODENDRON; 
BOTHRODENDRON (?) | ETC. 


43) 


ee 


part 3 | CARBONIFEROUS PLANTS FROM PERU. 283 


BIBLIOGRAPHY. 


Dovetas, J. A. (21) ‘ Geological Sections through the Andes of Peru & Bolivia: 
III—From the Port of Callao to the River Perene’ Q.J.G.S. 
vol. Ixxvii (1921) p. 246. 

Enocxk, R. (07) ‘The Andes & the Amazon’ London, 1907. 

Fucus, F. (00) ‘ Nota sobre el Terreno Carbonifero de Paracas’ Boletin de Minas, 
Lima, 1900. 

Lisson, C.I. (18) ‘Edad de los Fosiles Peruanos & Distribucion de sus Depositos 
en la Republica’ Lima, 19138, p. 15. 

Marsters, V. F. (09) ‘Informe sobre la Costa Sur del Peru’ Bol. del Cuerpo de 
Ingenieros de Minas del Peru, No. 70, 1909. 

Neumann, R. (07) ‘ Beitrage zur Kenntniss der Kreideformation in Mittel-Peru ’ 
Neues Jahrb. Beilage-Band xxiv (1907) p. 69. 

Natuorst, A.G. (20) ‘Zur Kulmflora Spitzbergens’ Stockholm, 1920. 

Renter, A. (10) ‘Documents pour Il’ Etude de la Paléontologie du Terrain houiller’ 
Liége, 1910. 

Sewakp, A.C. (97) ‘On the Association of Sigillaria & Glossopteris in South 
Africa’ Q. J. G.S. vol. liti (1897) p. 315. 

Weiss, E. (93) ‘Die Sigillarien der Preussischen Steinkohlen- & Rothliegenden- 
Gebiete: II’ Abhandl. k. Preuss. Geol. Landesanst. pt. 2 (1893). 

Waitt, D. (99) ‘Fossil Flora of the Lower Coal Measures of Missouri’ U.S. 
Geol. Surv. Monogr. xxxvii (1899). 

Waits, D. (08) ‘Final Report of Dr. I. C. White, Chief of the Brazilian Coal 
Commission : Pt. 11J—Fossil Flora of the Coal Measures of Brazil 
(by D. White)’ Rio de Janeiro, 1908. 

ZEILLER, R. (95) ‘Note sur la Flore Fossile des Gisements Houillers de Rio 
Grande do Sul’ Bull. Soc. Géol. France, ser. 3, vol. xxiii (1895) p. 601. 

ZEILLER, R. (10) ‘Sur quelques Plantes Wealdiennes du Pérou’ C. R. Acad. 
Sci. Paris, vol. cl (1910) p. 1488. 


EXPLANATION OF PLATE XIII. 


[The drawings, with the exception of that reproduced in fig. 5, 
were made by Mr. T. A. Brock. | 


Fig. 1. Sphenopteris sp. Branched axis and pinnule. Natural size. 
2. Sphenopteris sp. Smaller pinnule. Natural size. 
3. Sphenopteris sp. Piece of rachis, enlarged. 
4, Lepidodendron sp. Natural size. 
5. Lepidodendron sp. Leaf-cushion ; slightly enlarged. lt = impression 
of a leaf above the leaf-scar. 
6. Lepidodendron sp. Natural size. 
7. Sigillaria sp. or Lepidodendron sp. Natural size. 
8. Sigillaria sp. or Lepidodendron sp. Natural size. 
9. Bothrodendron sp. Leaf-scars as depressions. Natural size. 
10. Planta incertz sedis. Natural size. 


DISCUSSION. 


Mr. J. A. Dovetas, after expressing his gratitude to the 
Author for the trouble which he had taken in studying this 
collection, pointed out the possibility that the Wealden species 
recorded by Fuchs among Carboniferous forms were derived from 
the Cretaceous plant-bearing beds of the Island of San Lorenzo, 
and had become mixed with a collection from Paracas. 

The speaker considered that the correct identification of these 
plants was of great importance, apart from clearing up the 
anomaly in Fuchs’s list. David Forbes’s statement that nowhere 
have we evidence of the existence of Carboniferous beds on the 


284. CARBONIFEROUS PLANTS FROM PERU.  [Vvol. lxxviil, 


whole coast of South America was now definitely shown to be 
incorrect. 

This isolated Paleozoic fragment of Paracas must clearly be 
assigned to the coastal Cordillera, as distinct from the Mesozoic 
Western Cordillera, and its existence led to the hope that other 
fossiliferous Paleozoic rocks still awaited discovery among the 
crystalline rocks of the coast of Southern Peru. Further, the 
evidence afforded by these fossil plants seemed to support the view 
that the Andean Cordilleras, although subjected to repeated oscil- 
lation, had their origin at a very much earlier date than was 
generally recognized. 

On stratigraphical grounds, it could be shown that an uplift 
took place during the interval between Mid-Devonian and Permo- 
Carboniferous times; and the surmise that the altitude of the chain 
thus formed fell little short of that attained by the existing range 
was suggested by the dissimilarity between the flora of the Pacific 
region “and that of Gondwanaland on the east. At the present 
day the snowclad chain forms an effective barrier to plant- 
migration. Was it too much to suggest that similar conditions 
prevailed in late Paleozoic times? At any rate, it seemed to the 
speaker that the argument in favour of a land-barrier separating 
the two regions was more plausible than one in favour of a barrier 
formed by the sea. 


part 3] A NEW PLESIOSAUR FROM THE WEALD CLAY. 285 


10. Description of a NEw Presiosaur from the WEALD CLAY 
of BERwick (Sussex). By CHarLes WILLIAM ANDREWS, 
B.A., D.Sc., F.R.S., F.G.S. (Read February 22nd, 1922.) 


[Puates XIV & XV.] 


THE Plesiosaurian remains which form the subject of the present 
paper were contained in a large septarian nodule from the Upper 
Weald Clay of Berwick (Sussex). This nodule was found in the 
excavations made by the Cuckmere Brick Company ; 1t was broken 
into many fragments which were, so far as possible, collected by 
Mr. 8. Tooth, M.Inst.C.E., and by him presented to the British 
Museum (Natural History). The pieces, many of which clearly 
contained portions of bones, were reunited, and the gaps resulting 
from the loss of fragments filled in with plaster of Paris, so that 
the original form of the nodule was restored. The extremely hard 
matrix was then slowly and with great skill chiselled away by 
Mr. L. Parsons, who found that it enclosed a mass of bones for 
the greater part thrown together in the utmost confusion, with the 
result that their removal was a matter of extreme difficulty. 
Despite these drawbacks, however, he succeeded in getting out the 
hinder part of the skull and a nearly complete shoulder-girdle, all 
the elements of which seem to be quite undistorted by pressure, 
a most unusual circumstance. The humeri were also found, but 
the rest of the paddles, which probably projected beyond the limits 
of the concretion, was lost. Numerous cervical and dorsal ver- 
tebrze, ribs, and ventral ribs were found ; but of the pelvis and hinder 
limb nothing remained, and only one or two imperfect caudal 
vertebree were preserved. 


The skull (Pl. XIV, figs. 1 & 2).—This is unfortunately very 
imperfectly preserved, the region in front of the orbits being 
entirely lost. The posterior portion is also imperfect; but one 
exoccipital bone and the supra-occipital were found lying apart. 
The palate is well preserved on the right side, as far forwards as 
the anterior end of the sub-orbital vacuity. 

The occipital condyle (oc.c.) forms nearly a hemisphere, its 
upper border only being somewhat flattened. Ventrally and 
laterally it is bounded by a slight groove, but one cannot say that it 
is pedunculate. In front of this groove the face of the bone curves 
sharply downwards below and outwards at the sides. Ventrally it 
terminates in the sharp posterior border of the flat platform pre- 
sumably formed mainly by the basisphenoid, but actually covered 
by the parasphenoid, which conceals the relations between the basi- 
occipital and the basisphenoid. Laterally the basioccipital passes out 
into a pair of tuberosities (tubera sphenoccipitalia) which terminate 
in flat facets looking outwards and a little downwards. The 
portion of the parasphenoid (pas) covering the basioccipital 


286 DR. C. W. ANDREWS ON A NEW PLESIOSAUR _ [Vvol. lxxviii, 


and basisphenoid has a peculiarly roughened surface; externally it 
must have joined the pterygoids, but the line of junction between 
the two elements cannot be seen. Between the posterior inter- 
pterygoid vacuities (p.z.v.) the parasphenoid becomes Y-shaped, 
the Y being considerably above the general level of the palate; 
while the other arm of the Y forms a high sharp crest separating 
the two vacuities, in front of which the bone widens out into a flat 
plate wedged in between the converging anterior ends of the 
pterygoids. These (pt) are very large bones, of the usual 
triradiate form. The posterior wing is long, and extends back to 
the quadrate, which is situated far back behind the level of the 
occipital condyle: this portion of the bone is a nearly vertical 
plate, with a thickened ventral border. Anteriorly it joins the 
basis cranii, but its relations to the parasphenoid and basisphenoid 
are obscure: it is possible that the pterygoid may have overlapped 
the basis cranii to a considerable extent. In front of this Junction 
the bone is wide, and its inner and outer borders are nearly 
parallel for some distance: there is some evidence that in this 
region there was a median longitudinal ridge on the palatal surface. 
About opposite the middle of the interpterygoid vacuities the outer 
border turns sharply outwards to form the posterior border of the 
lateral wing, and at the same time the anterior limit of the tem- 
poral vacuity. Towards its outer angle this border is raised into a 
ridge and thickened for union with the transpalatine bone, which 
unfortunately is not preserved on either side. As already men- 
tioned, the pterygoid unites with the parasphenoid in front of the 
posterior interpterygoid vacuities (p.z.v.) and, in front of this 
again, meets its fellow in the middle line, there being no anterior 
interpterygoid vacuity such as occurs in some Plesiosaurs (for 
instance, P. capensis). Externally the anterior wing unites with 
the palatine, and doubtless anteriorly joined the vomers; but this 
portion is broken away. 

The palatine (pal.) unites with the outer border of the 
anterior wing of the pterygoid, extending back to the anterior border 
of the lateral wing. It forms the inner and anterior border of the 
suborbital vacuity, uniting with the maxilla in front. The upper 
surfaces of the pterygoids about opposite the level of the middle 
of the interpterygoid vacuities unite with the bases of a pair of 
bones, the epipterygoids, which run up towards the lower edge 
of the parietals; but their union with those bones has been broken, 
in consequence of the compression that the skull has undergone. 

The quadrates (qg.) unite with the pterygoids by their inner 
border, and are extensively overlapped by the squamosals on their 
posterior and outer sides. Their anterior face is concave from side 
to side. The articular surface for the mandible is wide from side 
to side and narrow from before backwards, thickening a little 
towards its outer end. 

The squamosals (sq.) are of the usual triradiate form; their 
lower limb overlaps, and unites closely with, the quadrates: the 
anterior (zygomatic) process runs forwards, doubtless uniting with 


part 3] FROM THE WEALD CLAY OF BERWICK (SUSSEX). 287 


the jugal and postorbital, completing the outer boundary of the 
relatively large temporal fossa. The upper limb of the squamosal 
runs upwards to the parietals, forming with them the posterior 
border of the temporal fossa; it cannot be determined whether the 
squamosals meet in the middle line above the parietals. 

The parietals (par.) form a A-shaped roof to the cranial 
cavity ; posteriorly (as just described) they are overlapped by the 
squamosals: towards the middle of the temporal fossa they narrow, 
and rise in the middle line into a high sagittal crest which is 
interrupted by a rather large pineal foramen, situated about 
opposite the hinder border of the orbits. In front of this the ridge 
is continued forwards on to the frontal, and, at the level of the 
front of the orbits (orb.), it is very prominent, and is separated 
from those openings by a well-marked channel. On the left side 
in this specimen there is an element uniting with the outer edge 
of the frontals, and forming a rounded upper border to the hinder 
part of the orbit: this is doubtless the post-frontal. Between this 
element and the temporal arcade is seen the impression of another 
bone, the postorbital; this probably united with the jugal 
below, but the limits of these elements are obscure. 

On the right side the anterior border of the orbit seems to be 
formed by a bar of bone (p.f.) curving up from the maxilla towards 
the frontals (Pprefrontals); it cannot be determined whether 
this bone is entirely composed of maxilla, or whether it may in- 
clude other elements, such as the prefrontals. 

The maxilla forms most of the lower border of the orbit; 
on the palatal surface it widens out, and seems to have borne teeth 
about as far back as the middle of the suborbital vacuity. In 
front its suture with the palatine can be seen, but posteriorly its 
relations with the jugal and other elements are obscure. 

The unfortunate loss of the anterior portion of the skull makes 
it impossible to say to what extent the snout was elongated, and 
renders comparison with other forms difficult. In a general way, 
it may be stated that the temporal fossa is larger in proportion to 
the orbit than in most Plesiosaurs. The region of the para- 
sphenoid and the posterior interpterygoid vacuities seem to be the 
most useful for purposes of comparison, the thin sharp edge of the 
parasphenoid separating the vacuities being very characteristic. 
In most Plesiosaurs this region of the parasphenoid is wide: for 
example, in Plestosaurus macrocephalus' it is broad, and has a 
flattened ventral surface: this seems to be the case also in Crypto- 
cleidus, Murenosaurus, Tricleidus, Dolichorhynchops, and to a 
less degree in Brachauchenius. The skull of Microcleidus homalo- 
spondylus (Owen) from the Upper Lias of Whitby approaches the 
specimen here described in the form of its parasphenoid; but 
differences in the skull and other parts of the skeleton exclude the 
possibility of any near relationship. Unfortunately, the skull of 
P. arcuatus is not known. In the nearly contemporary Branca- 


1 Q.J.G.S. vol. lii (1896) p. 246. 


288 DR. C. W. ANDREWS ON A NEW PLESIOSAUR _ | vol. lxxviil, 


saurus brancat Wegner, from the Wealden of Germany,! the para- 
sphenoid seems to have been very similar to that of our specimen ; 
but, on the other hand, the orbits are much larger in proportion 
to the temporal fossa and much longer than high. Plesiosaurus 
capensis Andrews® is also very similar to the specimen now 
described in the form of its parasphenoid; but here also differences 
are noticeable. Thus in P. capensis the orbits are larger, the 
pterygoids narrower where they form the outer side of the posterior 
interpterygoid vacuities, and there is an anterior interpterygoid 
vacuity ; this last character may, however, be a consequence of the 
crushing to which the skull has been subjected. On the whole, 
the skull seems to resemble that of P. capensis most nearly, and 
probably like it had a short snout somewhat widened in the pre- 
maxillary region. It willbe shown below (p. 291) that the likeness 
to P. capensis extends to the cervical vertebre. 


The vertebral column is represented by (1) ten cervical 
centra from the anterior part of the neck, eight being developed 
out of the matrix; (2) nine or ten posterior cervicals (the anterior 
five being represented by the arches only); three or four transitional 
(thoracic) vertebrz (both arches and centra preserved), then six 
dorsals, the posterior three represented by the arches only: all 
these form a united series; (3) about six dorsal centra free from 
the matrix; and (4) two or three caudal centra. Some neural 
arches still remain in the block of matrix. 

The anterior cervical centra (Pl. XIV, figs. 3 & 4) are very 
imperfect, and much deformed by cracks filled with calcite. The 
smallest one must have come from immediately behind the axis: 
it is too imperfect to supply complete measurements, but its length 
in the mid-ventral line is about 19 mm., and its width is consider- 
ably greater than its height. From this and two or three rather 
larger specimens of cervical centra it ean be seen that the articular 
face was formed by a broad, somewhat convex, outer rim within 
which is a well-defined deep central concavity. At the bottom of 
this concavity in some cases, a pit marking the position of the 
notochord can be seen. The third of the cervical centra (Pl. XIV, 
fig. 3) in point of size measures 2°2 cm. in length, 2°5 cm. 
in width, and 25cm. in height. Another from rather farther 
back in the neck is better preserved, and shows very well the deep 
concavity of the central portion of the articular surface surrounded 
by a broad convex border. The central depression on the anterior 
face seems to be more strongly defined than that of the posterior 
face. The dimensions of this centrum are :—length=25 mm. ; 
width=38mm.; height=30mm.; one of the cervical nbs is 
present, its base occupying nearly the whole length of the centrum. 
The last of the free centra (Pl. XIV, fig. 4) also has one rib (%.) 
attached; its dimensions are:—length in mid-ventral line=about 


1 Branca-Festschrift, Leipzig, 1914, p. 235. 
2 Ann. S. Afr. Mus. vol. vii (1911) p. 309. 


part 3] FROM THE WEALD CLAY OF BERWICK (SUSSEX). 289 


32mm.; width=43mm.; height=40mm. In none of these 
vertebre is the neural arch preserved. 

Of the posterior cervicals (Pl. XIV, fig. 5) only the last four 
are complete, the five in front of these being represented by their 
arches only. The fact that these vertebre are still articulated one 
with the other and are still partly embedded in matrix renders 
their description difficult; but it can be seen that the form of the 
articular surface is similar to that of the anterior cervicals described 
above. On the sides of the centrum the single rib-facets form 
well-marked prominences which occupy rather more than half the 
length of the centrum, and are situated nearer the posterior than 
the anterior border. Beneath them on each side of the middle line 
there is a depression into which a vascular foramen opens; these 
depressions are separated by a well-marked rounded hemal ridge. 
The borders of the centra are raised into fine rugosities running at 
right angles to the articular faces. The length of the centra is 
less than the length or width, and the width is greater than the 


height. 


The form of the thoracic vertebre seems to be much like © 


that of the centra of the posterior cervicals, but doubtless there ts 
a transition to the condition seen in the dorsals. In these the 
ventral surface becomes rounded from side to side, the hzmal ridge 
disappearing. The ventral surface of the centrum is perforated 
near the middle line by a pair of small vascular foramina, and 
there is another larger pair situated about a third of the way up 
the sides. The articular surfaces are evenly concave, wanting the 
convex border seen in the cervicals, so that there is a sharp edge 
between the articular surface and the sides of the centrum 
which are strongly concave from before backwards. The dimen- 
sions of a dorsal centrum are :—length=31mm.; width=55 mm. ; 
height =48 mm. 

In the posterior cervical and thoracic regions the neural arches 
are large and massive. In the last two cervicals the base of the 
pedicle is produced into a short process which extends down to the 
costal facet, but does not seem to have reached the rib: in the next 
four vertebrz (thoracic) this process of the arch increases in size, 
and forms more and more of the rib-articulation until in the first 
dorsal it wholly supports it, becoming a prominent diapophysis (d.). 
This is compressed from before backwards, and is enlarged towards 
its outer end, which bears a somewhat convex facet considerably 
higher than wide. Followed back in the series, the dorsal rib- 
facets become smaller, and their vertical and horizontal diameters 
more nearly equal. 

In the posterior cervical and thoracic regions the zygapophyses 
are large, and their articular surfaces are flat (making an angle of 
about 45° with the median plane). They project so far anteriorly 
and posteriorly that, when they were in articulation one with the 
other, there must have been a small interval between the articular 
surfaces of the centra. This interval is, in fact, shown in this 
specimen, and it can be seen that it was partly occupied by a dise 


2 te ee eee rw | i eo eee ee ee Se 


—— 


———————— ————— 


290 DR. C. W. ANDREWS ON A NEW PLESIOSAUR _ [Vvol. Ixxviul, 


of hard substance which under the microscope seems to show some 
resemblance to calcified cartilage. There does not seem to be any 
question of epiphyses on the ends of each centrum, but merely of a 
simple intervertebral disc (Pl. XIV, fig. 5, z.v.d.). Possibly the 
large size and prominence of the zygapophyses, the convexity 
of the outer rim of the articular surface of the centra, and the 
presence of this disc may be connected with increased flexibility 
in some directions of this region of the column. In the dorsal 
region the thickened rounded border of the articular surface is 
wanting, and the concavity begins at the edge which is sharply 
defined, also the zygapophyses are smaller and less prominent : 
here the successive centra must have been in direct contact, and 
the degree of flexibility consequently much less. The neural 
spines of the cervical region are rather short, and curve backwards ; 
the convexity of their anterior border is greater than the concavity 
of the posterior, so that they narrow towards their apex, which is 
occupied by a deep pit, probably marking the position of a carti- 
laginous extension in life. Towards the dorsal region the neural 
spines (27.sp.) lengthen somewhat, become more upright, and are of 
nearly the same width throughout their length. 

The cervical and thoracic vertebre are specially valuable for com- 
parison with those of other Plesiosaurs, since in very many cases the 
skull and other characteristic portions of the skeleton are unknown. 
The most notable character of these vertebre in the species now 
described is the form of their articular surfaces. A similar type of 
vertebra occurs in Plesiosaurus capensis Andrews, P. degenhardti 
Koken, P. bernardi Owen, Cimoliosaurus valdensis Lydekker, and 
in Brancasaurus brancat Wegner; also to some degree in Plesio- 
saurus arcuatus Owen. Plesiosaurus degenhardti is, however, 
distinguished from this species by the possession of cervical centra 
which are higher than wide, neural spines with their anterior and 
posterior borders parallel, and relatively small zygapophyses. 
In P. degenhardti, if the length of the centrum be taken as 
100, the width is 127, the height 152; in the specimen here 
described, if the length be taken as 100, the width is 155, the 
height 146. 

Comparison with the cervical of the type-specimen of Cimo- 
liosaurus valdensis Lydekker! shows that the form now described 
resembled that species in the shape of its neural spine, but differed 
in several other respects. Thus, in C. valdensis the zygapophyses 
are relatively smaller and less prominent, and their articular faces 
are more nearly horizontal. The articular ends of the centrum are 
more nearly circular, and their central concavity (though deep) 
has not the sharply-defined central depression. In Cimoliosaurus 
bernardi the articular ends of the cervicals are deeply concave 
with a convex outer border, but here also the sharply-defined 
central depression is wanting; in this species, too, there is a 
strongly-marked oblique ridge running up the side of the arch from 


1 Cat. Foss. Rept. Brit. Mus. pt. ii (1889) p. 188, fig. 61. 


part 3] FROM THE WEALD CLAY OF BERWICK (SUSSEX). 291 


the anterior angle of the pedicle to the posterior zygapophysis, a 
ridge which is wanting in our specimen. 

Comparison with the vertebre of  Plesiosaurus capensis 
Andrews,! from probably the nearly contemporary beds of Uiten- 
hage (Cape Province), shows very considerable similarity in several 
respects. The form of the neural spines in the posterior cervical 
and thoracic vertebre is nearly the same; the articular face of the 
centrum has the same sudden depression of its middle portion ; 
the zygapophyses in both are large, and project considerably in 
front of and behind the articular surfaces. The proportions of 
the centra are almost the same: thus if, as already mentioned 
above, the length of the centrum in the new specimen be taken as 
100, the width will be represented roughly as 155, the height 
as 146. In P,. capensis, under the same conditions, the width 
would be 156, the height 188. Differences in the skull, apart 
from other considerations, make it very unlikely that the specimen 
here described is specifically identical with the South African 
species. 

Se ccsaisrus brancat Wegner is by far the best known of the 

Wealden Plesiosaurs, most of the skeleton having been described 
and figured.2 The vertebree resemble to a considerable degree 
those described above, the neural spines and the articular surfaces 
of the centra being very similar. On the other hand, the length 
of the cervical centra is greater in proportion to their width and 
especially to their height; the proportions are :-—length=100, 
width=140, height=112. Other differences in the skull and 
shoulder-girdle are mentioned elsewhere (pp. 288, 296). 


The shoulder-girdle (Pl. XV, figs. 1-8) is in a remarkable 
state of preservation, the bones, even the thin clavicular arch, being 
quite undistorted. Some parts of the bones are wanting; but the 
remaining portions being retained in their original positions in 
the matrix, it has been possible to fill up the gaps, and to restore 
accurately the original form of the various elements. 

The general form of the shoulder-girdle will be best understood 
from the figures. It will be seen that the coracoids (cor.) are 
long and narrow. Opposite the glenoid and scapular surfaces they 
are thickened, the thickening extending to their median border, 
where it ends in a symphysial surface, convex longitudinally above 
and concave in the same direction below. The glenoid (g/.) sur- 
face, which forms about two-thirds of the articular surface for the 
humerus, is nearly flat, and makes an angle of about 120° with the 
facet for the scapula. The post-symphysial portion of the coracoids 
is thin, and in this region they must have been separated by a short 
interval in the middle line. The thin inner border of the left 
coracoid just behind the symphysis is cut into by a notch with a 
thin sharply-defined border: this notch seems to represent the 


1 Ann. §. Afr. Mus. vol. vii (1911) p. 309. 
2 Branca-Festschrift, Leipzig, 1914, p. 235. 


— ee ee ee _SV7"7" ~~ 


292 DR. C. W. ANDREWS ON A NEW PLESIOSAUR _ [ vol. lxxviii, 


foramen often seen between the coracoids in this position. 
Posteriorly the coracoids widen little, but do not seem to have 
been produced outwards into postero-lateral processes. In front of 
the surface for the scapula the sharp border of the bone describes 
nearly a semicircle, of which the anterior end forms the somewhat 
inwardly projecting angle of the long narrow anterior prolongations 
of the bone. The symphysial union seems to have been continued 
to the slightly-expanded anterior end of these prolongations. There 
is no evidence that there was any anterior union of the median 
processes with the scapule, but there probably was one with the 
posterior processes of the clavicles. 

The scapula (se.) is much thickened at its posterior end where 
it bears the surface for union with the coracoid, and also that 
forming the upper third of the glenoid surface. These two surfaces 
meet at an angle of about 95°. In front of this thickening the 
ventral ramus of the bone first narrows, and then widens out intoa 
broad plate; the outer border of this is straight, the inner concave, 
the antero-internal angle being produced inwards to a rounded 
point, which however did not reach the anterior prolongation of 
the coracoid or the corresponding expansion of the opposite scapula, 
as happens in the Elasmosauride. There was consequently no 
closed scapulo-coracoid fenestra. The ventral face of the inferior 
ramus of the scapula is concave, both from side to side and from 
before backwards. The base of the dorsal ramus of the scapula 
arises as a high plate of bone from the upper surface of the anterior 
two-thirds of the ventral ramus, into which it passes by a gentle 
curve on its outer side; while on the inner the curve is much 
sharper, especially towards the anterior end, the shelf-like surface 
thus formed supporting the similarly curved outer end of the 
clavicle which fits closely against it. The upper limb of the 
dorsal ramus forms a narrow backwardly directed blade of bone, 
narrowing towards its upper end : its upper edge is thin and sharp, 
the lower somewhat thickened and rounded. 

The interclavicle (z.cl.) is a transversely elongated plate of 
bone, the upper surface of which is gently concave, as also is its 
anterior border. It is widest in the middle line, and terminates 
laterally in blunt points. Its postero-lateral borders interlock with 
the anterior borders of the clavicles in a close suture: there may 
have been a notch or foramen in the interclavicle near its hinder 
end, at, or a little in front of, the point where the clavicles meet, 
but the bone is incomplete in this region. Such a foramen is seen 
in the clavicular arch (B.M. R.1322) of a Liassic Plesiosaur 
referred by Lydekker to Thaumatosaurus megacephalus? (see 
text-figure, p. 295). 

The clavicles (c/.) are rather large bones, the form of which 
is shown in the figure. Their anterior borders interlock with the 


1 For instance, in Cryptocleidus: see C. W. Andrews, Cat. Marine Rept. 
Oxford Clay (Brit. Mus.) pt. i, 1910, p. 180, where it is suggested that it 
probably transmitted a blood-vessel. 

2 This is referred below to the new genus Eurycleidus. 


part 3] FROM THE WEALD CLAY OF BERWICK (SUSSEX). 293 


posterior border of the interclavicle, behind which they seem to 
have met in a median suture. Their thin posterior edge is strongly 
concave; towards their outer ends they become thickened and 
bent sharply upwards, the convex surface thus produced fitting 
closely into the shelf-like projection on the inner side of the scapula 
(as described above). Apparently the posterior ends of the united 
clavicles overlapped the upper surface of the anterior ends of the 
forward prolongations of the coracoids, thus closing a large 
fenestra bounded by the coracoids, scapula, and clavicles. 

The dimensions of this shoulder-girdle (in millimetres) are as 
follows :— 


Greatest length of the whole girdle as mounted.................. 426 
Width of the same at the glenoid cavity ...............ccseenees 278 
Coracoid. 
PRE EC SE OMA TIM Am icte aria. seule taunaeabaerav nisin ta es See ME UNC RRELn Te aia 332 
Width at the elenoid prominence ...............ceccesscdese sven 153 
Least width of anterior prolongations.................. HAE Aad be 30 
Width of-anterior ends of the Game ......sdii..dseonest) ableecass 45 
Meneha Of SlenOld>faceb: ioiceunkycb ows hancs sed ea vage ee about 70 
ADB PH Ol: DHE AIO: ch ity slots nar dee see ea aee Bea awa oR Ne vas Roa 37 
Monee Ore SGA Pilar taCO bic sectors decors « ostecun oatenemeaneae suas se 39 
Greatest depth of symphysial surface..................... about 28 
Scapula. 
Greatest length of ventral ramus ......:.0.50hssvwcsetsoovse sev uas 150 
Wactheorebie' proximal! endl, 02%. sis soa ues oh can hie Funes ms dates Ses 61 
VATU ELA CET GT el Socal eRe ine peter i kA COR Rn ORR about 37 
Length of the upper border of the dorsal ramus ............... 193 
Greatest width of the anterior border of the ventralramus... 81 
Clavicular arch. 
Greatest width of the arch as a whole ....................0.0000 218 
Antero-posterior length of the same in the middle line about 125 
NRC MOk Inher ClawiCle:..-. jac. sc ams ncalc cd scram taslwkd vue Sealalee about 140 
Greatest width: of left’ clavicle, oy.ceecescrss ccvesteeetosinagesnesne ss 135 


This type of shoulder-girdle seems to be a primitive one, being 
very similar to that of a girdle from the Lower Lias of Street 
(Somerset) which is represented in the text-figure (B, p. 295). 
This is part of the type-specimen of Plestosawrus arcuatus,! 
figured in Hawkins’s ‘Sea-Dragons’ (1834) and described in part by 
Richard Owen in the British Association Report on Fossil Reptiles 
for 1839 (1840) p. 76. This skeleton, which is in the British 
Museum (Natural History), was registered under several numbers 
(2028*—29*, R. 1817-18), and it is only recently that the bones 
of the shoulder-girdle have been developed and mounted with the 
advice of Prof. D. M.S. Watson. As now figured (text-fig. B), 
it is one of the best specimens of the shoulder-girdle of a 
Liassic Plesiosaur known. ‘The clavicular arch has already been 
described and figured in detail by H. G. Seeley.” In its general 
structure it is very similar to that described above; but the inter- 
clavicle (¢.c/.) is wider, and the concavity of its anterior border 


1 Referred below to the new genus Hurycleidus. 
2 Proc. Roy. Soe. vol. li (1892) pp. 128-30 & text-figs. 2-8. 


Q. J. G.S. No. 311. x 


294: DR. C. W. ANDREWS ON A NEW PLESIOSAUR _ [Vol. lxxviil, 


is more marked. There is a trace of the anterior border of a fora- 
men in the posterior prolongation, most of which however has 
been broken away. The clavicles (cl.) interlock with the inter- 
clavicle in a complex overlapping suture (see Seeley’s figure). 
Posteriorly they are crushed and incomplete, and so it cannot be 
seen whether they met in the middle line or extended back to over- 
lap the coracoids. In the very similar but more complete clavicular 
arch ascribed by Lydekker to Lhaumatosaurus megacephalus + 
and referred to above, the posterior portions of the clavicle and 
interclavicle are much more complete (text-fig. A, p.295). Here it 
can be seen that the interclavicle (7.c/.) is prolonged backwards 
into a bifurcated process, enclosmg a foramen or notch (for.). 
The clavicles (¢/.) overlap on to the ventral face of this prolon- 
gation, and appear to meet below it in the middle line, extending 
behind it. If the coracoids in this Plesiosaur were similar to those 
of P. arcuatus, the clavicles must have overlapped the upper surface 
of their anterior prolongations, thus enclosing a fenestra as in the 
specimen here described. 

The scapula in P. arcuatus is generally similar in form and in 
its relation to the clavicular arch, but differs in details from the 
Wealden specimen: the ascending ramus is much stouter, and 
widens instead of narrowing towards its upper end, which is 
occupied by a surface for a supra-scapular cartilage. The anterior 
portion of the ventral ramus does not widen out to the same 
extent; its inner face against which the clavicle fits is gently 
curved, and slopes downwards instead of forming a sharply angu- 
lated shelf. The coracoids of P. arcwatus differ in having their 
anterior prolongations much broader and shorter. Behind the 
thickened symphysial surface in the interglenoid region the median 
borders of the coracoids, instead of having their sharp edges 
separated by a considerable interval, were fringed with cartilages 
which must have met in the middle line. In Brancasaurus also 
they are separated for a considerable distance behind the symphysis, 
but unite posteriorly at the postero-internal angles. 

Liydekker has referred both the reptiles originally described as 
Plesiosaurus arcuatus and P. megacephalus, to H. von Meyer’s 
genus Thaumatosaurus, without any apparent justification. This 
genus was founded on some imperfect dorsal and cervical centvra, 
some fragments of jaw with broken teeth, and some pieces of rib 
from the Great Oolite of Wiirtemberg. Nothing is known of the 
structure ef the skull, shoulder-girdle, or pelvis; and it is 
exceedingly improbable that the Lower Liassic species above men- 
tioned are referable to it. It seems better, therefore, that those 
Liassic species possessing the type of shoulder-girdle described 
above, as also a double costal facet on the cervical vertebre in the 
centra of which there is no sharply-defined central concavity, 
should be referred to a new genus, for which the name Hury- 
clezdus may be suggested, the type species being #. arcuatus. 


1 See Cat. Foss. Rept. Brit. Mus. pt. ii (1889) p. 167. 


—— 


part 3] FROM THE WEALD CLAY OF BERWICK (SUSSEX). 295 


The Wealden species that forms the subject of this paper, while 
possessing a shoulder-girdle of similar type, has single costal facets 
and a deep central depression in the cervical centra; this may 


on Wes 
A 

=~ —Z2 i | 

IOS A 


— 


~ 
S> > 
—————— 


\, 
\ 


SS 


\\ 


: 
Sy 

7 

W' we 
WU 


A=Clavicular arch of Eurycleidus megacephalus (Stutchbury) from below, 
about a sixth of the natural size. (B.M. R.1322.) 

B=Shoulder-girdle of Hurycleidus arcuatus (Owen) from above, about a sixth 
of the natural size. (B.M. 2028*—29*, R. 1317-18.) 


cl.=clavicle ; cor.=coracoid ; for.=foramen or notch in the 
interclavicle ; 2,cl.=interclavicle ; sc.=scapula. 


296 DR. C. W. ANDREWS ON A NEW PLESTIOSAUR _ [vol. lxxvii, 


be referred to a second genus, Leptocleidus, its specific name 
being Leptocleidus superstes. Probably Plesiosawrus 
capensis is also referable to this genus. 

The relationship of Leptocleidus to the other Wealden genus 
Brancasaurus, if Wegner’s account of the shoulder-girdle is correct, 
must be remote, since in the latter the scapulz extend inwards and 
join the anterior prolongations of the coracoids so that the shoulder- 
girdle is of Hlasmosaurian type: it seems, however, by no means 
impossible that Wegner’s restoration of this region may be mistaken. 

The dimensions of the shoulder-girdle of Hwrycleidus arcuatus 
(in millimetres) are :— 


Greatest length in the middle line. (770i c00.0...... hk 
Coracoid. 

uene'th:. of coracoid: % 2.05 s ss a oe 2 ay ee 456 

Width of coracoid at the glenoid cavity... ca ileiieieais rou dole, gesagt ee 

Width of the Dies ae ee ONT OST 

Length of the glenoid surface... COMM 
Scapula. 

Length of the body of the scapula... 280 

Length from the top of the dorsal ramus s to the anterior end. about 244, 

Width of thesmiddle of the dorsaliblade | y.---9-0 eee 53 
Clavicular arch. 

Greatest width .. Siopsinsa, avian dg bade beaedeenteteaiayi ts elects oat ee ae 

Width of the caskemel amaicla,. ES ORO AAL _. about 261 


The dimensions of the eu arch of Burge mega- 
cephalus (in millimetres) are :— 


Greatest: widths fi eee Oe Re ee ee 464. 
Width ofthe interclavicle | 20 sn 5h aks dete bern eee eee 324 
Kength inthe middie line. 1.03: 0e. see see ces cee ae sek elec utes eee eee 153 


The humerus of Leptocleidus (Pl. XV, fig. 4) was much 
expanded distally, but seems to have articulated with the radius 
and ulna only. The anterior border of the bone is nearly straight, 
the posterior strongly concave. The upper surface is gently 
concave in the direction of the long axis, the ventral face being 
correspondingly convex. None of the other bones of the paddle 
are preserved. 

The dimensions of the humerus (in millimetres) are :— 


Neb a M he cet OU acne tae Stan cen nee era ee Rem 245 
Wadithsot head, cme. f cnr Gae nw ee eee nn EE ete 50 
Width of upper end, with trochanter..................... 62 
Wadthvof-middle ofishattir ye. enon cose acca ence: 60 
Widthiok distaliexpansion: jena. keane ce nen anaes es 124 


The chief interest of Leptocleidus superstes lies in the circum- 
stance that, although of Wealden age, it has retained a very 
primitive type of structure of the shoulder-girdle, similar to that 
found in the Lower Liassic species Hurycleidus arcuatus and 
HE. megacephalus. In all these the clavicular arch is large, and the 
ventral rami of the scapule remain widely separated in the middle 
line. This type of shoulder-girdle is probably a direct inheritance 
from the Triassic ancestors of the group. The tendency to the 
reduction of the clavicular arch and its funztional replacement by 
the expansion of the scapule to form a median symphysis was 


part 3] FROM THE WEALD CLAY OF BERWICK (SUSSEX). 297 


manifested very early, so that even in the Upper Liassie Mzcro- 
cleidus homalospondylus there already appears an Elasmosaurian 
shoulder-girdle of a type nearly as advanced as that found in 
Cryptocleidus, Murenosaurus, etc. from the Oxford Clay: that | 
is to say, the enlarged ventral rami of the scapule form a median 
symphysis continuous posteriorly with the symphysis of the cora- 
eoids, while the reduced clavicular arch has already become 
functionless or nearly so, and lies within and upon the anterior 
scapular expansion. The retention of the primitive condition in 
Leptocleidus (and probably in Brancasaurus, if, as seems likely, 
Weegner’s interpretation of its shoulder-girdle is erroneous) may be 
a consequence of the freshwater, probably fluviatile, habitat of this 
Plesiosaur, which resulted in its leading a life sheltered from the 
great competition that seems to have resulted in more rapid 
evolution among the marine Plesiosaurs. A somewhat similar case 
occurs among the Cetacea, of which the freshwater Platanistide 
retain numerous primitive characters derived from their Squalodont 
ancestors. This circumstance H. Winge! explains as the con- 
sequence of their comparatively sheltered habitat in rivers and 
estuaries, where they evaded crowding-out by the higher Cetaceans. 
It must be admitted, however, that the clavicular arch is not 
reduced in all cases in the marine forms: for instance, in Dolicho- 
rhynchops osborni Williston, from the Niobrara Chalk of Kansas, 
the clavicles and interclavicle are of relatively large size, and 
the general structure of the shoulder-girdle is not unlike that of 
Leptocleidus although the form of the interclavicle is different. 
Possibly this Cretaceous species may have originated from a fresh- 
water form, which had secondarily adopted a marine life at a 
comparatively late period. 

The specimens described in this paper, which is published by the 
permission of the Trustees of the British Museum, are all pre- 
served in the Geological Department of that institution. 


EXPLANATION OF PLATES XIV & XV. 
Leptocleidus swperstes, gen. et sp. nov. 
PuaTe XIV. 
[ All figures, except 5, are a third of the natural size. | 


. 1. Posterior portion of skull, lateral view. 
2. The same, palatal view. 
3. Centrum of an anterior cervical vertebra. 
4, Centrum of a middle cervical vertebra. 
5. Posterior cervical, thoracic, and anterior dorsal vertebre. A quarter 
of the natural size. 


[col.=columella ; d.=diapophyses ; 1.v.d.=intervertebral disc ; mz.=maxilla ; 
n.sp.—=neural spine; oc.c.=occipital. condyle; orb.=orbit; pal.=pala- 
tine; par.=parietal; pas.=parasphenoid; .f.=? prefrontal; pin.= 
pineal foramen; .1.v.=posterior interpterygoid vacuities ; pt.=ptery- 
goid ; q.=quadrate ; 7.=:cervical rib ; sq. =squamosal. | 


1 <Udsigt over Hvalernes indbyrdes Slegtskab’ Vidensk. Medd. Dansk 
Naturh. Foren. vol. lxx (1919) p. 104. 


298 A NEW PLESIOSAUR FROM THE WEALD CLAY.  [vol. lxxvill. 


PLATE XV. 
[ All figures are a quarter of the natural size. | 
Fig. 1. Shoulder-girdle, from above. 
2. The same, from the front. 
3. The same, from the side. 
4, Right humerus, from above. 
5 


. Two middle ventral ribs. ? 


(cl.=clavicle; cor.=coracoid ; for.-==opening bounded by scapula, coracoid, and 
clavicle; f.=notch or foramen in coracoid; gl.=glenoid cavity; h.= 
head of humerus; 7.cl.=interclavicle; sc.=scapula ; tw.—tuberosity of 
humerus. | 


DISCUSSION. 


Dr. A. SmirH Woopwarbp referred to the interest of the section 
at Berwick, which shows the top of the Weald Clay and the base of 
the Lower Greensand. The Plesiosaur was found by Mr. Tooth in 
a well-defined layer of septarian nodules in the Weald Clay. The 
specimen was valuable, as showing the true shape of the clavicles 
and scapule: they were isolated in the nodule; but, when they 
were extricated and placed in contact, their curved surfaces fitted 
exactly. The neck was evidently more flexible than in the earlier 
Plesiosaurs. He agreed with the Author that the new specimen 
might be regarded as representing an ancient type, which owed its 
survival to its retreat to life in a river or lake. 

Baron F. Nopcsa pointed out that in most groups of marine 
reptiles one can distinguish a long-necked and a short-necked 
series. Among the Lacertilia the short-necked forms are repre- 
sented by the Aigialosauria and their descendants the Mosasauria, 
the long-necked by the Dolichosauria; of the Ichthyosauria, 
which are short-necked, the long-necked relatives seem to be the 
Mesosauria. 

The PrestpEnt (Prof. A. C. Szwarp) referred to the close 
resemblance between the Wealden plants of Sussex and those found 
in the Uitenhage Series of the Cape Province, a resemblance that 
appears to be shown also by the reptilian remains from the two 
regions. 

The AvuTHoR expressed his thanks for the reception accorded 
to his paper. 


[September 23rd, 1922. ] 


QuarRT. JOURN. GEOL. Soc. VOL. LXXVIII,PL. XIV. 


‘AOU ds yo"Uds ‘SALSHadNS SNGIa1TOO1d]7 


par 


410 
‘wl up 


“140 “M'*W'D 


Quart. JOURN, GEOL. Soc. VoL. LXXVII,PL. XV. 


"AOU ‘ds 490° 


U85 “S31SYHY3dNS SNGIg1900i1d37 


“130 “"M'‘W'S 


» ae pil 
un ee 
hs | 


peel Pe beuMiNnsy 
Cieyr iy 


1 


JAMES SWIFT & SON L!: 


Makers of the Highest Grade 


MICROSCOPES AND ACCESSORIES 


Contractors to all Scientific Departments of H.M. Government. 


Ready end cf September 


NEW CATALOGUE 


OF 


PETROLOGICAL ACCESSORIES 


and Instruments for Mineralogy and Crystallography, 
including Refractometers, Goniometers, etc. 


Applications invited for copies to be sent post free on pubiication. 


University Optical Works 
81 TOTTENHAM COURT ROAD, LONDON, W.I. 


Publications to be had of the Geological Society, 
Burlington House. 


QUARTERLY JOURNAL (Vols. III to LXXVII, inclusive), Price to 
Fellows, £1 0s. 0d. each: £1 5s. Od. in cloth. [Postage 1s.] Separate parts (2,5, 
7, 9, 55, & 273 are out of print) 7s. 6d. each: price to Fellows 5s. [Postage 5d.| 


GENERAL INDEX TO THE FIRST FIFTY VOLUMES OF THE 
QUARTERLY JOURNAL (1845-1894). Part I (A-La). Part II (La-Z), 
Price 7s. 6d. each. To Fellows 5s. each. [ Postage 6d. each. } 


GEOLOGICAL LITERATURE added to the Geological Society’s Library 
during the years ended December 31st, 1894-1913 and 1920, 1921. Price 5s. each. 
To Fellows, 3s. 9d. each. [Postage 7d.] (Nos. 8 & 14 are out of print.) 


THE HISTORY OF THE GEOLOGICAL SOCIETY OF LONDON 
by H. B. Woopwarp, F.R.S. Price 3s. net. [Postage 1s. ] 


’ 


THE CENTENARY OF THE GEOLOGICAL SOCIETY OF 
LONDON, recorded by W. W. Warts, F.R.S. Price 2s. [Postage 5d.] 


HUTTON’S ‘THEORY OF THE EARTH,’ Vol. III, edited by Sir 
ARCHIBALD GEIKIE, D.C.L., F.R.S Price 3s, 6d. To Fellows, 2s. [Postage 9d.] 


THE GEOLOGY OF NEW ZEALAND. Translated by Dr. C. E 
FIscHER from the works of MM. HocustrerTeR & PRTERMANN. With an Atlas of 
Six Maps. Price 10s. net. [Postage 9d. ] 


CONTENTS, 


PAPERS READ. 


Page 

6. Dr. H. H. Thomas on Xenolithic Tertiary Minor Intrusions in Mull 
(Plates VI-X) 0.1... SS es eee 
7. Mr. W. G. Shannon on a Composite Sill at Newton Abbot (Plate XI) ...... 261 


8. Prof. A. C. Seward & Mr. R. E. Holttum on Jurassic Plants from Ceylon 


(Plate. XUV) <scc.0 3.5 od een ee ee 271 

9. Prof. A. C. Seward on Carboniferous Plants from Peru (Plate XU See 278 
10. Dr. C. W. Andrews on a New Plesiosaur from the Weald Clay of Berwick, 

Sussex (Plates XIV &XMe-2 2 eee its See 285 


(The Editor of the Quarterly Journal is directed to make it known to the Public 
that the Authors alone are responsible for the facts and opinions 
contained in their respective Papers. | 


*,.* The Council request that all communications intended for publication by the 
Society shall be clearly and legibly written on one side of the paper only, with 
proper references, and in all respects in fit condition for being 
at once placed in the Printer’s hands. Unless this is done, it will be in 
the discretion of the Officers to return the communication to the Author for revision. 


The Library at the Apartments of the Society is open every Week Day from Ten 
o’clock until Five, except on Saturdays (when the hours are from 10 to 1), and 
except during the fortnight commencing on the first Monday 
in September, when the Library is closed for the purpose of 
cleaning. 


Fellows are reminded that they are not entitled to receive the 
Quarterly Journal regularly, unless a subscription at the rate of 
10s. per volume is paid in advance Those Fellows who do not 
require the compiete series can obtain separate parts at the price 
of 3s, each, if applied for within one year of the date of issue. 


~ 


| 
ae 
( 
é 
( 
| 
( 
¢ 
¢ 
¢ 
¢ 
¢ 
( 
( 
¢ 
¢ 
¢ 
; 
\ 
¢ 
¢ 
¢ 


PARDRAADRAADARR AAI AAP AR ARSE AARAARARAAARABAARARANRARARARARRARARARRARRARRRARRAY 


Vol. LX XVIII. ra No. 312. 
Part 4. <. a Ae 
His 7 5 
aa 
THE Spe P aa 


QUARTERLY JOURNAT? 


OF THE 


GEOLOGICAL SOCIETY. 


BEN 


Os 
ay 
— § 1992 ss 


ig ZU LY 
- nef 
thy 1922. . 
“ Leia, vl use 


r 


{With Four Plates, illustrating Dr. F. Dixey’s Paper, and one 
Folding Table, illustrating Mr. S. 8. Buckman’s Paper.] 


LONDON : 
LONGMANS, GREEN, AND CO. 


PARIS :—CHARLES KLINCKSIECK, 11 RUE DE LILLE. 
SOLD ALSO AT THE APARTMENTS OF THE SOCIETY; 


Price Seven Spaldings and Sixpence, 


| 
— 


FI IOI aes? 


6a 


LIST OF THE OFFICERS AND COUNCIL OF THE 


GEOLOGICAL ee ee LONDON. 


NAR RAAN 


ae 


on™ 


Vy 


Prestdent. 
Prof. Albert Charles Seward, Sc.D., F.R.S., F.L.S. 


Gice-Presivents. 


Prof. Edmund Johnston Garwood, M.A.,,) George Thurland Prior, 


Se.D., F.R.S. 
Richard Dixon Oldham, F.R.S. 


M.A., D.Se.; 
F.R.S. 


/ Herbert Henry Thomas, M.A., Se.D. 


Secretaries. 


Walter Campbell Smith, M.C., M.A. 


Foreign Secretary. 
Sir Archibald Geikie, O.M., K.C.B., D.C.L., 
LL.D., Se.D., F.R.S. 


| James Archibald Douglas, M.A., B.Se. 


Creasurer. 
Robert Stansfield Herries, M.A. 


UNICIL, 


Frederick Noel Ashcroft, M.A., F.C.S. 

Francis Arthur Bather, M.A. ans, Se., F.R. S.| 

Prof. Percy George Hamnall Boswell, 
O.B.E., D.Se. 

Prof. William S. Boulton, D.Sc., Assoc.| 
Roy. Coll. Sci. 

Thomas Crosbee Cantrill, B.Sc. 

James Archibald Douglas, M.A., B.Sc. 

John Smith Flett, O.B.E., 
D.Se., M.B., F.R.S. 


Sce.D., F.R.S. 


Prof. Owen oie: Jones, M.A., D.Se. 

William Bernard Robinson King O.B.E., 
M.A. 

| Richard Dixon Oldham, F.R.S. 

George Thurland Prior, M.A., DBe; 
F.RS. 


| Prof. Sidney Hugh Reynolds, M.A., Se.D. 
| Prof. Albert Charles 
M.A., LL.D.,! 

_ Walter Campbell Smith, M.C., M.A. 
Prof. Edmund Johnston Garwood, M.A..,, 


Seward, Sc.D., 
F.R.S., F.L.S. 


Sir Aubrey Strahan, K.B.E., Se.D., LL.D., 
F.R.S. 


Sir Archibald Geikie, O.M., K.C.B.,D.C.L.,. Herbert Henry Thomas, M.A., Se.D. 


LL.D., Se.D., F.R.S. 
John Frederick Norman Green, B.A. 
Frederick Henry Hatch, O.B.E., Ph.D. 
Robert Stansfield Herries, M.A. 


'| Prof. William Whitehead Watts, M.A., 


Se.D., LL.D., F.R.S. 
Henry Woods, M.A., F.R.S. 


Permanent Secretary. 
L. L. Belinfante, M.Sc. 


Librarian. 
Arthur Greig. 


Clerk. 
M. St. John Hope. 


STANDING PUBLICATION COMMITTEE. 
Prof. A. C. Seward, President. 


Mr. W. Campbell Smith 
Mr. J. A. Douglas. 
Prof. P. G. H. Boswell. 
Prof. W. S. Boulton. 
Dr. J. S. Flett. 
Prof. E. J. Garwood. 
Mr. J. F. N. Green. 


5 } Secretaries. 


Dr. F. H. Hatch. 
Prof. O. T. Jones. 
Mr. R. D. Oldham. 
Dr. G. T. Prior. 
Prof. S. H. Reynolds. 


Mr. R. S. Herries, Treasurer. 


Sir Aubrey Strahan. 
Dr. H. H. Thomas. 
Prof. W. W. Watts. 
Mr. H. Woods. 


ORDINARY MEETINGS OF THE GEOLOGICAL SOCIETY 
TO BE HELD AT BURLINGTON HOUSE. 
Session 1922-1923. 


1923. 
Wednesday, VANULEY Eo. co ewe toe cases 10*—24* 
> (Anniversary, Friday, 

Feb. 16th) February...... 7*—28* 
- RPO S coon oat «loetees eae ved 14 —28* 

Hs AQT Ste ss ands meee 18* 
- Misty Siete ian ee eevee oom 2 —16* 
TE Re SnD eg 8, ee oe 6 =20* 


part 4] THE NORITE OF SIERRA LEONE. 299 


1l. The Norte of StERRA Leone. By Frank DIxeEy, 
D.Sec., F.G.S. (Read June 22nd, 1921.) 


[Puatres XVI-XIX. ] 
CONTENTS. 


aD raO CUG ELOIN tras cides aR ete ee Pee cay Ae he 299 
(1) General Remarks. 
(2) Physiography of the Area. 
(3) Size and Form of the Occurrence. 
(4) The Intrusions. 
(5) Age of the Rocks. 
(6) Relation to other West African Basic Rocks, and also 
to a West African Magnesian Province. 


Mie Biel dl Opsemmar tO Nadas ia tres nau desde esse Suc sek oe aupaoe ate 307 
(1) The Older or Normal Norite. 
(2a) The Younger Norites and Norite-Pegmatite. 
(2 b) Field Relations of the Older and Younger Norites. 
(3) The Beerbachite. 
(4) The Norite-Aplite. 
(5) The Dolerites. 
(6) Differentiation and Assimilation. 
(7) Weathering and Lateritization of the Norite. 
(8a) Iron-Ores in the Norite. 
(86) Other Economic Minerals. 
iiebetrosrapnical INOGES ...o.i02.seliccesepoes ces cetaed ess tecusutveness 322 
(1) The Norite-Aplte. 
(2) The Dolerites. 
(3) The Minerals of the Complex. 
(4) Intergrowths of the Common Minerals, and Crystalliza- 
tion of the Norite-Magma. 
(5) The Order of Crystallization. 
(6) Contact-Metamorphism within the Complex. 


TV. Summary and Conclusions 


I. Inrropvucrion. 
(1) General Remarks. 


THe norite of Freetown (Sierra Leone) was briefly described by: 
G. Girich! in 1887, and in 1918 Prof. S. J. Shand? translated 
Giirich’s work, adding a few notes. I had the opportunity of 
making a detailed examination of this norite, as part of an investi- 
gation into the mineral resources of Sierra Leone ; and in the course 
of this work it was found that the norite exposed around Freetown 
was only one end of a huge noritic complex which made up the whole 


1 ¢QOlivingabbro von Freetown (Sierra Leone)’ Zeitschr. Deutsch. Geol. 
Gesellsch. vol. xxxix (1887) p. 108; further reference to this paper will be 
made below, p. 305. 

2 «The Norite of the Sierra Leone’ Geol. Mag. 1918, p. 21. 


@ JG Ss. No. 312. Y 


EI Wins] 
ORCL?” earn 
ACs { Teste Sse i tea 
ZEEE S \Havelock Plat 

s Leicester_ 
Peak IN 


Chari oile 


ital 


ls 
— 


‘SUSSEX 


a ; wale a SENS 
Nag eee) 


(ae CRS: 
“~MACD 


-- Of. 
o 


i eta 
ATLANTIC Egg Ht 


OCEAN ae 


JOHN OBEY. 


© C. Shilling 


ae Geological Map ue 
of the 
SIERRA LEONE PENINSULA,. 
Banana By ih. Dicey: 
Is. [Based on W.D. Topographical Map No, G.S., G.S.1764.] 


Scale of Miles 
° x 3 


Pleistocene & Norite 
Recent Beds eee Complex [eal 


13°|10’W 


ea: 


part 4] THE NORITE OF SIERRA LEONE. 301 


mountain-mass of the Colony.! Moreover, during the examination 
several features of considerable interest were observed, including 
the assimilation of one phase of the norite by another, the well- 
developed banding in the norite, the segregation of the iron-ores, 
and the sequence of the minor intrusions. These features, together 
with an important series of intergrowths of the common minerals, 
will be described in the course of this paper. ‘The investigation 
into the mineral resources of the Colony, with especial reference to 
the iron-ores, formed part of the work of the Geological Survey 
of Sierra Leone during the latter part of 1918 and during 1919. 


(2) Physiography of the Area.? 


The Colony is a mountainous peninsula adjacent to the undu- 
lating low ground which forms the southern part of the Protec- 
torate. Its mountains rise steeply from a narrow coastal plain 
to a height of nearly 38000 feet; this plain is continuous with that 
of the mainland, and is due to a Pleistocene uplift of a little 
over 300 feet. It is made up of marine deposits derived largely 
from the denudation of the mountains. (See Pl. XVI, fig. 1.) 

The mass of the Colony has been carved by erosion from an 
elongated stock of norite. As now exposed, the stock is 24 miles 
long and 8 miles wide, but it was doubtless much greater originally. 
From the southern end of the mass, the Banana Islands (forming 
a part of the same intrusion) extend away to the south-south-west. 
In the course of erosion a number of platforms have been carved 
into the mountain-mass. The more important of these platforms 
attain maximum altitudes of about 2400, 1800, and 1300 feet 
respectively ; they can all be distinguished without difficulty from 
the summit of one of the higher peaks of the northern end of the 
Colony, such as Leicester Peak, and also from the sea a few miles 
west of Freetown Harbour. The highest platform can be seen 
at a level shghtly below the summit of Sugarloaf Mountain (see 
Pl. XVL, fig. 1), the second shghtly below that of Leicester Peak, 
while the third forms the high ground that encloses Freetown 
on the south-east, south, and south-west. The oldest platform 
forms the upper limit of the broad middle ridge that traverses the 
‘Colony from end to end, whereas the second platform is recognized 
principally in the even tops of the two lateral ridges that run 
parallel with the main ridge; the third platform can be traced at 
intervals around the flanks of the higher platforms. Several 


' The term Colony refers to that part of Sierra Leone which lies along 
the coast ; it forms principally a peninsula about 25 miles long and 9 miles 
wide. At the northern end of this peninsula is situated the port of Freetown. 
The remaining part of the country is known as the Protectorate; it 
extends inland for a considerable distance, and covers an area almost as 
large as Ireland. 

2 See also F. Dixey, ‘The Physiography of Sierra Leone’ Geogr. Journ. 
‘vol. lx (1922) p. 41. 

3 See F. Dixey, ‘ Pleistocene Movements in Sierra Leone’ Trans. Geol. Soe. 
S. Africa, vol. xxii (1920) p. 112. 


y 2 


302 DR. F. DIXEY ON THE (vol. lxxviil, 


smaller platforms can be recognized at intermediate and at 
lower levels; those at the lower levels correspond in height to. 
low flat-topped hills and ridges that occur in neighbouring parts 
ot the Protectorate. 

Erosion of another type has given the stock a deeply-incised 
drainage-system which is remarkable for its regularity and sim- 
plicity (see map, fig. 1, p. 300). The principal watersheds are the 
three parallel ridges which traverse the Colony from end to end, 
and two groups of spurs which spread fan-wise from each end of 
the mountain-mass; the ridges throw off at right angles a 
number of short spurs which, on the northern side at least, are of 
very regular form. This arrangement is probably due to the fact 
that the master-joints of the stock are parallel and transverse to. 
its length in the middle portion, but radial at the ends; and, as the 
original covering was denuded, the young streams discovered these 
lines of weakness and developed along “them. Many bold cliffs 
and joint-faces rising out of the dense bush extend along the sides 
of the main and the ‘tributary valleys 


(3) Size and Form of the Occurrence. 


The Sierra-Leone norite is remarkable in being the largest 
intrusion of norite of which I have been able to trace any record. 
Elsewhere, however, norite is well known to occur in intimate 
association with other rocks which form intrusions of enormous. 
size. The norite of Sierra Leone is further remarkable in that it 
occurs as a stock which, regarded as a basic stock, is of very con- 
siderable size. Most of the known greater basic and ultrabasice 
masses, with the exception of certain anorthosites, occur not as 
stocks, but rather in the form of enormous laccolites and sills = 
as, for example, the Bushveldt,! Insizwa,” and Sudbury? intrusions. 
Both as a basic mass and as a norite- -mass, the Sierra- Leone norite 
is far more considerable than any of the British oceurrences.* 

It is worthy of notice that, so far as can be observed, the Sierra- 
Leone norite as a whole shows neither a marginal nor a stratiform 
structure; and further, that it is isolated from all other igneous. 
rocks, apart from the dolerite-dykes and the small veins of norite- 
aplite which traverse it. In these respects it differs considerably 
from the important basic masses already mentioned: the Bushveldt, 
Insizwa, Sudbury, and many other great laccolites and sills ecules 

a stratiform arrangement ascribed to the control of differentiation 


1 G. A. F. Molengraaff, ‘ Géologie de la République Sud-Africaine du Trans- 
yaal’ Bull. Soe. Géol. France, ser. 4, vol. i (1901) p. 62. 

2 A. L. Du Toit, ‘Report on the Copper-Nickel Deposits of the Insizwa, &c.” 
15th Ann. Rep. Geol. Comm. Cape of Good Hope (1910) p. 111. 

3 A. E. Barlow, ‘ Nickel & Copper Deposits of the Sudbury Mining District’ 
Ann. Rep. Geol. Surv. Canada, vol. xiv, pt. H (1904). 

* Note, however, that certain norites recently described from the neigh-. 
bourhood of Huntly in Aberdeenshire attain a considerable size—teste H. H. 
Read, in‘ The Contaminated Magmas of Aberdeenshire’ (paper read to Sect. C. 
Brit. Assoc., Edinburgh, 1921) 


part 4] NORITE OF SIERRA LEONE. 303 


by gravity, while others show a similar arrangement due to succes- 
Sive intrusions, as exemplified by the basic and ultrabasic Tertiary 
igneous rocks of Rum.1 

Other norites, such as those associated with the Galloway masses,” 
occur as basic phases of granitic intrusions; while yet others, 
such as those of St. David’s Head,? form one member of a series 
that comprises a wide range of rock-types. The norite which we 
are now considering possibly possessed a marginal phase, long since 
swept away; but it never had any stratiform arrangement, unless 
it was originally very extensive as compared with its present 
size. The following observations on the specific gravity of the 
norite, ranging from sea-level to the summit of the highest 
peak (an altitude of nearly 3000 feet), indicate great irregularity 
of structure. 

Approximate Gingehize. 


Locality. elevation, gah: 
in feet. TANNA 

1. Landing-stage, Murray Town ......... 0 3°06 
Pe Amana CUS ATIOS soc tiye), sch eu eene seek ais ) aL 
SP eBOOt Ol: Charlothen ball Ss ...2 .mersecenu seins 4.00 3°23 
ZNSE AV Wl OH (e172) 1 0) en ee A rs a SO 2°99 
‘Sie, TRMATUN GER ETCOR CIS Re aa eS on Bo 700 2°95 
GamVWVaberloo= VOrk TOA” ~.:foesc8 sec celv seen 1000 P e726 
Wa lieveesterdeealc’ \ secs. A dis oh omaboe eee 1400 2°87 
Beelneieester. Peake. qin Sel yet eaten te 1950 2°98 
SPEIER MEE sy iss ahaa ede cine, ck Nomen eee 2900 2°80 

Average of twelve specimens ............ 3°01 


Numerous observations on mineralogical composition yield a 
similar result. Nevertheless, in the variety and sometimes the 
succession of its associated minor intrusions, the norite bears a 
strong resemblance to certain other basic masses, such as the 
norites of St. David’s Head, the Duluth laccolite,* and the gabbro 
of the Cuillin Hills.® 


(4) The Intrusions. 


The stock is thus made up essentially of a noritic intrusion of 
fairly uniform character. But the original mass was invaded 
in quick succession by several cognate sub-magmas, all of which 
were, however, relatively of very small volume; ata later date the 
complex was cut by numerous dykes of enstatite-dolerite. Most 
of these rocks are well exposed along the south-western foreshore 


1 «Geology of the Small Isles of Inverness’ Mem. Geol. Surv. Scot. 1908, 
p. 68. 

2 «The Silurian Rocks of Scotland’ Mem. Geol. Surv. 1899, p. 618. 

3 J. V. Elsden, ‘The St. David’s Head Rock Series’ Q.J.G.S. vol. lxiv 
(1908) p. 278. 

4M. L. Nebel, ‘The Basal Phases of the Duluth Gabbro near Gabamichi- 
gami Lake (Minnesota), & its Contact-Effects’ Econ. Geol. vol. xiv (1919) 
p. 367. See also N. H. Winchell on this intrusion in Final Rep. Geol. Surv. 
Minnesota, vols. iv & v (1899-1900). 

° ‘The Tertiary Igneous Rocks of Skye’ Mem. Geol. Surv. 1904, p. 82. 


304 DR. F. DIXEY ON THE (vol. Ixxviu, 


of the Colony ; inland, where the bush is very dense, they rarely 
crop out elsewhere than in stream-beds and as joint-faces along the 
sides of valleys. ‘The various rock-types will be described below 
in their order of intrusion, namely :— 

(i) Older or normal norite. 

(ii) Younger norites and norite-pegmatite. 

(iii) Beerbachite. 

(iv) Norite-aplite. 

(v) Dolerite. 


(5) Age of the Rocks. 


Although no field evidence has been found to throw any certain 
light upon the age of the complex, there are many considera- 
tions of a general character which indicate that, while probably 
later than pre-Cambrian, it is nevertheless of very great age- 
The only rocks seen in contact with the complex are the 
Pleistocene sediments which rest upon it on all sides, and the 
nearest rocks of an earlier age than these are representatives of am 
ancient series of crystalline schists and gneisses which are exposed 
in the Port Lokko Creek, 11 miles east-north-east of the north- 
western end of the norite-complex. Since the complex is elongated 
in a direction parallel with the general trend of the coast of this 
part of West Africa, it is obvious that few, if any, rocks will be 
exposed on the strike, or on the seaward side, of the complex. 
The only crystalline rock known to occur actually on the strike of 
the complex is the nepheline-syenite of the Los Islands, situated 
off the coast of French Guinea at a distance of 72 miles from 
Freetown; it is important to notice, however, that the mainland 
opposite the Los Islands consists of a series of diabases, st, 
and peridotites intruded into ancient crystalline schists.!_ In the 
Protectorate of Sierra Leone, crystalline rocks approach the strike 
of the complex in the locality already mentioned. and also in the 
mainland off the north-eastern corner of Sherbro Island. Thus it 
is seen that all the old rocks exposed on the strike, and on the 
landward side, of the norite, over a large area extending from 
Liberia to the Los Islands, belong to an ancient series of 
crystalline schists and gneisses generally considered to be of 
pre-Cambrian age.* The following considerations show that the 


1 A. Lacroix, ‘Les Syénites Néphéliniques de Archipel de Los’ Nonuy. 
Arch. Mus. Hist. Nat. Paris, ser. 5, vol. ii (1911) p. 108. Note, moreover, 
that along the coast of Liberia there are at least four masses of gabbro and 
associated basic rocks which are largely similar in occurrence to the norite of 
Sierra Leone, but of smaller size. See also H. Hubert, ‘Carte Géologique 
de 1 Afrique Occidentale’ 1920, and Delafosse, ‘Le Libéria en 1907’ pt. ix: 
Géologie, Bull. Com. Afrique Frane. 1907, No. 12. 

2 See A. E. Kitson, ‘The Gold Coast: some Considerations of its Structure, 
People, & Natural History’ Geogr. Journ. vol. xlviii (1916) p. 377; id. ‘ Out- 
lines of the Geology of Southern Nigeria (British West Africa), with especial 
reference to the Tertiary Deposits’ Abs. Proc. Geol. Soc. 1918-19, pp. 100— 
105. Also J. D. Falconer, ‘The Geology & Geography of Northern Nigeria’ 
1911, and ‘The Geology of the Plateau Tin-Fields’ Bull. No. 1, Geol. Surv. 
Nigeria (1921) p. 33. 


part 4] NORITE OF SIERRA LEONE. 305 


norite-complex itself, while probably later than pre-Cambrian, 
is nevertheless of very great age !:—— 


(1) The complex as a whole is unfoliated, although half-enclosed by highly- 
foliated crystalline rocks. 
(2) The complex is elongated parallel to 


(a) The foliation of the crystalline rocks ? ; 

(6) to the general trend of the coastline ; since, however, there is 
good reason to suppose that the present coastline was deter- 
mined by a series of important more or less parallel faults, it 
is probable that the complex originated as an intrusion along 
a considerable fault running parallel with the foliation of the 
erystalline rocks. 


(3) In its great size, its highly-magnesian character (resulting in the 
frequent development of rhombic pyroxenes), and the extraordinary freshness 
of its constituent minerals, it presents a close analogy with the Charnockite 
Series of India,’ certain phases of the anorthosites of Canada,’ the Scandi- 
navian norites, and other well-known intrusives all considered to be of pre- 
Cambrian age. 

(4) The enormous amount of erosion undergone by the norite and the rocks 
into which it intruded must have required a very long period of geological 
‘time in which to take place. For instance, the complex, which despite 
erosion is even now exposed to a depth of 3000 feet, must have been covered 
at the time of its intrusion by a thickness of many thousands of feet of 
sediments or other rocks, all of which have now completely disappeared. 
Also, within a radius of 70 miles on the landward side of the complex the only 
rocks so far observed to rise above the low-lying Pleistocene sediments are 
the crystalline schists. These schists, which rarely attain a height of more 
than 700 feet above sea-level, were invaded by gneissose granites such as now, 
on account of their superior resisting powers, make up the high land forming 
the north-eastern part of the Protectorate. But even these hard granites, 
which have been carved into a series of plateaux one above the other, do not 
in general attain a height of more than 2000 feet above sea-level anywhere 
within 130 miles of the isolated complex. Finally, certain of the extensive 
platforms carved into the ancient crystalline masses have their counterparts 
on the norite-complex (see above, p. 301); one of these platforms of the 
Protectorate, standing at a height of about 1200 feet, supports a thickness of 
nearly 700 feet of (?) early Palzeozoic sediments, which even now are prac- 
tically horizontal.’ Thus it is seen that the erosion of the complex and the 
surrounding rocks must have been very great indeed. 


It may be noticed at this point that Girich, in the paper cited above (p. 299), 
concluded that the norite was considerably younger than the pre-Cambrian. 
This conclusion, however. was based on several errors of observation: for 
instance, he failed to distinguish the laterite formed by alteration of the 
Pleistocene sediments from that derived from the norite by lateritization in 
place (see below, p. 321), and, further, he regarded both of them as volcanic 
rocks or tuffs. He was, nevertheless, fairly well acquainted with laterite, as 
we know it, from many parts of the West Coast. In consequence of this 
confusion he considered the norite to be a laccolitic intrusion into the ‘ tuffs,’ 


1 It is proposed to give additional evidence as to the great age of the 
complex, based upon certain important relations existing between the norite 
and the large basic intrusions of French Guinea (referred to below) in a 
subsequent paper on the geology of Sierra Leone. 

2 F. Dixey, Rep. Geol. Surv. Sierra Leone, 1919, p. 8; also ibid. 1920. 

3 T. H. Holland, Mem. Geol. Surv. India, vol. xxviii (1900) p. 119. 

4 A. Lawson, Amer. Geologist, vol. vii (1891) p. 153. 

5 F, Dixey, Rep. Geol. Surv. Sierra Leone, 1920. 


305 DR. F. DIXEY ON THE (vol. xxvii, 


and, where certain bands of the norite were deeply lateritized at their outcrop 
while neighbouring bands were practically unaltered, he concluded that the 
norite had locally forced its way along the bedding of the ‘ tuffs’ in the form 
of small sills. Finally, since the crystalline schists of pre-Cambrian age 
would probably be steeply inclined if at all present in the country, whereas 
the ‘ tuffs’ were horizontally bedded and were presumably ‘younger’ than 
the schists, therefore the norite itself, ‘ younger’ than the ‘ tuffs,’ must neces- 
sarily be much later than pre-Cambrian in age. Giurich considered these 
conclusions to be supported by analogy with the conditions prevailing in 
the Los Islands. 


(6) Relation to other West African Basic Rocks, and 
also to a West Africau Magnesian Province. 


Two great petrographic groups, both made up of rocks rich in 
hypersthene, are known? to exist in West Africa; they consist 
respectively of a series of peridotites, gabbros, and diabases 
occurring principally in French Guinea, and of a series ranging 
from norites to hypersthene-granites, which has been compared 
by Prof. Lacroix with the Charnockite Series of India. The 
peridotites of the first series approach wehrlite in composition, 
and consist mainly of olivine with a little diallage. The diabases 
include types rich in olivine, hypersthene, and a monoclinic 
magnesian pyroxene respectively. The gabbros form the most 
important group, and they occur in large masses: they consist of 
felspars ranging from labradorite to bytownite, diallage, and small 
quantities of a magnesian pyroxene (pigeonite); hypersthene and 
olivine occur in variable amounts. The felspars are free from 
zoning, and show a tendency to parallelism; the structure 
approaches an ophitic character. These gabbros show varieties 
approaching towards norite and troctolite respectively ; but, so far, 
actual examples of these types are not known in French Guinea. 
From the limited information available at the time when he wrote, 
Prof. Lacroix considered that the rocks around Freetown were 
sunilar to these gabbros; and it is interesting to note that the 
present investigation not only confirms this similarity, but goes to 
show that the Sierra-Leone norite is probably part of the same 
great basic series. 

The other great series, analogous to the Charnockite Series, was 
considered by Lacroix to occur almost exclusively on the Ivory 
Coast; but from recent Survey work in the Sierra-Leone Protec- 
torate it appears that rocks similar to these are well developed 
among the old crystalline schists in this area also.” The Sierra- 
Leone norite, however, differs considerably from this series, notably 
in the absence of granular structure, in the absence of quartz, and 
in the abundance of olivine and augite. These mineralogical differ- 
enees are expressed chemically to some extent in that the Sierra- 


1 A. Lacroix, ‘Les Syénites Néphéliniques de lArchipel de Los’ Nouy, 
Arch. Mus. Hist. Nat. Paris, ser. 5, vol. iii (1911) p. 108; id. ‘Sur Existence 
a la Céte d'Ivoire d’une Série Pétrographique comparable 4 celle de la 
Charnockite ’ C. R. Acad. Sci. Paris, vol. ce] (1910) p. 18. 

2 F. Dixey, Reps. Geol. Surv. Sierra Leone, 1920-21. 


part 4] NORITE OF SIERRA LEONE. 307 


Leone norite is poorer in alkalies, generally in silica, and richer in 
magnesia and lime. 

Not the least interesting feature concerning the study of the 
Sierra-Leone rocks of both the Colony and the Protectorate is that 
they tend to confirm Lacroix’s statement as to the widespread 
occurrence in West Africa of rocks rich in magnesium. ‘This 
richness in magnesium, which generally gives rise to more or less 
abundant hypersthene,! leads to the conception of a West 
African Magnesian Province.? 


Il. Freitp OBSERVATIONS. 


(1) The Older or Normal Norite. 


The oldest and most important member of the series is a norite 
of medium texture, which in succeeding paragraphs will be defined 
as the normal norite. It varies but little in composition, 
texture, and macroscopic characters from one end of the Colony 
to the other. It consists essentially of a plagioclase (labradorite), 
augite, and a rhombic pyroxene, with varying proportions of 
olivine; iron-ore and apatite occur in small amount, and are 
unevenly distributed. An analysis of a single specimen of this 
rock has given the following result :— 


Per cent. 
SSIET fs ease fark Sato iraud Ainsttet ape eee oe 48°25 
PNM PISO UOTE hi eat MN el el heh Mae acca a 21°54 
erro Oxides * meee eerie takes Sage) 
(WET O WSs O RIG ON ia: hs: LeeAnn aa 3°60 
ManoamousiOxide@s (a. sc .ciseteonsnben <8 0°45 
1 DITTO RR lp, ee eran aa SY ge en 9°73 
UP OME STON nackte Mince 1 ness Aas aa eae 6:30 
OLAS IER ere Fe note es Mee aes 1°60 
SOG CR A MOR SRE Ara Iie va Li at ek ee 1:76 

Phosphoric aeids 02.532. 06. Bieee: 0:015 
MGA SACI ONS sic AS, eects trp eee cet 0:26 
Water above 105° C. 2°90 
URoballeercrscs.s cae 99:99 


Prof. 8. J. Shand has classified the rock as a leucocratic olivine- 
norite (/,—to /;—subnorite).? The norite is tough and commonly 
grey in colour; with increasing iron content it becomes denser, 
harder, and splintery, gives a strong metallic ring when struck 
with the hammer, and weathers with a black lustrous surface. 


1 The formation of hypersthene will depend chiefly on the relative amounts 
in the magma of both alumina and lime; hypersthene would be unlikely to 
form until these oxides had been taken up by felspars and augite. A mono- 
clinic magnesia-pyroxene may accompany and even replace hypersthene in 
the highly magnesian rocks: see A. Lacroix, Nouv. Arch. Mus. Hist. Nat. 
Paris, ser. 5, vol. iii (1911) p. 117, and also J, V. Elsden, ‘The St. David’s 
Head Rock Series’ Q. J. G.S. vol. lxiv (1908) p. 288. 

2 See F. Dixey, ‘The Magnesian Group of Igneous Rocks’ Geol. Mag. 
1921, p. 485. 

3 «The Norite of the Sierra Leone’ Geol. Mag. 1918, p. 23. 


308 DR. F. DIXEY ON THE [vol. ixxviii,. 


Sometimes it becomes granular, and its texture then approaches 
that of a coarse dolerite. The rock is characteristically banded,. 
but to a variable degree, and it usually exhibits a parallelism of 
the felspars which is often obvious even when the banding is 
obscure. Finally, all minerals are beautifully preserved, in 
common with those of succeeding intrusions. 

The banding of the norite occurs to a greater or less degree: 
throughout the Peninsula, and is essentially a flow-banding pro- 
duced during intrusion. Individual bands are generally constant 
in thickness, and their strike remains uniform over distances of 
several hundred yards; any contortion of the banding is quite 
exceptional, but it has been observed in one case around the margin. 
of a pegmatite intrusion. The bands become more obvious with. 
weathering, on account of varying resistance or colour: the colours. 
of weathered bands range through dark-grey, brown, red-brown 
to almost black; whereas the slight differences in composition. 
between one band and the next are but faintly indicated on a fresh 
surface. 

Locally the fluxion-structure is more pronounced; individual 
crystals within the bands are broken down and rolled out, and the 
bands themselves, while keeping a more or less uniform strike, 
tend to form lenticles which sometimes thin out completely in 
about half a dozen yards. Some of the bands contain streaks and 
seams of magnetite, or of magnetite and felspar, ranging up to- 
3 inches in thickness. Where a marked difference in composition 
of neighbouring bands occurs, it is due to varying proportions of 
the felspathic and ferromagnesian constituents, leading in a few 
extreme instances to bands of coarse granular olivine. The bands. 
vary in texture as well as in composition; the coarser of them are 
generally dark, and they thus bear a close resemblance to the coarse: 
norite which was intruded at a slightly later date (see below, 
p- 810). The darker tinge of the coarse bands is seen in thin 
section to be due to the abundance of minute opaque inclusions in 
the felspars, as well as to the greater proportion of ferromagnesian: 
constituents. 

The relation between the banding and the separation of the 
constituents of the magma is clearly displayed at a point on the: 
foreshore near No. 2 Town. The normal norite assumes a patchy 
character, due to the separation of the ferromagnesian constituents 
(chiefly pyroxenes); these constituents segregate into numerous. 
dark clots, up to 2 inches in diameter, which remain enclosed in a 
pale felspathic matrix. Where movement occurred at a late stage, 
the clots are elongated or drawn out into more or less parallel 
streaks, and in some cases the structure can be seen to pass into: 
regular banding of the characteristic type. 

The banding of the Sierra-Leone norite is similar to, but 
better developed than, that observed in the St. David’s-Head 
norite by Dr. J. V. Elsden.! A banding with many parallel 


1 <The St. David’s Head Rock Series (Pembrokeshire) ’ Q.J.G.5S. vol. lxiv 
(1908) p. 277. 


part 4] NORITE OF SIERRA LEONE. 309° 


features has been noted also in the gabbro of the Cuillin Hills by 
Sir Archibald Geikie and Dr. A. Harker,! and in the Carrock-Fell 
gabbro by the latter.” 

In many localities the grey surface of the norite shows numerous 
ill-defined brown and black spots: the former being composed of 
pyroxene and felspar in ophitic relation, and the latter of spongy 
growths of magnetite. 

The principal intrusions into the normal rock are:—(1) the 
younger norites; (2) norite-pegmatite ; (3) beerbachite ; (4:) norite- 
aplite; and iG dolerite; but, in addition to these, there are two 
series of veins and a felspar- olivine rock, all, however, of rela- 
tively little importance. I may now briefly describe these smaller 
intrusions :— 


The veins may be divided into two series, according to their 
age and occurrence. 

(a) The veins of the first series occur in the older norite; they 
have ill-defined margins and a complex branching and tapering 
habit which causes them to weather into a rough network on an 
exposed surface. An example of this may be seen on the foreshore 
at Kent opposite the Court House. The veins rarely exceed 
2 inches in width; they are generally less basic than the enclosing 
rock, and are of rare occurrence as compared with the veins of the 
later series. They closely resemble the segregation-veins figured 
and described in the Skye Memoir (p. 78). 

(b) The veins of the second series are related in occurrence and 
in composition to the younger norites; they are consequently 
younger than those of the first series, and may sometimes be seen 
to cut them. They are distinguished from the segregation-veins 
by their composition, by the inclusion of ragged black bronzites 
measuring up to an inch in diameter, by their sharper margins, 
more regular form, and greater width. 


The felspar-olivine intrusions consist of a tough pale-green 
rock of medium texture, rich in olivine and containing numerous 
small black grains of iron-ore. One of its best exposures is near 
the foot of Charlotte Falls, where it forms a small lenticle in the 
normal norite about 4 feet thick. 


Exposures. 


(i) Murray Town, foreshore.—Normal norite banded and spotted, 
and traversed by black pyroxenic veins. 


(ii) Aberdeen Creek, foreshore. — Southern point: the norite is 
banded and highly ferruginous ; it contains numerous seams rich 
in magnetite, and the laterite resting upon it is a good hematite-ore. 

(ii) Sussex, foreshore.—Flow-banding well developed; some bands 
show crushed felspars, others consist largely of coarse granular 
olivine. 


eee Tertiary Igneous Rocks of Skye’ Mem. Geol. Surv. 1904, p. 91. 
2 *Carrock Fell; a Study in the Variation of Igneous Rock-Masses: Part I 
—The Gabbro’ Q. J.G.S. vol. 1 (1894) p. 319. 


310 DR. F. DIXEY ON THE [ vol. Ixxvin, 


(iv) Toke.—Both normal and coarse norite cut by pegmatite-veins up to 
18 inches in width ; normal norite pierced by fine siliceous veins. 


(v) Kent, Cape Shilling, and Banana Islands.—Normal norite 
variably ferruginous ; banding beautifully developed on the foreshore 
near the church at Kent. 


(2a) The Younger Norites and Norite-Pegmatite. 


The younger norites form two or more series of intrusions 
cutting the older or normal norite; but they are collectively of 
small bulk, as compared with the original intrusion. In compo- 
sition they differ little from the normal norite, and are distinguished 
principally by their texture, which is generally much coarser than 
that of the older rocks. There are, however, several other 
distinguishing features of great use in the field: when weathered, 
the younger norites are dark grey in colour, and possess a curious 
scaly surface. Moreover, since jointing is but slightly developed 
in them, their outcrops form large hummocks or well-rounded 
residual boulders. The older norite, on the other hand, becomes 
dull-red on exposure and, being well jointed, weathers into 
rectangular blocks with sharp edges and clean-cut surfaces. One 
other important point of difference remains: the normal norite is 
generally well-banded, whereas the younger norites are free from 
banding, although the felspars in them are often more or less 
parallel. (See Pl. XVI, fig. 2.) 

When thin sections of the two rocks are compared, however, 
certain other differences are apparent: the coarser or younger 
norite is seen in most cases to be richer in hypersthene, in iron- 
ores, and in micrographic intergrowths of felspar and augite ; 
whereas it is generally poorer in felspar and also in apatite. 
Moreover, the felspars of the coarser rock are slightly more basic, 
and the darker colour which they give to the hand-specimens is 
seen to be due to innumerable opaque inclusions of very small 
size. 

A typical specimen of the younger norite consists essentially of 
grey sub-parallel felspars, dark-brown to black hypersthene, and a 
little granular augite, commonly diallage. The felspars are 
tabular in habit, and generally about 1 inch long. Olivine and 
magnetite are usually common : the former as rounded grains and 
sometimes as plates enclosing felspar-laths, and the latter both as 
grains and as interstitial growths. . Locally, the magnetite forms 
lenticular streaks and abundant small masses ranging up to 2 inches 
in length. A partial analysis of a single specimen of this rock 
shows :—Silica 41°16 per cent., alumina 11:13, iron oxides 22°30, 
and magnesia 17°68. ‘This norite also contains small brown and 
black patches, similar in some respects to those in the older rock, 
but more numerous. The black patches are pyroxenes in some 
cases, and spongy growths of magnetite in others. ‘The brown 
patches are of two types which differ considerably in origin. 
Those of the first type are small and fairly evenly distributed, and 
they each consist of a rounded augite-crystal, typically about 


part 4: NORITE OF SIERRA LEONE. eoilils 


an inch and a half in diameter; they enclose felspar-laths more 
or less completely, and this, in the case of the larger spots of this 
type, gives rise toa beautiful ophitic structure. ‘The brown spots 
of the second type vary considerably in size, but they are generally 
larger and of finer texture than those of the first type; they are 
often irregular in shape, and of a reddish tinge. ‘These patches are. 
altered and highly-corroded fragments of the older norite, and are. 
naturally most numerous near the margins of the parent rock ; 
they were saved from complete destruction only by the freezing of 
the invading magma. 

The younger norites formed several series of intrusions into the 
older norite, but only two of them, referred to below as the first 
and second series respectively, can be recognized with certainty. 
The first of these, much smaller in volume than the second, was. 
intruded after the older mass had largely cooled down ; conse- 
quently it shows a chilled margin in many cases. It generally 
possesses a texture similar to that of the main mass, but its. 
characteristic colour and weathering, as well as the lack of banding 
and jointing, readily distinguish 1b “from the normal norite. Like 
the intrusions of the later series, 1t generally broke through the 
older norite quite irregularly; nevertheless, cases do occur in which. 
it took on the habit of the later dolerite-dykes and gave rise to a 
rock, which, in the hand-specimen, is distinguished from the. 
coarser dolerites only with great difficulty. Fortunately, the. 
relations of these rocks are made clear at several exposures wherein 
the dolerite is seen to cut all phases of the norite (see below, 
p- 317). The second and more important series is typically much 
coarser than the older or normal norite, and is consequently coarser: 
than the first series also. Since the exposed portions of the 
second series are probably only the upper part of one or more. 
very large intrusions, this greater coarseness may be due either to. 
the oreater volume and consequent greater capacity for heat and 
higher temperature of the second series, or even to the preliminary 
heating of the older norite by the first series. Hither one of these. 
causes “would have enabled the second series to cool slowly, and 
thus acquire a coarser texture. It is at least certain that the 
second series contained an enormous reserve of heat, because to 
it is due most of the extensive alteration produced in the older 
norite. Another possible factor in producing the coarser texture 
was the slight difference in composition, such as induced an even, 
greater degree of coarseness in the next set of intrusions, and. 
caused them to crystallize as norite-pegmatite. 

Norite-pegmatite cuts both the older and the younger norites, 
and takes the form of veins as much as 18 inches wide and irregular. 
tongues several yards across: one of the latter has apparently 
induced foliation parallel to its margin in the older or normal 
norite enclosing it. The norite- -pegmatite consists essentially of 
augite and bronzite, with some felspar and a small quantity of 
magnetite. 


312 DR. F. DIXEY ON THE [vol. lxxviil, 


(26) Field Relations of the Older and Younger Norites. 


Apart from several small isolated exposures on the mountains of 
the peninsula, the younger norites crop out only along the fore- 
‘shore on the south-western side. ‘They occur as numerous 
intrusions, many of them continuous under a thin roof of the 
invaded rock. These intrusions are of all sizes up to a quarter of 
.a mile in length, and several of the larger ones form low capes, as 
for example, Godrich Point (see p. 313). They do not show any ~ 
regular arrangement, except in one case, where four of them are 
elongated along a line running east and west. They send out 
numerous veins and tongues into the surrounding rocks. The 
grey rounded masses of the coarse or younger norite are in most 
‘eases readily distinguished from the well-jointed, dull-red blocks 
-of the normal type. (See Pl. XVI, fig. 2.) 

In certain places the junction of the older and younger norites 
‘can be followed for as much as 100 yards, and is seen to consist of 
.a confused jumble, or intrusion-breccia, of the two types; it 
includes wide areas of the normal rock shattered and pierced by 
numerous veins and tongues of the younger norite, and also 
scattered blocks and fragments of the normal norite, some sharp 
and angular and others deeply corroded. Even where the junction 
is not so broad, it is often vague and difficult to follow because the 
older rock has in some places been heated to a high temperature 
-and even re-fused. Away from the junction are a few isolated 
masses of younger norite and of older norite, the former repre- 
‘senting cupolas of the main intrusion, and the latter the remains 
of pendants from the roof. Also, within the younger norite are 
‘stoped blocks of the older norite, some of them moved only slightly 
-out of place, while others, carried a considerable distance, lie in 
all directions. Moreover, some present clean fresh joint-faces to 
‘the younger norite, and others show slight corrosion or deeply- 
indented outlines. Yet others, while retaining their form, have 
lost their original texture and composition ; finally, many are 
-completely shattered, and so permeated by the younger norite that 
‘they are represented by a mixed rock showing only shreds of the 
original norite. (See Pl. XVIL fig. 1.) 

The invading magma nearly always cuts across the banding of 
‘the older mass, and only in exceptional cases does it appear to 
have induced any foliation. In these instances a few bands and 
numerous streaks are associated with and arranged parallel to the 
margins of the coarse intrusions, as if a gneiss, comparable in 
-structure and origin with (a) the ‘streaky’ phase of the Kennack 
-gneiss,! and (3) the gneisses of Rum,? had been formed by the 
‘intermixture of plastic fragments of the older norite with the 
viscous invading magma. ‘This banding and streakiness is distin- 
-guished from the ordinary banding of the older norite by greater 


1 «The Geology of the Lizard & Meneage’ Mem. Geol. Surv. 1912, p. 132. 
2 ¢The Geology of the Small Isles of Inverness’ Mem. Geol. Surv. Scot. 
1908, p. 69. 


° 


part 4:] NORITE OF SIERRA LEONE. 313 


differences in the texture of the component parts, and by the 
rough weathering and darker colour of the bands and streaks of 
the younger rock. Nevertheless, where relations between the 
different rocks are very complex, it is not always possible to assign 
the banding to one cause rather than to another. 


Important Exposures. 


(i) Godrich Point.—Four elongated domes of coarse norite, long axes 
running east and west, rise through a roof of normal norite. One 
of them, forming Godrich Point, cuts across the banding of the 
normal norite, and is itself pierced by a dolerite-dyke; another, 
5 yards long and 3 yards wide, sends out a number of little tongues 
into the normal norite. 


(ii) Point north of No. 2 Town.—Striking complex of coarse and 
normal norites displayed; blocks and fragments of normal norite 
measuring several feet across are enclosed within the coarse norite ; 
a few of them are angular, with sharp boundaries, but the greater 
number show corroded outlines with large re-entrant curves. Many 
normal norite-fragments are small and ill-defined, and in some 
places are, with the enclosing rock, drawn out into wavy streaks. 
There are also several exposures of a rock of mixed texture due to 
‘permeation of normal norite by coarse norite. 

(ii) Toke Point.—Intrusion phenomena well exposed. 

{iv) York foreshore.—Large area of coarse norite richly impregnated 
with magnetite. In it are numerous stoped blocks of normal norite 
measuring as much as 10 yards in diameter ; they exhibit all stages 
of disruption, corrosion, and absorption, some of them having been 
liberally eaten into by the invading magma, and others thoroughly 
impregnated by it (see Pl. XVII, fig. 1). Large outcrop also of a 
rock consisting of an intimate mixture of older and younger 
norites. Farther north, a dolerite-dyke is seen cutting older and 
younger norites. 

{v) Point on the south side of Whale River, York.—One ex- 
posure shows coarse dolerite invading and shattering normal norite ; 
the junction is crossed by a later fine-grained dolerite-dyke. 


(vi) John Obey Point.—A large intrusion of coarse norite : its junction 
with the normal rock on the north side is almost 100 yards wide, 
and shows a great intermingling of the two types. Innumerable 
veins, tongues, and masses of the coarse pierce the normal rock, 
which shows signs of a very high temperature and much absorption. 


{viil) Samuel Island.—Numerous intrusions of younger norite along the 
foreshore. The margins exhibit banding and streakiness due to the 
intermixture of the two rocks. 


(3) The Beerbachite. 


In the northern half of the Colony, and principally in the 
Freetown district, occur intrusions of a fine-grained, granular 
rock,! dark grey when fresh and weathering to pale grey; it some- 
times shows a faint parallel structure in thin section. It consists 
essentially of labradorite, with subordinate hypersthene and mag- 
netite, and its specific gravity is 3°08. A partial analysis of a 


1 The ‘ norite-aplite’ of Prof. S. J. Shand, Geol. Mag. 1918, pp. 22, 23; he 
classifies the rock as melanocratic norite (m,—micronorite). 


314 DR. F. DIXEY ON THE [vol. lxxviii, 


single specimen of the rock shows :—Silica 47:96 per cent., alumina 
12°78, iron-oxides 11:19, and magnesia 16°58. This rock is a 
variety of beerbachite (Chelius), defined by H. Rosenbusch! as 
a fine-grained to compact granular rock, composed of labradorite 
(sometimes bytownite) and diallage, with variable amounts of 
hypersthene and magnetite; he states also that olivine-bearing 
varieties occur. Similar rocks are figured by Dr. J. S. Flett 
in the Lizard Memoir, and by Rosenbusch in his ‘ Elemente der 
Gesteinslehre.’? The rock described by the former, referred to. 
again below, occurs in the form of dykes in the Lizard gabbro, and. 
consists mainly of granular labradorite and augite.? Rosenbusch 
states * that beerbachites occur in the gabbros of Rum and Kilhoan 
(Ardnamurchan) and in that near Harzburg. Similar rocks occur 
also among the Skye gabbros, and are described as ‘ granulitic 
gabbros’ or ‘ pyroxene-granulites.’ ° 

The ‘fine-grained granulitic gabbro’ of the Duluth laccolite 6 
so closely resembles the Sierra-Leone beerbachite that microphoto- 
graphs of the two rocks can scarcely be distinguished. This 
granulitic gabbro, like the beerbachite now considered, is both 
darker and denser than the rock of the main intrusion. 

J. W. Judd? has explained the texture of certain Tertiary 
granulitic gabbros as due to crystallization of a moving magma in 
which the grains as they were formed were rolled around and 
prevented from interlocking freely. The Sierra-Leone beerbachite 
evidently crystallized under somewhat similar conditions, because 
fragments of norite caught up in it were softened, disintegrated, 
and drawn out into streaks (see below, p. 315). On the other 
hand, a few coarsely-crystallized and fresh-looking patches, free 
from obvious signs of contact-metamorphism, probably represent 
portions of the magma which crystallized under static conditions.® 

With further reference to the Lizard beerbachite, it is evident, 
from some interesting notes recently published by Prof. T. G. 
Bonney,? that this rock is very closely paralleled in occurrence and 
field-characters by the Sierra-Leone beerbachite. For instance, the 
Sierra-Leone rock also is frequently ‘ brown-speckled’ on a weathered 
surface, and it occurs in small masses rather than as veins; it 
invades a gabbro-mass made up of separate intrusions of different 
textures, and is followed by a series of dolerite-dykes. But the 
closest parallel of all appears in the way in which the Sierra-Leone 


1 « Hlemente der Gesteinslehre’ 3rd ed. (1910) p. 282. 

“Thid. is G. Paw le 

3 «The Geology of the Lizard & Meneage’ Mem. Geol. Surv. 1912, p. 110. 
coypl. ax hissy o: 

4 «HWlemente der Gesteinslehre’ 3rd ed. (1910) p. 288. 

° «The Tertiary Igneous Rocks of Skye’ Mem. Geol. Surv. 1904, p. 115. 

6 M. L. Nebel, Econ. Geol. vol. xiv (1919) p. 879 ; also pl. xiv, fig. d. 

7 *On the Gabbros, Dolerites, & Basalts of Tertiary Age in Scotland & 
Treland’ Q.J.G.S. vol. xi (1886) p. 49. 

8 Compare the coarse-grained rock cutting the Duluth granulitic gabbro: 
see M. L. Nebel, op. supra cit. p. 381. 

9 * Beerbachite at the Lizard’ Geol. Mag. 1920, p. 339. 


part 4] NORITE OF SIERRA LEONE. d15 


rock has, like that of the Lizard, incorporated fragments of the 
gabbro, softened and disintegrated them, and isolated their felspars 
into clusters and strings. 

The exposures of the Sierra-Leone beerbachite are generally 
small, and often are only a few yards across. Nevertheless, those 
which occur on Wilberforce Spur, on Mount Aureol, and in the 
Congo Valley respectively, corresponding to as many separate in- 
trusions, are much more extensive than the others, and probably 
range up to 150 yards in length. ‘The first of these intrusions 
Baws interesting contact- phenomena with the younger norites, 
while the second “displays a wide zone of hybrid rock produced by 
interaction with the older norite. 

The Wilberforce Spur intrusion crosses an ill-defined junction 
between the older and younger norites over part of its outcrop, 
but elsewhere it is confined to the younger norite. It affects the 
two invaded rocks in the same manner. The beerbachite-magma 
possessed a great power of corroding and even of assimilating “the 
norite, and all stages in the process from a fairly sharp junction to 
complete permeation can be observed in the field. The beerbachite 
contains blocks and fragments of norite, some of them having 
clean outlines and traversed by veins, others with deeply-indented 
margins showing where they have been attacked; yet others, 
more completely altered, are merely ‘ghcsts,’ in that (although 
their original outlines are still fainthy defined) they actually 
consist of mixtures in which corroded crystals and small fragments 
of the original rock are enclosed in a mass of beerbachite. This 
alteration has proceeded on a much larger scale at Mount Aureol, 
where the intermixture of normal norite and beerbachite has 
produced a hybrid rock. This hybrid rock is exposed along the 
sides of the unfinished motor-road, which runs around the flank of 
the hill from a kind of barrack-square to the G.O.C.’s house on 
the top. The road passes in turn over normal norite, beerbachite, 
the hybrid rock, and, finally, near the G.O.C.’s house, normal 
norite again. The hybrid rock forms an ill-defined zone about 
150 yards wide, and, although it is better developed on Mount 
Aureol than in any other known locality, the actual exposures are 
poor, and consist chiefly of small masses and residual boulders more 
or less enveloped in laterite. The hybrid rock is darker than 
either of the parent rocks; it usually contains numerous small 
spots and streaks, most of which are dirty brown to black, while 
many others are pale grey. The rock is tough, and possesses an 
irregular fracture. 

The invading magma continued to flow after the enclosed norite- 
fragments had become plastic and more or less disintegrated ; 
these fragments were consequently so drawn out into streaks that 
many of their crystals became isolated, or were broken down into 
‘augen.’ Specimens collected show strings of augite-fragments. 
and of white lenticles that represent felspars, and also the attenuated 
remnants of sponge-like crystals of magnetite (see Pl. XVII, fig. 2). 


Ce Gs. No vo2: is 


316 DR. F. DIXEY ON THE | vol. Ixxviil, 


(4) The Norite-Aplite. 


Veins of norite-aplite may not infrequently be seen in the norite: 
generally they do not exceed 1/8 inch in thickness, but in one 
locality, near York, they are much thicker, and one of them for a 
short distance attains a thickness of as much as 9 inches. The 
veins tend to become more acid with diminishing width, and so 
at their terminations they may consist almost wholly of quartz 
and micropegmatite. The thin veins, however, which are white, 
are very persistent, and can be traced ‘for 20 yards or more across 
an- outcrop of norite without altering appreciably in thickness. 
The thicker veins consist of a medium-grained rock having a 
pale greenish tinge, and made up chiefly of acid soda-lime felspar, 
orthoclase, quartz, and micropegmatite, with small quantities 
of pyroxene (both rhombic and monoclinic), hornblende, biotite, 
and apatite (see Petrographical Notes, p. 322). 

The norite-aplite-veins are the youngest intrusions of the norite- 
complex, apart from the dolerite-dykes which are seen to traverse 
the veins in several localities; evidence for this may be seen in 
the Congo Valley, below Hill Cot, where thin veins of norite-aplite 
traverse a large intrusion of beerbachite. It is interesting to note, 
moreover, that the veins are fr Si associated with the master- 
joints of the norite, often running parallel with, or along them, for 
considerable distances. ‘Ihe affinity of the norite- -aplite to the 
preceding intrusions is indicated, however, in the similarity of 
these rocks as regards certain chemical and mineralog ical characters, 
such as the presence of rhombic pyroxenes. Nevertheless, there is 
reason to believe (see p. 317) that the aplite is related to the 
dolerite as much as, or even more than, to the norite; also that 
the aplite bears a magmatic relation to the dolerite similar to 
that which the segregation-veins and norite-pegmatite do to the 
norite. If this be so, the norite and dolerite of the complex show 
an interesting parallel with the olivine-dolerites and quartz-dolerites 
of the Edinburgh district, in that these Scottish dolerites are each 
cut by segregation-veins related in composition to the parent mass.! 
Moreover, this view of the magmatic relations of the aplite is 
supported by the fact that typical gabbro-aplites, such as those 
occurring in the gabbros of Rum and Skye (see below), are free 
from quartz and micropegmatite. 

The specific gravity of a specimen of norite-aplite taken from 
one of the thicker veins was found to be 2°72. 

The aplite-veins generally effected a slight alteration of the 
wall-rocks, shown chiefly in silicification of the felspars and 
uralitization of the pyroxenes; this alteration appears in the hand- 
specimen as a whitish zone, nearly an inch wide, enclosing the finer 
veins. The margins of the veins are, nevertheless, sharply defined. 

The thin aplite-veins of St. David’s® differ considerably from 


1 <The Geology of the Neighbourhood of Edinburgh’ Mem. Geol. Surv. 
1910, pp. 289 & 307. 

2 J. V. Elsden, ‘The St. David’s Head Rock Series’ Q.J.G.S. vol. lxiv 
(1908) p. 283. 


part 4] NORITE OF SIERRA LEONE. 317 


those of Sierra Leone; for instance, they apparently cut only the 
more acid members of the series, and they are free from ferro- 
magnesian constituents and micropegmatite. Moreover, they are 
considered to be of contemporaneous origin, whereas those of 
Sierra Leone are distinctly younger than the enclosing rock. 


(5) he Dolerites. 


The injection of a number of dolerite-dykes, seen in different 
localities to cut ail the earlier intrusions, marked the last phase in 
the history of the complex. It is quite possible, however, since in 
the field the dolerite is distinguished with difficulty from certain 
of the later intrusions of norite, that a few of the dolerite-dykes 
were intruded considerably in advance of the main series.! The 
dykes not infrequently made their way along the same channels as 
the preceding aplite-veins; this procedure is indicated chiefly by 
the presence of patches of aplite still adhering to their original 
walls, and also by a whitish alteration-product of the walls due to 
a contact-effect characteristic of the aplite. Sometimes, however, 
the invading magma forced its way between an aplite-vein and its 
wall-rocks, resulting in the inclusion of long irregular lenticles of 
aplite parallel to the margins of the dolerite. An excellent ex- 
ample of this may be seen on the foreshore a short distance north 
ot York; here, included in a dolerite-dyke about a foot wide, is a 
highly-corroded aphte-xenolith 4 feet long and about 6 inches in 
maximum width. Several smaller xenoliths accompany the bigger 
one. 

The dykes show interesting affinities to both the norite and the 
norite-aplite. To the norite they are related by the presence of a 
rhombic pyroxene which accompanies the augite: in the smaller 
dykes and in the marginal phases of the larger dykes, the rhombic 
pyroxene is enstatite ; ‘but in the central portion of the larger dykes 
the enstatite is replaced by a form approaching the common 
hypersthene of the norite [C.121].? In this connexion it may be 
pointed out that, with regard to certain basic rocks near St. David’s, 
Dr. J. V. Elsden ® considered the thin intrusions of enstatite-diabase 
to differ from neighbouring quartz-norites only in ophitic tendency. 
To the norite-aphte, on the other hand, the dolerite-dykes are related 
by the presence of interstitial orthoclase and acid plagioclase, often 
considerable in amount, together with enstatite, some biotite, and 
innumerable needles of apatite. Thus it would appear, par ticularly 
in view of the field relations, that the aplite-veins were the fore- 
runners of the dolerite, rather than the successors of the norite. 


1 Tn one locality, near York, coarse dolerite is traversed by a dolerite-dyke 
possessing chilled margins. 

2 Throughout this paper the numbers in square brackets refer to thin 
sections of the ‘ Colony’ rocks. 

3 “On the Igneous Rocks occurring between St. David’s Head & Strumble 
Head (Pembrokeshire)’ Q. J. G.S. vol. lxi (1905) p. 591. 


FQ, 


318 DR. F. DIXEY ON THE [vol. xxvii, 


The dykes consist essentially of a more or less ophitic enstatite- 
dolerite, free from olivine and containing variable amounts of 
interstitial acid felspar (see Petrographical Notes, p. 324). The 
specific gravity of the rock is 3°08. Thus we have in the one 
area two series of basic intrusions, not differing greatly in age, 
the one (norite) characterized by the presence of olivine, and the 
other (dolerite) free from this mineral; this association recalls 
the well-known occurrence of both olivine- and quartz-dolerites 
among the Carboniferous intrusives of the Edinburgh district.1 

The Sierra-Leone dolerite-dykes are exposed in considerable num- 
bers along the foreshore on the south-western coast of the Colony ; 
elsewhere, with the exception of one cutting across the lower part 
of the Charlotte Falls, they have not been observed, probably 
because the rocks that may contain them are poorly exposed. 
Most of them are of an irregularly branching habit, and single 
examples can rarely be traced far enough to give a mean strike, 
The average direction of a number of which the strike could be 
measured, was north 35° west. They vary in width from a few 
inches to nearly 20 feet; the narrower are fine-grained and compact 
with sharp chilled margins, whereas the wider are coarse and are 
sometimes porphyritic with augite and felspar. Near York, one of 
the coarser dykes contains numerous skeletal crystals of ilmenite, 
and it cuts a series of fine aplitic veins in the norite. Others 
are vesicular, either along their margins or centrally. 

In addition to the dykes several masses of coarse dolerite oceur 
in the norite; their boundaries are ill-defined, and their exact 
relation to the enclosing rocks difficult to determine. 

The Sierra- Leone dolerites closely resemble certain quartz- 
dolerites which occur elsewhere. For instance, they are similar im 
several respects, particularly the strong zoning of the felspars,? to 
the Carboniferous quartz-dolerites of Scotland; but the pale 
rhombic pyroxene of these Scottish rocks is identified as a form of 
hypersthene. Despite the obvious resemblances, however, it should 
be noted that both quartz and micropegmatite are apparently 
absent from the Sierra-Leone dolerite, the interstitial matter of 
which consists only of orthoclase and highly-acid plagioelase. 
Like that of the Scottish quartz-dolerites and of most of the rocks 
mentioned below, this interstitial matter is riddled with long 
needles of apatite. Other similar rocks are the Ratho intrusion 
near Edinburgh,’ the Whin Sill,4 certain intrusions of the Kilsyth- 
Croy district (Dumbartonshire),® and those of Arran.® 


1 «The Geology of the Neighbourhood of Edinburgh’ Mem. Geol. Sury. 
1910, p. 289. 

2 Ibid. p. 305 & pl. ix, fig. 3. 

3 J. J. H. Teall, ‘ British Petrography’ 1888, p. 190; & J. S. Flett, ‘ The. 
Geology of the Neighbourhood of Edinburgh’ Mem. Geol. Surv. 1910, p. 306. 

4 J.J. HH. Teall, Q.J.G.S. vol. xl (1884) p. 640. 

> G. W. Tyrrell. Geol. Mag. 1909, p. 305. 

6 A. Harker, ‘The Geology of North Arran, &c.’ Mem. Geol. Surv. 1903, 
pe lie 


part 4] NORITE OF SIERRA LEONE. 319 


The mineralogical differences between the various members of 
the norite-complex may be summarized in tabular form as 


follows :— 
| 3 2) 5) 5 ia 
S| o | : ® | 
= o o a 3 nR ge, we 
ea “= Wb . 2 o 5 ted 
S ISTO = o I 5 o = 5 
° 4 2 = [=| S) ne re nS fo) 
aa g BS on a 7 a= q 3B S 
& = i) Py =) 2) aS) fo} [oy oy 
A, | 64 = S) 4 aa ss <q fo) 
Worite  ...... = =m = | =] — — 
Beerbachite..} mss fs] = | ne) Tas 
Norite-Aplite] mam — — | Lg pe || a es 
eS Soe Stor iS. | peek eS) 5s ee 
Dolerite eeecee [ico] | aa es | —- SSS | —- —<<s 
| 
| 


The relative frequency of occurrence of the minerals is represented by the 
magnitude of the dashes as follows, in descending order:— —_— —-— — 


(6) Differentiation and Assimilation. 


Several stages of differentiation may be distinguished in the 
Sierra-Leone rocks :— 


G) Deep-seated differentiation.—The source from which 
the complex was derived underwent differentiation into a series of 
cognate sub-magmas which, on the whole, became intruded in the 
usual order of decreasing basicity: norites proper, rich in olivine, 
were the first to arrive, and they were followed in turn by norite- 
pegmatite and beerbachite. Then came the aplite-veins, very small 
in total volume, and finally a series of enstatite- dolerite dykes 
free from olivine and often rich in interstitial acid felspars. 


Gi) Differentiation during intrusion.—The cooling of 
the normal norite-magma on intrusion led to a partial separation of 
the felspathic and ferromagnesian constituents in the liquid state. 
This heterogeneous product was drawn out, as intrusion proceeded, 
into bands which were fixed later by crystallization. 'The resultant 
banding was sometimes accentuated by small quantities of the 
coarse norite- -magma, which were caught up by the normal norite- 
magma and interbanded with it. 


Gu) Differentiation in place.—This phase led to the 
segregation of magnetite from the remaining constituents, and 
produced the streaks of this mineral within and parallel to the 
margins of certain bands. Locally, the ferromagnesian constituents 
‘separated out In a similar manner, and formed small patches, 


Quartz. 


320 DR. F. DIXEY ON THE (vol. xxvii, 


Assimilation.—An interesting feature of the Sierra-Leone ' 


complex is the great amount of absor ption suffered by earlier rocks 
when invaded by later intrusions; the absorption of older norite 
by younger norite and of both older and younger norites by beer- 
bachite are examples of these reactions. With regard to the 
norites themselves, this absorption is the more remarkable, in that 
the composition of the later rocks differed but slightly from that 
of the earlier rocks. Cases have been described in which a basic 
magma has been modified by the absorption of an acid rock,! or, 
more frequently, in which an acid magma has been modified by the 


absorption of a basic rock: as, for example, the Lizard Kennack 


gneiss* and the ‘marscoite’ of Skye?; but a basic magma has. 
rarely been known to absorb basic rocks. Even where a. basic: 
magma has invaded an ultrabasic rock, as where the basic rocks 
vf Rum#+ and Skye® break through and invade the ultrabasic: 
rocks, the older rock has simply been broken up and finally involved. 
with the newer rock to form an intrusion-breccia, while little or no 
corrosion has taken place. As may be expected, however, a magma 
has sometimes been known to absorb a rock of similar composition: 
~ locally, as where the Skye gabbro has ‘ to a limited extent actually 
fused and incorporated’ basaltic lavas. As a general rule, how-- 
ever, it is not to be expected that a magma should absorb readily 
a rock of similar composition, since there is little scope for mutual! 


chemical reaction, and the inv ading magma would carry little, if: 


any, excess of heat.6 


The unusual extent to which absorption has proceeded in the: 
case of the Sierra-Leone norites is ascribed chiefly to the great: 


depth at which the action took place; this would result in a high . 


initial temperature for the invaded rocks, and would also prevent 
the heat of the intrusions from escaping, ther eby allowing a great: 
length of time during which the reactions could proceed. Mare: 
over, there is considerable evidence to show that many .of the: 
numerous later intrusions of norite are all parts of one or more: 
large intrusions, of which only the irregular upper portions are at 
present visible. 


(7) Weathering and Lateritization of the Norite. 


Where the products of weathering are unable to accumulate, 
the highly-resistant magnetite stands out in sharp relief on the 
surface of the nor ite, which in the majority of cases consists largely 
of felspar, since the pyroxenes disintegrate most easily. Along the 
foreshore the norites rich in iron weather to a smooth surface 


1 As, for example, Purcell Sills; see R. A. Daly, Mem. 38, Geol. Surv.. 
Canada, 1912, pt. i, p. 248.-— 
2 «The Geology of the Lizard & Meneage’ Mem. Geol. Surv. 1912, p. 132.. 
* ©The Tertiary Igneous Rocks of Skye’ Ibid. 1904, p. 183. 
Ibid. p. 69. 
Ibid. p. 64. 
° A. Harker, ‘ The Natural History of Igneous Rocks’ 1909, p. 357. 


Oe 


part 4] NORITE OF SIERRA LEONE. 321 


covered with a thin ferruginous skin, which possesses a dull 
metallic lustre, is dark brown to black, and very hard. 

A more detailed account of the lateritization of the norite has 
been given elsewhere.! 

At Mount Aureol the lateritization of well-jointed norite has 
given rise to a curious effect, in the form of a slightly-raised and 
regular lattice-work on what is now the floor of an old barrack- 
square. Originally the norite at this place was strongly affected 
by two series of joints which ran at right angles ; hens was also a 
third series of jomts, running obliquely to the others, and less well 
developed. The joints at right angles formed a large number of 
small rectangles with sides measuring as much as ‘12 inches in 
length. Lateritization first proceeded along the joints, and then 
it attacked the intervening spaces; the first-formed laterite was, 
however, more resistant than the second, and this relative Wonanens 
ultimately led to the formation of a raised rectangular pattern on 
the floor of the square. 


(8 a) Iron-Ores in the Norite. 


The iron-ores that occur in the norite are magnetite, ilmenite, 
and intermediate forms : octahedra of magnetite have been collected, 
as also skeletal crystals of ilmenite ; but in general the ores are 
not distinguishable in the field, and the term ma gnetite will be 
used here to include them all. All forms of the norite include 
magnetite as larger or smaller disseminated grains: it is developed 
also as spongy growths, or as dense shapeless masses measuring up 
to 3 inches in length, and even as streaks and seams several inches 
thick, formed as the result of segregation in the banded norite. 
Locally, the norite is very rich in such ores; but the occurrences 
are not individually large enough for commercia] exploitation. 

A considerable quantity of iron-ore crops out along the fore- 
shore west of Aberdeen Creek. ‘The norite at this place is highly 
ferruginous, and contains numerous seams of magnetite intergrown 
with variable amounts of felspar; nevertheless, almost all the iron 
exposed is in the form of red oxide (hematite) due to the alteration 
of the magnetite 7m sifw, and to concentration of the iron with 
progressive lateritization of the norite. The foreshore south of 
York over a distance of some hundreds of yards consists of coarse 
norite richly impregnated with magnetite as small segregations 
and as narrow seams, and north of John Obey is a small exposure 
of coarse norite showing magnetite-crystals (measuring as much 
as 8 inches in length) intergrown with pyroxenes. 

The coastal plain which extends intermittently all around the- 
Colony consists almost entirely of detritus washed down from 
the mountains, and since the iron-ore (when weathered out from 
the parent rock) is carried downwards in the form of grains and 


1 See F. Dixey, ‘Notes on Lateritization in Sierra Leone’ Geol. Mag. 
1920, p. 211; also W. M. Davis, ‘ Physiographic Relations of Laterite’ Geol. 
Mag. 1920, p. 429. 


322 DR. F. DIXEY ON THE [ vol. Ixxvinl, 
lumps of magnetite, it follows that the detrital accumulations 
should contain a large quantity of iron, a quantity which decreases 
as the distance from the mountains increases. Investigation 
confirms this, and in one locality on the north side a considerable 
deposit of iron-ore has been found along the inner margin of the 
plain. Most of this iron occurs now in the form of strings and 
small masses of hematite running through the laterite, which is 
itself in places sufficiently rich to rank as an ore. Elsewhere in 
the beds constituting the plain there is always quite enough iron- 
oxide to form a thick hard crust of laterite. 


(86) Other Economic Minerals. 


Economic minerals other than iron-ores seem to be developed 
only to a very slight extent in the norite, and this is probably due 
to the absence of basic and marginal phases to the intrusion. 
An analysis published in 1910 indicated 3-28 per cent. of chromic 
oxide in a specimen of iron-ore taken from ‘ Bathurst Mountain.’ ! 
The present investigation, however, has not furnished any indica- 
tion of this amount, either in the field or in thin section, and 
recent analyses carried out in connexion with the work do not 
show in any one case more than 0:1 per cent. of the oxide; in 
most cases it is altogether absent. Nickel-oxide also has been 
shown by one of these analyses to be present up to 0-1 per cent. 
Sulphides are represented only by grains of pyrites which are seen 
but rarely in thin sections of the rock. A series of concentrates of 
stream-gravels has yielded nothing of interest, other than small 
quantities of rutile.2 Bauxite containing over 51 per cent. of 
alumina occurs in considerable quantity, as a decomposition-product 
of the norite.3 


III. Perrocraputcat Notes. 
(1) The Norite-A plite. 


An examination of a series of thin sections has shown that the 
veins of norite-aplite become increasingly acid in composition 
towards their extremities. 

A specimen from a vein of medium texture, 9 inches wide, was 
found to consist chiefly of acid plagioclase, with much micropeg- 
matite and a little quartz and orthoclase. In addition, it contained 
small amounts of enstatite, brown hornblende, brown biotite, 
magnetite, and apatite (the last-named as clouds of minute parallel 
needles in the felspar, and as fairly numerous grains and prisms). 
The plagioclase, from consideration of the refractive index and 


1 *Tron-Ore Resources of the World’ Stockholm, vol. ii (1910) p. 1029. 


* Note the occurrence of rutile in certain anorthosites of Quebec: 
J. P. Iddings, ‘Igneous Rocks’ vol. ii (1913) p. 358. 

3 For an analysis of bauxite, see F. Dixey, Rep. Geol. Surv. Sierra Leone, 
1921, p. 18. 


part 4] NORITE OF SIERRA LEONE. 323 


extinction-angles, is determined mainly as oligoclase; its twin- 
lamelle are exceedingly fine. The felspar of the micropegmatite 
is frequently seen to possess twin-lamelle. The enstatite is pale 
green, and generally non-pleochroic ; it occurs as skeleton-crystals 
and also as rods and grains, all associated with numerous grains of 
iron-ore. Some crystals, densely. crowded with minute rod-lke 
inclusions, exhibit a slight greenish pleochroism. Certain other 
vague pyroxenic patches occur in the slide, crowded with grains 
of iron-ore. These, like most of the ferromaguesian minerals in 
the gabbro-aplites of Skye,! are probably the remains of pyroxenes 
caught up by the norite-aplite during intrusion. A variety of 
medium-grained norite-aplite is seen as an inclusion in a dolerite- 
dyke, which has forced its way along the same course as an earlier 
aplite-vein. This variety is more acid; it consists chiefly of 
granular quartz and orthoclase; an acid plagioclase occurs as well, 
and generally takes the form of relatively large crystals. The 
following minerals are also present :—pale-green pyroxene, brown 
and green hornblende, brown biotite, magnetite, and apatite, all 
showing much the same characters as in the first variety. The 
fine extremities of the veins are rich in quartz and micropegmatite, 
but the other minerals are generally represented in them also. 


(2) The Dolerites. 


The dolerites are not often wholly of the ophitic type ; frequently 
they show both ophitic and intersertal structure in the same slide, 
due to the variable habit of the pyroxenes. 

The ophitic structure is developed when the felspars penetrate 
uregular plates of pale-brown augite; these augites, commonly 
about 1/8 inch in diameter, are scattered through the rock in great 
numbers, and, owing to the numerous grains of iron-ore and small 
flakes of brown biotite associated with them, they impart to the 
slice a markedly patchy appearance. They are sometimes almost 
opaque with inclusions. In the remaining parts of the slice, the 
pyroxene is chiefly a colourless to pale-green enstatite, which is 
shghtly pleochroic and of granular habit; grains of this mineral 
are often involved in the areas of clear felspar interstitial to the 
common felspars of the rock. 

The finer varieties of dolerite, occurring near chilled margins 
and in thin dykes, consist of minute laths of felspar, with inter- 
stitial granules of augite and iron-ore. The coarser types, 
developed in the large dykes, sometimes contain phenocrysts of 
felspar possessing fine twin-striation and occasional signs of cor- 
rosion [C 125}. In one other variety [C121], from the middle of 
a dyke 18 feet wide, the pyroxenes were developed in the form 
of coarse grains showing a tendency to idiomorphism ; a number 
of these grains belonged to the rhombic system, and possessed a 
pale-brownish pleochroism. This rock is of especial interest, as 
showing an intermediate stage between dolerite and norite. 


1 «The Tertiary Igneous Rocks of Skye’ Mem. Geol. Surv. 1904, p. 119. 


d24 DR. F. DIXEY ON THE [ vol. Ixxviu, 


The felspar of the ordinary dolerite is of two kinds: the one 
occurs as laths and is turbid, whereas the other is interstitial and. 
almost water-clear. The laths have a narrow clear margin, which 
is optically continuous with parts of the adjacent interstitial 
felspar. Thus the felspar-laths are strongly zoned, and where the: 
margin of the microscope-slide cuts across one of them, it is. 
interesting to trace the variation in refractive index from the 
centre of the crystal to its outer limit. In this way it can be- 
demonstrated, just as in the case of certain Scottish quartz- 
dolerites,| that the composition of the laths ranges from basic 
labradorite to acid oligoclase. The felspars making up the larger 
areas of clear interstitial material range from oligoclase to ortho- 
clase in composition, and they are riddled by numerous needles of 
apatite. Some of these felspars, determined as orthoclase, have- 
short rectangular outlines; while others possess a curious streaky 
appearance suggestive of anorthoclase. 

These clear areas, and the minerals that compose them, closely 
resemble the interstitial portions of the Whin Sill and of certain 
Scottish and Pembrokeshire quartz-dolerites. 

The iron-ores of the dolerite are generally abundant; they 
occur as microliths, grains, and irregular masses, the last-mentioned. 
sometimes assuming curious wheel-like and other forms similar to. 
those described by Mr. G. W. Tyrrell from the Kilsyth-Croy 
intrusions.* 


(3) The Minerals of the Complex. 


Quartz.—In the aplite-veins, particularly at their fine extremities, 
quartz occurs in fair amount, both in granular form and in graphic 
intergrowth with felspar; the felspar of the intergrowth is. 
generally orthoclase, but in some cases it is an acid plagioclase. 
In the norite, however, quartz does not occur as a normal product: 
of crystallization; this is only to be expected, since olivine is so- 
commonly developed. Nevertheless, it is interesting to note that 
there was probably a small excess of silica at the time when the 
felspars were crystallizing; later, as the felspars attained a lower 
temperature, this excess was precipitated in the form of strings. 
and small patches of granular quartz. 


Felspars.—The only felspar of the normal norite is an acid. 
labradorite the composition of which is near Ab,An,, as determined 
by extinction-angles and refractive index. This felspar is fresh and 
clear, grey in colour, and free from zoning. It generally forms. 
pinacoidal tables with Carlsbad and albite twinning; pericline 
twinning occurs also. The grey colour of the felspars in the hand- 
specimen is due to the presence of innumerable minute opaque 


1 ¢The Geology of the Neighbourhood of Edinburgh’ Mem. Geol. Surv. 
Scot. 1910, p. 305. 

2 “Geology & Petrology of the Intrusions of the Kilsyth-Croy District. 
(Dumbartonshire) ’ Geol. Mag. 1909, p. 308. 


part 4] NORITE OF SIERRA LEONE. B25 


black inclusions ; these inclusions, which apparently consist of iron- 
ore, may occur in parallel bands along certain twin-lamellz, or’ 
may be scattered irregularly throughout the crystal. They 
generally lie parallel to two or more crystallographic planes in the: 
felspar. They are much more numerous in the felspar of the 
younger norites ; this felspar, approaching bytownite in com- 
position, is more basic than that of the normal norite. The felspar 
of the beerbachite, on the other hand, is slightly more acid than: 
that of the normal norite. The plagioclase occurring in the aplite-: 
veins belongs to the acid end of the soda-lime series; it exhibits: 
minute twin-striation, and its variable composition is indicated by 
strong zoning. Finally, the plagioclase of the dolerite ranges iv 
composition from basic labradorite to acid olgoclase ; the basic 
portions of a crystal form a turbid central area, whereas the acid 
parts make up a clear relatively narrow external zone. Porphyritic 
plagioclases oceur locally in some of the finer-grained phases of the 
norite ; the phenocrysts do not differ appreciably from the ground- 
mass felspars in composition, and moreover, like the latter, they 
contain occasional strings and clots of granular quartz.! 

Orthoclase occurs as a constituent of the aplite-veins, and it is 
present also in the interstitial material of the dolerite. 

Locally, the more basic felspars show small quantities of a 
pale-brown alteration-product, which is developed in fibrous form 
transversely to cracks; sometimes it appears as minute rhombs 
[C48]. It is of low refractive index and feeble bir efringence. 

The felspar of the norite enters into an interesting series of 
intergrowths with magnetite, augite, and olivine (see _ below, 


p- 829). 


Pyroxenes.—Next to the felspars the pyroxenes are the most 
abundant minerals in the norite. They consist essentially of augite 
and hypersthene; but there occur also small quantities of a pale 
augite, and of a colourless rhombic pyroxene. Enstatite occurs in 
both the aplite and the dolerite. 

The augite of the norite is chiefly a smoky-brown diallage ; 
schillerization, as lines of minute grains and rods of iron-ore, is 
sometimes strongly developed. Not infrequently the augite faintly 
simulates the pleochroism and polarization-colours of ‘the hyper- 
sthene ; a similar feature has been noted in the augite of the 
Kilsyth-Croy intrusions by Mr. G. W. Tyrrell, who attributes it 
to the presence of the hypersthene-molecule in the augite.2 A 
colourless to pale-green augite, probably diopside, occurs in the 
aplite-veins. 

The hypersthene of the norite is associated with the augite in 
variable proportions ; generally, however, it is subordinate ‘to. the 
augite. It is less common in the older than in the younger norite.. 


! Quartz occurs similarly in the Charnockite Series: see <A. Harker, 
‘The Natural History of Igneous Rocks’ 1909, p. 261. 


-*Geology & Petrology of the Intrusions of the Kilsyth-Croy District 
(Dumbartonshire) ’ Geol. Mag. 1909, p. 306. 


B26 DR. F. DIXEY ON THE (vol. Ixxviil, 


It occurs in various tints of brown: the pleochroism, which is 
generally strong, has been observed as follows :-— 


X, light-red to brownish red; Y, reddish yellow; Z, pale green. 


Bastite is of rare occurrence only; sometimes, however, the hyper- 
sthene contains yellow-green fibrous veins of a mineral which 
resembles delessite: this mineral has been noted in the hypersthene 
of the Charnockite Series! and in the ‘ pyroxene-leptynite ’ (Lacroix) 
of Ceylon. Hypersthene is practically the only pyroxene of the 
beerbachite, in which it is more or less granular, with a tendency 
to idiomorphism. In the coarser rocks, on the other hand, 
sometimes forms large irregular plates enclosing all the other 
minerals, after the manner of the big ragged augites in the 
eucrites of Rum.? Schillerization is developed much as in the 
diallage, except that the inclusions are larger and consist of a 
brown birefringent mineral. The hypersthene sometimes shows a 
fine multiple twinning : this is probably secondary, and analogous 
to that sometimes produced i in felspars by local pressure. Rarely, 
two sets of twinning are seen, crossing at right angles. * 

The enstatite of the aplite-veins is often idiomorphic, and 
not infrequently it takes the form of bladed skeleton-crystals ; 
in the dolerite it generally occurs as anhedral grains. It 1s 
colourless to pale green, optically positive, and the pleochroism, 
when visible, is X and Y yellowish or yellow-green and Z green; 
this pleochroism is rarely detected, except in those forms that are 
crowded with minute rod-like inclusions. 

The order of crystallization of the various pyroxenes is dis- 
tinctly variable. The pale augite usually appears as rounded 
patches in the diallage ; small and fairly idiomorphic hypersthenes 
are sometimes enclosed in augite ; but, on the other hand, plates of 
hypersthene may enclose augite- -crystals.2 At other times, the 
monoclinic and rhombic pyroxenes are intergrown ; they then show 
the customary interpositions, and occasionally have a common 
crystallographic orientation. Several examples were seen, similar 
to those observed by Sir Jethro Teall in the dolerite of the Whin 
Sul,* and by Dr. J. V. Elsden in the norite of St. David’s.? The 
latter observer has already shown (op. c7t. p. 286) that 


‘the simultaneous separation of these minerals, as shown by intergrowths 
and compound twins, might represent the eutectic composition,’ 


in accordance with Prof. J. H. L. Vogt’s view that in certain cases, 
the sequence of these minerals should be dependent upon the 


1 Mem. Geol. Surv. India, vol. xxviii (1900) p. 141. 

2 «The Geology of the Small Isles of Inverness’ Mem. Geol. Sury. Scot. 
1908, p. 100. 

3 Similar relations are noted in the norite occurring near Huntly: see 
W. R. Watt, ‘The Geology of the Country around Huntly (Aberdeenshire) ’ 
Q.J.G.S. vol. lxx (1914) p. 266. 

+ Tbid. vol. x] (1884) p. 649. 

> Ibid. vol. xiv (1908) pl. xxxii. 


part 4] NORITE OF SIERRA LEONE. 327 


relative proportions of lime and magnesia present in the magma.! 
Vogt has shown elsewhere ® that, when magnesia is largely in excess. 
of lime, the rhombic pyroxene would crystallize first; 1t would 
then be followed by intergrowths of mix-crystals, and these latter 
by monoclinic pyroxene Silene. in accordance with Roozeboom’s. 
7 ype IV of mix-erystals. There occurs also a lamellar inter- 

evowth of hypersthene and augite similar to that of the pyroxenes. 
in the St. David’s, Whin Sill, and Duluth rocks; it ranges from 
strong banding down to lamellation so thin as to be scarcely per- 
ceptible microscopically. The finer phases of the lamellation and 
other intimately-related forms are probably due to deposition from 
solid solution. With reference to similar phenomena in the. 
labradorite-norite of the Lofoten Islands (Norway), Prof. J. H. L. 
Vogt has stated that 3 


‘By cooling down to ordinary temperature, ... the enstatite-hypersthene. 
cannot carry so much Ca(Mg, Fe)Si,O, in solid solution. Consequently some 
augite is separated out in perthitic lamelle 
lamelle of albite or albite-oligoclase in perthitic striated microcline.’ 


The relations of the pyroxenes to the other principal minerals. 
depend upon the relative proportions of the various minerals in the 
magma: when the pyroxenes are greatly in excess of the general 
eutectic, they are the first minerals to crystallize out. The eutectic: 
minerals enter into an interesting series of intergrowths. 

The pale augite occurring as rounded inclusions in the diallage. 
is probably the monoclinic form of magnesia-pyroxene which Dr. 


‘S) 
Elsden has briefly discussed in his St. David’s paper. 


Olivine.—This mineral is a common and often important con- 
stituent of both the older and the younger norites; some varieties. 
of the norite consist almost entirely of olivine and felspar, while. 
certain bands in the banded norite are very rich in coarse granular- 
olivine. Olivine is absent from the normal beerbachite; but it 
is frequently present in this rock as xenocrysts derived from the. 
norite. Olivine occurs principally in the norite, both as rounded 
blebs and as more or less idiomorphic cry ‘stals; sometimes it 
occurs as strings and clusters of contiguous crystals which, while 
presenting rounded outlines to other minerals, are themselves. 
separated by clean crystal-faces. The crystallization of the olivine. 
extended over a considerable period: where the mineral was rela-. 
tively abundant it formed fairly idiomorphic crystals, but where it: 
was in relatively small proportion, on the other hand, it developed as. 
skeleton-crystals enclosing pyroxenes and laths of felspar. Locally, 
however, it formed a beautiful pcecilitic structure with the pyro- 
xene; sometimes it entered into intergrowth with other minerals. 
(see below). 


1 Die Silikatschmelzlésungen’ pt. i (1903) p. 129 & pt. ii (1904) p. 109,. 
Vidensk. Selsk. Skrifter, Christiania. 

2 See J. V. Elsden, ‘ Principles of Chemical Geology’ 1910, p. 166. 

3 On Labradorite-Norite with Porphyritic Labradorite-Crystals, &c.’’ 
Cee ool, xv (LIO9}p. LO: 

4 Ibid. vol. lxiv (1908) p. 288. 


328 DR. F. DIXEY ON THE (vol. Ixxviil, 


Normally, the olivine is remarkably fresh; apart from pale- 
green and yellow-green serpentine, the most important alteration- 
product is iddingsite. This mineral replaces the olivine first along 
eracks and then in the intervening spaces, until it forms large 
plates, varying in colour from deep brownish-red to yellow and 
green. It has strong double refraction, and is distinctly pleochroic, 
the colours ranging ‘from red-brown to a yellowish tint, vibrating 
respectively parallel with, and at right angles to, the short axis of 
the polarizer. In the hand-specimen it has a good cleavage in one 
direction; the flakes, which can be separated with a knite, are 
-somewhat brittle. It occurs most commonly in norite relatively 
wich in iron. The mineral bears a close resemblance to that 
described by Prof. W.S. Boulton from an intrusion of monchiquite 
in the Old Red Sandstone of Monmouthshire! Well-marked 
reaction-rims are sometimes developed between the olivine and 
‘the felspar. The olivine frequently encloses beautiful dendritic 
magnetite. 


Tron-Ores.—-The chemical analyses of the norite indicate that 
the iron-ore contains a considerable percentage of titanium; 
examination of thin sections shows, however, that the amount of 
‘titanium varies in different specimens of the ore. Lateritic iron- 
ores derived from. the norite have been found to contain as much 
as 20 per cent. of titanium dioxide. The ore apparently consists 
of a mechanical mixture of magnetite and ilmenite; for, when 
a sainple is powdered, the two minerals can readily be separated 
by means of a magnet.? It is thus a titaniferous magnetite or 
‘titanomagnetite. The magnetite (titanomagnetite) is present 
in all phases of the norite, but it is not always developed to 
the same extent. It occurs generally as irregular grains, some 
-of which are rounded while others are more or less idiomorphice. 
It is often interstitial to other minerals of the rock, showing that 
its crystallization extended to a very late stage; on the other 
-hand it not infrequently forms, as in the Skye gabbros, large plates 
in which all other minerals are embedded.? It enters into intimate 
intergrowth with other minerals (see below, p. 332), and even 
“appears in some cases to be replacing them; similar relations 
‘have been noted in the Duluth gabbro, concerning which M. L. 
Nebel states that 


‘Maenetite is often later than augite and olivine, as well as plagioclase, as 

-it is found surrounding and penetrating them in such a manner as to suggest 

corrosion of the older mineral and partial replacement by the magnetite 
(pl. xiii,c).’ Econ. Geol. vol. xiv (1919) p. 372. 


1 “Qn a Monchiquite Intrusion in the Old Red Sandstone of Monmouthshire’ 
‘EQ. J. G.S. vol. Ixvii (1911) p. 472 

2 See J. H. L. Vogt, ‘On bates foie Norite with Porphyritic Labora 
- Crystals, be. Oran G. S. vol. Ixv (1909) p. 87; and also J. J. H. Teall, ‘ The 
‘Geology of the Glasgow District’ Mem. Geol. Surv. Scot. 1911. p- 128. 

3 A. Geikie & J. J. H. Teall, ‘On the Banded Structure of some Tertiary 
«Gabbros in the Isle of-Skye’ Q.J.G.S. vol. 1 (1894) pl. xxvii. 


part 4] NORITE OF SIERRA LEONE. 329 


The presence of ilmenite can occasionally be detected by charac- 
teristic cleavage and also by white patches and highly refracting 
borders of leucoxene; generally, however, the iron-ore is too fresh 
to show these decomposition-products. Narrow borders of pyroxene 
also occur, and scraps of biotite are often associated with the ore. 

Although iron-ore occurs so persistently in the norite, and may 
even occur locally as numerous ‘schlieren’ up to 3 inches in thick- 
ness and as small masses several inches long, there are unfortunately 
no indications that the ore is anywhere sufficiently concentrated to 
be worth working for industrial purposes. 


Hornblende.—Green and brown varieties of this mineral oceur 
in the aplite-veins, frequently intergrown with biotite. It is 
absent from the norite, except where produced by contact-meta- 
morphism (see below, p. 34:1). 


Biotite.—This mineral normally occurs in the norite only as 
minute brown flakes associated with the iron-ores; it is fairly 
common in the aplite-veins and in the dolerite, and frequently 
arises in the norite as a result of contact-metamorphism. 


Apatite.—As in the 8t. David’s Head rocks,! this mineral 
oecurs post commonly in the more acid members of the complex. 
It forms countless minute needles and numerous grains and prisms 
in the aplite-veins, and occurs abundantly also as small needles in 
the interstitial matter of the dolerites. 


Zircon occurs sparingly in the complex 1 in the form of minute 
prisms, and rutile hes. been found in concentrates of river- 
gravels. 


(4) Intergrowths of the Common Minerals, and 
Crystallization of the Norite-Magma. 


One of the most interesting features of the Sierra-Leone norite 
is the number of binary and ternary intergrowths occurring 
between the principal minerals. 

Prof. J. H. L. Vogt,? in an investigation into the sequence and 
process of crystallization in gabbroidal rocks, noted in the 
labradorite-norite of the Lofoten Islands a ternary system which 
is closely paralleled in the Sierra-Leone norite. The labradorite- 
norite referred to was ‘chemically and mineralogically on the 
boundary between labradorite-rock and norite’; it was porphyritic, 
with xenocrysts of labradorite. Vogt says (op. cit. pp. 91, 95) :— 


‘We may distinguish between the following three stages of crystallization :— 


[A.] (1) Plagioclase alone (phenocrysts) ; 
(2) then plagioclase and magnetite contemporaneously ; 
(38) plagioclase, magnetite, and pyroxene-minerals, besides biotite, 
contemporaneously. 


1 J. V. Elsden, ‘The St. David’s Head Rock Series’ Q.J.G.S. vol. lxiv 
(1908) p. 289. 2 Toid. vol. lxv (1909) p. 81. 


330 DR. F. DIXEY ON THE [ vol. lxxvin, 


‘If then we confine ourselves to the chief minerals, and if we provisionally 
look upon the two pyroxenic minerals as one unit, we find here the same three: 
stages of crystallization as in the ordinary ternary systems consisting of 
independent components.’ 


In extension of this idea, the following series might also be 
expected to occur, since magnetite crystallized earlier than felspar, 
when the ratio of the magnetite-component to the felspar- 
component exceeded a certain quantity : 

B. (1) Magnetite. 
(2) Magnetite and felspar (simultaneously). 
(3) Magnetite, felspar, and pyroxene (simultaneously). 


Both series A and B actually occur in the Sierra-Leone norite, 
even to the development of phenocrysts of labradorite in the 
less coarse varieties. Moreover, with a slightly increased propor- 
tion of pyroxene, the following modification of A is sometimes 
produced :— 

C. (1) Felspar. 
(2) Felspar and pyroxene. 
(3) Felspar, pyroxene, and magnetite. 

(The minerals of stage 2 and of stage 8, respectively, in this. 
and the following systems crystallize simultaneously. ) 

There is also evidence in the formation of other binary and 
ternary intergrowths, graphic or otherwise, of the existence in the 
magma of a number of additional ternary systems, due to the 
inclusion of olivine as a fourth independent component; they 


include :— 
D. (1) Felspayr. 
(2) Felspar and olivine. 
(3) Felspar, olivine, and magnetite. 
And E. (1) Pyroxene. 
(2) Pyroxene and olivine. 
(3) Pyroxene, olivine, and magnetite. 


The development of the above ternary systems in the porite: 
does not imply great variations in the composition of the magma 
as originally intruded, nor general variations in the resultant rock. 
There is evidence from the experiments of A. L. Day & E. S. 
Shepherd,! that magmas differing slightly in composition will, at. 
different temperatures, give rise to a limited number of eutectics. 
which differ considerably one from the other in composition and in 
the character of the minerals formed. 'The experiments referred 
to were made upon the lime-silica series of minerals, for which 
three eutectic points were found; but it seems very probable that 
the results would apply in some measure at least to the crystal- 
lization of more complex magmas such as occur in nature. Local. 
variations in composition within the magma were probably set 
up in the course of crystallization and cooling, and they were. 
doubtless compheated also by more or less superfusion and possibly 
viscosity.” 


1 Amer. Journ. Sci. ser. 4, vol. xxii (1906) pp. 265 et seqq. 
2 J. V. Elsden, c Principles of Chemical Geology’ 1910, p. 151, 


part 4] NORITE OF SIERRA LEONE. 331 


In rare cases there is even seen a quaternary intergrowth 
composed of felspar, pyroxene, olivine, and magnetite. This inter- 
growth is of considerable interest, because it contains all the 
principal components of the rock, and is therefore practically the 
theoretical eutectic of the magma. An almost parallel case has 
been described by Dr. R. H. Rastall! from the Skiddaw Granite, 
in which a quaternary eutectic of four of the five principal com- 
ponents is sometimes developed. 

The following is a brief description of the more interesting 
intergrowths of the Sierra-Leone norite and of corresponding 
intergrowths in other rocks :— 


(i) Binary.—Plagioclase and augite form two types of 
graphie intergrowth in the norite. The first type is obviously 
primary, and oceurs as narrow parallel rods of augite penetrating 
the felspar (see Pl. XVIII, figs. 5 & 6); a similar intergrowth has 
been noted in the Norwegian syenite-pegmatites,? in the coarser 
parts of the Rowley basalt, and in the Duluth gabbro.4 The 
second type is of slightly later age, and generally takes the form 
of a corona or reaction-rim partly enclosing biotite and olivine at 
their contact with plagioclase; it is developed only on the plagio- 
clase side of the contact and spreads more or less fanwise into 
the felspars (see Pl. XIX, fig. 1). It is in general readily distin- 
guished from the first type of intergrowth, both by its finer texture 
and by its relation to other minerals, particularly the felspars. The 
manner in which it is seen to spread through the felspars indicates 
that, although formed after the main period of crystallization of 
the rock, it was nevertheless connected in some way with the final 
stages of consolidation. That it is unlikely to be a secondary 
alteration-product is shown by the beautifully fresh condition of 
all the minerals of the rock. With regard to the production of a 
similar intergrowth in the Duluth gabbro, Nebel writes (op. cit. 
pp. 3876-77 & pl. xiv, fig. b) :— 


‘The evidence is not conclusive, however, as to whether the process was a 
phase of the last stages of crystallization of the magma, or whether it 
was a metamorphic process which took place after the rock was completely 
solidified. The fact that the gabbro shows many metamorphic features 
indicates that a metamorphic origin for the intergrowth is possible.’ 


Dr. J. J. Sederholm clearly favoured the theory that this inter- 
growth was of a secondary nature, and formed by metamorphic 


1 ¢The Skiddaw Granite & its Metamorphism,’ Q. J.G.8. vol. Ixvi (1910) 
xp. 120. 

2 W. C. Brogger, ‘ Die Mineralien der Syenitpegmatitginge der Siidnorweg- 
ischen Augit- & Nephelinsyenite’ Zeitschr. Krystallogr. vol. xvi (1890) 
pp. 149-52. 

3 J.J. H. Teall, ‘ British Petrography ’ 1888, p. 212 & pl. xxiii, fig. 2. 

4M. L. Nebel, ‘The Basal Phases of the Duluth Gabbro, near Gabami- 
chigami Lake (Minnesota) & its Contact-Effects’ Econ. Geol. vol. xiv (1919) 
p. d/l & pl. xiv, fig. a. 


Q.J.G.8. No. 312. 2A 


352 DR. F. DIXEY ON THE [vol. Ixxvitl, 


agencies later than the solidification of the rock.! In the Sierra- 
Leone norite, however, the intergrowth is frequently developed in 
places where it appears very unlikely that metamorphic agencies. 
have had any influence. It is, nevertheless, true that intergrowths. 
of felspar and augite are more abundant in the contact-altered 
norite than in the unaltered rock (see below, p. 338 & Pl. XIX, 
fig. 6), but this is due only to the increased opportunity of crystal- 
lization afforded by the great heat of the intrusions, whereby 
the two minerals, already present in eutectic proportions, could 
recrystallize in graphic intergrowth. 

Plagioclase and olivine form relatively rare intergrowths 
that follow a crude graphic plan; the small blebs and short laths 
of olivine embedded in the felspar are in optical continuity with a 
neighbouring larger crystal of olivine. Similar occurrences have 
been noted and figured by W.S. Bayley? and M. L. Nebel. These 


minerals occasionally enter also into simultaneous crystallization 


after the fashion of anorthite and olivine in the allivalites of 


Rum.* : 
Olivine and magnetite form a graphic intergrowth recalling 


the dendritic inclusions of magnetite in olivine. Inter growths of 
these minerals have been de abed from certain tocie in Aln6.® 
In highly ferriferous parts of the norite, augite and magnetite 


may frequently be seen to bear simular interstitial relations to the 


remaining minerals; sometimes, in these circumstances, they enter 


into a graphic intergrowth consisting of rods of the two minerals 


lying parallel to the cleavage of the augite. Intergrowths of these 


two minerals have also been noted by Dr. A. Harker.6 Moreover, 


hypersthene and magnetite enter into intergrowth in the 


form of plates of hypersthene enclosing graphic magnetite (see 
Pl. XIX, fig. 2). Intergrowths of the monoclinic and rhombic 
pyroxenes have been described above (see p. 326). 

The following intergrowths also occur—(a) pyroxene and 
olivine’; (d) plagioclase and magnetite, in which rods and 
narrow tongues of magnetite penetrate the felspar’; and (c) in 
the aplite-veins, hornblende and biotite. 


(ii) Ternary.—The most striking ternary intergrowth is that 


composed of parallel rods of felspar, pyroxene, and magnetite 


1*On Synantectic Minerals & Related Phenomena’ Bull. Comm. Géol. 
oe No. 48 (1916) pp. 9-46. 
‘The Basic Massive Rocks of the Lake Superior Region’ Journ. Geol. 
a i (1898) p. 709. 
3 Econ. Geol. vol. xiv (1919) p. 372. 


4 <The Geology of the Small Isles of Inverness’ Mem. Geol. Sury. Seot. 


1908, p. 88. 

®»> R. Workman, ‘ Calcite as a Primary Constituent of Igneous Rocks’ 
Geol. Mag. 1911, p. 193. 

6 «The Natural History of Igneous Rocks’ 1909, p. 271. 


7 Compare Vogt’s researches on slags, ‘ Die Silikatschmelzlésungen’ pt. i 


(1903) Vidensk. Selsk. Skrifter, Christiania. 


8 See also J. H. L. Vogt, ‘On Labradorite-Norite with Porphyritic Labra~ 


dorite-Crystals, &c.’ Q. J. G. 8. vol. lxv (1909) p. 81. 


Ss 


part 4] NORITE OF SIERRA LEONE. 500 


[C 68]. (See Pl. XVII, fig. 6.) <A second system, similar in 
many respects to one seen in the Duluth gabbro,! consists of 
pyroxene, olivine, and magnetite. In certain of these intergrowths 
minerals form angular and wedge-shaped masses, which irregularly 
interlock and enclose one another. Interlocking rhombic and 
monoclinic pyroxenes are occasionally associated with one other 
mineral in ternary intergrowth. 


(iu) The principal minerals of the quaternary intergrowths are 
pyroxene, magnetite, and olivine; felspar enters in as the fourth 
constituent, intergrown graphically with one or more of the other 
minerals. These intergrowths are similar to the quaternary 
eutecties noted by Prof. Lacroix in certain pyroxene-gneisses.2 
Intergrowths of this order are sometimes formed also from the 
ternary intergrowths of the norite, through replacement of a simple 
pyroxene by intergrown rhombic fn Pienoclinie pyroxenes.? 


It is possible to express in a simple diagram the variable minera- 
logical composition of the norite and also the development of the 
ternary systems from magma of a given composition. It will be 
assumed in that part of the diagram which illustrates the sequence 
of crystallization in the ternary systems, that the crystallization 
proceeds without interruption or modification due to the formation 
of mix-crystals. This assumption is justified, with the minor 
exceptions of olivine and pyroxene taking up small amounts of 
magnetite and titanic oxide respectively, and of felspar absorbing 
small quantities of other minerals. Substances held in solid 
solution in this manner tend to be precipitated on the cooling of 
the rock, and to assume the form of minute inclusions. 

Suppose that the oxides $i0,, Fe,O,. FeO, CaO, MgO, Na,O, con- 
stituting the principal chemical components of ie norite-magma, 
are placed at the angles of a pentagon (see fig. 2, p. 8334). The 
area, as a whole, repr esents the general composition ‘of the magma ; 
the point SiO, represents the mineral quartz, and the points AJ,O, 
and Fe,0,. FeO, represent the minerals alumina and magnetite 
respectively ; the areas CaO, Al.O,, Si0,, and MgO, Fe,O,, 810,, 
felspar and olivine respectively ; whereas the central part of the 
pentagon represents pyroxene. Now take a felspar, magnetite, and 
a pyroxene represented by the points A, B, C respectively; any 
point P within the area A BC will represent a combination of 
these three minerals, in proportions which are inversely as the 
distances of the point P from the points A, B,C. The composition 
of the magma representing the ternary eutectic of the felspar, mag- 
netite, and pyroxene can then be found as follows :—Erect ordinates. 


1M. L. Nebel, Econ. Geol. vol. xiv (1919) Dp: a2. 
2 « Gontributions al’Etude des Gneiss 4 Pyroxéne & des Roches 4 Wernérite ” 


Bull. Soc. Min. France, vol. xii (1889) p. 83. 
3 See J. H. L. Vogt, ‘On Labradorite-Norite with Porphyritic Labradorite-- 
Crystals, &c.’ Q. J. G. S. vol. Ixv (1909) p. 81. 
4 See J. V. Elsden, ‘Principles of Chemical Geology’ 1910, p. 182. 
242 


334 DR. F. DIXEY ON THE [vol. lxxviii, 


Aa, Bb, Ce to represent temperatures, the points a, b, ¢ being the 
freezing-points of the minerals A, B,C. Through a, b, and e will 
pass freezing-point surfaces for mixtures of A, B, and GC, 
respectively, with the remaining two minerals.! Lach pair of 
surfaces will intersect in a line formed by the eutectic points of 
the pair of minerals at different temperatures; the three lines 
representing the binary eutectics will meet in a point representing 
the ternary eutectic. Let EH, E,E, and E,E, represent the 
projection of the three lines on the area A B C; then the inter- 
section (E) of these lines is the required point. 

The order of events in the cooling and crystallization of a 
magma of composition P will be as follows: an indicating point 
vertically above P will descend as the magma cools, until it reaches 


Fig. 2. 
MeO. 


D 


Olivine 


Coy, 
Alo OF 25 


the freezing-point surface of the felspar (A) ata; then the felspar 
will begin to crystallize out, and the indicating point will travel 
along the surface on a line (the projection of which is PQ) directly 
away from a, until it meets the felspar-magnetite eutectic line at a 
point projected at Q. The magnetite will then crystallize in 
eutectic proportions with the felspar, and the indicating point will 
travel down the binary eutectic line of these two minerals until it 
reaches the ternary eutectic point, which is projected to HE. Then 


1 See A. Harker, ‘The Natural History of Igneous Rocks’ 1909, fig. 60, 
p. 202. The notation which I have adopted is after that employed by him. 


part 4:] NORITE OF SIERRA LEONE. 335 


the three minerals (felspar, magnetite, and pyroxene) will proceed 
to crystallize together in eutectic proportions and at a stationary 
temperature. 

Similarly, it will be seen that in the cooling of a magma of 
composition X, magnetite would crystallize out “until the residue 
attained the composition Y, when felspar would crystallize with 
the magnetite; the remaining magma would finally attain the 
composition H, at which the ternary eutectic (magnetite, felspar, 
augite) would be formed, as before. 

The ternary eutectic of a pyroxene C,, an olivine D, and 
magnetite B, or of any other of the ternary systems described 
above, could also be expressed by this method. 


(5) The Order of Crystallization. 


It is obvious from the foregoing remarks that the order of 
erystallization in the norite-magma was far from being in 
accordance with the law, named after Rosenbusch, which commonly 
holds in the case of basic magmas. ‘The principal departures from 
this law rest in the early crystallization of the labradorite, the 
prolonged and generally late crystallization of the iron-ores, 
the variable crystallization of the olivine and pyroxenes, and the 
late crystallization of the biotite. The order of crystallization 
followed the law, however, in the early precipitation of the apatite 
and zircon. Prof. J. H. L. Vogt’s more sunple law, demonstrated 
by his researches on slag,! that the order of crystallization 1S 
determined by the relative proportions of the several minerals 
present as compared with the eutectic proportions, seems to have 
been the leading principle. This is confirmed by the occasional 
development of a quaternary intergrowth which closely approaches 
the theoretical eutectic of the magma. With regard to similar 
conditions in the Skiddaw Granite (see above, p. 831) Dr. R. H. 
Rastall concludes :-— 


‘Tt appears to follow that the order of crystallization was that which finally 
led to a eutectic composition which expressed itself as a graphic intergrowth 
of a varying number of components. The large and comparatively pure 
phenocrysts represent the excess of certain components over this eutectic 
raion = (Q.3.G. S. vol. Ixviy LOLO; p.. 1211;) 


Nevertheless, certain anomahes in the crystallization of the 
norite were possibly due to the influence of superfusion and 
viscosity; superfusion itself may cause not only reversals in 
the order of crystallization, but also may result in a marked 
sequence of separation in place of the simultaneous crystallization 
of the eutectic.2 The early precipitation of the apatite and zircon 


1< Die Silikatschmelzlésungen’ pt. i (1903) Vidensk. Selsk. Skrifter, 
Christiania. 

2 W. Meyerhoffer, ‘Schmalzpunkte & Ausscheidungsfolge von Mineralien ’ 
Zeitschr. Krystallogr. vol. xxxvi (1902) pp. 5938 et seqq.; see also J. V. Elsden, 
‘Principles of Chemical Geology’ 1910, pp. 151 & 154. 


336 DR. F, DIXEY ON THE [ vol. lxxviii, 


is probably due to the immiscibility of these compounds with the 
common aluminous silicates.! 

The actual order in which the minerals crystallized when not 
too strongly affected by eutectic proportions was as follows :— 
(1) apatite ‘and zircon; (2) magnetite, olivine, and pyroxenes in 
small amount; (3) labrador ite; (4) most of the olivine; (5) most 
of the pyroxenes; (6) most of the magnetite; (7 ‘) biotite ; 
(8) micropegmatite and quartz. This ai agr ee closely with 
that of the Duluth gabbro.2 The periods of crystallization, other 
than (1) and (8), were not always sharply detined, however, and a 
considerable amount of overlapping ensued: for instance, the 
magnetite, although occurring principally as in the above list, 
nevertheless kept up a certain amount of crystallization all through 
the periods, almost to the end. The magnetite of certain basalts 
in Franz Josef Land behaves in a similar way, as was pointed out 
by Sir Jethro Teall as long ago as 1897 (Geol. Mag. p. 554). 

A little biotite formed on some of the earlier crystals of mag- 
netite, and, in general, diallage preceded hypersthene. 

Finally, it is worthy of note that, although crystallization in the 
principal member of the norite-complex did not follow the common 
sequence, the different members of the complex were nevertheless 
intruded in the normal order: that is, that of decreasing basicity. 


(6) Contact-Metamorphism within the Complex. 


Some interesting contact-effects have been observed as a result 
of the invasion of one member of the complex by another. These 
effects generally took the form of corrosion and recrystallization. 
Sometimes, however, small quantities of hornblende and biotite 
were developed; but, apart from these, no new minerals were 
produced. This is in the main what would be expected, since the 
composition of any one principal intrusion did not differ very 
much from that of another. 

Such changes as have taken place at the Sierra-Leone contacts 
are more closely paralleled in certain cases described by Prof. 
Lacroix? from the Central Plateau of France and elsewhere, 
rather than in any of the British occurrences. Most of the latter 
refer to the alteration by plutonic masses, not of basic plutonie 
rocks, but only of basic lavas; and it is uncertain in most instances 
how far secondary products had developed in the lavas before they 
‘were invaded by the new magmas.* Consequently, these cases 
cannot often be taken as good examples of the thermal metamor- 
phism of basic rocks. The British occurrences to which I would 


1 A. Harker, ‘The Natural History of Igneous Rocks’ 1909, p. 200; and 
J. V. Elsden, ‘ Principles of Chemical Geology ’ 1910, p. 135. 

2M. L. Nebel, Econ. Geol. vol. xiv (1919) p. 372. 

3 ‘Tes Enclaves des Roches Voleaniques ’ Ann. Acad. Macon, ser. 2, vol. x 
(1893). 

* See A. Harker & J. E. Marr, ‘ Re een Notes on the Metamorphic 
‘Rocks eee the Shap Granite’ Q. J. G. S. vol. xlix (1893) p. 360. 


bn 


part 4 | NORITE OF SIERRA LEONE. Dol 


particularly refer are the alteration of basic lavas by the Shap 
Granite,! and of the Eycott lavas by the Carrock-Fell Gabbro ? and 
the Eskdale Granite. The changes induced in these cases * consist 
essentially of the conversion of pyroxenes to amphiboles, the 
recrystallization of the felspars into a mosaic, and the development 
of brown biotite; locally, garnets, sphene, and other minerals are 
developed. The Grainsgill Granophyre® altered the augite of 
the Carrock-Fell Gabbro locally into compact brown hornblende, 
formed granular sphene from the ilmenite and felspar, and con- 
verted the felspar into secondary minerals. Examp'es are given 
in the Skye Survey Memoir of gabbro enclosing xenoliths (1) of 
ultrabasic rocks and (2) of an earlier gabbro; in the first case 1t 
is stated that no clear indication of thermal metamorphism occurs, 
and in the second no mention of any alteration is made.® 

Prof. Lacroix, in the course of his studies on the inclusions of 
voleanie rocks, was able to consider in detail the effect of basic 
magmas on coarse-grained basic and ultrabasic xenoliths; but, so 
far as possible, he satisfied himself beforehand that the xenoliths 
were still fresh at the time when they were first attacked by the 
magma. In a later paragraph his results will be compared with 
those obtained in Sierra Leone. Although in the course of building 
up the complex deseribed above, the invading magmas themselves 
did not suffer any appreciable modification, the rocks invaded were 
nevertheless subjected to a more or less intense contact-metamor- 
phism. The more important examples of this metamorphism occur 
in the older or normal norite where it has been invaded by :— 


(1) A. The younger norite. 
B. The beerbachite. 


(2) A. The norite-aplite. 
B. The dolerite. 


The alteration of the older norite effected by (1) the younger 
norite and the beerbachite, differed from that due to (2) the 
aplite-veins and the dolerite, in the following important par- 
ticulars :— 


(a) Magnitude.—tThe intrusions of the first group were much larger than 
those of the second group, and accordingly they were able to incorporate 
innumerable large and small fragments of the older norite. An examination 
of these fragments has afforded excellent material for a study of the corrosion 


1 A. Harker & J. E. Marr, Q. J. G. S. vol. xlix (1898). 

2 A. Harker, ‘ Carrock Fell; a Study in the Variation of Igneous Rock- 
Masses: Part I—the Gabbro’ Ibid. vol. 1 (1894) p. 334. 

3B. E. Walker, ‘Notes on the Garnet-bearing & Associated Rocks of 
the Borrowdale Volcanic Series’ Ibid. vol. lx (1904) p. 102. 

4 See also the metamorphism of basalt by Cuillin gabbro, ‘ The Geology of 
Glenelg, Lochalsh, & the South-Hast Part of Skye’ Mem. Geol. Surv. Scot. 
1910, p. 145; and ‘ The Tertiary Igneous Rocks of Skye’ Mem. Geol. Surv. 
1904, p. 122. 

5 A. Harker, Q. J. G.S. vol. 1 (1894) p. 334. 

6 «The Tertiary Igneous Rocks of Skye’ Mem. Geol. “trv. 1904, pp. 99 
& 121, 


538 DR. F. DIXEY ON THE [vol. xxvii, 


of xenocrysts. The results obtained confirm the remarks made above as to 
the importance of assimilation in certain of the Sierra-Leone intrusives, and 
are, moreover, of especial interest in that they reveal the mechanism by which 
an intrusion may, in certain cases, absorb large quantities of the rock which 
it invades. No xenocrysts have been found, however, in the norite-aplite and 
the dolerites ; consequently, alteration due to these intrusions has to be 
looked for in those portions of the norite which are near the contacts. 

(6) Character.—tThe intrusions of the second group, particularly the 
norite-aplite, were relatively more acid in composition than those of the first 
group. Hence the contact-effects of the second group are locally due to the 
infusion of new material, even more than to the application of heat; this has 
resulted near the margins of these intrusions in a more intense alteration, 
combined with the development of new minerals in the norite, especially 
hornblende and biotite. The alteration produced by the younger norite and 
the beerbachite, on the other hand, may be ascribed almost entirely to the 
effect of heat, since any infusion that might have taken place could not have 
altered the composition of the invaded rock very appreciably. 


(1) A. Alteration of norite by younger norite.—The 
various minerals of the normal norite have been aftected as 
follows :— 


(a) Felspar.—This mineral shows corrosion and recrystal- 
lization, distortion and irregularity of twin-lamellz, and also a 
patchy, irregular extinction. Cleavage and other cracks are abun- 
dant. The magnetite-microliths of the felspar are sometimes: 
replaced ‘by exceedingly minute, pale-green, rounded grains, and 
much micrographic felspar and augite are developed, both fine and 
coarse. The coarse micrographic augite is enclosed in clear felspar, 
in which twin-lamelle may sometimes be distinguished. A string 
of augite-granules, associated with a little magnetite and biotite, 
frequently runs along the contact between adjacent felspars, and on. 
each side of the string is a narrow zone of clear recrystallized 
felspar. The fine variety of intergrowth generally originates at 
the contact of a ferromagnesian mineral with felspar, and then 
spreads fanwise into the felspar. It should be noticed that the 
above two intergrowths of felspar and augite occur also in the 
unaltered norite (see p. 332), in which the coarse variety can 
frequently be demonstrated to be of primary origin; whereas 
the fine variety is often secondary (that is, formed shortly after 
the main period of consolidation of the rock). In the thermally- 
altered rock, however, these structures are abundantly and charac- 
teristically developed, probably because the gentle heat-gradient 
set up at comparatively low temperatures gave the minerals a 
better opportunity for growing together in oraphic fashion than 
they had had in the cooling of the main body. 


(6) Augite.—Outlines are embayed and irregular; there are 
lines of granules of felspar and magnetite apparently due to 
cr ystallization along cracks and in pores ic 120]. A granular recry- 
stallization of the augite may occur. Magnetite is thrown down 
in abundance ; much of it takes the form of long narrow lenticles 
generally parallel to the cleavage, with numerous thread-lke 


part 4] NORITE OF SIERRA LEONE. — 309" 


microliths projecting at right angles. The magnetite occurs also 
as numerous crystallites and clots ; individually these are generally 
parallel to the cleavages, but they are more closely packed along. 
certain twin-lamelle. All these features are confined principally 
to the inner portions of the crystals, since there is generally a 
narrow outer zone free from them; this outer zone points to 
renewed crystallization (see Pl. XIX, fig. 4). The normal faint 
pleochroism of the augite is sometimes increased. A patchy 
polarization may be produced, generally in bright colours, but 
sometimes in greys. Flakes of brown biotite appear in some cases: 


[C1720]. 


(c) Hypersthene.—Magnetite thrown down, both as clots and 
in coarse dendritic form. The hypersthene is less susceptible to 
alteration than the augite 1s. 


(d) Olivine.—Deeply embayed outlines are common; cracks are 
unusually numerous, and many of them show ferruginous stains. 
Much serpentine is produced, particularly near the margins of the 
erystals. In some cases numerous flakes of biotite are formed 
around the margins [C170]; in others, where the olivine has 
recrystallized in granular form near the edges, small crystals of 
augite! and of hypersthene may be included (C 172 5]. Magnetite 
is frequently thrown down as clouds of dust or in dendritic 
fashion. 


(1) B. Alteration of norite by beerbachite.—The most 
important feature of this alteration is the extensive corrosion of 
xenocrysts; olivine and hypersthene, in particular, have been 
attacked so vigorously that they have been reduced to mere 
spongy skeletons (see Pl. XIX, fig. 3). Augite also has been 
actively corroded, with deposition of much iron-ore. Where cry- 
stallization has recommenced, however, the augite-crystals have 


5 
grown again; they have thus acquired wide clear zones free from 


inclusions of magnetite (see Pl. XIX, fig.4). No biotite has been 
detected in this type of alteration. Apart from these features, 
the contact-action of the beerbachite has been similar to that of 
the younger norites. 

From consideration of numerous xenoliths in the beerbachite, 
it appears that the various minerals of the norite were dissolved in 
the following order :—magenetite, felspar, augite, hypersthene, and, 
finally, olive. Large magnetite-xenocr ysts are rarely seen —they 
disappear at an early stage; numerous small and more or less: 
rounded xenocrysts of augite may be recognized, and the felspars 
are in due course reduced to the average size of the beerbachite 
felspars; hypersthene and olivine are the most resistant, and large 
spongy masses of these minerals, with their rounded pores occupied 


! Prof. C. Deelter has shown that the formation of augite from olivine and 
felspar is a reversible action; see A. Harker, ‘The Natural History of Igneous 
Rocks’ 1909, p. 168. 


~b40 DR. F. DIXEY ON THE (vol. lxxviul, 


by granules of felspar and augite, are a common feature of the 
beerbachite near its contact with the norite. This interesting 
succession corresponds fairly well with the order of fusibility 
of these minerals as determined experimentally,! and may be 
compared with similar results obtained by Prof. Lacroix ? from 
observations on the behaviour of olivine-nodules and other basic 
fragments enclosed-in coarse basic dolerites. 

Lacroix’s observations on the manner in which the individual 
minerals of basic xenoliths are affected by inclusion in a highly 
heated basic magma do not differ in any important respect from 
those given above; the chief differences noted in his work are as 
follows :— 


(a) Felspar.—Micrographic intergrowths of felspar and augite are not 
developed ; these minerals recrystallized in granular form. 

(8) Hypersthene.—In general, this mineral was converted into granular 
augite; but sometimes it recrystallized into both augite and olivine. 

(y) Olivine.—Commonly converted into granular augite and magnetite, 
or into granular olivine; no biotite was developed. A notable 
feature in the alteration of this mineral was that it frequently 
recrystallized into a form quite unlike that habitual to it: it assumed 
the habit of long-bladed crystallites which, elongated in the direction 
of the C-axis, locally grew parallel one to the other so as to form 
the skeleton of a large crystal of clivine. 


These differences, particularly the greater frequency of fine 
granular forms in the minerals described by Lacroix, may be 
ascribed to the fact that the xenoliths studied by this author had 
been incorporated in a magma crystallizing under voleanie or at 
most hypabyssal conditions; whereas those considered in the pre- 
sent paper were attacked by magmas subjected wholly to plutonic 
conditions. Further, the phenomena observed in the Sierra- Leone 
rocks took place on a much larger scale. 


(2) A. Alteration of norite by norite-aplite. — The 
zone of alteration produced in the norite by a narrow vein of 
norite-aplite is generally many times wider than the vein itself. 
The zone may be regarded as consisting of an inner zone, in which 
the alteration is most intense, and an outer zone, with less 
intense alteration. The minerals of the two zones are affected as 
follows :— 

Felspar: inner zone.—The crystals are rendered clearer; 
this is probably due to loss of magnetite-microliths, to the intro- 
duction of silica and orthoclase, and also to a certain amount of 
recrystallization. Further, the twin-lamelle of the felspars become 
vague or even indistinguishable, the outlines of the crystals become 
sinuous, and a curious patchy extinction is developed. The refrac- 
tive index of the felspar does not, however, fall below that of 
Canada balsam. Numerous shreds of hornblende are developed. 


1 A. Harker, ‘ The Natural History of Igneous Rocks’ 1909, p. 156. 


9° 


* *TLes Enclaves des Roches Voleaniques’ Ann. Acad. Macon, ser. 2, vol. x 
(1893) pp. 483 et seqq. 


part 4] NORITE OF SIERRA LEONE. 341 


Outer zone. —On the whole, the felspar shows shghtly 
increased turbidity, but many erystals are crossed by clear bands 
containing granules and microliths of hornblende and pyroxene. 
The felspar-crystals are, moreover, separated by narrow clear areas 
in which granules of the ferromagnesian minerals occur. Much 


p. 5 . . . . . 
additional micrographie felspar and augite is developed. 


Augite: inner zone.—This mineral is represented only by 
plates and spongy masses of green hornblende, crowded with 
innumerable regularly-arranged microliths and clots of iron-ore ; 
even the iron-ore tends to disappear finally (see Pl. XIX, fig. 5.) 

Outer zone.—The augite shows the usual additional precipi- 
tation of iron-ore, and moreover is often traversed by brown 
streaks developed along cracks and along certain twin-lamelle 
[C 172d]. These streaks are frequently accompanied by numerous 
flakes of brown biotite; the flakes, which are often parallel, may 
extend outwards into the adjacent felspar [C 172d]. The augite is 
generally fringed with green hornblende, which increases in amount 
as the inner zone 1s approached. 


Hypersthene: inner zone.—Generally, only the inclusions 
of iron-ore are recognizable; the mineral itself has been converted 
into green hornblende. 

Outer zone.—Often heavily charged with iron-ore, both in 
the form of microliths and of a coarse graphic intergrowth with 
the hypersthene. The mineral shows increasing alteration to green 
hornblende. 


Olivine: inner zone.—The crystals are not recognizable in 
this zone. 

Outer zone.—Much iron-ore is thrown down, with the develop- 
ment of numerous brown and green serpentinous veins. The 
mineral is sometimes converted into a mesh of yellow-green 
serpentine, with flakes of brown biotite around the margins 
[C 172d}. Generally, however, it alters to green hornblende (see 
PIOXTX, fig:6). 


(2) B. Alteration of norite by dolerite.—In one speci- 
- men [C72] taken from near the contact, the felspars are turbid 
and extensively altered, both to calcite and to a colourless mica in 
the form of minute tufted flakes. The pyroxenes are more or less 
completely altered to green hornblende; and much magnetite is 
thrown down as a fine dust along innumerable cracks. 

In another specimen [C1726] hornblende is developed only 
about the contact; elsewhere the ferromagnesian minerals are 
largely altered to flakes of brown biotite. Much corrosion of the 
older rock has taken place, and the junction with the dolerite is 
very ill-defined. The felspar, for some distance from the contact, 
shows evident recrystallization, and in its turbidity and minute 
twin-lamellation it is scarcely to be distinguished from the felspar 


342 DR. F. DIXEY ON THE (vol. lxxvii,. 


of the dolerite. Also, micrographic structures are abundantly 
developed in the felspar, and granules of augite run along its 
boundaries. The pyroxenes and the olivine all show more or less 
complete recrystallization, and they are, moreover, granulated 
around their margins; the granules along the margins of each 
mineral are intermixed with those of the other minerals, and also 
with flakes of biotite. 


In conclusion, I wish to express my thanks to Prof. A. H. Cox, 
of the University College of South Wales & Monmouthshire, 
Cardiff, to whom I am indebted for many valuable suggestions in 
the course of this work and also for laboratory accommodation. 
during the examination of specimens. 


IV. SUMMARY AND CONCLUSIONS. 


(1) The norite of Sierra Leone constitutes a complex, of which 
the oldest and most important member is an olivine-norite. The 
complex forms the mountainous mass which, together with a 
narrow coastal plain of Pleistocene sediments, makes up the Sierra- 
Leone peninsula. 

(2) The norite differs from other well-known noritic intrusions 
in its great size, apparent batholithic character, and lack of exposed. 
marginal or basic phases; its junction with older rocks is obscured 
by the Pleistocene sediments. 

(3) The complex is believed to be of very great age, although 
probably later than pre-Cambrian; it belongs to the West African 
Magnesian Province. 

(4) The main intrusion of norite was invaded in succession by 
the following related minor intrusions:—(1) norites of slightly 
different character, which generally possessed a coarser texture 
(11) norite-pegmatite; (111) beerbachite; (iv) norite-aplite; (v) 
dolerite. 

(5) The first intrusion of norite consists essentially of labra- 
dorite, augite, and hypersthene, with varying proportions of olivine 
and iron-ore (titanomagnetite). The rock is beautifully fresh, and 
it generally exhibits stiong flow-banding. In addition to the 
intrusions already mentioned, the older norites are cut by segre- 
gation-veins, by veins emanating from the younger norites, and by 
small lenticles of a felspar-olivine rock. 

(6) An interesting series of binary and ternary intergrowths of 
the common minerals has been observed in the ‘norite, indicating 
the importance of eutectic conditions during the crystallization 
of the magma. Consequently, there was very often no regular 
order of crystallization among the principal minerals. 

(7) The younger norites form two or more series of intrusions 
cutting the older norite; but they are collectively of small bulk 
as compared with the original intrusion. in addition to their 
greater coarseness, they are as a rule readily distinguished in the 
field from the older norite by their grey colour and scaly weathered 
surface, and also by the absence of banding and jointing. In thin 


part 4: NORITE OF SIERRA LEONE. 343 


section the younger norites, as compared with the older norite, 
generally seem to be richer in hypersthene, in iron-ore, and in 
micrographic intergrowths of felspar and augite; moreover, the 
felspars of the younger norites are slightly more basic. 

(8) The field-relations of the older and younger norites are 
often very complex, owing to the irregular manner in which the 
younger rocks broke through the older; highly-corroded xenoliths 
of the older norite are abundantly included in the younger norites, 
and there is much evidence to show that the older rock has been 
incorporated on a large scale in the younger rocks. 

(9) The beerbachite intrusions are generally small, and of 
irregular form. The larger masses actively disintegrated and 
assimilated the preceding “intrusions of norite. The beerbachite 
consists essentially of fine-grained granular labradorite and hyper- 
sthene, with some augite and iron-ore. 

(10) The norite-aplite veins occur in the norite in the form 
of fine threads consisting mainly of quartz and micropegmatite. 
These threads are, however, only the relatively-acid terminations 
of wider veins which are sometimes seen to attain a thickness of as 
much as 9 inches. The thick portions of the veins are light in 
colour, and possess a pale greenish tinge; they are of medium 
texture, and consist chiefly of acid soda-lime felspar, orthoclase, 
quartz, and micropegmatite, with small quantities of pyroxene, 
hornblende, biotite, and apatite. 

(11) The norite-aplite veins were succeeded by a series of more 
or less ophitie enstatite-dolerite dykes, free from olivine and rich 
in interstitial acid felspars. In many respects, these dykes closely 
resemble the well-known British quartz-dolerites ; they are, never- 
theless, free from quartz and micropegmatite. 

(12) Several stages of differentiation may be distinguished in 
the complex: for instance, deep-seated differentiation, differentia- 
tion during intrusion, and differentiation in place. 

(13) The older norite was subjected to varying degrees of meta- 
morphism by the younger members of the complex. A resultant 
effect of frequent occurrence in the norite was the recrystallization 
of parts of the felspar and augite into a graphic intergrowth of 
these two minerals. ‘The intrusives more closely related to the 
norite in composition caused active corrosion and recrystallization, 
but did not set up new minerals; the relatively acid intrusions, 
however, such as the norite-aplite and the dolerite, converted the 
pyroxenes to hornblende and modified the felspars. besides effecting 
other changes. 

(14) Iron-ores occur in the norite only as disseminated grains, 
as small masses up to several inches in length, and as narrow 
schlieren; they are highly titaniferous. Sulphides and other 
economic minerals often associated with noritic intrusions are rare 
or absent. 

(15) Laterite is developed on the rocks of the complex only to a 
shght degree; it occurs principally on ancient platforms carved 
into the mass, and on parts of the complex characterized by 
numerous joints. 


344 


ig. 1. 


c=) 


Fig. 1. 


* DR. F. DIXEY ON THE [ vol. lxxviii, 
EXPLANATION OF PLATES XVI-XIX. 
PLATE XVI. 


. The Sierra Leone Mountains, looking southwards from the 


summit of Leicester Peak. The photograph shows part of the 
middle ridge of the mountain-mass with Sugar-Loaf Mountain, 
capped by a cloud, on the right. Regent village is seen in the 
upper part of the Orugu Valley on the left of the photograph. (See 
p. 301.) 


. Contact of coarse norite with older or normal norite, 


foreshore, near Godrich. The left-hand portion of the photograph 
shows normal norite, easily recognized by its jointing; in the right- 
hand portion is seen part of a big intrusion of coarse norite, charac- 
teristically free from jomting. (See p. 310.) 


PLATE XViit: 


Coarse norite invading normal norite, foreshore, near York. 
The photograph shows fragments of normal norite embedded in 
coarse norite. The fragments, recognized by a relatively-smooth 
weathered surface, possess deeply corroded outlines due to the 
corrosive action of the invading rock ; moreover, at the right-hand 
end of the big block of norite shown in the photograph, there is 
seen a rock of patchy character due to incomplete assimilation of 
the fragments by coarse norite. (See p. 312.) 


: Beerbachite invading and incorporating normal norite, 


Wilberforce Spur. The photograph shows a large residual 
boulder, about 6 feet long, consisting mainly of beerbachite; on 
the weathered surface may be seen irregular streaks and patches, 
due to fragments of normal norite which have been softened, drawn 
out, and partly incorporated by the beerbachite magma. (See 
p. 315.) 

PuaTe XVII. 


Normal norite [C3], summit of Leicester Peak. A large 
fresh crystal of olivine is cut by the lower end of the vertical wire, 
and two crystals of schillerized diallage are cut by the horizontal 
wire ; hypersthene partly enwraps the olivine and diallage. The 
clear mineral is labradorite. In the top left-hand quadrant is part 
of a crystal of labradorite which is completely enclosed in diallage. 
and hypersthene ; the labradorite of the central part of the section 
is later than the hypersthene. Several grains of iron-ore are 
enclosed in hypersthene. Ordinary light. x 28. (See p. 325.) 


. Beerbachite [C93], Wilberforce. The following minerals are 


present, mostly in granular form :—pyroxenes, chiefiy hypersthene, 
felspar, and black iron-ore. Some of the pyroxenes possess idio- 
morphic outlines. The minerals show a slight tendency to paral-. 
lelism. Ordinary light. x 28. (See p. 326.) 


. Norite-aplite [C172a], near York. The photograph shows 


much fine micropegmatite and also two large crystals of acid 
plagioclase, one of which possesses two sets of twin-lamellz inter- 
secting at right angles. Small patches of felspar with twim striz 
are enclosed in the micropegmatite. The plagioclase, as well as 
the orthoclase,is seen to be graphically intergrown with the quartz. 
At the intersection of the cross-wires is a fine spongy mass of 
enstatite, and adjacent to it, in the lower right-hand quadrant, is 
another mass which is of somewhat less dense texture; the second 
mass consists of minute, more or less parallel, prisms of enstatite, 
all embedded in a plate of felspar which shows fine twin-lamelle. 
Crossed nicols. X 28. (See p. 322 


part 4: NORITE OF SIERRA LEONE. 345. 


Fig. 4. Enstatite-dolerite [C163], south of York. The section con- 
sists principally of zoned plagioclase and augite. In the top left- 
hand quadrant is a patch of clear petted felspar, enclosing 
erains of pyroxene and numerous small needles of apatite; also it 
partly encloses an irregular plate of augite. Interstitial to the 
laths and prisms of plagioclase, and also enclosed in the areas of 
clear felspar, is much granular pyroxene, in the form of both com- 
mon augite and enstatite. The felspar-prisms each consist of 
turbid labradorite surrounded by a narrow zone of clear felspar. 
In the lower half of the figtre is a spongy mass of iron-ore.. 
Ordinary light. x 28. (See p. 324.) 

5. Primary intergrowth(binary) of felspar and augite [C 68], 
Lumley. The figure shows fine and coarse grains of augite 
embedded in clear labradorite. The bigger grains, arranged in rude 
graphic fashion, are in optical continuity with the augite that 
encloses the large plates of magnetite. Ordinary hght. x 28. 
(See p. 331.) 

6. Primary intergrowth (ternary) of felspar, augite, and 
magnetite |C68], Lumley. The minerals depicted are felspar 
(white), augite (dark grey), and magnetite (black). The magnetite 
is largely enclosed in augite that shows characteristic cleavage.. 
Parallel tongues of augite, each enclosing a rod of magnetite, 
project into the felspar. Near the intersection of the cross-wires, 
in the upper right-hand quadrant, the ternary intergrowth is beauti-. 
fully developed in the form of fine parallel rods of the three com- 
ponent minerals. In several parts of the slide the augite is seen to 
be graphically intergrown with the felspar. Ordinary hght. x 28. 
(See p. 333.) 


PruatTEe XIX, 


Fig. 1. Corona-like intergrowth of felspar and augite [C68], 
Lumley. The intergrowth was formed later than that shown in 
Pl. XVII, fig. 5. The minerals depicted are labradorite (pale grey), 
the intergrowth (dark grey), and magnetite (black). The clear 
spaces represent fragments of the slide that have broken away. 
The intergrowth is seen spreading fan-wise into the felspar from 
the contact of this mineral with the magnetite. The felspar- 
erystal enclosed in the magnetite has been largely replaced by the 
intergrowth. Ordinary light. x 28. (See p. 331.) 

2. Graphic intergrowth of hypersthene and magnetite, 
normal norite !C1], Leicester Peak. The minerals depicted 
are the following:—felspar (white), with faint cleavage-cracks ;. 
olivine (pale grey), with thick irregular cracks; hypersthene 
(medium grey), with a fine parallel cleavage ; augite (dark grey), 
locally almost opaque with schiller inclusions; and magnetite 
(black). In the central portion of the section, with a large crystal 
of olivine on the left and a smaller crystal of augite on the right, 
is an irregular plate of hypersthene in which abundant graphic 
magnetite isembedded. The magnetite appears also as large plates 
in the hypersthene. Ordinary light. x 28. (See p. 332.) 


Figs. 3 & 4 illustrate the alteration of norite by a magma of similar 
composition ; namely, beerbachite. 


3. Corroded xenolith of olivine and hypersthene in beer- 
bachite [Mount Aureol, No. 3]. The left-hand half of the 
section consists mainly of olivine, and the right-hand half mainly 
of hypersthene. These minerals have been so extensively corroded 
that they now form merely a spongy network with interstices 


iy t 


Se 


346 


DR. F. DIXEY ON THE [ vol. Ixxviii, 


occupied by granules of felspar. The hypersthene has largely 
recrystallized in granular form; it contains much magnetite, of 
which some at least has been thrown down as a result of thermal 
metamorphism. The olivine has recrystallized locally, but only 
to a slight extent. Along its contact with the hypersthene the 
olivine has been converted into serpentine; this serpentine appears 
black in the photograph. Magnetite has been thrown down along 
cracks in the olivine. Ordinary light. xX 28. (See p. 339.) 


Fig, 4. Corroded augite-xenocryst in beerbachite [C 101], 


Wilberforce.—The minerals seen here are felspar, pyroxene, 
and magnetite. The augite-xenocryst, derived from the norite, 
underwent much corrosion; in the course of this corrosion the 
crystal was rendered almost opaque by the deposition of much 
iron-ore in the form of schiller-inclusions. Then, as the en- 
closing magma began to crystallize, a clear outgrowth formed 
around the xenocryst; this outgrowth is in optical continuity with 
the augite of the original crystal. Ordinary light. xX 28. (See 
p. 339.) 


Figs. 5 & 6 illustrate the alteration of norite by a relatively-acid 
magma: namely, norite-aplite. 


.5. Alteration of augite by norite-aplite [Cll6a], near 


York. In the centre of the field of view is a crystal of augite 
which has been almost completely replaced by hornblende in the 
form of ragged green plates; remnants of the original crystal may 
still be seen as small light-coloured shreds. Numerous grains of 
iron-ore are scattered through the crystal; they were thrown down 
as a result of the alteration. In the lower part of the figure the 
pseudomorph is seen to be in communication with a small vein of 
norite-aplite that was chiefly responsible for the alteration. The 
felspar shown in the figure has also been considerably modified ; 
it has on the whole become less turbid, although numerous inelu- 
sions have developed in it. In the upper left-hand quadrant is a 
patch of turbid felspar which shows only slight traces of alteration. 
Numerous shreds and small spongy masses of hornblende are 
enclosed in the felspar; at least some of these represent augite 
which originally had been intergrown with the felspar in graphic 
form. Ordinary light. X 28. (See p. 341.) 


«6. Alteration of olivine in norite [C 172d], near Mowe 


An altered crystal of olivine is seen in the centre of the field 
of view; part of another similar crystal is seen in the upper left- 
hand quadrant. The olivine has been rendered almost opaque by 
the deposition of much magnetite in the form of dust and small 
grains. Around the margins the olivine has been largely replaced 
by flakes of brown biotite, as may be seen in the upper right-hand 
quadrant and along the lower margin of the crystal of olivine. 
An interesting feature of this contact-alteration of the norite is the 
abundant development of micrographic felspar and augite, which 
in the present instance almost completely surrounds the altered 
olivine, and spreads more or less fan-wise into the adjacent felspar. 
The intergrowth takes the form of small curved rods and blebs of 


_augite embedded in felspar. Ordimary light. xX 28. (See p. 341.) 


Quart. Journ. Geol. Soc. Vol. LXXVIII, Pl. XVI. 


Pia. 1.—-TuHeE Sierra LeonE Mounrains, LOOKING SOUTHWARDS 
FROM THE SUMMIt OF LEICESTER PAK. 


: 
Fig. 2.—Conracr or coarse Norive witit OLDER OR 
NORMAL Norire, ForESHORE, NEAR Gopricn. 


7 


Quart. Journ. Geol. Soc. Vol. LX XVIII, Pl. X VII. 


F. D. photo. 


Fic. 1.—Coarsk NoritE INVADING NORMAL NORITE. 
FORESHORE, NEAR York. 


I. D. photo. 


Pig. 2.—BEERBACHITE INVADING AND INCORPORATING 
NORMAL Norire, WILBERFORCE Spur. 


Cima 
be 


‘ Lenk he 4 4 
Na x _- * y r 
< Ve Bet ; 
ne ’ 
Weg i 
7 - y ‘ : 
< » ~~ * Js : 1 |) — 
2 oo. i a a? 
. Te: ‘? ox 
Z ’ \ ¢ = - 
‘ “ ‘ see i - 
. < J o y. 
5 E + Y cM, . 
“ - re " x 9 c 
2a 4 
P; 
‘ ; tte en 
.. 
e 
UW ti 
‘ 
x ar ‘ 
. ‘ ls , wh 
sa A E 
. 
i 2 is Ey, 
‘ cy ‘ 
: c * 
5 
. : ei Fa 
= . ; 
; ’ 


a a aoe pS. 


QUART. JOURN.GEOL.SOC.VOL.LXXVIII,PL. XVII, 


iz 


i Lye 
re 


4 

By descr Vis see De 

(j ‘ ib 8 
; he 


F.0. PROTOMICRO. 


ROCKS FROM THE NORITE-COMPLEX OF SIERRA LEONE. 


QUART.JOURN.GEOL.SOC.VOL.LXXVIII,PL. XIX, 


F.D, PHOTOMICRO. 


ROCKS FROM THE NORITE-COMPLEX OF SIERRA LEONE. 


—_—<-——--— 


——————— ae 


I Ne a er 


a, 
AAD, 


, j 


a i ay) 
ane 
ae SY i a 


part 4] NORITE OF SIERRA LEONE. 347 


DIscuUSSI0ON. 


Dr. J. W. Evans expressed his sense of the value of this paper, 
which described an intrusion of norite of unusual size and excep- 
tional characters. The comparatively fine grain of the main 
intrusion might be explained by the fact that its present form 
corresponded approximately to its original contours. The subse- 
quent intrusions might owe their coarser texture to the fact that 
the mass into which they were intruded was still hot. It would 
have been more satisfactory if the contact of the norite with the 
surrounding rocks could have been investigated by means of 
borings through the alluvium, but that was impracticable at the 
present time. 

My. A. E. Kirson congratulated the Author on the valuable 
work that he had done in Sierra Leone. He agreed with the view 
that the norite-mass of the Colony had probably been intruded 
along a zone of faulting, and thus had determined to some extent 
the character of the coast-line of this part of West Africa. 

Since the norite-mass had not been foliated, he believed that it 
was later than pre-Cambrian in age. The remnants of old plat- 
forms showed strong evidence of at least four successive uplifts, 
aggregating more than 1000 feet above sea-level. He agreed with 
the Author that there had been a good deal of assimilation of the 
older norite by the younger intrusions. Examples of assimilation, 
though of different rocks, occurred in the Gold Coast. He added 
that it was interesting to note that, although in the area between 
Freetown and the Hill Station there was a considerable amount of 
bauxitie ‘ laterite,’ he had not seen any blocks of bauxite. 

The AurHor thanked the Fellows present for the kind reception 
given to the paper, and the speakers for their remarks. With 
reference to Dr. Evans's observation concerning the coarse texture 
of the younger intrusions, the Author thought that this texture 
was accounted for, not only by the fact that the younger norites 
were intruded into a mass that had not entirely cooled down, but 
also by the fact that they were themselves probably of considerable 
size, although commonly giving rise only to small outcrops. Also, 
when intruded, they had sufficient reserve of heat to incorporate 
the older rock in considerableamount. Finally, the coarser texture 
was possibly induced by a slight difference in composition, much 
as in the case of the succeeding intrusions of norite-pegmatite, 
which were even coarser in texture. In reply to Mr. Kitson, 
the Author stated that iron-ores occurred in the younger norites 
much as in the older norites, except that in the former they were 
present only as grains and simall segregations, and not as schlieren. 
Also, bauxite occurred in small amount on outcrops of both the 
older and the younger norites. 


On. G.s. Nop di2- 28 


348 MR. H. B. MILNER ON THE [vol. xxvii, 


12. The Nature and Orictn of the PriocEeNE Deposits of the 
County of Cornwatt, and their BEARING on the PLIOCENE 
GEoGRAPHY of the SourH-West of Enetanp. By HENRY 
BrewER Miner, M.A., D.I.C., F.G.S., Lecturer in Oi 


Technology at the Royal School of Mines. (Read December 
21st, 1921.) 


CONTENTS. 

Page 

TV. Wntrodection: (5.0.35 ee ae ee ee 348 

II. Mode of Occurrence of the Deposits .................. 349 

TIL Methods of Snivesiteation “25.00.2024: eaee, oe 353 

IV. Lithological Character of the Deposits ............... 354 

V. Petrography of the Sediments. ...................-..... 357 

VI. Comparison and Correlation of the Deposits ...... 364 
VII. Source of Origin of the Deposits ..................... 366 . 

VIII. Pliocene Geography of the South-West of England. 367 

IX. Summary and Conclusions. ...5:s5:.5¢5- <5 sstestcaeee 373 


I. INTRODUCTION. 


THE present paper deals more particularly with the petrography of 
the younger Tertiary deposits of Western Cornwall which, by 
reason of their mode of occurrence and lithological similarity to 
the well-known fossiliferous beds of St. Erth, have been generally 
assigned to the Plocene age. These deposits occur as isolated 
patches of detrital material resting in eroded hollows in Paleozoic 
or older rocks in the south-western part of the county, and are 
located at St. Agnes, 4 miles due south-west of Perranporth; at 
St. Erth and Canons Town in the Hayle-River valley; at Crousa 
Common, a mile and a half south-west of St. Keverne (Lizard 
district); and at Polcrebo, near Crowan, 4 miles south of Cam- 
borne, where an outcrop of gravels of alleged Pliocene age is found. 
With the exception of the St. Erth Beds, no fossils have hitherto 
been discovered in these deposits, and their stratigraphical position 
has been inferred entirely from general geological and topographical 
considerations. 

The object of this paper is not only to establish the relative 
ages of these deposits by petrographic methods, but to demonstrate 
the importance of this mode of attack in problems of stratigra- 
phical correlation where paleontological evidence is in some cases 
lacking. In addition, the mineralogical factors concerned are used 
as a guide to the determination of the origin of these sediments 
and, in conjunction with topographic data, to restore the geo- 
graphy of this part of Britain in early Pliocene times, in so far as 
the evidence at our disposal allows. 


part 4] PLIOCENE DEPOSITS OF CORNWALL. d49 


II. Mops or OccURRENCE OF THE DEPosIts. 


(a) St. Agnes. 


Flanking the northern part of St. Agnes Beacon, and having the 
form of a crescent-shaped outcrop, the Pliocene deposits are here 
resting on a belt of metamorphosed Ladock Beds (Lower Devonian ) 
surrounding the St. Agnes granite-mass, except in the extreme 
south-western corner where they abut on the granite itself. The 
height of the Beacon is 628 feet O.D., and, as we descend the 
northern slope, the deposits are first met with approximately along 
the line of the 4:20-foot contour, whence they are continuous down 
a gentle gradient to the 350-foot level. Both the upper and the 
lower limits of the outcrop are concealed by several feet of ‘Head,’ 
a conspicuous feature of the Quaternary geology of this region, and 
consequently the actual contacts between the sands and the under- 
lying rocks are everywhere hidden. 

In addition to several minor natural exposures of the sands and 
clays, which here, as elsewhere, constitute the principal facies of 
these deposits, there are several overgrown pit-sections visible ; 
while two more recent pits giving greater facilities for study occur : 
one at Higher Bal, half a mile west of St. Agnes Church, and 
the other at Beacon Cottage, at the foot of the Beacon on the 
south-western side. In the former case about 12 feet of clay and 
sand are now seen overlain by 3 to 4 feet of ‘Head’; in the latter 
exposure only 10 feet of grey clay are seen, the section being now 
much obscured. The older sections mentioned by Sir Henry 
De la Beche! and quoted by Clement Reid? are certainly no longer 
in existence, and are either quite overgrown or obliterated by 
waste-material thrown out from the adjacent mines. 


(b) St. Erth. 


On the eastern side of the Hayle-River valley, and situate about 
haif a mile from the river itself, at the village of St. Erth, is the 
classic occurrence of Pliocene deposits, so well known for their 
peculiar (indeed, in this country, unique) fauna. The actual 
boundaries of the outcrop are again somewhat difficult to define 
precisely, on account of the thick capping of ‘Head’ developed. 
Roughly elliptical in shape, about half a mile long and a quarter 
of a mile broad, the St. Erth sands le on a gentle valley-slope 
between the 170- and the 50-foot contours; the mass has a 
discernible north-easterly and south-westerly strike, and is bounded 
by a well-marked quartz-porphyry dyke (‘ Mellanear’ or ‘ Long- 
Rock’ elvan)* on the north-west, and by an elongated sill of 
greenstone on the south-east, both intrusions having this dominant 


1 «Report on the Geology of Cornwall, Devon, & West Somerset’ 1839, 
pp. 258-60. 
2 ¢The Pliocene Deposits of Britain’ Mem. Geol. Surv. 1890, pp. 66-67. 
3¢The Geology of the Land’s End District’ Mem. Geol. Surv. 1907, 
pp. 63-64. 
2 Bee 


Woy SJUdLIN9 
BULIVOG=]UITULLpoOs 
JO OSINODH 


QUI9AI¥ 4S fivg 


prezry oy quNUyUOD MuoIouta y 
ra 

DT 

is] 


Y 
Y a 
yy 0) 919[Oq ona: 


) 
ONS UOUUBD OfZy 


Cab Qi haces enw 


“5 


‘ou 


‘YOU! [ 0} SOTIU Hf -!aJeIS 
‘d'O 392) 00b mojaq s9vsIns-pury juUsSoId 
9} JUWesatcda1 seare papeysuA “jsom-y}nNOos pure 
JSP9-Y}AOU WO SJUdTIMD SulIvaq-jusurpes’ syy 
jo Sasinoo siqeqoid ay} pue ‘(pepeys) suotsn.s7 
“Ul aRUBIG D1OZOM]Lg IVT VY} JO UOlQNqLIWSIp 
aya ‘(payop) sow sue00I[q A[1va Ul [AaAZT 
“POS DAOC BOPj.INS-pur, oyeunrxoidde suyoidaq 


‘GNVIONY AHO LSHM-HLNOS AHL 
HO dVIN-HOLAMS I'D 


or 


part 4:] THE PLIOCENE DEPOSITS OF CORNWALL. B01 


strike direction. Between the dyke and the sill is the basin- 
shaped depression in the Mylor Slates (Lower Paleeozoic) in which 
the deposits were laid down, and thus we can explain the disposi- 
tion of the sands in a north-easterly and south-westerly direction 
at this place. Natural sections in these deposits are rare, and 
evidence for mapping has to be based largely on quarry-exposures, 
on the character of the soil, and on topographic data. The arti- 
ficial sections at present visible differ greatly from those examined 
even 14 years ago, when the Geological Survey Memoir of the 
district was first published; in no case is the fossiliferous clay 
to be seen now, and, of the three exposures available for study, 
that of the Cornish Sand Company’s pit (an enlargement of the 
better known ‘ Harvey’s Pit)! is by far the best. This pit 1s 
situated immediately south of and adjacent to the road leading 
from the village to the vicarage. he old pits west of the vicarage, 
in which the fossils were originally obtained, are now much over- 
grown, and all traces of the fossiliferous clays obscured.? 

The Cornish Sand Company’s pit, which has been rapidly ex- 
tending during the last few years, shows an excellent section 
in the Phocene sands. The character of these deposits varies 
considerably both laterally and in depth, and apart from minor 
irregularities in bedding, there is a marked dip of the whole series 
of 5° north-westwards which determines the gentle slope of the 
fields between the vicarage and the village, and further explains 
the non-appearance of the fossiliferous clay in this pit. The 
section shows an average thickness of 20 feet of sand resting upon 
the eroded Palzozoic floor, which consists of slate intersected in 
one place by an ‘elvan’ dyke. The sand is overlain by 2 to 4 feet 
of ‘ Head’ and surface-soil. 

The late Clement Reid, writing in 1890, suggested that possibly 
a similar outcrop to that of St. Erth would be ultimately discovered 
on the watershed separating the Hayle River from the streams 
draining into Mounts Bay at Newtown; but careful search, both 
here and at likely horizons throughout the whole valley from north 
to south, failed to reveal any trace of Pliocene material other than 
that recorded by the Geological Survey as occurring at Lelant 
Downs, half a mile north of Canons Town, 2 miles west-north-west 
of St. Erth. In this case the evidence of outcrop, though indefinite, 
is certainly in favour of the occurrence of a small patch of Pliocene 
deposit resting upon metamorphosed Mylor Slate at a height of 
about 150 feet above O.D., and having much the same topo- 
graphical aspect as the St. Erth Beds opposite. No pits have ever 
been dug in this deposit, and the only evidence obtainable is from 
the sandy nature of the soil and from a few blocks of ferruginous 
sandstone found occasionally on the valley-slope. 


' Mem. Geol. Surv. 1907, p. 72. 

2 At least 5 feet of sand and rubbly surface-material now filling the pits 
would have to be removed before exposing the clay again, even if the thick 
and overgrown vegetation were cleared. 

3 *The Pliocene Deposits of Britain’ Mem. Geol. Surv. p. 60. 


MR. H. B. MILNER ON THE [ vol. Ixxvill, 


co 
Or 
ho 


(c) St. Keverne. 


Resting on the Lizard ‘platform’ at a height of 364 feet above 
O.D., and situated a mile and a half due south-west of the village 
of St. Keverne, are the Crousa-Common gravels first noticed by Sir 
Henry De la Beche in his report of 1839. Unlike the similar deposits 
of St. Agnes and St. Erth, the extent of this outcrop is easy of 
definition. It consists of a rectangular mass 1 mile long by half a 
mile broad, lying upon the eroded surface of the Lizard gabbro, 
the conspicuous feature of this area. As Mr. J. B. Hill has pointed 
out,! the extent of the deposits is naturally defined by the sharp 
change in vegetation noticeable as the boundaries between the 
sands and the igneous rocks are traversed; in the former case the 
marked development of gorse, heather, and pine contrasts strongly 
with the more fertile grassy soil yielded by the gabbro, and the 
typical weathered blocks of this rock scattered everywhere over 
the surface of its outcrop constitute a further element of differen- 
tiation between the two. 

Both natural and artificial sections in these sands are common, 
but only one pit is at present worked, the others being overgrown 
and often under water. The best exposure is that in a shallow 
quarry adjoining the main road to St. Keverne, immediately east 
of the 9th milestone from Helston. This pit shows a section of 
coarse sands and gravels varying from 8 to 10 feet in thickness, 
and capped by a few inches of mixed surface-detritus and peaty 
soil. The deposits are remarkable for their varying texture, the 
strong current-bedding exhibited, and for the quantity of big 
pebbles and even boulders associated with them. ‘Their base is not 
seen in this particular pit, although it cannot be far below the 
floor of the present working (1920), as in wet weather the pit is 
badly flooded, pointing to the existence of impervious clayey rock 
at no great depth, such as would be yielded by the superficial 
decomposition of the gabbro. In some of the older abandoned 
pits on the north side of the road in the wood, blocks of gabbro 
may be seen in dry weather protruding from underneath the sandy 
floor, thus showing the vertical extent of the deposits. 

No fossils have ever been found in the Crousa-Common deposits, 
and their Pliocene age has been inferred from their relations to 
the Lizard ‘platform,’ a remnant of the characteristic Pliocene 
features of the West Country. 


(d) Polcrebo. 


The curious pebble-bed found at Polerebo near Crowan, 4 miles 
south of Camborne, originally described by W. Tyack ? and later 
referred to by Mr. J. B. Hill,’ is of very limited extent; it consists 
principally of well-rounded jpebbles of quartz, resting at a height 


1 «The Geology of the Lizard & Meneage’ Mem. Geol. Sury. 1912, p. 230. 
2 Trans. Roy. Geol. Soe. Cornwall, vol. ix (1875) p. 177 
3 «The Geology of Falmouth & Camborne’ Mem. Geol. Surv. 1906, pp. 88-89. 


part 4:] PLIOCENE DEPOSITS OF CORNWALL. 3598 


of 480 feet upon an eroded granite-surface, and is of very obscure 
origin. Beyond the doubtful topographical evidence as to its 
alleged Pliocene age, there is little whereby to reconcile its sug- 
gested relationship to the St. Agnes, St. Erth, or Crousa-Common 
deposits.! 


JIJ. Mernonps or INVESTIGATION. 


In all cases the individual outcrops of the deposits were sampled 
both laterally and vertically: in the first instance, suitable ex- 
posures were located along the strike of the beds, from which 
samples were taken at regular intervals if outcrops allowed; in the 
second case all the principal quarry-sections at each locality were 
visited, and the sands sampled from top to bottom at intervals 
depending on the lithological variations noted in the beds. As an 
example of this method of procedure we inay take the St. Erth 
occurrence—in a lateral distance of some 800 yards measured 
along the strike of the beds, four samples were taken at approxi- 
mately equal intervals of 200 yards, the material being furnished 
by stream, hedgebank, and quarry. For sampling in depth, the 
Cornish Sand Company’s pit was chosen, since it showed the 
maximum vertical section exposed in the beds at the time. In 
this case samples were taken from 6, 8, 14, 20, 22, and 23 feet 
below the surface, these horizons representing the more marked 
lithological variations in the deposit as seen in this pit. The 
samples referred to in all instances consisted of about 200 grammes 
of material, having regard to the average grade-size of the par- 
ticles, about 0-2 mm. In the case of much coarser material, as 
at Crousa Common, at least 500 grammes were taken for each 
sample, in order to ensure a reasonable yield of ‘ heavy’ residue. 

The usual method of procedure for the qualitative examination 
of the mineral constituents of the deposits was adopted; the 
samples were first sifted free of the larger grains and pebbles, by 
using a 80-mesh sieve and later a 60-mesh sieve; in this way each 
sample was divided into three rough grades, a coarse, medium, and 
fine. The coarse material, rejected by the 30-mesh sieve, included 
the grains greater than approximately 0°5 mm. in diameter; the 
medium material, passed by the 30-mesh but rejected by the 60- 
mesh sieve, included the grains between approximately 0°25 mm. 
and 0°5 mm., while the fine material, passed by the 60-mesh, in- 
eluded all fine sand, silt, and clay: that is, particles less than 
0°25 mm. in diameter. It is desirable to emphasize that such 
grading is only of a very approximate character, and is used solely 
for convenience in qualitative work, but not for accurate mechanical 
analysis, which must be obtained by the ordinary methods of 
elutriation; it is of use, however, in giving a rough impression 
of the relative proportions of the particles between the chosen 
limits. 


1 Tbid. p. 88; also Trans. Roy. Geol. Soc. Cornwall, vol. ix (1875) p. 181. 


B54 MR. H. B. MILNER ON THE [vol. lxxvili, 


The coarse material was first carefully examined with a lens, and 
subsequently with the microscope, in the latter case by placing 
some of the grains on a black card and viewing them by reflected 
light. The medium and fine material were both washed with 
water, treated with a dilute solution of hydrochloric acid so as to 
remove any ferruginous coating of the grains, and in the case of the 
fine grade, boiled previously with a 1 per cent. solution of sodium 
carbonate to deflocculate silty and clayey matter. The clean sand 
was then dried, and about a third taken for treatment with ‘heavy’ 
liquids. (A preliminary trial in all cases showed that these Pho- 
cene deposits yielded abundant residue, and consequently there was 
no necessity for utilizing the whole of the cleaned sample, except 
for quantitative determination. ) 

For the ‘heavy’ mineral concentration bromoform of specific 
gravity 2°82! and, in special circumstances, cadmium borotungstate 
of specific gravity 3°28 were used. The residues thus obtained 
were separated with an electromagnet into magnetic and non- 
magnetic crops; these were then mounted in Canada balsam for 
microscopical examination, and a slide was made in each case of 
the ‘light’ material of the sample, s.g. <2°82. The total amount 
of ‘heavy’ mineral concentrate obtained in all cases was mounted, 
to ensure inclusion of the rarest mineral species present in the 
sediment. 

In many instances the samples were examined quantitatively, 
and the percentages of light material, ferruginous cement, silt and 
clayey matter, and ‘heavy’ residue determined. Such results are 
included below in the section dealing with the petrography of the 
various deposits. 


IV. LirHoLtoeicaL CHARACTER OF THE DEPOSITS. 


(a) St. Agnes. 


The deposits here consist of a well-marked series of yellow, 
white, and brownish-red sands overlain by grey and mottled clays. 
The principal section exposed at Higher Bal, west-north-west of 
St. Agnes Church, shows about 6 feet of sand, yellow at the base, 
and passing up into an almost pure white sand, which gives place 
to a brownish-red facies immediately beneath the clay. The 
transition from the sand to the clay is actually not so sharp as at 
first sight appears, and an intermediate arenaceous clay separates 
the sands below from the true clays above. The sandy clay and 
overlying grey and mottled clays in this section vary from 7 to 
8 feet in thickness, although this facies tends to thicken south- 
westward; as in the Beacon Cottage pit no sand is exposed, and 
8 to 10 feet of clay occur. The sands themselves occasionally 


1 Since the War, the bromoform put on the market has varied considerably 
in purity, and the specific gravity has fluctuated accordingly ; in some cases 
it was possible to raise the gravity to 2°9 by repeated distillation, but not 
without great trouble and a certain amount of loss. 


proc’ 


part 4] PLIOCENE DEPOSITS OF CORNWALL. BOD 


yield interesting well-worn quartz-pebbles, quartz-veined ‘killas’ 
pebbles, rounded schorlaceous fragments, and (more rarely) rounded 
pebbles of cassiterite.! A few coarser grit-bands are present in the 
red sand, but they are essentially local and discontinuous. 


(6) St. Erth. 


Here again the clay and sand facies are both developed in these 
deposits, although unfortunately we are no longer able to study 
the former zz sttw; for the old section in the Vicarage pit at 
St. Erth, in which the fossiliferous clay occurred (see p. 351), we 
must refer to Prof. P. F. Kendall & R. G. Bell’s diagram in their 
paper dealing with the Pliocene fauna of this deposit.2 From 
the details given it is evident that the section showed a few feet 
of ‘ Head’ capping about 6 feet of yellow sand overlying the clay ; 
the latter rested upon a much thicker band of ‘fine quartzose 
sand’ persistent to the base of the series. Unfortunately, the 
authors of this paper omitted all details as to thickness of these 
beds; but, from particulars given by Clement Reid,? and from 
the section which it is now possible to measure in the uppermost 
yellow sand at the Vicarage pit, it would seem that the clay varied 
from 7 to 8 feet in thickness, and the underlying sand from 10 to 
12 feet in thickness. The pit now only shows about 5 feet of 
the uppermost yellow sand. 

The St. Erth sands vary lithologically in much the same manner 
as those of St. Agnes do: the same yellow, white, and brownish- 
red facies, with an associated grit-band, are present, and the last- 
mentioned contains several small pebbles of quartz, killas, green- 
stone, and schorlaceous material; this grit-band, although quite 
conspicuous where it does occur, 1s non-persistent and of little 
value as a definite horizon. Other seams of pebbles are scattered 
promiscuously throughout the red sand; such pebbles are always 
well worn and by far the greater percentage consists of quartz. A 
loamy sand underlying the grit-band is a well-differentiated facies, 
of a consistency contrasting markedly with the white and yellow 
sand above; it constitutes the best moulding-sand from the pit, 
and is of importance on that account. Current-bedding in this 
series is not a well-marked feature, although it may be detected 
in some instances on a small scale; in this respect, we may note a 
further similarity to the St. Agnes deposits. On the whole, the 
St. Erth deposits are of finer texture than those of St. Agnes or 
St. Keverne, particularly the last-named; their general lithology 
suggests deposition in deeper water than in the other cases. 

The lithological character of the deposit alleged to occur on the 
opposite side of the valley at Lelant Downs, has already been 
mentioned (p. 351). 


1 «The Geology of the Country around Newquay’ Mem. Geol. Surv. 1906, 
p. 64. 

2 Q.J.G.8. vol. xli (1886) fig. 1, p. 202. 

3 “The Pliocene Deposits of Britain’ Mem. Geol. Surv. 1890, pp. 59 et seqq. 


356 MR. H. B. MILNER ON THE [vol. lxxviu, 


(c) St. Keverne. 


In the Crousa-Common gravels there is a much wider variation 
noticeable, and the deposits are coarser and more heterogeneous in 
composition than in the preceding cases. The material consists 
largely of subangular fragments of quartz, occurring both as 
grains 0°25 mm. in diameter and as pebbles 2 or 38 inches in 
diameter, with every intermediate grade present; associated with 
the quartz are pebbles of slate of the ‘killas’ type, schorlaceous 
fragments, and pebbles of roeks recognizable in the Lizard series. 
Impersistent seams of finer sand and occasionally grey clay accom- 
pany the gravels, but they are by no means common. Reference 
must be made to the contortions and local folds exhibited by these 
gravels: this feature is certainly most striking in the pit adjacent 
to the main St. Keverne road (p. 352); but it would seem to be more 
easily explicable as due to current-bedding of coarse detritus in 
shallow water, than as the result of later subsoil movements.! The 
coarse texture and the very mixed character of these deposits tend 
to accentuate such inequalities of bedding; but, when examined 
closely, their true neritic charaeter 1s quite apparent. The amount 
of material that has to be sifted in order to produce a sand 
concentrate for petrographical investigation is more than double 
that used in the other cases; this in itself bears testimony to the 
nature of these gravels. 


(d) Polcrebo. 


Mr. J. B. Hill has already pointed out a similarity existing 
between the Polerebo gravels and those of Crousa Common, in so 
far as the quartz-pebbles are concerned, a feature remarked by 
W. Tyack in his original description of the former deposit. The 
greater mass of the Polcrebo material is quite unlike that of 
Crousa Common, however, and consists of well-worn rounded 
pebbles of quartz and granite, the latter of local origin. These 
pebbles are extremely variable in size; Tyack says that ‘they are 
of all sizes, from boulders as large as a pumpkin to pebbles as small 
as hazel-nuts.’ Sand in association with the gravels is of rare 
occurrence, and what fine-grained material (2 mm. in diameter) 
there is, has been derived solely from the weathering and disinte- 
eration of the pebbles themselves. But for their elevation at 
480 feet above sea-level, there is little lithologically to suggest a 
relationship to the deposits under consideration, although their 
mode of occurrence is certainly significant. 


1 «The Geology of the Lizard & Meneage’ Mem. Geol. Surry. 1912, p. 230. 
2 Trans. Roy. Geol. Soc. Cornwall, vol. ix (1875) p. 175. 


part 4] PLIOCENE DEPOSITS OF CORNWALL. 357 


V. PETROGRAPHY OF THE SEDIMENTS. 


(a) St. Agnes. 


The determination of the mineralogical composition of the 
St. Agnes deposits yielded the following results :— 


JIAO see oaetonsal al ilesay lige Welle ae cIeN Ge 

coe Minerals. 
2s me QuartZ fou. ss. stcserecsseess...f Hssential constituent throughout. 
SV LENO TIA ere pai ae Se — — 4, 4. 
ane mee ovite ct SURE A rr bd 4 Fi 5 i 
= APAWCOMMEE... svacrerscarense saetee vee A dl 
2 Ferruginous cement ............ Essential authigenous material. 
ae Meiometnieny is uals. cette aria 7 a aa 7 
BA I]menite d 5 a | 5 
ee Garnet —_ — — = 
27 Tourmaline sn SoC 8 8 8 8 
ae LAMMONIC. # sicrod vase aes lau oees 3 il 3 3 
aes HAIL O HER estas tehc 5 satcpiehcaue See seces 4, 4A — 4, 
SS @hlorite sss. ae. 1 2 2 2 
FA Biotite 1 2 2 2 

ol Wem COON Cie nchkaseifis couse Ff VA 7 i 

a Zircon Se ee abe oe EGE 4. 6 5 5 
AN JODIE Seah fata elie a atatanicarnar te 6 5 iL Orel 
Bok Paialsens, Paris > cree) — — 2 2 
5 Rutile seo dh Snood cee 2 4 2 2 
wn . 
& 3 Esrnokattie wert 9.01, eaten ON Ae — = = a 
s AN CSU Aes Sens Re BES taste con eters 4, Seip 5 5 
a Andalusite ....... i 5 | 8 if 
Se Nnascovite ssc. .oe. 5 5 6 ie 
r= @o wom daarig sys. Sh. cemcdhoatn Bee as — — p e 

a CaS Sib ENTHES ceniealatedine odvigsenae oxenak ee i 1 

F, DCM OIA tue, fee 6b eae oss a jesacrsss y — pol al le 


[9=‘ Flood’ of a particular species. 8=Very abundant. 7=Abundant. 
6= Very common. 5=Common. 4==Scarce. 
3=Very scarce. 2= Rare. 1=Very rare. | 
I. Yellow Sand. Base of series now exposed, 15 feet below the surface, quarry 
at Higher Bal. 
II. Red Sand. 12 feet below the surface, quarry at Higher Bal. 


IIT. ‘Candle Clay.’ Average of several samples taken laterally, including quarries 
at Higher Bal and Beacon Cottage. 


IV. Average composition of the St. Agnes deposits. 


The residues yielded by all the members of the St. Agnes 
deposits are characterized by the abundance of the magnetic 
minerals as compared with the non-magnetic, and the bulk of this 
magnetic crop consists of tourmaline and iron-ores. In the case 
of the red sand, the actual proportions are 0°81 per cent. magnetic 
to 0:086 per cent. non-magnetic. The ‘light’ material is mainly 


358 MR. H. B. MILNER ON THE [ vol. Ixxvili, 


quartz, which in the case of the sands constitutes about 94:1 per 
cent. of the facies; associated with the quartz are kaolin, mus- 
covite, glauconite, and authigenic matter, chiefly limonite in the 
yellow sands, and hematite in the red sands. In the clays the 
percentage of mica and kaolin is higher, though much of the 
material is an indeterminate ‘ paste,’ probably consisting of quartz, 
sericite, chiastolite, rutile, and iron-ores; the mottling of these 
clays is due to variations in the degree of secondary oxidation 
undergone by the original iron-compounds. 


DESCRIPTION OF THE MORE IMPORTANT MINERAL SPECIES. 


Quartz.—This mineral occurs in subangular to well-rounded grains, 
usually coated with ferruginous cement. On clarification with acid, the 
grains are seen to be highly turbid, owing to inclusions of apatite, tourmaline, 
and rutile. No fluid cavities were noted in the grains. 


Muscovite.—Varies considerably in composition. The ‘light’ material 
(S.G.< 2°82) presents fairly-clear irregular flakes of low polarizing character, 
usually showing good interference-figures. The denser facies which sinks in 
bromoform is turbid with inclusions of ferric oxide, tending to parallel 
arrangement within the flakes. 


Magnetite is a noteworthy constituent of all facies of the deposit, and 
occurs in subangular grains ; rounded octahedra are rare, though noted more 
in the red sands than in the other facies. In the clays, sharply angular 
fragments are seen (magnetite dust), of a brilliant silvery-grey colour by 
reflected light. 


Ilmenite.—Owing to the absence of well-individualized magnetite, some 
little difficulty is experienced in differentiating ilmenite from that mineral ; 
the abundance of leucoxene in the non-magnetic residue, however, is some 
measure of the proportion of original ilmenite, though in many cases rounded 
irregular grains of the species are clearly defined by their dull reddish-grey 
lustre in reflected light. 


Tourmaline is exceedingly abundant, and in the yellow and red sands 
constitutes the greater part of the residue. It usually occurs as large, 
irregular, and rounded grains, of blue, green, brown, intermediate shades, 
and parti-coloured. The brown varieties show the strongest pleochroism, 
and many of the grains, being basal, exhibit uniaxial interference-figures. 
Besides the grains (though by no means so common), well-formed prisms are 
seen, generally striated, and having rhombohedral terminations and basal 
partings ; doubly terminated varieties are rare. Such prisms are usually 
blue, green, or bluish-green, and are pleochroic: they have an average 
length of 0°3 mm., while the grains frequently measure 0°25 mm. along 
their greatest length. 


Staurolite.—Only a few grains of indubitable staurolite were recognized, 
although the mineral may actually occur in greater proportion, or at least 
with more constancy than is to be inferred from the determinations made. 
The grains are well rounded, but characterized by their deep brownish-yellow 
colour, making distinction between them and the abundant brown tourmaline 
a dificult matter. The weaker pleochroism and higher refractive index than 
the tourmaline were noted; but the most reliable differentiating factor was 
the biaxial positive interference-figure revealed in some cases. 


Leucoxene.—Abundant and occurring in dull whitish-grey pellets (as 
seen by incident light), well rounded, with occasionally a core of unaltered 
ilmenite. The grains have a rough and pitted appearance, and are frequently 
stained brown. 


part 4] PLIOCENE DEPOSITS OF CORNWALL. 309 


Zircon.—Two very distinct varieties of zircon occur, well-rounded doubly- 
terminated prisms averaging 0°3 mm. in length, and short, ‘stumpy, but 
well-defined prisms with irregular terminations. The former type is usually 
full of inclusions, and is more characteristic of the basal yellow sands ; the 
latter variety is almost transparent with only occasional fluid (?) cavities, 
and is more typical of the red sands and clay. The scarcity of this mineral 
at the base of the series, its common occurrence at the middle horizons, and 
its gradual diminution in proportion as the upper beds are reached, considered 
together, are noteworthy features. 


Kyanite.—These crystals are certainly distinctive, and occur as elongated 
prisms determined by the (100) cleavage and (001) parting; traces of a well- 
marked (010) cleavage are nearly always present; the grains have suffered 
much abrasion, and are on the whole extremely well rounded. They strongly 
resemble analogous grains found in the Lower Greensand. (Fig. 3, p. 371.) 


Anatase.—Absent in the sands and occurring but rarely in the clay as 
indigo-blue tabular erystals perfectly formed. Probably secondary, derived 
from the decomposition of ilmenite. 


Rutile.—This mineral is not so common as might be expected, and well- 
formed crystals are rare. Generally it occurs in well-rounded ‘foxy’ red 
grains, rudely prismatic, measuring 0°25 mm., and void of any pleochroism. 


Topaz.—Well-rounded, clear, and transparent grains of topaz are charac- 
teristic of the red sands and clay, but less common in the yellow sand. The 
grains are variable in size, measuring from 0:1 to 0°2 mm. in diameter, but 
occasionally attaining 0°5 mm. ‘They are easily distinguishable from the 
andalusite by their transparency, absence of pleochroism, and by the biaxial 
positive interference-figures exhibited by many of them. 


Andalusite.—This mineral constitutes the most important, and at the 
same time the most interesting, feature of the St. Agnes deposits. It occurs 
both as irregular rounded grains and as well-formed prismatic crystals, and 
in most cases exhibits characteristic blood-red pleochroism, more intense 
and striking than in the case of any other known occurrence of the 
mineral in British sedimentary strata. The ‘rounded grain’ type, which is 
the commoner mode of occurrence, presents marked subconchoidal fractures, 
and is usually less turbid with inclusions than the crystals; the latter 
consist of well-formed rhombic prisms terminated by (011) faces or by the 
basal pinacoid ; grains flattened parallel to (001) are normal to the acute 
bisectrix, and show biaxial negative interference-figures. The inclusions are 
mainly quartz, graphite, and magnetite, and in some cases secondary alteration- 
products are developed, the chief of which has been recognized as kaolin. 
The pleochroism and usually turbid character of the mineral serve to differen- 
tiate it easily from the associated topaz, to which it is only subordinate in 
quantity in the red-sand horizon. 


Cassiterite.—This mineral is by no means common, and no trace of it 
was found in the red sand. In the basal sands and also in the clay, however, 
one or two dusky-brown, rounded, tetragonal crystals were observed, measur- 
ing about 0°2 mm. in length. 


(6) St. Erth. 


As stated previously (p. 358) six definite lithological horizons 
were recognized in the St. Erth sands, in the vertical section 
shown in the Cornish Sand Company’s pit; but only the upper 
yellowish-brown sand was traceable laterally. The following table 


560 


MR. H. 


B. MILNER ON THE [vol. lxxviii, 


shows the results of the microscopical examination of the various 


facies :— 


Light Material: | 
S.G. <2'82 


Heavy Residue : 
Magnetic. 8.G. >2'82. 


Heavy Residue; 
8.G. >2'82. 


Non-Magnetic. 


TOnizon ee | A B | (Ge | D 
| | 
Minerals. 
Quartz Essential and dominant constituent throughout. 
Kaolin 5 & 5 5 5 5 5 
Muscovite. 49 2252 6 6 4, 4, A, 5 4 
Glauconite ............ 4 4 4, 3 A 3 4, 
Ferruginous cement.) Essential authigenous material throughout. 
Macnetite (2. Seay ner of 7 5 6 6 a 
imeniter a). s ee: 5 i 5 5 6 5 5 
Garnet 1 1 1 —_ — — ik 
Tourmaline) (lens... 8 8 8 8 8 8 8 
Staurolite i s/n..0.: 5 5 4, 2 A, A, 4, 
Hipidotes scene 4, 4 4 4, A 4, 4 
Chiorite: 5 aes) pS — — — — — — 
Biotite 4 1 2 1 — — 1 
WewGoxene i... 100%: 7 Oe eller 5 8 6 7 
Zircon Ree 9 95 29 5 4, 8 9 
Way ante an. on sonia ce 5 Ae 5 6 6 3 5 
AM AEASC te cece iy —- — — 1 — it 
Rutile faLte a 61 46 4, 6 A 6 
Brookite ............0..) Dy ieee — — P 1 
Mowpag eee senkesen ce 5 4 | 4 4 3 3 4, 
Amidalusite tanec. 288) iD Bil 46 5 6 7 5 
Miuseovite:.. Saar 5 Son As 4 4 4 4, 
Corundum .. — —- | — — — — — 
Cassiterite ........:.-- 4 Ae ese — —- — 4, 
Xeno iumen oases P — | — —- | = P 


. Yellowish-brown sand, 6 feet below the surface. 
taken laterally, including one from the Cornish Sand Company’s pit. 


Average of several samples 


. Brown sand, 14 feet below the surface, Cornish Sand Company’s pit. 


A 

B. White sand, 8 feet below the surface, Cornish Sand Company’s pit. 
C 

D 


. Reddish-brown sand and grit, 20 feet below the surface, Cornish Sand Com- 


pany’s pit. 


EK. Chocolate moulding sand, 22 feet below the surface, Cornish Sand Company’s 


F, Reddish-brown sand, 23 feet below the surface, bottom of the pit, Cornish 


Sand Company’s pit. 


G. Average composition of the St. Erth sands. 


[Conventional signs as in the previous table, p. 357.1] 


A quantitative estimation of the mineralogical composition of 
an average sample of St. Erth sand gave the following results :— 


Quartz, 91°6 per cent. 
Authigenous and other ‘ light’ material, 8:26 per cent. 


Magnetic residue, 


0°120 per cent. 


Non-magnetic residue, 0°025 per cent. 


part 4] PLIOCENE DEPOSITS OF CORNWALL. 361 


From this it will be seen that the relative proportion of the 
magnetic to the non-magnetic residue is less, and the percentage 
of authigenous matter (chiefly limonite) greater than in the case 
of the St. Agnes deposits. The principal mineralogical features of 
the St. Erth sands consist in the abundance of tourmaline (variable 
in colour, size, and shape), in the marked ‘flooding’ of well- 
rounded zircon-crystals at the upper horizons, in the rare appearance 
of garnet (absent both at St. Agnes and St. Keverne), in the 
presence of staurolite in variable quantity throughout, and in the 
occurrence of the same type of kyanite (fig. 2, p. 3871) as that 
remarked at St. Agnes, though generally less common here than 
at that locality. Quartz, muscovite, magnetite, ilmenite, kyanite, 
and cassiterite all present characteristics similar to the species 
occurring at St. Agnes (p. 3858), and need not be further mentioned. 
The following minerals, however, are specially to be noted :— 


Garnet.—This mineral, although very rare, is distributed laterally 
throughout the upper yellowish-brown sand, and also vertically through the 
upper facies of the deposits. It occurs as subangular grains never longer 
than 0°2 mm., with a refractive index of about 1°76, pink in colour, and 
traversed by numerous intersecting cracks. Its isotropic character is con- 
sistent. 


Tourmaline.—This most abundant mineral is present both as large and 
as small grains and prisms, the prisms averaging about 0°3 mm. and the grains 
0°25 mm. in length. The colours range from grey, blue, purple, green, brown, 
to yellow, and in many cases parti-coloured grains are seen. A noteworthy 
feature of the species is its occurrence as basal sections yielding good uwni- 
axial interference-figures ; such basal grains are extraordinarily common at 
all horizons, and are indistinguishable from similar grains noticed in the 
St. Agnes deposits (p. 358). Extreme rounding, subangularity, and irregular 
outline of grains are all developed, the last-mentioned feature being 
especially noticeable in the reddish-brown sand near the base of the series. 


Staurolite.—Common in the upper beds, but scarcer at the lower 
horizons, this mineral is well individualized at St. Erth. It occurs as rounded 
brownish-yellow grains, exhibiting weak pleochroism, high refractive index 
and, in some instances, biaxial interference-figures. The average grains 
measure 0°2 mm. 


E pidote.—Grains of this minéral, when present, are all very similar. 
They present sharply angular outlines, clear greenish-yellow coloration in 
transmitted white light, a refractive index equal to that of the garnets, and 
weak pleochroism. Between crossed nicols, third-order birefringence colours 
(chiefly blues and yellows) are exhibited, and many of the grains show typical 
partial interference-figures for cleavage-flakes parallel to (100). 


Zircon.—Reference has already been made above to the ‘ flooding’ of this 
mineral at certain horizons, which is its most characteristic feature. Com- 
pared with the other minerals associated with it, and also with the similar 
species occurring at St. Agnes, the St. Erth type is much smaller, and much 
more rounded ;. the average size is 0'1 mm. 


Rutile.—In contrast to the St. Agnes deposits, this mineral occurs very 
commonly at St. Erth; the prevalent type of grain is a well-rounded, dusky 
reddish-brown prism, with faint pleochroism and a very high refractive index. 
Sharply angular and irregular fragments of clear yellow rutile also occur, 
especially in the uppermost yellowish-brown sands. 


362 MR. H. B. MILNER ON THE [vol. Ixxviul, 


Brookite.—Only one grain of this mineral was identified in the samples 
examined, and this occurred in the white sand associated with rutile, as an 
irregular, well-rounded, yellow crystal flattened parallel to (100) and striated 
parallel to the vertical axis. The grain is feebly pleochroic, possesses a very 
high refractive index and birefringence, and exhibits a well-defined biaxial 
positive interference-figure. 


T opaz.—This mineral is commoner in the upper horizons than in the lower, 
and in some cases occurs in excess of the andalusite. The grains are usually 
somewhat irregular, but are clear, colourless, void of inclusions, and average 
from 0°2 to 0°3 mm. in diameter. 


Andalusite.—The chief feature of this species, as occurring here, is the 
intense blood-red pleochroism exhibited by nearly all the crystals, which are 
in this, and in all.other respects, similar to those of St. Agnes. 


(c) St. Keverne. 


Microscopical examination of the Crousa-Common deposits gave 
the following results :— 


SORIZON ae ree ee A. | B. | C. | D. 
| 
Zoi Minerals. 
2A Quartz 20... ce .ecccceeseseceeesn|  HSsential constituent thronehoute 
Ss V Nitiscowitel.. ee ee ee ee 5 5 5 
2a Gilacomite ney ca setae eae 3 2 3 2 
"Shey Ferruginous cement ............| Essential authigenous material. 
| 4 
a } 
CON SaMao me tile wt, werent a csatter er es 7 7 fl ay 
S x Ilmenite 8 8 8 8 
see Garnet eat nyeteaiten sad peed scl age tee — — — 
oa Fourmalinesaes ee eee 8 8 Sia, 8 
ray. Staunolites e825 aeccs wede ek eeere 1 1 Le 1 
aoe J Dh ots Opes te heen oat na ere Remon zi A en eaerk zi 
Som |*Chloritere ee woe i ibis aes TL 1 
5 | Biotite 2 mea uy 1 
= a f bes 
a Leucoxenee tsi ee aceon eT) aig Gia 7 
® Zircon 4. A 5 4 
| aS Kivanities a=: ae Sn eee ene — = — = 
Oc JA TIAT ASE: Bey steer ee! ae 5 5 8 5 
Pe siicniabiloe 2% Atavus lean 4. 5 4 
& TSROOKIGE! 3 Fosse cd, Sees eicacecct s deee — a 2 2 
Pas opiate ane oie tn, 2 2 By lage ie 
a AG alusite:s5ecce en ootees ace 8 oy a 9 
3 én Misco vate se cee ee. ce eee 5 5 ri 5 
mS Gorundiimy seo tee alee Bee = = | = an 
a Cassitienite! War. cc kx seh Ones A, A, | ler zy 
2 XOMOUIMEg Peeoe ee eee eh eee el alee =— oe BE? 
al | 


A. ‘Gravel’ from Crousa Common, St. Keverne. (Average of several samples.) 

B. ‘Gravel’ from a quarry by the main road, 9th milestone from Helston, 
St. Keverne. 

C. ‘Gravel’ from the base of the deposits. same locality as B. 

I. Average composition of the St. Keverne deposits. 


[Conventional signs as in the previous tables. | 


part 4] PLIOCENE DEPOSITS OF CORNWALL. 363 


The mixed, unsorted character of these Crousa-Common deposits 
renders quantitative data of little direct value in this case. Mention 
has already been made of the variation in grade-size shown by 
the constituent particles, and the general coarse texture of the 
sands has been commented upon; such variations are reflected in 
the elutriation-curve for these sediments plotted by Prof. P. G. H. 
Boswell, which compares very unfavourably with the curve of 
the St. Erth facies.!. It would be expected that the coarser the 
sample is in the aggregate, the less will be the yield of heavy 
residue, and this was found to be the case in the present instance. 
The samples collected varied considerably in average texture, and 
the residues fluctuated quantitatively from O-U1 to 0:2 per cent. 
The only uniform feature was the mineralogical composition of the 
sediments, which was found to be constant qualitatively through- 
out. The characteristic minerals, poth in frequency of occurrence 
and in optical properties, are undoubtedly ilmenite, anatase, and 
andalusite; the last-mentioned species particularly predominates 
in some samples, and in one non-magnetic residue, save for three 
erains of anatase, it constituted the entire crop. The negative 
characteristics that should be noted are the extreme rarity of 
staurolite, the entire absence of kyanite and garnet, and the 
dropping-off of zircon, when these minerals are considered with. 
reference to the other deposits. 


Ilmenite.—The contrast between this mineral and the magnetite tends 
to be most marked in these deposits ; whereas the latter frequently occurs in 
well-recognizable octahedra, the former is invariably irregular, fragmental, 
and ragged. In some cases the partial alteration to leucoxene can be seen, 
and secondary outgrowths of tiny ultra-microscopic crystals (probably of 
rutile or anatase) from the opaque nucleus are by no means uncommon. 


Tourmaline.—This species is again remarkable for the abundance of 
basal grains, as at St. Agnes and St. Hrth. Prismatic crystals are present, 
but less frequently than at those localities. The prevalent type is usually 
rounded but subangular, and many grains show a marked fracturing. The 
average grain measures 0°4 mm. in diameter. 


Anatase.—This species is undoubtedly the distinctive feature of these 
deposits, and in no other sediment known to me are the crystals so abundant, 
so beautifully formed and preserved, and so conspicuous as they are here. 
The common type is tabular parallel to (001), either as individual crystals or 
as composite groups of tabular forms in parallel growth. The colour is. 
invariably indigo-blue or greyish-blue ; refractive index extremely high ; bi- 
refringence high, but tending to be masked by the natural colour ; interference- 
figure (uniaxial negative) remarkably clear. These crystals average about 
9-25 mm. in longest diameter. Occasionally fractured grains occur which are 
almost opaque in white light, but easily studied and identified with convergent. 
light. 


Andalusite.—On the whole, grains of this mineral assume much larger 
dimensions here than in former instances. They are rarely well crystallized, 
but tend to occur as rudely triangular fragments, fractured, with ‘ pitted ’ 
surfaces, and often crowded with inclusions. The blood-red pleochroism is 


1 «British Resources of Sands & Rocks used in Glass-making’ 2nd ed. 
GSi8) pp. 30, 31: 


@ 1G. S. No. 312. I¢ 


364 MR. H. B. MILNER ON THE [ vol. Ixxviu, 


shown by many individuals, but several of the grains are bereft of this pro- 
perty, particularly the clearer and more irregular types. The refractive index 
and birefringence are normal, and a few grains exhibit biaxial negative inter- 
ference-figures. The average diameter of the grains is 0°5 mm., and fre- 
quently natural grading factors have produced local concentrates composed 
of individuals almost without exception conforming to this dimension. In both 
physical and optical characters, this andalusite differs in no fundamental 
respects from that found in the more northern localities. 


The other minerals present call for no special remark, save possibly the 
brookite, of which two grains were identified in the samples taken from 
the base of the deposits. ‘These grains exhibited characteristics similar to 
those possessed by the same mineral noted in the St. Erth Beds. 


VI. CoMPARISON AND CORRELATION OF THE DEPOSITS. 


Average mineralogical composition. 

| | 

| 

Species. | wSteAenest |) OSt.sbirth: St. Keverne. | 

| } 
Maonetiteni once se eee 7 7 7 
Ilmenite 5 5 8 
| Garnet eee PeRpnee Y 8 3. — i — 
hed Wesph wooo Fitrys) eueyh, san inne ee Ast 8 | 8 8 
SGaMrolikerms-os ee te cece 3 A 1 
IRB pYdobe irc cacti: coset lacs 4 4 4 
Ole cite ee oer cee 2 — it 
Bionibe bait rely 5 Oe 2 | 1 1 
| Leucoxene............ G if v 
| Zircon sr BR eae Bone Cr oa te 5 9 4. 
Kivamitie steeds res eee 6 5 -— 
| SAGHAGASE ., Be Saco cierse ss = spastewils or 2 1 5 
| Rutile 2 6 4. 
| Brookite — 1 2 
| PAOD Zy oP Ee ceca anne are ea 5 4, 5 
Anpainsitie: © Sie ohare oe: 76 5 9 
Muscovite.......... 5 4A 5 
Corundum 2 — = 
Cassiberite ~.-..::.. i A 4 
Xenotime ......... Pala P = 


[Conventional signs as in the previous tables. | 


If we compare the average mineralogical composition (heavy 
residues) of the St. Agnes, St. Erth, and St. Keverne deposits, 
taking also into account the crystallographical, physical, and optical 
properties of the component species, it soon becomes abundantly 
clear that, although we are dealing with isolated outcrops, the 
deposits all present marked petrographical affinities suggesting a 
close genetical relationship. Reviewing the results of the inves- 
tigation from the purely mineralogical standpoint, and in order to 
validate subsequent questions of correlation and source of origin of 
material, we have to enquire into the cumulative evidence furnished ~ 
by the more important mineral species, especially with regard to 
their frequency or infrequency of occurrence, their persistence in 
each locality, and the constancy of their mineralogical properties 
as an indication of consanguinity of origin. 


part. 4] 


PLIOCENE DEPOSITS OF CORNWALL. 365 


(i) Frequency of Occurrence (deduced from the average 
mineralogical composition of each deposit). 


Abundant. 
Common. 
Scarce. 
Rare. 
Doubtful. 


Maenetite, tourmaline, leucoxene, andalusite. 
Ilmenite, zircon, topaz, muscovite, kyanite. 
Epidote, rutile, cassiterite, staurolite, anatase. 
Garnet, chlorite, biotite, brookite. 

Corundum, xenotime. 


Gi) Persistence of Species: (Distribution Diagram). 


Mineral. 


| Tourmaline 


Biotite 


Zircon 


Maenetite ........ 
WWhameraane sea ceees 
Garnmete manseae: 


Staurolite Geese 
Bipidote: 2/2. ..04: 
(OUT O TE cee ee ee 


Bee eee ee 


IMEWRCORCIG!... ii cccdssunacce- 
1) 20) ee 
| ASIC ICIS ee ae rr 
RUGS .2. cists d.. 
ISTOOKIGE — asec auc 
MOA oo. vk 
Andalusite ..... 
| Muscovite ........ 
@Cormmoum .....0.. 
Cassiterite ..... 
Menotime ........ 


Cn ea 


wee eee 


| 
St Agnes. St. Erth. St. Keverne. 


Hl 
IM AIM 


(iii) Constant Mineralogical Features noted throughout. 


Magnetite. 
Ilmenite. 
Tourmaline. 


Ss taurolite. 


Epidote. 


Zircon. 


Kyanite. 


Anatase. 
Rutile. 


Rounded octahedra or grains. Not ragged. 

Commonly irregular and ragged. 

Prismatic and rounded grains both present. Deep-brown 
basal sections with interference-figures very common. 
Numerous and parti-coloured grains occur in all cases. 

Well-worn grains with similar physical and optical pro- 
perties noted throughout. 

Always sharply angular, greenish-yellow in colour, many 
grains exhibiting partial interference-figures. Weak 
but characteristic pleochroism. 

Extremely well-rounded prismatic forms characteristic in 
all cases. A few of the ‘stumpy’ prismatic varieties 
always present. 

Precisely the same type present in both the St. Agnes and 
the St. Erth deposits. The rounded, elongated prisms, 
showing traces of a well-marked (010) cleavage, are 
always present. 

Commonly in indigo-blue tabular crystals, seldom abraded. 

Well-rounded ‘foxy ’-red or reddish-brown, feebly-pleo- 
chroic grains are the prevalent type, with a subordinate 
amount of yellow translucent fragments. 

Die 


366 MR. H. B. MILNER ON THE [ vol. Ixxviu, 


Topaz. Invariably clear, colourless and transparent, and in irre- 
cular fiakes. 

Andalusite. Well individualized as regards optical properties, parti- 
cularly that of pleochroism, which is intense in many 
grains. Often well crystallized, although usually turbid. 
with alteration-products such as kaolin; contrasting 
strongly with topaz in these respects. 


The foregoing species have been selected on account of their 
importance quantitatively, and on account of their value for 
purposes of correlation. 

The marked similarity of the mineralogical composition of the 
sediments and of the more precise physical and optical characters. 
of the component species, can only be explained on the assumption 
of derivation from a homogeneous ‘ distributive province,’ using 
the phrase to include both igneous and sedimentary facies.! Quite 
apart from general lthological resemblance and similar mode of 
occurrence of the deposits, we find that (excepting two doubtful 
minerals), in only four cases are species non-persistent throughout, 
and of these all save one (kyanite) are comparatively unimportant.. 
Such uniformity in composition is most significant, and is parti- 
cularly striking in the present case. Again, the frequency of 
occurrence of the important species, and the similarity of 
erystallographical and optica] properties, and of the nature and. 
degree of abrasion exhibited by these species, are factors which 
we cannot overlook, and are strongly suggestive of the truth of 
the hypothesis above postulated. It would seem not unreason- 
able, then, to conclude that the deposits were the product of a 
definite period of erosion, subsequent denudation, probably marine, 
determining the removal of much of the detritus which had aecu- 
mulated; the survival of a few isolated remnants of the original 
mass was by no means fortuitous, but due entirely to favourable 
conditions of deposition. On this basis, the initial assumption of 
contemporaneity of the deposits under discussion becomes a distinct 
probability, and receives support not only from the mineralogical 
evidence cited above, but also from topographical criteria to be. 
discussed in the sequel. 


VII. Sourck or ORIGIN OF THE DEPOSITS. 


Any well-differentiated phase of detrital deposition must of 
necessity give rise to sediments reflecting the nature and composi-. 
tion of the parent rocks of the comminuted material; it follows, 
therefore, that an enquiry into the normal petrological habitat of 
the several mineral species concerned should furnish us_ with, 
valuable evidence as to the source of origin of the deposits, which, 
when combined with the geological and physiographical knowledge: 
obtainable from the area to-day, should serve as a foundation for 
a geographical reconstruction of this part of Britain at the epoch 
under consideration. The results of such an enquiry show that. 


1 The term ‘ distributive province’ was suggested by Mr. A. Brammall, M.Sc. 


part 4] PLIOCENE DEPOSITS OF CORNWALL. 367 


with the exception of two or possibly three of the mineral species 
‘characterizing these deposits, all occur in rocks known to-day in 
the vicinity of the outcrops under consideration: in other words, 
85 per cent. of the species are probably of local origin. With 
regard to xenotime, this mineral has not been recorded from the 
acid i2neous rocks of Cornwall, so far as Lam aware; but there is 
no reason why it should not be present in some of the granite- 
masses, even though sparsely distributed. Staurolite is essentially 
an ‘ Armorican ’ mineral, with a type-locality in Britanny to-day: 
its origin may be more pertinently discussed hereafter (p. 370). 
“The ky: anite is probably in the nature of a remanié mineral, and 
its ery ‘stallographic features point to derivation from a pre-existing 
sediment, such as the Lower Greensand; the nearest outcrop of the 
Lower Greensand, containing kyanite similar to that of St. Agnes 
and St. Erth, is in the north-west of Wiltshire: the significance 
of this fact will be ultimately apparent. 

The cumulative evidence to be obtained from a study of these 
minerals shows them to have belonged originally to a distributive 
province which included acid, basic, and ultrabasic rock-types, 
with an associated suite of metamor phie rocks: such a province 
clearly is only indicative of Cornwall itself where all such rock- 
facies are known to occur. Excluding, then, the staurolite and 
kyanite, the origin of which is discussed hereafter, the mainly 
local derivation of the deposits under investigation 
would seem to be most decidedly indicated. 


VILL. Priccene GEoGRAPHY OF THE SOUTH-WEST 
oF ENGLAND. 


One of the most difficult problems facing the geologist in 
Western Cornwall is the adequate explanation of certain out- 
standing physical features, for which there is but little direct 
geological evidence; among such features that of the St. Erth 
valley (see map, fig. 1, p. 350) i is probably the most striking, and 
a close investigation of its nature and origin 1s a sive gud non in 
any intellig ent discussion of Pliocene palzeogeography. 

The late Clement Reid, in a paper ‘On the Probable Occurrence 
of an Eocene Outlier off the Cornish Coast,’! drew attention to this 
valley, and interpreted its origin in the light of his theory of the 
existence of Eocene gravels on the floor of Mounts Bay. He in- 
clined to the view that the St. Erth valley was the site of an old 


Eocene river tlowing probably from north to south through this. 


valley, the river being responsible for the transportation and 
accumulation of the material in the bay. Thence the inference is 
that the initiation of this valley was the product of early Hocene 
erosion, the material, composed of 86 per cent. of Chalk flint and 
2 per cent. of Greensand chert, being derived from Cretaceous 
rocks, presumably from the north-east. That the valley was not 
originally formed in Pliocene times is apparent from careful 


1 Q. J.G.S. vol. lx (1904) p. 113, 


RO eR ee 


368 MR, H. B. MILNER ON THE [vol. Ixxviii, 


observation to-day; the mode of occurrence and the elevation of the 
St. Erth deposits at 170 feet above sea-level both tend to suggest 
a greater antiquity, and hence a probable Eocene age. On this. 
assumption, the belief is justified that the whole of South- Western 
Cornwall, if not the greater part of the South-West of England, 
stood then ata higher level than it does now, and the forces of 
denudation probably continued to act unchecked throughout Oligo- 
cene and early Miocene times, and possibly later. Such a belief 
can only be based on analogy with contiguous areas to the east, 
since in Cornwall itself we have really no direct evidence of the 
trend of geological events between Upper Paleozoic and Pliocene 
times—an enormous gap in the stratigraphical record. 

With the renewal of folding along the ancient Armorican lines 
in Miocene times, the effects of which are so marked in the Isle 
of Wight and the Isle of Purbeck, it is possible that considerable 
changes were wrought in the surface-relief of Southern Cornwall, 
although it is by no means certain that these tectonic influences 
extended so far westwards; if they did, it would be expected that 
the tendency was towards the production of asymmetrical fold- 
features with a gentler dip southwards, as is the case in these 
localities, though of far less severity in the west where the move- 
ments were decreasing in intensity. In Cornwall the effect of 
such deformation would probably only be an accentuation of the 
chief Paleogene physical features, and such drainage-courses as. 
the St. Erth valley with its southerly gradient would receive an 
increased impetus in the work of discharging material into the 
English Channel. Even if the original early Tertiary drainage of 
the St. Erth valley were in the opposite direction: that is, from 
south to north (which is extremely unlikely from all that we can 
gather of its history), the result of Miocene earth-movement 
would still tend to produce drainage into the English Channel in 
South-Western Cornwall, and at the close of Miocene times the 
St. Erth valley would in any case be well differentiated. What- 
ever happened during this pre-Pliocene period, two factors stand 
out very clearly from the late Miocene records: namely, the general 
subsidence of the area in ear ly Pliocene times and the concomitant 
accentuation of the ‘400-foot platform’ asa result of marine erosion.! 
This Phocene platform is one of the most characteristic physical 
features of Cornwall, and it can be traced at heights varying from 
370 to 420 feet above O.D., at intervals along both the southern 
and the northern coasts of the county, extending from the coastal 
margin well inland to the higher Paleozoic country which consti- 
tuted the ancient Pliocene land-area (shown in the accompanying 
sketch-map, fig. 1, p. 350). It is difficult to estimate the precise 
amount of this subsidence from the data at present available; but, 
taking into account the St. Erth deposits, the character of their 
fauna, their lithology, and present topographical evidence, Clement 


1 See Mem. Geol. Surv. Sheets 351, 358, 346, 359, and H. Dewey, Q. J.G.S. 
vol, lxxii (1916-17) p. 63, 


part 4] PLIOCENE DEPOSITS OF CORNWALL. 369 


Reid’s estimate of 340 feet is probably under rather than over the 
actual amount, particularly when the St. Agnes and St. Keverne 
deposits are also considered.! The difference in topographie level 
of the St. Erth deposits compared with those of the other localities 
ean only be due to the former having been laid down in a pre- 
existing hollow which, as subsidence progressed, became ultimately 
modified by the sculpturing of the ‘4.00-foot’ feature: this implies 
deposition of the St. Erth material in deeper water than in the 
ease of the other deposits, a belief justified both on paleontological 
and on petrographical grounds, and by comparison with the 
markedly shallow-water character of the St. Agnes and especially 
the St. Keverne facies. Thus the St. Erth valley, from its initiation 
as an early Tertiary river-course, became transformed at the close 
of Miocene times into a strait separating the Land’s End area from 
that of the main Cornish land-mass ; and it is noteworthy that, 
even with the present configuration of the land, a subsidence of 
only about 150 feet would be required to re-establish such condi- 
tions. With a subsidence of 364 feet, as above postulated, the 
geography of Western Cornwall assumes widely different aspects, 
the peninsula becoming in fact a series of large and small islands, 
a miniature West Indies. 

Under such conditions of submergence the present nature and 
disposition of the Pliocene sediments is readily understood : 
St. Agnes, the Beacon, rising to a height of 628 feet, becomes 
part of a large island, the northern and leeward shoreline of which 
formed the upper limit of the submarine shelf upon which the 
deposits were laid down. In the vicinity of St. Erth and Lelant, 
much larger islands existed on the west and on the east, the former 
comprising the Land’s End granite and metamorphic area, the 
latter comprising the petr ographically similar uplands of Carn Brea, 
Carn Menellis, and the Godolphin Hills. AtSt. Keverne, the Carn- 
Menellis mass 1s again the dominant background feature, and the 
gently sloping Lizard platform on the south becomes the locus of 
deposition of a mass of sand and gravel thence derived, a remnant 
of which has survived late Tertiary erosion and is preserved in the 
form of the present outcrop on Crousa Common. Then, as regards 
the Polerebo deposits, in my opinion their occurrence at a height 
of 480 feet above O.D. can only with extreme difficulty be recon- 
ciled to this phase of geological history: if, in the absence of all 
other evidence, they are to be regarded as having been formed 
collaterally with the Phocene deposits, how can we expiain their 
present anomalous position above sea-level, and the absence of any 
like feature, either lithological or topographical, elsewhere in the 
county? ‘Their general character would seem to fit in far more 
reasonably with Quaternary erosion than with any older period of 
denudation. 

Hence, reviewing the chief features of these deposits of South- 
Western Cornwall in the light of the foregoing topographical data, 


1 «The Pliocene Deposits of Britain’ Mem. Geol. Surv. 1890, p. 65. 


370 MR. H. B. MILNER ON THE [ vol. Ixxviil, 


it is not difficult to understand the similarity of composition of 
their constituent minerals, their predominantly local derivation, 
the physical features of the grains (particularly quartz) as con- 
noting the abrasive action oo sea-water, and the indication of the 
deeper- water character of the St. Erth facies when compared with 
that of the other localities. 

Lastly, the sources of origin of both the staurolite and the 
kyanite as ‘foreign’ minerals, and their bearing on the wider 
problem here discussed, have to be considered. In the case of 
the staurolite, the most probable source of derivation would seem 
to be from the south, from a land-mass, though now submerged, 
having geological and tectonic connexion with Britanny and the 
North-West of France, where we know this mineral to occur in 
quantity. The researches of Dr. H. H. Thomas! have shown that 
the staurolite in the Bunter deposits of Devon and Somerset 
probably had this origin, in view of the fact that the Armorican 
massif is generally considered to have extended much farther north- 

estward at the close of Upper Paleozoic times; although it was 
afterwards largely submerged, particularly in the late Cretaceous 
Period, renewed folding and uplift j in Miocene times, involving the 
South of England and the North-West of France, determined 
the reappearance of this Armorican continent in the area now 
occupied by the English Channel, and the concomitant westward 
recession of the sea. The effect of this uplift would be to promote 
drainage both from Southern England and from the Armorican 
continent into a broad central river flowing westwards along the 
basin of the English Channel, and probably emptying itself into 
the sea at a point some miles south of the present Cornish coast. 
The action of marine currents bordering the western shore-line of 
this uplifted area would tend to the lateral distribution of the 
detritus brought down, although with a strong inclination to a 
northerly direction, owing to the probable south-westerly on-shore 
wind prevailing from the open ocean, as is the case at the present 
time. The comparative rarity of staurolite in the deposits, how- 
ever, suggests a gradual alteration in potency and direction of these 
marine “currents, such as would be occasioned by subsidence of 
the land-areas and the advance of the sea eastwards up the 
Channe: once more; such factors reflect the initial conditions 
caused by the early Pliocene submergence to which reference has 
already been made. 

In the case of kyanite, the evidence is much more obscure, 
though derivation from the north-east is suggested on petrogra- 
phical erounds. Attention bas already been called to the marked 
similarity between the Pliocene kyanites and those commonly 
occurring in the Lower Greensand; in order to test this point 


further, it became advisable to examine the non- magnetic residues 
from the Lower Greensand in the vicinity of Seend (Wiltshire), 
some 4 miles west of Devizes. Accordingly, such an investigaticn 


1 Q. J. GS. vol. lviii (1902) p. 630. 


part 4:] PLIOCENE DEPOSITS OF COlNWALL. 371 


was made, and with entirely satisfactory results. In figs. 2-4 
from drawings of species occurring in St. Erth, St. Agnes, and 
North-Western Wiltshire, it will be seen that there is a marked 
similarity in crystal form in all cases; this is supplemented by a 
eareful comparison of all the kyanite-grains in the Seend residue 
with those in the Cornish deposits. On the whole, the Pliocene 
grains are more worn and rounded than those of the Lower Green- 
sand, as we should expect; but the species from Seend show the 


Fig. 2.— Kyanite from Pliocene, Wig. 3.-—Kyanite from Plio- 
St. Erth (x70). cene, St. Agnes (x70). 


same characteristic prisms terminated by (001) or basal parting, 
- further individualized by cleavage-traces parallel to (010) and 
(OO1), as well as the short ‘stumpy’ varieties met with in the 
Phocene deposits. This mineralogical similarity seems, at first, to 
be but slender evidence on which to found a source of origin for 
the kyanite occurring in the Cornish deposits; but, when certain 
other factors are taken into account, it soon becomes apparent that 
this north-eastern region was indeed the most probable source of 


supply. 


372 MR. H. B. MILNER ON THE [vol. Ixxvii,. 


The evidence of the existence of an ancient Eocene drainage 
from north to south has already been mentioned; as a matter of 
fact, the occurrence of scattered Chalk flints is by no means un- 
common in Cornwall, as, for example, at Newquay! and Padstow,” 
where in the latter locality it 1s associated with Greensand chert. 
as at Ludgvan,? the area examined by Clement Reid. The 
significance of this association of chert and flint is too strong 
to be ignored; clearly it suggests derivation from the east or- 
north-east, where the typical developments of Cretaceous strata 
are known to occur. If we bear in mind the probably greater 
westward extent of such strata prior to early Tertiary erosion, it 
would seem not unreasonable to conclude that this material was. 
carried south-westwards by an ancient river-system draining approx- 
imately along the line of the present Bristol Channel, receiving 
affluents possibly from South Wales on its northern bank and from 
Exmoor Forest and the northern part of Dartmoor on its southern: 
bank, and finally emptying itself into the English Channel by way 
of the St. Erth valley. Such a drainage would be quite compatible 
with the little knowledge that we possess of the physiography of 
the South-West of England in the Paleogene epoch, before the 
renewal of folding in Miocene times. There is little evidence to. 
show that this folding entirely obliterated any western drainage 
previously existing ; on the contrary, such a trend was accentuated 
rather than opposed, for a north-and-south watershed was probably 
well established west of the Cotswold Hills in early Tertiary times, 
differentiating a main east-and-west drainage, the former into the 
Thames basin, the latter into the Bristol-Channel and Exmoor 
region, although the precise location of this watershed is at present 
obscure in so far as our information goes. Miocene folding 
merely cut short the southward extension of that watershed by the: 
superposition of an east-and-west line of flexures, now indicated by 
such tectonic features as the Kingsclere, Pewsey, and Mendip: 
uplifts. Consequently, the ancestral drainage of rivers draining: 
westwards, far from being obliterated, was resculptured to form 
the basis of the present river-system in this region, and material 
continued to be borne westwards until the Plocene submergence: 
set in. With this submergence came the gradual advance of the 
sea over the Miocene land-areas of the South-West of England, 
the drowning of pre-existing lowland topography, and the limita- 
tion of the distributive power of the sediment-bearing rivers. If 
the apportionment of the kyanite-grains in the Pliocene deposits. 
is any criterion of these fundamental physiographical changes, then: 
we can readily appreciate the absence of the species at St. Keverne, 


! Mem. Geol. Surv. Sheet 346 (1906) p. 65. 

2 Tbid. Sheets 335 & 336 (1910) p. 93. 

3 Ibid. Sheets 351 & 358 (1907) p. 68. 

4 Some light was thrown on this problem recently by Mr. W. D. Varney 
at a meeting of the Geologists’ Association on May 6th, 1921, when a paper 
on the ‘ Geological History of the Pewsey Vale’ was read (Proc. Geol. Assoc.. 
vol. xxxii, p. 189). 


part 4 | PLIOCENE DEPOSITS OF CORNWALL. 373: 
its appearance at St. Erth, and its still commoner occurrence at 
St. Agnes, St. Keverne being the southernmost and St. Agnes. 
the northernmost locality. The probable trend of advance of the 
Pliocene sea was from south-west to north-east, and obviously as 
this sea deepened, a constantly diminishing amount of material 
would find its way from the north-east to the submerged parts of 
Cornwall, and the influence of the drainage would be progressively 
curtailed. Again, the occurrence of staurolite at St. Agnes would. 
seem to suggest the influence of south-westerly marine currents at 
least as far north as that locality, which further explains the 
absence of kyanite in the extreme south. 

Finally, the possibility of derivation of the kyanite from any 
pre-existing Eocene deposits in South-Western Cornwall is nega- 
tived by the entire absence of Chalk flints and Greensand cherts 
from the Phocene deposits, indicating the extensive erosion that 
the older deposits suffered in early and Middle Tertiary times. 


IX. SumMMARY AND CONCLUSIONS. 


The results of the investigation may now be summarized as. 
follows :— 


(1) The petrographical characters of the St. Agnes, St. Erth, 
and St. Keverne deposits have been shown to be substantially the 
same, and they suggest derivation from rocks belonging to a 
homogeneous distributive province’ such as would be “furnished 
by the Paleozoic rocks of Cornwall. 


(2) The geological age of the St. Erth Beds has previously been 
proved palzontologically to be early Pliocene. Although no fossils 
occur in the St. Agnes or St. Keverne deposits, and since the latter 
are the products of contemporaneous erosion, by petrographical 
correlation their Plocene age is established. 


(3) In the case of the alleged occurrences of Pliocene material 
at Canons Town and Polcrebo, in neither instance is the evidence 
sufficient to establish relationship with the other deposits, although 
there is little doubt that, if an outcrop exists at the former locality, 
it has a direct connexion with the St. Erth material on the 
opposite side of the valley. The Polerebo gravels are, in my 
opinion, the product of much later erosion. 


(4) The topographical evidence furnished by the ‘400- foot’ 
plateau has been used, in conjunction with the petrographical 
investigation of the deposits associated with it, as a means of 
reconstructing the early Pliocene geography of Cornwall. The 
occurrence and distribution of the two species, staurolite and. 
kyanite, have been shown to be of specific value in this connexion, 
the former as connoting the existence of the ancient Armorican 
land-mass, the latter as indicating the direction of drainage from 
the north-east, at the close of Miocene and the commencement of 
Pliocene times. 


il| 


B74: MR. H. B. MILNER ON THE [vol. Ixxviii, 


In conclusion, it is hoped that the methods employed and the 
results obtained from this investigation will indicate a possible 
solution in other cases where paleontological evidence is scanty or 
wanting; used cautiously and over limited areas, such petrographic 
methods should prove invaluable to the stratigrapher. 


It remains for me to record my thanks to Prof. W. W. Watts, 
Dr. H. H. Thomas, and Prof. P. G. H. Boswell for their kind help 
during the course of this work. Mr. G. M. Part very kindly 

. . . 2 ¢ = 2 
reproduced the kyanite-grains shown in figs. 2-4, and I am also 
indebted to Mr. G. S. Sweeting for the assistance which he has 
rendered during the preparation of the manuscript. 


DISCUSSION. 


Prof. P. G. H. BoswEtn welcomed the paper as a contribution 
‘to our knowledge of the petrology of the Tertiary deposits of the 
West of England. The Author’s records of the characteristic 
minerals were confirmed by the speaker’s own identifications. It 
was a noteworthy but unexplained fact that, while the Permian, 
Trias, Lias, and Inferior Oolite of the West of England were 
characterized by an abundance of garnet, that mineral was absent 
from, or extremely rare in, the Cretaceous, Eocene, Oligocene, 
and Pliocene of Devon and Cornwall, although it was a common 
constituent of the Paleozoic rocks of Devon, Cornwall, and 
Britanny. ) 

He enquired why the Author desired to have the staurolite in 
the St. Keverne sands derived from the old ‘ Armorican’ land on 
the south-west, unaccompanied by its usual associate, kyanite ; 
while, in the case of the sands at St. Agnes and St. Erth, he 
assumed a Lower Greensand origin in the north-east for the same 
two minerals. The absence of kyanite at St. Keverne seemed to be 
inconclusive in the matter of the derivation of the material. 

The paper was valuable for the additional light which it threw 
upon early Phocene geography. The Author’s conclusions, although 
novel, were not necessarily inconsistent with our ideas of the 
distribution of Plocene land obtained from a study of the fauna 


“of the St. Erth clays and the Crags. That the migration of 


southern species into the North-Sea basin over what is now the 
English Channel and Kent was possible in early Pliocene times, 
but was prevented during the deposition of the Middle and Upper 
Pliocene, presumably by the raising of a land or shallow-water 
barrier, is indicated by the molluscan faunas. Further, the totally 
different character of the mineral assemblages of the Cornish and 
East Anglian Pliocene (the latter having apparently been derived 
from the south-east) supports the view. Moreover, the similarity 
in certain mineralogical respects (notably in the rarity of garnets) 
of the Lenham Beds and the Cornish Pliocene may prove to be 
more than mere coincidence. 


part 4] PLIOCENE DEPOSITS OF CORNWALL. 375: 


Mr. G. M. Parr said that he had hoped that this paper might 
suggest some reasonable source for the very similar assemblage of 
minerals found in some of the Glacial drifts of Pembrokeshire, 
which it could only be surmised were derived from some similar: 
Tertiary deposit. As the Pembrokeshire drifts were derived from 
a westerly or north-westerly direction, this would involve a northern. 
drainage, if Cornwall had provided any of the material for a deposit 
from which the blood-red pleochroic andalusite had found its way 
into the gravels. 

Dr. H. H. Tuomas agreed with the Author that the kyanite. 
of these deposits was most probably derived from sediments of 
‘ Cretaceous age. ‘The absence of kyanite from the Western New 
Red rocks was strong evidence in support of the Author’s view. 
He thought that the grains of staurolite with frayed-out cleavages. 
owed this distinctive character to their derivation from laree. 
erystals: that was tne conclusion to which he had come from a 
study of the abundant and large grains found in the Western: 
Trias. With regard to the great similarity between the mineral 
assemblages of the Cornish Pliocene and the drift-sands of Pem-- 
oe nae. he had many years ago suggested the latter’s partial 
derivation from Tertiary deposits occupying, in Glacial times, a. 
position in the bed of the Irish Sea. 

Mr. G. Barrow was greatly interested in the Author’s work, as. 
it dealt with what are commonly known as Pliocene deposits, now 
occurring at heights much above sea-level. The Geologists’ 
Association had taken a special interest in these high-level deposits,. 
which occurred in patches extending from the south-east of Kent 
at least as far as the neighbourhood of the Guildford gap in the 
Chalk. Apart from the Lenham Beds, fragments of undoubtedly 
marine shells have been found in the gravels of Headley Heath.. 
The mode of occurrence of these is important, as it suggests where 
to look for further casts. They occur only when the sand. sur- 
rounding the original shell-fragments has been firmly cemented 
by iron-oxide, and nowhere else. The examination of the deposit 
on the ground suggests that these gravels are beach-deposits, with: 
broken shell-fragments once common in them. All these frag-. 
ments have been dissolved; but, where iron-oxide filtered in and 
cemented the sand before solution took place, casts of some frag- 
ments are preserved. It is in such cemented patches that we must 
in future look for evidence of the age and nature of these beds ; 
this knowledge not only shows where to look, but where it is use- 
less to look, and this so far covers more than 90 per cent. of the 
whole. 

Mr. T. Croox said that it was particularly pleasing to note the- 
emphasis laid on sampling. Failure to record a mineral in such 
deposits did not necessarily mean that the mineral was absent; 
but, with careful sampling, such as the Author appeared to have: 
earried out, much better results were likely to be obtained than: 
when records were made on specimens collected haphazard. It 


HN 


alll 


376 THE PLIOCENE DEPOSITS OF CORNWALL.  |[ vol. Ixxviu, 


seemed to the speaker that 1t would be very usefulif the Author 


would include in his paper an account of the method of sampling 
adopted, so that other workers might be able to obtain results 
that were fairly comparative. 

With regard to the mineral composition of the sediments 


described, 1t was perhaps worth while to enquire whether the rela- 


tive amounts of such minerals as garnet, staurolite, and kyanite, 
which were all metamorphic minerals in the ordinary sense, and 
might have emanated from the same ultimate source, were in any 
way connected with differences in chemical stability. Such 
differences had a good chance of asserting themselves where the 
proximate derivation was from other sediments, and where con-. 


sequently the minerals had suffered repeatedly from the action of 


processes incidental to detrital sedimentation, possibly under a 
wide range of climatic variation. 
Dr. J. W. Evans thought it improbable that the relative level 


of land and sea changed to the same extent throughout the area 
-concerned. He did not believe that material could have been 


transported from the north-east by coastal wave-action. On the 
northern coast of Devon and Cornwall it was the north-westerly 
winds that produced the most important transporting action, and 
the movement was from south-west to north-east, not vzce versa. 
If the kyanite had travelled from the Lower Greensand of the 
neighbourhood of Devizes, it must have been by means of river- 


-action, when the Bristol Channel was a tract of alluvium and 


Devon and Cornwall were at a relatively low level. 
Mr. Kt. B. Newton declined on paleontological grounds to 


accept a Pliocene age for the St. Erth deposits. His own studies 
(see Journ. Conch. vol. xv, 1916) of the shells from those beds 


were all in favour of their Upper Miocene horizon, and as exactly 
the same facies was apparent among the Lenham mollusca, he was 


-of opinion that the St. Erth and Lenham Beds were of contempo- 


raneous origin. He mentioned the occurrence of similar species 


-of shells in the Gourbesville deposits of Normandy which had 


been described by M. Gustave Dollfus (Bull. Soc. Géol. Normandie, 
1880, and C. R. Assoc. Frang. Av. Sci., Cherbourg, 1906, 


‘pp. 858-70), and referred to the Redonian stage of the Miocene, 


which is considered the equivalent of the Upper Vindobonian- 
horizon of Europe. The speaker was also of opinion that the 
paleontological evidence proved that the St. Erth as well as the 
Lenham deposits represented fragmentary remnants of the Conti- 
nental Miocene development, which extended from Holland, Den- 
mark, Northern Germany, and Northern France. 

The AurHor, in reply to Prof. Boswell, said that it appeared to 


‘him unlikely that the staurolite could have come from the north- 
east, in view of the absence of kyanite from St. Keverne; if both 


the minerals had been derived from the north-east, kyanite should 
have penetrated at least as far as the staurolite, and it should 


vappear with the latter at St. Keverne, which is actually not the 


part 4] THE PLIOCENE DEPOSITS OF CORNWALI« 377 


ease. In reply to Mr. Part, the Author said that he had noticed 
the resemblance of the andalusite from the Glacial gravels of 
Pembrokeshire to the Pliocene species, but there was little doubt 
in his mind that the source of the Pembrokeshire deposits was from 
the west, possibly from an area now covered by the Irish Sea, 
In answer to Mr. Crook, the Author discussed the methods of 
sampling in the field at equal intervals along the strike of the 
deposits, and at each change of the lithological facies in vertical 
sections. The Author agreed with Dr. Evans that probably 
Cornwall stood at a higher level in early Tertiary times than it does 
at present; he specially drew attention, however, to the distinction 
necessary between the Tertiary deposits now occurring on either 
side of Dartmoor. In reply to Mr. Newton’s criticism of the use 
of the word ‘Pliocene’ in connexion with these deposits, the 
Author said that this was essentially a paleontological aspect of 
the subject, and one which he did not feel competent to discuss ; 
he had always been content to accept as a basis the admirable 
work of the late Mr. Clement Reid on the Pliocene Deposits of 
Britain. 


378 ; MR. 8S. 8S. BUCKMAN ON [ vol. Ixxviii,. 


15. Jurassic Curonotoey: I1—Pretiminary Srupies. Cer- 
tain JURASSIC Strata near EypEsMoutH (Dorset); the 
Juncrion-Bep of Warton Crirr and AssoctaTED Rocks. 
By 8. 8. Buckman, F.G.S. (Read December 7th, 1921.) 


CONTENTS. 
Page 
L, introduction. 2.672 fess nee 378 
It. Stratal and: Paunal Details (22 25 ee ee 380 


(A) Watton Cliff—the main mass. 
(B) The Junction-Bed of Watton Cliff in detail—the 
Watton Bed. 


Ill. The Dating of the Junction-Bed of Watton Cliff ............ 390- 
(A) Upper Lias Succession in other Areas. 
(B) Additional Details concerning the Junction-Bed of 
Thorncombe Beacon. 
(C) The Middle Lias (Domerian) of Thornecombe Beacon. 
(D) Analyses of the Junction-Beds, and Theories as to 
the Watton Bed. 


IV. The White Bed of Burton Bradstock................. Pe A2.0> 


V. Certain Chronolosical Studtes:s .0.3s2. 3 sce eee 431 
(A) Milborne Wick and the Green-grained Marl. 
(B) Haselbury and Hammatoceratids. 
VI. A Palzontological Note—Tetrarhynchia thorncombiensis, 
TROT, NOV's > svayrcsiwelnds nates seek here Gee aoe eo 435 
VIL “Bibliography: a. 52300 od stecdie cosets hoe Sica ee ORE 435. 
VIII. Appendices. 
I: Sections of the Junction-Bed and Contiguous Deposits, 
by James Frederick Jackson. Diam 


Notes by 8. 8. Buckman.] ....... . 436. 
he rhe Upper Lias Succession, by =, E See J. 
Pringle, A. Templeman, and S. 8. Buckman ......... 449 - 


(A) Introduction, by S. S. Buckman. 

(B) Upper Lias Succession near Ilminster (Somerset), 
by L. F. Spath. 

(C) Two New Sections in the Middle and Upper Lias 
at Barrington, near Ilminster (Somerset), by 
J. Pringle and A. Templeman. 

(D) Upper and part of the Middle Lias Succession, and 
Correlation, by 8. 8. Buckman. 


PX. SM Mary.) idates, cee bones Lae eee 454 


I. InrRoDUCTION. 


THIS paper is to a certain extent a supplement to the communi-- 
cation on ‘ Certain Jurassic Strata of South Dorset’ published by 
the Geological Society in 1910, in so far as it contains a study of 
another exposure of the Junction-Bed (Domerian, Whitbian, 

Yeovilian)! discovered farther east along the coast; but it also- 


1 T, 5, pp. 61, 64, 82. These numerals in the footnotes throughout refer to- 
the Bibliography, § VII, pp. 435-36. 


part 4: JURASSIC CHRONOLOGY. 379 


gives a sketch of other Jurassic strata, the sequence of which 
should be of some importance in connexion with the chronology of 
the Oolite rocks. 

About three-quarters of a mile east of Thorneombe Beacon ! 
is Eypesmouth, a break in the cliffs where the little stream, the 
Eype (pronounced ‘ Hep,’ to rhyme with sheep) runs into the sea. 
Kast of Eypesmouth is a lofty cliff of about a mile extending to 
West Bay—the harbour of Bridport. 

The cliff is interesting geologically for three considerable faults : 
one, seen best about a quarter of a mile east of Eypesmouth is 
a fault of about 500 feet, bringing down Forest-Marble Beds to 
about the level of Yeovilian strata (Duwmortieria hemera) ; another, 
about the same distance west of West Bay, brings down beds of 
Fullers’ Earth almost vertically—it is a fault of about 120 feet ; 
the third, immediately west of West Bay, shows Yeovilian strata 
of Dumortierva hemera or later—they are later than those on the 
west 01 the cliff—lying level with lower beds of Fuller’s Earth: 
a drop as regards the latter of about 150 feet. So the middle 
and main portion of the cliff is let down by these faults. (See 
diagram 1, sketch-elevation of Watton Cliff, p. 383.) 

The cliff faces about south-west by south and the faults run 
approximately west and east. The effect of the westerly fault is 
that destruction of the cliff has been hastened—there is a con- 
siderable recess from Eypesmouth to where the fault is now most 
visible. the cliff having been cleared back roughly along the line 
of fault, except for some more or less tumbled strata at its foot. 
The line of fault can be traced from what may be called ‘the fault- 
corner’ more or less towards HKypesmouth, where it disappears 
beneath the sand and shingle of the shore. This fault is shown on 
the l-inch Geological Survey map, Sheet XVII, extending inland 
some 8 miles to the foot of the Chalk Downs—the Dorset 
Heights—near Long Bredy. It and the other faults run approxi- 
mately parallel to, and are no doubt part of, the system of the 
Weymouth Anticline. <A ground-plan of the cliff and the faults 
is appended (diagram 2, p. 384). 

In the autumn of 1916 a large block lying on the platform of 
the fault-corner attracted my attention.2 A portion that had 
been broken off exposed a weathered surface with Theczdelle, 
and in some marl connected with them were unfamiliar ammonites 
in a fragmentary condition— Hildoeeratids presenting almost the 
appearance of those which characterize the discites-beds of the 
Inferior Oolite. Further search showed specimens of Tetra- 
rhynchia thornecombiensis, nom. nov.,? evidently derived. 

Here was a case for investigation. The rock from which this 
block had fallen was subsequently located high up in the cliff-face, 
with also other tumbled blocks lying on a higher platform. Work 


ie ora Oo- 
* This block has disappeared, buried perhaps by a slide of clay. 
3 See later, Paleontological Note, p. 435. 


Q. J.G.8. No. 312. 2D 


a 


SS a a ee a ee 


a ee CO a 


380 MR. 8. 8S. BUCKMAN ON [vol. lxxviu, 


on these blocks showed that they were in the position of the 
Junction-Bed of Down Cliffs to Thorncombe Beacon, described 
in my former paper,! but that this exposure of the bed differed 
remarkably from what is found only about three-quarters of a mile 
away to the west. The Eype type consists mainly of strata with 
Grammoceras striatulum (G. striatulum sensu lato + thouar- 
sense and other species)—-a thin layer with these fossils was 
seldom present at the top of the Down Cliffs Junction-Bed ; and 
here, at Eype, it is mainly a fine white lithographic stone, 
weathered faces of which show it to have been laid down as a 
fine white mud in paper-like layers: it is a very finely laminated 
bed. As I had, in my former paper, described at Burton Brad- 
stock another white lithographic-stone bed, also associated with a 
fault and connected with Bridport Sands, the various interesting 
questions which arose will be readily understood. 

Further work was done on this bed and the neighbouring strata 
during short holidays in the autumns of 1917, 1919, and 1920. A 
preliminary account of these investigations will, it is hoped, be 
equally interesting to the Society. 

This paper was commenced on my return in 1917, but it was 
mainly written in the winters of 1918-19, 1919-20. During my 
visit to Eypesmouth in 1920, inspection of the bed showed that 
another investigator had taken details of 1t; a few days after- 
wards there came to Eypesmouth a letter from Mr. J. F. Jackson, 
enclosing a section. He kindly agreed to my suggestion that the . 
account of his quite independent discovery and of his researches 
in the Junction-Bed of the western area should form an Appendix 
to this paper. Therefore, I have divided this communication 
into two parts: the present paper, mainly concerned with these 
accounts of the Junction-Bed; and a proposed later paper, to give 
a fuller study of the main mass of Eype (Watton) Clitf—Fuller’s 
Earth to Cornbrash—or the upper portion of the Lower Oolites. 


Il. Srratat ann Fatwat DETarIts. 


(A) Watton Cliff: the main mass.® 


The following is a section of the beds exposed in Watton Cliff 
down to the ‘margaritatus bed,’ showing the sequence, with 


1 T, 5, pp. 61, 64, 82. 

2 Mr. J ackson’ S account i in his Section VIII does not seem to bear out my 
statement made above about Grammoceras ; but then he might not feel suffi- 
cient confidence to identify Grammoceras by peripheries and cross-sections 
showing in a rock-mass. His Section IX would appear to deal with a block 
which I have not seen. 

3 The local name for the hill of which the cliff is the face is ‘ Fourfoot Hill’ 
and for the cliff itself ‘Clay Knapp.’ E.C. H. Day has the name ‘ Fourfoot 
Hill’ (III, p. 286). It may be suggested that the name is really ‘ Forefoot 
Hill,’ from the tumbled platform at the base of part of the cliff, though this is 
to trespass dangerously near the usual error of folk-etymology. It seems 
advisable to distinguish it as Watton Cliff, from the name of the farm 
which lies behind it. 


part 4] 


the position of the Junction-Bed. A consideration of the post 


JURASSIC CHRONOLOGY. 


381 


Inferior-Oolite strata will be reserved for a later communication. 


Section I—WaTTON CLIFF, BETWEEN EYPESMOUTH AND WEST Bay 


Beds 1—4 in 


the middle part 


of the Cliff. 
Bis 


2. 
3. 


(BRIDPORT), DorsET.! 


Thickness. 
Sequence of Strata. (approximate) 
un feet 
Forest MARBLE: massive shelly blocks with clay- 
PALTIMOSS o. oanacaaee tenes Chase oman fa sieeinans si 30 
WirPin: Gin bie MEARE cre etc cee ueeae tare meets 30 


a. MicRoMORPH Osrred BuD: a mass of small) 


C. 
. WHITE MARL. 


d 


oysters rarely 5 mm. long. Pedicle-valve of 
Dictyothyris ; fragments of Acanthothiris (A. 


bradfordiensis Walker ?), 


. The ‘ soverr’ or ‘ RHYNCHONELLA BED’: about 
16 inches thick, mainly brown, crumbly, but 


white and compact for about 2 to 38 inches } 5) 


from the bottom. Full of specimens of Gonio- 
rhynchia,? often crushed; also occasional ex- 
amples of Orinithella sp., crushed, Rarely 
Terebratula langtonensis Walker. 

LOWER GREY MARL. 


LARGE CONCHOIDAL BED: clays which break into 


large pieces. Measured up the cliff from 5 
CORD Oe Les Br CR ane Rabi erate eae ame. oka nee 


OSEREA-ACUMINATA CTHANIS) \ Lite cits owls otidooaceh ack s sedes 
BRACHIOPOD-BEDS*?: stone bands 6 to 12 inches 


thick separated by clays 18 to 30 inches thick. 
The contents of the stone-bands vary, and their 
sequence is somewhat supposititious ............ 


. ORNITHELLA BED. 
. LARGE surrnHit BED: Rhynchonelloidea aff. 


smithvi, rather large examples without other 
brachiopods. 


. ACANTHOTHIRIS BED: A. powerstockensis, Rhyn- 


chonelloidea. smithw, and a large ammonite 
(Parkinsonites ?).4 


. THE GLoBATA BED or TEREBRATULA BED: Stiph- 


rothyris spp.= Terebratula ef. twmida Davidson 
(T. globata auctt. non Sow.)+T. cf. nunney- 
ensis S. Buckman. Rhynchonelloidea cf. smithit. 


. SHELLY BED: small forms of Rhynchonelloidea 


ef. smithit. 


25 


1 See diagram 1, p. 383. 
2 For these and other brachiopod names see Bibliography, I, 6. 
3 Details of the Brachiopod-Beds were mostly obtained from blocks 


scattered on the beach during 1916. 


In 1917 and 1919 these blocks were 


nearly all buried ; towards the end of my visit in 1920 a high tide removed 
shingle, disclosing the upturned edges of some stone-bands along the line of 
fault immediately east of Eypesmouth. These gave some help in the inter- 
pretation of the succession; but, owing to faulting and dislocation, they are 
A detailed section will be given in a later communication. 

+ Now in the Museum of the Geological Survey, Jermyn Street, London. 


not too reliable. 


2D2 


ia VR 897 KOSS tO 


382 JURASSIC CHRONOLOGY. [ vol. lxxvi. 


Thickness 
Sequence of Strata. (approximate) 
in feet. 
6*, OsrreEA-KNORRI CLAYS: possibly Bed 6e is a layer 


im THESE. : ii aah es oe ee a cee eee 


The following beds (7-9) are to be seen in the 
eastern part of the cliff beyond the second 


fault :— 
7. UmBer Bep: umber-coloured clays with a nodular 
Poa 58. cshec ce asapara dele wale ee) agnee Ee CEO ee eT 20 
8. SmALL CONCHOIDAL BED: clays with conchoidal 
fracture, breaking into small lumpg............... 40 


9. OcHRE BAND: a yellowish marly band about 
1 foot thick, seen near the top of the cliff, 
eastern end, resting on 


LAMINATED CLAYS: with light bands of somewhat 
caleareous rock. Occasional lumps of pyrites . 50 


Of the following beds, details for 10 and 11 are 
furnished by Burton Bradstock. There are 
Inferior Oolite rocks, the Black Rocks, just out 
to sea west of the third fault (see later, p. 430). 
Then part of No. 12 is seen immediately east of 
the third fault, and the basal part of No. 13 is 
found with succeeding beds in the cliff west of 
the first fault : 


5 Oy 10.. | BELEMNETHSODAYS)...c.se.cseens eee tee ae heres 15 
feces ‘ 11. Limestone-mass of the Inferior Oolite ............... 16 
VWeovilian 12) BRIDPORTISANDS | 25. c-5--eoce ceeee eee ee eee eee 130 

13, DowN-Crinr OuAY 1.28) Wie een eee 70 


14, JuNcTION-BED: fine creamy-white lithographic 

stone in paper-like lamine (Grammoceras of 

Whitbian striatulum-thouarsense types) and yellowish 
conglomerate, somewhat sandy, with derived 

Tetrarhynchia thorncombiensis. For detailed 

Section,-see later D.OOG.. ocn.-nasn see nee eee 


Domerian 15. THORNCOMBE SaANDs!: yellow sands with doggers 30 
16: Blue: claly 22 eee es Bi ee eee 2 

17. Marecariratvs BED: a prominent and easily re- 
cognized -datum=line 20h. 0).2 seas hen eee eee 1 


18. Down-CuiFF SAnps.? 


Approximate total ...... 624 
The accompanying diagram (1) embodies a general sketch of 
Watton Cliff. 


1 Name given from their occurrence at Thorncombe Beacon. 

2 The equivalent of this at Down Cliffs was wrongly placed in my section 
(I, 5, p. 66) copied from Day, and the thickness omitted. The f should be 
just ‘above g there, and the thickness (6 feet) should be put opposite; see 
BC. Hs Day, TLL. 285: 

> From their occurrence at Down Cliffs; see Day, as above. 


a 0 i i ee ee eee a wee 


439} 08 31 "399J OO =YOUT I [POTIIOA “yoeoq ot} Jo 
jnoqe “OTT I= SOYOUT O [CJUOZIO FY yied sty} Suoje punoj oq Aew 
Ul 32] soyewutxoidde ‘ateas spegq podoryoeig ‘o]sulys pue 
‘ore ya0j OSt i apt} JO sazejs ule}I99 JW 
spurs jnoqge 399} OZI 3293 00S 
oh a eal ee ok re 
pod? 6 I 
=I a 
[== 
SSS 
keg i QUOISOWI-T SjJt[IJopu() pue 
SoM oe podoryse1g PAEMUOR ESD) |pe(aMeen gy, noursadAy 
Were 
6 
BUPUIWINIe é 
/ <— PoSQ , <—‘jsuvse | 8! 
spies . eVeEUIUNIE 
yiodpiig sdum] a31e, ut 


j 
Y 0 
Oll 
Ol 
at 
9/0 
o/b 
0/0 
0¥/0 
oe /-0 
0 
O 
sjueuse1j podomsrig 9 
2JBUIWINIE BIIISQ S 
4 


skyjQ yeploysu0g ¥% 


p9d7,/790,,¢ 


ee qzodpiig 0} oouvi}uq 


(qaeg ssddy) 
spurs yrodpiug 


qney 


ype_ S197 


yney 


Vas 


[ay I-soyout g: BRIG] 


S SFT VOWe M JO. 
26 5 \SH20W API ueyd-puno1s y9}a4S 


— -ZWIeIseIGq 


<a S a104S 


peg-uor oun yo 
S¥I0[q poyquin yp 
9-3Da ISTO 
sped 


S[QIeTA S910 4 CIN PIBUIWUNOG BIIISO 


Souojsounry 
peq*yunf ‘sxyso07q poyquin y podorporig 


BIPJ-JJND Ul pog*joun[- 


oSPA I 
peg-uorjounf jo a yynowsadAg 


StIT 
do194no oyewrxoiddy SIPPHAL 


ynez 


spues yodpiig jesegq 


part 4] JURASSIC CHRONOLOGY. 385 


The general features of the strata of this cliff will be discussed 
in a later paper. Now the Junction-Bed claims attention. 


Diagram 3.—Watton Cliff, near Bridport (Dorset), view of 
Junction-Bed (Watton Bed) in face, looking northwards. 


Bluish sandy clay, with hard beds 
‘at intervals. Basal Bridport 


ie Sands or Down - Cliff Clay Difference 
equivalent. ; 
ie PA rl ed tnveolour 
2,|Junction-Bed. Hard compact | ) 4 oon 
block. 
| Beds 1 & 3 


Yellow sands of Middle Lias, 
faced on a very steep batter by very noticeable 
an irony conglomerate-bed, con- 

3.| taining belemnites (slickensided), | jn the clift- 

ammonites (Dwmortieria, and 
some Whitbian species). Some | face. 

20 feet of fault-face visible. 


Blue Clay. Under about a foot 

of clay is Day’s margaritatus- 
4,| bed—a bluish marly stone: 
fallen blocks may be seen under 
the cliff on the left. 


Diagram 4. 


Watton Cliff, 
near Bridport : view of 


fault, looking westwards Bluish sandy 


clay, with hard 

layers at intervals 
(Basal Bridport Sands 
or Down Cliff Clay 
equivalent) 


Junction-Bed, 
about 5 feet 


Face 
Yellow Sands 


of the 


Fault , 3 
Lip Middle Lias 


Foundered Block 
of Junction-Bed 


of 


\ 
p 


ee ee ee ee 


— eee 


‘sopUuWajIg pure (aisasjs0uINnGq ) 
soytuowMy jo dn=x1ur & 

YM aye19uI0]3u09 Auosrjo 
JoAL] UY} B :990eF-}]Ne,Y 


WIO}}2Iq 


7995 
punois bi sili 7 & 
peyquimny 


A. (pag-voroun{) 
=A peg u037eA jo 
=A = sx9019 PIP 


| 


| 


do} ay} ye Wnayoyey 
"yye sesd Ode py Ves STPAIOJUT Je 
(FR aN s10fe] prey 
yy shes 
ee: Apues ysiniq 
EE TD, 


pessodes Si ‘spremjsom BUIDe; amD oy} jo yred sty 7 ee 


SpleMyyNOS Sure; ‘IHD 
‘spavmgsna buryoo) ynvf fo mara : (gassoq,) quodpug wwaw “frp uojqwy—'G Weiseiq 


part 4] JURASSIC CHRONOLOGY. 387 


Diagrams 3, 4, & 5 are sketches to show the general position of 
the Junction-Bed. Of the face view it may be remarked that on the 
left, as the Junction-Bed runs out to the grassy slope of the cliff, 
it diminishes in thickness and seems to peter out at the surface : 
this is possibly due to solution by humic acid and the removal of 
the clayey capping, and it accounts for the bed not making any 
feature in the slope of the cliff. However, the bed was detected 
in the ploughed field immediately north of the fault-corner, a 
little way down the hill. 


(B) The Junction-Bed of Watton Cliff, or, as it may more 
conveniently be called, the Watton Bed, is now described in 
detailed section, the result of my investigation. For the results of 
Mr. J. F. Jackson’s examination Appendix I, p. 445, should be 
consulted. 


Section II—Warton CLIFF, NEAR BRIDPORT (DorRSET). The Junction- 
Bed, generalized section with details from various fallen blocks. 


Thickness in feet «wnches. 
Layer 1. Irony scale! Dwmortieriz (D. aff. regularis S. Buck- 
man, but more distant ribs, D. cf. falcofila Quen- 
stedt sp.) and belemnites (of tripartitus style)...... 0 1 
2. White lithographic stone, weathered at the edge to 
look like paper-shales. Peripheries of ammonites 
of the Grammoceras-striatulum type, but scarce. 
A Dactyloid (No. 3002) cf. Ammonites crassus Du- 
mortier, non Young & Bird, ‘ Bassin du Rhone’ 
vol. iv (1874) pl. xxvii, figs. 5-7, non cet. Similar 
forms occur in the variabilis beds of the Cottes- 
NTONGCIST 34 sue ak piste Ga gaee Rete tino A Shit cna One gt nee 0 6 
3. Yellow conglomerate, whiter towards the base. Bro- 
ken fragments of Grammoceras-striatulwm type, 
belemnites, Tetrarhynchia thorncombiensis. About 
2 inches from the base Dumortieria sp.?, and about 
at the base Hildoceras of bifrons type ............... 0 7 
4, White lithographic stone like No. 2, with a conspicuous 
line of Grammoceras of striatulum type in section, 
showing peripheries and partly weathered—mostly 
in the topmost 2 inches. Section of a small Naw- 
tiulus in the rock—an unusual form with a large 
umbilicus, a cordate whorl-section, the periphery 
somewhat sharpened. Hammatoceras of insigne 
VCs OAS tEOPORS tee ta ee eee Rian cna eh a tig 0 4, 
5. Similar to No. 4, sharply irregular at the base. At 
one place the bed is 9 inches thick, at another 
14 inches. Many specimens of Grammoceras 
thowarsense-striatulum type, about 4 inches from 
the top, in a somewhat muddy yellowish seam 


1 Tn one case, irony scale presumably lacking, fine yellow sands rest on 
layer 2. They contain small belemnites like B. quadricanaliculatus (Quen- 
stedt), which are also cemented to the top of layer 2. 

2 A micromorph with rounded whorls, undeveloped as to keel, ribs coarse ; 
like the inner whorls of Dumortieria novata 8. Buckman, but the ribs rather 
more approximate. 


DD ee ne 


388 MR. 8. 8. BUCKMAN ON [vol. Ixxviul, 


Thickness in feet inches. 
resting on lithographic stone—some of them are 
in the lithographic stone. These ammonites are 
mostly lying horizontal, but not in all cases.! 
At the same horizon as these ammonites a re- 
versed gastropod, Cirrus sp. ef. ‘ Turbo bertholeti ? 
D’Orbigny’ Moore.? Nautilus with squared whorls. 9°' to 1’ 2” 
Layer 6. Yellowish-brown shelly, more or less conglomeratic 
bed of variable thickness, running up into the bed 
above.” The variation in thickness seems to be 
due to a bodily transported block of the Tetra- | 
rhynchia-thorneombiensis Bed deposited in this 
layer ; specimens of this species broken and whole; 
belemnites. This bed rests upon yellowish shales 
containing peculiar micromorph Hildoceratid am- 
monites (seeps AOS) Ces. 8 ee ee 1 A" toss 
Partly conglomerate, that is, broken up T. thorncom- 
biensis Bed redeposited, and partly lithographic 
stone. Small Dactyloid ammonites; at the top 
Rhynchonella cf. moorei Davidson—a flattish form, 
subcirecular, with coarse ribs, rectimarginate. This 
is the horizon yielding T. thorncombiensis in a 
fallen block on the lowest platform under the cliff, 


and Thecidelle on the weathered top-face............ 0 8 

8. Lithorraphie, stone <..8.5 32 hc. tes oe ee 0 3 
9. More or less conglomeratic with a pink tinge, some 

yellow sand-rock ©2422) oc Ms eee 0 5 

10. Lithographic stone 2 oh ee ee ee 0 6 

Potall:, | cates ee 5 5 


Remarks on the above section.—Fallen blocks gave a 
thickness of about 5 feet 6 inches, and a block measured 77 s7fw in 
the cliff-face gave a similar thickness. Attached to the upper 
surface of some blocks there is a certain portion of sandy deposit 
which really belongs to the sandy marls above (see diagram 3). 

The irony scale which forms the top of the J unction-Bed is 
very noticeable in the fault-face, where this is formed by a partly 
foundered block (see diagram 5, p. 386). In this scale were found 


fragments of Dumor tieria, and there are slickensided belemnites. 


From this scale on a block in the cliff I obtained a broken and 
worn fragment of Harpoceras of mulgravium type, and another 
example came from this top irony scale of a fallen block. 

Blocks of the Junction-Bed on the beach gave the following in- 
formation. Towards the base of blocks, especially where polished 


_ by wave-action, the stone has a pink tinge very suggestive of the 


pink stone (bifrons) of the Western Cliffs.4 “Some matrix sug- 
gestive of spinatum marlstone was seen, but no fossils. A large 
Harpoceras of mulgravium type was obtained from the base of a 
block—the specimen was in fair condition, but was not lying 


1 From ‘about middle of Watton Bed,’ which would be this layer 5, two 
examples of Stolmorhynchia bothenhamptonensis (Walker) with a yellowish- 
white matrix. 

2 VII, p. 210 & pl. vi, figs. 7-8. 

3 From a loose block examples of Stolmorhynchia bouchardi Davidson, with 
a yellow matrix (presumably this bed), were obtained. 25, 


part 4] JURASSIC CHRONOLOGY. 389 


horizontally. A striate Wawtilus of fair size was obtained from 
just below the [lower?] sfrvatulum layer and a little Zeillerva 
(the so-called * Waldheimia lycetti’) was obtained from this 
striatulum layer. 


A comparison of this detailed section of the Watton Bed with 
that of the Junction-Bed in the cliffs west of Hypesmouth 
(Thorneombe Beacon, Doghus and Down Cliffs), as recorded in 
my previous paper,! will show how unlike the former is to the 
latter. 

Thus the former is as much as 54 feet thick, whereas the latter 
is about 2 feet, although it may amount to somewhat more 
occasionally. The former is mainly made up of a lithographic 
stone with Grammoceras-striatulum types, derived examples of 
Tetrarhynchia thorncombiensis, and derived lumps of this 
Ethynchonella Bed. The latter shows sometimes about 2 inches 
of the striatulum layer, while the 7.-thorncombiensis Bed is, 
according to my measurements, some 8 feet below the Junction- 
Bed at Thorncombe Beacon: according to my interpretation of 
Day’s measurements at Down Cliffs,? it is more than 18 feet 
below, about which something will be said later. 

Further, the Watton Bed shows no sign of Marlstone,® which is, 
when present, a richly fossiliferous horizon in the Western Cliffs ; 
nor did I find any strata with the bifrons type of ammonite— 
only examples redeposited along with strzatulum forms. 

In the Watton Bed there are certain pecuhar Whitbian ammo- 
nites—unfortunately in fragmentary condition, mainly only body- 
chambers. Nothing of such forms has been noted for the Junc- 
tion-Bed of the Western Cliffs ; but, of course, all the ammonite 
fauna of that deposit has not been fully examined, for removal 
of this matrix is very tedious. In the Watton Bed, however, 
there were enough specimens to attract attention at once, and 
yet I imagine that they are unusual for Whitbian deposits of the 
South of England. About that it is inadvisable to speak too 
positively as yet—first, because these ammonites require study ; 
and secondly, because the Whitbian ammonite fauna of the South 
of England has been very imperfectly illustrated. These ammo- 
nites have something of the appearance of certain small species 
from the Jet-Rock of Yorkshire—at any rate, they suggest an 
early date in Whitbian, about exaratum, or even before that.* 
The Theeidelle, which are just below them, call to mind the 
micro-brachiopod horizon below the Fish- and Insect-Beds of the 
South-West of England. . 

These matters will require further study later, see p. 400. 


Pleas pe OAc 2 Tipe Sp: 

> J, 5, p. 65. Later observations of a fallen block suggested some Marl- 
stone matrix used up, but no fossils were found. Naturally, blocks may 
vary considerably in their constituents. 

* Curiously enough, Mr. J. F. Jackson has found similar ammonites ata 
higher horizon in the Watton Bed; they are in a much better state of pre- 
servation, and have a white matrix. 


any ie eer 


AS 8. a So Eee, Ate Ogee ae A 


ae 


390 MR. S. S. BUCKMAN ON (vol. lxxviil, 


Ill. Toe Datine OF THE JUNCTION-BED OF WaATTON CLIFF. 
(A) Upper Lias Succession in other Areas. 


In order to understand the evidence of the Junction-Bed of 
Watton Cliff, it is necessary to investigate the stratal and faunal 
sequences of other areas. And, as this Junction-Bed contains 
fauna belonging to dates which range from pre-spimatum to 
Dumortieria—in other words, contains fauna of Domerian, Whit- 
bian, and Yeovilian ages—it is necessary to make a somewhat 
extended investigation (1) as to the sequence of the Upper Lias 
(Whitbian, Yeovilian); (2) as to the Middle Lias of about 
spinatum (Domerian) date in the Junction-Bed elsewhere; and 
(3) as to the Domerian pre-spinatum beds which are found in 
Thorncombe Beacon. First and most important, then, is the 
Upper Lias (Whitbian) succession, so far as the lower part of 
it is concerned. This is given in the following summaries. 


I. SuccEssION IN NORMANDY (according to E. Eudes-Deslongchamps).! 


' bifrons- Marnes moyennes. Ammonites bifrons [| Hildoceras spp.| et 
faleiyerum. serpentinus | =Harpoceras faleiferum et aff. |. 

murleyt ? Argile 4 poissons. 

Leptzna. Couches a Leptezna: L. moorei, L. liasina, Terebratula globu- 

globulina. | lina, Rhynchonella pygmeza. 


Il. StUccESSION IN THE EARLY PART OF THE UPPER LIAS OF 
ILMINSTER (according to Charles Moore, 1867).* 


( bouchardi- (6) ‘ Zone of Rhynchonella bouchardit.’ 
murleyi. | (5) ‘ The Saurian and Fish Zone.’ 
/(4) ‘Zone of Leptena [Pseudokingena| granulosa, Spirifera 
ilminsterensis, and Zellania liassica.’ 
‘The (3) ‘Zone of Alaria wnispinosa.’ 
‘Zone of Thecidium rusticum,’ 


eee * (2) ‘Zone of Leptzna bouchardii, L. moorei’) ‘ Terebratula 
a | (the first abundant, the second rare). globulina. 

(1) ‘ Zone of Leptzna bouchardii, L. moorei’ { Rhynchonella 
\ (and several other Leptznz), p. 170. pygmeza. 


For the time when it was written this is a remarkably detailed 
record, to our present advantage, and we may be grateful to 
Moore for it. But it is interesting to note that, although he 
gives so much detail with regard to these lower beds, he groups 
the sixteen beds above his Saurian zone under the one term ‘ The 
Upper Cephalopoda Beds,’ making only one zone of the basal 
2 inches, and leaving the rest unseparated. Yet now we have 
those beds separated out into some eight or nine zones. There is 
reason to suppose that the zone of Rhynchonella bouchardi was 
rightly separated from these by Moore, and, at any rate for record- 
ing purposes, it should be kept distinct. Moore shows that it is 
post-Saurian Bed—that is post-mur/ey7, if the Alderton Fish-Beds 


' V, pp. 60 et seqg. 
* VII, pp. 132, 170 et seqq. 


part 4:] JURASSIC CHRONOLOGY. 391 


are on the same horizon as his Saurian Bed, which seems likely, 
but is not yet definitely proved. From Moore’s evidence it may 
be judged that the Rhynchonella-bouchardi Zone is pre-falci- 
ferum; but how much earlier? Was it pre-exaratum? Its 
absence from Yorkshire and the failure of exaratum-like ammo- 
nites from the South-West of England, where Rhynchonella 
bouchardi is not unusual, seem to indicate that it is not actually 
of exaratum date. . 


Ill. SuccEssIoN IN THE UppErR LIAS OF STROUD, GLOUCESTERSHIRE 
(according to E. Witchell).! 


[ bifrons- [Ammonites] bifrons, A. serpentinus [ Harpoceras falciferum 

falevferum. et aff.], A. communis [Dactylioceras spp.]. Fish-remains 
rare. 

globulina. | Terebratula globulina and Rhynchonella pygmexa in consider- 


able numbers. 


IV. SUCCESSION IN THE UPPER LIAS OF CHURCHDOWN, GLOUCESTERSHIRE 
(according to F. Smithe, cited by E. Witchell).? 


‘Crustacean ’ [ Bed]. 


[murleyt. ‘ Fish- Bed.’ 
‘ Alga-Bed.’ 
Leptena. ‘ Leptena Bed.’ 


globulina. | ‘Terebratula globulina’ [Bed]. 


V. SUCCESSION IN THE UPPER LIAS AT ALDERTON (DUMBLETON), sum- 
marized from various authorities? and from personal observations. 


| bifrons- 3. Shales with Hildoceras, Harpoceras, Dactylioceras, 
falciferum. 44 feet. 
miley. 2. Saurian, Fish- and Insect-Bed: Light-ochre Fissile Bed ; 


Ammonites murleyi,* 1 foot. 
globulina. | 1. Paper-shales, ‘ Leptena Shales,’ Terebratula globulina, 
Rhynchonella pygmexa, 15 feet. 


This is the northernmost point, I think, at which the small 
Terebratula and Rhynchonella have been observed; but their 
range is wide, as it extends into Normandy. Therefore they are 
good for dating. It is doubtful whether Leptena has been found 
at Alderton (Dumbleton). Moore mentioned the Leptena Clays 
as 15 feet thick there; but he may have recognized them only by 
the presence of the Terebratula and Rhynchonella. H. B. Wood- 
ward’s citation of Leptena from these clays may have no other 
basis than Moore’s remark. I. have collected the Terebratula 
and Rhynchonella, but have seen no other brachiopods there, and 
Mr. Linsdall Richardson mentions the two former, though he does 
not cite Leptena.> Witchell’s evidence from Stroud is to the 
same effect. 


XC ps 2a, 2) QUE 10s ZG: 

NE p49 Xe apaoG: XC leap. oO.) KTV, p. 26: 

‘ The following species of ‘ Ammonites murleyi’ have now been published. 
Murleyiceras murleyi (1, 8, cexvi), M. forte (ecxlv), M. aptwm (ecexvi). 

2 IDG Ios Die 


Sa oe oa Oe 


— - 


a 


Riza EE 


392 MR. 8. S. BUCKMAN ON [vol. lxxvni, 


The point is that, if the various species of Leptena are absent 
from the thick deposit of Alderton, which contains the minute 
Terebratula and Rhynchonella, it may be because the Leptena- 
Beds are not really synchronous with the globulina beds, although 
in places (Ilminster and Normandy) the two may be mixed owing 
to paucity of sedimentation. Smithe’s record at Churehdown 
supplies some evidence in this connexion, giving the globulina- 
bed below Leptena. The data are insufficient, and further 
research is required. But they show that, in the case of Church- 
down, records from the Leptena Bed and from the globulina bed 
should be kept separate, and that while the strata of Alderton 
(Dumbleton) and Stroud are known to be of globulina date, they 
are not known to be of Leptena date. The same may be the case 
with other localities whence Leptena Clays have been cited. 

The next piece of evidence for the separation of the Leptena 
and globulina beds would be to find the former without the latter. 
Moore’s records of his finds at Whatley come in here: he obtained 
three species of Leptena, but makes no mention of Terebratula 
globulina nor of Rhynchonella pygmea+ He gives a similar 
result for Sandford [ Orcas, Somerset }. 

Therefore the faunal analysis works out as follows :— 


NoRMANDY. SOMERSET. GLOUCESTERSHIRE. 
(SS ee EE TE ara iz 
Caen. Ilmimster. Sandford Whatley. Stroud. Church- Alderton. 
Orcas. down. 
Leptena ...... x x x x — x P 
T. globulina... X x ee “ne x x x 


These records give at least sufficient reason for a theory of two 
deposits at two different dates. At any rate, it is necessary to 
keep them distinct for recording purposes, and not to credit a 
locality with the possession of the Leptena Beds? merely on the 
evidence of the Terebratula and Rhynchonella. 

Similar mistakes to this we have all made in the past—a relic of 
the old teaching, which consistently obstructed all increase in the 
number of names—of zones, of genera, or of species. ‘That was a 
wrong doctrine; because it will now give much more trouble 
than the other course, and impairs the value of many old records. 
It is to be hoped that what may be called the ‘ analytical method’ 
will be pursued in the future, even if it does involve the use of 
many names. 

A consideration of these Upper Lias records suggests the fol- 
lowing succession ; but it is incomplete. There are gaps (non- 


LOVER ard 
2 The Leptena Beds have a wide range—the South-West of England, 


Normandy, Wiirtemberg; also Sicily, according to the title of a paper by 
G. G. Gemmellaro, ‘ Sugli Strati con Leptena nel Lias Superiore di Sicilia’ 
Boll. Com. Geol. Ital. vol. xvii (1886) pp. 156, 341. 


part 4] JURASSIC CHRONOLOGY. 393 


sequences), as is known from other areas, and where these really 
occur is a matter of some surmise :— 


falciferum, 
bouchardt, 
murleyt, 
granulosa, 

Alana, 

Thecidella rustica, 
Leptxena, 
globulina, 
spinatum. 


(B) Additional Details concerning the Junctiou-Bed of 
Thorncombe Beacon. 


One of the most remarkable features of the Junction-Bed is 
that it may always spring surprises in the matter of its faunal 
contents. This apparently arises from the fact that fragments of 
so many different beds have been preserved. These fragments are 
in many cases quite small, and in other cases it would seem that 
the fragments themselves were disintegrated, but that some of 
their contents became incorporated in other strata. 

During one visit I was fortunate enough to find a small block 
in which were preserved various specimens of Ahynchonella 
serrata and like forms (Prionorhynchia spp.) in a sort of pale 
Marlstone-Rock matrix. This presumably is only a fragment of a 
once widespread bed. One may search many of the ordinary 
Marlstone blocks under Thorncombe without finding it. Walker 
found many specimens of /th. serrata at the temporary exposure 
near Bothenhampton. Moore says that near Ilminster (Somerset) 
the species is found only at Moolham!; these Somerset and Dorset 
localities are, I think, the only places where the fossil is known in 
England, and consequently destruction of this bed must have been 
considerable. On the Continent a species allied to Rh. serrata— 
Prionorhynchia quingueplicata (Zieten) is found in Wiirtembere, 
indicating, perhaps, that an original spread of the bed was to that 
country. This species is also found in England with Rh. serrata. 

From the same block I obtained a remarkable Terebratulid— 
somewhat of Terebratula punctata style, but anteriorly sulcate, 
and therefore an inverted form. Further search for other blocks 
to yield another example was quite unsuccessful. During my visit 
in 1920 I could not find a single block showing the serrata bed. 
Myr. Jackson, however, has another inverted example, though it 
is a different species. I hope to deal with these in a future 
paleontological portion of this paper. 

Rhynchonellids, small, rather flat, with few coarse plice, were 
found in this bed: such forms have passed by the names Rhyncho- 
nella egretta, Rh. fallax, but the identifications are open to doubt. 
Also from this block I extracted examples of (or allied to) 


1 VIL, p. 164. 


| 
| 
| 
| 
: 
| 


SS eee ee ee ee ee 


394 MR. S. S. BUCKMAN ON (vol. lxxviul, 


Rhynchonella bouchardi (Stolmorhynchia spp.). As Rh. bou- 
chardi was, according to Moore, of post Saurian-bed date, this 
seems to show that the serrata bed and the bouchardi bed were 
being destroyed tugether, and that some of their fossils were being 
mixed up. The serrata bed holds evidence of such a mix-up, for 
in this same block were Rhynchonella (Homeorhynchia) acuta, 
the large form, Quadratirhynchia crassimedia,! Q. att. spherot- 
dalis,* and other species which properly belong to the lower bed— 
the brown marlstone. I have seen Rhynchonella acuta in the 
Upper Lias part of the Junction-Bed. 

For successful collecting, it is important to note the distinction 
in matrix between the pale, rather soft serrata bed and the dark 
hard Marlstone below, as it is of little use to look in the latter for 
the special fossils of the former. The hard Marlstone is a fairly 
constant bed at the base of blocks of Junction-Bed: the serrata 
bed is rarely found, having been denuded from the Marlstone 
before the Upper Lias was cemented to it. The planed-off top of 
the Marlstone shows the denudation: it is also a guide as to 
which is the upper part of odd Marlstone blocks. 

However, this lower Marlstone is not necessarily homogeneous, 
and may be compounded of beds of various dates. In my former 
paper I mentioned my lack of success in finding Day’s Plewro- 
tomaria Bed. During my visit in 1920 I was successful. A 
Marlstone block about 1 foot thick gave the following :— 


(a) Hard, finely ironshot, blue marlstone decomposing to a rusty brown, 
about 3 or 4 inches. 


(b) Hard ironshot in the middle of the bed. 


(c) Hard ironshot in the lower part of the bed, enclosing big blue sand- 
stone-pebbles ; they look like fragments of the Starfish-Bed. See 
later, p 397. 


The following is a rough sketch of the faunal contents :— 


Upper 3 or 4 inches—Pleurotomaria spp. Specimens so crowded that, in 
the course of the extraction of one, others were broken. Mainly acuminate 
species, but one or two more depressed. See p. 400. 

Just under the Plewrotomariz various species of Paltoplewroceras and 
Quadratirhynchia crassimedia occur. 

In about the middle of the block a fragment of Ammonites cf. kurrianus 
Oppel and various lamellibranchs were found. 

In the lower part of the block, a depressed Plewrotomaria was seen. 


In another small block—a rather soft brown matrix, quite 
unfamiliar—I found a single small example of another inverted 
Terebratulid: it is like Terebratula bakerie Davidson, which 
comes from the acutwm bed (Transition-Bed) of Northamptonshire, 
but very rarely, only two or three specimens being known.* 


1, G¢pk. sais hes ee. 2 1,6, pl an, figs 2: 3-1, Dap) See 

+ Its rarity may be due to the fact that it really comes in the bottom layer 
of the Transition-Bed—the athleticum horizon, which has been almost 
destroyed in Northamptonshire. The finding of a like form in the Junction- 
Bed supports this idea, for Tiltoniceras has not been found in this bed, 
although there are evidences of the athleticwm fauna. 


~ 


part 4:] JURASSIC CHRONOLOGY. 395 


Edward Wilson found, in the Junction-Bed of the Dorset coast 
some 25 years ago, when in company with Mr. Tutcher, a similar 
species: the specimen is nowin the Bristol Museum. Accompany- 
ing my example were some unfamiliar Harpoceratoid ammonites 
which will require further investigation. This bed possibly yielded 
those Dactylicerates, of about Transition-Bed date, which have 
been obtained from the Dorset coast.1 

We may now return to Rhynchonella bouchardi: three or four 
examples can sometimes be obtained from a small lump—from 
rock which belongs to the base of the Upper Lias: that is to 
say, it is to be sought for in the blocks immediately above the 
Marlstone, which is congruous with Moore’s post-Saurian-Bed 
position. It also comes from the Watton Bed, from Ilminster, 
South Petherton, and other places. It is a form of fairly wide 
distribution: Thomas Davidson figures an example from Cromarty 
(Scotland) ; Hudes-Deslongchamps mentions it from Normandy. 
But all mention and figures of it are not to be trusted. I seem to 
recollect the name being applied to some figured examples which 
have nothing to do with Davidson’s species. And I have my doubts 
about the Cromarty shell. 

The widespread occurrence of strata with Rhynchonella bou- 
chardi indicates a period of perhaps greater quiescence after their 
deposition than had been the case with some previous deposits. 
But the bouwchardi deposit did not escape wholly, for there are 
some large areas without it—I think that the Cotteswolds might 
be cited ; and in some eases where Rh. bowchardi is found, it has 
certainly been taken from its own bed and redeposited. 

It is from such evidence as this—some of it particularly frag- 
mentary—that a reconstruction of the course of events in the 
matter of deposition had to be made. 

The Dorset coast, then, seems to supply evidence, by fossils and 
sometimes by fragments of strata, for the following sequence :— 

(6) Harpoceras-falciferum Bed. 
(5) Rhynchonella-bouchardi Bed. 
(4) Dactylioceras-athleticum Bed. 
(3) Rhynchonella-serrata Bed. 
(2) Plewrotomaria Bed. 

(1) Spinatum Bed. 


(C) The Middle Lias (Domerian) of Thorncombe Beacon. 


As the Junction-Bed of Watton Cliff contains Domerian strata 
of earlier date than those found in the bed under the Western 
Cliffs, it becomes necessary to give a short summary of these 
deposits. The information is required for several reasons, as will 
be seen later. 

Blocks and pebbles scattered along the shore from under Thorn- 
combe Beacon to Hypesmouth show various matrices, and con- 
tain many different fossils. In fact, some of the small pebbles 


Tope. 
Q.3.G.8. No. 312. 2 


Se eee ee 


2 3 Agee 


= {5 ae ae 


Fe 


396 MR. 8S. S. BUCKMAN ON [vol. Ixxvin, 
yield quite a rich harvest, and are well worthy of investigation. 
Only in certain cases have I been able to attack the beds in 
the cliffs, a very laborious and unsatisfactory task, and therefore 
my information is not so complete as it might be. But, with the 
help of Day’s section, 16 may be possible to work out the main 
sequence—sufficiently, at any rate, for present purposes. 

Table I, below, gives in the two left-hand columns a summary 
of Day’s information, with my interpretation of his fossil names in 
square brackets, and in the two right-hand columns are the details 
which I have collected—the sequence and position in certain cases 


being supposititious. 


a 


TABLE I—DOMERIAN (PRE-SPINATUM) DEPOSITS, THORNCOMBE BEACON? 


Day’s Details. 


Strata. 


Brown sands 
and sandstones. 
(Blocks of 
indurated sand.) 


Margaritatus- 
Stone. 

(Margaritatus- 
Bed.) 


Grey and brown 
sands with 
nodules. 


Fauna. 


Anmonites. 
spinatus 

[| Amaltheus 
armiger ? | 

Ammonites 
bechet. 

[ Anisoloboceras 
nautiliforme. | 


Ammonites 
margaritatus. 

[ Amaltheus cf. 
amaltheus 

and other 
species. | 

A. fimbriatus. 

[ Lytoceras 
Jimbriatum 
Wright (non 
Sowerby), Mon. 
Lias Amm, 1888, 
pls. 1xxi, lxxii= 
L. postfimbria- 
tum Prinz- 
Vadasz. | 

‘ Masses of 
Rhynchonelle 
of small size.’ 

‘In some places 
lumps composed 
chiefly of the 
stems and arms 
of Pentacrinus 
Johnsonit 
Austin.’ 


S.S. Buckman’s Details. 


Strata. 


Thorncombe Sands 
with Tetrarhynchia- 
thorncombiensis Bed. 
(The ‘ Rhynchonella- 
northamptonensis ’ 
Bed.) Compact 
brown marly rock 
with numerous 
Rhynchonellids, 
which weather out 
cleanly. 

Margaritatus Bed : 
Yellowish-brown, 
blue-centred, marly 
stone. 


Down-Cliff Sands with 
massive sandstone- 
bed. Pale yellow 
with blue centre. 
Mainly unfossili- 
ferous, but fossils 
crowded in certain 
small portions which 
resist wave-action : 
these supply the 
fossiliferous pebbles. 
The yellow pebbles 
can be assigned to this 
bed with fair cer- 
tainty; but the blue 
may be mistaken for 
Starfish-Bed or for 


Shell-Bed, or vice versa. 


Fauna. 


Tetrarhynchia 
thorncombiensis= 
‘ Rhynchonella 
northamptonensis’ 
(auctt.) Walker in 
Davidson. A thin 
sharp-edged Zeil- 
leria. 


Amaltheus spp., and 
Lytoceratid, as 
cited in col. 2. 


Numerous small 
Rhynchonelle of 
cuboidal form, with 
sharp-cut ribs. Some 
small Amaltheids in 
the yellow stone, 
Many small Amal- 
theids in blue peb- 
bles. Pebbles with 
numerous fragments 
of crinoids in a blue 
sandstone —__ perhaps 
belong here. 


part 4] 


JURASSIC CHRONOLOGY. 


Day’s Details. 


Strata. 


The Starfish-Bed. 


The Shell-Bed 
inmediately 
underlies the 
Starfish-Bed. 


Muadstone, with 
nodular concre- 
tions and shells. 


Marls, with a 
layer of small 
nodules. 


Fauna. 
Ophioderma. 


Rich in Conchi- 
fera. 
Ammonites 
margaritatus. 
[Amaltheus spp. | 
A. thouarsensis. 
| Seguenziceras 
cf. algovianum. | 


Ammonites of 
several species. 


397 


S. S. Buckman’s Details. 


Strata. 


Blue somewhat marly 
sandstone. 


The black nodule-bed. 
(Bluish marly stone, 
with black nodules.) 

Light-blue clay. 

Dark blue conchoidal 
clay, with two lines 
of small nodules near 
the top. 


Fauna. 
Ophioderma. 


Amaltheids. 


Fragmentary Amal- 
theids in the nodules. 
Body-chambers _ of 
Amaltheus aff. cleve- 
landicus, Ammonites 
cf. boscensis and like 
forms in the clay in 
line with the nodules 
(Tragophylloceras 
aff. loscombei J. Sow- 
erby sp. [derived ?]). 


The Three Tiers. A. margaritatus. The Three Tiers form 
A. loscombii. ‘The Ledges, which 
A. fimbriatus. show on the foreshore 
under the eastern end 
of Thorncombe Bea- 
con at low tide. (Hard 
blue sandstone.) 


Most of the hard beds mentioned in Table I are separated by 
thick masses of unfossiliferous or poorly fossiliferous sands or clays 
(see Day). From the Junction-Bed to his ‘ blocks of indurated 
sand,’ which I take to be the 7.-thorncombiensis Bed, Day makes 
18 feet at Down Cliffs and from the Junction-Bed to the Marlstone- 
Bed 92% feet; whereas my measurements at Thorncombe Beacon 
give 8 and 68 feet respectively. More will be said about this 
difference presently, for it is a rather interesting point. 

In certain pebbles of blue sandy matrix I found small Amaltheids, 
which are very similar, if not identical, with Amaltheus levis 
(Quenstedt), and their matrix certainly recalls that of the Scalpa 
Sandstone. In my first paper on Jurassic Chronology I proposed 
a zone of Amaltheus levis,! placing it directly beneath that of 
Paltopleuroceras spinatum. Recent researches have tended to 
confirm the value of the zone, but have raised considerable doubt 
as to its position. According tothe Thorncombe evidence the first 
zone below that of P. spinatwm is that of the so-called Rhynchonella 
northamptonensis (Letrarhynchia thorncombiensis) and the next 
is that of the margaritatus bed with large examples of Amal- 
theus. ‘The specimens supposed to be A. /evis may come from the 


DG 262: 
2b 2 


398 MR. 8. 8S. BUCKMAN ON [vol. lxxviil, 


massive sandstone-beds of the Down-Cliff Sands at the latest, but 
may come from the Starfish-Bed or the Shell-Bed. No sign of the 
rest of the fauna which was found in the Scalpa Sandstone in the 
same matrix as that of Amaltheus levis was seen at Thorncombe. 

If A. levis comes from the Starfish-Bed and Seguenziceras 
from the immediately subjacent Shell-Bed, while there seems to 
be some 200 feet between A. /e@vis and Seguenziceras at Raasay 
in the Scalpa Sandstone,! a very interesting position would be 
reached, but one quite analogous to what is found in other cases. 
Some 200 feet would have to be added where now is little or no 
deposit. The already thick strata of the Domerian on the Dorset 
coast would still lack representatives of a time-interval equal to 
some 200 feet of deposit. The length of time required for the 
deposition of the Domerian is thus lkely to increase to a Jarge 
figure: it is already considerable. 

These points cannot be pursued now, but they show where 
further work is required. It is obvious that the complication of 
the Domerian succession will be much greater than has been 
expected, and that the dating of the Scalpa Sandstone may have 
to be considerably modified. But that will not be at all a simple 
matter: it will immediately raise many other questions, among 
them perhaps that of local denudation of the Scalpa Sandstone— 
and that presents difficulties. 

The Domerian succession disclosed by Thorncombe may be stated 
as follows :— 

(7) Paltoplewroceras spinatuwm. 

(6) ‘ Tetrarhynchia thorncombiensis.’ 
(5) Lytoceratid (the margaritatus bed). 
(4) Cuboidal Rhynchonellids. 

(3) Starfish-Bed (=? Amaltheus levis). 
(2) Seguenziceras. 

(1) Amaltheus clevelandicus. 


This sequence is, in all probability, incomplete for the Domerian 
strata of Thorncombe Beacon—because, for one thing, it has not 
yet been possible to extract from their matrix all the specimens 
collected,—and it is certainly incomplete so far as the full 
Domerian sequence is concerned. Faunal elements of the 
Hebrides and Yorkshire are missing, and will have to be accounted 
for; but, more important still, faunal elements of the Ilminster 
district and of Normandy—that is, faunal elements immediately 
north and south of Dorset—are lacking. 

Some interesting examples of this are the collomanee :—Aula- 
cothyris resupinata and A. moore: occur at Ilminster and in 
Normandy. ‘The absence from the Dorset-coast sections of Tere- 
bratula |Aulacothyris| resupinata (including T. [A.] moored 
presumably) is particularly noted by Day.? Walker mentions 
this, and says nothing to the contrary. My experience is the 


UAT. 7oaos 26 2 IIL, p. 293. 3 XII, p. 442. 


part 4: JURASSIC CHRONOLOGY. 399 


same. Yet Walker found eight specimens of the first and one 
of the second at Bothenhampton. Now, it is fairly certain that 
these forms occupy a rather low position in the spinatum zone 
—may even indicate a deposit which is actually of pre-spinatum 
date. If so, the explanation of the geographical record is easy 
—this earlier bed, or some of its contents, is preserved at Bothen- 
hampton ; and the bed is certainly present at other places, but 
it and its contents have been lost from the coast-sections. 

On the other hand, Walker notices particularly that not a 
fragment of Rhynchonella acuta rewarded his work at Bothen- 
hampton. Considering that it is a not uncommon fossil at the 
coast-sections in certain cases, and that it is a usual fossil of Marl- 
stone localities, this is rather remarkable—more especially as what 
seem to be its customary associates were found by Walker. Here 
further analyses may be particularly interesting, with this addi- 
tional reason—that there are two forms of Ah. acuwta—a small form, 
to which the name was originally applied, anda large form. There 
is reason to think that the small form is not actually the young 
(the brephomorph) of the large form, but that it is earlier in date— 
is an earlier stage (an anamorph) of the large form. If so, small 
forms and large could be used as chronological indices, the small 
form possibly marking a fairly early Marlstone (spinatum) date ; 
while the large form certainly marks a particularly late date, 
although it must be earlier than Rh. serrata, for it occurs where 
the serrata deposit has been lost. Is it to be assumed that there 
was no record of either of these two dates at Bothenhampton ? 

It is, I think, correct to say that the small form ranges from the 
Dorset coast to the Hebrides; but, before such a statement could 
be made positively, a critical study would be necessary: for there 
must be a true brephomorph of the large form, and this must 
be so synilar to the presumed adult small form that they would 
only be distinguishable by what would be regarded as very trivial 
features, This shows why it is so necessary in paleontology 
to be precise in noting and naming quite small details, if the 
full value from the different forms is to be obtained, so as rightly to 
apprehend what they indicate regarding the history of deposition 
and denudation. If small biological details be passed over without 
notice, much of considerable chronological value may be lost. 

It follows, then, that if the idea be correct that the two 
differently-sized forms of Rhynchonella acuta indicate different 
dates, the finding of the large form may be taken as fair 
evidence for the later date; but the finding of the small form 
is not necessarily good evidence for the earlier date—not unless 
we work out precisely whether the small form is a brephomorph 
or an anamorph, a labour only to be accomplished when the 
respective distinguishing characters have been properly ascer- 
tained. Proof that the anamorph and the large form lived at 
different dates should be found by faunal analysis, as well as by 
direct observation. The former should reveal areas where the 
anamorph existed by itself without any trace of the large form ; 


ee 


ee A ee 


a eS 


Sea SS Te 


4.00 MR. S. 8S. BUCKMAN ON 7 [ vol. Ixxvitl, 


while the latter would find small forms occupying strata at a lower 
horizon than the large forms. This shows how much scope there 
is for further work. 

Mr. J. W. Tutcher has kindly supplied the following identifica- 
tions of gastropods and scaphopods which I have been able to give 
him, as the result of chance collecting during various years at and 
near Thorncombe Beacon. Some of the labels require a little 
interpretation, because in earlier years I was necessarily less 


conversant with the details of the sequence or the matrices of odd 
blocks :— 


‘ Junction-Bed, bifrons layer.’. 
Discohelia dunkert Moore. 
‘ Junction-Bed.’ ; 
Trochus nodulatus Moore. 
‘ Rhynchonella-serrata Bed.’ 
Trochus lineatus Moore. 
‘Pleurotomaria Bed.’ [See p. 394. | 
Pleurotomaria mirabilis Deslongchamps. 
Pleurotomaria ef. subnodosa Goldfuss= possibly the form recorded 
by Day as P. precatoria Deslongchamps. 
‘Marlstone’ [=spinatum ?]. 
Amberleya cf. gaudryana A. @Orbigny. 
‘ Spinatum.’ 
Trochus ef. flecicostatus Moore. 
‘Lower part of Marlstone’ [see p. 394]. 
Pleurotomaria mirabilis Deslongchamps. 
‘ Margaritatus zone” | =possibly the T.-thorncombiensis Bed }. 
Cryptena cf. solarioides (J. Sowerby). : 
‘ Margaritatus’ (possibly margaritatus bed]. 
Cerithium liassicwm Moore. 
Ataphrus cinctus Moore. 
‘Massive Sandstone-Bed.’ 
Turbo aciculus (Stoliczka). 
Dentalium elongatum Moore. 
‘With Amaltheus cf. levis, pebble on beach, near Hypesmouth.’ 
Dentalium elongatum Moore. 
‘Pebble on beach, Eypesmouth, below margaritatus et possibly 
Starfish- Bed.’ 
Actzonina ilminsterensis Moore. 


(D) Analyses of the Junction-Beds, and Theories 
as to the Watton Bed. 


From the faunal sequences arrived at in the foregoing studies, 
and from the results which have been already obtained and 
published elsewhere,! it is possible to present a detailed list of 
successive hemerz, noting at the same time which of these 
hemere are represented by the fauna of the Junction-Bed at 
various places. But, as a preliminary, it is advisable to present an 
epitome of Walker’s section at Bothenhampton. 

About 2 miles somewhat east by north of Watton Cliff was the 
site of an exposure of the Junction-Bed at Shipton Long Lane, 


Leeper ote 


7 


part 4 | JURASSIC CHRONOLOGY. 401 


Bothenhampton, described by J. F. Walker in 1892.1! The 
following may be constructed trom his description :— 


Section IIJ—Suipton Lone LANE, BOTHENHAMPTON (J. F. Walker). 


Thickness in feet. 
1. ‘White stone. Ammonites germani [ germaini | 
(1D) Oral OER aN) Vcc ess desc RURAL cero aR NORE rc 1 
[striatulum.| 2. ‘ Brown stone. Ammonites (Grammoceras) thou- 
arsensis (D’Orbigny), Rhynchonella jurensis 


var. DothennaMptonenstS” 2.2... .cccccccctscceecss i) 
[ bifrons. | 3. ‘Brown conglomerate, often with pink stone at 
the base. Ammonites (Hildoceras) bifrons, 
faleiferum. worn specimens of Harpoceras falciferwm, 
bouchardi. Eehynchonelta, DOWN dt” 6... cece owen ohn hnoms 2 
Harpoceratoid 4. ‘Marlstone. Rh. serrata’ in the upper 3 inches. 
(serrata). ‘Rh. tetrahedra’ and many other brachiopods 
[spinatum]. io), Chelowenpavbincncsacaauensiwendics ca vdaeaaae pene a 


The possible faunal sequence ascertained by the foregoing studies 
may now be given and results compared. 


TasLE [[—Faunat Contents oF THE JUNCTION-BED. 
Localities. 
[Cea PO —— nN 
Western Cliffs, 
Hemeree. Thorncombe, etc. Bothenhampton. Watton. 


NN EO eee 


(GMOOret. |i ade kes 

Catulloceras............ 
Dumortieria............ as sas x 
Hammatoceras......... 3 ps x 

YEOVILIAN. ~ dispansum............... 

StruckManns ............ 

PEDUCUNTS 60 05 5 cours Decne 

CSCRUA RE eras ee 

L striatulwm ...... 0.065 x x x 
(Peamiabelis) 32. -na ses 
NLU Cee ae heen mee. ON accictes & 
braunianum ............ 
SJE WI CHL UME Peenaclias a) 
DU RON Str e tte mene os: x x x 
SUWOCATINGEG en. eee: 2 on x< 
x 
x 


Von. al ( 


pseudovatum...... sc... 
Saleiferum ar 
Wee te AN DOWOCRATOU ics se nace 
exaratum ..... 
murleyi ... Ree ie 
tenuicostatum ......... 


< 
UCL: (eset eek se ee pM 
e 


xX 
xX 
' POs en ee ae Ce =: —e, 


GlODUTINA «2.6 .e sete ses 
COUN Sovnod o.anqnanonoe 
CITA SICH Onenn ORD PH Ope 
Harpoceratoid 
(serrata bed)......... 


Pleurotomaria......... 
DoMERIAN. + spinatum ............... 
T'. thorncombiensis ... te ae x 


KK EX 
x 
as ger Be BE 


1 XT, p. 441. 

See also the hemeral table given in Appendix II. 

Ammonites germaint. 4 Mr. Jackson’s discovery. 
Small Hildoceratid ammonites. 6 Thecidelle. 


a ow w 


QUO ISIC IAL 


oy M pue 
UMOIg ia 


UMOTE 


UMOIG 
USIMO]IIA 


SFU MN. Pe 


oT. 
YSImo]]a Az 


UMOIG 


oUt 


MMe 


SIO wx0989 “VW 


oyu M 
49947 


a eae (oye tf) 
uoydweyueyj0g'q 


‘(qaaf &@=Yyour T + 9798 1091}190) 
peg-uoyoune ay2 fo hhojoyzv—T—9 wieiseig 


part 4:] JURASSIC CHRONOLOGY. 403 


Table II records the faunal contents of the Junction-Bed at 
three places—-I have drawn upon Mr. Jackson’s evidence for one 
date (swbcarinatum) in regard to Watton Cliff,—and it shows 
that, while there is substantial agreement between sections of the 
Western Cliffs and of Bothenhampton, yet that Watton Cliff 
differs considerably from both. This divergence is more noticeable 
than a mere list of fossil contents would indicate; for the points 
that are most strikingly in contrast between Watton Cliff and the 
other sections are, so far as Watton Cliff is concerned, 


(1) The continuity through most of the Watton Bed of white-stone 
deposits. 

(2) The considerable thickness of strata in which forms of Grammoceras- 
striatulum type are found. 

(3) The presence of the Thecidelle and unfamiliar Hildoceratids. 

(4) The absence of the Marlstone-Rock bed—serrata and associated beds. 

(5) The presence of the Tetrarhynchia-thorncombiensis Bed and many 
derived examples of that fossil. 


These points are illustrated in Table II and diagrams 6 & 7. 

The section at Bothenhampton is near a line of fault, parallel 
to and north of the Watton Cliff (Eype) Fault, heading in the 
direction of Thorncombe Beacon. The Junction-Bed of Bothen- 
hampton reproduces the Thorncombe type, and shows Marlstone 
at the base. The Junction-Bed of Watton Cliff is closer to the 
axis of the Weymouth Anticline, and shows not only denudation 
and destruction of the Marlstone, but denudation carried down 
many feet lower—to below the Tetrarhynchia-thorncombiensis 
Bed (diagram 6, p. 402). 

Now, Day’s measurements (see above, p. 397) were made at Down 
Cliffs,! and by the increase in thickness it can be estimated that 
his exposure was some 4250 feet—approximately 6 furlongs— 
to the west of mine; it would, owing to the trend of the coast, 
take him some distance farther away from the axis of the 
Weymouth <Anticline, and this would account for the greater 
thickness which he obtains between the Junction-Bed and the 
margaritatus bed. The following sections (diagram 8, p. 404) 
are placed in order from west to east, which direction, though not 
at true right angles to the anticlinal axis, is sufficient to illustrate 
the effect of it. 

Thus in the Down-Cliffs section, as there are some 92 feet 
between the Junction-Bed and the margaritatus bed, in the 
middle section about 68 feet, and at Watton Cliff about 40 feet, 
the Tetrarhynchia-thorncombiensis Bed must have disappeared 
in the post-Marlstone denudation immediately east of Thorn- 
combe Beacon, and the denudation has, at Watton Cliff, been 
carried down into the pre-Tetrarhynchia-thorncombiensis-Bed 
Sands (diagrams 8 & 9, pp. 404, 405). 

The Marlstone Rock is only preserved at Thorneombe Beaecon— 


1 There is reason to think that he took details, if not measurements, from 
Doghus as well as from Down Oliffs—the latter name originally covered both, 


Diagram 7.—Faunal comparison. 
(Vertical scale: linch=2 feet.) 


A.Western Cliffs B. Watton Cliff 


V4 Strata with 
striatulum forms, 
often missing in 

the Western Cliffs 


Diagram 8.—Comparative sections at Thorncombe Beacon and 
Watton Cliff, on the scale of 30 feet to the inch. 


Junction-Bed INNTAEAAAH 


occasional 
doggers 


Sands with ee Pierre ness 
occasional |: 
doggers 


(a)Line showing the extent to 
which denudation of the pre-Watton 
Bed was carried in regard to 


Blue Clay the strata at Thorncombe Beacon. 


Margaritates-| ie cana 
Bed 


SOYIUI OF = ‘Wu T 
J99} OF = “ULI TeoMIIA 
329] 009 = "WII 
}29] OOST = ‘UT I TeyUOZTIO PY : saeas 


i] 


JNO woe mM ye pores seM 


LOT}epNnusp YOIYM 0} sul] (e) 
PId-SMIBIIAICSAB Y/ 


P9g-SHICUIBSIP IY 


1983 89 Pea 

-SHIBUAICSAP WW 
0} 

peg-uonounl 


199] OV pog 
-SNJEMACSAP 
073 
peg-uornounf 


IafhP] 


SISUDIGuUIOIUJOY-]. ¢ 


\ 


p2g-uonounf{ 
UD VOWeEM'D equmoour0y yg 


‘aunoyup yynouha sy ay) fo poaffa ayy 


P°g-SNIBUIBSAIB YY 


qooy 126 pog 
-SHIBILIC SIP 
03 
peg-uonounf 


IDLE] 
SISUdIGuIosusOY I 
jo 
uorl}isod jnoqy 


JID UMOg'’y 


hurnoys ‘(suormpnuap 10) worynpnuap UniuaUog, agv'T—G WeIserg, 


4.06 MR. 8. S. BUCKMAN ON (vol. lxxviu, 


not at Down Cliffs nor at Watton Cliff; this implies slight syn- 
clination of the Thorncombe area at the time of post-Marlstone 
denudation, just sufficient to preserve the thin bed. But there 
was a_pre-Marlstone denudation—the presence of big blue 
sandstone-pebbles in the marlstone of the Junction-Bed of 
Thorncombe Beacon is evidence for that. The matrix of these 
pebbles suggests that they come from the Starfish-Bed; they 
certainly did not come from the margaritatus bed. Taking the 
data of denudation ascertained in the above sections—that is, pnccs 
30 feet to a mile,—then the Starfish-Bed was elevated to the line 
of erosion some 54 milesfrom Thorncombe Beacon. The main axis 
of the Weymouth Anticline runs out to sea some 7 miles south of 
Thorncombe Beacon. Another mile and a half would involve a 
further 45 feet, which would be insufficient to bring the Three 
Tiers—the next hard bed—to the surface along the axis. 

The fact that the margaritatus bed is not found in pebbly 
condition, while the Tetrarhynchia-thorncombiensis Bed is found 
as pebbles in Watton Cliff (and probably it is the Starfish-Bed 
that furnished the pebbles for the Junction-Bed of Thorncombe 
Beacon), may suggest that movements of post-margaritatus-bed 
time allowed that bed to be covered up and protected by thorn- 
combiensis or pre-thorncombiensis-bed deposits. 

It will thus be seen that the Watton Bed is a deposit of excep- 
tional interest geologically: 1t would also be rich palzontologically, 
if one had the means of breaking up the massive blocks. That it 
is different from the Thorncombe Beacon Junction-Bed, only about 
7 furlongs away to the west, is due to the fact that it really belongs 
to a different piece of territory—to what was some half-mile 
nearer the anticlinal axis. This little transverse difference is more 
important than a considerable lateral distance, as is shown by the 
similarity of the Down Cliffs (Thornecombe) and Bothenhampton 
Junction-Beds, which are from 23 to 3 miles apart, but along the 
same line of fold. 

The movements of the Weymouth Anticline and the distribution 
of strata which they produced in the Englhsh Channel suggest 
an interesting line of enquiry—as to the present- -day distribution 
of strata out to sea. Some 20 years ago I collected certain data 
on the subject; but there has not yet been any opportunity of 
working out details, and the enquiry must be deferred for the 
present. However, such an enquiry should have a certain prae- 
tical bearing. The distribution of areas of hard rocks—that is, 
areas of sandstone or limestone—is important to fishermen, as 
affording good ground for the setting of thew lobster-pots. A 
geological investigation should indicate likely places to sound for 
such areas, and might lead to the discovery of some which are not 
yet known to the fishermen: they had, at the time I speak of, just 
made a chance discovery of a new ground. 

To return to the Junction-Bed—it is evident from the various 
sections that the Weymouth Anticline was in a state of constant 
movement with consequent denudation making non-sequences. 


part 4] JURASSIC CHRONOLOGY. 407 


Yet the non-sequences are not always synchronous in the different 
sections. 

The main feature of the Watton Bed is the amount of deposit 
containing forms of Grammoceras-striatulum type, which, how- 
ever, there is reason to suppose, are redeposited specimens. Yet 
the main mass of the Watton Bed is a hardened paper-shale—thin 
laminz of very fine mud,—indicating a slowly-accumulating tranquil 
deposit. These statements appear to conflict: so does the state- 
ment about tranquillity with the observation that some ammonites 
are found more or less on edge, and that fairly large blocks of 
Tetrarhynchia-thorncombiensis Bed have been redeposited in the 
Watton Bed. 

The reason for supposing that the forms of striatulwm type are 
redeposited are the following :—-At Bothenhampton J. F. Walker 
found stréatuluwm forms in a brown bed, and a species of later date 
(Ammonites germaint) in white stone. The striatulwm forms of 
the Watton Bed show evidence of a yellowish-brown matrix, 
although they are found in a very white lithographic stone. Asso- 
elated with the strzatulwm forms are species of Hammatoceras—one 
form of the H. cnstgne type found in the rock itself and another 
(a rare and peculiar form somewhat removed from ¢zsigne, but 
certainly a Hammatoceras) was found on the lower platform loose : 
the matrix of the Watton Bed, however, is unmistakable. Now, 
Hamimatoceras is of considerably later date than the striatulum 
forms, as may be seen from the synopsis on p. 401 (Table II). 
It is presumably of about the same date as the Ammonites 
germaint quoted by Walker, but is of pre-Dumortieria age. 
Therefore, the evidence appears to show that the forms of strva- 
tulum type which are found in Bed 4 are redeposited in strata 
of Hammatoceras date. 

In the layer above, No. 3, is found evidence of a still later 
horizon, Dumorfserta. The specimen belongs to a species with 
coarse, rather distant ribs, and is closely allied to Dumortieria 
novata S. Buckman (p. 387). But im this layer there is very 
plain evidence of redeposition apart from the s¢réatulum forms : 
there is Hildoceras of bifrons type, which is a species some five 
hemere earlier than strzatuluwm forms and some eleven hemerze 
earlier than Dumortieria; and there are specimens of Tetra- 


rhynchia thorneombiensis, which is some twenty hemerz earlier 


than s¢rzatulum and some twenty-six earlier than Dumortieria. 

It is evident, then, that the fossils which the Watton Bed yields 
are not to be trusted as evidence of the date of its deposition. 
And there is further evidence on this head in the finding of Harpo- 
ceras aft. falciferum (mulgraviwm) in the top of the bed (p. 388). 
Therefore, it 1s only possible to date the bed by the latest fossils 
which it contains. 

What is, however, of considerable interest is the faunal 
inversion :— ) 

Harpoceras at the top of the bed. 


Hildoceras aff. bifrons in layer 3. 
Grammoceras of striatulwm type in layers 4 & 5, 


a a i eg 


— 


a) ae ee 


408 MR. 8. S. BUCKMAN ON | vol. lxxviil, 


This is an exact reversal of their true sequence. The possi- 
bility of such inversion had been already surmised from consider- 
ation of other cases.1 

The important point is, however, that the evidence of ammonites 
is not to be trusted too implicitly in certain cases: it shows that 
special care has to be taken in reading records. That there was 
such an inversion might be explained on the supposition that, as 
the latest bed (that of strzatulum date) was destroyed, its removal 
exposed an earlier bed (bzfrons) which, in turn being removed, gave 
opportunity for the denudation of falciferum. But this is only 
a part of the history : it is fairly evident that strata of these dates 
and of others were all exposed to denudation at the same time. 
Thus faleiferum contributed to the fauna of the lower part of one 
fallen block; 6zfrons (so far as the evidence of pink matrix goes) 
was laid under contribution for layer 9; thorncombiensis provided 
materials for layer 3, and for various earlier layers. While the 
middle and upper part of the bed was being laid down, however, a 
stratum which yielded striatulum forms was being considerably 
raided to provide materials, though it was not the only bed from 
which they were obtained. 

The latest date for the deposition of the Watton Bed is deter- 
mined by the date of the superimposed areno-argillaceous stratum 
(No. 13) ?: the Watton Bed was laid down before that date. 
This sandy stratum is, from its position, presumably a less argilla- 
ceous representative of the Down-Clitt Clay which caps the 
Junction-Bed in the cliffs west of Eypesmouth ; or it may be the 
equivalent of the basal part of the Bridport Sands, which in 
Burton Cliff assume a bluish colour in their lower part. But, 
again, these basal Bridport Sands may be really a less argillaceous 
condition of Down-Cliff Clay. 


The Down-Cliff Clay has yielded Dumortieria,> the irony scale at 
the base of the Watton-Cliff sandy stratum has also vielded Dumor- 
tieria, the basal Bridport Sands at Burton have given no evidence. 
The Watton Bed is, therefore, earlier than the Dumortieria 
hemera, in the main. But an early form of Dumortieria occurs 
in the Watton Bed, layer 3; the irony scale which caps that bed 
and the complete lthic change between the Watton Bed and 
the sandy stratum point to a non-sequence of greater or less dura- 
tion. It is to be concluded that the Dumortieria hemera contains 
really more episodes than our present time-scale allows for: first, an 
episode of early Dumortieria, during which calcareous conditions 
of lithographic-stone deposition obtained—a tranquil deposition of 
fine sediment resembling paper-shales ; secondly, an episode of 
possible denudation while a lithic change was accomplished; thirdly, 
the episode of argillaceous or areno-argillaceous conditions of the 
Down-Cliff Clay and its equivalents, passing higher up into 


1 T, 9, p. 74, footnote 1. 
? Section I, p. 382. 
37,2. p. 519, 15. pike: 


sab 


part 4] JURASSIC CHRONOLOGY. 4.09 


the completely arenaceous conditions of the middle part of the 
Bridport Sands. In this third episode Dumortveria ranges up some 
60 feet in the Down-Cliff Clay,! and it may range as far again 
into the Bridport Sands before Catulloceras appears, while there 
is about 170 feet before evidence of moore? is obtained.” 

The other supposition is that the upper part of the Watton 
Bed was deposited contemporaneously with the Down-Clff Clay. 
This is possible, but it follows that a curiously limited horizontal 
extension of the lithographic-stone deposit in a westerly direction 
would have to be allowed, and, moreover, would have to be accounted 
for ; because north-eastwards, so far as the Bothenhampton evi- 
dence goes, there 1s reason to suppose considerable horizontal range 
of lithographic-stone conditions. 

On this theory the non-sequence between the Watton Bed and 
the sandy stratum might correspond to the time of deposition of 
the upper part of the Down-Chff Clay. 

So far, then, as our present time-scale allows, the three upper 
layers of the Watton Bed were deposited in the earliest part of 
the Dumortieria hemera. The layer below, which contains 
Hammatoceras, could be dated as Hammatoceras hemera, which 
precedes Dumortieriva. The layer below (No. 5), which contains 
striatulum forms, can be dated as certainly later than strzatulwm 
hemera; but whether it can be dated as contemporaneous is a 
question for consideration later. 

In this layer I obtained a reversed gastropod (Cirrus), thinking 
it to beanew record; but Moore hasalready figured and described? 
a similar form as Turbo bertholeti from the ‘Upper Lias at 
Compton [near Sherborne, Dorset] . . . from the highest bed of that 
place in association with Ammonites walcotti.’ There is reason 
to suppose at Compton and the neighbourhood a close association 
of Grammoceras-striatulum forms with A. walcottii | cf. bifrons|, 
owing to paucity of sediment, erosion, or both; therefore the evi- 
dence is not necessarily against this gastropod being a contemporary 
of striatulum forms.* 

The peculiar Hildoceratid ammonites of layer 6 have some 
resemblance to species which occur in the Jet-Rock of the York- 
shire Upper Lias (evaratum hemera), as, for instance, Ammonites 
rugatulus and A. multifoliatus Simpson ; there is also likeness 
to A. similis Simpson, but this is perhaps trom the faledferwm 
horizon. ‘The species of Hlegantuliceras, HE. elegantulum and 
LE. ovatulum (Simpson), ‘ Yorkshire Type-Ammonites ai, piss xenm 
& cvi, may also be cited as similar, but they are thicker: they are 
from Jet-Rock, exaratwm hemera. 


Ona. OA Spee oe pmoas 

3 VII, p. 210 & pl. vi, figs. 7-8. 

+ There is little doubt that, at Compton, Moore worked a higher (later) 
deposit of Upper Lias than at Ilminster. Analysis of his finds should show 
this. The forms common to Ilminster and Compton would presumably be 
from the lower bed; those peculiar to Compton might be expected to indicate 
the higher bed. 


== YS Lo eee 


St Soe Tes 


410 MR. S. S. BUCKMAN ON [ vol. lxxvin, 


It is rather tempting to assign to the rock that date, because it 
would bring the Lhecidelle of the top of layer 7 into line with 
Moore’s zone of Thecidium rusticum (p. 390). But there seem to 
be difficulties in this course:—(1) there is lithographic stone in 
layers 8 & 10 below; (2) there is pink rock suggestive of bifrons 
date in layer 9; and (8) the possibility that the Harpoceras aff. 
mulgravium tound in a fallen block (p. 888) came from about the 
horizon of layer 8 or even below it. 

The ammonites of layer 6 may have been derived, especially as 
Mr. Jackson. has found similar peculiar Hildoceratid ammonites 
in far better condition nearly in the top of the bed (corresponding 
to layer 3, or possibly 4). His specimens are in quite unworn 
condition, showing no signs of derivation, yet it seems quite 
impossible to imagine that they were contemporaries of Hammato- 
ceras—that is contrary to all our experience: there is admittedly 
a difficulty here. As to YLhecidelle, they can only have been 
derived if the whole slab to which they are attached had been 
derived. This is possible; but it is also possible that the Thecz- 
delle ave a different species and of a different date from Thecidella 
rustica: they are too much weathered to make any determination 
of species satisfactory. And, even if such were made, our know- 
ledge of the range of Thecidelle is admittedly very incomplete : 
possibly several Whitbian horizons yield Theczdelle, but there has 
been, since Moore’s time, too little systematic search for them. 
The washing process, by which alone they can be obtained in 
satisfactory condition, is particularly tedious ; and there are cases, 
as their occurrence in this calcareous Watton Bed shows, where 
it could be applied only with much difficulty. 

If it be supposed that the peculiar Hildoceratids are 7m sitw in 
the lower part, or not much removed from their original position, 
that they were at least deposited originally in a lithographic-stone 
matrix, then we are confronted with a difficult position, and the 
following theories may be put forward to account for the facts. 
A recapitulation of the facts may be helpful: they are :— 


(1) A laminated lithographic-stone bed about 5% feet thick shows in 
the face of Watton Cliff, separating Yeovilian from Domerian 
sandy deposits. : 

(2) It is in the same position as the Junction-Bed of Down-Thorncombe 
Cliffs ; but it is quite different in lithic character, and disagrees 
greatly in its fauna. 

(3) The white-stone matrix runs through the bed, but there are other 
matrices mixed up with it. 

(4) Striatulwm forms occur in a yellowish matrix, but are redeposited 
in lithographic stone ; they are associated with Hammatoceras and 
Dumortieria. : 

(5) There is faunal inversion. 

(6) Certain small species of Hildoceratids of pre-falciferwm aspect are 
found at two levels, towards the base and towards the top; those 
at the top are in the best condition and in a white-stone matrix. 

(7) A white-stone matrix is not found in the Junction-Bed of the western 
cliffs; but it occurs at Bothenhampton and is post-striatulwm 
in date. 


ggg o.5- 


part 4] JURASSIC CHRONOLOGY. 411 


These are the facts which have to be accounted for, and the 
following theories may be put forward :— 


(1) The lithographic or white-stone matrix began to be de- 
posited in pre-‘alczferum time, and its deposition continued until 
the time of early Dumortieria, although there were various breaks 
in the record due to penecontemporaneous erosions. 

(2) The whole of the Watton Bed was laid down at about one 
date, Hammatoceras (that is, the base of the Watton Bed corre- 
sponds in date with the upper part of Walker’s Bothenhampton 
section—it is post-strzatulum wholly): therefore, it is altogether 
later in date than any part of the Junction-Bed of the Western 
Cliffs, and than all but the last layer of the Bothenhampton 
section. So far as agreement in lithic character is concerned, 
this would appear to be correct. So far as faunal contents are 
concerned, nothing that has been found lower than layer 4 is in 
favour of it. | 

(3) The Watton Bed consists of two similar deposits of rather 
widely different dates; there was a deposit of, say, pre-falciferwm 
date which had the characters of white lithographic stone, and 
made up the lowest part of the bed, then there was a break, 
possibly with denudation, while falcezferum and post-falciferum 
deposits were being laid down in surrounding areas, as for instance 
in that of Thorneombe; then followed a time of deposition at 
Watton Cliff, for which materials were obtained from falciferum 
and later deposits, from the pre-falciferum white-bed deposit, 
and even from the thorncombiensis rock. Afterwards came 
another period of definite lithographic-stone deposition beginning 
in Hammatoceras hemera (laid down not only at Watton Cliff, 
but at Bothenhampton), and continuing to the earliest part of 
the Dumortieria hemera. In other words, two deposits of similar 
character, but of widely different dates, have coalesced. 


Let us consider these theories: the first theory carries various 
difficulties. That a homogeneous deposit existed for many nemere, 
and at the same time shows various non-sequences, is not a diffi- 
culty; for such a deposit, lasting through more than thirty 
hemerz with various non-sequences, is illustrated in my last paper.! 
But the difficulty is to suppose that during all the long time of 
about eighteen hemerze which the fauna of the Watton Bed 
would require for its deposition (that is, from pre-falciferwm 
to early Dumortieria), the white-stone conditions had so remark- 
ably restricted a geographical range—not extending to the cliffs 
west of Eypesmouth, about 7 furlongs westwards, where different 
conditions of deposit obtained, nor to Bothenhampton, about 
2 miles away to the north-east, until the time of Hammatoceras 
hemera, when white-stone conditions did come in at that locality. 

Next there is evidence that white-stone conditions were not 
really continuous. Some small Dactyloids are in a_ brownish 


11,9, p. 100. 
Orw.G.S. No. $12. Ze! 


VS 


y 
ij 


412 MR. S. 8S. BUCKMAN ON (vol. lxxvin, 


matrix, and the Grammoceras-striatulum forms are in a yellowish 
deposit. 


The second theory requires that Hammatoceras or some contem- 
porary should be found low in the Watton-Cliff Junction-Bed. This 
theory does not account for the white matrix of the presumed pre- 
falciferum Hildoceratids, found by Mr. Jackson towards the top 
of the bed. And it involves the following corollaries: (@) that 
any strata deposited in the Watton-Cliff locality during the 
making of the Junction-Bed of the Western Cliffs and elsewhere 
were broken up and redeposited during Hammatoceras time; 
(6) that all such strata were removed, and then that the Hamma- 
toceras sea had access to exposed strata of thorncombiensis, pre- 
exaratum, falciferum, bifrons, striatulum, and other hemere, in 


order to gather materials for the making of the bed—to a certain 


extent layers 2, 3, & 4 give evidence that somé portions of such 
strata were available for supplies; (c) that all the faunal contents 
of the Watton Bed which are of older date than Hammatoceras 
have been derived. 


The third theory sounds rather elaborate, but it seems helpful 
in many ways: (a) it will account for the pre-falciferwm Hildo- 
ceratids having a white-stone matrix, so that they have the same 
matrix after derivation as that in which they are enclosed; (0) it 
synchronizes this presumed pre-evaratum deposit of fine-grained 
lithographic stone with the paper-shales of North Gloucestershire— 
as, for instance, Alderton-Dumbleton : and such synchronization 
of two fine-grained, thinly-laminated deposits is rather an interest- 
ing point; (¢) it accounts for the absence, except as derivatives, 
of deposits of post-falciferum to pre-Hammatoceras hemere, and 
also for their irregularity ; (d) 1t brings into accord the commence- 
ment of the second white-stone deposit at Watton Cliff with the 
same deposit at Bothenhampton—there it is post-striatulum, 
presumably Hammatoceras; at Watton Cliff it is post-striatulum 
certainly Hammatoceras, so far as layer 4 is concerned. 

If, however, this theory be correct, then there will be the 
following corollaries awaiting acceptance :— 

(a) The presumed pre-faleiferum Hildoceratids should be found 
in the basal part of the blocks: unfortunately, this basal part is 
the most difficult portion to attack; (6) this pre-falciferum 
white-stone deposit has been removed by denudation from the 
Junction-Bed of the Western Cliffs and of other places: there 
certainly is a non-sequence in the required position—both stratal 
and faunal failure; this is not asking too much, because denuda- 
tion of the pre-falciferum deposits occurs over a considerable 
area—south of North Gloucestershire they are only preserved in 
patches, and in stratal sequence those are often incomplete ; (c) that 
the main constituents of the Junction-Bed of the Western 
Cliffs were not deposited in the Watton Bed in a regular order, 
but are only represented by chance faunal and stratal elements in 


Se ee 


part 4] JURASSIC CHRONOLOGY. Ald 


disorder ; (d) that the second white-stone deposit has practically 
come into contact with the first one, giving the Watton Bed a 
false appearance of stratal continuity, whereas it masks a con- 
siderable non-sequence—say, some dozen or more hemere ; (¢) that 
the second white-stone deposit which is of two dates at Watton 

Clift—Hammatoceras and earliest Dumortierta—has been partly 
removed, so far as the Dumortierva portion is concerned, from 
Bothenhampton, and has been wholly removed from the Thorn- 
combe area: there, indeed, removal has gone further—it has taken 
off the deposit of s¢réatulum date from Thorncombe Beacon, and 
yet left it as witness for original deposition at the neighbouring 
Doghus and Down Cliffs, in places. 

The phenomenon of what may be called stratal r epetition— 
the occurrence of like deposits after greater or less intervals of inter- 
ruption by deposition of strata unlike them—is quite well known. 
It occurs in two*forms which may be called local and non- 
local—the first is stratal repetition in the same district, the 
second is stratal repetition, but not in the same district; there is 
stratal repetition in time, although what may be called the 
depositional focus has shifted. In the first case the lke 
deposits are superunposed, in the second case they are not. 

Those Jurassic sands of the South-West of England which 
makes a first appearance in Middle Lias (Domerian), and continue 
with various changes of locality until early Inferior Oolite 
(Aalenian), illustrate remarkably well stratal repetition, both local 
and non-local, as Table IIT (p. 415) will show. 

Before illustrating this point I may say just a word about the 
so-called ‘Midford Sands’ or ‘Oolite-Lias Sands’ (Cotteswold, Mid- 
ford, Yeovil, etc. Sands). From what one may read concerning 
lithological lithic] evidence about change of deposit producing 
change of ammonite fauna, and when one sees assumptions made 
that the same formation stretehing across country may be taken 
as evidence for the same date, I fear that the lessons to be 
gathered from the ‘Midford Sands,’ the secret of which I un- 
ravelled some years ago,! have not yet been learnt. Here is a 
formation of sand stretching across country for some 90 miles, 
from near Gloucester to the Dorset coast, resting upon clay below 
and capped by limestone rocks above. But the sand formation 
is not uniform in date—as it passes from north to south it 
gradually becomes later in time. Diagram 10 (p. 414) will illus- 
trate this—the numbers refer to the hemere in Table III; but, 
of course, the diagram does not illustrate all the complexities due 
to differential thickening of deposits, to penecontemporaneous 
erosions, and so on. But it shows how unreliable a guide the 
lithic character may be. In this case clay, sand, and limestones are 
all being deposited synchronously during several hemere without, 
so faras 1s known, any effect on the ammonite faunas. But it can 


1 «On the Cotteswold, Midford, & Yeovil Sands, &c.’ Q.J.G.S. vol. xlv 
(1889) p. 440. 
aree 


414, MR. S. S. BUCKMAN ON [ vol. Ixxviil, 


Diagram 10.—Non-local stratal repetition. 


(N.) Gloucester. Dorset (S.) 
Scissum. Limestone. Limestone. 
ee 
99-97 Limestone. /“ ~ Sands: 


96-24 Limestone. Zz Clay. 


23-21 Limestone, _ 
17-15 
14-12 Glay. 


hardly be supposed that these simultaneous depositions of different 
strata form an isolated case—ain fact, there is every reason to think 
that the phenomenon is rather frequent. It may, however, be 
masked by paucity of exposures and by hemeral sequences not having 
been so fully worked out. In the case of paucity of exposures, 
faunal analyses of the contents of a formation should be made. 
If they reveal notable discrepancies at certain localities, the 
explanation may be sought in the phenomenon of synchronous 
deposition of different strata—the similar lithic character as 
evidence of date should be regarded with suspicion. 

It is not surprising that, in the Midford Sand case, observers 
disputed fruitlessly for many years as to the position of the Sands. 
But the paleontology of these observers was much at fault—a 
name like Ammonites variabilis was applied to species of such 
diverse genera as Haugia, Phlyseogrammoceras, Pleydellia, 
Hammatoceras, Sonninia, and Fissilobiceras. 

I hope to be able to illustrate the phenomena of these Sands 
and synchronous deposits more fully in some subsequent paper of 
this series—much of it is written. Meanwhile, I return to the 
subject of stratal repetition, as illustrated by the geographical 
shifting of the focus of deposition (see Table IIT, p. 415). 


part 4] JURASSIC CHRONOLOGY. 415 


Taste I1I—Dates anp LOCALITIES OF SANDS. 


Dates. Localities. 
re | wel hae Sell thes 
Hemere. ae ae 2 si 2 2s 
eS) palais || Ses ad osc AMM eS 
29. venustula ......... x ) > 
MRO CII Goes isles ses .4 | 
27. [Cotteswoldia } | xX | See 
DMREIOOREU Ne. «0c acscas +4 x x | s 
25. Catulloceras ...... We x ee : 
24. Dumortieria ...... la x } S 
23. Hammatoceras ...| ... ed x | Pruhouy Lg 
22. dispansum ......... | ve ae 3 
21. struckmanni ...... Rae ects Say viiieose | Sands, z 
BO! pedicwm ............ sensu stricto. = 
110), C022) Seen = 
18. striatulum ......... ane ee oe Ae x Sodbury 
ta. Sands. 
17. variabilis ......... “hf cae ae ae are x } Cotteswold 
WG. GAO pee eee ee na Se, ae sae x Sands. 
15. braunianum ...... | | 
14, fibulatum ......... 
Me OUMONS .. 0.2.10... 
12. subcarinata ...... 
11. pseudovatum ...... 
10: faleiferum ......... 
CLOT ACUI, © oc... ss. 
8. tenuicostatun...... | | 
PERGCWLOME occ cncves | 
6: athleticum ......... 
5. Harpoceratoid 
4. spinatum............ Thotne 
3. thorncombiensis...| XX combe 
2. margaritatus bed. | | Sands. 
1. pre-margaritatus on Ce 
SH ec Cu CR el aca es 


So far as the Dorset coast is concerned, there are three sets of 
sands which are remarkably alike—so much so, in fact, that it is 
matter of common knowledge how difficult it is to be sure of the 
date (even in a general way) of a chance opening, especially as the 
strata are rather barren in a fossiliferous sense. The three sets of 
sands are those of pre-margaritatus bed (Down-Cliff Sands), 
separated by a thin calcareous and by a thin argillaceous bed from 
a post-margaritatus sand-deposit (thorncombiensis, the Thorn- 
combe Sands), which is in some cases separated from the third 


416 MR. 8S. 8S. BUCKMAN ON [vol. lxxvii, 


sandy deposit (Bridport Sands) only by the thin Junction-Bed. 
This is local stratal repetition. Non-local stratal repeti- 
tion is illustrated by the other areas, which show the depositional 
focus of sands travelling northwards into Gloucestershire and then 
returning southwards to the Dorset coast again. 

In Table III the interval between the Thorncombe Sands and 
the Bridport Sands seems to be very great, and so it is in the time- 
scale as well, if the full thickness of deposits made contemporane- 
ously elsewhere were brought in—250 feet or so in Jzlli-variabilis 
hemerz of the Cotteswold Sands alone; but in actual section on 
the Dorset coast the break is a very small matter, sometimes only 
about 2 or 3 feet—so little, indeed, that it may reasonably be 
inferred that the movements of the Weymouth Anticline have 
resulted in places in such small upheaval as was necessary to bring. 
about the removal of the Junction-Bed and the superposition of. 
Bridport Sands on Thornecombe Sands in one bed, with a wholly 
false appearance of sequence. 

In argillaceous deposits stratal repetition is shown in Fuller’s 
Harth, Upper Lias, Lower Lias—to name only a few. And in 
calcareous rocks the phenomenon occurs—certain strata of especially 
similar appearance may be named :— 


Minchinhampton Stone (Great Oolite). 


Notgrove Oolite. 
Upper Freestone. 
Lower Freestone. 


Inferior Oolite. 
Lower Limestone. | 


These are strata of white freestone with oolitic grains: they are 
separated by deposits having other characters. 

But to ask for a stratal repetition of a fine- grained white litho- 
graphic stone of two dates, one pre-falezferuwm and the other 
Hammatoceras, to be deposited in the same area so as to form one 
bed masking a non-sequence of considerable duration, is to make 
a somewhat extravagant demand, because of the great degree of 
similarity involved. There are two phenomena to consider—the 
first is repetition, and the second is coalescence. It will greatly 
strengthen the case to bring forward evidence of the first phe- 
nomenon in regard to lithographic stone, and of the second in 
regard to another stone; because, although the phenomena occur 
separately, there is obviously only one further step towards finding 
them occurring together. 

Remarkable confirmation in regard to stratal repetition of a like 
deposit comes from a neighbouring place, Burton Bradstock!; there 
is found a white bed (which it will be necessary to discuss presently) 
of much later date, so similar to the white bed of Watton Cliff 
that, if portions without their fossils were mixed, separation could 
doubttully be made. Now, the greater will include the less. Here 
is definite evidence that white- -stone—lithographic- stone—conditions 


LT, 5, p. 69. 


part 4] JURASSIC CHRONOLOGY. 417 


recurred in the same area after the lapse of some fifteen to twenty 
hemerz, so there is ground for surmising that the same phenomenon 
occurred at Watton Cliff after the lapse of some ten to fifteen 
hemerz. But the two beds of presumed different dates at Watton 
Cliff have coalesced. Yet coalescence of unlike deposits separated 
by more than twenty hemere in time is well known; rarer, of 
necessity, is the coalescence of two like deposits of different dates, 
because the chances against it are greater: but it is known, and 
was commented upon in my last paper! Possibly, other cases of 
such coalescence have been overlooked, because they were not 
understood. If only such coalescence of the Burton and Watton 
white beds had taken place, it would have provided a problem and 
a crop of ingenious surmises before it was understood. And there 
is just the possibility that such coalescence did occur—somewhere 
out to sea, in the neighbourhood of the axis of the Weymouth 
Anticline. 

The third theory seems to have the greatest weight of proba- 
bility in its favour; but, at present, that is all that can be said 
for it. If correct, it makes the Watton Bed contemporaneous, so 
far as its earliest part is concerned, with the pre-falciferum paper- 
shales of the North Gloucestershire Whitbian, and, so far as its 
later part is concerned, contemporaneous with the top layer of the 
Bothenhampton section, with the Yeovilian Sands of Cole, near 
Bruton (Somerset), and with the middle part of the Gloucestershire 
Cephalopod-Bed. It will be noticed that at present there is no 
ammonite evidence for any of the hemerz between Hammatoceras 
and streatulum-—in descending order: dispansum, struckmannt, 
pedicum, esert. 'They have not been found at Bothenhampton nor 
in the Western Cliffs—at the former place towards the top of the 
Junction-Bed there is, consequently, a gap of four hemere; at 
Doghus and Down Cliffs, between the Junction-Bed and the over- 
lying Down-Clitff Clay, there is a gap of five hemere; at Thorn- 
combe Beacon there is a gap of six hemere. At Shoot’s Lane, 
Symondsbury (Dorset), evidence for eser? has been found— 
Haugia fascigera® (now Esericeras).8 

Stress has been laid on stratal repetition and coalescence, because 
this may possibly explain some of the Continental deposits which 
are puzzling, on account of inclusion in the same matrix of species 
of very widely different dates. 

In the Red Ammonite-Limestone of Lombardy are species of 
various dates, from Domerian through Whitbian to Yeovilian, with 
certain notable omissions. The dates of the species are fairly 
obvious, and the deposit possibly represents a continuity of 
hike conditions with erosions. But were the gaps—the portions 


Beto. De SO: ZOOM pao: 

3 J, 8, pl. elxxxii. Since this was written, examination of Mr. Jackson’s 
finds gives evidence for fauna of esert and pedicwm (or struckmanni hemera, 
perhaps) in the Western Cliffs. Such finds are always possible, and do not 
invalidate my statements ; they stand good for many exposures of the Junc- 
tion-Bed of the Western Cliffs. 


418 MR. 8. S. BUCKMAN ON [vol. lxxviul, 


unrepresented by fauna—filled by strata of like or of unlike 
character? There is no evidence. 

The stratum of Posidonomya alpina of Sette Comuni (Italy) 
contains a fauna described and figured by Parona! which appears 
to show considerable mixture of Bajocian (and early Vesulian) 
with Callovian species. Parona notes that such an admixture has 
been recognized by H. Bése & H. Finkelstein for the same stratum 
in Southern Tyrol and by E. Jiissen for the Klaus Beds of the 
Northern Alps.” In Parona’s case the admixture seems to be greater 
than he supposes, even allowing for the well-known and deceptive 
homceomorphy of certain Bajocian-Vesulian and Callovian species, 
and therefore the following remarks on some of his faunal elements 
may be of service. It is interesting to note how many of them 
have a distinctly Bajocian-Vesulian aspect. Before claiming them 
as Callovian species it would be very necessary to show that they, 
or very similar forms, occur in deposits which are unquestionably 
Callovian. If that could be done, then they would still remain of 
much interest, as illustrating heterochronous homceomorphy. But 
at present the majority of the forms about which the following 
notes are made appear to be strangers to Callovian, though not to 
Bajocian-Vesulian deposits. 


TasLteE [V—ReEMARKS ON PARonaA’s FIGURED SPECIES OF 
Basocian—VESULIAN ASPECT. 


Reference. Species. Remarks. 
Pu. I, fig. 11. ‘ Oppelia subtilicostata.’ Similar to Oppelids of niortense 
date. 
12. ‘ Heotraustes minor, Possibly a catamorph of Cado- 


moceras. Cf. C. costatum, I, 8, 
pl. clxxxix: niortense. 


13. ‘ Cadomoceras nepos.’ Good evidence for niortense date. 
14,15. ‘Spheroceras pilula.’ Possibly catamorph of Labyrin- 

thoceras, I, 8, pls. cXXX1V, CXxxv; 
C. Sauzet. 

16. ‘Spheroceras auritum,’ Like species of blagdeni-niortense 
dates. 

17. ‘Spheroceras? disputabile. Vike species of niortense date. 

18. ‘Stephanoceras rotula.’ Like the coronate stage of Pevi- 
sphinctids of niortense-truellei 
date. 


19. ‘Stephanoceras gibbum.’ Very like Tvrilobiticeras, 1, 8, 
pl. exl; Discites. 

21. ‘Stephanoceras venetum. Remarkably similar to Polyplec- 
tites sp. of the truellei bed of- 
Burton Bradstock. 

22. ‘ Parkinsonia bonarellii. Not a Parkinsonia, as the furea- 
tion is on the edge of the peri- 
phery; it should be about 
medio-lateral. A phaulomorph 
Perisphinctid ; tiwellei ? 

Pu. II, fig. 1. ‘ Cosmoceras pollux, Like certain forms of Strenoceras 
of niortense date. But there is 
a great resemblance between the 
Bajocian—Vesulian genera St7e- 
noceras, Baculatoceras, Garan- 
tiana, and the Callovian Cosmo- 
ceras in certain cases. 


ft avannie 2 VIII, p. 5. 


part 4] JURASSIC CHRONOLOGY. 4.19 


Reference. Species. Remarks. 
Pu. Il, fig. 2. ‘ Cosmoceras n. f.’ Much like Baculatoceras of nior- 
tense date. 
3. ‘ Perisphinctes subtilis’ Not P. subtilis. Very like P. 
Neumayr. pseudomartinsii Siemiradzki— 
Prorsisphinctes, I, 8, pl. cc; 
garantiana. 


9. ‘ Peltoceras chauvini- More like Caumontisphinctes, I, 8, 
anum’ A. VOrbigny. pls. elxix & excii; niortense. 
21-23. ‘ Waldheimia behmi.’ ‘Has the most remarkable likeress 


to Waldheimia brodiei 8. Buck- 
man (in Davidson, Mon. Brit. 
Jur. Brach. App. to Suppl., 
Paleont. Soc. 1884, p. 266 & 
pl. xix, figs. 14-15, which is a 
species from the Irony Bed 
(blagdeni zoue) of Louse Hill.’ ! 

24, ‘ Waldheimia concava.’ An anamorph of Waldheimia 
haasi S. Buckman in Davidson 
as above, p. 265 & pl. xix, 
fig. 12; blagdeni zone of Louse 
Hill. 

19. ‘ Waldheimia beneckei.’ A catamorph of Zeilleria ferru- 
ginea S. Buckman, I, 4, p. 260 
& pl. xiii, lig. 4; from the Irony 
Bed (blagdeni zone) of Louse 
Hill. 


The stratum in question appears from Parona’s description ? to 
be homogeneous. It is making a rather large, though not 
impossible, demand on credulity to believe that this homogeneous 
deposit was persistent without alteration, through all the great 
number of hemerze which are contained in Upper Inferior Oolite, 
Fuller’s Earth, Stonesfield Slate, Great Oolite, Forest Marble, 
Cornbrash, Kellaways Rock to basal Oxford Clay. Continental 
authors, it is true, have failed to realize the great time-interval 
that exists between post-Bajocian and Callovian—being inclined to 
look upon them as merely a part of one deposit, because on the 
Continent so many of the stratal constituents are lacking. 
Quenstedt, for instance, placed such strata just in one division, 
Braun Jura e, as if they all made up quite a minor episode. But 
I shall have more to say on that point in a sequel to this paper. 

What concerns us now is the difficulty of thinking that the 
homogeneous deposit lasted persistently through all this time. 
There is perhaps no evidence against it in the shape of different 
deposits in the neighbourhood; but the great non-sequence 
suggests that a change of conditions occurred, that different 
deposits were laid down and were entirely swept away again—— 
then that there was stratal repetition and coalescence. 

The interesting point about the Burton White Bed and the 
Watton Bed is the definite evidence given for about 2380 feet of 
strata, different from them and differing among themselves— 
sands and various limestones—separating two like deposits. But 
it is not difficult to picture another condition—a difference of 
geological and geographical history—sea where now is land, the 


S15 ea (on AO, 29 Vali cos) oe 


420 MR. S. S. BUCKMAN ON (vol. Ixxviu, 


Dorset-Somerset evidence of these intervening strata swept away ; 
land where now is sea, with coalescence instead of separation 
of the two White Beds. Then we might be examining a rock of 
white matrix obtained some miles off the Dorset coast which would 
puzzle us, because it showed in a homogeneous deposit Whitbian 
and Vesulian ammonites. Given sufficient destruction of evidence, 
and we might be long in arriving at the solution—erosion, stratal 
repetition, and then coalescence. Yet such, I think, is the 
explanation of sundry Continental deposits, and also of the Watton 
Bed itself: stratal repetition for the Yeovilian deposit and coales- 
cence with a like Whitbian deposit—some of the intervening 
strata, of different lithic composition, being deposited in the 
Junction-Bed of the Western Cliffs and elsewhere. 

The subject of stratal repetition and coalescence must receive 
further attention at another time. 


lV. THe Waite Bep or Burton BRADSTOCK. 


The likeness of the Watton Bed to that which I described as 
found at Burton Bradstock naturally attracted attention.! That 
bed is in Bridport Sands and also connected with faulting. 
Accordingly, in the autumn of 1919, I paid another visit to 
Burton. The first blow of the hammer fortunately hit a block 
disclosing examples of the same latesuleate ‘ Garantiana’ as that 
which I mentioned before.” It is not properly a ‘ Garantiana’ ; 
but it isa well-known form of the néortense bed of Louse Hill,3 
the Astarte or Rotten Bed+ which les above the Irony Bed. A 
fine-ribbed Perisphinctid was also obtained. 

Subsequent labour, however, proved unproductive of more 
evidence for date. But a Gavrantiana—like form and a Peri- 
sphinctid in this bed are sufficient to show that, like as these two 
lithographic strata are in composition, yet they are of widely 
different dates—the Watton Bed is pre-Bridport Sands, the Burton 
Bed is on the border-line of Bajocian-Vesulian. not only post- 
Bridport Sands, but subsequent to the early and middle Inferior 
Oolite. What is interesting, however, is that the same fine muddy 
sedimentary conditions must have obtained at these two different 
dates: but, in the Burton ease, it is only by the accident of faulting 
that a fragment of the later deposit has been preserved; in the 
Watton-Cliff case the accident of faulting has doubtless preserved 
the southern extension of the Bed, although that must be buried 
at a steep angle some 300 feet below Watton Cliff. What has 
been preserved is the unfaulted part which lies practically hori- 
zontal in the cliff-face, in position between Middle Lias and basal 
Bridport Sands. 


LE Spy 68: 21,5, pp. (0 

3 1,5,p. 71. [The Louse-Hill species has, since this was written, been 
figured and named Hlawiceras platyrrymum (I, 8, pl. eexl).] 

+ T, 3, p. 488, Section VI, Bed 3. 


Se ee 


part 4] JURASSIC CHRONOLOGY. 421 


Mr. Linsdall Richardson has said that the Burton White Bed is 

in situ in Bridport Sands formed by percolation subsequently ! ; 
but while such an explanation might account for a deposit, it is 
quite inedaquate to account for the presence of fossils in that 
deposit. It seems hardly sufficient to account for such a deposit 
as this: percolation should produce calcite-veins (the so-called 
‘beef’), not a finely-laminated lithographic stone. The difficulty 
about the fossils in the deposit Mr. Richardson avoids by a sugges- 
tion which ignores my statements, and is totally at variance with 
the facts; he says: 
‘It was probably from this horizon [the top of the Red Beds] that the piece 
of “lithographic ” stone came that yielded to Mr. Buckman fossils indicative 
of late niortensis or garantiane hemera. Prof. S. H. Reynolds has very 
kindly examined microscopically one of the two pieces of ‘lithographic ” 
stone that Mr. Buckman gave to the Director [Mr. Richardson]. These two 
particular pieces probably come from the top of the Red-Bed horizon.’ ? 


My statements were 


‘The White Bed.... only occurs....in the bank at the beach opposite the 
villas.... Itis particularly exposed on the sort of pathway leading from the 
road to the beach, and’ just to the right hand as one reaches the beach.... 
The blocks on the beach were broken. The yield was several specimens [of 
fossils]....and a piece of a Garantiana sp. noy.... known as a species from 
the niortensis beds of Louse Hill, near Sherborne.’ ? 


The fossils and the rock-specimens were both obtained from the 
same place, from those blocks which are on the beach below the 
villas, blocks detached from the Bridport Sands, into which they 
have been faulted. The latest finds (1919)—similar ammonites, a 
Perisphinctid, and a Nawtilws—were all extracted from the bank 
in the pathway below the villas. 

Why Mr. Richardson made the assumption that the fossils and 
specimens came from the Red Bed it is difficult to imagine, 
especially as the cliff-section with the Red Bed is from a quarter to 
half a mile away ; and I had remarked that # 


‘there is [in the cliff-section] practically no sign of any deposit of a thick 
white bed of the character of the one that has just been described.’ 


But the assumption was necessary to fit the percolation theory, in 
pursuance of which Mr. Richardson remarks that ‘ the lithographic 
stone might be found associated with fossils of any hemera.’° The 
difficulty here is that the fossils would show internally the different 
matrices formed during the hemerz to which they rightly belonged. 
But these fossils from the white bed show inside and outside a 
white-bed matrix—they are evidently synchronous with the White 
Bed. Another objection is that the fossils of the White Bed are 
not found in any other of the Burton deposits. And there is a still 
further objection which may be taken to the percolation theory— 
that percolation would not produce a fine-grained, laminated 
lithographic stone. 


1 IX, 2, p. 56. 2 IX, 2, 
6 IX, 2 


p. 56-57, 3 T, 5, pp. 69-71. 
oro, pac. Ble 


422 MR. 8. 8S. BUCKMAN ON [ vol. Ixxviui, 


It is now known that a similar lithographic stone was deposited 
horizontally and normally at an earlier date at Watton Cliff, of 
which the unfaulted part is preserved and the faulted part rendered 
inaccessible: it 1s reasonable to conclude that the Burton White 
Bed was deposited in a normal manner, but that the unfaulted 
part has been destroyed and only the faulted part preserved. A 
somewhat complicated series of events which involves faulting and 
penecontemporaneous erosion must be postulated, but nothing that 
is not aiready known in other cases. 

Thus, in regard to the White Bed, there are the following 
facts :— 


(i) That the White Bed is of later date than the Red Bed, and earlier than 
the Astarte Bed. 


(ii) That the White Bed is not now found resting upon the Red Bed. 


(iii) That the White Bed is not found separating the Red Bed from the 
Astarte Bed. 


(iv) That the White Bed is a finely-stratified laminated deposit—in part, at 
any rate. 


(v) That the White Bed is let down in a fault some 50 to 60 feet below the 
top of the Red Bed: that is, below its original position. 


(vi) That it is mixed up with Bridport Sands. 


(vii) That hard rocks of dates subsequent to that of the White Bed—that 
is, strata of the hemere Garantiana to zigzag—are not found associated with 
broken-up masses of the Red Bed. 


Next, the following inferences may be drawn :— 
? 5 


(i) That the White Bed was deposited discontinuously on a surface of the 
Bridport Sands to the east, but with fair continuity on the surface of the Red 
Bed to the west. 


(ii) That the White Bed is a deposit of slow accumula‘ion, laid down under 
conditions similar to those of the lithographic stone of the Watton Bed. 


(iii) That the White Bed deposited on the Bridport Sands must have been 
protected from the denuding agency. 


(iv) That the preservation of the White Bed is apparently due to a small 
syncline in the Bridport Sands. 


(v) That the White Bed was removed from the top of the Red Bed before 
the commencement of the deposition of the Astarte Bed. 


(vi) That the hard rocks of the strata of hemerze Garantiana to zigzag 
must have been either not deposited on the top of the White Bed of the 
Bridport-Sands tract, or, if deposited, they must have been completely re- 
moved again, otherwise they would be found associated with fallen blocks 
of the White Bed. 


(vii) That non-deposition of these strata on the top of the White Bed 
would mean the elevation of the sand tract above the water, and would there- 


fore expose it to denudation, which would bring about the total loss of the 
White Bed. 


(viii) That deposition of these strata on the White Bed is a necessary 
supposition ; but that this requires a postulate as to their subsequent removal. 

(ix) That it is necessary to suppose that a soft rock easily yielding to 
denudation—a rock like the clays of the Fuller’s Earth—was the immediate 
covering of the White Bed at the time when faulting occurred, as such a rock 
could easily disappear from the face of the White Bed without leaving any 
traces. 


part 4] JURASSIC CHRONOLOGY. 4.23 


To account for these facts and inferences, the following history 
of events may be suggested. Diagram 11 (I-V), pp. 424-25, 
which illustrates the different events is merely a sketch, only 
approximately to scale. Vertical scales are, of course, greatly 
exaggerated in relation to horizontal :— 


(i) Towards:the end of the deposition of the Red Bed there was elevation 
of the tract of Bridport Sands on the east. 


Gi) There was denudation of this tract, and then the White Bed was 
deposited ona surface consisting of Red Bed in the west, passing across strata 
down to Bridport Sands in the east (I in diagram 11). 


(iii) Subsequent to the deposition of the White Bed, and before the deposi- 
tion of the Astarte Bed, there was a renewed clevation of the Bridport Sands. 
Then there was denudation of the White Bed and its complete removal off 
the Red Bed, but its preservation in the Bridport-Sand area in a small fold 
east of the anticlinal axis (II in diagram 11). 

(iv) Following this was the deposition of the Astarte to zigzag strata non- 
sequentially upon the various denuded surfaces (II). 

(v) Renewed elevation of the Bridport-Sand tract came next, with a denu- 
dation which removed from that tract all the zigzag to Astarte-Bed strata 
and possibly some more of the White Bed ; because the supposition that the 
later denudation should cease at the same place as the earlier makes too much 
demand upon coincidence (III in diagram 11). 

(vi) On such a denuded area Fuller’s Harth was deposited (III). 


(vii), At some date subsequent to the deposition of the Fuller’s Harth— 
perhaps at the same date as the faulting in Watton Cliff—the strata of the 
Bridport-Sand tract bearing the White Bed were let down considerably, and 
at as steep an angle as are the Junction-Bed and Fuller’s Harth of the faults 
in Watton Cliff. 

(viii) That the soft Fuller’s Earth would be rapidly removed, and would 
then leave the White Bed exposed. IV (diagram 11) gives a view of this 
condition of affairs, in profile at right angles to the supposed fault-face, the 
elevation of the rocks inland being of course greatly exaggerated in relation 
to the horizontal scale, and V gives a picture of the same looking on to the 
fault-face, omitting the rocks which lie directly inland, but showing those of 
the cliff which lie to the westward. In this case it should be noted that 
really, from the fault westwards along the cliff to where the Fuller’s Earth is 
preserved, is a distance of about half a mile. 

(ix) While the White Bed was being eroded from the surface of the Red 
Bed, there is the possibility that portions of it may have been broken off and 
cemented on to the top of the Red Bed with ground-up paste of the Red Bed; 
and there is always the possibility that pockets of the White Bed may yet be 
found in hollows of the Red Bed. A recent discovery of a possible White- 
Bed block in the Astarte Bed will be noticed presently. 


This theory of the course of events is, at any rate, superior to the 
percolation theory, in that 1t does not commence with an incorrect 
assumption as to the facts. The only real call that it makes on 
probability is in postulating two elevations and erosions, the second 
elevation being so adjusted that all the hard rocks of Garantiana 
to zigzag hemere are removed from the surface of the White 
Bed, but that some portion of the White Bed is left intact. Yet 
this is only asking for a phenomenon on a small scale, such as is 
exhibited on a large scale over a considerable area of the Cottes- 
wolds. There the strata which intervene between those of brad- 
fordensis and Garantiana hemerz were deposited after a period of 


{ tunssios 
0} ped Pea 
BUBIJUBIBI) 
0} OE ZOIZ 


Ye SIN 


Sunt 


UINSSIOS 
0} ped peu 
BUBIZUBIED 
0} SBZ517 


ne ee 
Se 


aes‘ psg STM Oe 
Tt Asis 
BUBIZUBIABY) i — —~ 
0} SBZOI7 —~ 2S 


“HLL 


: Capes 


eS) neta 


weeee seus 
Seeeneneeuegee 
oo 0} peg pery 
ped oy MA, 


Mien 
WML 


(ja: 10/ T) YIOPS) qd ? O qd ) a Y C y | t ay Ve S29 = | | U RAI 21 
y | DA l JAN whO G 
e € fo pa 
4 q PUTS bd 
I ¥ 
- e ° < Ss « . - " Iq 


yw P 


peg s7an0g ~"y 


‘eS 


aeerenrs oq 179n0g 
[were ~ ay} mojaq skeTZ 


310d] 


ets] 


LUIASSIDS 

0} pag pay 

pueluelen 
0} SEZ5IZ 


ype S19qny 
AN ‘M'S 


‘(panurqzu0o) TT urersriqy 


urer pue saaem 2400S 
Aq poaowar' ayy 


PUPIZUBIEN 
0} SEZOIIZ 


Ye S19] [Ny 


‘AN 


4.26 MR. 8. S. BUCKMAN ON [vol. Ixxviil, 


elevation and erosion, yet prior to another period of elevation and 
erosion. ‘The second erosion obliterated all these intervening strata 
from certain places, yet was so adjusted that they were preserved 
in other places. But the latest bed of all is only preserved over a 
rather limited area—Cleeve Hill,—and strata which ought to follow 
on this bed, preceding those of Giarantiana hemera, have been 
completely destroyed. 

It is not necessary to assume that a fault was present until the 
date which let down the White Bed to its present position—long 
after Fuller’s-Harth date presumably.! All that need be assumed 
is a gentle fold bringing Bridport Sands first to the level of the 
Red Bed, and then to the level of the Zigzag Bed : the actual anti- 
clinal axis being about on the present line of fault, which would 
give a syncline just beyond it sufficient to let in the White Bed. 

We do not know how thick the White Bed was originally. Say 
that there is now some 2 or 3 feet of it preserved. It may have 
been deposited to a thickness of 20 feet, of which only this small 
portion is preserved, and that by an accident. It is true that 
the other Inferior-Oolite rocks of Burton Bradstock are quite 
thin; but this is no measure of their original thickness—it is 
the result of the constant denudation that they underwent, being 
removed almost as fast as they were deposited. Some of them 
thicken considerably only a short distance inland—away from the 
axis of the Weymouth Anticline. 

For instance, at Vetney (Vinney) Cross, which lies about 3 miles 
north-east by north of Burton Bradstock (or about 8 miles from 
the axis of the Weymouth Anticline), the Garantiana Bed is 
18 inches thick as against 4 inches at Burton or an increase of 
43 times, while the truellei bed is about 6 feet as against less 
than 2 feet at Burton; this, however, is not the full thickness, 
for really the deposit of 6 feet corresponds to about the lower 
6 inches of the Burton ¢rwellez bed, or an increase of 12 times 
as much. Then at Nettlecomb, near Powerstock Station (about 
5 miles north-north-east of Burton Bradstock), a rather poorly- 
fossiliferous quarry shows some 20 feet of schlenbachi deposit, 
as against less than 5 feet at Burton; and yet there is no sign 
of the subjacent or superjacent beds, either in the quarry or in 
the stones of the fields around. 

We speak of the sequence blagdeni, niortense, Garantiana ; 
but we have no certainty that this is the complete sequence. 
It may be that, between the hemerz nzortense and Garantiana, 
strata were deposited of which no trace has yet been discovered.? 

1 The Forest Marble of Watton Cliff contains pebbles of a White-Bed 
matrix. Itis an interesting speculation whether these have been derived 
from destruction of the Burton or from that of the Watton White Bed. They 
appear to be rather too soft for either: but this is a case where chemical 
analysis might yield some evidence. 

2 The most likely place in this country for such a discovery is near Sherborne 
(Dorset), in the neighbourhood of the quarries of Frogden and Lower Clat- 
combe. Seeing how little distance at Burton Bradstock is required to bring 
in a bed not found in the other neighbouring sections, there is no telling what 
an excavation at 100 or 200 yards in certain directions near Sherborne might — 
not reveal. This subject may be more fully worked out in a later paper. 


lod 


part 4] JURASSIC CHRONOLOGY. 427 


It is fairly certain that, when the fauna of the strata now dated 
as niortense are submitted to analysis, several episodes will be 
found covered by the one name—deposits of one episode have 
been preserved at one place, and those of others in other places. 
And yet the sequence may not be complete. 

These remarks are made to show that, on theoretical grounds, 
there is reason to claim plenty of time between the dates of the 
deposition of the Red Bed and the Astarte Bed for the various 
events of elevation, denudation, deposition, to have occurred even 
in repetition. And there is actual evidence in support: in the 
Hebrides, according to the researches of Dr. G. W. Lee, some 
70 feet of strata separate deposits which are respectively the 
equivalents of the Red Bed and the Astarte Bed. Unfortu- 
nately, they are unfossiliferous, and so their dating as niortense 
rests only on the fact that they are between Ob/agdeni and 
Garantiana hemere. ‘Then in the Sherborne district there are 
poorly fossiliferous strata—the Building-Stones—which are post- 
mortense: they are reckoned as belonging to the Garantiana 
hemera, and so is the Astarte Bed of Burton Bradstock; but that 
they are strictly contemporaneous is possibly doubtful—the various 
Garantiana-like forms in the two deposits show certain differences, 
and therefore, at any rate, the species are not altogether identical. 
Possibly the Astarte Bed is, in part at least, of somewhat earlier 
date than the Building-Stone of Sherborne; but it is unadvisable 
to speak positively on this point, until the Garantzana-like forms 
ean be worked out in detail—a long task with such a mass of 
material as the strata yield. 

The point to be emphasized is the necessity of knowing whether 
our chronological datum-lines are strictly contemporaneous ; because, 
as it is only possible to estimate time by the amount of work per- 
formed during stated periods, it is necessary to be certain that the 
periods are identical For instance, was the thickness of 70 feet 
in the Hebrides laid down only prior to the Building-Stone of 
Sherborne, or was it laid down prior to the Astarte Bed of Burton, 
or, as 1s possible, was it laid down even prior to the White Bed of 
Burton ? These possible differences make considerable difference 
to the time-estimate ; for, in the last case, the 70 feet represent 
work done in a time-interval between the Red Bed and the White 
Bed, which was a period of upheaval and erosion at Burton; then 
between the Red Bed and the Astarte Bed, which reposes upon it, 
one could place a time-interval represented by work done in 
depositing 70 feet in the Hebrides, and, in addition, an unknown 
original thickness of White Bed. It is rather interesting to note 
that the White Bed, which is the sole representative at Burton of 
the time-interval between the Red Bed and the Astarte Bed, 
should not be found separating them, and has only been preserved 
by an accident. 

Bearing on the remarks about the possible original thickness of 
the White-Bed deposit, there is this to be said further: in the 
Sherborne district the pre-Garantiana zones of the Inferior 


Gd Gs, No, sie, 26 


428 MR. S. S. BUCKMAN ON [vol. lxxviul, 


Oolite are thin deposits, averaging about a foot or so, but the 
Garantiana deposit is a thick one—running up to 40 or 50 feet, 
perhaps more. The paucity of fossils accounts for such horizons 
as there may be not being followed out in the different quarries of 
Building-Stone, for these quarries may be really on various levels— 
if so, the thickness would be much greater. Therefore, a surmise 
of a possible 20-foot deposit for the original White Bed is not 
necessarily a gross overestimate. 

It is interesting to note that these lithographic-stone beds of the 
Yeovilian of Watton Cliff and of the late Bajocian of Burton 
Bradstock have a great likeness to the Continental strata termed 
White Jura, diphya-kalk or Alpenkalk, which form so conspi- 
cuous a feature of the Upper Jurassic over wide areas. It has, 
before now, been claimed for this country that its special geological 
interest lies in the fact that, small though its area be, yet it con- 
tains strata so fully representative as régards all the different dates 
and all the different structures. Yet it is correct, I think, to say 
that before the date of my discovery of the lithographic stone of 
Burton, it showed nothing to compare with strata laid down under 
those conditions which produced the Alpenkalk. Now, however, 
the Watton Cliff discovery adds another representative of such 
conditions—a deposit made ata still earlier date: so it is now 
unnecessary to travel outside England to obtain samples of strata 
like those of the Alpenkalk. And there will be this additional 
interest attached to them, that these beds of Watton Cliff and 
Burton indicate Alpenkalk conditions of deposition prevailing 
in Western Europe long prior to the date when they held good in 
Central Europe. How long these conditions endured in the 
English cases, and what respective thicknesses of strata were laid 
down, can only be matters of surmise; for penecontemporaneous 
erosion has removed so much, and came near to removing all. How 
wide an area was occupied by such deposits is also matter for 
surmise—they may have spread far over the area now occupied 
by the English Channel. Some parts of the deposits may now 
lie buried beneath its waters, other parts of them have certainly 
been destroyed by these waters. Only a very few cubic yards of 
deposit in the Burton case are now left available for investigation. 
In the case of the earlier deposit, there may be a good deal more. 
There is certainly not much in Watton Hill itself: for the bed is 
cut off on the south by the Watton Fault, and on the west, north, 
and east by the landward slope. There is, perhaps, ne more thay 
an area of some 250 to 300 square yards preserved. But this White 
Bed has been detected in Shipton Long Lane, Bothenhampton—say, 
about three-quarters of a mile inland from the east-and-west line 
of Eype—and there is a possibility that it can be found farther 
inland, as around Allington Hill. 


1 Mr. J. W. Tutcher rightly draws my attention to the Sun-Bed in the 
White Lias of the Radstock-Bristol district as being an earlier deposit of 
similar character. Therefore in the South-West of England there was a 


‘threefold repetition of this kind of rock. 


escapee htt — 


ethtetiata bi ieait tis aE I i 


* 


part 4] JURASSIC CHRONOLOGY. . 4.29 


Now, Bothenhampton is some 7 miles from the main anticlinal 
axis of the Weymouth Anticline. If it be a legitimate surmise that 
the conditions producing Alpenkalk extended as far south of the 
-axis of the anticline as they do north of it, then a north-to-south 
stretch of the White Bed was some 14 miles. ‘To put the east-to- 
west stretch at the same distance would be reasonable : this would 
give nearly 200 square miles of area as the original extent of 
White-Bed deposit. T’o put the east-to-west stretch at some three 
times the distance would not be really so very unreasonable: this 
would imply some 600 square miles of area—by no means an incon- 
‘siderable extent. 

The exploration north of Bridport of what is marked as the 
contact-line between Lias Marlstone (G2) and Lias Sand (G 4) 
would certainly be desirable, as giving a chance of finding further 
evidence of the Yeovilian White Bed. But there is little chance 
.of exploration in such an area giving more evidence for the Burton 
White Bed, as there are plenty of quarries opened at the necessary 
horizon and they have received much attention. However large 
may have been the surface over which the Burton White Bed was 
deposited, it seems unfortunately to be only too true that it has 
been wholly removed from all that area. If such removal did 
not wholly take place, as at Burton Bradstock, prior to the 
deposition of the Garantiana Bed, it was accomplished later, when 
it shared the fate which in some localities was meted out to 
the Garantiana or subsequent deposits. But there is reason to 
suppose that the main accomplishment of its destruction was a 
pre-Garantiana episode, for, unlike the strata of niortense date 
which are preserved only at afew isolated localities, the strata of 
Garantiana date have a remarkably wide spread. 

An observation made at the Black Rocks, west of West Bay, tends 
to confirm the theory that the White Bed was an original stratified 
deposit, broken up and redeposited later, where it was not preserved 
by being let down in the fault. At the Black Rocks, about the 
middle, close to the beach of pebbles, was found a big block (dia- 
gram 12, p. 480)—4 feet long, 18 inches broad and 26 inches thick 
—consisting of the <Astarte-Bed (Garantiana) matrix, recog- 
nizable by its lithic characters and the great number of various 
fossils shown in section. About in its middle it carries a large 
mass (1 foot thick), extending rather beyond the length and 
breadth of the block, because of its superior hardness. ‘This mass 
is a very compact, close-grained, grey rock, apparently quite un- 
fossiliferous. It differs from all other local Inferior-Oolite rocks 
except the White Bed; that it resembles in texture, hardness, and 
paucity (Gf not absence) of fossils. In colour it differs some- 
what, but not more, perhaps, than the difference of locality 
it is nearly 25 miles from the Burton White Bed—might explain. 
It certainly seems as if this included mass might be a local 
portion of the White or Nautilus Bed redeposited in the Astarte 
Bed. If such be the case, then, as the Black Rocks lie at the 
foot of Watton Cliff, we should have evidence of the second 
White Bed deposited in the same locality as the first. 

. 262 


430 MR. 8S. S. BUCKMAN ON [vol. xxviii, 


Mr. Richardson says with regard to the Burton White Bed : 
‘the very white aspect of the stone was due to weathering of the 
surface.’! This is not my experience—the true lthographie 
stone is beautifully white, inside as well as outside. 

The Black Rocks which lie out to sea immediately west of the 
first fault from West Bay, should repay investigation. Hitherto 
they seem to have been neglected; but this may be due to the 
fact that they are inaccessible, except during certain low tides. 
It was only during iy last visit to Eype (1920) that I found 
them sufficiently uncovered for a short examination, which en- 
abled me to collect certain specimens, indicative of the Astarte- 
Bed (Garantiana) of Burton Bradstock, from some masses well 


Diagram 12.—One of the Black Rocks (Dorset). 


Inches. 


A : 8 


LF 


A =Tronshot fossiliferous (Garantiana) rock. 

B= Very hard, fine-grained grey rock (White Bed equivalent). 

C =TIronshot, like the hard portion of the Vetney Cross Astarte Bed 
(Garantiana). 


off shore, but just landward of the big rock. At my next visit at 
low tide these rocks were not accessible; but, nearer the beach 
and among the shingle of the beach, were found certain blocks: 
which could be recognized as sczsswm bed, Red Bed, schlenbacht 
and z7gzag beds, besides the mass of Astarte Bed just mentioned. 
At certain states of the shingle these beds are buried; but, given: 
favourable conditions of tide and shingle, the Black Rocks should. 
yield a good harvest. However, heavy tools are a necessity. 


To return for a moment to the nzortense bed: there is reason: 
to suppose that this bed was originally of wide geographical 
extent—possibly from the Hebrides to Italy and the Carpathians, 
certainly from Sherborne (Dorset) through France to Wiirtemberg.. 
But, although it was originally deposited over so vast an area, the 
bed is now preserved only at a few widely-distant localities and is: 
lacking from intermediate areas—much of it was wholly destroyed. 


2h 1. rig pps Om 


part 4:] JURASSIC CHRONOLOGY. 431 


prior to the deposition of strata of the Garantiana hemera, much 
of what may have escaped that destruction perished in later 
erosions along with the whole of the Inferior Oolite. I have made 
a rough calculation that possibly not 1 part in 30,000 of the bed 
as originally deposited has been preserved. Ido not pledge myselt 
to the figures; they may be thought to be an under-estimate, but 
there 1s also reason to think that they may be an over-estimate. 
But to give an idea of what such destruction means: there are 
about 30,000 post-offices in the British Isles enumerated in the 
Postal Guide. If we were to destroy all these names but that 
of Sherborne (Dorset), we should thereby obtain a picture of the 
destruction of the néortense bed. Then consider how small a part 
a quarry or two forms of the preserved portion, and that, when 
such quarries are in work, many years may pass without anyone 
visiting them. On such a basis, it is easy to understand that the 
collected material may form no more than one part in a million 
of the entombed originals. 

Which deposit of the Jurassic hemere has suffered the most in 
regard to destruction may be a matter for future research, but the 
niortense. bed will, I expect, be one of those which make the 
greatest claim to such distinction. It is not proposed to enter 
into details on this subject here—the accumulation of evidence is 
no light task; but it is hoped to initiate in a sequel to this paper 
a geographical enquiry as to the original extent of the nzortense 
bed and as to its present preservation, which may fit in suitably 
with some other geographical studies suggested by stratal and 
faunal phenomena of the rocks to be dealt with in that sequel. 


V. CERTAIN CHRONOLOGICAL STUDIES. 


(A) Milborne Wick and the Green-grained Marl. 


In the Table! of his paper on the Doulting-Milborne-Port 
district, Mr. Linsdall Richardson places opposite blagdenz hemera, 
and under Milborne Wick, the following ‘ Marl with green grains, 
S. S. Buckman.’ The placing of it within inverted commas makes 
it appear like a statement taken from my paper,? or given on my 
authority. But this is incorrect: reference to my section of 
Milborne Wick will show that I divided the beds in more detail 
than did Mr. Richardson. I recorded 


Humphriesiam. 2. Soft white chalky limestone, 4 inches. 
[ blagdent }. 
3. Grey limestone with iron grains, 6 inches. 
Sawzed. 4. White limestone, 6 inches. 
5. [Soft limestone] speckled with green grains, Astarte 
spissa. 


Here I have said nothing about any green grains in the limestone 
of blagdent hemera: at the top that is white, without coloured 


1 TX, 3, facing p. 518. 27, 3, p. 502. 


432 MR. 8. §. BUCKMAN ON [vol. lxxviii,. 


grains, lower it has grains which are noticeably pink; ata still 
lower level, which I dated as sawzez, are the green grains. 

The point is of considerable importance for dating specimens. 
In the ’seventies (at the time, I understand, when the road-cutting 
was made at Milborne Wick), very extensive collections of fossils. 
were obtained from this place: many are in my collection, and it is. 
rare to see a public or private collection without noting fossils. 
which have the characteristic Milborne-Wick matrices. If my 
observations are correct, the lithic differences afford a means of 
ascertaining the sequence of these fossils. I have examined many 
ammonites in my collection, and they certainly tend to confirm 
this opinion. The very common fossil of the blag gdent zone 1s 
Pecilomorphus cycloides—common, because the name really covers 
a multitude of different forms; but Mr. Richardson does not 
mention this species. 

Not only is he incorrect in citing my authority for this dating of 
the marl with green grains; but I think that he may be incorrect 
in his dating of the beds which he saw. My ‘ Bed 2’ is given by 
me as 4 inches thick, and Beds 3 & 4, as 1 foot, total=16 inches. 
Mr. Richardson’s Beds 2 to 4 total 13 inches. So on that basis he 
may have hit a place where Bed 2 was only 1 inch thick. But 
there is a lack of 14 inches in the rest of his section—under Bed 6. 
I think it possible that where he took his section he saw, at the 
top, Bed 5—where I noted the green grains—with just some small 
portions of the higher beds and their fossils in more or less derived 
condition mixed up with the disturbed green-grained marl. This 
point might be settled by an examination of the matrix inside 
the fossils, not that outside. 

There are two further points for consideration :— 

That in this section local removal of the beds below the non- 
sequence—Beds 2—4—is possible, with the corollary that, at 
favourable places, Bed 2 may be thicker than is recorded in my 
section; and that there may be local variation in the thickness of 
any of the beds. Therefore, if Mr. Richardson found green-grained 
rock at the top of the section, whereas I found it at 16 inches below, 
the explanation may be local thickening at the top of Bed 5 and/or 
local denudation. 

One point I would add stout this section, from examination of 
ammonites from old collections—specimens of Witchellia show a 
matrix with green grains and Astarte spissa; wherefore it is 
possible that the top of Bed 5 should be reckoned as. Witchellia 
and notas sawzez, asitisinmy paper.! Actually I cannot recollect 
any example of ‘ Ammonites sauzet’ (Otottes) from Muilborne 
Wick. The species ‘Spheroceras perexpansum’ (Labyrintho- 
ceras) ® cited for Bed 4 may really indicate a somewhat different 
date. But whether just before or just after cannot be said yet: this. 
is a matter possibly to be considered in future faunal analyses. 


3, p. 5038. 
8,p t. 19, pls. exxxiv & cxxxv. 


part 4] JURASSIC CHRONOLOGY. 433 


(B) Haselbury and Hammatoceratids. 


In his paper on the Crewkerne district ! Mr. Linsdall Richardson 
reproduces W. H. Hudleston’s profile? of the now disused quarry 
near the church of Haselbury. When he was preparing his paper 
he asked my opinion as to the possible date of Bed 3: at that time 
I could only suggest bradfordensis hemera, adding, however, that 
I did not recollect any bradfordensis specimens from Haselbury. 
Since then the possible solution of the date has occurred to me. 
The keeled ammonites which Hudleston mentions from this Bed 3 
are perhaps represented by the species of Hammatoceras and 
Erycites, which have been obtained from Haselbury and the 
immediate neighbourhood—A. ef. planinsigne Vacek and Erycites 
aff. gonionotum (Benecke): similar forms have been figured by 
Vacek? & De Gregorio * from the strata at Cape San Vizilio, Lago 
di Garda (Italy). 

In the Bradford-Abbas district occur many species of these two 
genera °: they have come from strata mainly dated as murchisone 
hemera, though partly or occasionally as bradfordensis. But, when 
faunal analysis comes to be applied, it is seen that the synchroni- 
zation of such species with murchisone (that is, the Ludwigoids), 
on the one hand, or with bradfordensis (that is, Brasilia spp.), 
on the other, becomes rather doubtful. At the Italian locality 
are few or no species that can be properly reckoned as either 
Ludwigoids® or Brasilia, but Hammatoceratids are abundant. 
In the Hebrides the Ludwigoids are especially abundant, but the 
Hammatoceratids are unknown. In Dorset—Somerset Ludwigoids 
associated with the characteristic brachiopod, the so-called 
Waldheimia anglica, are of fairly wide distribution and not 
uncommon, but Hammatoceratids are localized and particularly 
rare. At Haselbury both Ludwigoids and Hammatocerata are 
rare, for the quarry was not a productive one so far as ammonites 
are concerned; but Hudleston notices for Bed 4 Ammonites 
murchisone and Waldheimia anglica, and for Bed 3 he records 
keeled ammonites: evidently they were unfamiliar to him—had 
they been of murchisone or concavus (that is, bradfordensis) type, 


Px 4. p. 165. 2 alnose Ale 

2a op Ne aloe. Gol; 

6 So far as the Ludwigoids are concerned, this statement may seem 
particularly rash, for De Gregorio says (iV, p. 11) ‘ L’Harpoceras Murchisonz 
est trés-commun 4 S. Vigilio, et c’est une des espéces les plus caractéristiques 
de cette faune.’ This illustrates the necessity for precision in paleontological 
identification. The specimens which he figures (IV, pl. 11) as Harpoceras 
murchisonz with various qualifying terms show no species agreeing strictly 
with Ammonites murchisone itself, possibly no Ludwigoids at all, certainly 
not the murchisone fauna of the Hebrides nor of Dorset—Somerset, nor of other 
areas ; while the list of species of widely different dates which he gives as 
possible varieties or mutations of A. murchisonz# show how greatly outward 
similarity may mislead. His Harpoceras murchisonxe includes a variety of 
types, some indicative of Ancolioceras date and perhaps of earlier hemerz, some 
more or less suggestive of bradfordensis date, some perchance indicative of 
strata of even later dates. 


= =z 


4.34 MR. 8. S. BUCKMAN ON [ vol. Ixxvin, 


it may be presumed that he would have appended some less general 
term. 

On the assumption that at Haselbury the Hamblatocenane 
occupy a higher bed, and are of later date than the Ludwigoids, 
which would lead to a theory of a Hammatocerate-Erycite horizon 
as distinct, that is, of later date than that of Ludwigoids, the facts 
of distribution could be explained. In Italy strata of murchisone 
hemera are absent or only represented in part, but those of a 
Hammatocerate-Erycite hemera are present. In the Hebrides, the 
former was very conspicuous, but there is as yet no evidence for 
the latter. In Dorset-Somerset the strata of the murchisone 
hemera are developed over a wide area ; but those of the Hammato- 
cerate date are wholly destroyed in some places, and partly removed 
in others. At Haselbury a bed assumed to be that of Hammato- 
cerate date is preserved in sufficient thickness to attract attention 
as a deposit superior to that yielding ‘ Ammonites murchisone. 

More analysis on these ines may suitably come later, and will 
be of greater value if and when some of the species of Hammato- 
cerates can be figured. But, meanwhile, it may be advisable to 
consider the possibility of a Hammatocerate-Erycite date, as inter- 
mediate between those of murchisone and bradfordensis, and to 
date Bed 8 of Haselbury in Hudleston’s (and Mr. Richardson’s) 
communications as Hammatocerate-Erycite? rather than as ‘brad- 
Sordensis?’ 

The Cotteswold evidence seems in favour of this view. There 
between the Pea Grit (murchisone hemera) and Oolite Marl 
(bradfordensis) is a thick mass of poorly fossiliferous, perhaps 
non-ammonitiferous freestone with a more or less eroded surface: 
this deposit, in part at any rate, and the subsequent erosion 
(prior to bradfordensis) would mark the time required for laying 
down a Hammatocerate-Hrycite stratum in other areas. 

A short record of the chronological succession and geographical 
occurrences of some of the principal Hammatoceratid forms shows 
the faunal repetition and the limited extent—in some areas— 
over which the particular strata have been preserved: it is the 
earliest of these horizons that has come so much into discussion 
in connexion with the Watton Bed (see p. 387). 


TABLE V—HAMMATOCERATIDS: CHRONOLOGICAL SUCCESSION AND 
GEOGRAPHICAL OCCURRENCE. 


Intervening hemerse omitted. 


Hemere. Fauna and Localities. 
Hudmetoceras. Eudmetoceras spp.', Euaptetoceras.1 ua 
[ discites/concava]. Bradford Abbas district; Beaminster district 


(Dorset) ; Lago di Garda (Italy). 


‘ Hammatocerate ’-Erycite. ‘ Hammatoceras, Erycites,. Abbasites,2 Ambersites? 

[bradfordensis/murchisone]|. Dorset and Somerset; Normandy; Lago di Garda; 
Lombardy (Italy) ; ; Monte Grappa (Italy) ; 
Bakony (Hungary). 


1 J, 8, pls. elxxix, clxxx, ccxcix. 21 8, pl. coxxxvi. 31, 8, pl: cexxxvie 


part 4] JURASSIC CHRONOLOGY. 435 


Hemere. Fauna and Localities. 


scisswin. Bredyia spp. 

{ Ancolioceras/opaliniforme}. Burton Bradstock (Dorset) ; Cotteswolds (Gloucester- 
shire) in the Sandy Ferruginous Limestone; 
Normandy ; Rhone Basin ? 


Hammatoceras (insigne). Watton Cliff, Eype (Dorset), Junction-Bed; Cole 
| Dumortieria/dispansum]. (Somerset), Bruton Sands; Cotteswolds, Gepha- 
lopod_Bed; widespread on the Continent. 


VI. A Patmontrotogican NorE—TETRARHYNCHIA 
THORNCOMBIENSIS, NOM. NOV. 


J. F. Walker identified specimens of this characteristic fossil 
from the Thorncombe bed as his Rhynchonella northamptonensis, 
and it is presumably the same form from the same deposit at 
North Allington, near Bridport, that he called ‘ Rhynchonella 
tetrahedra, var. northamptonensis.’! But the Thorneombe form 
certainly is not the same species as that from Northamptonshire, 
for that has clean-cut, sharp ribs; whereas the Thorncombe fossil 
has blunt, somewhat rounded-off ribs, and has a general smoothed- 
off appearance. As the two forms are certainly not contemporaries, 
as the Thorncombe fossil marks a particularly distinctive horizon 
on the Dorset coast, and as it has been frequently necessary to cite 
the name, it seems preferable to give it a distinct appellation rather 
than to perpetuate one which will require changing. I have 
selected a type to be figured in the proposed paleontological 
sequel to this paper, where it is also hoped to figure and describe 
various ammonites and brachiopods, which have special bearing on 
the chronology of Lias-Oolite rocks as well as giving evidence for 
widespread deposition of certain deposits. 


VII. BretioGRAPnHry. 


. Buckman, S.S. ‘The Descent of Sonninia & Hammatoceras’ Q.J.G.S. 

vol. xlv (1889) p. 651. 

. Do. ‘On the So-called “Upper-Lias Clay” of Down Cliffs’ Q.J.G.S. 

vol. xlvi (1890) p. 518. 

. Do. ‘The Bajocian of the Sherborne District: its Relation to Subjacent 
& Superjacent Strata’ Q.J.G.S8. vol. xlix (1893) p. 479. 

Do. ‘Homeomorphy among Jurassic Brachiopoda’ Proc. Cotteswold 
Nat. Field-Club, vol. xiii, pt. 4 (1901) p. 231. 

_ Do. ‘Certain Jurassic (Lias-Oolite) Strata of South Dorset, & their 
Correlation’ Q. J. G. S. vol. Ixvi (1910) p. 52. 

Do. ‘The Brachiopoda of the Namyau Beds, Northern Shan States 
(Burma) ’ Paleontologia Indica, n.s. vol. 11, No. 2, 1917. 

Do. ‘Jurassic Chronology : I— fase ONI: cea S. lk Ixxili *(1917-18) 
Dacor 

. Do. ‘Type-Ammonites’ III & IV, 1919-22. 

Do. ‘Jurassic Chronology: I—lLias: Supplement 1, West England 
Strata’ Q. J.G.S. vol. Ixxvi (1920) p. 62. 

II. Davipson, THomas. ‘Monograph of the British Fossil Brachiopoda ’ 

(Pal. Soc.) iv, pt. ii, no. 1, Suppl. to the Jurassic & Triassic Species, 

1876; no. 2, 1878: Appendix to Suppl., 1884. 


CO NM SB Mm BP w to 


Xi p: 438. 
2 II, 1878, p. 199, Suppl. pl. xxix, figs. 7-12: fig. 7 may be taken as lecto- 
type of the species. 


436 MR. J. F. JACKSON ON [ vol. Ixxvin, 


Ill. Day, E.C.H. ‘Onthe Middle & Upper Lias of the Dorsetshire Coast ’ 
Q. J.G. S. vol. xix (1868) p. 278. 
IV. Der Greeorto, A. ‘Monographie des Fossiles de S. wieaine Ann. Géol. 
Pal. livr. 5, 1886: 2: i. 
VY.  Evpers-Drestonecuames, BE. F.G. ‘Etudes sur les Etages Jurassiques 
inférieurs de la Normandie’ 1864. 
Vi. Hupiestoy, W. H. ‘A Monograph of the British Jurassic Gasteropoda : 
Part 1—Gasteropoda of the Inferior Oolite’ (Pal. Soc.) 1887-1896. 
VII. Moors, Cuarues. ‘On the Middle & Upper Lias of the South-West of 
England ’ Proc. Somerset Arch. Soe. vol. xiii (1867) p. 119. 
VIII. Parona, C.F. ‘Nuove Osservazioni sopra la Fauna & Eta degli Stratz 
con Posidonomya alpina nei Sette Comuni’ Paleont. Italica, vol. 1 
(1895) p. 1. 
1X, 1. Ricuarpson, L. ‘A Handbook to the Geology of Cheltenham & Neigh- 
bourhood’ 1904. 
2. Do. * Report of an Excursion to Bridport, Beaminster, & Crewkerne” 
Proc. Geol. Assoc. vol. xxvi (1915) p. 47. 
- Do. ‘ The Inferior Oolite & Contiguous Deposits of the Doulting-Milborne- 
Port District (Somerset)’ Q. J.G.S. vol. Ixxi (1915-16) p. 473. 
- Do. ‘The Inferior Oolite & Contiguous Deposits of the Crewkerne. 
District (Somerset) ’ Q. J.G.S. vol. Ixxiv (1918-19) p. 145. 
X. Srrickianp, H. E., & Buckman, J. ‘ Outline of the Geology of the- 
Neighbourhood of Cheltenham, by R. I. Murchison’ new ed. 1844. 
XI. Vacex, M. ‘ Fauna der Oolithe von Cap San Vigilio, &c.’ Abhandl.. 
K. K. Geol. Reichsanst. vol. xii (1886) p. 57. 
XII. Waker, J.F. ‘On Liassic Sections near Bridport (Dorsetshire)’ Geol. 
Mag. 1892, p. 437. 
XIII. Wircnerr, EK. ‘The Geology of Stroud & the Area drained by the Frome” 
1882. 
XIV. Woopwarp, H. B. ‘The Jurassic Rocks of Britain: vol. 111, The Lias of 
England & Wales’ Mem. Geol. Sury. 1893. 


= oD 


VIII. APPENDICES. 


Appendix I—Srcrions of THE JuNcTION-BED anDd ContTIGvOUS 
Deposits. By JAMES FREDERICK JACKSON. 


[In brackets remarks on ammonites and brachiopods, by 8. S. Buckman. } 


(1) Introduction. 


My acquaintance with the Junction-Bed dates from 1912; and, 
in the course of a number of visits to the sections between that 
date and 1920, several interesting facts bearing on the stratigraphy 
and paleontology of the bed have come under observation. Some 
of these facts appear to be new, and others, while not strictly new, 
have proved to possess a greater significance than had previously 
been recognized. 


(2) Characters of the Junction-Bed (Western Cliffs). 


In 1910 Mr. Buckman! recognized six separate layers in the 
Junction-Bed, and referred them to the hemere striatulum, lilli (7), 
bifrons, falciferum, and spinatum. Day’s ‘ Pleurotomaria Bed’ 
was not certainly identified. I have now noted, in addition to 
the above subdivisions: (i) a lenticular development of clay (O) 
between the falciferum and spinatum layers at Down Cliffs ; 


1 ‘Certain Jurassic (Lias-Oolite) Strata of South Dorset, & their Corre- 
lation’ Q.J.G.S. vol. lxvi (1910) pp. 57 et seqq. 


part 4] JURASSIC CHRONOLOGY. 437 


(i1) a thin ferruginous layer (N) with Harpoceratoides spp. below 
the limestones of the falczferum layer at Thorncombe Beacon ; 
and (iii) a marly layer (N,) at the top of the sevrata bed, also at 
Thorneombe Beacon. 

The blue sandy ‘Upper lias’ clays everywhere rest non- 
sequentially upon the limestones of the Junction-Bed with a 
remarkably sharp line of division; and, owing to the impersistence 
of the several layers of the Junction-Bed, the clays may rest upon 
any layer from the stréatulum to the falciferum zone. Usually, 
the basal 6 to 12 inches of the clays become more sandy and 
ferruginous than the main mass above, and this lower portion 
contains a good many coarse-ribbed Dumortier¢e in the condition 
of fmable casts—mostly too rotten to be preserved. A long 
slender Belemnites sp. occurs, but 1s uncommon. 

The lithic character of the strvatulum layer varies much. In 
parts it is a hard, rubbly, greyish-yellow limestone; in other 
sections it is a soft earthy marl, with harder lumps and many 
limonitic nodules and rolled fossils in a perished condition. At 
the base is a very thin, impersistent seam of dense laminated 
limestone showing an obscure mammillated structure, suggestive of 
‘Cotham Marble.’ Whenever the top of the stréatulum layer cam 
be examined 7m stu, it supplies plain evidence of long-continued. 
quiet erosion, prior to the deposition of the ‘ Upper Liassic ’ clays, 
being planed off perfectly level and displaying ammonites in sec- 
tion—the lower or embedded side well preserved, the upper 
removed by the erosion. From a fallen block below Down Cliff I 
collected a large alticarinate Haugia, 53221 | Hsericeras aff. esert 
(Oppel), more “umbilicate], almost as accurately cut through as if 
sliced on a lapidary’s wheel: the erosion has removed the shell 
down to within a few millimetres of the thin high keel almost 
equally around the greater portion of the circumference of the 
shell. The strzatulwm layer reposes upon the smoothed top of 
the bifrons layer with a well-marked plane of division, but 
in perfect conformity. There is no obvious indication of the 
extensive non-sequence known to exist—a good illustration of the 
kind of deceptive conformity that has led to so many under- 
estimates of the importance and real time-value of attenuated 
deposits. 

The b¢frons layer is a hard tough limestone, usually mottled 
pinkish-yellow and red. Sometimes the stone is wholly fine- 
grained and massive; generally 1t abounds in pebbles of a similar 
rock, together with derived H2ldoceras bifrons.2 The derived 
ammonites are often reduced to mere limonitic pebbles showing 
traces of the sutures, ete. Unworn examples of H. bifrons occur 
among the redeposited specimens. The upper 2 or 3 inches of the 
bifrons layer is a greyish-yellow rubbly limestone, abounding in 
small specimens of H. aff. bifrons and Dactylioceras ef. commune ; 


1 These numerals refer to my register of specimens. 
* Trivial names of species are used sensw lato. 


438 MR. J. F. JACKSON ON (vol. Ixxviil, 


two or three species of small, thick-ribbed, involute Dactyloids 
are not uncommon. The Ozfrons layer is separated into two 
or three minor seams by impersistent planes of erosion, and occa- 
sionally the ammonites subjacent to these planes are in a similar 
condition to those at the top of the striatulum layer. Some of 
the ammonites are embedded in a vertical or highly inclined 
position, and in one or two instances which came under my obser- 
vation the ammonite was seen to be truncated by a plane of 
erosion: a portion having been removed while the remainder con- 
tinued firmly fixed in the rock, which must have been completely 
consolidated previous to the erosion. The division between the 
bifrons and falciferum layers is often rather obscure, and falsely 
suggestive of a passage. 

The falciferum layer is a very hard tough limestone of a 
yellowish-pink colour, often mottled with red blotches ; occasionally 
it is rather sreenish. Sometimes the rock is fine- ‘grained and 
compact; more often it is highly conglomeratic, containing 
broken and rolled Harpocerata and pebbles of a similar limestone. 
As in the bzfrons layer, there are minor layers separated by planes 
of erosion. Apart from Harpoceras falciferum, fossils are un- 
common, and all are very difficult to extract on account of the 
intense hardness of the stone. 

The falciferum layer frequently rests directly upon a planed-off 
surface of ‘marlstone,’ but in places it appears to be transitional 
from a thin, earthy, ferruginous seam with Plewrotomarie and 
other gastropods—presumably the ‘ Plewrotomaria Bed’ of Day.! 

That writer’s description is so vague that it is difficult to 
determine whether he considered the ‘ Pleurotomaria Bed’ to be a 
separate layer at the base of the ‘Upper Lias’ limestones, or merely 
the highest portion of the ‘ marlstone.” Mr. Buckman has observed 
an lmpersistent seam with nests of Pleurotomarie at the top 
of the lower or spznatum layer of ‘marlstone,’ which he considers 
to be the true ‘ Pleurotomaria Bed.’ I have not noticed any 
very definite ‘ Pleurotomaria Bed, although Pleurotomarieé are 
fairly common in the serrata bed and rare in the main mass of 
the lower or spznatum layer. It is, perhaps, doubtful whether 
there is any particular horizon especially entitled to the term 
‘ Pleurotomaria Bed.’ 

At Down Cliff a lenticular deposit of clay intervenes between 
the falciferum layer and the ‘marlstone’; but, since no fossils 
were obtained, it is impossible to say eine as to the exact 
date of this clay-band. The occurrence of a lenticle of clay 
within the Junction-Bed is noteworthy. 

The spinatum layer or ‘marlstone,’ when fully developed, is 
divisible into two seams. The upper, Mr. Buckman’s sevrata bed, 
is usually a fine-grained oolitic limestone with a rich assemblage 
of well-preserved fossils: occasionally it is conglomeratic. The 


1 *On the Middle & Upper Lias of the Dorsetshire Coast’ Q. J. G.S. 
vol. xix (1863) p. 288. 


part 4] JURASSIC CHRONOLOGY. 439 


lower portion of the ‘marlstone’ is a harder and much coarser 
rock, and is often highly conglomeratic. Most of the pebbles are - 
made up of a similar coarse ‘marlstone’ ; but pieces of a compact 
blue limestone and micaceous sandstone occur. Many large flat 
pebbles are thickly encrusted with a small Serpu/a and riddled by 
Lithophagus borings, the shells sometimes remaining in the crypts. 
There does not appear to be any definite line of division between 
the two layers of ‘marlstone.’ The base of the ‘marlstone’ is. 
always highly irregular, on account of the presence of large 
branching and tuberous concretions of calcareous micaceous sand- 
stone. The ‘marlstone’ rests upon an impersistent stratum of 
blue sandy clay, or directly upon the underlying Yellow Sands. 


(3) Details of Sections (Western Cliffs). 


I think that it will be convenient to describe the sections from: 
Down Cliff to Thorncombe Beacon before dealing with the ex- 
posure at Watton Cliff, east of Eypesmouth, which exhibits so. 
remarkably abnormal a facies as to call for special consideration. 

The following scheme of lettering of the several layers of the 
Junction-Bed and the contiguous deposits will be utilized through-. 
out the sections to be described below :— 


SCHEME OF LETTERING. 


f A, WELL AS G2) Ban mee ce pei Sra dea Ae 

DDMOTUVETUG. <6 ...02.06 Be 1 B. DUpROTES ” Se wore aa hee eee al ON 1b 
C i KOMI GI RU TO oe Shae ee eee WE. 

OWUMOLOCETAS:......6-..... Ds DEC=(QULCUPfCT UM Joab oo denen N. 
GISPONSWIMN .......2. ae EK. clay within Junction-Bed...... ©, 
SP MIUGINUOIUIM s-...:s:0 60s «concen 7 AY CUGPIRUHITL © ee de teak eee Ee ane aren £3 
pedicum pas eae Or FVCUMOCONUGMUGA(S) 8. nc6. ee @7 
ERE nes aa ee oe jal SHO UIMOLUMIIRINIES 588 csc estas a's con R. 
SERUMEMLUHU. “hci ae bs il clay below Junction-Bed ...... Ss. 
DWOMMOUOUWAS «oc. cca lncse cas i yellow Sands"... | Le 
LROTNCOMDICISUS 4.022 4.4 tee. Ui. 


Section I—TueE JunctTion-BED anp ContIGuous DEPOSITS, MEASURED 
in situ NEAR THE WESTERN END oF Down CuiFF. 


Hemere. Strata and Fauna. Feet inches.. 
Dumortieria.| A+B. Blue sandy ‘Upper Lias’ clay. Coarse-ribbed 
Dumnortieria. 
Impersistent irony scale. 0 ot 
Lilli 2— K+. Pale conglomeratic limestone, with a very | 
Bifrons. smooth top. 0 2 


Hildoceras bifrons, Dactylioceras cf. com- 
mune, Rhynchonella. 

[5217, a fissicostate Rhynchonellid, gen. et 
sp. nov.; 521lla, Stolmorhynchia cf. bothen- 
hamptonensis (Walker); 52116, St. cf. bou- 
chardi (Davidson), but with too strong a 


fold. | 0 9 
Faintly-marked parting. 
Falciferum. M. Yellowish limestone with red __ blotches. 
Harpoceras falciferum, Dactylioceras sp. 0) 7-8. 
Spinatum. R. Oolitic conglomeratic ‘marlstone,’ with a very 
smooth top. Belemnites spp., Pseudopecten 
equivalvis. 0 6-9 


T. Soft, yellowish, clayey sands. 


440 MR. J. F. JACKSON ON [ vol. Ixxviul, 


On the coast the westernmost exposure of the Junction-Bed is 
near the western end of Down Cliff, where the falciferum layer 
rests directly upon the ‘marlstone.’ A few yards east of this 
first exposure a clay-band comes in between these layers, and 
continues almost to the eastern extremity of Down Cliff, where 
it passes into a band of earthy marly stone. Section I shows 
‘the sequence near the western end of Down Clif. 

Section II shows the sequence near the eastern end of Down 
Chiff. Here the striatulum layer is present, and the inter- 
_faleiferum-spinatum clay-band reaches its maximum thickness. 


Section II—Tue Junction-Bep anp Contiguotus DEposiITs, MEASURED 
in situ NEAR THE EsstTERN End oF Down CLIFF. 


Hemere. Strata and Fauna. - Feet inches. 


Dumortierie. A. Blue, sandy, ‘ Upper Lias’ clay. Coarse-ribbed 
Dumortierie in friable condition ; Belemnites 
sp. 
Transitional from 

B. Yellowish sandy clay with ferruginous streaks. 
Striatulum. I [G+H). Grey earthy limestone, with many rotten 
(Traces of limonitic nodules and rolled fossils. Grammo- 
pedicum and ceras spp. common. Top planed off very 
eseri faunas}. smooth. 0 
(5317, Grammoceras aff. audax S. B.; 4989, G. 
aff. comptum Haug; 53204, G. penestriatu- 
lum S. B.; 5319, G. aff. penestriatulum S. B.; 
5315, 5318, G. aff. penestriatulum S. B., but 
coarsely-ribbed; 5320, G. aff. striatulum; 5317 a, | 
G. att. thouarsense, rather strongly ribbed; 
5316, Pseudogrammoceras cf. pachu S. B., but 
more finely ribbed (pedicum fauna) ; 5322, Eseri- 
ceras aff. eseri (Oppel), but more umbilicate. | 
Strong parting. = 
Lilli (2) K. Whitish rubbly limestone. Small Hildoceras 
_bifrons, Rhynchonella sp. |e + 
5211 (2 examples), Stolmorhynchia bouchardi 
“bothenhamptonensis | 
Obscure parting. 
Bifrons. L.  Pinkish-yellow conglomeratic limestone. Hildo- 
ceras bifrons. 0 7 
Obscure parting. 
_Faleiferum. |M. Similar limestone, but showing bright red 
blotches. Harpoceras falciferum. 0 6 
- Pre- O. Very stiff, greyish-yellow, mottled clay. No 
R 


be 
=) 


bo 


faleiferum. fossils observed. ee 
Spinatum. Oolitic conglomeratic ‘marlstone,’ with a mark- 
edly planed-off top. Belemnites spp. common, 
Pseudopecten equivalvis, Rhynchonella tetra- 
édra. 0 
Resting with very irregular base upon 
T. Yellowish clayey sands. 


“I 


The ‘marlstone’ of the Down-Cliff sections is almost certainly 
-only the lower layer of that deposit. No evidence of the upper or 
serrata bed was tound at Down Cliff, and the planed-off top of the 
‘marlstone ’ points to a considerable non-sequence. 
The ‘marlstone’ at Down Cliff rests directly upon the Yellow 
Sands, without the intervention of the clay-bed seen in Doghus 
- Cliff and Thorncombe Beacon. At the eastern end of Down Cliff 


part 4] JURASSIC CHRONOLOGY. 4AT 


a thin laver of sandy clay is beginning to come in beneath the 
‘marlstone,’ and it thickens rapidly towards Doghus Cliff. 

Section III illustrates the thinning-out and change in lithic 
character of the lenticle of clay O. 


Section III—Tune Junotion-BepD, MEASURED IN A Block on THE TALUS 
AT THE EKastERN Knp oF Down CuiFF. 


Hemere. Strata and Fauna. Feet inches. 
Bifrons. |. Massive mottled limestone. Hildoceras bifrons.| 1 2 


Obscure parting. 
Falciferum. |M. Whitish and pink, conglomeratic, mottled lime- 


| stone; ferruginous near the base. 0 @ 
| Apparently transitional from 
Pre- O. Greyish-white, nodular, earthy store, with marly 
falciferuin ? | streaks and patches. O 3-5 
Apparently transitional from 
Spinatum. |R. Oolitic conglomeratic ‘marlstone.’ Belemmnites 


spp., Paltopleuroceras sp. [5168, Paltopleuro- 
ceras cf. pseudocostatum Hyatt, but more spi- 
nose.| Seen to 0 8 


The ‘marlstone’ here is almost certainly the lower or pre-serrata 
layer, and the ‘ Plewrotomaria Bed’ Q is undoubtedly absent: hence 
the appearance of a transition from R to M must be considered 
illusory. In formations where ‘ contemporaneous ’ erosion has been 
active (especially if the materials be at all soft) so much inter- 
mixture of matrices has taken place along the line of contact of 


Section I1V—TuHeE JunctTion-BED, MEASURED IN A BLOCK ON THE 
TaLus BELow Doeuus CLIFF. 


Hemere, Strata and Fauna. Feet inches. 

Bifrons. LL. Pinkish-yellow mottled limestone. Hildoceras 
bifrons, rolled fragments of Harpoceras sp., 

Rhynchonella ct. moore. O 8 
Obscure parting. | 
Falciferum. M. Similar, but harder, limestone. Very large | 
Harpoceras falciferum, Dactylioceras spp., | 

‘ Aulacothyris’ sp. | O i 


[An inverted Terebratulid (5218) a little like | 
T. aspasia Meneghini & Zittel, but rounder, | 
not nearly so sulcate, and possessing a smaller 
beak. | 

Obscure parting. 

Pleurotomaria ? | Q. Impersistent earthy ferruginous seam. Plewro- 

tomarie, Amberleya, Cylindrites, Trochus ; 


Plicatula ct. spinosa. Che, 
Resting upon a rather irregular surface of 
Spinatum. R. Rough conglomeratic ‘marlstone. Palto- 


pleuroceras spinatum, Discohelix sinister, 
Pteria ‘inequivalvis, Pseudopecten equi- 
valvis, Modiola, Ostrea, etc.; Rhynchonella 
acuta, Rh. furcillata, Rh. tetraédra, Tere- 
bratula punctata, Zeilleria cornuta, Z. 
quadrifida, Spiriferina oxygona. 0 9 

(5137, Paltopleuroceras ct. pseudospinatum 
Hyatt, but has many more ribs; 5689 (2 ex). 
Homeorhynchia acuta, medium size; 5361, 
Furcirhynchia furcata 8. B.; 18380, Tetra- 
rhynchia cf, tetraédra. | 


44.2 MR. J. F. JACKSON ON (vol. Ixxvil, 


newer with older deposits, which may be perfectly ‘conformable,’ 
that a false appearance of transition results: and such pseudo- 
sequences may give rise to very erroneous views as to the real 
importance of formations such as the Junction-Bed or the 
Inferior Oolite. 

I was unable to measure a satisfactory section 7m sztu at Doghus 
Cliff; but some blocks on the talus showed features of considerable 
interest. Section IV (p. 441), which is taken from one of these 
blocks, is important, in that it shows what Mr. Buckman considers 
to be the ‘ Pleurotomaria Bed.’ The ‘ marlstone’ here does not 
appear to include any representative of the serrata bed. The 
‘Pleurotomaria Bed’ (?) is impersistent—it is absent from the 
great majority of the blocks on the talus. 


As an illustration of the remarkably variable characters of the 
Junction-Bed, the following section (V) of another block on the 
talus of Doghus Cliff is of some interest. 


Section V—TuHeE JuNcTION-BED, MEASURED IN A BLOCK ON 
THE TaLus BELOW DocGuHuws CLIFF. 


Hemere. Strata and Fauna. Feet inches. 
Tilli (2). K. White rubbly limestone, with ochreous nodules. | 
Worn Hildoceras aff. bifrons, Dactylioceras spp. ; 
Rhynchonella sp. 0 3 


[1763, 4994, 5279, 5308, micromorph Dactyloids | 
presenting the aspect of Ammonites dayi Reynés; | 
5278, micromorph Dactyloid of Caloceras aspect ; 
1777, 4991, 5212 (2 examples), 5309, Stolmo- 
rhynchia bothenhamptonensis Walker. | 
Very obscure parting. 


Bifrons. L. eS mottled limestone. Hildoceras 
bifrons. - | 0 — 56 

Resting upon the markedly planed- off top of | 

Serrata- P+R. Light- brown oolitic ‘marlstone, becoming rough 

spinatum. and conglomeratic towards the base. Many small 


_gastropods (Amberleya, Cerithium, Plewroto- 
maria, Trochus, etc.) near the top. Rhynchonella | 
acuta, Rh. spp., Zeilleria cornuta, Spiriferina 
Sp., Serpule, etc. | 1 2 

[5155 (2 ex.), 5687, Homeorhynchia acuta (large); | 
5163 (2 ex.), Rudir hynchia sp. = Rhynchonella | 
fallax. (Deslongchamps) Walker, but presumably | 
new; 5360, Quadratirhynchia cf. crassimedia 
S. B.; 5686, Stolmorhynchia bouchardi; 5159, 
Spiriferina oxygona Davidson non Deslong- 
champs, cf. Sp. signensis Buvignier. | 


Here the falciferum layer and the ‘ Pleurotomarta Bed’ (?) are 
absent, while the serrata bed is present. The lower layer of the 
‘marlstone ’ appears to pass up into the serrata bed without any 
break ; probably this false appearance of transition is due to a 
mingling of matrices along the line of junction. 

Scattered about on the talus of Doghus Cliff are several large 
slabs of (presumably) ‘ Upper Lias’ limestone, showing a worn 
and pitted surface covered with the adherent valves of a small 
species of Ostvea and, more rarely, a large cristate Serpula. No 


part 4] JURASSIC CHRONOLOGY. 443 


‘satisfactory evidence sufficient to date the non-sequence thus 
indicated was observed, and the question must be left open to 
future research. 

The clay which was noted as coming in beneath the ‘ marlstone ’ 
at the eastern end of Down Chff thickens rapidly under Doghus 
Cliff, where a thickness of over 10 feet can be seen. The maxi- 
mum is at Thorncombe Beacon, where the thickness of clay is not 
less than 16, nor more than 20 feet. 

The ‘ marlstone’ attains its maximum thickness in the central 
portion of the Thorncombe-Beacon cliffs; but, unfortunately, owing 
to the steepness of these clitfs, it can scarcely be reached zn sztw. 
However, a very large recent slip has brought down to the shore 
an abundance of blocks: these yield a rich ane, and show most 
of the subdivisions of the Junction-Bed, with the exception of the 
striatulum layer, which, as Mr. Buekman has pointed out,! is 
probably absent from Thorncombe Beacon. Section VI shows 
two layers N, N, not previously noticed. 


Section VI—TuHE JuncTIoN-BED, MEASURED IN A VERY LARGE BLOCK ON 
THE TALUS BELOW THE CENTRAL PorRTION OF THORNCOMBE BEACON. 


Hemere. Strata and Fauna. Feet inches. 
Falciferwm. | M. Massive limestone, mottled greenish and pink. 
Harpocer as falcifer wm, Belemnites sp. 0 9 
| Transitional from 
Pre- _N. Earthy ferruginous seam. Harpoceras spp., Phyllo- 
Saleiferum. ceras sp., Pteria ‘inequivalvis, Trapezium sp., 
small gastropoda. O 2-3 


[5220a, Harpoceratoides kisslingi (2) Haug; 5219, 
5220, HA. cf. kisslingi Haug, but very finely 
ribbed; 5169, a micromorph Phylloceras, like 

Ammonites calypso D’Orbigny, ‘ Terr. Jurass.’ 

1845, pl. cx, but with straighter constrictions and 
a smaller umbilicus. | 

Obscure parting. 

Pre- |Ny. Hard, greyish-white, nodular, earthy limestone 
faleiferum. with Harpoceras spp. at the top of the bed. 
The lower portion of the bed is a soft yellowish- 
white rock with scattered oolitic grains, which 
appears to pass imperceptibly into the earthy 
limestone at the top. Rhynchonella acuta (2), 
Rh. sp. in the lower portion. A layer of large 
Belemnites spp. near the base. O 38-5 

From the top of the bed: [5222, Harpoceratoides ? 
aff. fellenbergi Haug; 5221, Harpoceratoides? sp. | 

Very obscure parting. 

Serrata. P. Finely oolitic, light-brown ‘marlstone. Palto- 
pleuroceras spinatum, large Harpoceras sp., 
large Belemnites spp., Cerithiuwm sp., Pleuvro- 
tomarieé spp., Trochus sp., etc.; Pseudopecten 
equivalvis ;- Rhynchonella acuta, Rh. furcil- 
lata, Rh. serrata, Rh. ‘ tetraédra, Rh. spp., 
Terebratula punctata, Zeilleria spp., Spiri- 
Ferina sp. 1 0 

[5167, a brephomorph of Paltoplewroceras cf. 
spinatum (D’Orbigny), showing the coronate stage 
strongly spined, before the advent of a carina; 
5231, Harpoceratoid = Harpoceras radians 
W right, Mon. Lias Amm. (Pal. Soc. 1882) xox, 
4-6. 


! Q: J. GS. vol. lxvi (1910) p. 82. 
© J GS No. 312. 2H 


did MR. J, F. JACKSON ON (vol. Ixxvii, 


Hemere. Strata and Fauna. Feet inches. 


1827, 5191 (4 ex.), Homeorhynchia acuta (large) ; 
5156, Furcirhynchia furcata S. B.; 5192, Priono- | 
rhynchia serrata ; 5195, P. quinqueplicata ; 5147 
(2ex.), Quadratirhynchia ct. crassimedia S. Buck- 
man; 5196, Stolmorhynchia cf. bothenhampton- 
ensis; 5197, S. bouchardi; 5192 a, Rudirhynchia 
(2) sp.=Rhynchonella fallax (Deslongchamps) 
Walker, but presumably new; 5194, Rudirhyn- | 
chia? cf. Terebratula triplicata fronto Quen- 
stedt ; 5160, Lobothyris punctata ; 1845, Zeilleria | 
cornuta; 1844, Z. aff. cornuta; 5343, Z. cf. 
quadrifida ; 5161, Z. sp. | ) 

Apparently transitional from 

Spinatum. R. Rough conglomeratic ‘marlstone,’ with large irre- | 

gular concretions of calcareous sandstone at the 

base. Rhynchonella ‘ tetraédra, Rh. spp., Tere-| 

bratula punctata, Zeilleria sp., Spiriferina | 

sp., ete. | O 46 
(3726, Tetrarhynchia media/tetraédra; 4956, 

Quadratirhynchia aff. crassimedia; 5359,' 

Q. crassimedia, spheroidalis ; 4523, Q. att. sphe- | 

roidalis ; 5199, Lobothyris punctata; 4951, L. sub- 

punctata; 1843, 5190, Aulacothyris florella ; 

1838, 5201, Zeilleria marie; 5202, Z. sub- 

niumnisimalass 5200, an inverted Terebratulid (gen. 

nov.) ; 5331, Spir iferina oxygona (Davidson); | 

1841, 5188, Sp. sp. like orygona Davidson, but | 

with a costate fold; 1840, Sp. sp. (smooth)—ef. | 

Spirifer rostratus Dav idson, Mon. Brit. Ool. Lias. | 

Brach. IIT, 1851, ii, 3.} 


The presence of Harpoceratoides and the absence of Palto- 
pleuroceras indicate that at least the upper portion of layer N, 
belongs to the ‘Upper Lias.’ The date of the lower portion of N, 
is very doubtful; no fossils of zonal importance were observed, and 
there are only very obscure indications of any break at the base. 

The presence in the serrata bed of a large Harpoceratoid bearing 
a superficial resemblance to Harpoceras falciferum is of some- 
interest, in that it enables us to understand how early observers 
came to record ‘ Ammonites serpentinus, etc., from the 
‘marlstone’ portion of the Junction-Bed. No real credence ean. 
be given to the alleged occurrence of typical ‘ Upper Lias’ species 
in the ‘ marlstone.’ 

Pleurotomarié cecur in the serrata bed, but no indication of 
any definite ‘ Pleurotomaria Bed’ was observed. 

The brachiopoda are the most abundant fossils of the two. 
‘marlstone’ layers. Rhynchonella serrata is fairly common in 
the serrata bed, to which it would appear to be confined. Large 
specimens of Rhynchonella acuta, often in a fine state of preser- 
vation, are common in the serrata bed; Rh. acuta is very much 
rarer in the basal ‘ marlstone ’ layer, so much so that an abundance 
of that form in a block is strongly suggestive of the serrata bed. 
Eh. acuta is markedly rare at Down Chiff, where the serrata bed is. 
considered to be absent. 

Section VII was measured zm situ near the eastern end of 
Thorncombe Beacon, only a short distance east of the position from 
which the block measured in Section VI must have fallen. It 


part 4] JURASSIC CHRONOLOGY. 445 


shows the most attenuated facies of the Junction-Bed yet observed 
on the coast. 


Section VII—TueE Junction-BepD and ConTIGuoUS DEPOSITS, MEASURED 
in situ CLOSE TO THE HASTERN HND oF THORNCOMBE BEACON. 
Hemere. Strata and Fauna. Feet inches. 


Dumortieria.| A+B. Blue sandy ‘Upper Lias’ clay. Coarsely- | 
ribbed Dumortieria sp. in friable condition. 
Thin irony scale (impersistent). 0 OF 
Bifrons. L. Earthy conglomeratic limestone, with a mark- 
edly-sraooth top. Hildoceras bifrons. 0) 5-6 
Strong parting. 
Serrata. P. _—_Light-brown, finely oolitic ‘ marlstone,’ yielding 
Rhynchonella acuta. ‘Top planed off very 
smooth. 0 6-7 
Resting upon 
S. Blue sandy clay. 


Sections I to VII will have served to illustrate some of the 
extraordinary complexities of the Junction-Bed; a larger number 
of sections would show still further variations. 

A few yards east of Section VII the outcrop of the Junction- 
Bed leaves the cliff-face, and turns inland ; when the Junction-Bed 
reappears at Watton Cliff, east of Eypesmouth, the whole facies 
is remarkably changed. 


(4) The Watton-Cliff Section. 


From the last appearance of the Junction-Bed at the eastern 
end of Thorncombe Beacon to its reappearance at the western 
end of Watton Chiff, east of Hypesmouth, there is a gap of some 
three-quarters of a mile. 

At Watton Cliff the Junction-Bed is exposed in a particularly 
inaccessible position. It forms a kind of projecting cornice, 
high up in the precipitous face of the cliff, on the upcast side of 
the great fault that throws ‘ Fuller’s Earth’ and ‘ Forest Marble’ 
against the ‘ Middle’ and ‘ Upper Lias.’ According to the Geolo- 
gical Survey,! the downthrow is ‘at least 4.25 feet.’ 

The only published reference to the existence of the Junction- 
Bed at Watton Cliff that I have been able to find is the following 
short and inconclusive note by the Geological Survey (Joc. cit.) : 

‘ Hast of Eype the cliff again exhibits a portion of the Middle Lias. Atthe 
base there are blue clays with Ammonites margaritatus, and these are suc- 
ceeded by the Starfish Bed, the Laminated Beds, and Yellow Sands. These 
are capped by grey shaly beds, that include a hard band that may be the 
Junction-Bed of Middle and Upper Lias, with perhaps some portions of 
the overlying Upper Lias. The higher strata were, however, difficult of 
access.’ 

It seems somewhat remarkable that this unique section should 
have escaped general notice for so many years. It is quite easy to 
observe from the talus that the Junction-Bed is much thicker here 
than in the Western Cliffs. 


1¢The Jurassic Rocks of Britain: the Lias of England & Wales’ Mem. 
Geol. Surv. vol. iii (1893) p. 200. 
2H 2 


See Ee a eee ee ee 


446 MR. J. F. JACKSON ON (vol. lxxviil, 


Section VIII—Tue Juncrion-BED, MEASURED ON A BLOCK ON THE TALUS 
AT THE WEsTERN END oF Watton CuiirF, East oF EYPESMOUTH. 


Hemere. Strata and Fauna, Feet inches. 
Dumertieria.| A. Traces of blue sandy ‘ Upper Lias’ clay. 
Transitional from 
B. Hard, sandy, ferruginous clay with ochreous veins. 
Coarsely-ribbed Dumortierieé in friable condition ; 
obscure gastropod-remains, fragments of Iso- 
erinus sp. 0 335 
Transitional from 
C. Whitish earthy limestone in two irregular layers. 
A seam of small slender Belemnites sp. 0 25 
Nou-sequence. 
D. Intensely hard, finely laminated, lithographic lime- | 
stone; the laming are of various shades ‘of pale 
brown and bluish grey. The lower portion splits 
readily into thin platy slabs, covered with fine 
dendrites; the upper portion is more massive, 
and breaks with a sub:conchoidal fracture.’ Many 
finely - preserved ammonites; no other fossils 
observed. Oo 7-8 
[5323, Alocolytoceras cf. germaini (D’Orbigny), 
a smooth cast, ornament almost obliterated; 5684, | 
Frechiella ati. subcarinata (Young & Bird), a) 
brephomorph, beautifully preserved, with white 
matrix in the body-chamber: this shows the) 
transition from Cymbites to the Paroniceras 
stage, but has not got to the Frechiella stage of 
bisuleate periphery ; 5324,5325a & 6, 5680, 5682, 
costate forms, more than one species, like Ammo- 
nites rugatulus Simpson, also lke A. multi- 
foliatus Simpson, but more costate; 5325, 5679, 
56796, costulate to capillate forms, like 4. similis | 
Simpson, bunt thinner ; 5325 ¢, a micromorph of, 
presumably, Grammoceras-striatulum type. | 
Impersistent earthy parting: non-sequence. 
D,;. Intensely hard, greyish -yeliow, non - laminated, 
sandy limestone. No recognizable fossils, except 
fragments of a small Rhynchonella sp. ' 0 56 
Apparently transitional from 
Dz. Massive cream-coloured limestone. Belemnitessp. 0 4% 
Apparently transitional from 
D3. Cream-coloured shelly limestone, crowded with 
Grammoceras sp. [5326, G. ail. thowarsense|, 
| especially at the base. Many minute gastropoda: 
Amberleya, Ataphrus, Cerithium, Cryptenia,| 
Trochus, etc.; Rhynchonelia sp., Isocrinus sp.,| 
| echinoid radiole. | O 34 5 
Apparently transitional from 
| Dy. Pale cream-coloured laminated limestone. 0 3 
Distinct parting. 
| Ds. Compact. cream-coloured, laminated limestone, 
somewhat false-bedded on a minute scale. 
| Fragment of Nautilus sp., Ataphrus sp. 0 2-10 
Resting unconformably upon 
[Thornecombi-- U. Hard vellowish-brown ferruginous rock of ‘ marl- 
ensis.| | stone’ type, almost identical in lithic characters 
with the T.-thorncombiensis Bed of the Western | 
Cliffs. Rhynchonella sp. | Tetrarhynchia thorn- 
combiensis| common; Belemnites sp.; fragment 
of large Harpoceras on the top of the bed, but 


| Hammato- 
ceras ? | 


probably not belonging to it. 0 010 
Very irregular base. 
Faleiferum ?| M? Massive, pinkish and yellow moitled limestone. ' 0 89 
Very obscure parting. | 
| Mj, ? Similar, but harder limestone. Seen to o 9-10 


part 4] JURASSIC CHRONOLOGY. 44,7 


There is only one place where it is at all possible to reach the 
Junction-Bed in sitw with any degree of safety—a kind of shallow 
gully or slide, descending from the top of the cliff along the line of 
fault to the top of the extensive overgrown talus between the inner 
cliff and the foreshore. JI measured a section in the Junction-Bed 
immediately above a group of three very big fallen blocks on the 
talus. The total thickness was 4 feet 7 inches; but the only part 
that could be reached was a joint-face, covered with stalactitic 
matter which prevented me from recording any detailed measure- 
ments. Another section, measured 77 ae a few yards farther 
east, will be described later on. The accompanying section (VIII) 
was taken from one of a group of three immense fallen blocks. 
Other blocks show a very similar development of the Junction- 
Bed, but with minor variations. 

The whitish earthy limestone (C) forms a kind of capping to 
the lithographic stone (D); buta close examination shows that 
there is no transition. The base of C fills shallow hollows in D, 
and cuts across the planes of lamination. The thickness of C 
is very variable. No fossils of zonal importance were seen. 

Little more can be said concerning layer D. Mr. Buckman 
writes (27 Jitt., February 1921): ‘the date of D is ver y uncertain.’ 
Layers D to D. are closely associated, and form a group with very 
similar lithic characters, but there are small non- -sequences. ‘The 
abundance of minute oastropoda i in D, is noteworthy. 

Below layer D, the sequence is very doubtful, and difficult to 
determine. The rock resembling ‘marlstone’ (U) is a wedge- 
shaped mass, about 3 feet long, presumably a portion of a large 
slab or lenticular cake of stone derived from some lower horizon. 
Mr. Buckman has determined the Ahynchonella to be Tetra- 
rhynchia thorncombiensis, which is abundant in a bed some little 
distance down in the Yellow Sands of the Western Cliffs. 
Presumably, therefore, this redeposited mass was derived from the 
T.-thorncombiensis Bed. Other blocks show similar lumps of 
stone, and derived specimens of J. thorncombiensis and Belem- 
notes sp. are common. 

The age of layers M (?) and M, (?) is doubtful. In lthic 
characters they remind one of the more massive portions of the 
faleiferum layer of the Western Cliffs, and are very similar to 
layer M, (?) of Section IX (which yielded some big specimens of 
Harpoceras falciferum). In some blocks pieces of the T.-thorn- 
combirensis Bed and redeposited Rhynchonelle are enclosed in the 
limestone layers M (?) and M, (?). 

At Watton Cliff the clay below the Junction-Bed is absent, 
and the presence of derived masses of the 7.-thorncombiensis Bed 
indicates that erosion has removed some thickness of the upper 
portion of the Yellow Sands. 

Section IX (p. 448) was measured 727 sztw immediately west 
of the point where the Lias is finally cut off by the faulted 
Bathonian rocks, and only some 380 feet east of the position from 
which the block measured in Section VIII must have fallen. 


4.48 JURASSIC CHRONOLOGY. [ vol. lxxviil, 


Secrion IX—TxHE JunctTion-BeEp anp Contiguous DEPOSITS, MEASURED 
in situ AT THE WESTERN EnD oF WaATTON CLIFF. 


Hemere. Strata end Fauna, Feet inches. 
Dumortieria.| A+ B. Blue sandy ‘ Upper Lias’ clay. 
Marked non-sequence. 
Falciferum ?| M(?). Very hard mottled limestone, with large Harpo- 
ceras falciferum common at the top. [5230, 
Harpoceras cf. mulgravium. | 
Top much iron-stained and planed off smooth. 0 Se 
Transitional from 
M,(?). Harder, conglomeratic, mottled limestone. Small 
Harpoceras ? sp. near the top. [Micromorph, 
not Harpoceras. | 0 10-11 
Irregular parting. 
[ Thorneombi-| U (?). Soft, earthy, yellowish ‘marlstone.’ Belemnites 
ensis. | sp., Syncyclonema sp., Rhynchonella sp. [5232, 
Tetrarhynchia thornceombiensis |. OG 
Apparently transitional from 
U,(?). Rough lumpy ‘marlstone, soft and highly 
ferruginous. 0. 46 
Apparently trausitional from 
U2(?). Soft earthy ‘marlstone’ crowded with crinoid 
fragments. 0) Ze 
Apparently transitional from 
Lumpy ironshot marlstone, somewhat oolitic. 
U3(?). Obscure fragment of Harpoceras in friable 
condition. Seen to 0:.% 
(About 1 foot obscured by slipped material.) 
T. Soft yellow sands, 


The most interesting feature of this section is the absence of 
the lithographic and other limestones of layers D to D,. That all 
these beds should have disappeared in so short a distance is a good 
example of the remarkably sudden variations to be met with in the 
Junction-Bed. 

Layers M(?) and M,(?) are almost certainly the continua- 
tion of layers M and M, (?) in Section VIII. Presumably layer 
M(?), at least, is of faleiferum date—unless the specimens of 
Harpoceras falciferum are redeposited. 

Layers U (?) to U,(?) are only separable with uncertainty, as 
the deposit is very irregular, Inmipy, and confusedly mingled. The 
fossils characteristic of the ‘marlstone’ of the Western Cliffs 
appear to be absent, and much of the rock is more hke the Tetra- 
ee econ Bed and other hard bands in the Yellow 
Sands than the true ‘ marlstone.’ Mr. Buckman considers that the 
whole of layers U (?) to U,(?) is pre-spinatum material redeposited. 

{During further work in 1921 J discovered a block of the 
Junction-Bed (about 8 feet long) lying partly embedded in talus 
some few feet west of the group of three blocks measured in 
Section VIII. At the eastern end, the Ue Tias’ clay A+B 
rests upon the earthy limestone C (4 to 5 inches) and the litho- 
graphic stone D (7 to 8 inches); at the western end it rests upon 
the grey limestone D,, Cand D having been removed by denudation 
prior to the deposition of the ‘ Upper Lias’ clay. | 


part 4] JURASSIC CHRONOLOGY. 449 


Appendix II—Tne Uprer Lirias Succession. By Leonarp 
Frank Spatu, D.Sc., F.G.S., Joun Prinerz, F.G.S., 
ANDREW TEMPLEMAN, and S. 8. Buckman, F.G.S. 


(A) Introduction. (S.S8. B.) 


Some time after my paper had been sent in to the Society, some 
important excavations were made in the Upper Lias of Somerset, 
which throw much hght on the points discussed in pp. 890-895. 
Dr. Spath sent me a summary of the results that he had obtained 
from a study of a collection made by Prof. D. M. 8. Watson: this 
communication is placed first, because 1t enters into considerable 
detail as regards the genera and species of ammonites. Later, 
Mr. Pringle and Mr. Templeman forwarded a summary of the 
results of their collecting—particularly important because the beds 
were collected from almost inch by inch, with extremely happy 
results. They did not attempt much particularization of ammonites, 
because their finds were placed in my hands for detailed work. 
But, although such detailed work eannot be undertaken for some 
time, it has been possible to make a general survey of the faunal 
succession. From this information and the study of some 
Northamptonshire specimens submitted by Mr. Pringle at the 
same time, it has been possible to construct succession and corre- 
lation tables, which, although necessarily imperfect in certain 
respects, will (it is hoped) become useful bases for further work, 


(B) Upper Lias Succession near Ilminster, Somerset. 
Ci2H=S:) 


This succession is based on material collected and submitted 
by Prof. D. M. 8. Watson, to whom grateful acknowledgments are 
tendered. Beds 18 to 15 were collected from at Barrington, beds 
4 to 12 at Stocklinch. The highest bed (‘'Top-Rock’ 13) at the 
latter locality did not yield any ammonites; but Prof. Watson has 
reasons for considering beds 13-15 to follow on bed 12, as in the 
succession tabulated below. 


Strata. Horizons. 
(a) Barrington. 
lb Basement Bed of Yeovil Sands” 2)... .0:.0.....2. -s- Dumortieria ? to 
(Phlyseogrammoceras dispansum, Dumor- | dispansum. 
tieria ? sp., Alocolytoceras ? sp. juv.) 
14, ‘ Black Clay,’ 6 inches (fauna derived) ................ struckmanni? to 
(Hammatoceras cf. insigne; Grammoceras [striatulum. 


spp.; Pseudogrammoceras? cf. grunowi 
(Dumortier,! nov Hauer); Haugia sp.; 
v-script Hildoceras.) 

13. ‘Top-Rock’ (Phymatoceras ? cf. werthi) ............ variabilis ? 


1 « Btudes Paléontologiques sur les Dépéts Jurassiques du Bassin du 
Rhone: Lias Supérieur’ vol. iv (1874) pl. xiv, figs. 6 & 7, 


450 DR. L. F. SPATH & OTHERS ON [vol., lxxviii, 


Strata. Horizons. 
(b) Stocklinch. 
12. Top 8 feet. (Zones not separated in collecting.) ... bifrons, ete. 
(Hildoceras bifrons, H. walcottii ete. ; a stout 
Dactyloid (Porpoceras 2?) and vermiform 
Dactylioceras.) 
. Above Fish-Bed 7 feet 6 inches ......................... Hildoceratoides. 
(Ammonites levisoni, auctt. non Simpson; a 
stout Dactyloid, more finely costate than 
‘ Celoceras’ | er Be i below.) 
. Above Fish-Bed 7 feet . falciferwm B. 
(Many young Harpocera as; | Hildoceratoides sp. 
(of Mercaticeras aspect) ; Dactylioceras 
cf. vermis ; ‘ Celoceras’ cf. fonticulum.) 
. ‘Brown Bed’ (above Fish-Bed 6 feet 6 inches) ... faleiferwm A. 
(Main dey elopment of Ha ar poceras Saleiferum 
sensu stricto ; ; ‘ Celoceras’ cf. crassoides.) 
. Above Fish-Bed 53 to 6 feet ...... Hildaites. 
(Hildaites levisoni Simpson and Hildaites cf. 
chrysanthemum Yokoyama sp.?). 
. Above Fish-Bed 43 to 5 feet .. anguinum. 
(Dactylioceras anguinum (Reinecke) and other, 
more vermiform, Dactylioceras.) 
. Above Fish-Bed 18 inches to 45 feet Hine es, ? 
. Above Fish-Bed 18 inches ....... ? 
(Harpoceratoides, iarge form, more finely 
costate than those below.) 
4, Above Fish-Bed 1 foot . Jadissetecsgiasesp ass LLOnpocemmain@ess 
(Harpoceratoides alternatus, TEL kisslingi, 
etc.; Lactylioceras helianthoides Yoko- 
yama, ‘ Celoceras’ sp. juv.; Phylloceras 
sp.; Hlegantuliceras sp.) 
. Fish-Bed (Clay 18 inches), no yield eter 
Leptena Bed" toot); novyield een ens eee P 
. Marlstone. 


i) 
— 
— 


i 
Oo 


ws 


ee) 


Beds 7-11 with falciferum-like 
Harpoceras throughout. 
To) 


I 


Ou oD 


me bo Co 


(C). Two New Sections in the Middle and Upper Lias at 
Barrington, near [iminster, Somerset. (J.P. &A.T. 
Notes in brackets by 8S. 8. B.) 


During the year 1920 a quarry was opened in a field on the west 
side of the cross-roads between Barrington and Stocklinch, and 
650 yards south-west of Barrington Church. In the excavation, 
which was carried down to the base of the Marlstones, about 
14 feet of Upper Lias clay and limestones are exposed. An 
abstract of these beds, numbered in ascending order from 1 to 26, 
is set forth in the appended section (p. 451). Fuller details await 
a critical examination of the ammonites. 

In the same year a smaller quarry was opened on the west side 
of Shelway Lane, Barrington; it shows a similar sequence of 
Middle and Upper deposits on the eastern side of the pit. Beds 
1—26 of the Upper Lias can again be seen; but in the roadside 
above the quarry and in the bank above the adit on the east side 
of the road additional strata are exposed, carrying the section up 


1 Celoceras sensu stricto, restricted to the pettos group, in L. F. Spath, 
‘Notes on .\nmonites’ Geol. Mag. 1919, p. 28. 

? ‘Jurassic Ammonites from Echizen & Nagato’ Journ. Coll. Sci. Tokyo, 
vol. xix, No. 20 (1904) pl. ii, fig. 1. 


part 4 | JURASSIC CHRONOLOGY. 451 


to the brown sands, which are believed to be of dispansum date. 
These higher beds are numbered in continuation with the larger 
quarry. It should be noted here that the Shelway-Lane quarry 
also shows a remarkable local wash-out, probably of recent geological 
date. On the west side of the pit all of the beds above the 
Marlstone have been broken up to the depth of 14 feet, and re- 
arranged. 


Section I—Lras, BARRINGTON (EPITOME). 


(A) SHetway Lanz, BARRINGTON. 


No. Strata, Thickness. Horizon, 
Upper Lias. Feet inches. 

32. NSHP DY LANMReg AMOR ea ee oe ae oe On 6 0 dispansum. 

oe i Clays and limestones, unfossiliferous... 2 6 

Be ; oastossilnterous. See 6695. ccsase 3 8 


(B) Barrineton Quarry. 


ae t Clays and limestones, Hildoceras...... 3 8 { Pee? ae 
23. Reddish-brown clay, pinkish-grey 
limestone ....... ert eee 0 7 [10, Spath. | 
22. Whitish clayey limestone .......0.0..-.- it 0 
21. Olive-grey clay, with Crania=Crania- 
Clays of Moore Rees (0) 5 
20. Pink-tinged, grey, oe a “limestone. 
Hildoceras ....... 0 5 [pre-bifrons. | 
[Not Hildoceras ; a a new form. il 
19. Clay and limestones. Large Harpo- [9, Spath. ] 
Gerais, Coloccras cscs bis. weeks: 0) 8 Falciferum 
18. et Aes ee RS ee 0) 3 (8, Spath.] 
LY. Woe Dactyltoceras te. 0. tec sai noses O 3 (7, Spath. | 
MM it al hee, led [6, Spath.] 
ine Wor A arpocsras 228 ie.s tel fae wc 0 5 
: as Do. .. f° More) to)": 5, Spath.| 
i Do., Bincvodsila 8 bouchardi, i, Harpo. [Moore’s zone of 
ceratoides......... é 0 vf Rh. bouchardi. | 
6. Do. , Harpoceratoides ... cae ae 0 3 r 
5. Laminated clay, Harpocer atoides ... 0 5) ; [eet] 
4, Mish-Bed. «....5...: psc ee re 0 3 exaratum. 
3 Leptena Clays of Moore... 0... 2 0 
2 Top bed of Moore’s Middle Nias: 
bed F of his Ilminster section. 
Dactylioceras spp.. 0 4, tenuicostatum. 
i Sandy marl; large belemnites, finely 
ribbed Dactyli ecerates ..... 0 7 


[Impression of Pa Itopleurc oceras 
cf. hawskerense. | 


Middle Lias. 


2. Marlstone Rock - Bed, 7 feet thick. acutum. 
A layer of sandy mail 6 to $ inches [regulare. | 
from the top [contains degenerate 
Paltopleurocerates, cf. Paltopleur 0- spinatum. 


wT 
© 


ceras regulare Simpson SPelbale 
1B Sands. 


452 MR. S. S. BUCKMAN ON (vol. lxxviul, 


(D) Upper and part of the Middle Lias Succession, and 
Correlation. (S.S. B.) 


In his examination of Prof. Watson’s collection Dr. Spath was 
able to make certain notable additions to the recognizable horizons 
of the Upper Lias: he was able to place as chronological indices 
two new genera which I had recently named and illustrated in 
‘'Type-Ammonites —Hildaites (pl. cexvii) and Hildoceratoides 
(pl. eexvii). Further, he distinguished a horizon by the name 
anguinum, and placed Harpocer atoides as marking a distinct date: 
species of this genus had been obtained by Mr. J. 'F. Jackson from 
the Junction-Bed of the Dorset Coast (see p. 443) ; but on that 
evidence alone it had not been considered advisable to distinguish 
Harpocerotoides as a separate time-term. Now all this evidence 
falls nicely into line. 

Adopting Dr. Spath’s terms, and using information otherwise 
obtained, it is possible to present the following sketch of the chron- 
ological sequence in the Upper Lias (and top of the Middle Lias) 
—Table VI, p. 453. At the same time, a series of stratigraphical 
terms are appended which may be useful to the memory, as well as 
indicating where certain beds are well exposed. It is not intended 
to say that the beds are only exposed at the places which give their 
names: it is known, for instance, that Whitby possesses far more 
than the strata of the three hemere which stand opposite ‘ Whitby 
Beds.’ But, on the other hand, deposits of certain hemerze seem 
only to have been preserved at the localities which give their 
names: thus the strata of psewdovatum date are only known in 
Yorkshire. 

A similar phenomenon attends exaratum: this species has not 
been recognized outside of Yorkshire. Several of the associates of 
exaratum in the Jet-Rock of Yorkshire have now been found in 
other localities—for instance, in the Leptena Bed of Somerset, 
where the inch-to-inch collecting gives them a certain sequence, 
which, however, must be taken with some reserve : first, because the 
present examination is necessarily rather cursory, and, secondly, 
because the specimens are somewhat crushed and ill-pr eserved. But 
the important fact is that, while the Leptena Bed yields these 
exaratum associates, another of the Jet-Rock genera (Harpo- 
ceratoides) is found in a bed of different lithic character at a 
distinctly higher level—separated from the Leptena Bed by the 
Fish-Bed. Therefore, it may be argued that the Yorkshire Jet- 
Rock is not a deposit of one date, but is polyhemeral. 

The exact chronological position of exaratum is, then, uncertain. 
There is little doubt that it has some affinity with the forms now 
spoken of as ‘ Grantham ammonites,’ and it is therefore concluded 
that its date is either just before or just after those forms. If 
placed before the Grantham ammonites, a non-sequence in the 
Leptena Bed is produced, and a considerable non-sequence in the 


part 4:] JURASSIC CHRONOLOGY. 453 


TapLte VI—SKETOH OF THE CHRONOLOGICAL SEQUENCE IN THE UPPER LIAS, 


Ages.. Bed Nos. Hemere. Strata. 
i 9. moorei. 
8. Catulloceras. i Yeovil Sands. 
Zz 7. Dumortieria. 
= 6. Hammatoceras. Bruton Sands. 
a2 ( 5. dispansum. Frocester Beds. 
6 | 4. struckmanni. Midford Sands. 
p| U L, 28. 4 3. [Paani ; Stinchcombe Beds. 
| 2. eseri. 
G L 1. striatulum. Sodbury Sands. 
(aU 2. 26. variabilis. 
25. lilli. i Cotteswold Sands. 
| U.L. 27 6. 24. semipolitum.} 
dy Olan 27, ais 23. subplanatum. 
22. braunianum. i Northampton Beds. 
ji 21. fibulatum. 
U.L. 26. 20. bifrons. 
HOt 25sr 19. subcarinata. i Whitby Beds. 
18. pseudovatum. 
ULL. 24. 17. Gen. and sp. nov. ) 
x (small Harpocerate.) | 
= | Wels 23. 16. Hildoceratoides. | 
@ } U.L. 19. 15. falciferum. the ; 
Ey Wl, 18. 14. ye b EEN VEO Ce 
ss | (Uf 105 LS 13. Cf. Pseudolioceras. | 
Wises 11 12. anguinum. 
welt. 11. Harpoceratoides. J 
10. murleyi. Dumbleton Insect-Bed. 
9. exaratum. Boulby Bed. 
8. biform Harpocerates. Grantham Bed. 
U.L. 3. } i ie HOSE ; Yorkshire Jet-Rock. 
. egantuliceras. 
5. tenuicostatum. Yorkshire Grey Shales. 
4. Tiltoniceras. . ; 
us 2 3. athleticum. } daltons Beds. 
(CLUS bg 2. fine-ribbed Dactyloids. Stocklinch Bed. 
‘.. 1. Harpoceratoid. Chideock Bed. 
= si Ae { 2. hawskerense. Hawsker Beds. 
eis 1. spinatum. South Petherton Beds. 


Jet-Rock; if placed afterwards, the former is avoided at the 
expense of a double non-sequence in the Jet-Rock. Further 
evidence is required. 

As the evidence for the chronology of the earlier hemera of the 
Upper Lias is somewhat imperfect, the correlation-table (VII, 
facing p. 454) has been drawn up, chiefly in order to compare 
Somerset with Yorkshire. The lacune in the intervening areas 
are not all to be read as non-sequences: they arise from lack of 
evidence, collection-failure, exposure-failure, nomenclature-failure, 
and so on. It is doubtful whether anything like the full sequence 
of the early hemere of the Whitbian has yet been obtained. 


1 Hildoceras semipolitum, S. Buckman, is the particular species of the lower 
part of the Cotteswold Sands; evidence for Lillia lilli and its associates in that 
deposit is lacking, but there is plenty of room for them. Very little collecting has 
been done in this deposit, which sometimes is 280 feet thick. 


454 MR. 8. 8S. BUCKMAN ON [vol. Ixxvin, 


A last word may be added concerning the Japanese species 
Dactylioceras helianthoides Yokoyama. Dr. Spath quotes this. 
from above the Fish-Bed: I quote something like it from. well 
below. The Dactyhocerates area very difficult group, occurring 
in the Upper Lias in seemingly endless variety. When it was 
thought that the Upper Lias was laid down at about three dif- 
ferent dates, such endless variety of apparently contemporaneous 
forms was difficult to understand. When itis seen that the dating 
of the Upper Lias (the Dactyliocerate part—the Whitbian) has to be 
multiplied some eight times and, therefore, the number of contem- 
poraneous species has to be divided eight times, the diversity of 
the Dactyliocerates becomes more comprehensible: they represent 
waves of more or less closely-allied lineages, which develop and 
tail-off (degenerate) in somewhat similar fashion, producing 
homceomorphous forms. 

It was necessary to be satisfied, in regard to the finely-ribbed 
Dactyliocerates below the Fish-Bed, that they were not D. tenuz- 
costatum of the Yorkshire Grey Shales: they are not, but they 
have a likeness to D. helianthoides. When really systematic 
vesearch-work on the Dactylioceratide can be undertaken—at 
present such work is only in its infaney,—it will doubtless be found 
that the points of difference between similar-looking forms are 
more important than is now suspected. When these differences 
can be recorded and brought to bear on species of known date 
in an extended time-scale, one may expect the Dactylioceratide to 
become very useful chronological indicators. 


IX. Summary. 


(1) The body of the paper deals with certain Jurassic strata 
near Hy pesmouth on the coast of Dorset; but, as it forms 
part of a series of preliminary studies in connexion with 
Jurassic chronology, certain details connected with other 
localities are noticed. 

(2) A general section of the main mass of Watton Cliff east of 
Hypesmouth is given. 

(3) A detailed section is recorded of a remarkable white litho- 
graphic bed in Watton Cliff, one in the same position as. 
the Upper and Mid-Uias Junction-Bed of the Thorncombe- 
Beacon area, but differing much in faunal and stratal 
details. 

(4) This bed shows faunal inversion, presumably due to re- 
deposition of material from older deposits. 

(5) The dating of this Watton Bed is discussed, after pre- 
liminary investigations into the sequence of horizons in 
the Upper Lias ‘of various areas, in the Junction-Bed and 
pre-J unction-Bed strata of Thorncombe Beacon. 

(6) A theory of stratal repetition and coalescence is discussed in 
regard to the Watton Bed. Its main date is taken to be 
Yeovilian, Hammatoceras hemera. 


[Quart. Journ. Geol. Soc. Vol. LX XVIII, Part 4. 


HEMER#) AND THE LAST PART OF THE DOMERIAN. | 


Worthamptonshire 
(Byfield, etc.). 


(Cf. Harpoceratoides. 


Fish-Bed 
pe SS 


(Grantham Amm.  . Grantham Ainm. 


» 


( Sandy clay. 


es 
ic) 

an : = 

= | Tiltoniceras. .° . ( Tiltoniceras. 
Pe) S24 

SSI DS as 

2 : lee : 

a | Dactyloids, cf. ps Dactyloids. 
i | athleticum. 


Lincolushire 
(Grantham, etc.). 


Yorkshire 


(Whitby neighbourhood). 


( Harpoceratoides. 


exaratum. 


Jet-Rock. 
KS 


Eleganticeras. 


| Elegantuliceras. 


tenuicostatum. 


Grey 
Shales. 


( Tiltoniceras. 


athleticum. 
| 


Middle Lias, auctt. 
a, 


P. hawskerense. 
IP. 


| P. regulare. 


11. 


10. 
9. 


Y 


To face p. 454.) (Quart. Journ. Geol. So . Vo : 


TABLE VII—CORRELATION OF THE Upper Lias (WHITBIAN—EARLIER Hemer 


be 


& Somerset Gloucestershire Gloucestershire | 
Dorset Coast. (Barrington). (Stinchcombe). (Dumbleton). | 


onshire _Lincoh 
te.). (Grantham, e 


11. Harpoceratoides. Harpoceratoides. 
10. Fish-Bed. Insect-Bed ; murleyi. ; 
9. 
8. rc (Grantham Ammonites 
roa) | (biform Harpocerates). 
3 3 Leptena-Bed equi- 
NA valent. 
Tho s | Hleganticeras. 
Sy 
6. BPlegantuliceras- 4 ( Blegantuliceras. : : 
like forms. 
5. 
4. Tiltoniceras 
(Thin seam resting on 
Middle Lias Rock-Bed). 
3. athleticum-like ( athleticum-like forms. 
forms. 
2. = | Fine-ribbed Dactyloids, 
S not D. tenuicostatum, 
af | cf. D. helianthoides. 
1. Harpoceratoid a : 
Ammonite. a5 bs 
aloe Fe 
2. Paltopleuroceras J | Paltopleuroceras- “4 P. ct. hawskerense. 
degenerates. = hawskerense and 3 
i | regulare forms. Fa 
| 
1. Paltopleuroceras- P.-spinatum forms. : P.-spinatum torms. 


spinatum forms. 


part 4] JURASSIC CHRONOLOGY. 4.55 


(7) The white lithographic bed of Burton Bradstock is cited as 
evidence of stratal repetition, and a theory as to the 
deposition and almost complete destruction of this Burton 
bed is put forward. 

(8) The Watton and Burton lthographic beds are cited as 
evidence of Alpenkalk conditions prevailing in Western 
Europe during two well-separated Jurassic dates, both of 
them far earlier than the times of Alpenkalk deposits in 
Central and Hastern Europe. 

(9) Certain remarks are made upon sections at Milborne Wick 
and Haselbury (Somerset) with regard to the dating of 
their deposits, and a table of the succession and distribu- 
tion of Hammatoceratids is given. 

(10) A paleontological note describes a new species of Rhyncho- 
nellid—Tetrarhynchia thorncombiensis (Rhynchonella 
northamptonensis auctt., passim)—a species marking a 
particular deposit at Thorncombe Beacon. 

~(11) An Appendix by Mr. J. F. Jackson gives the result of his 
studies of various sections of the Junction-Bed (including 
the Watton Bed) on the Dorset coast, 

(12) A second Appendix gives studies by Dr. L. F. Spath and 
by Mr. J. Pringle & Mr. A. Templeman of the stratal 
and faunal (ammonite) succession revealed by certain new 
exposures in the Upper and Middle Lias near Barrington 
(Somerset )—the collecting of specimens having been done 
almost inch by inch. 

(18) The evidence thus obtained, added to that gleaned elsewhere, 
some of it set out in the body of the paper, enables the 
Author (S. 8. Buckman) to put forward a tabular sketch 
of Upper Lias chronology more detailed than has yet been 
attempted—dividing the ages (Yeovilian and Whitbian ) 
of the Upper Lias into thirty-five hemerze—more divisions 
than were originally made by Oppel for the whole of the 
Jurassic. 


DIscusstIon. 


Mr. J. Prryvee said that, as joint author of one of the 
Appendices, he would refer to the salient features of the work 
done by Mr. A. Templeman and himself on the Upper Lias exposed ~ 
in certain quarries at Barrington. These beds had been described 
by Charles Moore, and the reopening of the sections after a long 
period of disuse had provided an opportunity of examining the 
Upper Lias, with a view to the correlation of the deposits with 
those of the Banbury district. The section was carefully col- 
lected over bed by bed, and more than a thousand specimens 
of ammonites were secured. The value of the results obtained 
had been much enhanced by Mr. Buckman’s co-operation. 

Dr. A. Moriry Davies said that the deposits in a shallow sea 
transgressing over an area where folding and faulting movements 


4:56 JURASSIC CHRONOLOGY. [ vol. xxviii, 


were still.in progress, might very well show such anomalies as the 
Author described. He found it difficult to believe, however, that 
the beautifully preserved ammonite exhibited, with similar matrix 
within its body-chamber and around it, could be a derived fossil. 
He enquired whether the 62/rons ammonites which appeared in 
inverted sequence in this new junction-bed showed the same pink 
colour as that which characterized their matrix in the well-known 
Junction-Bed. 

Dr. W. D. Lane asked whether a simpler explanation than that 
suggested by the Author might not be applicable to the inverted 
sequence ot ammonites in the Junction-Bed at Hype, namely, 
a long period of very slow deposition coinciding with an oscillation. 
of surface at about sea-level, allowing wave-action to mingle the 
fossils of successive faunas, and even to remove a certain amount, 
of sediment; the whole deposit being subsequently consolidated by 
a segregation of calcium carbonate. If the last-mentioned process. 
took place, it would be unnecessary to consider, as the Author 
apparently did, that the Junction-Bed was a deep-water deposit. 
That such segregation would not necessarily do away with the 
bedding-planes could be seen in the concretions of the birchi 
nodular deposit, occurring in the Lower Lias of the same district. 
Bedding was very apparent in these nodules. 

Prof. P. G. H. Boswetn said that, among the many interesting 
questions raised by the Author, that of the evidence of shallowing 
conditions and instability in the area during the period covered. 
by the hemere from /zllz to opaliniformis deserved emphasis. 
The sandy facies which stretched from the Cotteswolds to the 
Dorset coast, and included the uppermost zones of the Lias and 
lowermost zones of the Inferior Oolite, was characterized through- 
out by the constancy of its peculiar lithological and petrographical 
characters. As the Author had proved many years ago, this sandy 
and silty phase transgressed as a wave of shallowing conditions to. 
successively higher horizons as the observer travelled southwards, 
thus providing an admirable example of the transgression of time- 
planes by hthological planes. 

Mr. G. W. Lamptueu thought it improbable, on the evidence 
adduced, that the comparatively large and well-preserved ammonites. 
found in the narrow band of fine-grained laminated material at. 
Watton Cliff could have been derived at recurrent intervals from 
older strata and redeposited in inverted order. Judging from the 
specimens exhibited, he suggested that the bed might be a ‘con-. 
densed’ deposit, very slowly accumulated and covering a long 
period. In such beds the rare accident of preservation at intervals. 
was likely to bring about the juxtaposition of forms not truly — 
contemporaneous. The further accident of collecting-chances in 
beds of this kind rendered the basis for the hypothesis still narrower. 

The AurHoR, in reply, said that the Watton-Cliff bed presented a, 
mass of puzzles, and the point raised by Dr. Davies—the difficulty 
of derivation in the case of well-preserved fossils—was a problem 
already noted, not only for this instance. The solution in some. 


part 4] JURASSIC CHRONOLOGY, 457 


cases was that specimens had been redeposited inside lumps of 
original matrix. Big lumps of derived matrix were in the Watton 
bed, but there arose the difficulty of reconciling such lumps with a 
fine-grained laminated deposit—the former indicated violent action, 
the latter very tranquil conditions and, possibly, deep water. The 
tranquil conditions seemed to be inconsistent with the wave-action 
suggested by Dr. Lang. 

The Author agreed with Mr. Lamplugh that the Watton Bed 
was a highly condensed deposit: it had taken the whole of the 
time of the Upper Lias—some thirty hemerze—to deposit 5 feet ; 
but the question was whether the deposit as seen now represented 
anything like the original mass laid down. The Author supposed 
not. How much had been removed by penecontemporaneous erosion 
without leaving a trace behind? 

With reference to Mr. Pringle’s remarks, the Author wished to. 
acknowledge how greatly he was indebted to his collaborators for 
their Appendices, representing a mass of new observations supple- 
menting his paper. The most interesting point, perhaps, was that 
the system of many chronological divisions, which the Author 
might claim to have originated, had been extended by his col- 
laborators: that testimony to his moderation was welcome ; 
because his original number of subdivisions had often been con- 
demned as excessive. He had always been compelled to go on. 
asking for more, and now other investigators went farther still. 
This constant lengthening of the Earth’s chronology was particu- 
larly interesting. 

In noting that paleontological planes did not coincide with 
lithological planes, Prof. Boswell had, as he said, drawn atten-. 
tion to the Author’s work of many years ago. It was a vindication 
of the subdivisional method that it had given these results so. 
clearly, and had shown the uselessness of lithology as a guide. 
to date. 


GENERAL INDEX 


TO 


THE QUARTERLY. JOURN A 


AND 


PROCEEDINGS OF THE GEOLOGICAL SOCIETY. 


Aberdeen Creek (Sierra Leone), 309, 
321. 

Aberfoyle Slates, 98 et seqq. 

Albitization of diabases, 270. 

Alt-Ddu Mudstones, 139-40; faunal 
list, 171. 

Aluminous xenoliths in Mull rocks, 
240-54 & pls. vii—-x. ' 

Analyses of picrite, 265 ; of bostonite, 
266; of mugearite, 267 ; of Sierra- 
Leone norite, 307. 

Anatase in Plioc. deposits of Corn- 
wall, 359, 363, 365. 

Andalusite in Plioc. deposits of Corn- 
wall, 359, 362, 363-64, 366. 

_ANDREWS, C. W., Description of a 
New Plesiosaur from the Weald 
Clay of Berwick (Sussex), 285-98 
fig. & pls. xiv—xv. 

Annual General Meeting, vili et seqq. 

Anorthite in xenolithic intrusions of 
Mull, 244 et seqq. & pls. vili-x. 

Apatite in Sierra-Leone noritic com- 
plex, 329. 

Aplite, noritic, of Sierra Leone, 316— 
17, 322-23, 340-41 & pl. xix. 

Appin & Ballachulish Folds, 108-109 
fig. 

Appin Quartzites, &c., 104 et seqq. 

Araucarites cutchensis, 274 & pl. xii. 

Ardmeanach, see Mull. 

Ardmucknish (Argyll), geology of 
district betw. Ben Lui and, 108-25 
w. maps & sect. 

Ardrishaig Group, 96 et seqq. 

Asaphus-powisi fauna, 171. 

Ashgillian of the Bala district, 137 
et seqg.; A.fauna ibid., 164; A. of 


the Llangollen district, 180 et seqq. ; 
A. south of the Llangollen Syneli- 
norium, 186-88; relat. of do. to 
Caradocian, 188-89; A. north of the 
Llangollen Synclinorium, 190-91. 

Assets, statement of, xxxvil. 

Auditors elected, iv 

Augite in altered norites, 338-39, 
341 & pl. xix. 

Avonian of Broadfield Down, 227-28. 


Baruey, E. B., on the Structure of 
the South-West Highlands of Seot- 
land, 82-127 figs. & pl. i (geol. 
map), 130-31. 

Bala country (Merioneth), its struc- 
ture & vrock-succession, 132-75 
figs. & pl. i (geol. map); sect. to 
Moel-fryn, 136; Bala-Lake Dis- 
placement, 160; Bala formation in 
Llangollen district, 179 et seqq. 

Balance-sheet for 1921, xxxii-xxxiii. 

Ballachulish & Appin Folds, 108-109 
fig. 

Ballappel ‘ Foundation’ (tectonic), 
104-106, 111-12, 117 et seqq. 

Banana Is. (Sierra Leone), 303, 310. 

BARLOW-JAMESON Fund, list of re- 
clplents, xxix. 

Barrington (Somerset), sects. deser., 
449, 451. 

Basement-beds, see Gault. 

Bauxite in Sierra Leone, 322. 

Beerbachite of Sierra Leone, 313-15, 
339-40 & pls. xvli-xix. 

Beinn Bheula, see Ben Ledi. 

Ben-Lawers Schists, 98 et seqq. 

Ben-Ledi Grits, 98 et seqq. 


part 4] 


Ben Lui (Argyll), geology of district 
betw. Ardmucknish and, 108-25 
w. maps & sect. 

Berwick (Sussex), Leptocleidus super- 
stes from, 285-98 fig. & pls. xiv—xv. 

Berwyn Nodal Anticline, 217-18. 

Bibliography of S. W. Highlands, 
125-27 ; of Bala succession, 133-— 
35; of xenolithic minor intrusions 
in Mull, 229-31 ; of Jurassic plants 
fr. India, &c., 276-77; of Carboni- 
ferous plants fr. S. America, 283; 
of geology of Llangollen district, 
177; of Jurassic of Dorset coast, 
&c., 433-36. 

BiasBy medallists, list of, xxviii. 

Billington Crossing, see Pratt’s Pit. 

Biotite in Sierra-Leone norite, 329. 

Birkhillian of Llangollen district, 192 
et seqq. 

Blaen-y-cwm Beds, 180, 183-86. 

BLAKE, W., portrait presented, ]xxii. 

Bouton, H., award fr. Murchison 
Fund to, xliii. 

Bostonite of Knowles Hill, 264, 266— 
67 w. chem. anals. & pl. xi; use of 
term, 270. 

BoswEtu, P. G. H., 374. 

Bothrodendron (2)*sp. (Peru), 281 fig. 
& pl. xiii. 

Brachyphyllum mamillare, 274-75 & 
pl. xii. 

Broadfield Down (Somerset), Carb. 
Limest. of, 227-28. 

BROMEHEAD, C. BH. N., on the In- 
fluence of Geology on the History 
of London, v—vii. 

Brookite in Plioc. deposits of Corn- 
wall, 362, 364. 

Bryn’ Beds, 181, 182-83. 

Bryn-cut (Merioneth), sect. up, 142; 
Caradocian & Ashgillian of, 148-50 
w. lists of foss. 

Bryn-melyn, see Moel-fryn. 

Bryn-pig (Merioneth), Caradocian, 
&e. of, w. lists of foss., 152-538; 
vert. sect. at, 154.; sect. to Crei- 
giaujBychain, 158. 

Bryneglwys Synelinorium, 211; 
Bryneglwys Fault, 218-21 w. map. 

Buchites of Mull, 240 et seqq. & pls. 
vil—-x. 

BuckKMAN, S.S., on Jurassic Chrono- 
logy : JI—Preliminary Studies. 
Certain Jurassic Strata near 
Eypesmouth (Dorset); the June- 
tion-Bed of Watton Cliff and Asso- 
ciated Rocks, 378-455 figs. 

Burton Bradstock (Dorset), White 
Bed of, 420-81 figs. 


Ot Gos. No, 312, 


GENERAL INDEX. 


Buttresses, anticlinal, 225, 226. 


Cae-mawr (Denbigh), sect. to Pen- 
Creigiau Barcut, 185. 

Caer Drewyn (Denbigh), tectonics of, 
215; map of, 216. 

Caerhafotty Limestone, 137. 

Caledonoid (tectonic) structures in 
Llangollen district, 218-21 w. map. 

Calymene-planimarginata fauna, 170, 
IL7Ale 

Canons Town, see St. Erth. 

Caradocian of Bala district, 187 et 


seqq.; C. fauna <ibid., 162-64; 
Caradocian of Llangollen district, 
179-86 figs. 


Carboniferous Limestone of Broad- 
field Down, 227-28. 

Carboniferous plants fr. Peru, 278-— 
84 fig. & pl. xi. 

Carrick-Castle Fold, 101 et seqq. 

Carsaig Sandstone (Jurassic) 282 et 
seqq. 

Cassiterite in Plioc. deposits of Corn- 
wall, 359. 

Cefn-ddwy-graig Displacement, 163. 

Ceylon, Jurassic plants from, 271-77 
& pl. xii. 

Chamberlain-Barn Pit (Shenley Hill), 
sect. deser. & fig., 30. 

CHANDLER, (Miss) M. HE. J., 168. 

Charlotte Falls (Sierra Leone), 303, 
309. 

Chasmops fauna, 170. 

Chrome-iron-ore in Sierra Leone, 322. 

Cladophlebis denticulata, 272 & pl. 
xii, 


reversa, 271-72 & pl. xii. 

Claridge’s Pit (Shenley Hill), sect. 
descr. & fig., 26. 

Climacograptus-peltifer Zone (in Bala 
district), 137 et seqq. 

CiLIncH, G., obituary of, li. 

CuiouaGH, C. T., 85 et seqq. 

Clwydian Anticline, 210. 

Contact-metamorphism in _ Sierra- 
Leone noritic complex, 336-42 & 
ple xix 

Cordierite in xenolithic intrusions of 
Mull, 245-47 & pls. vii, x. 

Cornish Miners’ Relief Fund, Ixx. 

CoRNWALL, I. E., [on Desmostylus 
fr. Lr. Miocene of Vancouver I.], 
lxix—lxx, 

Cornwall, Pliocene of, 348-77 w. map 
& figs. 

Corundum in xenolithic intrusions of 
Mull, 242-43 & pl. viii. 

Corvedale (Shropshire), Downtonian 
fossils from, 11. 


21 


460 


Corwen Grit, see Glyn Grit; Corwen- 
Moel-Fferna area, Silurian, &e. of, 
197; Corwen Buttress, 215-17. 

Council, annual report of, viii-xx; 
(& Officers) elected, xxi. 

Cox, A. H., 342. 

Creigiau Bychain (Merioneth), Ash- 
gillian, &c. at, 153-55 w. lists of 
foss. ; sect. to Bryn-pig, 158. 

Crinan Grits & Quartzites, 96 et seqq. 

Cuil-Bay Slates, 105 et seqq. 

Cwm-Clwyd Ash, 179-82. 

Cwm-yr-Aethnen (Merioneth), Ash- 
gillian, &e. of, 155-57. 

Cyrn-y-Brain Beds, 180, 190-91 w. 
list of foss.; C.-y-B. Nodal Anti- 
cline, 211-12. 


Daz, A. B., 168. 

DANIEL-PiDGEON Fund, awards to 
F.S. Wallis & R. W. Segnit, ix ; 
award to H. P. Lewis, lxxi ; list of 
awards, XXix. 

Davison, C., Lyell Medal awarded 
to, xli—xii. 

DEELEY, R. M., elected Auditor. iv. 

Denbighshire Series in Llangollen 
district, 200-201, 202. 

Derfel Limestone (Llandeilian), 187- 
38; faunal list, 169. 

Desmiophyllum sp., 275 & pl. xu. 

Desmostylus (Lr. Miocene), lxix—lxx. 

Dicellograptus-anceps Zone in Bala 
district, 137 et seqq. 

Dicranograptus Shales, 137-38. 

Dinas-Bran Group, 208. 

Dixery, F., on the Norite of Sierra 
Leone, 299-346 figs. & pls. xvi—xix. 

Doghus Cliff (Dorset), sects. descr. 
441-43 ; see also Eypesmouth. 

Dolerite of Knowles Hill, 265-64, 
265-66, 267-69 w. chem. anals. & 
pl. xi; of Sierra Leone, 317-18, 
323-24, 341-42. 

Dolhir Beds, 180, 186-88. 

Domerian (Middle Lias) of Thorn- 
combe Beacon, 395-400. 

Donors to Library, lists of, xi—xvi. 

Double Arches Pit (Shenley Hill), 
sect. deser., 24. 

Dovuetas, J. A., elected Secretary, 
xxi; on the Carboniferous of Peru, 
283-84. 

Down Cliff (Dorset), sects. descr., 
439-41; see also Eypesmouth. 

Downtonian fossils exhibited, i. 

Drew, H., 168. 

Ducts, Earl of, decease announced, 
i; obituary of, xlix—l. 


GENERAL INDEX. 


[ vol. Ixxviil, 


DucKkwortH, H., obituary of, li. 
Dunoon Slates, 98 et seqq. 


Earuer, K. W., on the Lower Car- 
boniferous Rocks of West Cumber- 
land [title only], ii. 

Earthquakes, cause and character of, 
ly—lxviii. 

EKasdale Slates, 98 et seqq. 

Hilde Flags, 104 et seqq. 

Elatocladus plana, 275 & pl. xii. 

Election of Auditors, iv; of Council 
iv—v, lxix, ]xx, lxxii-lxxiii, Ixxiy— 
Ixxyv. 

Evuegs, (Miss) G.L.,176; on the Bala 
Country: its Structure & Rock- 
Succession, 132-73 figs. & pl. ii 
(geol. map), 175. 

ENnNos, F. R., 235. 

Enstatite in Sierra-Leone noritic 
complex, 326. 

Epidote in Plioc. deposits of Corn- 
wall, 361, 365. 

Erins Quartzite, 96 et seqq. 

Estimates for 1922, xxx—xxxi. 

Hurycleidus arcuatus, gen, noy., 294— 
95 fiz., 296. 

megacephalus, 295 fig., 296. 

Evans, J. W., Murchison Medal 
awarded to, xxxix—xl; obituary of 
E. H. O. K. Weinschenk, liii. 

Eypesmouth (Dorset). Jurassic near. 
378-457 fies. 


FEARNSIDES, W.G. ix. 

FEILDEN, H. W.. obituary of, li. 

Fellows elected, 11, iii, iv- v, lxix, lxx, 
lxxii—lxxiii, lxxiv-Ixxy; names read 
out, i, lxxv ; number of, xvii—xix. 

Felsite of Rudh’ a’ Chromain, 235 
& pl. vi. 

Felspars in xenolithic intrusions of 
Mull, 244-45 & pls. vili-x; in 
Sierra-Leone noritic complex, 324— 
25, 338, 340 & pls. xvili—xix. 

Ffridd-Defaid Displacement, 161. 

Financial report, XxxxX—XxXxXvll. 

Fuett, J. S., receives awards fr. 
Lyell Fund for A. Macconochie & 
D. Tait; xl. 

Foel-y-Ddinas (Merioneth), Ashgil- 
lian, &c. of, 155-57 w. sect. & lists 
of foss. ; Foel-y-Ddinas Mudstones, 
144, 

Foreign Members, list of, xxii; For. 
Correspondents, list of, xxiii. 

Frondderw Ash, 139. 


part 4] 


GaLLoway, T. L., obituary of, 1. 

GARDNER, EH. W., 168. 

GARFORTH, Sir WILLIAM H., obituary 
of, |-li. 

Garnet in Plioc. deposits of Cornwall, 
B61. 

Garside’s ‘old pit’ (Shenley Hill), 
sects. descr. & fig., 10-13; ‘new 
pit,’ sect. descr. & fig., 14. 

Gastropod (?), giant, fr. Wadhurst 
Clay, Ixxv. 

Gault & Lr. Greensand Junction nr. 
Leighton Buzzard, 1-81 w. maps 
& sects. 

GEIKIE, Sir ARCHIBALD, re-elected 
Foreign Secretary, xxi. 

Gelli-grin (Merioneth), Caradocian, 
&e. of, 148-50 w. lists of foss.; 
vert. sect. at, 154; Gelli-grin Cal- 
careous Ash Series, 141-42 w. 
sect.; faunal list, 170. 

GILLIGAN, A. (& others), on the Car- 
boniferous Rocks of the Deer-Lake 
District of Newfoundland [title 
only |, lxix. 

Glen-Orchy Anticline, 95; Glen 
Orchy (& Ben Lui), geol. map of, 
122. 

Glen-Sluan Schists & Grits, 98 et 
seqq. : 
Glyn-Ceiriog area, Silurian, &c. of, 

197-99. 

Glyn-Corwen Grit, 180, 188. 

Glyn-Dyfrdwy Group in Llangollen 
district, 204-206, 

Glyn-Gower Beds, 138; faunal list, 
ia 

Godrich (Sierra Leone), 313 & pl. xvi. 

GoDwIn, H., lxxiii. 

GORDON, J. M., obituary of, lii—liii. 

GREEN, J. F. N., elected Auditor, 


iv. 

Greenland (W.), geology of, lxxili— 
Ixxiv. 

Greensand (Lr.) & Gault junction nr. 
Leighton Buzzard, 1-81 w. maps 
& sects. 

Grovebury Pits (Shenley Hill), sects. 
descr. & fig., 31-35. 


Hammatoceratids, chronol. succes- 
sion & geogr. occurrence of, 434— 
30. 

HARKER, A., Wollaston 
awarded to, Xxxvili—xxxix, 

Harris’s Pit (Shenley Hill), profile 
sect., 7. 

HARRISON, B., obituary of, liii. 

Haselbury (Somerset), Jurassic of, 
433-34, 


Medal 


GENERAL INDEX. 


461 


HAwksHAw, J. C., obituary of, 1. 

Heath-House Pit (Shenley Hill), 
sects. deser., 27-29. 

HERRIES, R.S., re-elected Treasurer, 
POO 18 

Heterorthis-alternata fauna, 171. 

Highlands (S.W.) of Scotland, struc- 
ture of, 82-131 figs. & pl. i (map). 

Hirnant Beds, 144, 156. 

Houmss, Sir CHARLES J., on Leo- 
nardo da Vinci as a Geologist, 
Ixxi-lxxii. 

Horrrum, RK. H., 282; ( A.ee: 
Seward), on Jurassic Plants from 
Ceylon, 271-77 & pl. xii. 

Hornblende in Sierra-Leone aplte, 
329, 

HutTcHInson, H. N., presents model 
of Peloneustes philarcus, lxxii. 

Hypersthene in altered norite, 339, 
O41, 


Ilmenite in Sierra-Leone norite, 321, 
329; in Plioc. deposits of Corn- 
wall, 358, 363. 

Ilminster (Somerset), Upper & Middle 
Lias near, 449-51. 

Iltay Nappe, 98-104 fig., 112 et seqq. 

Inninmorite of Mull, 235 et seqq. & 
pl. vi. 

Intergrowths, mineral, in Sierra- 
Leone norite, 329-35 fig. & pls. 
XV1l1—Xix. 

Iron-ores in Sierra-Leone norite, 321— 
22, 328-29. 

Islay Anticline, 95; Islay Limestone 
& Quartzite, 99 et seqq. 


JACK, R. LoGan, obituary of, xlix. 

JACKSON, J. F., see Buckman, 8. S. 

John Obey Point (Sierra Leone), 313, 
321, 

JONES OF A tite 

Junction-Bed at Watton Cliff, 378 
et seqq.; sects. of do. descr., 387— 
88, 4386-48; at  Thorncombe 
Beacon, 393-95; and at Bothen- 
hampton, &c., 400 et seqq. figs. 

Jurassic plants fr. Ceylon, 261-70 & 
pl. xii; Jurassic strata nr. Eypes- 
mouth, 378-457 figs. 


Kent (Sierra Leone), 309, 310. 

Kine, W. B. R., 168, 176. 

Kine, W. W., exhibits Downtonian 
fossils, il. 

Knowles Hill (Devon), composite sill 
at, 261-70 figs. & pl. x1. 

Kyanite in Plioc. deposits of Corn- 
wall, 359, 365, 370-71 figs. 


462 


TakE, Peeliave 

LAMPLUGH, G. W., on the Junction 
of Gault & Lower Greensand near 
Leighton Buzzard (Bedfordshire), 
1-80 w. maps & sects. 

LANDELL-Miuus, T. (& others), on 
the Carboniferous Rocks of the 
Deer-Lake District of Newfound- 
land [ title only}, lxix. 

Lateritization of Sierra-Leone norite, 
320-21. 

Leadhills (Lanark), plattnerite from, 
ili. 

Leicester Peak (Sierra Leone), 301, 
303 & pls. xvi, xviii. 

Leighton Buzzard (Beds.), junction 
of Gault & Lr. Greensand near, 
1-81 w. maps & sects. 

Leny Grits, 98 et seqq. 

LEONARDO DA VINCI as a geologist, 
lxxi—]xxil. 
Lepidodendron 

pl. xii. 

Leptocleidus swperstes, gen. et sp. 
nov., 285-98 fig. & pls. xiv—xy. 

Leucoxene in Plioe. deposits of Corn- 
wall, 358. 

bn wis, SH oPeawards ire 
Pidgeon Fund to, lxxi. 

Lias (Upper), succession in, 390-93, 
449-54. 

Library, annual report of Committee, 
xxi; lists of donors to, xi—xvi. 
Littleworth Brickyard (Bucks.), sect. 

descer., 39. 

Llandderfel Syncline, 218. 

Llandeilian of Bala district, 137 et 
seqq. 

Llanelidan-Fault Syncline, 210. 

Llangollen district (Denbigh), Lr. 
Paleozoic of, 176-226 figs. & 
pls. ii—v (geol. maps & sects.). 

Llangower Displacement, 160-61. 

Llyn-Tegid Displacement, 160. 

Loch-Avich Slates, 96 et seqq. 

Loch-Awe Nappe, 95-98, 118; Loch- 
Awe Syncline, 95. 

Loch Creran (& Ardmucknish Bay), 
geol. map of, 110; contact of 
Ballappel Foundation & Iltay 
Nappe at, 111-17. 

Loch Etive (Argyll), &c., geol. map 
of, 116. 

Loch-Tay Limestone, 98 et seqq. 

London, influence of geology on 
history of, v—vii. 

Long Crendon (Oxon.), sects. descr. 
& fig., 40-44. 

Ludlow Series in Llangollen district, 
204-208. 


sp. (Peru), 280 & 


Daniel- 


GENERAL 


INDEX. [vol. Ixxvin, 

LYELL medallists, list of, xxvi;- 
recipients of L. Fund, list of, 
xxvii; L. Medal awarded to C. 
Davison, xli-xlii; L. Fund, awards 
to A. Macconochie & D. Tait, 
xlii—xliv. 


Macconocuigz, A., award fr. Lyell 
Fund to, xliti—xliv. 

Magnesian Province, West African, 
306-307. 

Magnetite in Sierra-Leone noritic 
complex, 315, 321, 328 & pl. xvii; 
in Plioe. deposits of Cornwall, 358, 
365. 

MaAIpwELt, F. T., obituary of, lu. 

MAuLet, F. R., obituary of, xlvii. 

Mammillatus Bed nr. Leighton 
Buzzard, 3 et seqq.; fossils of, 49- 
ol. 

Maol-an-Fhithich Quartzite, 
seqq. 

Map of neighbourhood of Leighton 
Buzzard, 2; of Shenley Hill Sand- 
pits, 5; of early ‘nappes 7m 
S.W. Highlands, 84; of meta- 
morphism of ‘ nappes, 92; of the 
two great folds of the Iltay Nappe, 
102; geol. map of Loch Creran & 
Ardmucknish Bay, 110; of Loch 
Etive, Pass of Brander, & Loch 
Awe, 116; of Glen Orchy & Ben 
Lui, 122; of S.W. Highlands, pl.i; 
of the country E. & S.E. of Bala 
Lake, pl. 11; showing tectonic 
features of Llangollen district, 178 ; 
geol. map of Bryn (Llangollen), 
184; of Caer Drewyn, 216; of 
Cyrn-y-Brain, &c., 220; of area 
round Mboel-y-faen, pl. ii; of 
country round Llangollen, pl. v; 
of the Ross of Mull & Ardmean- 
ach, 230; of Knowles Hill, 261 ; 
geol. map of the Sierra-Leone 
peninsula, 300; of S.W. England, 
350. 

Marr, J. E., 168. 

Mavres, H. B., 87. 

Metamorphism of nappes, 92—93. 

MiAut, L. C., obituary of, xlvi-xlvii. 

Milborne Wick (Somerset), green- 
grained marl of, 431-82. 

Miletree Farm (Shenley Hill), sects. . 
in pits near descr. & fig., 19-21. 
Miner, H. B., on the Nature & 
Origin of the Pliocene Deposits of 
the County of Cornwall, and their 
Bearing on the Pliocene Geography 
of the South-West of England, 

348-74 figs., 376-77. 


99 et 


part 4] 


Miocene (Lr.) of Vancouver I., Des- 
mostylus from, lxix—lxx. 

Mochleusis defined, lvii. 

Moel-Fferna-Corwen area, Silurian. 
&c. of, 197. 

Moel-fryn (Merioneth), sect. to Bala, 
136; Caradocian, &c. of, 150-52 
w. vert. sect. & list of foss.; vert. 
sect. at, 154; Moel-fryn Sand- 
stones, 143; Moel-fryn Displace- 
ment, 161. 

Monograptus-crispus & M.-sedgwicki 
Zones in Bala district, 137 et seqq. 

Mugearite in nomenclature, 270; 
mugearite of Knowles Hill, 264, 
267 w. chem. anals. & pl. xi. 

Mull (Argyll), xenolithic Tertiary 
minor intrusions in, 229-60 figs. & 
pls. vi-x (microscope-sects.). 

Mull-of-Oa Phyllites, 99 et seqq. 

Mullach Glace an t’Sneachda (Mull), 
sapphire, &c. at, 237. 

MURCHISON medallists & recipients 
of ME Fund, lists! of, xxv 3 M. 
medal awarded to J. W. Evans, 
xxxix—xl; M. Fund, award to H. 
Bolton, xliii. 

Murray Town (Sierra Leone), 303. 
309. 

Muscovite in Plioce. deposits of Corn- 
wall, 358. 

Mynydd-Cricor area (N. Wales), Va- 
lentian of, 195-96; M. C. Nodal 
Anticline, 210-11. 


Names of Fellows read out, i, Ixxv. 
Nant-Aber Derfel (Merioneth), Llan- 
deilian of, 144-45 w. list of foss. 
Nant Rhyd Wen (Merioneth), Cara- 

docian of, 145-46. 

Nant T’yn-y-twmpach 
shire), sect. deser., 198. 

Nant-y-Bache Group, 206-207. 

‘Nappes’ (tectonic) in S.W. High- 
lands, 82 et seqq. ; ‘nappe’ defined, 
87; metamorphism of, 92-93 w. 
map. 

Newton Abbot (Devon), composite 
sill at, 261—70 figs. & pl. xi (micro- 
scope-sects.). 

NicHouas, T. C., 168. 

Nickeliferous rocks in Sierra Leone, 
322. 

Nicolella-actoniz fauna, 170. 
Nine-Acre Pit (Shenley Hill), sects. 
descr. & fig., 16-18. 

Nodal anticlines defined, 209, 226. 

Norite of Sierra Leone, 299-347 figs. 
& pls. xvi—xix 

Number of Fellows, &c., xvii-xx. 


(Denbigh- 


GENERAL INDEX. 46: 


Officers (& Council) elected, xxi. 

OLDHAM, R. D., ix; retires fr. office 
of President, xx; addresses to 
medallists & recipients of Funds, 
xxxvui et seqq.; obituaries of de- 
ceased Fellows, &c., xlv—liv; on 
the Cause & Character of Earth- 
quakes, lv—lxviii. 
Olivine in Sierra-Leone noritic com- 
plex, 327-28, 339, 341 & pl. xix. 
Olivine-dolerite of Knowles Hill, 268 
& pl. xi. 

Orchesis defined, lvii. 

Ordovician, see Bala, &e. 

Orthis hirnantensis differentiated fr. 
Strophomena siluriana, 165-66 fig. 


Paleozoic (Lr.) of Llangollen district, 
176-226 figs. & pls. ili—v (geol. 
maps & sects.). 

Pandy Ash, 181, 182. 

Paracas (Peru), Carboniferous plants 
from, 278-84 fig. & pl. xii. 

Parsons, L., 285. 

PART, G. M., 374. 

Pegmatite, noritic, of Sierra Leone, 
310-11. 

Peloneustes philarcus, model presen- 
ted, Ixxiil. 

Pen-Creigian Bareut (N. Wales), 
sect. to Cae-mawr, 185. 

Pen-y-Dallgwm (Merioneth), Ashgil- 
lian, &c. of, 155-57 w. sect. & lists 
of foss. 

Pen-y-graig Ash, 181, 182-83. 

Peru, Carboniferous plants 
278-84 fig. & pl. xii. 

Perritt, N. E., appointment con- 
firmed, iy. 

Phacops-mucronatus fauna, see Foel- 
y-Ddinas Mudstones. 

Phillipsinella-parabola fauna, 172. 

Picrite at Knowles Hill, 262 et seqq.. 
264-65 w. chem. anal. 

PripGEOoN Fund, see Daniel-Pidgeon 
Fund. 

Pitlochry Schists & Grits, 98 et seqq. 

Plas-uchaf Grit, 180, 190-91. 

Plattnerite exhibited, iii. 

Plesiosauria, see Leptocleidus. 

Pliocene of Cornwall, 348-77 w. map 
& figs. 

Polcrebo (Cornwall), Pliocene (?) of, 
352-53, 356. 

Pont-y-Ceunant Ash, 140-41. 

Poplars Pit (Shenley Hill), 
deser. & fig., 23. 

Port-EHllen Phyllites, 99 et seqq. 

Portaskaig Conglomerate, 99 et seqq. 


from, 


sect. 


464. GENERAL 

Posidonomya venusta (Knowles Hill), 
263. 

Post-Devonian movements in Llan- 
gollen district, 222-28. 

Pratt’s Pit (Leighton Buzzard), sect. 
deser., 35-36. 

Pre-Carboniferous floor in Llangollen 
district, 222. 

PRESTWICH medallists, 
XXVIll. 

PRINGLE, J.,177; see also Buckman, 
Sass 

Pyroxenes in Sierra-Leone noritic 
complex, 325-27, 338-39, 341 & 
pl. xix; see also Augite, §c. 


list or 


(Juartz in dolerites of Knowles Hill, 
267, 270, & pl. xi; in xenolithic 
intrusions of Mull, 240 & pl. vu; 
in Sierra-Leone noritic complex, 
324; in Plioe. deposits of Corn- 
wall, 358. 


RADLEY, EH. G. | chem. anals. of Mull 
rocks], 229 et seqq. 

REYNOLDS, S. H., obituary of the 
Earl of Ducie, xlix—l. 

Rhiwlas Mudstone & Limestone, 
143; faunal list, 172. 

RHODES, J., 256. 

RomaAngEs, J. (Mrs.), 168. 

Rudh’a’Chromain (Mull), 232 et seqq.; 
sects. at, 234. 

Rudha Garbh (Argyll), 113. 

Rutile in Plioc. deposits of Cornwall, 
359, 361, 365. 

St. Agnes (Cornwall), Plioc. de- 
posits of, 349, 354-55; minerals 
in the same, 357-59. 

St. Erth -(Cornwall), Plioc. de- 
posits of, 349-51, 355; minerals 
in the same, 359-62. 

St. Keverne (Cornwall), Plioc. de- 
posits of, 352, 356; minerals in 
the same, 362-64. 

Salopian of Llangollen district, 200 
et seqq. 

Samuel I. (Sierra Leone), 313. 

Sandpit Cottages (Shenley Hill), sect. 
in pit near, descr. & fig., 15. 

Sapphire, occurrences of, in Mull, 
231 et. seqq. 

SeeNiIT, R. W., award from Daniel 
Pidgeon Fund to, ix. 

Selma (Argyll), 113. 

SEWARD, A. C., elected President, 
xx1; Geological Notes on Western 


INDEX. [vol. lxxvin, 
Greenland, Ixxiii-lxxiv ; on a Col- 
lection of Carboniferous Plants 
from Peru, 278-83 fig. & pl. xii; 
(& R. E. Holttum), on Jurassic 
Plants from Ceylon, 271-77 & pl. 
pai 

SHANNON, W. G. St. J., om a Com- 
posite Sill at Newton Abbot 
(Devon), 261-70 figs. & pl. xi 
(microscope-sects.). 

Shelway Lane, see Barrington. 

Shenley Hill, see Leighton Buzzard. 

Shilling, Cape (Sierra Leone), 510. 

Shira Limestone, 96 et seqq. 

Sierra Leone (W. Africa), norite of, 
299-847 figs. & pls. xvi—xix. 

Sigillaria sp. (Peru), 280-81 & pl. 
Xill. 

Siliceous xenoliths 
239-40 & pl, vii. 

Sillimanite in xenolithic intrusions of 
Mull, 243-44 & pls. vii-x. 

Silty Beds in Lr. Greensand, 55-57. 

Slides (=fold-faults), 86. 

SmitH, B. (& L. J. Wills), on the 
Lower Paleozoic Rocks of the 
Liangollen District, with special 
reference to the Tectonics, 176— 
223 figs. & pls. ii—v (geol. maps & 
sections). 

SmitH, W. CAMPBELL, exhibits platt- 
nerite, ii; re-elected Secretary, 
xxi; obituary of J. M. Gordon, 
li. 

Soda-syenite of, Knowles Hill, 266- 
67 & pl. xi. 

South-West Highlands of Scotland, 
structure of, 82-181 figs. & pl.i 
(map). 

Southeott Pit (Bucks.), sect. descr., 
37-38. 

SpatH, L. F., see Buckman, 8. 8. 

Sphenopteris sp. (Peru), 279-80 & 
Pleas 

Spinel in xenolithic intrusions of 
Mull, 247-48 & pls. vili-x, w. 
chem. anals. 

Spinney Pit, see Grovebury. 

Sramp, L. D., on the Base of the 
Devonian, with especial reference 
to the Welsh Borderland [title 
only |, ii. 

STANSFIELD, J., on Banded Precipi- 
tates of Vivianite in a Saskat- 
chewan Fireclay [title only], iii. 

Staurolite in Plioc. deposits of Corn- 
wall, 358, 361, 365. 

Stocklinch (Somerset), sect. deser., 
450. 


in Mull rocks, 


part. 4 


STRAHAN, Sir AUBREY, 1X. 

Strain-intervals between earth- 
tremors, lx. 

Strophomena siluriana differentiated 
fr. Orthis hirnantensis, 165-66 fig. 

Sugarloaf Mountain (Sierra Leone), 
301 & pl. xvi. 

Sussex (Sierra Leone), 309. 

Swceh-Gorge Ash, 179. 

SWEETING, G.5S., 374. 


Teniopteris spatulata, 273-74 & pl. xii. 

Tarr, D., 283; award fr. Lyell Fund 
to, xliii—xliv. 

Tarannon Series of Llangollen district, 
192 et seqq. 

Tayvallich Slates & Limestones, 96 
et seqq. 

TEALL, Sir JETHRO, obituary of M. 
E. Wadsworth, xlvii—xlviii. 

Tectonic earthquakes, lix et seqq. 

Tectonics of Llangollen district, 176 
et seqq. 

Teirw Beds, 179-82. 

TEMPLEMAN, A., see Buckman, 8. 8. 

Tertiary xenolithic minor intrusions 
in Mull, 229-60 figs. & pls. vi-x 
(microscope-sects.) ; see also Plio- 


cene, gc. 

Tetrarhynchia thorncombiensis, nom. 
nov., 435. 

Tholetites of Mull, 236 et seqq. & 
pole wae 

THomas, H. H., 374; retires fr. 


Secretaryship, xx; on _ certain 
Xenolithic Tertiary Minor Intru- 
sions in the Island of Mull (Arg yll- 
shire), 229-59 figs. & pls. vi-x 
(microscope Beene), 260. 

Thornecombe Beacon (Dorset), 379 et 
seqq.; Junction-Bed at, 393-95, 
443-45 ; Middle Lias of, 395-400. 

Titanomagnetite in Sierra-Leone 
noritic complex, 328. 

Todites williamsoni(?) = Cladophlebis 
reversa, 271. 

ToKE (Sierra Leone), 310, 313. 

ToorTtH, S., 285. 

Topaz in Plioe. deposits of Cornwall, 
359, 362, 366. 

Tourmaline in Plioc. deposits of Corn- 
wall, 358, 361, 363, 365. 

Tridymite in xenolithic intrusions of 
Mull, 2389-40 & pl. vii. 

Trimerocephalus (Knowles Hill), 2638. 

TrustT-FuNpDS, statement of, xxxiv— 
XXXVI. 


Tyn’-y-twmpath Beds, 180, 186-87, 


GENERAL INDEX. 465 


Valentian of Bala district, 137 et 
seqq.; of Llangollen district, 192— 
200. 

Vancouver I. (B. C.), Dessnoctoinacs fr: 
Lr. Miocene of, lxix—lx 

VINCI, DA, see itconandon 

Vivod Group, 207. 


Wadhurst Clay, giant 
from, Ixxv. 

WapswortH, M. E., Obituary of, 
xlvii—xlviii. 

Wautuis, F.S., on the Carboniferous 
Limestone (Avonian) of Broadfield 
Down (Somerset), 227, 228; award 
fr. Daniel-Pidgeon Fund to, ix. 

Watson, D. M.S., 293. 

Watton Cliff (Dorset), Junction- Bed 
of, 378 et seqq., 387-88, 445-48. 
Warts, W. W., 374; receives Lyell 

Medal for C. Davison, xl—xli. 

Weald Clay, Leptocleidus swperstes 
from, 285-98 fig. & pls. xiv—xv. 

Webster‘s Pit (Leighton Buzzard), 
sect. deser., 36. 

WEDD, C. B., 176, 209. 

WEINSCHENK, E. H. O. K., obituary 
Ole hai 

Wenlock Series in Llangollen district, 
201-204. 

West African Magnesian Province, 
306-307. 

Whale R.. see York. 

Whitbian, see Jurassic. 

White Bed of Burton Bradstock, 420— 
31 figs. 

Wilberforce (Sierra Leone), 303, 315 
& pls. xvii, xix. 

Wits, L. J., award fr. Wollaston 
Fund to, xli; (& B. Smith), on the 
Lower Paleozoic Rocks of the 
Llangollen District, with special 
reference to the Tectonics, 176-223 
figs. & pls. ii-v (geol. maps & 
sects.), 226. 

WOLLASTON medallists & recipients 
of W. Fund, lists of, xxiv; W. 
Medal awarded to A. Harker, 
XXXVili-xxxix ; W. Fund,award to 
1G Os aul sdhht 

Woopwakb, A. S., obituaries of H. 
Woodward & L.C. Miall, xlv—xlvii ;sx 
(& others) on the Carboniferous 
Rocks of the Deer-Lake District 
of Newfoundland [title only], 
aps, 

WoopwarRbD, Hrnry, decease an- 
nounced, i; obituary of, xlv—xlvi. 


gastropod (?) 


466 


Xenolithie Tertiary minor  intru- 
sions in Mull, 229-60 figs. & pls. 
vi-x (microscope-sects.) ; Xxeno- 
liths in dolerite of Knowles Hill, 
263-64, 265-66 & pl. xi. 


Garnedd (Merioneth), Caradocian 
of, 146-48 w. sect. & lists of foss. 


GENERAL INDEX. 


[ vol. Ixxviil. 


Yeovilian, see Jurassic. 
York (Sierra Leone), 313 & pls. xvii- 


X1X, 


ZABEL, C, F., obituary of, lii. 

Zircon in Sierra-Leone noritic com- 
plex, 323; in Phoc, deposits of 
Cornwall, 359, 361, 364, 365. 


END OF VOL. LXXVITI. 


| December 30th, 1922. | 


PRINTED BY TAYLOR AND FRANCIS, RED LION COURT, FLEET STREET. 


JAMES SWIFT & SON L®- 


Makers of the Highest Grade 


MICROSCOPES and ACCESSORIES 


Contractors to. all Scientific Departments of H.M. Government. 
JUST PUBLISHED 


NEW CATALOGUE 


OF 
PETROLOGICAL ACCESSORIES 


and Instruments for Mineralogy and Crystallography, 
including Refractometers, Goniometers, etc. 


A copy will be sent post free on request. 


University Optical Works 
81 TOTTENHAM COURT ROAD, LONDON, W.I. 


Publications to be had of the Geological Society, 
Burlington House. 


QUARTERLY JOURNAL (Vols. III to LXXVIII, inclusive). Price to 
Fellows, £1 0s. Od. each: £1 5s. Od. in cloth. [Postage ls.] Separate parts (2,5, 
7, 9, 55, & 273 are out of print) 7s. 6d. each: price to Fellows 5s. [Postage 5d.| 


GENERAL INDEX TO THE FIRST FIFTY VOLUMES OF THE 
QUARTERLY JOURNAL (1845-1894). Part I (A-La). Part II (La-Z), 
Price 7s. 6d. each. To Fellows 5s. each. [Postage 6d each. | 


GEOLOGICAL LITERATURE added to the Geological Society’s Library 
during the years ended December 31st, 1894-1913 and 1920, 1921. Price. 5s. each. 
To Fellows, 3s. 9d. each. [Postage 7d.] (Nos. 8 & 14 are out of print.) 


THE HISTORY OF THRE GEOLOGICAL SOCIETY OF LONDON, 
by H. B. Woopwakrp, F.R.S. Price 3s, net. [Postage 1s.] 


THE CENTENARY OF THE GEOLOGICAL SOCIETY OF 
LONDON, recorded by W. W. Warts, F.R.S. Price 2s. [Postage 5d.] 


HUTTON’S ‘THEORY OF THE EARTH,’ Vol. III, edited by Sir 
ARCHIBALD GEIKIE£, D.C.L., F.R.S_ Price 3s. 6d. To Fellows, 2s. [Postage 9d.] 


THE GEOLOGY OF NEW ZEALAND. Translated by Dr. GC. E. 
FiscHEeR from the works of MM. HocusteTTeR & PETERMANN. With an Atlas of 
Six Maps. Price 10s. net. [Postage 9d.] 


Py F 
¢ G - é t. Fz. ee 
CONTENTS. 
PAPERS READ. 
Page 


11. Dr. F. Dixey on the Norite of Sierra Leone (Plates KVI-XIX) ............... 299 


12, Mr. H. B. Milner on the Pliocene Deposits of Cornwall, and their Bearing 
on the Pliocene Geography of the South-West of England.................. 348 


13. Mr. 8. S. Buckman on Certain Jurassic Strata near Eypesmouth (Dorset) : 
the Junction-Bed of Watton Cliff and Associated Rocks .................. 378 


[Titte-Pacy, ConTEnTs, and InpEx to Vol. LXXVIIL. j 


[The Editor of the Quarterly Journal is directed to make it known to the Public 


that the Authors alone are responsible for the facts and opinions 
contained in their respective Papers. | 


*,* The Council request that all communications intended for publication by the 
Society shall be clearly and legibly written on one side of the paper only, with 


proper references, and in all respects in fit condition for being 


at once placed in the Printer’s hands. Unless this is done, it will be in 
the discretion of the Officers to return the communication to the Author for revision. 


The Library at the Apartments of the Society is open every Week Day from Ten 
o’clock until Five, except on Saturdays (when the hours are from 10 to 1), and 
except during the fortnight commencing on the first Monday 
in September, when the Library is closed for the purpose of 
cleaning. 


. 


Fellows are reminded that they are not entitled to receive the 
Quarterly Journal regularly, unless a subscription at the rate of 
10s. per volume is paid in advance. Those Fellows who do not 
require the complete series can obtain separate parts at the price 
of 3s, each, if applied for within one year of the date of issue. Sub- 
scriptions for Vol. LXXIX (1923) are now due, and Fellows who wish 
to subscribe for the same are requested to send the amount payable, 
as soon as possible, to the Clerk, Geological Society, Burlington 
House, W. 1. he fm 


"42" Wy 
r Li 


au 
i 


f, 


‘ ya) 
hy 


OE ae 


oo nie —- oq te e™ 1 elf As TOY IN LLk. ¥ 845. . 
5 geen. ol!) ba i $ . \ fe 4, ° % Ripe =< 2 a Pa, 9 vy aa Te) eo Rms 
An esn a, ~~ |” | | Rs - ra 4 Thee le. ha 
e e SA a 


~ 


3 . . 4 ips a By 
Wy uot ir ignill li] 5 ANN ARIMAP Age, %-%_1,, | TO YY PP 


ui aia sana aA® Die bol FT abhi 9) ASR Ee LTE aS A sala leleleiaer vert 


BA a z ea (aad Aa 
P TT abel | Ani Y v8 sate \, pana h onAnpat han’ er paal 
TOT, Le am ‘Aa Aa | a ~~ pan? a age’ A ery. 
Poe aN NapaoAneg RE | ay ot ~ eomSannl Phone act a f. ghiain! 
i s 1 led Py Wann. 4, i- thy, iy art wa. a _p whe Pe pasasae wis TAT el T' »! am aks 
a Shefel feat PTTL fal PHATE PRak br CPE Pipa ae, Dla bel ttt ale 
2 MARAAL Balt tiaRe TALL «, Ale Tepes) v AAA’ P aa pyle Ll. 
SE Ae Bt Lol eile | el hal 
Aira aa srpas tS | pallies 
a ’ ~~ am & » &a = 
eURAURAiReem ite, ADR < iA aaa Sawanan a aM Utne 
re ~ en nN A rev 


saa 8 . 


s- 


\y | TPAC i rrha eo ae 2 isha a8 nny 
lel | pW opaiabal. Wa Nae Se ral aaa Mitt siaehnsli a 
Fe irked nama pr owacee aee yal lip 


== Lae ABa Vvv¥ , » CAA SARIA MR | 


y » » es A eh na ie 


~ 


@ecasnneaninee OA aa || = a ae aegis) ata WW yA f RANA \ > 
OidGaprrvec cApaatIT aap bh taal eee hresee Pate. | nares I ATARAREAAUR 
a al agave War104 Manage’ MM epen, on MANNA 


a > rs 
ah: i es 
PUMA Rik smee eet) 2D 


THAT se Rak ee 
ul {| pre 


pe s yey San Ae 
aw nr A YN "AD iA ry: EN 
j eget Aine: Vr . 
APap tt ©: | Phy Ltt’ 
cy Rte tf gett | B23 
Mri errreal el Te 


~y ARERR RRR anor mnt 


Bit a aaah ir tas echt | 


fabet Uyehabanar's 


a] : “Henne | 


DA enaed ae || 


Nr ee 


‘ , Na 
aM Atty ais “py o 


(lil) it “i 


Aanann? A) 


vAARAL 


“eae ae 


AQF, 


wv 


aan 


= 


MEET TTD 
\n SB a 
mCapeeE| 
a iy r 
ae A 
AMAa abe 


LP. 


a Aw. aru 


ea auaninttOAy 
pn hiihoulte wiih 


pi 


gasagy 
MEET all 


> do Tod Pee bit jaiaehe 


Maia, al 


iA 


SAAR ab, AHA | qa ahl 


si 


2%. » 
Be Beaks ; ~@ Asrieaw AA ‘ 
wt * PELs tiaras 
uaa ry 


» fy ae 


Neel ts *ASAMNUng ana. 


abe ei Stee Oper 


Ma Ue f 
pf A Bhi vee * Wel 3 pith BPR Ll bt y Larry)! 


Viti 


3 9088 01350
Internet Archive Audio

Featured

Top

Images

Featured

Top

Software

Featured

Top

Books

Featured

Top

Video

Featured

Top

Mobile Apps

Browser Extensions

Archive-It Subscription

Save Page Now

Full text of "The Quarterly journal of the Geological Society of London"

See other formats
