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TEXT-BOOK OF GEOLOGY. 



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TEXT-BOOK OP GEOLOGY. 



BY 

ARCHIBALD GEIKIE, LL.D., F.B.S., 

DIKXCTOR-GBNEBAL OF TU£ OEOtOQICAL 8URTET OF GBEAT BRITAIN AND IICBLAND, 
AND DIBECTOR OF THE MI^BEUU OF PRACTICAL GEOLOGY, LONDON; 
FORMERLY Mt'RCBISON FROFE880R OF GEOLOGY AND MINERALOGY 
IK THE UNHERSITY OF EDINBURGH, 
AND DIRECTOR OF THE GEOLOGICAL SURVEY OF SCOTLAND. 



WITH ILLUSTRATIONS. 



SECOND EDITION, REVISED AND ENLARGED. 



Ji 



M A C M I L L A N AND CO. 

1885. 



f 



PEEFACE TO THE SECOND EDITION. 



In the endeavour to keep this Text-book abreast of the onward progress 
of Geology in all departments of the science, I have subjected the 
present edition to a thorough revision. By abridgment, where this 
was possible, space has been found for important additions, while 
by the adoption of a diflferent type from that used in the first edition, 
the amount of information given has been largely increased without 
any serious augmentation of the bulk of the volume. I have been 
particularly careful to add greatly to the number of references to special 
sources of information, which are of so much service to the student who 
seeks to carry his studies beyond the limits of a mere text-book. 

In the revision, I have derived assistance from several friends, to 
whom I desire to record my obligations, especially to Professor Bonney, 
Professor T. Rupert Jones, and Mr. W. Topley. 

28, Jeumtx Street, 
\Cjth MareJi, 1885. 







LONDON: 
PRINTED BY WILLIAM CLOWES AND SONS^ IjIMITBI^ 

STAMrOSD STRRKT AKO CBARIXQ CAOflB. 



FROM THE PREFACE TO THE FIRST EDITION. 



The method of treatment adopted in this Text-book is one which, while 
conducting the class of Geology in the University of Edinburgh, I have 
found to afford the student a good grasp of the general principles of the 
science, and at the same time a familiarity with and interest in details 
of which he is enabled to see the bearing in the general system of 
knowledge. A portion of the volume appeared in the autumn of 1879 
as the article " Geology " in the Encyclopsedia Britannica. My leisure 
since that date has been chiefly devoted to expanding those sections of 
the treatise which could not be adequately developed in the pages of a 
general work of reference. 

While the book will not, I hope, repel the general reader who cares 
to know somewhat in detail the facts and principles of one of the most 
fascinating branches of natural history, it is intended primarily for 
students, and is therefore adapted specially for their use. The digest 
given of each subject will be found to be accompanied by references to 
memoirs where a fuller statement may be sought. It has long been a 
charge against the geologists of Great Britain that, like their country- 
men in general, they are apt to be somewhat insular in their concep- 
tions, even in regard to their own branch of science. Of course, specialists 
who have devoted themselves to the investigation of certain geological 
formations or of a certain group of fossil animals, have made themselves 
familiar with what has been written upon their subject in other 
countries. But I am afraid there is still not a little truth in the charge, 
that the general Ixxly of geologists here is but vaguely acquainted with 
;:;oological types and illustrations other than such as have been drawn 
from tlie area of the British Isles. More particularly is the accusation 
true in regard to American geology. Comparatively few of us have any 
ade(iiiate conception of the simplicity and grandeur of the examples bj* 
which the principles of the science liave been enforced on the other side 
.»f tlio Atlantic. 

Fully sensible of this natural tendency, I have tried to keep it in 
crmstant view as a danger to be avoided as far as the conditions of my 
task would allow. In a text-book designed for use in Britain, the 
Illustrations must obviously Ije in the first place British. A truth can 
Ik? enforced much more vividly by an example culled from familiar 
irround than bv one taken from a distance. But I have striven to widen 
the vision of the student by indicating to him that while the general 



viii PREFACE. 



prhiciples of the science remain uniform, they receive sometimes a clearer, 
sometimes a somewhat different, light from the rocks of other countries 
than our own. If from these references he is induced to turn to the 
labours of our fellow- workers on the Continent, and to share my respect 
and admiration for them, a large part of my design will have been 
accomplished. If, further, he is led to study with interest the work of 
oiir brethren across the Atlantic, and to join in my hearty regard for it 
and for them, another important section of my task will have been 
fulfilled. And if in perusing these pages he should find in them any 
stimulus to explore nature for himself, to wander with the enthusiasm 
of a true geologist over the length and breadth of his own country, and, 
where opportunity offers, to extend his experience and widen his 
sympathies by exploring the rocks of other lands, the remaining and 
chief part of my aim would be attained. 

The illustrations of Fossils in Book VI. have boon chiefly drawn by 
Mr. George Sharman ; a few by Mr. B. N. Peach, and one or two by Dr. 
B. H. Traquair, F.E.S., to all of whom my best thanks are due. The 
publishers having become possessed of the wood-blocks of Sir Henry De 
la Beche's * Geological Observer,' I gladly made use of them as far as 
they could be employed in Books III. and IV. Sir Henry's sketches 
were always both clear and artistic, and I hope that students will not be 
sorry to see some of them revived. They are indicted by the letter (B), 
The engravings of the microscopic structure of rocks are from my own 
drawings, and I have also availed myself of materials from my sketch- 
books. The frontispiece is a reduction of a drawing by Mr. W. 11. 
Holmes, whose pictures of the scenery in the Far West of the United 
States are by far the most lemarkable exami^les yet attained of the 
union of artistic effectiveness with almost diagrammatic geological 
distinctness and accuracy. Captain Button, of the Geological Survey of 
the United States, furnished me with this drawing, and also requested 
Mr. Holmes to make for me the canon-sections given in Book VII. To 
both of these kind friends I desire to acknowledge my indebtedness. 



CONTETNTS. 



PAOR 

INTBODUCTION 1 



BOOK I. 
CosMicAL Aspects op Geology, 6. 

I. Relations op the Eabth in the Solar System 7 

IL Form and Size of the Earth 11 

III. Movements of the Earth in their Geological Relations 13 

1. RoUiion, 13—2. Revolntlon. 14—3. Precession of the Equinoxes, li— 4. Change in 
the obliquity of the Ecliptic. IS— 6. Subilityof the Earth's axis, 16—6. Changes 
of the Earth's centre of gravity, 18 — 7. Results of the Attractive Influence of Sun 
and Moon on the Geological Condition of the Earth, 19—8. Climate in its Geo- 
logical relation^ 21. 

BOOK IL 

Geognosy — An Investigation of the Materials of the 

Earth's Substance. 

Part I. — A General Description of the Parts of the Earth. 

I. The Envelopes 30 

1. The Atmosphere, 30—2. The Oceans, 32. 

II. The Solid Globe 3G 

1. The Outer Surface, 36—2. The Crust, 44 — 3. The Interior or Nucleus. 45; Evidence 
of Internal Heat, 47 ; Irregularities in the downward Increment of Heat. 49 ; 
Probable Condition of the Earth's Interior, 61—4. Age of the Earth and Measures 
of Geological Time, 66. 

Part II. — An Account of the Composition of the Earth's Crust — Minerals 

AND Rocks. 

I. General Chemical Constitution of the Crust .58 

II. ROCK-FORMINO MINERALS 62 

III. Determination op Bocks 7(5 

1. MacroBsopic Examination, 77 — 2. Microscopic Investigation, 86. 

IV. General Macroscopic Characters op Rocks 91 

1. Structure, 91 — 2. Composition, 96 — 3. State of Aggregation, 97— i. Colour and Lustre, 
97—5. Feel and Smell, 99—6. Specific Gravity, 99—7. Magnetism, 99. 

V. Minute or Microscoho Characters op Rocks 99 

1. Microscopic Elements, 100 — 2. Microscopic Structures, 109. 




CONTENTS, 

VL CtAaawwcATKis or Bocxs 114 

VII. A DuKBiniOt]r or the siobk nfPOBTANT Rocss of the Eabth's Ckst. 

A. OrtftimOime mmd TUrMmt 117 

1. Stimtifieil 117 

2L Sdikftoae or FoUmleii 123 

1. AnjOlitMb ISS— ft. OwBts^BockiL UT— 3. Plymiwif Itwtii, US— 4. Homblcod^ 

131—11. QMBto- ui Mka- 
131--1X g«Bt>- ui F^fap«rwEKks» 133—14. Qauts^ Fclif«r% And 
]lla4toclaw 133— IS^ Qoutx-^ Fd^ns a^ G«neC-1tocka, 133— l<w FUqwr 
ami )ficB-Rock% 13«— IT. Sctokwe Ooaplonente Bvcks 134. 

a. Mmbkb 136 

L Filipr Wartig Series: «, Or t ho ciMe B «k»-^l^ ^jw mUMU ims, 137— (3) QmiU- 

iHi^ «r pMT is q[«Hti» 143—1^ niciKlaav^hxta. 147—1. rh gi»c W -H< 

liffc t<r Xip»>) BsdcB, 14S--3. ng^Muc^Mka-Rocks, li4 

JL%». twl^M»— 4. nagiirlaw Kriky Pwfri, 1S4— «. nugMrl—r ftllnMl ir 

1S4— auXifiWBaf gtrtaw l»~liL Imcite>-B«iaL 1»— ir. 

Kfr^T. OBviMt-EKksv FHiteUteiv IM— ^ ary fit riofks 

a f>ljBiiiliF(Cfaii>r) .. 158 

L GimTclMdSndBockB^>teBBitet) 158 

2. CWj Bo^s (Peiites) .. .. 163 

S. Toleuie FnipMBlal Bods (Toft) lei 

4. FtigftilBoAsofOUgMioOqi^ 1C7 

{!> CiliwiiMU 10--<S> SMrnM> lt»-<S> y^f tgrtn. 1I«^(4> Ctetvumw^ 171 

— (S> FlumlMiiMu 174. 



BOOK m. 

Dtxuikjll Gkojogy^ 177. 



Pawt L— 

IS 



AcTK^ — Ax Ix^»T iviv> TWK Gbmakicai. Cha^cwis 

IteK.\TH THK S^rWACK \Xr THK E-\ETH, 17?^ 



I. Ti^iTASOB A3EP T«WAXjr A<TK«, 

K T<4nur TVcdvts 

1. «a— »MrfTMiii»ik 1J#-S. Wmtt. 1<#-X Ui««. 1^3—4. 



lj«. 



17S 



189 

31 Sii«rB»«rf^TtfiMawi» 523 

5. OiiiMM' «e' V.'dtrsak' A<«i\a . .. v. .. . 244 



IL K\^n»^ U(» » 



250 



CONTENTS. xi 



-*f- 



PAai 
UL SaOULAB VrBMkJAL AHD DlFBIHEOK 259 

UplMsnl, llI-8al»UMQt» M8-CraMi of UphetTal and SuteMnioeb MS. 

IV. HTroomi CUnMBi or Obaxqwb n Tm Tstcturs, SnnrcTrBs, axd 

OumuMTiow or Bocks 272 

1. Efbote of Heat 278 

RiMof TeipMBrtauBbj Dapwioa, «T»"^BIm of Twpcwtaw bj OwbImJ TnuMfor- 
autkNi, tT4— Bite of TonpinUirfl bj Book-ornahiiift 274— Blae of Tempen- 
tan bj Istraiioa of Bnmtod Bock, a7ft— EzpaoBioii, ITS— CiryitilHwtlon 
CMiMbX STt— ProdncUoa of Prinutlo Stmctora, a7t— D^ Fuiloii, tT6— 
Coirtnottoii oC Boelu in poaslDg ftom a Glaaj to a Stony 8UK SW-So^ 

2. Infloenoectf Heated Water ... .. 281 

FkvMOMor Watarln all Books, ass— SolTcnt FOww of Water aaoiw Bocki, as^Tlda 
Power iacnased bj Bealh MS— Cb*oDentloa oC Pieeeiire, wa ■ Aqi»lgneoiM 
Fuloii, aSi-JkitUkiiBl Fkodaeltoa of Mineral 384— Bzperiments In Ketamor- 
phlni.28B. 

8. Eifoet8ofOoiiipreaaioii,TeiiaioiiyandFraotaie 286 

Minor Boptoiei and Holaea. asT— GonooUdatlon and Wddttnc; asT— OeaTage, 388— 
Deihrm«aon,a>0— Pltotk»n,ata-^otntfngandPlBtocrtton,a8^ 

4. TheMetanunphianofBockB 294 

ProdneCkn ofliaiUo ikom limeetone, aas— Dokaillialloii, aOf— Ooofenlon ofYcge- 
taUe Snbotanee into Goal, 397— rrodnetioB of the Sdditoae atroctoie, 388. 

Pabt U.— Epigbnb OB Sttbtaoe AonoK, 800. 

L An 801 

1. Qeologioal Work on Land 808 

1 . Deetractive Action, 803— Effects of TJgiitntng, 303— Ellteta of Changes of Tempera- 
ture, S04-4£lliBcU of Wind, 304—3. Beprodnctlve Action— OiowUi of boat, 
308— Loess, 807— 8snd-H ills or Dunes, 308— Dost-showers ; Blood-rain, 311— 
Transportation of Seeds, 313— Effiorescenoe Products, 313. 

2. Influence on Water 813 

Ocean Currents, 813— Waves, 313— Alteration of Water-level, 814. 

II. Watbb 814 

1. Rain 815 

(1) Cbemical Action,315— Chemical Composition of Baiu-water, 318— Chemical and 
Mineralogical Changes produced by Bain, 817— Weatherinf, 319— Formation of 
Soil, 326— (2) Mechanical Action. 327— Bemoval and Benewal of Soil, 327— 
Movement of Soil-cap, 338— Unequal Erosive Action of Bain, 828. 

2. Undergronnd Water 831 

Springs, 331— (1) Chemical Action, 334— Alteration of Bocks, 338— Chemical Depo«itii, 
340— Subterranean Channels and Caverns, 341— (2) Mechanical Action, 343. 

3. Brooka and RiTers 345 

1. .<^uroes of Supply, 316—11. IHschane, 347— ill. Flow, 349— iv. Geological Action, 
361 : (1) Qiemical, 351 ; (2) Mechanical, 363— Transporting Power, 863— 
Excavating Power, 368— Beproductlve Power, 366— Cones de l>^)ectlon. 3«8— 
Biver-beds, 367— Flood-plains, 368— Deposits in Tjikes, 370— Ban and Lsgoon- 
barriers, 371— Deltas in the Sea, 373. 

4. Lakes 377 

Fnsh-water, 377— Saline, 380— Depodlts in Salt and Bitter Lakes, 883. 

5. Terrestrial Ice 885 

Ftast, 886— Frosen Bivers and Lakes, 386— Hall, 887— Snow. 387— Gladers and Ice- 
fibects, 388— Work of Glaciers : (a) Transport, 393— (fr) ^^On, 397. 



xii CONTENTS. 



FAGK 

6. Oceanic Waters 402 

I. MoTcments : (l)TideBp 402 -<a) Currents, 4(»-<3) Watm and Oroimd-gtreU. 405— 
(4) loe on tbe Sea, 407— ii. Geological Work :!(1) Influence on Climate, 400— 
(2)E^O0ion: (a) Chemical, 410--(6) Merhanicaf, 4ia-<3) Transport, 418^ 
C4) Beprodnction. 430— Chemical Depostts, 420 : Medumlcal Deposits, 421 : 



I 



i 



a) Land-derived or Terrigenous : Shore Deporits, 421, Infira-littoral and Deeper- 
water Deposits, 422 ; (6) Abysmal, 423. 

7. Denudation and Deposition — ^The Besnlta of the Action of Air and 

Water upon Land 426 

1. Subaerial Denudation : the guienl Lowering of Land, 42^—2. Sabaerial Denudation : 
tbe unequal Erosion of Land, 430—3. Marine Denudation, its comparative 
Bale, 432—4. Marine Denudation, its flnal Besnlt, 433—6. Deposition : the 
Frameworic of New Land, 435. 

ni. LiFs. 

1. Destructive Action of Plants and Animals 437 

2. OonservatiYe Action 440 

3. Reproductive Action 442 

Humus and Black Soils, 442— Pftai-Mosses and Bogs, 443— Mangrove Swamps, 445— 
Diatom earth, 440— Chemical Deposits formed 1^ Plant-agen<7, 446— Shell- 
marl, 448— Ooral-reefii, 448— Limestone and Cose, 465— Slioeous Ooie. 456— 
Phoq>hatic Dq[X)sits. 467. 

4. Man as a Geologieal Agent 457 



BOOK IV. 

GEOTEcrroNic (Structural) Geoloot, or the Architegturb of the 

Earth's Crust. 

Pabt I. — Stratification and its AccoMPANiMENTSy 461. 

Funis of Bedding. 461— False-bedding, 464— Intercalated Contortion, 465— lrregularitt«s 
of Bedding due to Inequalities of Deposition or of Erosion, 466— Sorfiue-Markings 
(lUpple-marlE, San<r«cks, Ike.), 4Ta--ConcTetloiis, 4T3— Altemations and Associa- 
tions of Strata, 476— Relative Perdstenoe of Strata, 4T8— Influence of the Attenu- 
ation of StraU upon apparent Dip, 480— Orerlap, 481— Relative Lapse of Time 
represented by Strata axid by the Intervals betweoi them, 481 — Ternary Sucoeasion 
of Strata, 484— Groups of Strata, 485— Order of Saperpoeition : the Foundation of 
Geological Chronology, 486. 

Part II.— Joints, 486. 

1. In Stratified Rocks, 48t-3. In Massive (Igneoos) BoAa, 49a— 3. In Foliated 
(Schistose) Rooks, 484. 

Part III. — ^Inclination of Bocks, 494. 

Dip, 494— Outcrop, 496— Strike. 497. 

Part IV. — Curvature, 499. 

Monoclines. 601— AnticUnes and SyncUncs, 609— Invenion, 509— Ornmpling, 504— 
Deformation, 606. 

Part V. — Cleavage, 506. 
Part VI. — Dislocation, 506. 

Nature of Faoltn, 509— Origin of Faults, 511— Normal Faults, 611— Revscsed Fanlt^ 
51 1— Tbruftplanes, 519— Throw of Faulta, 519— Dip-tkuHsand Sbike>Faults» 51S— 
Dying-out of Faults, 516— Groups of Faults, 51 T— Detection and Tracing of faults, 9 18. 



CONTENTS. xiii 



FAGB 

Pabt Vn. — ^Ebuftivb (laNEOUs) Rooks as Part of the Structure op the 

Earth's Crust, 521. 

L PLUTQinCI^ I WTH U ttlV E, OR SUBSEQUENT PHASE OF EbUFTIVITY 524 

1. BoBBee 526 

Gnnlto-bosMt, sae— BeUiloa of Onnlta to Gontignoiu Bocks. 627— Coonection of 
Qnnite wilh Vokanie Bodca, 629— Metamorphlc Origin of some Granite, 630 
— DIorite, Iecm Effects on ConUgnoiis Rocks, 632— Connection with Volcanic 
Action and vtth CrystalUne Schists, 633. 

2. Sheets 533 

GfliMnlOhancttr, 638— Effects on Oontigoons Bocks, 636— CopnectJon with Vol- 
eanle Action, 686. 

3. Yeins and Dykes 537 

BnqittT» or Intmrtya, 637— Veins, 638— Qykes^ 640-Segr«B*tlon Veins, 642. 

4. Necks 544 

II. Intebbedded YoLGAKio OB ()oiiTEMF<HiAinous Phase ov Ebuftiyitt. 

1. Grysialline or Lavas 548 

2. Fiagmental or Tuffs 551 

Part YIIL — ^Thb Crtbtalukb Schists as Part of the Architecture of 

THE Earth^9 Crust. 

L GemebalOhaeaokebs .. 554 

IL Local Metamobfhish (Mbtamobfhish OF CoETAor OR JuxTAPOsnnoK) .. 557 

Floarhlng. 667-^Coloratlon, 667— Induration, 668— BzpolslaB of Water, 668— Prismatic 
StractnTe, 65&— Calcination, Melting, CoUng, 669— Mannarosls, 661— Production 
of New Minerals, 662— Production of Foliation, 663— Summary of Facts, 666. 

in. Bbgiokal (Normal) Metamobphish 568 

Illustrated by the Ardennes, 669— Taunus, 670— Scandinayia, 670— The Alps, 671— 
Scottish Highlands, 673— Greece, 677— Green Mountains, 677— Table showing the 
wide Range of Geological Qystems affected by Regional Metamorphism, 677— Num- 
mary, 678. 

IT. The Archjban Crtbtalline Sohists 580 

Part IX.— -Orb Deposits, 682. 

1. Mineral Veins or Lodes, 683— Variations 'In breadth, 684— Structure and Contents, 

684— Socoeesiye infilling, 686— Connection with faults, 687— Relation of Contents 
to Surrounding Bocks. 687 — ^Decomposition and Recomposition, 688. 

2. Stocks and Sto^-works, 688— Origin of Mineral Veins, 690. 

Part X. — ^Unoonformability, 591. 



BOOK V. 
pALiE0lnx)L0GicAL Geology, 695. 

DeflniUon of the term Fossil, 696. i. Conditions for the Entombment of Organic 
Remains, 606 : on Land, 606 — ^in the Sea, 608 — ^i1. Preservation of Organic R^nains 
in Mineral Masses, 600 — 1. Influence of Original Structure and Composition, 600 
—2. Fossilization, 600— iii. Relative Pakeontological Value of Organic Remains, 
sot— iv. Uses of Fossils in Geology, 603: They show (1) Changes in Physical 
Geology, 603 ; (2) Geological Chronology, 604 ; (3) Subdivisions of the Geological 
Record, 612 — ^v. fiearing of Pahrontologlcal Data upon Evolution, 614— vl. Doctrine 
of Colonies, 618— vli. 'Ilie Collecting of Fossils, 621. 



xiv CONTENTS, 



BOOK VI. 

Stratioraphical Geology. 

1>AGE 

Gkxeral PuxopLES 626 

Table of the Stratified Fonnatioiis oonBtitating the Crecdogical Record . 

2b/<Kej>. 631 



Part I. — ^Archjsak. 

1. General Cbaraotera 632 

2. Local Development 636 

BriUin. rw SrandinivU, M*— Gentnl Europe, 6lt— Anerica, fia— India, »U— 
China. MS— Anstralaida, UZ. 



Part IT. — Paleozoic 

1. Cambrian (Pboiordxal Silurian). 

1. General Characters 644 

2. Local Development 649 

Mtain, •l»-Crtnthiental EArope. C&l->Karth America, •67— China, CS4. 
n. SltrRIAX. 

1. €}eneral Charaeiera 659 

2. Looal Development 665 

Britain, M»->BaBlB of the Baltic, Riusia and Scandinavia, C^ss— Bohcnia, €«7— 
Weatarn Europe, f9S— Oenaanj, ftc^ CM->N<icth America, C9I~A5ia, Cfl— 
Anstralasia, CS3. 

ILL Devoxiax AKD Old Red Saxdstoxe 693 

(I.) Deronian T)fpf. 

1. General Ghaiacters 61H 

2. Local Developmeiit 699 

Britain, C9»-Ontra] Eorope. TOO— Ruteia, ;«3— N\4th Amenca, ?•«— Asia, 7«&. 
Anatralaria, 7 1&. 

(ii. / Old Bed Sanddome l^pe, 

1. General Chiiraciers '^qq 

2. Local Development 710 

Britain, 71»— Xorvaj. *c., 716— North Amenca, 717. 
IV. CARBONIFEROUS. 

1. General Characters^ .. 717 

•2. Looal Development 735 

British 1*)ca, 73»— Frauce and Belgium. 74.V-Qerman;>. T4S— AInk T45— l&»t«tii 
Eimi|»e. 74«— Atia. 74«— AnstraU^iia, Y47— Konh AmeriM. 747. 



CONTENTS, 



XV 



I'AGE 

Y. Pebmiak (Dyad). 

1. General Oharacters 748 

2. Local Deyelopment 753 

BriUio, ?S3— Oermany. &c., Tfii^RuggU, 755— France, 756— North America, 756 
—India, 757— Aitstralla, 757. 

Part III. — Mesozoic or Secondary. 

I. Tbiassio. 

1. General Characters 757 

2. Local Development 762 

Britain, 762— Central Korope, 765— Alps. 767— Scandinavia, 769— Xorth America, 770 
—Asia, 770— Aiutralia, 771— New Zealand, 771. 

IL JUBABSIC. 

1. General Characters 772 

2. Local Development .. .. 786 

Britain, 787— France and .the Jora, 709— Germany, 80a— Alps, 805— Riusia, SOS- 
North America, 805— Asia, 806— Australaeia, 806. 

IIL Cb£TACEOUS. 

1. Greneral Characters 806 

2. Local Development 819 

Britain, 820— France and Belgium, 829 — Germany, 834— Switserland and the Alp^ 
836 — Basin of the Me«lit4>rranean, 837— India, 837— North America, 837— Austral- 
asia, S40. 

Part IV. — Caikozoio or Tertiary, 840. 

I. KiK'EXE. 

1. Geuerul Characters 844 

2. Local Development 849 

Britain, 849— Northern France and Belgium, 852— Southern Europe, 855— India, &c., 
857— North America, 857 — Australasia, 858. 

II. Oligooene. 

1. General Clmiacters 858 

2. Local Development 861 

Britain, 861— Parin Basin, 863— Germany, 864— Switzerland, 865— Central France, 
865— Vienna Basin, 866— Italy. 866— North America, 866. 

Iir. Miocene. 

1. General Characters ^Q^ 

2. Local Development 870 

France, 870 — Relgium, 871 — Mainz Basin, i*71 — Vieuiia Ba^iu, bil — Switxeriaiul, 
872— Italy, 873— Greenland, 873— India, 874— North America, 874— New Zea- 
land, 875. 

IV. Pliocene. 

1. General Characters 875 

2. Local Development S7S 

Britain, 878— France, ssi— Belgium, ftal— Mainz Ba»iu, 8h1— Vienna Basin, K^2 
—Italy, 882 — Greece, k»3 — India, H«4 — North America, 886- Australia, 8S6— 
New Zealand, 886. 



xvi CONTENTS. 



PACK 

Part V. — Quaternary or Post-Tertiary, 

I. Pleistocene OR Glacial. 

1. General Gbaracters 887 

Pre-glacial Land-ourfaces, 888 — The Nortbern loeHBbeet, 889— loe-crumpled Bocks, 
893— Detritus of the Ice-sheet, Boulder-clay, Till, 894— Inter-gladal beds, 
895 — Evidences of Submergence, 897— Second Glaciation, Re-elevation, Raised 
Beache^ 898. 

2. Local Development 1M)1 

Britain, 901— Scandinavia, 903— GermAn}', 904— France, 90&— Belgium, 906— Alps, 
906— North America, 907— India, 911— Australasia, 912. 

II. Recent OR HuKAN Period. 

1. General Chaiaoten 912 

F»Icolithic Alluvia, 91fr— Brick-Earths, 916— Cavern Deposits, 916— Calcareous 
Tulas, 916— Loess, 916— PalsK}litbic Fauna, 917— Neolithic, 919. 

2. LoonlDeyelopmcnt 920 

Britain, 920— France, 921— Germany, 922— 6witierland, 922— Denmark, 922— North 
America, 922— Australasia, 923. 



BOOK VII. 
Physiographical Geology, 924. 

1. Terrestrial Features due more or less directly to Disturbance of the Crust, 927— 
9. Terrestrial Features due to Volcanic Action, 934—3. Terrestrial Features due 
to Denudation, 936. 

List or Aitthors quoted or betbrred to 9i4 

Index 953 



i 



GEOLOGY. 



INTRODUCTION. 

GvjoiJOGY itt tho scionce whioh inveBtigates the hiBtory of tho Earth. Its 
o])ject iti to traco tho progress of our planet from the earliest beginnings 
of its separate existence, through its various stages of growth, down to 
tho present condition of things. It unravels the complicated processes, 
involving vast geographical revolutions, by which each continent and 
country has been built up, tracing out the origin of their materials and 
the manner in which their existing outlines have been determined. It 
likewise follows into detail the varied sculpture of mountain and valley, 
crag and ravine. 

Nor does this science confine itself merely to changes in the inorganic 
world. Geology shows that the present races of plants and animals are the 
descendants of other and very different races that once peopled tho earth. 
It teaches that there has been a i>rogre88 of the inhabitants, as well as 
one of tho globe on which they have dwelt ; that each successive period 
in tho earth's history, since the introduction of living things, has l>een 
marked by characteristic types of the animal and vegetable kingdoms ; 
and that, how imperfectly soever they may have been preserved or may 
1x3 deciphered, materials exist for a history of life upon tho planet. The 
geographical distribution of existing faunas and floras is often made 
clear and intelligible by geological evidence; and in a similar way, 
liglit is thrown upon some of the remoter phases in tho history of man 
himself. 

A subject so comprehensive as this must require a wide and vaiied 
basis of evidence. One of the characteristics of geology is to gather 
evidence from sources which, at first sight, seem far removed from its 
scope, and to seek aid from almost every other leading branch of science. 
Thus, in dealing vnth the earliest conditions of the planet, the geologiKt 
must fully avail himself of the lalwurs of the astronomer. Whatever is 
ascertainable by telescope, Bi)ectrosoope, or chemical analysis, regarding 
the constitution of other heavenly bodies, has a geological Iwariug. 
The experiments of the physicist, undertaken to determine conditions of 
matter and of energy, may sometimes be taken as the starting-point 
of geological investigation. The work of tho chemical lalM)ratory fVjnns 
the foundation of a vast and increasing mass of geological in(|uiry. To 
tho botanist, tho zoologist, oven to the unscientific, if observvwil. 



JNTBODUCTION. 



t- .- 



traveller by land or sea, the geologist turns for information and 
asBistance. 

But while thus culling freely from the dominions of other sciences, 
geology claims, as its peculiar territory, the rocky framework of the 
globe. In the materials composing that framework, their composition 
and arrangement, the processes of their formation, the changes which 
they have individually undergone, and the grand terrestrial revolutions 
to which they bear witness, lie the main data of geological history. Ik 
is the task of the geologist to group these elements in such a way that 
they may be made to yield up their evidence as to the march of events 
in the evolution of the planet. Ho finds that they have in large 
measure arranged themselves in chronological sequence, — the oldest 
lying at the bottom and the newest at the top. Kelics of an ancient 
sea-floor are overlaid with traces of a vanished land-surface ; these are in 
turn covered by the deposits of a former lake, above which once more 
appear proofs of the return of the sea. Among these rocky records, lie 
the lavas and ashes of long-extinct volcanoes. The ripple left upon 
a sandy beach, the cracks formed by the sun's heat upon the muddy 
bottom of a driod-up pool, the very imprint of the drops of a passing 
rain-shower, have all been accurately preserved, and often l)ear witness 
to geographical conditions widely diflferent from those that exist where 
such markings are now found. 

But it is mainly by the remains of plants and animals iml>edded in 
the rocks that the geologist is guided in unravelling the chronological 
succession of geological changes. He has found that a certain order 
of appearance characterises these organic remains ; that each successive 
group of rocks is marked by its own special types of life ; that these 
types can be recognised, and the rocks in which they occur can be corre- 
lated, even in distant countries, where no other means of comparison are 
available. At one moment, he has to deal with the bones of some large 
mammal scattered through a deposit of superficial gravel, at another 
time, with the minute foraminifers and ostracods of an upraised sea- 
lx)ttom. Corals and crinoids, crowded and crushed into a massive 
limestone on the spot where they lived and died, ferns and terrestrial 
plants matted together into a bed of coal where they originally grew, 
the scattered shells of a submarine sand-bank, the snails and lizards 
that left their mouldering remains within a hollow tree, the insects 
that have been imprisoned within the exuding resin of old forests, the 
footprints of birds and quadrupeds, or the trails of worms left upon 
former shoi-os — these, and innumerable other pieces of evidence, enable 
the geologist to realise in some measure what the vegetable and animal 
life of successive periods has been, and what geographical changes the 
site of every land has undergone. 

It is bvident that to deal Buccessfully with these varied materials, a 
t?i '»«i<^^?qMto.§igjpgri|ifaTice with different branches of science is desirable. 

kte the knowledge which the geologist has of 
V the more interesting and fruitful will bp 




INTRODUCTION. 3 



his own reBearohes.. Pi-oin ite very nature, geology demands on the 
part of its votaries, wide sympathy with investigation in ahnost every 
branch of natural science. Especially necessary is a tolerably largo 
acquaintance with the processes now at work in changing the surface of 
the earth, and of at least those forms of plant and animal life whose 
remains are apt to be preserved in geological deposits, or which, in their 
structure and habitat, enable us to realise what their forerunners were. 

It has often been insisted upon that the present is the key to the 
past ; and in a wide sense this assertion is eminently true. Only in 
proportion as we understand the present, where everj^thing is open 
on all sides to the fullest investigation, can we expect to decipher. the 
past, where so much is obscure, im|)erfectly preserved, or not preserved 
at alL A study of the existing economy of nature ought evidently to 
be the foundation of the geologist's training. 

While, however, the present condition of things is thus employed, 
we must obviously be on our guanl against the danger of unconsciously 
assuming that the phase of nature's operations which we now witness 
has been the same in all past time ; that geological changes have taken 
place, in former ages, in the manner and on the scale which wo behold 
to-day, and that at the present time all the great geological processen, 
which have produced changes in past eras of the earth's history, are 
still existent and active. Of course, we may assume this uniformity of 
action, and use the assumption as a working hypothesis. But it ought 
not to be allowed a firmer footing, nor on any account 1)0 suifered to 
blind us to the obvious truth that the few centuries, wherein man has 
been deserving nature, form much too brief an interval, l)y which to 
measure the intensity of geological action in all past time. For aught 
we can toll, the present is an era of quietude and slow change, compared 
witli some of the eras that have preceded it. Nor can we bo sure that 
when we have explored every geological process now in progress, we 
have exhausted all the causes of change wliich, even in comparatively 
recent times, have ]>een at work. 

In dealing with the Geological Record, as the accessible solid part of 
the globe is called, wo cannot too vividly realise that, at the iKist, it 
forms but an imperfect chronicle. Geological history cannot Iw com- 
piled from a full and continuous series of documents. Owing to the 
very nature of its origin, the reconl is necessarily from the first frag- 
mentary, and it has been further mutilated and obscured by the 
revolutions of successive ages. Even where the chronicle of evontH is 
continuous, it is of very unequal value in different places. In one 
case, for example, it may present us with an unbroken succession of 
deposits, many thousands of feet in thickness, from wliich, however, 
only a few meagre facts as to geological history can be gleaned. In 
another instance, it brings before us, within the compass of a few yards, 
the evidence of a most varied and complicated series of changes in 
physical geography, as well as an abundant and interesting suite of 
.organic remains. These and other characteristics of the geological 

1^ 1 



INTRODUCTION. 



record will become more apparent and intelligible to the Btudent as ho 
proceeds in the study of the science. 

In the present volume the subject will be distributed under the 
following leading divisions. 

1. The Cosmical Aspects of Geology, — It is desirable to realise some 
of the more important relations of the earth to the other members of 
the solar system, of which it forms a part, seeing that geological pheno- 
mena are largely the result of these relations. The form and motions of 
the planet may be briefly touched upon, and attention should be 
directed to the way in which these planetary movements influence 
geological change. The light cast upon the early history of the earth 
by researches into the composition of the sun and stars deserves notice 
here. 

2. Geognosy — An Inquiry into the Materials of the EartKs Svhstance, — 
This division describes the constituent parts of the earth, its envelopes 
of air and water, its solid crust, and the probable condition of its interior. 
Especially, it directs attention to the more important minerals of the 
crust, and the chief rocks of which that crust is built up. In this way, 
it lays a foundation of knowledge regarding the nature of the matoriais 
constituting the mass of the globe, whence we may next proceed to 
investigate the processes by which these materials are produced and 
altered. 

8. Dynamical Geology embraces an investigation of the operations 
which lead to the formation, alteration, and disturbance of rocks, and 
calls in the aid of physical and chemical experiment in elucidation of 
these operations. It considers the nature and operation of the processes 
that have determined the distribution of sea and land, and have moulded 
the forms of the terrestrial ridges and depressions. It further investi- 
gates the geological changes which are in progress over the surface of 
the land and floor of the sea, whether these are due to subterranean 
disturbance, or to the effect of operations above ground. Such an 
in([uiry necessitates a careful study of the existing economy of nature, 
and forms a fitting introduction to the investigation of the geological 
changes of former periods. This and the previous section, including 
must of what is embraced under Physical Geography and Petrogeny or 
Geogeny, will here bo discussed more in detail than is usual in geologi- 
cal treatises. 

4. Geotectonic, or Structural Geology — the Architecture of the Earth. — This 
hection of the investigation, applying the results arrived at in the 
previous division, discusses the actual arrangement of the various 
materials composing the crust of tlie earth. It proves that some have 
l)eeu formed in beds or strata, whetlier by the deposit of sediment on 
the floor of the sea, or by the slow aggregation of organic forms, that 
others have been poured out from subterranean sources in sheets of 
molten i-ock, or in showers of loose dust, which have been built up into 
mountains and plateaux. It further showD that rocks originally laid 
down in almost horizontal beds have subsequently been crumpled. 



INTRODUCTION. 



contorted; dislocated, invaded by igneous masses from below, and ren- 
dered sometimes intensely crystalline. It teaches, too, that wherever 
exposed above sea-level, they have been incessantly worn down, and have 
often been depressed, so that older lie buried beneath later accumu- 
lations. 

5. Palseontologtcal Geology, — This branch of the subject deals with the 
organic forms which are found preserved in the rocks of the ci-ust of the 
earth. It includes such questions as the manner in which the remains 
of plants and animals are entombed in sedimentary accumulations, the 
relations between extinct and living types, the laws which appear to 
have governed the distribution of life in time and in space, the nature and 
use of the evidence from organic remains regarding former conditions 
of physical geography, and the relative importance of different genera of 
animals and plants in geological inquir}\ 

6. Stratigraphical Geology. — This -section might be called Geological 
Ilistory, or Historical Geology. It works out the chronological succession 
of the great formations of the earth's crust, and endeavours to trace the 
sequence of events of which they contain the record. More particularly, 
it determines the order of succession of the various plants and animals 
which in past time have peopled the earth, and thus, by ascertaining what 
has been the grand march of life upon the planet, seeks to unravel the 
story of the earth as made known by the rocks of the crnst. 

7. Physiographtcal Geology, starting from the basis of fact laid down 
by stratigraphical geology regarding former geographical changes, 
oiiibraces an inquiry into the history of the present features of the earth's 
surface — continental ridges and ocean basins, plains, valleys, and moun- 
tains. It investigates the structure of mountains and valleys, compares 
tlie mountains of different countries, and ascertains the relative geological 
dates of their upheaval. It explains the causes on which local differences 
of scenery depend, and shows under what very different circumstances, 
and at what widely separated intervals, the varied contours, even of a 
single country, have l)een produced. 



(; COSMIC AL ASPECTS OF GEOLOGY. [Book I. 



BOOK I. 

COSMICAL ASPECTS OF GEOLOGY. 

Bkfouk goulogy had attained to the position of an inductive science, 
it was customary to begin all investigations into the history of the earth 
hy propounding or adopting some more or less fanciful hypothesis, in 
explanation of the origin of our planet or of the universe. Such pre- 
liminary notions were looked npon as essential to a right understanding 
of the manner in which the materials of the glohe had been put together. 
To the illustrious James KntUm (1785) geoh)gi8ts are indebted, if 
not for originating, at least for strenuously upholding the doctrine that 
it is no part of the province of geology to discuss the origin of things. 
lie taught them that in the materials from which geological evidence is 
to be compiled there can be found " no traces of a beginning, no prospect 
of an end." In England, mainly to the influence of the school which he 
founded, and to the subsequent rise of the Geological Society (1807), 
which resolved to collect facts instead of fighting over hypotheses, is due 
the disappearance of the crude and unscientific cosmologies of previous 
centuries. 

But there can now be little doubt that in the reaction against the 
visionary and often grotesque speculations of earlier writers, geologists 
were carried too far in an opposite direction. In allowing themselves to 
believe that geology had notliing to do with questions of cosmogony, 
tliey gradually grew up in the conviction that such questions could never 
be otlier than mere speculation, interesting or amusing as a theme for 
tlie employment of the fancy, but hardly coming within the domain of 
sober and inductive science. Nor would thev soon have been awakened 
out of this belief by anything in their own science. It is still true that in 
the data with which they are accustomed to deal, as comprising the sum 
of geological evidence, there can l)e found no trace of a beginning, though 
there is ample proof of constant, upward progression from some invisible 
starting-point. The oldest rocks which have been discovered on any 
part of the globe have probably been derived from other rocks older than 
themselves. Geology by itself has not yet revealed, and is little likely 
ever to reveal, a portion of the first solid crust of our glolje. If, then, 
gec^logical history is to be compiled from direct evidence furnished by 
the rocks of the earth, it cannot l>egiu at the })eginning of things, but 
must be content to date its first chapter from the earliest period of which 
any record has been preserved among tlie rocks. 

Nevertheless, though, in its usual restricted sense, geology has been, 
and must ever be, unable to reveal the earliest history of our jdanet, it 



Book I.] THE EABTIl IN THE SOLAR SYSTEM. 



no longers ignores, as mere speculation, what is attempted in this subject 
by its sister sciences. Astronomy, physics and chemistry have in late 
yeai-s all contributed to cast much light on the earliest stages of the 
earth's existence, previous to the beginning of what is commonly regarded 
as geological history. Whatever extends our knowledge of the former 
conditions of our globe may be legitimately claimed as part of the domain 
of geological inquiry. If Geology, therefore, is to continue worthy of its 
name as the science of the earth, it must take cognisance of these recent 
contributions from other sciences. It can no longer be content to begin 
its annals with the records of the oldest rocks, but must endeavour to 
gro[>o its way through the ages which preceded the formation of any 
rouks. Thanks to the results achieved with the telescope, the spectro- 
scope, and the chemical laboratory, the story of these earliest ages of our 
earth is every year becoming more definite and intelligible. 

I. ItELATlOKS 01' TilK EaUTII IN THE SoLAU SYSTEM. 

As a prelude to the study of the structure and history of the earth, 
some of the general relations of our planet to the solar system may here 
1k) noticed. The investigations of recent years, showing the community 
of substance between the different members of that system, have revived 
and have given a new form and meaning to the well-known nebular hypo- 
tlicji^is of Kant, Laplace and W. Herschel, which sketched the progress of 
the system from the state of an original nebula to its existing condition 
of a central incandescent sun with surrounding cool planetary bodies. 
According to this hyix)thesis, the nebula, originally diffused at least 
as far as the furthest member of the system, began to condense towards 
the centre, and in so doing threw off or loft behind successive rings. 
These, on disruption and further condensation, assumed the form of 
planets, sometimes with a further formation of rings, which in tlie case 
of Saturn remain, though in otlier planets they have broken up and 
united into stitellites. 

Accepting this view, we should exi)eet the matter composing the 

various members of the solar system to be everywhere nearly the same. 

Tlie fact of condensation round centres, however, indicates probable differ- 

cuc .s of density tliroughout the nebula. Tliat the materials compiising 

the nebuhi may liavc arranged themselves according to their respective 

<\«i»sities, tlie lightest occupying the exterior, and the heaviest the 

iiitt'iior of the mass, is suggested by a comparison of the densities of the 

Vivrions planets. These densities are usually estimated as in the follow- 

iii'j; table, that of the earth being taken as the unit : — 

u-25 

1 12 

I'iV.i 

l-0(» 

0-70 

. 0-2i 

o-i:j 

017 



Density 


of tlie Sun 




Mrrcury 




VlMlU8 . 




Kartli . 




Mars 




JupitcT . 




Saturn . 




Uninu8 . 


•I 


Neptune 



COSMIC AL ASPECTS OF GEOLOGY. [Book I. 



It 10 to be olMerved, however, that " the densities here given are mean 
densities, a88tiniing that the apparent size of the planet or sun is tho true 
hize, i.e.j making no allowance for thousands of miles deep of cloudy 
atmosphere. Hence tho numbers for Jupiter, Saturn, and Uranus are 
certainly too small, that for the sun, much too smalL" ^ Taking the 
figures as they stand, while they do not indicate a strict progression in 
the diminution of density, they state that the planets near the sun 
jKiHsess a density about twice as great as that of granite, but that those 
U-ing towards the outer limits of the s\-st«m are composed of matter as 
light as cork. Again, in some cases, a similar relation has l)een observed 
lietween the densities of tho satellites and their primaries* The moon, 
fiir example, has a density little more than half that of the earth. Tho 
first satellite of Jupiter is less dense, though the other three are found to 
lie more dense than tho planet. Further, in the condition of the earth 
itsf^lf, a very light gaseous atmosphere forms the outer portion, l)eneath 
which lies a heavier layer of water, while within these two envelopes the 
materials forming the solid substance of the planet are so arranged that 
tlie outer layer or crust has only about half the density of the whole 
glolie. Mr. Lockyer finds in the sun also evidence of the same tendency 
towanls a stratified arrangement in accordance with relative densities, as 
will 1)0 immediately fui*ther alluded to. 

There seems, therefore, to be much probability in the hypothesis that, 
in the gradual condensation of the original nebula, each successive mass 
left iKshind represented the density of its parent shell, and consisted of 
jirogi'oasively heavier matter.^ The remoter planets, with their low 
densities and vast absorbing atmospheres, may l)e supposed to consist of 
metalloids, like the outer parts of the sun's atmosphere, whDe the interior 
planets are no doubt mainly metallic. The rupture of each planetary 
ring would, it is conceived, raise the temperature of the resultant 
nebulous planet to such a height as to allow the va|)our8 to rearrange 
tliemselves by degrees in successive layers, or rather shells, according to 
densities. And when the planet gave off a satellite, that body might be 
expected to possess tho composition and density of the outer layers of its 
primaiy.^ 

For many years, the only evidence available as to the actual com- 
position of other heavenly bodies than our own earth was furnished by 
the aerolites^ meteorites, or fallen stars, which from time to time have 
entered our atmosphere from planetary space, and have descended upon 
the surface of the globe.* Sulrjected to chemical analysis, these foreign 

' Profesnor Tait, M8. note. 

■ On the origin of Sattjllites, see the researches of Prof. G. H. Darwin, Phil, Tram, 
(18710 olxx. i>. 535. Proc. Hoy. Soc. xxx. p. 1. 

^ l.«M'kvcr in Prestwicli's Jnavgural Lecture, Oxford, 1875, and in Manche8t<>r 
lirrturcM, Why (h** KariKs Clufinistry in a» it id. Readers interested in the historical 
ih'V(;lo{»inent of geological opinion will tind much suggestive matter bearing on the 
queHti(in» diicniwed alx)vo, in Do la Keche's 'Kesearches in Theoretical Geology,' 1834, 
—a work noialtly in culvanco of its time. 

* On mtteoriiei oontnlt Partsch, * Die Metcoriten,* Vienna, 1843. Rose, Ahhand, htminl 

Akwk B«iUn, lfi88. Bammelsberg, * Die Chcmiflchc Natnr der Meteoriteu,' 1870. Tscher- 

.iifflM!'* ^**«* Wimen. Vienna (1875) Ixxi. Daubree'd 'Etudes Synthc^tiquefl do 



Book L] SPECTROSCOPIC ANALYSIS. 9 

bodies show considerable divei*8ities of compoBition ; but in no case have 
they yet revealed the existence of any element not already recognised 
among terrestrial materials. Upwards of twenty of our elements have 
been detected in aerolites, sometimes in the free state, sometimes com- 
bined with each other. More than half of them are metals, including iron, 
nickel, manganese, calcium, sodium, and potassium. There occur also, 
carbon, silicon, phosphorus, sulphur, oxygen, nitrogen, and hydrogen. 
In some of their combinations, these elements, as found in the meteoric 
stones, differ from their mode of occurrence in the accessible parts of the 
earth. Iron, for example, occurs as native metal, alloyed with a variable 
proportion (6 to 10 per cent.) of metallic nickel. But, in other respects, 
they closely resemble some of the familiar materials of the earth's rocky 
crust. Thus we have such minerals as chromic iron, pyrite, apatite, 
olivine, augite, enstatite, hornblende and labradoritc. Some meteorites, 
have l)een found to contain large quantities of occluded gas, particularly 
hydrogen and carbonic oxide. ^ No more convincing proof could ho 
ilesired that some at least of the other members of the solar system arc 
ftirmed of the same materials as compose the earth. 

But, in recent years, a far more precise and generally available method 
of research into the composition of the heavenly bodies has been found in 
the application of the spectroscope. By means of this instrument, the 
light emitted from self-luminous bodies can be analysed in such a way as 
to show what elements are present in their intensely hot luminous vapour. 
When the light of the incandescent vapour of a metal is allowed to 
j>ji88 through a properly arranged prism, it is seen to give a spectrum 
eoiiHisting of transverse bright lines only. This is termed a radinfion- 
»l)4tetrwn. Each element appears to have its own characteristic arrange- 
ment of lines, whicli in general retain the same relative position, intensity 
and colours. Moreover, gases and the vapours of solid bodies are found to 
intercept those rays of light which they themselves emit. The spectrum 
c»f wxlium- vapour, for example, shows two bright orange lines. If there- 
fore white light, from some hotter light-source, passes through the vapour 
of sodium, these two bright lines become dark lines, the light being 
exactly cut off which would have been given out by the sodium itself. 
I'his is called an ahsorption'Spectrmn. 

From this method of examination, it has been inferred that many of 
the elements of which our earth is composed must exist in the state of 
incandescent vapour in the atmosphere of the sun. Thirtj'-two metals 
have l)een thus identified, including aluminium, barium, manganese, lead, 
calcium, cobalt, potassium, iron, zinc, copper, nickel, sodium and 
magnesium. These elements, or at least su})staiices which give the same 
groups of lines as the terrestrial elements with which they have been 



(;.'-.»lof^ic Experimentalo/ 1879. W. Flight, Geol Mng. 1875, Pop. Scl. Jiev. new 
*^'r. i. j>. WM). J. Gttllo and A. vun Liwanlx, MonntAhericht hi'mujl. Akad. BorHn, July, 1 879. 
K. Vo«jjt, on Huppospd ornranisnis in Meteorites, Mem. Tnxt. Nat. Getievd, xv. (l88Ji). 
' S-e A. AV. Wrij,'ht, Amr. Jovrn, ser. H, xi. p. 2'y^: xii. p. IG.'i. W. Flifrht, /Vrx?. 



10 COSMIOAL ASPECTS OF GEOLOGY. [Book I. 



identified, do not occur promiscuously diffused throughout the outer 
mass of the sun. According to Mr. Lockyev's observations, they appear to 
succeed eacli other in relation to their respective densities. Tims the 
coronal atmosphere which, as seen in total eclipses, extends to so prodigious 
a distance beyond the disc of the Bun, consists mainly of subincandescent 
hydrogen and another element which may he new. Beneath this external 
vaporous enveloi)e lies the chromosphere, where the vapours of incan- 
descent hydrogen, calcium, and magnesium can be detected. Further 
inward the spot-zone shows the presence of sodium, titanium, &c. ; while 
still lower, a layer (the reversing layer) of intensely hot vapours, lying 
pro})al)ly next to the inner brilliant j)hot08phere, gives spectroscopic 
evidence of the existence of inciindescent iron, manganese, cobalt, nickel, 
chopper, and other well-known terrestrial metals.* 

It is to be observed, however, that in these spectroscopic researches the 
decomposition of the elements by electrical action was not considei^ed. 
The conclusions embodied in the foregoing paragraph have been founded 
on the idea that the lines seen in the spectrum of any element are all due 
to the vibrations of the molecules of that element. But Mr. Lockyer has 
suggested that this view may after all be but a rough approximation 
to the truth, and that it may be more accurate to say, as a result of the 
facts alrea<ly acquired, that there exist basic elements common to 
calcium, iron, <fec., and to the solar atmosphere.^ 

The spectroscope has likewise l)een successfully applied by Mr.Huggins 
and others to the observation of the fixed stars and nebula*, with the 
result of establishing a similarity of elements between our own system 
and other bodies in sidereal space. In the radiation spectra of nebulae, 
Mr. Huggins finds the hydrogen lines very prominent ; and he conceives 
that they may be glowing masses of that element. Professor Tait has 
suggested, on the other hand, that they are more probably clouds of 
stones frequently colliding and thus giving off incandescent gases. Sir 
William Thomson appears to favour this view. Among the fixed stara, 
absori)tion-spectra have been recognised, pointing to a structure resem- 
1)1 ing that of our sun, viz., an incandescent nucleus which may be solid or 
liquid or of very highly compressed gas, but which gives a continuous 
spectrum and which is surrounded with an atmosphere of glowing 
vapour.^ 

' On spectroscopic research as upplietl to the sun, see Kirchhoff and Buusen, 
* Kesoarches on Solar Si>cctrum,' &c., 1863 ; Angstrom, * Rechcrches sur le Spectre 
normal du Soleil'; Lockyer, * Solar Physics/ 1873, and 'Studies in Spectrum Analysis * 
(lutornational Series), 1878; Huggins and Miller, Proc, Roy. Soc. xii. Phil. Tram. 1864; 
Ilo;3C(>o's * Spectrum Analysis,' witli authorities there cited. An ingenious theory to 
account for the conservation of solar energy was suggested by the late Sir O. W. 
Siemens {Proc, Roy. Soc. xxxiii, (1881) p. 389), It reipiires the presence of aqueous 
vajwur and carbon compounds in stellar spaco, which are dissociated and drawn into 
the solar photosphere, where tlicy buret into flame with a large development of heat, 
and then passing into aqueous vapour and carlnniic anhydride or oxide, flow to the solar 
equator whence they are projected into space. 

^ See also the opposite views of Dewar and Jiivcing, Proc. Roy. Soc. xxx. p. 93, and 
II. W. Vogel, Nalnrey xxvii. p. 233. 

* Huggins, Proc. Roy. Soc. 1863-6G, and Brit. Aswc. Lecture (Nottingham, 1866) ; 
Huggins and Miller, Phil. Trau9. 1864. 



Book IJ FOBM AND SIZE OF THE EARTH, 11 

According to Mr. Lockyer, those stars which have the highest temperature 
have the simplest spectra, and in proportion as they cool their materials 
become more and more differentiated into what we call elements. He 
remarks that the most brilliant or hottest stars show in their spectra only 
the lines of gases, as hydrogen. Cooler stars, like onr sun, give indications 
of the presence, in addition, of the metals — ^magnesium, sodium, calcium, 
iron. A still lower temperature he regards as marked by the appearance 
of the other metals, metalloids, and compounds.^ The sun would thus bo 
a star considerably advanced in the process of differentiation or asso- 
ciation of its atoms. It contains, so far as we know, no metalloid except 
carlxin, and^ possibly oxygen, nor any compound, while stars like Sirius 
show the presence only of hydrogen, with Init a feeble proportion of 
metallic vapours ; and on the other hand, the red stars indicate by their 
spectra that their metallic vapours have entered into combination, 
whence it is inferred that their temperature is lower than that of our sun. 

11. Form and Size of the Earth. 

Further confirmation of the foregoing views as to the order of 
planetary evolution is furnished by the form of the earth and the 
arrangement of its component mateiials. 

ITiat the earth is an oblate spheroid, and not a perfectly spherical 
glol)e, was discovered and demonstrated by Newton. He even calcu- 
lated the amount of ellipticity long before any measurement had 
confirmed such a conclusion. During the present century' numerous 
arcs of the meridian have been measured, chiefly in the northern 
hemisi)here. From a scries made by different observers between the 
latitudes of Sweden and the Cape of Goo<l Hope, Bessel obtained the 
following data for the dimensions of the earth : — 

Ejiuutoriul diameter . . 41,847,192 feet, or 7925-004 miles. 
Polar diameter . . . 41,707,314 „ 7899-114 „ 
Amount of polar flattening . 139,768 „ 20*471 „ 

The efj[uatorial circumference is thus a little less than 25,000 miles, 
and tlie difference between the polar and equatorial diameters (nearly 
2o.J miles) amounts to about ^jJo^-^^ ^^ ^^^^ equatorial diameter.^ More 
recently, however, it has been shown that the oblate spheroid indicated 
by these measurementH is not a symmetrical body, the equatorial cireum- 
fen.nce being an ellipse instead of a circle. I'he greater axis of tlie 
equator lies in long. 8^ 15' W. — a meridian passing through Ireland, 
Portugal, and the north-west corner of Africa, and cutting off the n(^»rtli- 
east Corner of Asiji in the opj)osite hemisphere.^ 

The polar flattening, established by measurement and calculation, as 
that which would necessarily have been assumed b}' an originally plastic 
>:lobe in obe<lienee to the movement of rotation, has been cited as 
eviilcnee that the earth was once in a plastic condition. Taken in 

' L«jckyer, Compie« i'eiulutt^ iJoe. 1873. 

- Ilemohel, * Astronomy,' [k 139. 

* A. R. Clarke, riiiL Miuj. Auguat 1878; Eiuyclopxdia BriUuintcHj Utli edit. x. 172. 



12 C08MT0AL ASPECTS OF GEOLOGY, [Book I. 

connection with the analogies supplied by the sun and other heavenly 
bodies, this inference seems well grounded.^ 

Though the general spheroidal form of our planet, and possibly the 
general distribution of sea and land, are referable to the early effects of 
rotation on a fluid or viscous mass, it is certain that the present details 
of its surface-contours are of comparatively recent date. Speculations 
have been made as to what may have been the earliest character of the 
solid surface, whether it was smooth or rough, and particularly whether 
it was marked by any indication of the existing continental elevations 
and oceanic depressions. So far as we can reason from geological 
evidence, there is no proof of any uniform superficies having ever 
existed. Most probably the first formed crust broke up ' irregularly, 
and not until after many successive corrugations did the surface 
ac({uiro stability. Some writers have imagined that at first the ocean 
spread over the whole surface of the planet. But of this there is not 
only no evidence, but good reason for believing that it never could have 
taken place. As will be alluded to in a later page, the preponderance 
of water in the southern hemisphere, seems to indicate some excess of 
density in that hemisphere. This excels can hardly have been proiiuced 
by any change since the materials of the interior ceased to \re mobile ; it 
must therefore be at least as ancient as the condensation of water on the 
earth's surface. Hence there was probably from the beginning a 
tendency in the ocean to accumulate in the southern rather than in the 
northern hemisphere. 

That land existed from Ihe earliest ages of which we have any 
record in rock-formations, is evident from the obvious fact that these 
formations themselves consist in great measure of materials derived from 
the waste of land. When the student, in a later part of this volume, is 
presented with the proofs of the existence of enormous masses of 
sedimentary deposits, even among some of the oldest geological systems, 
he will perceive how important must have been the tracts of land that 
could furnish such piles of detritus. 

The tendency of modern research is to give probability to the 
conception, first outlined by Kant, that not only in our own solar system, 
but throxLghout the regions of space, there has been a common plan of 
evolDtion, and that the matter diffused through space in nebulce, stars, 
and planets is substantially the same as that with which we are familiar. 
Henco the study of the structure and probable history of the sun and 

* It was opposed, however, by Mohr (* Geschichte der Erde,* p. 472), who, adopt- 
ing a suggestion long ago mode by Playfair, endeavoured to show that the 
polar flattening can bo accounted for by greater denudation of the polar tracts, ex- 
posed A8 tiiese nave been by the heaping up of the oceanic waters towards the equator 
ui conaeqaenoe of rotation. He dwelt chiefly on the eficcts of glaciers in lowering 
the londflmt as PfaflT has pointed out, the work of erosion is chiefly jierformed by other 
ttbnnspbciio icacm that operate rather towards the equator than the poles (* Allgemeine 
Geokttia i^ e^el^tnMiseliall,' p. 6). Compare Kaumann, News Jahrb. 1871 , p. 250. 
Nui iJilllillii l^;liMaW ^^^ Recalled attention to a conceivable cause by which, 

ft^ilttHBMHMHlipA^^ equatorial subsidence, the external form of the planet 



* .. ' 



Book L] THE EARTH'S MOVEMENTS. 13 



the other heavenly bodies oomes to possess an evident geological interest, 
seeing that it may yet enable ns to carry back the story of our planet 
far beyond the domain of ordinary geological evidence, and upon data not 
lees trustworthy than those furnished by the rocks of the earth's crust, 

III. The Movements of the Earth in their Geological Kelations. 

We are here concerned with the earth's motions in so far only as they 
materially influence the progress of geological phenomena. 

§ 1. Rotation. — In consequence of its angular momentum at its 
original separation, the earth rotates on its axis. The rate of rotation 
has once been much more rapid than it now is (p. 20). At present a 
complete rotation is performed in about twenty-four hours, and to it is 
due the succession of day and night. So far as observation has yet gone, 
this movement is uniform, though recent calculations of the influence of 
the tides in retarding rotation tend to show that a very slow diminution 
of the angular velocity is in progress. If this be so, the length of the 
day and night will slowly increase until finally the duration of the day 
and that of the year will be equal. The earth will then have reached 
the condition into which the moon has passed relatively to the earth, 
one half being in continual day, the other in perpetual night. 

The linear velocity due to rotation varies in different places, according 
to their position on the surface of the planet. At each pole there can be 
no velocity, but from these two points towards the equator there is a 
continually increasing rapidity of motion, till at the equator it is equal 
to a rate of 607 yards in a second. 

To the rotation of the earth are due certain remarkable influences upon 
currents of air circulating either towards the equator or towards the 
jH>les. Currents which move from polar latitudes travel from parts of 
the earth's surface where the velocity due to rotation is small, to others 
where it is great. Hence they lag behind, and their course is bent more 
and more westward. An air current, quitting the north polar or north 
teiui>crato regions as a north wind, is deflected out of its course, and 
iKxomes a north-oast wind. On the^opposite side of the equator, a similar 
current sotting out straight for the eijuator, is changed into a south-east 
wind. Hence, as is well known, the Trade-winds have their charac- 
teristic westward deflection. On the other hand, a current setting 
out northwards or southwards from the equator, passes into regions 
liaving a less velocity due to rotation than it possesses itself, and hence 
it travels on in advance and appears to be gradually deflected eastward. 
The aerial currents, blowing steadily across the surface of the ocean 
towar<ls the C(iuator, i)roduce oceanic currents which unite to form the 
westward-flowing E([uatorial current. 

It has been maintained by Von Baer,* that a certain deflection is 

* " Ueber ein aU^omcincs Gcsetz in der Gestaltunp: der Fhissbetten." Bull, Acad, 
Hi. Ptieribourg, ii. (IHGO). See also Ferrcl on the motions of fluids and solids relatively 
to the earth's surface. Caml). (Mass.) Math. Monthly, vols. i. and ii. (1859-60). Dulk. 
Z. Dfni$ch. G(ol. Ges. xxxi. (1879) p. 224. The Kiver Irtiscli is said in flowing north- 
ward to have cat eo much into its right bank that villages arc gradually driven ea^t- 



14 COSMIC AL ASPECTS OF GEOLOGY. [Book I. 

exporienced by rivers that flow in a meridional direction, like the Volga 
and Irtisch. Those travelling polewards are asserted to press upon their 
eastern rather than their western banks, while those which run in the 
opposite direction are stated to be thrown more against the western than 
the eastern. When, however, we consider the comparatively small 
volume, slow motion, and continually meandering course of rivers, it 
may reasonably be doubted whether this vera causa can have had much 
eft'ect generally in modifying the form of river-channels. 

§ 2. Revolution. — Besides turning on its axis, the globe performs 
a movement round the sun, termed revolution. This movement, 
accomplished in rather more than 365 days, determines for us the 
length of our year, which is, in fact, merely the time required for one 
complete revolution. The path or orbit followed by the earth round the 
sun is not a perfect circle but an ellipse, with the sun in one of the foci, 
the mean distance of the earth from the sun being 92,800,000, the present 
aphelion distance 94,500,000 and the perihelion distance, 91,250,000 miles. 
By slow secular variations, the form of the orbit alternately approaches to 
and recedes from that of a circle. At the nearest possible approach 
l)etween the two bodies, owing to change in the ellipticity of the orbit, 
the earth is 14,308,200 miles nearer the sun than when at its greatest 
possible distance. These maxima and minima of distance occur at vast 
intervals of time.^ The last consideralde eccentricity took place about 
200,000 years ago, and the previous one more than half a million 
years earlier. Since the amount of heat received by the earth from 
the sun is inversely as the square of the distance, eccentricity may 
have had in past time much effect upon the climate of the earth, as will 
be i^ointed out further on (§ 8). 

§ 3. PreceBsion of the Equinoxes. — If the axis of the earth were 
porpeudicular to the plane of its orbit, there would be equal day and 
night all the year round. But it is really inclined from that position at 
an angle of 23° 27' 21." Hence our hemisphere is alternately presented 
to and turned away from the sun, and, in this way, brings the familiar 
alternation of the seasons. Again, were the earth a perfect sphere, of 
uniform density throughout, the position of its axis of rotation would 
not be changed by attractions of external bodies. But owing to the 
protuberance along the equatorial regions, the attraction chiefly of the 
moon and sun tends to pull the axis aside, or to make it describe a 
conical movement, like that of the axis of a top, round the vertical. 
Hence each pole points successively to diflerent stars. This movement, 
called the precession of the equinoxes, in combination with another 
smaller movement, due to the attraction of the moon (called nutation), corn- 



ward^, Boinianak havlDg been shifted about a mile in 240 years {Nature, xy. p. 207). But 
this may be acoonnted for by local causes. See an excellent paper oa this subject with 
speoial roferenw to the regime of some rivers in northern Germany, by F. Klockmaun. 
Mfh* iVrtiM. (ML XoiulttKiml. 1882. Bee also £. Dunker, Zeitsch, fur die gesammten 

' 6pfit^jf^^ismi^m^ Gh»p«. XY., XIX, 



Book I.] THE EABTH'S MOVEMENTS. 15 

pletes its cycle in 21,000 years, the animal total advance of the equinox 
amounting to 62." At present the winter in the northern hemisphere 
coincides with the earth's nearest approach to the sun, or perihelion. 
In 10,500 years hence it will take place when the earth is at the 
farthest part of its orbit from the sun, or in aphelion, This movement 
acquires great importance when considered in connection with the 
secular variations in the eccentricity of the orbit (§ 8). 

§ 4. Change in the Obliquity of the Ecliptic— The angle at 
which the axis of the earth is inclined to the plane of its orbit does not 
remain strictly constant. It oscillates through long periods of time to 
the extent of about a degree and a half, or perhaps a little more, on 
either side of the mean. According to Dr. Croll,^ this oscillation must 
have considerably affected former conditions of climate on the earth, 
since, when the obliquity is at its maximum, the polar regions receive 
about eight and a half days' more of heat than they do at present — that 
is, about as much heat as lat. 76° enjoys at this day. This movement 
must have augmented the geological effects of precession, to which 
reference has just been made, and which are described in § 8. 

§ 5. Stability of the Earth's Axia.— That the axis of the earth's 
rotation has successively shifted, and consequently that the poles have 
wandered to dififerent points on the surface of the globe, has been 
maintained by geologists as the only possible explanation of certain 
remarkable conditions of climate, which can be proved to have formerly 
obtained within the Arctic Circle. Even as far north as lat. 81'^ 4.5', 
abundant remains of a vegetation indicative of a warm climate, and 
including a bed of coal 25 to 30 feet thick, have been found in situ.'^ 
It is contended that when these plants lived, the ground could not liave 
been permanently frozen or covered for most of the year with thick 
snow. In explanation of the difficulty, it has been suggested that the 
north pole did not occupy its present position, and tliat tlio locality 
wliere the jdants occur lay in more southerly latitudes. Without at 
jiresent entering on the discussion of the question whether the geological 
evidence necessarily requires so important a geographical change, let us 
considc^r how far a shifting of the axis of rotation has been a possible 
cause of change during- that section of geological time for which there 
are records among the stratified rocks. 

From the time of Laplace,^ astronomers have strenuously denied 
the possibility of any sensible change in the position of the axis of 
notation. It has l)een urged that, since the planet acquired its present 
oblate spheroidal form, nothing but an utterly incredible amount of 
<leformation could overcome the greater centrifugal force of the 
equat(»rial protuberance. It is certain, however, that the axis of 
rotation does not strictly coincide with the principal axis of inertia. 
Though the angular difference between them must always have been 

» Ooll, Trans. Geol Soc. GlnMjow, ii. 177. * Climate and Time,* Chap. xxv. 
■ Fielden and Heer, Qmirt. Journ. Geol. Soe. Nov. 1877. 
' 'Mecanique Cclcsto,' tome v. p. 14. 



16 C08MICAL ASPECTS OF GEOLOGY. [Book L 



small, we can, without having recourse to any extramundane influence, 
rocogniso two causes which, whether or not they may suffice to produce 
any change in the position of the main axis of inertia, undoubtedly tend 
to do so. In the first place, a widespread upheaval or depression of 
certain unsymmetrically arranged portions of the surface to a consider- 
able amount would tend to shift that axis. In the second place, an 
analogous result might arise from the denudation of continental masses 
of land, and the consequent filling up of sea-basins. Sir William 
Tliomson freely concedes the physical possibility of such changes. 
" We may not merely admit," he says, " but assert as highly probable, 
that the axis of maximum inertia and axis of rotation, always very near 
one another, may have been in ancient times very far from their present 
geographical position, and may have gradually shifted through 10, 20, 
30, 40, or more degrees, without at any time any perceptible sudden 
disturbance of either land or water." ^ But though, in the earlier ages 
of the planet's history, stupendous deformations may have occurred, and 
the axis of rotation may have often shifted, it is only the alterations 
which can possibly have occurred during the accumulation of the 
stratified rocks, that need to be taken into account in connection with 
former changes of climate. If it Can be shown, therefore, that the 
geographical revolutions necessary to shift the axis are incredibly 
stui^endous in amount, improbable in their distribution, and not really 
demanded by geological evidence, we may reasonably withhold our 
belief from this alleged cause of the changes of climate during 
geological history. 

It has been estimated by Sir William Thomson " that an elevation 
of 600 feet, over a tract of the earth's surface 1000 miles square and 10 
miles in thickness, would only alter the position of the principal axis by 
one-third of a second, or 34 feet." ^ Prof. George Darwin has shown that, 
on the supposition of the earth's complete rigidity, no redistribution of 
matter in new continents could ever shift the pole from its primitive 
l)08ition more than 3°, but that, if its degree of rigidity is consistent 
with a periodical re-adjustment to a new fonu of equilibrium, the pole 
may have wandered some 10° or 15° from its primitive position, or have 
made a smaller excursion and returned to near Its old place. In order, 
however, that these maximum effects should be produced, it would be 
necessary that each elevated area should have an area of depression 
corresponding in size and diametrically opposite to it, that they should 
lie on the same complete meridian, and that they should both be situated 
in lat. 45°. With all these coincident favourable circumstances, an 
effective elevati<m of r^l^y of the earth's surface to the extent of 10,000 feet 
would shift the pole 11^ ; a similar elevation of tj'^ would move it 1° 46^' ; 

of ^V> ^^ 1*^' J ^^^ ^^ ^» ®^ ^l* ^^' I^arwin admits these to be superior 
limits to what is possible, and that, on the supposition of intumescence or 

» Brit, Asmc. Rep. (1876), Sections, p. 11. 

' Tnins. Geol. Soc, Glaagow^ iv. p. 313. Tho situation of tho supposed area of up- 
heaval on tho earth's surface is not stated. 



Book L] CHANGES OF THE EAIiTITS AXIS, 17 



contraction tinder the regions in question, the deflection of tlio pole 
might be reduced to a quite insignificant amount.^ 

Under the most favourable conditions, therefore, the possible amount 
of deviation of the pole from its first position would appear to have been 
too small to have seriously influenced the climates of the globe within 
geological history. If we grant that these changes were cumulative, and 
that the superior limit of deflection was reached only after a long series 
of concurrent elevations and depressions, we must suppose that no move- 
ments took place elsewhere to counteract the eflect of those about lat. 45° 
in the two hemispheres. But this is hardly credible. A glance at a 
geographical globe suffices to show how largo a mass of land exists now 
both to the north and south of that latitude, especially in the northern 
hemisphere, and that the deepest parts of the ocean are not antipodal to 
the greatest heights of the land. These features of the earth's surface 
are of old standing. There seems, indeed, to be no geological evidence in 
favour of any such geographical changes as could have produced even 
the comparativaly small displacement of the axis considered possible by 
Prof. Darwin. 

In an ingenious suggestion, Dr. John Evans contended that, even 
without any sensible change in the position of the axis of rotation of the 
nucleus of the globe, there might be very considerable changes of 
latitude due to disturbance of the equilibrium of the shell by the 
upheaval or removal of masses of land between the e(j[uator and the 
l>ole8 and to the consequent sliding of the shell over the nucleus until 
the equilibrium was restored.* Subsequently he precisely formulated 
his hypothesis as a question to be determined mathematically ; ^ and 
the Holuti(m of the problem was worked out by the Kcv. J. F. 
Twisden, who arrived at the conclusion that even the large amount of 
geographical change postulated by Dr. Evans could only displace the 
earth's axis of figure to the extent of less tlian 10' of angle, that a dis- 
placement of as much as 10^ or 15^ could Ik) effected only if the heigh t^i 
and depths of the areas elevated and depressed exceeded by many times 
tlie heights of the highest mountains, that under no circumstances couhl a 
displacement of 20^ Ix) effected by a transfer of matter of less amount than 
about a sixth jmrt of the whole equatorial bulge, and that even tliis extreme 
amount would not necessarily alter the position of the axis of figure.'* 

Against any hypothesis which assumes a thin crust enclosing a li(|uid 
or viscous interior, weighty and, indeed, insuperable objections liave 
Ur<n urged. It has been suggested, however, that the almost universal 
ti;mi*h of present or former volcanic action, the evidence from the com- 
l^ivB8e<l strata in mountain regions that the crust of the earth must have 
a capacity for slipping towards certain lines, the great amount of 
horizontal compression of strata which can 1k) proved to have l>een 

» Pka, Trwiw. Nov. 1876. * Pn)C. Hoy, S^^r. xv. (IHO?) p. Ht. 

' Q. /. OtoL StMU xxxii. (1870) p. G2. 

• <i, J. QtoL 8oe. zxxW, (1878) p. 41. Soo also K. Hill, (irol ^hnf. v. (iiiMl h/.) 
{*\\ 2138, 47a O. Fiahor, op, ciL pp. 201, .'>.'> 1. 

C 



18 COSMWAL ASPECTS OF GEOLOGY. [Book I. 



accomplished, and tho Becular changes of climate — notably the former 
warm climate near the north pole — furnish grounds for inquiry whether 
tho doctrine of a fluid substratum over a rigid nucleus, which has been 
urged by several able writers, would not be compatible with mechanical 
considerations, and whether, under these circumstances, changes in lati- 
tude would not result from unequal thickening of the crust.^ This 
question of the internal condition of the globe is discussed at p. 54. 

§ 6. Changes of the Earth's Centre of Gravity.— If the centre 
of gravity in our planet, as pointed out by Herschel, be not coincident 
with the centre of figure, but lies somewhat to the south of it, any 
variation in its position will affect tho ocean, which of course adjusts 
itself in relation to the earth's centre of gravity. How far any redis- 
tribution of the matter within the earth, in such a way as to affect the 
present equilibrium, is now pos8il>le, we cannot tell. But certain 
revolutions at the surface may from time to time produce changes of 
this kind. The accumulation of ice which, as will be immediately 
described (§ 8), is believed to gather round one pole during the 
maximum of eccentricity, will displace the centre of gravity, and, as the 
result of this change, will raise the level of the ocean in the glacial 
hemisphere.^ Dr. Croll has estimated that, if the present mass of ice in 
the southern hemisphere is taken at 1000 feet thick extending down to 
lat. 60^, the transference of this mass to the northern hemisphere would 
raise the level of the sea 80 feet at the noi-th pole. Other methods of 
calculation give different results. Mr. Heath puts the rise at 128 feet; 
Archdeacon Pratt makes it more ; while the Rev. 0. Fisher gives it at 
409 feet.^ More recently, in returning to this question, Dr. Croll remarks 
" that the removal of two miles of ice from the Antarctic continent [and 
at present the mats of ice there is probablj^ thicker than that] would 
displace the centre of gravity 190 feet, and the formation of a mass of 
ice equal to the one-half of this, on the Arctic regions, would carry the 
centre of gravity 95 feet farther ; giving in all a total displacement 
of 285 feet, thus producing a rise of level at the north pole of 285 feet, 
and in the latitude of Edinburgh of 234 feet." A very considerable 
additional displacement would arise from the increment of water to the 
mass of the ocean by the melting of the ice. Supposing half of the two 
miles of Antarctic ice to be replaced by an ice-cap of similar extent and 
one mile thick in the northern hemisphere, the other half being melted 
into water and increasing the mass of the ocean, Dr. Croll estimates that 
from this source an extra rise of 200 feet would take place in the general 
ocean level, so that there would be a rise of 485 feet at the north pole, 
and 434 feet in the latitude of Edinburgh.* An intermittent submer- 

» O. Fisher, Ged, Mag. 1878 p. 552, and his ' Physics of the Earth's Crust,' 1882. 

' Adhemar, * Revolutions de la Mer,' li^40. 

» Croll, ill Jieader for 2nd Sept<.mber, 18G5, and riul Mag. April, 186G; Heath, 
Phil Mag. April, 18G9; Pratt. Phil Mag. March, 18G6; Fisher, BendeVy 10th 
Febrnary, 18(1(5. 

* Croll, Gcol Mag. new sericH, i. (1874) p. 347; * Cliiuuio and Time,' chops, xxiii. 
and XX iv. 



Bo(« I.] GEOLOGICAL INFLUENCE OF SUN AND MOON. 19 



genoe and emergence of the low polar lands might bo due to the alternate 
Bhifting of the centre of gravity. 

To what extent this cause has actually come into operation in past 
time cannot at present be deterniinod. It has been suggested that the 
"raised beaches" or old sea- terraces, so numerous at various heights 
in the north-west of Europe, might bo due to the transference of the 
oceanic waters, and not to any subterranean movement, as generally 
believed. But if such had been their origin, they ought to have 
shown evidence of a gradual and uniform decline in elevation from 
north to south. No such feature, however, has been detected. On the 
contrary, the levels of the terraces vary within comparatively short 
distances. Though numerous on both sides of Scotland, they disappear 
among the Orkney and Shetland islands, although these localities 
were admirably adapted for their formation and preservation.* The 
conclusion must be drawn that the " raised beaches " cannot be adduced 
as evidence of changes of the earth's centre of gravity, but are due 
to local and irregular subterranean movement. (See Book III. Part I. 
Section iii. § 1.) 

§. 7. Results of the Attractive Influence of Sun and Moon on 
the Geological Condition of the Earth. — Many speculations have 
been offered to account for supposed former greater intensity of geological 
activity on the surface of the globe. Two causes for such greater 
intensity may be adduced. In tlie first place, if the earth has cooled 
down from an original molten condition, it has lost, in cooling, a vast 
amount of potential geological energy. It deep not necessarily follow, 
however, that the geological phenomena resulting from internal tem- 
perature have, during the time recorded in the accessible part of the 
earth's crust, been steadily decreasing in magnitude. We might, on 
the contrary, contend that the increased resistance of a thickening 
cooled crust may rather have hitherto intensified tlie manifestations of 
subterranean activity, by augmenting the resistance to be overcome. In 
the second place, the earth may have been once more powerfully affected 
by external causes, such as the greater heat of the sun, and the greater 
proximity of the moon. That the formerly larger amount of solar heat 
received by the surface of our planet must have produced warmer 
climates and more rapid evaporation, with greater rainfall and the 
important chain of geological changes which such an increase would 
introduce, appears in every way probable, though the geologist has not 
yet been able to observe any indisputable indication of such a former 
intensity of superficial changes. 

Prof. Darwin, in investigating the lx>dily tides of viscous spheroids, 
lias brought forward some remarkable results bearing on the question 
of the possibility that geological operations, both internal and superficial, 
may have l^en once greatly more gigantic and rapid than they are 
now.^ He assumes the earth to be a homogeneous spheroid and to have 

» Nature, xvi. (1877) p. 41.5. ' Phil Tran», 1870, parts i. and ii. 

c 2 



20 COSmCAL ASPECTS OF GEOLOGY. [Book I. 



ffosflcmed a certain ftmall viflcosity,^ and he calculates the internal tidal 
friction in snch a mass exposeil to the attraction of moon and inm, and 
the con»eqncnr;e8 which these bodily tides have produced. He finds 
that the length of onr day and month have greatly increased, that the 
m^Kin'fl distance has likewise angmented, that the obliquity of the 
ecliptic has diTnininhcd, that a large amount of hyi)ogene heat has been 
gcTicrate^l by the internal tidal friction, and that these changes may all 
havfj transpired within c^miparatively so short a period (57,0<)0,000 years) 
as to place them quite probably within the limits of ordinary geological 
hintfiry. Ac»^>rding to his estimate, 46,300,000 years ago the length of 
the sidereal day was fifteen and a half hours, the moon's distance in mean 
radii of the earth was 40*8 as compared with 60-4 at the present time. 
1^1 1 50,810,000 years >)ack, the length of a day was only 6f hours, or 
loss than a quarter of its present value, the moon's distance was only 
nine earth's radii, while the lunar month lasted not more than about a 
day and a half Cl'58), or -j^ of its present duration. He arrives at the 
deduction tliat the energy lost by internal tidal friction in the earth's 
muss is c(mvcrtod into heat at such a rate that the amount lost during 
57,000,000 years, if it were all applied at once, and if the earth had the 
sp(M;iric heat of iron, would raise the temperature of the whole planet's mass 
1, 7 (JO'' Fahrenheit, but that the distribution of this heat-generation has 
l»o<ui such as not to interfere with the normal augmentation of tempera- 
tun* downward due to secular cooling, and the conclusion drawn there- 
fn)m by Sir William Thomson. Mr. Darwin further concludes from his 
liypothcsis tliat the oUiptioity of the earth's figure having been con- 
tinually diminishing, " the polar regions must have been ever rising and 
the o([uatorial ones falling, though as tlio ocean followed these changes, 
thoy might cpiito well have left no geological traces. The tides must 
have been very much more frequent and larger, and accordingly the rate 
of oceanic denudation much accelerated. The more rapid alternation of 
day and niglit^ would probably lead to more sudden and violent storms, 
an<l the incroase<l rotation of the earth would augment the violence of the 
tra<le-winds, wliich in their turn would affect oceanic currents." ^ As 
al)ovo stated, no facts yet revealed by the geological record compel the 
admission of more violent superficial action in former times than now. 
Kut though the facts do not of themselves load to such an admission, it 
is proper to enquire whether any of them are hostile to it. It will be 
shown in Book Vl. that even as far kick as early Palaeiizoic times, that 
is, as far int<> the jvust as the history of organiseil life can l>e traced, 
HO«limonta<ion Ux^k plaet> very much as it does now. Sheets of fine mud 

' T\\o i1ofrrrH> of viw»osily rt«s«mo<l is nwch that •* thirtoon and a half tons to tho 
-.m-m- inch a.»tin>r for (wt^nty-fonr honra on a slab an in«^h tliick di8i)la<v8 the uppcT 
->nJ!io«» iv1aii\ir1y to iho lower 1hn>u^h »>no-lon1h of an inch. It is oWioua," sava Mr. 
I^«i«in, ** thai woh a ^nlwtanco as this wonM Iv oalli><l a s.Mid in ordinary lUiflanoo, 
an.l in Iho tidal pi>f>Wom this nwsl Iv n>ffaHcd as a vcnr small viscwdtr!" 0/». riV. 

• AMVii>lin|t t« Hm OftkMilaiinm, th«^ yoM .'»7,000,0i>0 of voara a^o c»-»ntftinrtl 1800 dav* 
ImIvM of *», » Oju fit. \x .S32. 



Book L] CLIMATE AND QEOLOGY. 21 



and Bilt were pitted with rain drops, ribbed with ripple-marks, and 
furrowed by crawling worms, exactly as they now are on the shores of 
any modem estuary. These surfaces were quietly buried under succeed- 
ing sediment of a similar kind, and this for hundreds and thousands of 
feet. Nothing indicates violence; all the evidence favours tranquil 
deposit.^ If, therefore, Mr. Darwin's hypothesis be accepted, we must 
conclude either that it does not necessarily involve such violent super- 
ficial operations as he supposes, or that even the oldest sedimentary 
formations do not date back to a time when the influence of increased 
rotation could make itself evident in sedimentation, that is to say, on 
Mr. Darwin's hypothesis, the most ancient fossiliferous rocks cannot be 
nearly as much as 57,000,000 years old. 

§ 8. Climate in its Geological Relations. — In subsequent parts 
uf this volume the data will be given from which we learn that the 
climates of the earth have formerly been considerably different from 
those which at present prevail. A consideration of the history of the 
»jlar system would of itself suggest the inference that, on the whole, the 
climates of early geological periods must have been warmer. The sun's 
heat was greater, probably the amount of it received by the earth was 
likewise greater, while there would be for some time a sensible influence 
of the planet's own internal heat upon the general temperature of the 
whole globe.^ Although arguments based upon the probable climatal 
necessities of extinct species and genera of plants and animals must be 
used with extreme caution, it may be asserted with some confidence that 
from the vast areas over which many Palseozoic moUusks have been 
traced, alike in the eastern and the western hemisplieres, the climates of 
the glol)e in Palaeozoic time were probal)ly much more uniform than thc.y 
now are. There appears to have been a gradual lowering of the general 
temiwrature during past geological time, accompanied by a tendency 
towards greater extremes of climate. But there are proofs also that at 
longer or shorter intervals cold cycles have intervened. The (jllacial 
lV*ri<Kl, for example, preceded our own time, and in successive geological 
formations indications, of more or less value, have been found that point 
to a prevalence of ice in what are now temperate regions. 

* Tin* al>ovo paHsagc ha<l been printed off when Professor R. S. Ball's remarkablo 
l«?<!turo apj)eare<l {NfUure, xxv. 1881, pp. 79, 108), in wliicli, starting from Professor 
Darwin's data, ho pushed his conclusions U^ such an cxtrcnio as to call in the agency of 
tides more than (JOO feet liigh in early geological times. In repudiating this ai)plioation 
of his H'sults, Mr. Darwhi {Ndturf^ xxv. p. 213) employs the argument I have here used 
from the absence of any evidence of such tidal action in the geological formations, and 
fr^m the indicjition, on the cr)ntrary, of (rancjuil dejK)sit. 

- Sir William Thom«<m believes that the hypothesis that terrestrial temperature was 
form«Tly higher by reason of a hotter sun ** is rendered almost infmitely probalde by 
iiid»;iM.-ndent physical evidence and mathematical calculation." ('rr<in:<. GtoL Soc. 
Glnsij'nr^ v. p. 2.'>8.) l*rofessor Tait, however, has suggested, that the former greater 
heat of the sun may have raised such vast clouds of absorbing vapour round thai 
lmninar>' as to prevent the effective amount of radiation of heat to tlie earth's 
iurfaee from l>eing greater than at present; while on the other hand, a similar supj)osi- 
ii«»n may be mjule with reference to the greater amount of vapour which increased solar 
radiation would raise to l>e ccmdenaed in the earth's atmosphere. *Eecent Advance.^ 
in Physical Science,* 1870, p. 174. 



22 COSMIC AL ASPECTS OF GEOLOGY. [Book I. 

Various theories have been proposed in explanation of such 
alternations of climate. Some of these have appealed to a change in the 
position of the earth's axis relatively to the mass of the planet (antCj § 5). 
Others have been based on the notion that the earth may have passed 
through hot and cold regions of space. Others, again, have called in the 
effects of terrestrial changes, such as the distribution of land and sea, on 
the assumption that elevation of land about the poles must cool the 
temperature of the globe, while elevation round the equator woxdd raise 
it.^ But the changes of temperature appear to have affected the whole 
of the earth's surfac^e, while there is not only no proof of any such enor- 
mous vicissitudes in physical geography as would be required, but good 
grounds for believing that the present terrestrial and oceanic areas have 
remained, on the whole, on the same sites from very early geological time. 
Moreover, as evidence has accumulated in favour of periodic alternations 
of climate, the conviction has been strengthened that no mere local 
changes could have sufficed, but that secular variations in climate must 
be assigned to some general and probably recurring cause. 

By degrees, geologists accustomed themselves to the belief that the 
cold of the Glacial Period was not due to mere terrestrial changes, but 
was to be explained somehow as the result of cosmical causes. Of various 
suggestions as to the probable nature and operation of these causes, one 
deserves careful consideration — change in the eccentricity of the earth's 
orbit. Sir John Herschel ^ pointed out many years ago that the direct 
effect of a high condition of eccentricity is to produce an unusually cold 
winter, followed by a correspondingly hot summer, in the hemisphere 
whose winter occurs in aphelion, while an equable condition of climate 
at the same time prevails on the opposite hemisphere. But both hemi- 
spheres must receive precisely the same amount of solar heat, because 
the deficiency of heat, resulting from the sun's greater distance during 
one part of the year, is exactly compensated by the greater length of that 
season. Sir John Herschel even considered that the direct effects of 
eccentricity must thus be nearly neutralised.^ As a like verdict was 
afterwards given by Arago, Humboldt, and others, geologists were 
satisfied that no important change of climate could be attributed to 
change of eccentricit3^ 

It is to the luminous memoirs of Dr. James CroU that geology is 

indebted for the first fruitful suggestion in this matter, and for the 

subsequent elaborate development of the whole subject of the physical 

causes on which climate depends.* He has been good enough to draw 

' UJ3 the following abstract of them for the present work. 

" Assuming the mean distance of the sum to be 92,400,000 miles, 
then when the eccentricity is at its superior limit, • 07775, the distance 
of the sun from the earth, when the latter is in the aphelion of its orbit, 

* In Lyell's 'Principles of Geology,' this doctrine of the influence of geographical 
changes is maintained. 

* Trans, Geol Soc. vol. iii. p. 293 (2nd series). 

» * Cabinet Cyclopiedia,' sec 315 ; * Outlines of Astronomy/ sec. 368. 

* His researches will be found in detail in his Yolume * Climate and Time,' 1875. 



Hook L] 



ECCENTRICITY OF EARTIPS ORBIT. 



23 



in no leas than 99,584,100 miles, and when in the perihelion it is only 
85,215,900 miles. The earth is, therefore, 14,368,200 miles farther from 
the sun in the former than in the latter position, The direct heat of the 
sun being inversely as the square of the distance, it follows that the 
amount of heat received by the earth in these two positions will bo as 
19 to 26. The present eccentricity being '0168, the earth's distance 
during our northern winter is 90,847,680 miles. Suppose now that, 
from the precession of the equinoxes, winter in our northem hemisphere 
should happen when the earth is in the aphelion of its orbit, at the time 
that the orbit is at its greatest eccentricity ; the earth would then be 
8,736,420, miles farther from the sun in winter than it is at present. 
The direct heat of the sun would therefore, during winter, be one-fifth 



N.P. 





NJ>. 



*V. WiiUtr Solstice in Aphelion. y. Wintmr Solstice in Perihelion. 

Fig. 1.— Eccentricity of the Earth's Orbit in Relation to aimatc. 

loss and during summer one-fifth greater than now. This enormous 
difference would necessarily affect the climate to a very great extent. 
Were the winters under those circumstances to occur when the earth was 
in the perihelion of its orbit, the earth would then bo 14,308,200 miles 
nearer the sun in winter than in summer. In this case the difference 
between winter and summer in our latitudes would bo almost annihilated. 
But as the winters in the one hemisphere correspond with tho flummers 
in the other, it follows that while the one hemisphere would be enduring 
the greatest extremes of summer heat and winter cold, the other would 
]je enjoying perpetual summer. 

*' It is quite true that, whatever may be tho eccentricity of the earth's 
orbit, the two hemispheres must receive equal quantities of heat per 
annum ; for proximity to the sun is exactly compensated by the effect of 
swifter motion. The total amount of heat received from the sun 
between the two equinoxes is, therefore, the same in both halves of the 



24 COSMIC AL ASPECTS OF GEOLOGY. [Book I. 

-■■■ —■■■■ !■■■ -y ■ ■ I ■ I ^^M^— ^—^ . 

year, whatovor tho ocoentricity'; of tho earth's orbit may be. Por 
example, whatever extra heat the. Houthem hemisphere may at present 
receive per day from the sun during its summer months, owing to 
greater proximity to the sun, is exactly compensated by a corresponding 
loss arising from the shortness of the season ; and, on the other hand, 
whatever deficiency of heat we in the northern hemisphere may at 
present have per day during our summer half-year, in consequence of 
the earth's distance from the sun, is also exactly compensated by a corre* 
spending length of season. 

*' It is well known, however, that those simple changes in the summer 
and winter distances would not alone produce a glacial epoch, and that 
physicists, confining their attention to the purely astronomical effects, 
were perfectly correct in affirming that no increase of eccentricity of the 
earth's orbit could account for that epoch. But the important fact was 
overlooked that, although the glacial epoch could not result directly 
from an increase of eccentricity, it might nevertheless do so indirectly 
from physical agents that were brought into operation as a result of 
an increase of eccentricity. The following is an outline of what these 
physical agents were, how they were brought into operation, and the 
way in which they may have led to the glacial epoch. 

** With tho eccentricity at its superior limit and the winter occurring 
in the aphelion, tho earth would, as we have seen, be 8,736,420 miles 
farther from the sun during that season than at present. The reduction 
in tho amount of heat received from the sun owing to his increased 
distance, would lower the midwinter temperature to an enormous extent. 
In temperate regions the greater portion of the moisture of ^e air is at 
present precipitated in the form of rain, and the very small portion 
which falls as snow disappears in the course of a few weeks at most. 
But in the circumstances under consideration, the mean winter- 
temperature would be lowered so much below the freezing point that 
what now falls as rain during that season, would then fall as snow. 
This is not all ; the winters would then not only be cooler than now, 
but they would also be much longer. At present the winters are nearly 
eight days shorter than the summers ; but with the eccentricity at its 
superior limit and the winter solstice in aphelion, the length of tho 
winters would exceed that of tho summers by no fewer than thirty-six 
days. The lowering of the temperature and the lengthening of the 
winter would both tend to the same effect, viz., to increase tho amount 
of snow accumulated during the winter ; for, other things being equal, 
the longer the snow-accumulating period, the greater the accumulation. 
It may be remarked, however, that the absolute quantity of heat 
received during winter is not affected by the decrease in the sun's heat 
for the additional length of the season compensated for this decrease.* 
As regards the absolute amount of heat received, increase of the sun's 

* When the occeutricity is at its superior limit, the absolute quantity of heat received 
by tho earth during the year is, however, about one three-hundredth part greater than 
at present But this does not affect the question at issue. 



Book L] CAUSES DETERMINING CLIMATE. 25 



diHtance and lengthening of the winter are compensatory, but not bo 
in regard to the amount of snow accumulated. The consequence of this 
state of things would be that, at the commencement of the short summer, 
the ground would be covered with the winter's accumulation of snow. 
Again, the presence of so much snow would lower the summer tem- 
perature, and prevent to a great extent the melting of the snow. 

"There are three separate ways whereby accumulated masses of 
snow and ice tend to lower the summer temperature, viz, : — 

'* Firsty By means of direct radiation. No matter what the intensity 
of the son's rays may be, the temperature of snow and ice can never rise 
above 32®. Hence, the presence of snow and ice tends by direct 
radiation to lower the temperature of all surrounding bodies to 32°. In 
Greenland, a country covered with snow and ice, the pitch has Ijeen seen 
to melt on the side of a ship exposed to the direct rays of the sun, while 
at the same time, the surrounding air was far below the freezing point ; 
a thermometer exposed to the direct radiation of the sun has been observed 
to stand above 100°, while the air surrounding the instrument was 
actually 12° below the freezing-point. A similar experience has been 
recorded by travellers on the snow-fields of the Alps. These results, 
surprising as they no doubt appear, are what we ought to expect under 
the circumstances. Perfectly dry air seems to be nearly incapable of 
absorbing radiant heat. The entire radiation passes through it 
almost without any sensible absorption. Consequently the pitch on the 
side of the ship may be melted, or the bulb of the thermometer raised to 
a liigh temperature by the direct rays of the sun, while the surrounding 
air remains intensely cold. The air is cooled by contact with the 
snow-covered ground, but is not heated by the radiation from the sun. 

"When the air is charged with aqueous vapour, a similar cooling 
effect also takes place, but in a slightly different way. Air charged 
with aqueous vapour is a good absorber of radiant heat, but it can 
ouly absorl) those rays which agree with it in period. It so happens 
that rays from snow and ice are, of all others, those which it al>sorbs 
best. The humid air will absorb tlie total radiation from the snow and 
ice, but it will allow the greater part of, if not nearly all, the sun's rays 
t«) pass unabsorlx)d. But during the day, when the sun is shining, the 
radiation from the snow and ic^ to the air is negative; that is, the 
Huow and ice cool the air by radiation. The result is, the air is cooled 
by radiation from the snow and ice (or rather, wc should say, to the 
snow and ice) more rapidly than it is heated by the sun ; and as a 
consequence, in a country like Greenland, covered with au icy mantle, 
tlie temperature of the air, even during summer, seldom rises above the 
freezing-j)oint. Snow is a 'good reflector, l)ut as simple reflection does 
not change the character of the rays, they would not be absorbed by the 
air, but would pass into stellar space. Were it not for the ice, the 
summers of North Greenland, owing to the continuance of the sun 
above the horizon, would be as warm as those of England ; but instead 
of this, the Greenland summers are colder than our -winters. Cover 



26 COSMIC AL ASPECTS OB' GEOLOGY. [B«)ok J. 



India with an ice sheet, and its summers would be colder than those of 
England. 

" Second^ Another cause of the cooling effect is that the rays which 
fall on snow and ice are to a great extent reflected back into space. 
But those that are not reflected, but absorbed, do not raise the tem- 
perature, for they disappear in the mechanical work of melting the ice. 
For whatsoever may be the intensity of the sun's heat, the surface of 
the ground will bo kept at 32° so long as the snow and ice remain 
unmelted. 

" Thirds Snow and ice lower the temperature by chilling the air and 
condensing the vapour into thick fogs. The great strength of the sun's 
rays during summer, due to his nearness at that season, would, in the 
first place tend to produce an increased amount of evaporation. But the 
presence of snow-clad mountains and an icy sea would chill the 
atmosphere and condense the vapour into thick fogs. The thick fogs 
and cloudy sky would effectually prevent the sun's rays from reaching 
the earth, and the snow, in consequence, would i-emain unmelted during 
the entire summer. In fact, we have this very condition of things 
exemplified in some of the islands of the Southern Ocean at the present 
day. Sandwich Land, which is in the same parallel of latitude as the 
north of Scotland, is covered with ice and snow the entire summer ; and 
in the island of South Georgia, which is in the same parallel as the centre 
of England, the perpetual snow descends to the very sea-beach. Captain 
Sir James Boss found the perpetual snow at the sea-level at Admiralty 
Inlet, South Shetland, in lat. 64° ; and while near this place the thermo- 
meter in the very middle of summer fell at night to 23° P. The 
reduction of the sun's heat and lengthening of the winter, which would 
take place when the eccentricity is near to its superior limit and the 
winter in aphelion, would in this country produce a state of things 
perhaps as bad as, if not worse than, that which at present exists in South 
Georgia and South Shetland. 

" The cause which above all others must tend to produce great 
changes of climate, is the deflection of great ocean currents. A high 
condition of eccentricity tends, we have seen, to produce an accumu- 
lation of snow and ice on the hemisphere whoso winters occur in 
aphelion. The accumulation of snow, in turn, tends to lower the 
summer temperature, cut off the sun's rays, and retard the melting 
of the snow. In short, it tends to produce, on that hemisphere, a 
Btiite of glaciation. Exactly opposite effects take place on the other 
liemispliero, which has its winter in perihelion. Thei-e the short- 
ness of the winters, combined with the high temperature arising 
from tho nearness of the sun, tends to prevent the accumulation of 
Know. Tho general result is thit tlie one hemisphere is cooled and the 
other heated. This state of things now brings into play the agencies 
which lead to the deflection of the Gulf-stream and other great ocean 
currents. 

*' Owiug to the great difference between the temperature of the 



Book L] THE GULF-STBEAM, 27 



equator and tho poles, there is a constant flow of air from the poles to 
the equator. It is to this that the trade-winds owe their existence. 
Now, Bs the strength of these winds will, as a general rule, depend upon 
the difference of temperature that may exist between the equator and 
higher latitudes, it follows that the trades on the cold hemisphere will 
be stronger than those on the warm. When the polar and temperate 
regions of the one hemisphere are covered to a large extent with snow 
and ioe, the air, as we have just seen, is kept almost at the freezing-point 
during both summer and winter. The trades on that hemisphere will, 
of necessity, be exceedingly powerful ; while on the other hemisphere, 
where there is comparatively little snow or ice, and the air is warm, the 
trades will consequently be weak. Suppose now tho northern hemi- 
sphere to be the cold one. The north-east trade- winds of this hemisphere 
will far exceed in strength the south-east trade-winds of the southern 
hemisphere. The median line between the trades will consequently lie 
to a very considerable distance to the south of the equator. We have a 
good example of this at the present day. The difference of temperature 
between the two hemispheres at present is but trifling to what it would 
be in the case under consideration ; yet we find that the south-east 
trades of the Atlantic blow with greater force than the north-east trades, 
sometimes extending to 10° or 15° N. lat., whereas the north-east trades 
seldom blow south of the equator. The effect of the northern trades 
blowing across the equator to a great distance will be to impel the wann 
water of the tropics over into the Southern Ocean. But this is not all ; 
not only would the median line of tho trades be shifted southwards, but 
the great equatorial currents of the globe would also bo shifted south- 
wards. 

*' Let us now consider how this would affect the Gulf-stream. The 
South American continent is shaped somewhat in the form of a triangle 
with one of its angular corners, called Capo St. Ko(j[Uo, pointing 
eastwards. The equatorial current of the Atlantic impinges against this 
comer ; but as the greater portion of the current lies a little to the north 
of the corner, it flows westward into the Gulf of Mexico and forms the 
Gulf-stream. A considerable portion of the water, however, strikes the 
land to tho south of the capo, and is deflected along the shore of Brazil 
into the Southern Ocean, forming what is known as tho Brazilian 
current. Now, it is obvious that tho shifting of the equatorial current 
of the Atlantic only a few degrees to the south of its present position — a 
thing which would certainly take place under the conditions which we 
have been detailing — would turn the entire current into the Brazilian 
branch, and instead of flowing chiefly into the Gulf of Mexico, as at 
y»reijent, it would all flow into tho Southern Ocean, and the Gulf-stream 
would consequently >)e stopped. The stoppage of the Gulf-stream, 
combined with all those causes which we have just been considering, 
would place Europe under a glacial condition, while at the same time 
the temperature of the Southern Ocean would, in consequence of the 
enormous quantity of warm water received, have its temperature 



28 COSMICAL ASPECTS OF GEOLOGY. [Bwmc I. 

(already high from other causes) raised enormously. And what holds 
true in regard to the currents of the Atlantic holds also true, though 
perhaps not to the same extent, of the currents of the Pacific. 

'*If the breadth of the Gulf-stream be taken at 50 miles, its depth 
at 1000 feet, its mean velocity at 2 statute miles an hour, the tempera- 
ture of the water when it leaves the Gulf at 65°, and the return current 
at 40^ F.,^ then, the quantity of heat conveyed into the Atlantic by this 
stream is equal to one-fourth of all the heat received from the sun by 
that ocean from the Tropic of Cancer to the Arctic Circle.^ From 
principles discussed at considerable length in * Climate and Time ' it is 
shown that, but for the Gulf- stream and other currents, London would 
have a mean annual temperature 40^ lower than at present. 

" But there is still another cause which must be noticed : — a strong 
undercurrent of air from the north implies an equally strong upjKjr 
current to the north. Now if the effect of the undercurrent would be to 
impel the warm water at the equator to the south, the effect of the 
upper current would be to carry the aqueous vapour formed at the 
equator to the north ; the upper current, on reaching the snow and ice of 
temperate regions, would deposit its moisture in the form of snow ; so 
that it is probable that, notwithstanding the great cold of the glacial 
epoch, the quantity of snow falling in the northern regions, would bo 
enormous. This would be particularly the case during summer, when 
the earth would be in the perihelion and the heat at the equator great. 
The equator would be the furnace where evaporation would take place, 
and the snow and ice of temperate regions would act as a condenser. 

" The foregoing considerations, as well as many others which might 
be stated, lead to the conclusion that, in order to raise the mean 
temperature of the globe, water should be placed along the equator, and 
not landy as was contended by Sir Charles Lyell and others. For if land 
\)o placed at the equator, the possibility of conveying the sun's heat 
from the equatorial regions by means of ocean currents is prevented." ^ 

Inter-Glacial Periods. — Allusion has already been made to the 
accumulating evidence that changes of climate have been recurrent, and 
to the deduction from this alternation or perimlicity that they have 
probably been due to some general or cosmical cause. Dr. Croll has 
ingeniously shown that every long cold period arising in each hemis- 

• Sir Wyville Thomson states that in May, 1873, the Chalhmger cxpetlitiou found 
the (iulf-streani, at tho point where it was crosswl, to l)o alxmt sixty miles in widtli, 
KM) fathoms (lr<;i), and flowinp: at the mte of three knots i)er hour. This makes the 
volume of the strenm one-fifth fjreater than the aljove estimate. 

'^ The (juantity of heat conveyed by the Gulf-stream for distribution is oquul to 
77,479,t>o0,000,00(),()0(),()0() foot-pounds i)er day. The quantity received from the sun 
by tho North Atlantic is 310,923,000,000,000,000,000 foot-pounds. «CUnmto and 
Time,' chap. ii. 

* That cllmato, however, may be considerably affected by changes, such as are known 
to havo taken place in the distribution of land and sea, must bo frankly conceded. 
This has been recently cogently argued by Mr. Wallace in his * Island liife,* 1880. Mr. 
CtoII'b yiews, summarised above, have recently l)oen adversely criticised by Prof. 
Newcomhe, for whose papers and Mr. CroU's replies see A met. Joum. Seicnce^ 1876, 
1883, 1881. 



Book Ll INTER-OLACIAL PERIODS, 29 

phere from the circumstances sketched in the preceding pages, must 
have been interrupted by several shorter warm periods. 

** When the one hemisphere," ho says, " is under glaciation, the other 
is enjoying a warm and equable climate. But, owing to the precession 
of the equinoxes, the condition of things on the two hemispheres must bo 
reversed every 10,000 years or so. When the solstice passes the aphelion, 
a contrary process commences; the snow and ice gradually begin to 
diminish on the cold hemisphere and to make their appearance on the 
other hemisphere. The glaciated hemispliero turns by degrees warmer, 
and the warm hemisphere colder, and this continues to go on for a period 
of ten or twelve thousand years, until the winter solstice reaches the 
I)erihelion. By this time the conditions of the two hemispheres have 
been reversed ; the formerly glaciated hemisphere has now become the 
warm one, and the warm hemisphere the glaciated. The transference 
of the ice from the one hemisphere to the other continues as long 
as the eccentricity remains at a high value. It is probable that, during 
the warm inter-glacial periods, Greenland and the Arctic regions would 
be comparatively free from snow and ice, and enjoying a temperate and 
equable climate," 



30 GEOGNOSY. [Book U. 



BOOK IL 

GEOGNOSY. 

AN INVESTIGATION OF THE MATERIALS OP THE EABTITS 

SUBSTANCE. 

Part I. — A General Description of the Parts of the Earth. 

A DISCUSSION of tho geological changes which our planet has nndeigono, 
ought to be preceded by a study of the materials of which the planet 
consists. This latter branch of inquiry is termed Geognosy. 

Viewed in a broad way, the earth may be considered as consisting 
of (1) two envelopes, — an outer one of gas, completely surrounding tho 
planet, and an inner one of water, covering about three-fourths of the 
globe ; and (2) a globe, cool and solid on its surface, but possessing a 
liigh internal temperature. 

I. — The Envelopes, 

It is certain that the present gaseous and liquid envelopes of the 
planet form only a portion of the original mass of gas and water with 
which the globe was invested. Fully a half of the outer shell or crust of 
the earth consists of oxygen, which, there can be no doubt, once existed 
in the atmosphere. The extent, likewise, to which water has been 
abstracted by minerals is almost incredible. It has been estimated that 
already one-third of the whole mass of the ocean has been thus absorbed. 
Eventually tho condition of the planet will probably resemble that of 
the moon — a globe without air, or water, or life of any kind. 

1. The Atmosphere. — The gaseous envelope to which the name of 
atmosphere is given, extends to a distance of perhaps 500 or 600 miles 
from the earth's surface, possibly in a state of extreme tenuity to a 
still greater height. But its thickness must necessaril}'- vary with lati- 
tude and changes in atmospheric pressure. The layer of air lying over 
the poles is not so deep as that which surrounds the equator. 

Many speculations have been made regarding the chemical composition 
of the atmosphere during former geological periods. There can indeed 
be no doubt that it must originally have differed very greatly from its 
present conditicnu' Besides the abstraction of the oxygen which now 
forms folly a half of the outer crust of the earth, the vast lieds of coal 



Part L] THE ATMOSPHERE. 31 



found all over the world, in geological formations of many different ages, 
doubtless represent so much carbon-dioxide (carbonic acid) once present 
in the air. According to Sterry Hunt, the amount of carbonic acid 
absorbed in the process of rock-decay, and now represented in the form 
of carbonates in the earth's crust, probably equals two hundred times the 
present volume of the entire atmosphere.^ The chlorides in the sea, 
likewise, were probably carried down out of the atmosphere in the 
primitive condensation of aqueous vapour. It has often been stated 
that, daring the Carboniferous period, the atmosphere must have been 
warmer and with more aqueous vapour and carbon-dioxide in its com- 
position than at the present day, to admit of so luxuriant a flora as that 
from which the coal-seams were formed. There seems, however, to be at 
present no method of arriving at any certainty on this subject. 

As now existing, the atmosphere is considered to be normally a 
mechanical mixture of nearly 4 volumes of nitrogen and 1 of oxygon 
(N 79*4, 20' 6), with minute proportions of carbon-dioxide and 
water- vapour and still smaller quantities of ammonia and the powerful 
oxidising agent, ozone. These quantities are liable to some variation 
according to locality. The mean proportion of carbon-dioxide is about 3*5 
parts in every 10,000 of air. In the air of streets and houses the pro- 
portion of oxygen diminishes, while that of carbon-dioxide increases. 
According to the researches of Angus Smith, verj'- pure air should 
contain not less than 20*99 per cent, of oxygen, with 0*030 of carbon- 
dioxide ; but he found impure air in Manchester to have only 20*2 1 of 
oxygen, while the proportion of carbon-dioxide in that city during fog 
was ascertained to rise sometimes to 0*0679 and in the pit of the theatre 
to the very large amount of 0*2734. As plants absorb carbon- dioxide 
during the day and give it off at night, the quantity of this gas in the 
atmosphere oscillates between a maximum at night and a minimum 
during the day. During the part of the year when vegetation is active, 
it is Ixjlieved that there is at least 10 per cent, more carbonic acid in 
the air of the open country at night than in the day.^ Small as 
the normal percentage of this gas in the air may seem, yet the total 
amount of it in the whole atmosphere probably exceeds what would be 
disengaged if all the vegetable and animal matter on the earth's surface 
were burnt. 

Tlie other substances in the air are gases, vapours, and solid particles. 
Of these by much the most important is the vapour of water, which is 
always present, but in very variable amount according to temperature.^ 
It is this vapour which chiefly absorbs radiant heat.* It condenses into 

» Brit. A9»oe. Rep. 1878, Sects, p. 544. 

» Prof. G. F. ArmatroTig. Froc. Roy. Soc. xxx. (1880), p. 843. 

• A cubic metre of air at the frecziDg point can liold only 4*871 granimea of water- 
vapoar, but ut 40'' C. can tuko up 50*70 gminmcs. One cubic mile of air satuniled with 
vapour at 35° C. will, if cooled to 0°, deposit upwanls of 140,000 tons of water as rain. 
RuBcoe aod Schorlemmer'a *■ Chemistry/ i. )). 452. 

* Seo TyndalVa roaearches which established this important function of the aqueous 
vap(»ar of the atmoaphere, and tlieir confirmation by meteorological observation. S. A. 
Hill, Proe, Rfrtj. 8or. xxxiil 210, 435. 



32 GEOGNOSY, [Book H. 



dew, rain, hail, and snow. In assuming a visible form, and descending 
through the atmosphere, it takes up a minute quantity of air, and of the 
different substances which the air may contain. Being caught by the 
rain, and held in solution or suspension, these substances can be best 
examined by analysing rain-water. In this way, the atmospheric gases, 
ammonia, nitric, sulphurous, and sulphuric acids, chlorides, various salts, 
solid carbon, inorganic dust, and organic matter have been detected. To 
the fine microscopic dust so abundant in the air, great importance in 
the condensation of vapour has recently been assigned. (Book III. 
Part II. Section ii.) 

The comparatively small, but by no means unimportant, proportions 
of these minor components of the atmosphere are much more liable to 
variation than those of the more essential gases. Chloride of sodium » 
for instance, is, as might be expected, particularly abundant in the air 
l)ordering the sea. Nitric acid, ammonia, and sulphuric acid appear most 
conspicuously in the air of towns. The organic substances present in the 
air are sometimes living germs, such as probably often lead to the pro- 
pagation of disease, and sometimes mere fine particles of dust derived 
from the bodies of living or dead organisms.^ 

As a geological agent, the atmosphere effects changes by the chemical 
reactions of its constituent gases and vapours, by its varying temperature, 
and by its motions. Its functions in these respects are described in 
Book in. Part II. Section i. 

2. The Oceans. — Bather less than three-fourths of the surface 
of the globe (or about 144,712,000 square miles) are covered by the 
irregular sheet of water known as the Sea. Within the last ten years, 
much new light has been thrown upon the depths, temperatures, and bio- 
logical conditions of the ocean-basins, more particularly by the Lighininffy 
Porcupine, Cliallengsr, Tuscarora, Blake and Gazelle expeditions fitted out 
by the British, American and Grerman Governments. It has been ascer- 
tained that few parts of the Atlantic Ocean exceed 3000 fathoms, the 
deepest sounding obta.incd there being one taken about 100 miles north 
from the island of St. Thomas, which gave 3875 fathoms, or rather less than 
4J miles. The Atlantic appears to have an average depth in its more 
open parts of from 2000 to 3000 fathoms, or from about 2 to 3 J miles. 
In the Pacific Ocean H.M. Ship Challenger got soundings of 3950 and 
4475 fathoms, or about 4^ and rather more than 5 miles. Since then the 
U.S. Ship Tuscarora obtained a still deeper sounding (4655 fathoms), to 
the east of the Eurile Islands. This is the deepest abyss yet found in 
any part of the ocean. But these appear to mark exceptionally abysmal 
depressions, the average depth being, as in the Atlantic, between 2000 

• 
1 The air of towns is peculiarly rich in impurities, especially in manufacturing districU, 
whore mnoh coal is used. These impurities, however, though of serious consequence to 
the towns in ft sanitary point of view, do not sensihly affect the general atmosphere, 
seeing thftfc t^ y agg pro bably injneat measure taken out of the air hy rain, even in tlio 
^^^^'^^'^^ ^ri ddi ||g j|iw tiwpa. Tkey possess, howeyor, a special geological significance, 
oniil In lM^iMH|iMyMliBB9ortaiit eoonomio hearings. See on this whole suhjec^ 
ArijeaMaKlKKtKKf& ^ account of Rain in Book III. Part H. 8ect. iL 



Pabt L] the oceans. 33 



and 8000 fathoms. We may thoreforo assume, as probably not far from 
the truth, that the average depth of the sea is about 2,500 fathoms, 
or nearly 3 miles. Its total cubic contents will thus be about 400 millions 
of cubic miles. 

With regard also to the form of the bottom of tlie great oceans, much 
additional information has recently been obtained. Over vast areas 
in the central regions, the sea-floor appears to form great plains, with 
comparatively few inequalities, but with lines of submarine ridges, com- 
X>arablo to chains of hills or mountains on the land. Kccent soundings, 
however, taken at short distances, have revealed, in parts of the Atlantic 
that were supposed to be deep and witli a tolerably uniform bottom, sub- 
marine peaks rising to within 50 fathoms from the surface.^ A vast central 
ridge has also been traced down the length of this ocean, from which a 
few lonely peaks rise above sea-level — the Azores, St. Paul, Ascension, and 
Tristan d*Acunha. In the Pacific Ocean, the lines of coral-islands ap2)ear 
to rise on submarine ridges, having a general north-westerly and south- 
easterly trend. It is significant that the islands which thus appear far 
from any large mass of land are either coral-reefs or of volcanic origin, 
and contain none of the granites, schists and other ordinary continental 
rocks. St. Helena and Ascension in the Atlantic, and the Friendly and 
Sandwich Islands in the Pacific Ocean are conspicuous examples. 

Another important result of recent deep-sea research is the determi- 
nation of the relation of mediterranean seas to the main ocean. These 
l^asins, such as the North, Mediterranean, and Black Seas, the Gulf of 
^lexico, Caribbean Sea, Baffin's Bay, Hudson's Bay, Sea of Okhotsk, and 
< -hincse Sea, belong rather to the continental than the oceanic areas of the 
earth's surface. An elevation of a few hundred fathoms would convert 
Hiost of them into laud, with here and there deep water-tilled basins. 

The water of the ocean is distinguished from ordinary terrestrial 
waters by a higher specific gravity, and the presence of so large a pro- 
j>ortion of saline ingredients as to impart a strongly salt taste. The 
average density of sea- water is about 1*026, but it varies slightly in 
difterent jmrts even of the same ocean. According to the observations 
uf J. Y. Buchanan during the ChalleiKjer ex2)edition, some of the 
heaviest sea-water occurs in the pathway of the trade-winds of the 
North Atlantic, where evaporation must be comparatively rapid, a density 
uf 1-02781 being registered. Where, however, large rivers enter the 
sea, or where there is much melting ice, the density diminishes; 
Buchanan found among the broken ice of the Antarctic Ocean that 
it had sunk to 1-02418.2 

The greater density of sea-water depends, of course, upon the salu 
wldch it contains in solution. At an early period in the earth's history, 
the water now forming the ocean, together with the rivers, lakes and 
snowficlds of the land, existed as vapour, in which were mingled many 
other gases and vapours, the whole forming a vast atuiosphere 

' Thnti, 7tli Deer. 18«3. [.). Y. lUiohanuu.J 
- Uuclijuiau, Vi'oc, Jioij. S(k: (1S7(')« vd. xxiv. 

1) 



34 GEOGNOSY. [Book H. 



surrounding the still intensely hot glol^e. Under the enormous pressure 
(»f the primeval atmosphere, the first condensed water might have had 
the temperature of a didl red heat.* In condensing, it would carry 
do^-n with it many substances in solution. The salts now present in 
tjea- water are to hci regarded as principally derived from the primeval 
constitution of the sea, and thus we may infer that the sea has always 
been salt. It is probable, however, that, as in the case of the atmosphere, 
the composition of the ocean-water has acquired its present character 
only after many ages of slow change, and the abstraction of much mineral 
matter originally contained in it. There is evidence, indeed, among 
llic geological fonuations that large quantities of lime, silica, chlorides, 
and sulphates have in the coui>ic of time been removed from tho sea.- 

](ut it is manifciit also that, whatever may have K'eu the original 
rumpositiou of the oi-eans, they have for a vast section of geological time 
1>een constantly rueeiving minenil matter in solution from the land. 
Every spring, brook, and river removes various salts from tho rucks 
over which it moves, and these substances, thus dissolved, eventually 
find their way into the sea. Consequently sea-water ought to contain 
more or less traceable proportions of every substance which the 
terrestrial waters can remove from the land, in short, of probably every 
element present in the outer shell of the globe, for there seems to be no 
constituent of the earth which may not, under certain circumstances, bo 
held in solution in water, ^loreover, unless there be some counteracting 
process to remove these mineral ingreilients, the ocean-water ought to Ije 
growing, insensibly perhaps, Salter, for the supply of saline matter from 
the land is incessant. It has been ascertained indeed, with some 
approach to certainty, that the salinity of the Baltic and Mediterranean 
is gradually increasing.^ 

The average projwrtion of saline constituents in the water of tho 
great oceans far from land is about three and a half }>arts in eveiy 
hundred of water.* But in enclosed seas, receiving much fresh water, it 
is greatly reduced, while in those where evaporation predominates it is 
correspondingly augmented. Thus the Baltic wat^r contains from one- 
seventh to nearly a half of the ordinary proportion in ocean water, while 
the Mediterranean contains sometimes one-sixth more than that propor- 
tion. Forchhammer has shown the presence of the follo\\^ng twenty-- 
seven elements in sea-water : oxygen, hydrog-eu, chlorine, bromine, 
iodine, fluorine, sulphur, phosphorus, nitrogen, carbon, silicon, boron, 
silver, copi)er, lead, zinc, cobalt, nickel, iron, manganese, aluminium, 

» q, J, OtiJ, Sor. xxxvi, Oi^O) pp. 112, 117. 

' Dr. Sit-rry Hunt uven snppofieii th»t the saliue watera of Canada au<l tlio iiortlicrii 
suite's ditiTL' their luiueral ingzedicnts frum the salts still rutaiued amoug tlie sdUnienlfl 
Hi id predpitadrs of the aia-ie ut aca iu which the earb'or Pa]fix>ic»ic rocks were dttpooitctl. 
— * trtrokskal and Chemical EMuyV p. lOi. 

■ ndu in W«tt|r«* Dictionary of Chemisiry,* v. p. 1^^ 

* Dittnn'a wlilwMitt n mn tib m on the aunplee of ocean water collected by the Chal- 
lengtr apedBloiti ft^aw. ObtA tiie lowoit peroentage of salts obtaiQe<l was 3*301, from 
Uk mtiOShmimfMjAiggk^ Ooean, tonth of kt. 66^, whUe the highest was S'737, 
fi< m the tamSmKKKKK^Mn^ at about lat 23^ 



7^^-' 



Part I.] 



COMPOSITION OF SEA'WATEn, 



35 



magnesium, calcium, strontium, barium, sodium, and potassium.^ To 
these may be added arsenic, lithium, caesium, rubidium, gold, and 
probably most if not all of the other elements, though present in pro- 
portions too minute for detection. The chief constituents have been 
determined by Dittmar to be present in the proportions shown in the 
first column of the subjoined tables. Assuming them to occur in the 
combinations shown in the second column, they are present in the average 
ratios therein stat^jd ^ ; — 



I. 

Chlorine 55*292 

KromiDc 0188 

Sulplmric acid, W3 . . . 6410' 

Carbonic acid, CO.. . . . 0152 

Lime, CaO ..".... l-67(i 

Magnesia, MgO .... 6*209 

Potash, KO 1-332 

Soda, NajO 41*234 



II. 

Chloride of sodium . 
Chloride of magueHiiim 
Sulphate of magaesia 
Sulphate of lime . 
Sulphate of potash 
Bromide of magnesium 
Carbonate of lime. • 



77*758 
10-878 
4-737 
O-600 
2-465 
0-217 
0-345 



Total Salts . . 100000 



Subtract Basic Oxygen \ ^o.-qq 
equivalent to the Halogens/ ^^*^^ 

Total SalU . . 100-000 



Sea- water is appreciably alkaline, its alkalinity being duo to the 
presence of carbonates, of which carbonate of lime is one.^ In addition 
to its salts it always contains dissolved atmospheric gases. From the 
researches conducted during the voyage of the Bonite in the Atlantic 
and Indian Oceans, it was estimated that the gases in 100 volumes of 
sea- water ranged from 1*85 to 3*04, or from two to three per cent. 
Fdjui observatious made during the Porcii}>ine cruise of 18 08, it was 
ascertained that the proportion of oxygen was greatest in the surface 
water, and least in the bottom water. The dissolved oxygon and niti*o- 
^cn are doubtless absorbed from the atmosphere, the proportion so 
aljsurlxMl l>eing mainly regulated by temperature. According to Ditt- 
niar's recent determinations, a litre of sea-water at 0'^ C. will take up 
15*00 cubic centimetres of nitrogen and 8-18 of oxygen, while at 30° C 
the pixjportions sink respectively to 8*30 and 4*17. He regards the 
carlx)nic acid as occurring chiefly as carbonates, its presence in the free 
state being exceptional. During the voyage of the Challenger^ Buchanan 
aiscertained that the proportion of carbonic acid is always nearly the 
Hame for similar temperatures, the amount in the Atlantic surface water, 
lietween 2(f and 25"^ C, being 00400 gramme per litre, and in the 
burface Pacific water 00208 ; and that sea-water contains sometimes at 
least tliirty times as much carbonic acid as an equal bulk of fresh 

' Forchhiuuuier, PhU. 'Dans. civ. p. 205. According to Thorpe ami Morton (Cfiem. Hin\ 
Jonni, xxiv. p. .507), the water of the Irish Sea contains in summer rather more salti* than 
in winter. In 1000 grammea of the snmmer water of the Irish Sea they found 0-04754 
grammcflof cnrhomite of lime, 0-0050.T of ferrous carbonate and traces of silicic acid. An 
exhauKtiyc chemical iuventigation regarding the chemistry of ocean water is that by 
Dittmar in vol. i. *' PhyHicH and Chemistry,'* nt'porf o/Voijatjc 0/ the Challeutjer^ 1884. 

• Dittmar, nj*. rit. p. 203, tt M^q. ' ' ' 3 Dittmar, op, n't. p. 200. 

D 2 



3G OEOOXOSr. [Book H. 

wjitor wuiild do.' A wippoBcd greater proportion of carbonic acid in 
the deeper and colder waters of the ocean lias l>een snggested as the 
main cauee of the diKippoaranco of tlie largtT and more delicate cal- 
carooTUt pelagic orgaiiiBUis from abymiinl depoaitfi, these forms being 
luoru readily attacked and carried away in solution ; but according 
to Dittiuar, even alkaline sea-wntcr, if given Bofficient time, will 
take \ii) carlwrnate of lime in addition to what it already coutainB,' 
Another of the conBtituentB of sea-water is diffused organic mutter, 
derived from the Itodiea of dead plants and animals, and no doubt of 
great importance as furniehiug food for the lower grades of animal life.= 

n.—The S«m Globe. 

Within the atmoaplieric and m-canic envelopes lies the inner solid 
globe. The only jwrtioii of it whii-h, rishig above tlio sea, is visible 
to us, and forms what wo term Tutud, oeiuijies rather more than one- 
foTirtli of the total supcrEieieB of the glolie, or about 5:i,000,000 wjaare 
miles. 

S 1. The Outer Surface. — The land is placed chiefly in the northern 
homisphore and is disjtosed in large masses, or continents, which taper 
southwards to about half the distance between the equator and the 
Kowth pole. No adequate cause has yet been assigned for the present 
distribution of tho land. It can bo shown, however, that jMrtious of 
the coiitiiientH are of extreme gculogical antiquity. There la reason 
to believe, indeed, that the present terrestrial areas have on tho whole 
been land, or have, at least, never been submerged beneath deep water, 
from tho time of fho earliciit stratified formations; and that, on the 
other hand, the ocean-basins have ahvays been vast areas of depression. 
This subject will l>e diseussed in subsequent i>ages. 

In fho Kow Wtirld, tho continental trend is approximately north 
aud south ; in tho Old World, though less distinctly marked, it ranges 
on tlie whole east and west. The intimate relation which may be 
observed between this general trend and the direction of mountain 
i-hiiiiis, is best exhibited by the Amerieau continent. Euroi^e and 
Africa may lie considered as forming, with Asia, the vast continental 
mass of the Old World. The existing sevcram.'c of Africa and Euro{>e is 
of eoinparatively recent date. On the other hand, Europe and Asia 
were not always so continuous as at present. But even where the 
continents of the Old World are separated by sea, the intervening 
hollo\vB, though now coverwl by oceau-water, must bo reganled as 
essentially part of tho continental aveus. Asia is linked with Australia 

W,.- f. -■ ^ i.l; I.) ilr. IViiw f.Yor<eejj«iii JforIA ^Wa«(i« Kt- 

ficiiili"" !- ' ' 'I tliu racliuiixc Hcid of BM-wnter 18 iucoubiiiutioti 

Willi -"■'■ Ilia niGiiKiir for nucitinute of tlic iiruportiou uf 

nir hi >. Sminrf., XIt. p. 3^ft, Dittmw, oj>. rt). p. 209. 

• 1 ■ iwTi, lujtlc^Uieuniportloii of niguDiomnttcr. AcMnliii'' 

■<>>«iMiK'<fUk8m£s|H'<litloi,,iS7S-S(L, S.-)iiiu>lrl:,rAnu. 
) <|in<1 jppHmalnlM e.n. nf wukr, 




Pabt I.] TERRESTRIAL CONFIOURATIOK 37 



by a chain of islands. The groat contrast between the Asiatic and 
Australian faunas, however, affords good grounds for the belief that, at 
least for an enormous period of time, Asia and Australia have been 
divided by an important barrier of sea. 

While any good map of the globe enables us to see at a glance 
the relative positions and areas of the continents and oceans, most 
maps fail to furnish any data by which the general height or volume 
of a continent may be estimated. As a rule, the mountain-chains are 
exaggerated in breadth, and incorrectly indicated, while no attempt is 
made to distinguish between high plateaux and low plains. In North 
America, for example, a continuous shaded ridge is placed down the axis 
of the continent and marked " Kocky Mountains," while the vast level 
or gently rolling prairies are left with no mark to distinguish them from 
the maritime plains of the eastern and southern states. In reality 
there is no such continuous mountain-chain. The so-called "Rocky 
Mountains" consist of many independent and sometimes widely 
separated ridges, having a general meridional trend, and rising above 
a vast plateau, which is itself 4000 or 5000 feet in elevation. It 
is not these intermittent ridges which really form the great mass of 
the land in that region, but the widely extended lofty plateau, or 
rather succession of plateaux, which supports them. In Europe, also, 
the Alps form but a subordinate part of the total bulk of the land. 
If their materials could be spread out over the continent, it has been 
calculated that they would not increase its height more than about 
twenty-one fcet.^ 

Attempts have been made to estimate the probable aA'^orage height 
which would be attained if the various inequalities of the land could be 
levelled down. Humboldt estimated that the mean height of Europe 
must be about C71, of Asia 1132, of North America 748, and of Soutli 
America 1151 feet.^ Herschel supposed the mean height of Africa to bo 
1800 feet.^ These figures, though based on the best data available at 
the time, are no doubt much under the truth. In particular, the average 
height assigned to North America is evidently far less than it should be ; 
for the great plains west of the Mississippi valley reach an altitude of 
a>x>ut 5000 feet, and serve as the platform from which the mountain 
ranges rise. The height of Asia also is obviously much greater than 
this old estimate. G. Leipoldt has computed the mean height of Europe 

> M. De Lapparent (* Traits de G^ologie,* p. 62) gives the following estimate of tlio 
relative heights of different continental zones. 

Zont^ I. (from sea level to 200 metres) covers 32 per cent, of the whole surface. 
„ II. „ 200 „ 500 „ 19 

„ III. „ 500 ,,1000 „ 28 

„ IV. „ 1000 ,,2000 „ IG 

„ V. „ above „ 2000 „ 5 „ 

100 
It is obviouB that in the total bulk of a continent the mountain-chains count for quito 
a subordinate part. 

« • Asie Centrale,' torn. 1, p. 168. * ' Physical Georgaphy,* p. 119. 



38 GEOGNOSY. [Book II. 



to be 296-838 nietres (973*(>28 feet).^ According to a recent computation 
from the data in Stieler's * Atlas ' M. A. De Lapparont makes the mean 
height of the land of the globe 2120 feet, which is double what it has 
been supposed to be. He estimates the mean height of Europe to bo 
958 feet, Asia, 2884, Africa, 1975, North America, 1952, and South 
America, 1762.^ It ip of some consequence to obtain as near an 
approximation to the truth in this matter as may be possible, in 
order to furnish a means of comparison between the relative bulk of 
dififorent continents, and the amount of material on which geological 
changes can be oflFected. 

The highest elevation of the surface of the land is the summit of 
Mount Everest, in the Himalaya range (29,002 feet); the deepest 
depression not covered by water is that of the shores of the Dead Sea 
(1300 feet below sea-level). There are, however, many subaqueous 
portions of the land which sink to far greater depths. The bottom of 
the Ca8j)ian Sea, for instance, lies about 3000 feet below the general 
sea-level. The vertical difference between the maximum height of the 
land and the maximum known depth of the sea is 56,932 feet or nearly 
eleven miles. 

There are two conspicuous junction-lines of the land with its over- 
lying and surrounding envelopes. First with the Air, expressed by 
the contours or relief of the land. Second, with the Sea, expressed 
by coast-lines. 

(1) Contours or Belief of the Land. — While the surface 
of tlie land presents endless diversities of detail, its leading features 
may 1x3 generalised under the designations of mountains, table-lands, 
plains. 

Mountains, — The word "mountain" is, properly speaking, not a 
«cientific term. It includes many forms of ground utterly different 
from each other in size, shape, structure, and origin. It is popularly 
applied to any considerable eminence or range of heights, but the 
height and size of the elevated ground so designated vary indefinitely. 
In a really mountainous country the word would be restricted to 
the loftier masses of ground, while such a word as hill would 1x3 
given to the lesser heights. But in a region of low or gently 
undulating land, where any conspicuous eminence becomes important, 
the term mountain is lavishly used- In Eastern America this 
habit has been indulged in to such an extent, that what are, so to 
speak, mere hummocks in the general landscape, are dignified by the 
name of mountains. 

It is hardly possible to gi^'e a precise scientific definition to a term 
so vaguely employed in ordinary language. When a geologist uses 

' * Die Mittlcro Hobo Europas,' Leipzig, 1874. In this work tlie mean height of 
Switzerhind is put down as 1299-91 metres; Spanish i>enin8ula» 70060; Austria, 517*87; 
Italy, 517-17; Scandinavia, 428-10; France, 393-84; Great Britain, 217-70; German 
Empire, 213-6G ; Russia, 167-09 ; Belgium. 163-36 ; Denmark (exclusive of [oeland), 35-20; 
the Netherlands (exclusive of Luxembourg and the tracts below sea-level), 9-61. 

» * Traits de Geologie/ p. 60. 



/ 



Part I.] TYPES OF MOUNTAINS. 39 

the word, he must either be content to take it in its familiar vague 
sense, or must add some phrase defining the meaning which he attaches 
to it. He finds that there are three loading and totally distinct types 
of elevation which are all popularly termed mountains. 1, Single 
eminences, standing alone uj^on a plain or table-land. This is essentially 
the volcanic type. The huge cones of Vesuvius, Etna, and TenerifFe, as well 
as the smaller ones so abundant in A^olcanic districts, are examples of it. 
There occur, however, occasional isolated eminences that stand up as 
remnants of once extensive rock-formations. These have no real 
analogy with volcanic elevations, but should be classed under the 
next lype. The remarkable hntfcs of Western America are good 
illustrations of them. 2. Groups of eminences connected at the sides 
or base, often forming lines of ridge between divergent valleys, and 
owing their essential forms not to underground structure so much 
as to superficial erosion. Many of the more ancient uplands, both in 
the Old World and the New, furnish examples of this type, such as the 
Highlands of Scotland, the hills of Cumberland and Wales, the high 
^)unds between Bohemia and Bavaria, the Laurentide Mountains of 
(^*anada, and the Green and White Mountains of Now England. 3. Lines 
of lofty ridge rising into a succession of more or less distinct summits, their 
general external form having relation to an internal plication of their 
component rocks. These linear elevations, whose existence and trend 
have been determined immediately by subterranean movement, are the 
true mountain-ranges of the globe. They may l)e looked upon as the 
rrest^ of the great waves into which the crust of the earth has been 
thrown. All the great mountain-lines of the world belong to this type. 
Leaving the details of mountain-form to be described in Book VII., 
we may confine our attention hero to a few of the more important 
general features. In elevations of the third or true mountain type, 
there may Ik? either one line or range of heights, or a scries of parallel 
and often coalescent ranges. In the Western Territories of the UnitrMl 
Stites, the vast plateau has been, as it were, wrinkled by the ujirise of 
long intermittent ridges, with broad plains and basins between them. 
Each of these forms an independent mountain-range. In the heart of 
Europe, the Bernese 01>erland, the Pennine, liC^pontine, Ilhaetic, and 
••ther ranges form one great Alpine chain or system. 

In a great mountain-chain, such as the Alps, Himalayas, or Andes, 

IW© IB one general i)ersi8tent trend for the successive ridges. Here 

iii\<l there, lateral offsh(X)ts may diverge, but the dominant direction of 

x\\*i axifi of the main chain is generally observed by its component ridges 

m\til they disappear. Vet while the general parallelism is preserved, 

110 single range may l>e tracealde for more than a comparatively short 

•ViHtanee; it may be found to pass insensibly into another, while a tliird 

DWJ be seon to Ix^gin on a slightly different line, and to continue with 

^H name dominant trend until it in turn becomes confluent. The 

^'^rioan ranges are thus apt to assume an arrangement en /'(Man, 

The ranges an? separated by hngitudlvnl valleys, that is, depressions 



40 GEOGNOSY. [Book II. 



coincident with the general direction of the chain. These, though 
BometimoB of great length, are relatively of narrow width. The valley 
of the Khone, from the source of the river down to Martigny, oflFers an 
excellent example. By a second series of valleys the ranges are 
trenched, often to a great depth, and in a direction transverse to the 
general trend. The Rhone furnishes also an example of one of these 
tramverse valleys, in its course from Martigny to the Lake of Geneva. 
In most mountain regions, the heads of two adjacent transverse valleys 
are connected by a depression or pass (col, joch). 

A large block of mountain ground, rising into one or more 
dominant summits, and more or less distinctly defined by longitudinal 
and traverse valleys, is termed in French a massif — a word for which 
there is no good English equivalent. Thus in the Swiss Alps we 
have the massifs of the Ghirnisch, the Todi, the Matterhorn, the 
Jungfrau, &c. 

Very exaggerated notions are common regarding the angle of 
declivity in mountains. Sections drawn across any mountain or 
mountain-chain on a true scale, that is, with the length and height on 
the same scale, bring out the fact that, even in the loftiest mountains, 
the breadth of base is always very much greater than the height. 
Actual vertical precipices are less frequent than is usually supposed, 
and even when they do occur, form minor incidents in the general 
declivity of mountains. Slopes of more than 30^ in angle are likewise 
far less abundant than casual tourists believe. Even such steep 
declivities as those of 38° or 40° are most frequently found as 
/a/w«-filopes at the foot of crumbling clififs, and represent the angle 
of repose of the disintegrated debris. Here and there, wheie the 
blocks loosened by weathering are of large size, they may accumulate 
upon each other in such a manner that for short distances the average 
angle of declivity may mount as high as 65°. But such steep slopes 
are of limited extent. Declivities exceeding 40°, and bearing a largo 
proportion to the total dimensions of hill or mountain, are always found 
to consist of naked solid rock. In estimating angles of inclination from 
a distance, the student will learn by practice how apt is the eye to bo 
deceived by perspective and to exaggerate the true declivity, sometimes 
to mistake a horizontal for a highly inclined or vertical line. The 
mountain outline shown in Fig. 2 presents a slope of 25° between a and 
6, of 45° between h and c, of 17° between c and d, of 40° between d an^ e, 
and of 70° between e and/. At a great distance, or with bad conditions 
of atmosphere, these might be believed to be the real declivities. Yet if 
the same angles be observed in another way (as on a cottage roof at B), 
we may learu that an apparently inclined surface may really be 
horizontal (as from a to 6 and from c to d), and that by the effect 
of perspective, slopes may be made to appear much steeper than they 
really are.* 

» Mr. Rufikin hfiA well illustratcfl tin's point. Soc * Modern Painters,' vol. iv. p. 183, 
wlienee the illusimtions in the text are tnkon. 



Pakt I.] 



TABLE-LANDS. 



i/Lach. evil haa resulted in geological leBearch from the 
ext^gereted angles of dope in seotionB and di^ramg. It 
ia therefore desirable that the student should, from the be- 
ginning, accoBtom himself to the drawing of outlinee as nearly 
as pnmtble on a true scale. The accompanying section of 



41 

use of 




tlio Alps by De la Heche (Fig. H) is of interest in this 
respect, as one of the earliest illustrations of the advanfiige 
of constructing geological sections on a true scale as to the 
relative proportions of height and length.' 

TabU-lanih or Plafeaitx are elevated regions of flat or 
tindalating country, rising to heights of 1000 feet and np- 
wanls above the level of the sea. They ore sometimes 
bordered with steep slopes, which desccml from their edges, 
as the table-land of the Spanish peninsiila docs into the sea. 
In other cases, they gradually sink into the plains and have 
no definite boundaries ; thus the prairie-land west of the 
MiMonii slowly and imperceptibly ascends until it becomes 
a-VABt plateau from 4000 to 5000 feet above the sen. Occa- 
Hionally a high table-land is encircled with lofty mountaine, 
iiM in those of Quito and Titicaca among the Andes, and 
that of tho heart of Asia ; or it forme in itself the platform 
on vrbich lines of mountains stand, as in North America, 
where the ranges inclnded within the Rocky Mountains reach 
c-levatioBB of from 10,000 to 14,000 feet above tlio sea, but 
ti«t more than from 5000 to in,000 foot above tho table-land. 



t..tAG. 



n awl Views, illiiBlmtivf of nciil"gi«i! PIict 



' iRao, ctfii ohiif 



42 GEOGNOSY. [Book IT. 

Two types of table-land Btnicture may be observed. 1. Table-lands 
consisting of level or gently undulated sheets of rock, the general surface 
of the country corresponding with that of the stratification. The Rocky 
^Mountain plateau is an example of this type, which may be called that 
of Deposit, for the flat strata have been equably upraised nearly in 
the position in which they were deposited. 2. Table-lands formed out 
of contorted, crystalline, or other rocks, which have been planed do^vn 
hy superficial agents. This type, where the external form is independent 
of geological structure, may be termed that of Erosion. The fjelds of 
Norway are portions of such a table-land. In proportion to its antiquity, 
a plateau is trenched by running water into systems of valleys, until in 
the end it may lose its plateau character and pass into the second type 
of mountain ground above described. This change has largely altered 
the ancient table-land of Scandinavia, as will be illustrated in Book VII. 

Plains are tracts of lowland (under 1000 feet in height) which skirt 
the sea-board of the continents and stretch inland up the river valleys. 
The largest plain in the world is that which, beginning in the centre of 
the British Islands, stretches across Europe and Asia. On the west, it is 
bounded by the ancient table-lands of Scandinavia, Scotland, and Wales 
on the one hand, and those of Spain, France, and Germany, on the other. 
Most of its southern boundary is foimed by the vast belt of high ground 
which sprcjids from Asia Minor to the east of Sibeiia. Its northern 
jnargin sinks beneath the waters of the Arctic Ocean.. This vast region 
is divided into an eastern and western tract by the low chain of the 
T'ral Mountains, south of which its general level sinks, until underneath 
the Caspian Sea it reaches a depression of about 3000 feet below sea- 
level. Along the eastern sea-board of America, lies a broad belt of low 
plains, which attain their greatest dimensions in the regions watered by 
the larger rivers. Thus they cover thousands of square miles on the 
north side of the Gulf of Mexico, and extend for hundreds of miles up 
the valley of the Mississippi. Almost the whole of the valleys of tho 
Orinoco, Amazon and La Plata is occupied with vast plains. 

From the evidence of upraised marine shells, it is certain that largo 
portions of the great plain of the Old World comparatively recently 
fomied part of the sea-floor. It is likewise probable that the beds of 
some enclosed sea-basins, such as that of the North Sea, have formerly 
been plains of the dry land. 

It is obvious, from their distribution along river- valleys, and on the 
areas between the base of high grounds and the sea, that plains are 
essentially areas of deposit. They are the tracts that have received tho 
detritus washed down from the slopes above them, whether that detritus 
has originally accumulated on the land or below the sea. Their surfaco 
presents everywhere loose sandy, gravelly, or clayey formations, indica- 
tive of its comparatively recent subjection to the operation of running 
water. 

(2) Coast- lines. — A mere inspection of a map of the glolje brings 
before the mind the striking differences which the masses of land present 



Part L] COAST-LINE^ OF CONTINENTS, 43 



in their lino of junction with the sea. As a rule, the southern conti- 
nents possess a more uniform unindonted coast-lino than the northern. 
It has been estimated that the ratios between area and coast-line among 
the different continents, stand approximately as in the following tiible : — 

f Europe has 1 geographical m\]o of coast-line to 143 square miles of Hiirfaop. 
Xorth America „ „ 2(m „ 

Asia, including tlio islands „ 4G9 ., 

1 Africa ., ,, 895 ,, 

South America „ „ 4:H ., 

Australia „ „ 382 „ 



In estimating the relative potency of the sea and of the atmospheric 
agents of disintegration, in the task of wearing down the land, it is 
evidently of great importance to take into account the amount of surface 
respectively exposed to their operations. Other things being equal, 
there is relatively more marine erosion in Europe than in North America. 
But we require also to consider tlie nature of the coast-lino, whether flat 
and alluvial, or steep and rocky, or with some intermediate blending of 
these two characters. By attending to this point, we are soon led to 
t»bserve such great differences in the character of coast-lines, and such 
an obvious relation to differences of geological structure, on the onci 
hand, and to diversities in the removal or deposit of material, on the 
other, as to suggest that the present coast-lines of the globe cannot he 
aboriginal, but must be referred to the operation of geologi(;al agents still 
at work. This inference is amply sustained by more detailed investi- 
gation. While the general distribution of land and water must un- 
doubtedly be assigned to terrestrial movements affecting the solid glulx), 
the present actual coasts of the land have chiefly been produced 
by lor,-al causes. Ileadlands project from the land because, for th(^ most 
part, they consist of rock which has been better able to withstand the 
shfxjk of the breakers, liays and creeks, on the other hand, have been rut 
by the waves out of less durable materials. Again, })y the f^inking (»f 
land, ranges of hills have become capes and headlands, while the valleys 
have passed into the condition of bays, inlets, or fjords. By the uprise of 
the sea-bottom, tracts of low alluvial ground have been added to the laud. 
Hence, speculations as to the history of the elevation of the land, 
based merely upon inferences from the form of coast-lines as oxpressMl 
upon ordinary maps, to l>e of real service, demand a careful scrntiny 
of the actual coast-lines, and an amount of geological invcHtigutioii 
which wonld require long and patient toil for its acooinpliHimionl. 

Passing from the mere external form of the land to the comjMJsitlun 
and fitnictiire of its materials, we may begin by considerinj;- lln- ;:.<'iieial 
«l-n«ity of the entire globe, computed from obsei-vations aii«] <(»inj.ai«d 

•:. iLuit of the outer and accessible portion of tlie planrt. Ii'<t<;n m «• 
ill- already Ijeen made to the aunparative density of th«.- jjhiIj ajjK^ng 
*':«i other memlyers of the solar system. In inqtiiri'.-h: r<';iardiii^ iho 
liistory of our globe, the density of the whole jna^ oi ih« jilajnH, as 



44 GEOGNOSY. [Book H. 



compared with water — the standard to which the specific gravities of 
terrestrial bodies are referred — ^is a question of prime importance. 
Various methods have been employed for determining the earth's 
density. The deflection of the plumb-line on either side of a mountain 
of known structure and density, the time of oscillation of the pendulum 
at great heights, at the sea level, and in deep mines, and the comparative 
force of gravitation as measured by the torsion balance, have each been 
tried with the following various results : 

Plumb-line experiments on Schiehallien (Maakelyne and Playfair) 

gave as the mean density of the earth ..... 4*713 

Do. on Arthur's Seat, Edinburgh (James) ....'. 5'316 

Pendulum experiments on Mont Cenis (Carlini and Giulio) . 4*950 

Do. in Ha^rton ooal-pit, Newcastle (Airy) 6*565 

Torsion balance experiments (Cavendish, 1798) 5 '480 

Do. do. (Reich, 1838) 5*49 

Do. do. (Baily, 1843) 5-660 

Do. do. (Comu and Bailie, 1872-3) . . . 5-50-5'56 

Though these observations are somewhat discrepant, we may feel 
satisfied that the globe has a mean density neither much more nor much 
less than 5*5 ; that is to say, it is five and a half times heavier than one 
of the same dimensions formed of pure water. Now the average 
density of the materials which compose the accessible portions of the 
earth is between 2*5 and 3 ; so that the mean density of the whole globe 
is about twice as much as that of its outer part. We might, therefore, 
infer that the inside consists of much lieavier materials than the outside, 
and consequently that the mass of the planet must contain at least two 
disBiniilar portions — an exterior lighter crust or rind, and an interior 
heavier nucleus. But the effect of pressure must necessarily increase 
ihe specific gravity of the interior, as will be alluded to further on. 

§2. The Crust. — It was formerly a prevalent belief that the exterior 
and interior of the globe diifered from each other to such an extent that, 
while the outer parts were cool and solid, the vastly more enormous 
inner intensely hot part was more or less completely liquid. Hence the 
term " crust" was applied to the external rind in the usual sense of that 
word. This crust was variously computed to be ten, fifteen, twenty, or 
more miles in thickness. In the accompanying diagram (Fig. 4), for 
example, the thick line forming the circle represents a relative thickness 
of 100 miles. There are so many proofs of enormous and wide-spread 
corrugation of the materials of the earth's outer layers, and such 
abundant traces of former volcanic action, that geologists have naturally 
regarded the doctrine of a thin crust over a liquid interior as necessary 
for the explanation of a large class of terrestrial phenomena. For 
reasons which will be afterwards given, however, this doctrine has been 
opposed by eminent physicists, and is now abandoned by most geologists. 
Nevertheless the term " crust " continues to be used, apart from all 
theory regarding the nucleus, as a convenient word to denote those cool, 
upper, or outer layers of the earth's mass in the structure and history of 
which, as the only portions of the planet accessible to human obser- 



Past L] DISTRIBUTION OF TERRESTRIAL DENSITY. 45 



vation, lie the chief materials of geological investigation. The chemical 
and mineral constitution of the crust is fully discussed in later pages 
(p. 58, ei seq.). 

§ 3. The Interior or Nucleus. — Though the mere outside skin of 
our planet is all with which direct acquaintance can be expected, the 
irregular distribution of materials beneath the crust may be inferred 
from the present distribution of land and water, and the observed 
differences in the amount of deflection of the plumb-line near the sea and 
near mountain chains. The fact that the southern hemisphere is almost 
wholly covered with water, appears explicable only on the assumption of 
an excefls of density in the mass of that half of the planet. The existence 
of such a vast sheet of water as that of the Pacific Ocean is to be 
accounted for, says Archdeacon Pratt, by the presence of " some excess 




Fig. 4.— Supposed Crust of the Earth, 100 Miles thick. 

<»r matter iu the solid parts of the earth between the l*iicitic Ocean and 
the earth's centre, which retains the water iu its place, otherwise the 
ocean would flow away to the other parts of the earth." ^ The same 
writer points out that a deflection of the plumb-line towards the sea, 
which has in a number of cases been observed, indicates that **the 
density of the crust beneath the mountains must bo less than tliat Ijelow 
the plains, and still less than that below the ocean-bed." '^ Apart, 
therefore, from the depressions of the earth's surface, in wliich the oceans 
lie, we must regard the internal density, whether of crust or nucleus, to 
bo somewhat irregularly arranged, — there being an excess of heavy 
materials in the water-hemisphere and beneath the ocean-beds, as com- 
pared with the continental masses. 

It has been argued from the difference between the spccitic gravity 

' ' Figure of the Earth,* 4th edit. p. 2,S0. 

- Op. cit. p. 200. Sec also Herschel, * Phvs. Geog.' § 13 ; O. Fisher, Cambridge 
Phil Trans, xii., part ii. ; * Physics of the Earth s Crust,* p. 75. ^ 



4G GEOGNOSY. [Book II. 



of the wholo globe aud that of the crust, that the interior must consist 
of heavier inatei'ial, and may be metallic. But the effect of the enonnous 
internal presHure, it might be supposed, should make the density of the 
nucleus much higher, oven if the interior consisted of matter which, on 
the surface, would bo no heavier than that of the crust. In fact, we 
might, on the contrary, argue for the probable comparative lightness 
of the substance composing the nucleus. That the total density of the 
planet does not greatly exceed its observed amount, may indicate that 
some antagonistic force counteracts the effect of pressure. The oidy force 
we can suppose capable of so acting is heat, though to what extent this 
eounterbalaneing tukes place i.s still unknown. It must be admitted 
that we are still in ignorance of the law that regulates the compression 
of solids under such vast pressure as must exist within the earth's 
interior. We know that gases and vapours may be compressed into 
licpiids, sometimes even into solids, and that in the liquid condition 
another law of compressibility begins. AVe know also from experiment 
that some substances have their melting-point raised by pressure.^ It 
may be that the same effect takes place within the earth ; that pressure 
increasing inward to the centre of the globe, while angmenting the 
density of each successive shell, may retain the whole in a solid condition, 
yet at temperatures far above the normal melting points at the surface. 
Hence, on this view of the matter, it is conceivable that the difference 
l)etween the density of the whole globe and that of the crust may be duo 
to pressure, rather than to any essential difference of composition. Dr. 
Pfaff, indeed, offers a calculation to show that the mean terrestrial density 
of iy'o is not incom2)atible with the notion that the whole globe consists 
of materials of the same density as the rocks of the crust.^ It is possible 
that the gases which, largely given off at volcanic foci, must exist dis- 
solved in the hot magma of the nucleus and possess a very high tension, 
may counteract the effects of compression and thus reduce density. 

Analogies in the solar system, however, as well as the actual struc- 
ture of the rocky crust of the globe, suggest that heaver metiillic 
ingredients possibly predominate in the nucleus. If the materials of tho 
globe wore once, as they are believed to have been, in a liquid condition, 
they would then doubtless be subject to internal arrangement, in accord- 
ance with their relative specific gravities. We may conceive that, as in 
the case of the sun, as well as of the solar system generally (ante, p. 8), 
thei*e would be, so long as internal mobility lasted, a tendency in the 
denser elements to gravitate towards the centre, in the lighter to 
accumulate outside. That a distribution of this nature has certainly 
tiiken place, to some extent, is evident from the structure of the envelopes 
and crust. It is what might be expected, if the constitution of the glolje 
resembles, on a small scale, the larger planetary system of which it forms 
a part. The existence even of a metallic interior has been inferred from 

* Under a pressure of 792 atmospheres, spermaeoti 1ms its melting jwint raised from 
51^ to 80-2^ ami wax from 64-5° to 80-2°. 

• » Allgemeine Greologio als exacte WisseiiBchnft,' p. 42. 



Part I.] THE EARTIPS INTEHNAL HEAT, 47 



iho metalKferous veins which traverse the crust, and which are commonlv 
supposed to have been filled from beluw.^ 

Evidence of Internal Heat. —In the evidence obtainable as to 
the former history of the earth, no fact is of more importance than the 
existence of a high temperature l)eiieath the cnist, whicli has now been 
placed beyond all doubt. Tliis feature of the planet's organisation is 
made clear by the following proofs : — 

(1.) VoUaiwes, — In many regions of the earth's surface, openings exist 
from which steam and hot vapours, ashes and streams of molten rock 
are from time to time emitted. The abundance and wide diflfusion of 
these openings, inexplicable by any niei-e local causes, nmst be regar<le<l 
as indiciitive of a very high internal temperature. If to the still active 
vents of eruption, we add those wliich have formerly Ijcen the channels 
of communication between the interior and the surface, there are 
probably few largo regions of the globe where proofs of volcanic 
action cannot be found. Everywhere we meet with masses of molten 
ruck which have risen from IkjIow, as if from some general reservoir. 
The phenomena of active volcanoes are fully discussed in Book III. 
Part I. 

(2.) Hoi Springs, — "Where volcanic eruptions have ceased, evidence of a 
high internal temperature is still often to be found in springs of hot water 
wliich continue for centuries to maintain their heat. Thermal springs, 
however, are not confined to volcanic districts. They sometimes rise even 
in regions many hundreds of miles distant from any active volcanic vent. 
Tlie hot springs of Bath (temp. 120" Fahr.) and BuxUm (temp. 82Tahr.) 
in England are fully 9o0 miles from the Icelandic volcanoes on the one 
side, and 1100 miles from those of Italy and Sicily on the other. 

(3.) BorimjSf Wells and Mines. — The influence of the seasonal changes 
uf temperature extends downward from the surface to a depth which 
varies according to latitude, to the thermal conductivity of soils and 
nnrks, and i)erhaps to other causes. The cold of winter and the heat of 
Mimmcr may be regarded as following each other in siiecessive waves 
downward, until they disappear along a limit at which the temperature 
remains constant. This zone of invariable temperature is commonly 
K-licved to lie at a depth of somewhere Ijctween GO and 80 feet in tem- 
l)erate regions. At Yakutsk in Eastern Silwria (lat. 62° N.), liowever, 
as shown in a well-sinking, the soil is peiinanently frozen to a depth of 
ulMmt 700 fect.2 jj^ j^ya, on the other hand, a constant temperature is 
sjiid to 1k3 met with at a depth of only 2 or 8 feet.^ 

It is a remarkable fact, now verified by observation all over the 

* L«*j<L'iulre HUpiwsc-il tliut lliu density being U'.") iit the surface, it is i>'5 ut liiilf 
tlio k-uj^Ui of the rudius imd 11*3 ut the centre. More recently E. licolio calcuhited 
ihe*c tlensiticH to be 2*1, S'."), and lOMj respectively. Thclate David Forbes 8Uggcste<l 
that the planet might be supposed to consist of three layers of uniform densities, 
fucktsed one within the other, llie density increasing towards the centre in arithmetical 

froRressiou. Allowing 2-.') as the ppeeillc gravity of the crust or outer layer, he assigned 
2*0 or thcrcal)Out8 na that rf tlie middle layer, and supposed that llie inner nucleus 
migiit po68C8S one avemging 20'0. (Popuhir Sr^teuo' lierinr^ April, ISO'J.) 

* IIclmer»?on, Jhit. AmK-. lieiKtrt^ 1S71, p. 22. •* Junj^diuhn's Mnvn,* ii. p. 771. 



48 OEOONOSr. [Book H. 

world, that below tho limit of the influence of ordiuaiy ucasonal changes 
the temperature, so far aa we yet know, in nowhere found to diminish 
ilownwarde. It always riaoa ; and its rate of increment never falU niucli 
below the average. The only eYceptional cases occur under circam- 
Btauces not difficult of e!£j,)lanation. On the one hand, the neighbourhood 
of liot-springs, of large masBes of lava, or of other manifestations of 
volcanic activity, may raise the subterranean toniperatnro much above 
lU) normal condition; and this augmentation may not disapjicar for many 
tliimisaud years oftcr the volcanic aclivity has wholly ceaseil, since tlio 
cooling down of a subtenituean maun of lava would necessarily bo a Tory 
slow process. It has oven been proi)oeed to estimate the age of subter- 
ranean masses of intrusive lava from their excess of temperature above 
the noiinal amount for tlicir isogeotherms (luica of equal earth-tem- 
perature), some probable initial temperature and rate of cooling being 
assumed. On the otlier hand, the spread of a thick mass of snow and itxi 
over any considerable area of the earth's surface, and its coutiuuanco 
there for several thousand years, would so depress the isogeothcrmB that, 
for many centuries afterwards, tlicro would bo a fall of temperature for » 
certain distance doivnwards. At the present day, in at least the more 
northerly j>arts of the northern hemisphere, there are such evidences of a 
former moro rigorous climate, as in the well-siuiiug at Yakutsk just 
ruferred to.' Sir William Thomson^ has calculated that any conaiderablo 
ai'ea of the earth's surface covered for several thousand years by suow 
or i**, aiid retaining, after the disappearance of that frozen covering, an 
average surface temperature of 13° C., " would during 900 years show a 
decreasing temperature for some depth down from the surface, and 3600 
years after the clearing away of the ice would still show residual 
effect of the ancient cold, in a half rate of augmentation of temperature 
downwards in the upper strata, gradually increasing to the whole 
normal rate, which would be sensibly reached at a depth of COO 
metres." 

Beneath the limit to which the iofluonce of the changes of the seasons 
extends, ohservations all over the globe, and at many diffcrout elevations, 
give a rate of increase of temperature downwai-ds, or " temperature gin- 
dient," which has been usually taken to bo 1° Fahr. for every 50 or 60 
feet of deaoent, this computation being based especially on obser\'atious 
iu deep mines and borings. According, however, to data collected by 
a Committee of the British Association during the last sixteen years, 
the average gradient appears to be 1° Fahr. for every C4 feet, or -^f of u 
degree per foot. Isogeotherms near the surface follow approximately the 
B of the snrface, but ore flatter than these, and "their flattening 
!■.. lower ones, until at a considerable depth they 

! iiiiiigiiral Lccfurc, l^T.'i.]!. -trolms KUg^'iitod Uiat tutiui tnuni 

atli's KUifocc iluriijj^ tliU cold period, and ti> the oonseqacnt 

. I ' i.ii'AD Uotbennul liuug. tl».t alleged present coniparatiTO iiuietiHle 

i< M 1 1 \a ui br Httribnieil. tlitinliiiial licat not Imviiig jot iccoTered its 

liu- uulat eru*t 

.-"-- HwpniU. 1870, &vUwK, i-. v.. 




Part IJ 



THE EARTH'S INTERNAL HEAT. 



49 



become sensibly horizontal planes. Tbo temperature gradient is con- 
sequently steepest beneath gorges and least steep beneath ridges.'* ^ 

Irregularities, in the Downward Increment of Heat. 
— While there is everywhere a progressive increase of temperature 
downwards, its rate . is . by no means uniform. The more detailed 
ol)servations which have been made in recent years have brought to 
light the important fact that considerable variations in the rate of 
increase take place, oven in the same bore. The tempei-atures obtained 
:it different depths in the Hose Bridge colliery shaft, Wigan, for instance, 
road as in the following columns : — 

TuiniKsrature 
(Kulir.) 

89 

IKJ^ 

• • • a «/ 1 2 

i)2 

. . . . . 93 

93J 

..... 94 



liepthin 


Temperature 
(Fahr.) 


Depth ill 
Yards. 


•)Oo . • . 


... 78 


745 


G05 . . . 


... 80 


761 


630 . . . 


... 83 


775 


663 ;. . 


.• . . 85 


783 


671 .. . 


... 86 


800 


679 .. . 


... 87 


806 


734 .. . 


. . . 88i 


815 



At La Chapelle, in . an important well made for the water-supply of 
Paris, observations have been taketi of the temperature at different 
depths, as shown in the subjoined table : — ^ 



Depth in 
Ifftrea. 


Ti'mperature 
(Fahr;). 


100 .. . 


. . . 59-5 


200 .. . 


. . • 618 


•Hill . . • 


. . . (;5-5 


400 .. . 


. . . (;90 



Depth ill 
'Metros. 

.500 . 

600 . 

660 . 



Temperature 

(Fahr.). 
. 72-6 
. 75*0 
. 760 



In drawing attention to the temi>orature-ol>HorvationH at the Rose 
Bridge collier}'— the deepest mine in Great BriUiin — Professor Everett 
jNjiuts out that, assuming the surface temperature to be 49° Fahr., in the 
first 5.">8 yards the rate of rise of temperature is l"" for 57*7 feet; in the 
next 257 yards it is 1"* in 48*2 feet ; in the portion between 605 and 671 
vanls— a distance of only 198 feet—it is l' in 33 feet; in the lowest 
Jiortion of 432 feet it is 1"* in 54 feet. * When such irregularities occur in 
the same verticar shaft, it is not surprising that the average should 
vary so much in different places. 

There can l)e little doubt tliat (»ne main caust* of these variations 
Ls to be sought in the different thermal couducjtivitit^s of the rocks of 
the earth's crust. The first accurate measurements of tlie conducting 
l^^wers of rocks were made by the late J. 1). Forbes at Edinburgli 
( 1H37-1845). He selected three sites for his thermometers, one hi 
** trap-rock " (a i>orphyrite of Lower Carl)oniferous age), one in loose 




lH7y-80. Min, Pntf, N, Enghtnd Innt. Mining-Mfchtn. Knghi. xxxil (1883) p. 19. 
• R«:porti»<>f Committw on ITiidrrjp-oiuul ToiiiiM'ratnre/ Brit. Aanor. Ufp. from 1S68 to 
1882, with mimmary of rwnUH in \ho volnino for 18S2. 

* BrH.Atim:, Rrp. 1873, SectioiiP, p. 2:j4. ^ lliit. A^^or. li, !>. \H7ii, S-i^tious, p. :;i. 



50 OtJOQNOSr. [Book It. 



band, and one in sandstone, eaoh set of instruments being sunk to depthfi 
of 3, 6, 12 and 24 French feet from the surface. He found that the 
wave of summer heat reached the bulb of the deepest instrument 
(24 feet) on 4th January in the trap-rock, on 25th December in the sand, 
and on 3rd November in the sandstone, the trap-rock being the worst 
conductor and the solid sandstone by far the best.^ 

As a rule, the lighter and more porous rocks oflfer the greatest 
resistance to the passage of heat, while the more dense and crystalline 
offer the least resistance. The resistance of opaque white quartz is 
expressed by the number 114, that of basalt stands at 273, while that of 
eannel coal stands very much higher at 1538, or more than thirteen 
times that of quartz.^ 

It is evident alsoj from the texture and structure of most rocks, that 
the conductivity must vary in different directions through the same 
mass, heat being more easily conducted along than across the " grain," 
the bedding, and the other numerous divisional surfaces. Experiments 
have been made to determine these variations in a number of rocks. 
Thus, the conductivity in a direction transverse to the divisional planes 
l)eing taken as unity, the conductivity parallel with these planes was 
found in a variety of magnesian schist to be 4*028. In certain slates 
and schistose rocks from central France, the ratio varied from 1 : 2*56 to 
1 : 3*952. Hence in such fissile rocks as slate and mica-schist, heat may 
travel four times more easily along the planes of cleavage or foliation 
than across them.^ 

In reasoning upon the discrepancies in the rate of increase of 
subterranean temperatures, we must also bear in mind that convection 
by percolating streams of water must materially affect the transference 
of heat from below.* Certain kinds of rock are more liable tlian others 
to be charged with water, and, in almost every boring or shaft, one or 
more horizons of such water-bearing rocks are met with. The effect ot 
interstitial water is to diminish thermal resistance. Dry red brick has 
its resistance lowered from 680 to 405 by being thoroughly soaked in 
water, its conductivity being thus increased 68 i)er cent. A piece of 
sandstone has its conductivity heightened to the extent of 8 per cent, by 
being wetted.* 

Mallet contended that the variations in the amount of increase 
lu subterranean temperature are too great to permit us to believe 
them to be due merely to differences in the transmission of the general 

* Ttans, Bity. Soc Edin. xvi. p. 211. 

* Hcrschel and Lebour (British Aaaociution Committee on Thennal Couduotivitios of 
llocks), Bi-it. Assoc, Bep. 1875, p. 59. The final Report is in the vol. for 1881. 

* •* Report of Committee on Thermal Conductivities of Rock,"-Brt<. Aaaoc Hep, 1875, 
p. 61. Jannettaz, BulL 8oc, 04oL France (April-June, 1874)* ii. p. 264. Thia observer 
has carried out a scries of detailed researches on the propagation of heat through rocks 
which will bo found in BuXL Soc GM, Prance^ tomes i.-ix. (.3rd series). 

* In the great bore of Sperenberg (4172 feet, entirely in rock-salt, except the firut 
283 feet) there is evidence that the water near the top is warmed 4 J° Fahr. by convection. 
BriL Amoc 1882, jx 78. 

* Ilerschd and Lebour, BrU. Atwd. Bep, 1876, p. 58. 



Pabt 1.] THE EAttTH'S INTERIOR, 51 



internal heat, and that they point to local accessionB of heat arising 
from transformation of- the mechanical work of compression, which 
is due to the constant cooling and contraction of the globe. ^ But it 
may be replied that these variations are not greater than, froni the 
known divergences in the conductivities of rocks, they might fairly be 
exjiected to be. 

Probable Condition of the Earth's Interior. — Various 
theories have been propounded on this subject. There are only three 
which merit serious consideration. (1.) One of these supposes theidanet 
to consist of a solid crust and a molten interior. (2.) The second holds 
that, with the exception of local vesicular spaces, the globe is solid and 
rigid to the centre. (3) The third contends that while the mass of thd 
globe is solid, there lies a liquid substratum beneath the crust. 

1. The arguments in favour of internal liquidity may be summed up 
as follows, (a.) The ascertained rise of temperature inwards from the 
surface is such that, at a very moderate depth, the ordinary melting 
jiuint of even the most refractory substances would be reached. At 20 
lutles the temperature, if it increases progressively, as it does in the 
depthfl accessible to observation, must be about 1700° Fahr. ; at 50 miles 
it must be 4600°, or far iiigher than the fusing-point even of so stubborn 
a metal as platinum, which melts at 3080° Fahr.^ (6.) All over the 
world volcanoes exist from which ste^m and torrents of molten ' lava are 
from time to time erupted. Abundant as are the active volcanic vents, 
they forin but a small proportion of the whole which have been in 
operation since early geological time. It has Ikjcu inferred, therefore, that 
these numerous funnels of communication with the heated interior could 
not have existed and poured forth such a vast amount of molten rock, unless 
they drew their supplies from an immense internal molten nucleus, (c.) 
When the products of volcanic action from different and widely-separated 
ri'gions are compared and analysed, tliey are found to exhibit a i*emark- 
able uniformity of character. Lavas from Vesuvius, from Hecla, from 
the Andes, from Japan, and from New Zealand present such an agTee- 
iiient in essential particular as, it is contended, can only be accounted 
for on the supposition that they have all emanated from one vast 
cimnion source.^ {d.) The abundant earthquake-shocks which affect 
large areas of the globe are maintained to Ije inexplicable unless on the 
huppisition of the existence of a thin and somewhat flexible crust. 
TheHC arguments, it will 1x5 observed, are only of the nature of inferences 
<lniwn from observations of the present constitution of the globe. They 
are based on geological data, and have been frequently urged by geo- 
h^jpsts as supporting the only view of the nature of the earth's interior, 
Kupi»08ed by them to be compatible with geological evidence. 

' - Volcanic Energy," PhU. Tram, 1875. 

^ But Sir W. Thomson lias shown that if the rate of increase of temperature is taken 
to be 1** for every 51 feet for the first 100,000 feet, it will begin to diminish below that 
limit, being only 1° in 2550 feet at 800,000 feet, and then rapidly lessening. Tram, 
Roy, Sac, Edh\, xxiii. p. 163. 

■ 8c« D. Forbi?H, Vopular Science Hcvieir, April, 1861). 

E 2 



52 QE00N08Y. [BooK.n, 



2. The arguments against the tniemal fluidity of the earth are based on 
physical and abtronomical considerations of . the greatest importance. 
They may be arranged as follows :— 

(a.) Argument from precession and nutation.— The problem of the 
internal condition of the globe was attacked as far back as the year 1839 
by Hopkins, who calculated how far the planetary motions of precession and 
nutation would be influenced by the solidity or liquidity of the Cfarth's 
interior. He found that the processional and nutational movements 
cuuld not possibly be as they are, if the planet consisted of a central core 
of molten rock surroimded with a crust of twenty or thirty miles in 
thickness ; that the least possible thickness of crust consistent with the 
existing movements was from 800 to 1000 miles ; and that the whole 
might even be solid to the centre, with the exception of comparatively 
small vesicular spaces filled witli melted rock.^ 

M. Delaunay,^ threw doubt on Hopkins' views, and suggested that, if 
the interior were a mass of sufficient viscosity, it might behave as if it 
were a solid, and thus the phenomena of precession and nutation might 
not be aflfected. Sir William Thomson, who had already arrived at the 
conclusion tliat the interior of the globe must be solid, and acquiesced 
generally in Hopkins* conclusions, pointed out that M. Delaimay had not 
worked out the problem mathematically, otherwise he could not have 
failed to see that the hypothesis of a viscoiis and quasi-rigid interior 
*• breaks down when tested by a simple calculation of the amount of 
tangential force required to give to any globular portion of the interior 
mass the processional and nutational motions which, with other physical 
astronomei's, he attributes to the earth as a whole." ^ Sir William, in 
making tliis calculation, holds that it demonstrates the earth's crust down 
to depths of himdreds of kilometres to be capable of resisting sueli a 
tangential stress (amounting to nearly iV^h of a gramme weight per 
square centimeti-e) as would with great rapidity draw out of sha][>e any 
plastic substance which could properly be termed a viscous fluid. " An 
angular distortion of 8" is produced in a cube of glass by a distorting 
stress of about ten grammes weight per square centimetre. We may 
tlierefore safely conclude that the rigidity of the earth's interior sub- 
stance could not be less than a millionth of the rigidity of glass without 
very sensibly augmenting the lunar nine teen-yearly nutation." * 

In Hopkins' hypothesis he assumed the crust to be infinitely rigid 
and unyielding, which is not true of any material substance. Sir 
William Thomson has recently returned to the problem, in the light 
of his own researches in vortex-motion. He now finds that, while 
the argument against a thin crust and vast liquid interior is still 
invincible, the phenomena of precession and nutation do not decisively 
settle the question of internal fluidity, though the solar semi-annual 

> PhU, Trans. 1839, p. 861 ; 1840, p. 198 ; 1842, p. 43 ; BriL As800. 1847. 

' In a paper on tho uypotheeifl of the interior fluidity of the globe, Ccmptes rendu*, 
July IS, 1868. QtoL Mag, v. p. 507. See also H. Hennessy, Comples rendw^ 6 Harch, 
]87], Oeo/. Mag, viii. p. 210. Naiurt^ xv. p. 78. 

• Naiurt, Febnmiy 1, 1871. * Loe» oit. p. 258. 







Part L] THE EABTH'8 INTERIOR. 53 



and lunar fortnightly nutations absolutely disprove tlio existence of 
a thin rigid shell full of liquid. If the inner surface of the crust 
or shell were rigorously spherical, the interior mass of supposed liquid 
could experience no precessional or nutational influence, except in so 
far as, if heterogeneous in composition, it might suffer from external 
attraction due to non-sphericity of its surfaces of equal density. But 
" a very slight deviation of the inner surface of the shell from perfect 
sphericity would suffice, in virtue of the quasi-rigidity due to vortex- 
motion, to hold back the shell from taking sensibly more precession 
than it would give to the liquid, and to cause the liquid (homogeneous 
or heterogeneous) and the shell to have sensibly the same precessional 
motion as if the whole constituted one rigid body." ^ 

The ■ assumption of a comparatively thin crust requires that the 
crust sliall have such perfect rigidity as is possessed by no known 
substance. The tide-producing force of the moon and sun exerts 
such a strain upon the substance of the globe, that it seems in the 
highest degree improbable that the planet could maintain its shape 
as it does unless the supposed crust were at least 2000 or 2500 miles 
in thickness.^ That the solid mass of the earth must yield to this 
strain is certain, though the amount of deformation is so slight as to 
have hitherto escaped all attempts to detect it.^ Had the rigidity been 
even that of glass or of steel, the deformation would probably have 
been by this time detected, and the actual phenomena of precession and 
nutation, as well as of the tides, would then have been very sensibly 
diminished.* The conclusion is thus reached that the mass of the earth 
" in on the whole more rigid certainly than a continuous solid globe of 
jclass (if the same diameter."^ 

(b.) Argument from tlie tides. — The phenomena of the oceanic tides 
jire only explicable on the theory that the earth is either solid to the 
fjeiitre, or possesses so thick a crust (2500 miles or more) as to give 
t.> the planet practical solidity. Sir William Thomson remarks that 
'* were the crust of continuous steel, and 500 kilometres tliick, it 
wimld yield very nearly as much as if it were india-rubljer to the 
deforming influences of centrifugal force, and of the sim's and mcK)n's 
attracti<»ns." It would yield, indeed, so freely to these attractions 
" that it would simply carry the waters of the ocean up and down 
with it, and there would be no sensible tidal rise and fall of water 
Tt-latively to land."® Prof. G. H. Darwin, in the series of papers 
alreaily referred to, has investigated mathematically tlie Iwidily tidt's 
of virtcous and semi-elastic spheroids, and the character of the ocean 
title** on a yielding nucleus.^ Ilis results tend to increase the force 

» Kir W. Thomson, Brit. Am)c, Rep. 1870, Sections, p. 5. 

2 Thomson, Proc. Roy. Soc. April. 1802. 

■ Sw AMotiiation Franfaine pour T Avancement (hn Scteufei*^ v. p. 2Sl. 

• Thomiioii, lor. eit. 

• Thomeou, 2'ran*. Roy. Soc. Edin. xxiii, p. 157. 

• Thomson, Brit A$ioc. Rep. 1870, Sections, p. 7. 

' FkQ, Trantt. 1879, Part I. See also Brit Assoc. Rep. 1882, Sects, p. 473. 



54 GEOGNOSY. [Book IL 



of Sir William Thomson's argument, since they show that " no very 
considerable portion of the interior of the earth can even distantly 
approach the fluid condition," the effective rigidity of the whole globe 
being very great. 

(c.) Argument from relative densities of melted and solid rock. — 
The two preceding arguments must be considered decisive against 
the hypothesis of a thin shell or crust covering a nucleus of molten 
matter. It has been further urged, as an objection to this hypothesis, 
that cold solid rock is more dense than hot melted rock, and that even 
if a thin crust were fonued over the central molten globe it would 
immediately break up and the fragments would sink towards the 
centre.^ This argument, however, does not appear to be well founded. 
Experiment has shown that at least in the cases of glass, iron, brass, 
copper, "whinstone," (probably some kind of diabase or basalt) and 
granite, the snbsta-nce is denser in the melted than in the solid state.^ 
Moreover as has been already pointed out, the specific gravity of the 
interior is at least twice as much as that of the visible parts of the 
crust. If this difference be due, not merely to the effect of pressure, 
but to the presence in the interior of intensely heated metallic sub- 
stances, we cannot suppose that solidified ix)rtion8 of such rocks as 
granite and the various lavas could ever have sunk into the centre of the 
earth, so as to build up there the honey-combed cavernous mass which 
might have served as a nucleus in the ultimate solicLification of the 
whole planet. If the earliest formed portions of the comparatively 
light crust were denser than the underlying liquid, they would no doubt 
descend until they reached a stratum with specific gravity agreeing with 
their own, or until they were again melted.^ 

3. Hypothesia of a liquid svbstratum between a solid nuclem and the 
crust, — Since the early and natural belief in the liquidity of the 
earth's interior has been so weightily opposed by physical arguments, 
geologists have endeavoured to modify it in such a way as, if possible, 
to satisfy the requirements of physics, while at the same time 
providing an adequate explanation of the corrugation of the earth's 
crust, the phenomena of volcanoes, <fec.* The hypothesis has been 
proposed of " a rigid nucleus nearly approaching the size of the whole 
globe, covered l)y a fluid substratum of no great thickness, compared 
with the radius, upon which a cnist of lesser density floats in a state of 
equilibrium." The nucleus is assumed to owe its solidity, to "the 

* This objection has been, repeatedly urged by Sir William Thomson. See Tmntt, 
Hoy. Sor. Eclin. xxiii. p. l.*)?; and Brit. Ansae. Rep. 1870, Sections, p. 7. 

^ Sir W. Thomson, Tram. Ge^d. Sftc. Glfvtgow, vi. (1878) p. 40. 
^ See D. Forbes, GeoL Mag, vol. iv. p. 4.S5. 

* See Dana in Silh'man*8 Journal^ iii. (1847) p. 147. Amer. Joum. Seience (1873). 
The hypothesis of a finid snbstratnm has been advocattHl by Shaler. Pioc. Best, Nat, 
Hut. Soc. xi. (18G8) p. 8. Geol Mag. v. p. 511. J. Le Conte, Ainer. Jtmrn. S&i. 187*2, 
1873. O. Fisher, Geol. Maq, v. (new series) pp. 291 and ,551. • Physics of the Earth's 
iJniBt,' 1883. See also Hill, Gaol, Mag, v. (new series) pp. 2C2, 479. The idea of 
a viscous layer between the solidifying central mass and the crust was prcf-ont in 
Jlopkimf mind. Brit, Awh;, 1848, Reports, p. 48. 



Part L] THE EARTH'S INTEBIOIt. 55 



enormous pressuro of tlie superincumbent matter, while the crust 
owes its solidity to having become cool. The fluid substratum is not 
under sufficient pressure to l)e rendered solid, and is sufficiently hot to 
1)6 fluid, being probably more viscous in its lower portion through 
pressure and likewise passing into a viscous state in its upper parts 
through cooling, until it joins the cnist." ^ The contraction and con- 
solidation of this substratum are assumed as the explanation of the 
plication which the crust has certainly undergone. 

It must be admitted that the wide-spread proofs of great crumpling 
of the rocks of the crust present a difficulty, for they indicate a 
capability of yielding to strain such as has been supposed impossible 
in a globe possessing on the whole the rigidity of steel or glass. But 
this difficulty may Ix) more formidable in appearance than in reality. 
Tho earth miist certainly possess such a degree of rigidity as to resist 
tidal deformation. Professor Darwin has calculated the limiting rigidity 
in the materials of the earth which is necessary to prevent the weight 
of nioimtains and continents from reducing them to the fluid condition or 
else cracking, and has found that these materials must be as strong as 
granite 1000 miles below the surface or else much stronger than 
granite near the surface.^ But high rigidity, that is, elasticity of form, 
is not contradictory of plasticity. Even bodies like steel may, under 
suitable stress, be made to flow like butter, (see postea. Book III. Part I. 
Sect. iv. § 3.) While, therefore, tho earth may possess as a whole the 
rigidity of steel, there seems no reason why, under sufficient strain, 
the outer portions may not be plicated or even reduced to tho fluid 
condition. It is important " to distinguish viscosity, in which flow is 
caused by infinitesimal forces, from plasticity, in which permanent 
distortion or flow only sets in when the stresses exceed a certain limit." ^ 

In speculating on the plication of the earth's crust, we ought not to 
forget that, from the earliest times, the existing continental regions 
seem to have specially suifered from the efforts of the planet to adjust 
its external form to its diminishing diameter and lessening rapidity 
of rotation. They have served as lines of relief from the strain of 
compression during many successive epochs. It is along their axial 
lines, — their long dominant mountain-ranges, that wo should naturally 
look for evidence of corrugation. Away from these lines of weakness the 
ground has been upraised for thousands of square miles without 
plication of the rocks, as in the instructive region of the Western 
Territories of North America. N(»r is there any proof that cornigation 
takes place beneath the great oceanic areas of subsidence. 

It api^ears highly probable tliat the substance of the earth's 
interior is at the melting p<»int proper for the pressure at each depth. 
Any relief from pi*essure, therefore, may allow of the liquefaction of tho 
matter so relieved. Sucli relief is doubtless afforded by the corrugation 
of mountain-chains and other terrestrial ridges. And it is in these 

» Fi«her, * Physics of Earth's Crust,' p. 2(^>9. ^ Froc. Roy, Soe, 1881, p. 432. 

' Professor Darwin in a letter to tho author, 0th January. 1884 



56 OEOGNOST. [Book II.' 



lines of uprise that volcanoes and other manifestations of subterranean 
heat actually show themselves. 

§ 4. Age of the Earth and Measures of Geological Tizae. — 

The age of our planet is a problem which may be attacked either 
from the geological or physical side. 

The geological argument rests chiefly upon the observed rates 
at which geological changes are being oifcc ted at the present time, 
and is open to the obvious preliminary objection that it assumes 
the existing rate of change as the measure of past revolutions,- — an 
assumption which may bo entirely erroneous, for the present may be 
a period when all geological events march forward more slowly than' 
they used to do. The argument proceeds on data partly of a physical 
and partly of an organic kind, (a.) The physical evidence is derived 
from such facts as the observed rates at which the surface of a 
country is lowered by rain and streams, and new sedimentary deposits 
are formed. These facts will be more particularly dwelt upon in 
later sections of this volume. If we assume that the land has been 
worn away, and that stratified deposits have been laid down, nearly 
at the same rate as at present, then we must admit fhat the stratifie*! 
portion of the crust of the earth must represent a very vast period 
of time.^ (h,) On the other hand, human experience, so far as it 
goes, warrants the belief that changes in the organic world proceed 
with extreme slowness. Yet in the stratified rocks of the terrestrial 
crust, we have abimdant proof that the whole fauna and flora of the 
earth's surface have passed through numerous cycles of revolution, — 
species, genera, families, orders, ap^Kjaring and disappearing many 
times in succession. On any supposition, it must bo admitted that 
these vicissitudes in the organic world can only have been effected 
with the lapse of vast periods of time, though no reliable standard 
seems to be available whereby these periods are to be measured. 
The argument from geological evidence indicates an interval of 
probably not much less than 100 million years since the earliest forms 
of life appeared upon the earth, and the oldest stratified rocks began to 
l>e laid down. 

2. The physical argument as to the age of our planet is based 
by Sir William Thomson upon three kinds of evidence : — (1) the 
internal heat and rate of cooling of the earth ; (2) the tidal retardation 
of the earth's rotation ; and (3) the origin and age of the sun's heat. 

(1.) Applying Fourier's theory of thermal conductivity, he pointed 

* Dr. Croll puts this period at not law, but possibly much more, than 60 millioa 
years. Dr. Haughton gives a much more extended period. Estimating the present 
rate of deposit of strata at 1 foot in 8(>16 years, assuming the former rate to have been 
ten times more rapid, or 1 foot in 861*6 years, and taking the thickness of the stratifie<l 
rocks of the earth's crust at 177,200 feet, he obtains a minimum of 200,000,000 years 
for the whole duration of geological time : • Six Lectures on Physical Geography,' 1880^ 

L94. Dr. Haughton has also proposed another geological measure of past time^' 
ed upon the assumed effects of continental uplicaval (Proc, Roy, Soc. xxvi. (1877) 
p. 534). But Professor Darwin has shown it to be imulmissible. (Ow. cit, xxvii. 
(1878) p, 179.) ^ ^ 



Pabt II.] THE AGE OF THE EAETff. 57 



out some years ago (1862) that in the known rate of increase of 
temperature downward beneath the surface, and the rate of loss of heat 
from the earth, we have a limit to the antiquity of the planet. He 
showed, from the data available at the time, that the superficial 
consolidation of the globe could not have occurred less than 20 million 
years ago, or the underground heat would have l>een greater than it is ; 
nor more than 400 million years ago, otherwiBo the underground 
temperature would have shown no sensible increase downwards. He 
admitted that very wide limits were nepessary. In more recently 
discassing the subject, he inclines rather towards the lower than the 
higher antiquity, but concludes that the limit, from a consideration of 
all the evidence, must be placed within some such period of past time as 
100 millions of years;^ 

(2.). The reasoning from tidal retardation proceeds on the admitted 
fact that, owing to the friction of the tide-wave, the rotation of the earth 
13 retarded, and is therefore slower now than it must have been at one 
time. Sir William Thomson contends that had the globe become solid 
some 10,000 million years ago, or indeed any high antiquity beyond 100 
million years, the centrifugal force due to the more rapid rotation must 
have given the planet a very much greater polar flattening than it 
actually possesses. He admits, however, that though 100 million years 
ago that force must have been about 3 per cent, greater than now, yet 
'* nothing we know regarding the figure of the earth and the disposition of 
land and water would justify us in saying thatab<.)dy consolidated when 
there was more centrifugal force by 3 per cent, than now, might not now 
be in all respects like the earth, so far as we know it at present." ^ 

(3.) Tlie third kind of evidence loads to results confessedly less 
emphatic than those from the two previous lines of reasoning. It is 
Ixised upon calculations as to the amount of heat that would be available 
by the falling together of masses from space, which gave rise by their 
impact to our sun, and the rate at which this heat has been radiated. 
Assuming that the sun has been cooling at a uniform rate, Professor 
Tait comes to the conclusion that it cannot have supplied the eartli, 
even at the present rate, for more than about 1 5 or 20 million years.*^ 

Part II. — An Account of the Composition of the Earth's 

Crust— Minerals and Rocks. 

The earth's crust is composed of mineral matter in various aggregates 
included under the general term Rock. A rock may be defined as a 
mass of matter composed of one or more simple minerals, having 
usually a variable chemical composition, with no necessarily symmetrical 

* Trans. Roy. Soe. Edin. xxiii. p. 157. Trans. Geol. Sor. GW/om?, iil. p. 25. Professor 
Tait reduces the period to 10 or 15 millions. * Recent Advances in Physical Science,* 

P- ^*^- 

^ Trans, Geol. Soc. Gla»goWy iii. p. 16. Professor Tait, in repeating this argument 

concludi« that, taken in connection witli the previous one, " it probably reduces the 

iioAsible periml which can Ije allowed to geologists to something less than 10 millions of 

years.'' Op. cit. p. 174. ^ Op. eif. p. 174. 



58 OEoaifosT. ^M% n. 

external form, and ranging in cohesion from mere loose cl61)ris up to 
the most compact stone. Granite, lava, sandstone, limestone, graTcl, sand, 
mud, soil, marl and peat, are all recognised in a geological sense aa rocks. 
The study of rocks is known as Lithology, Petrography or Petrology. 

It will bo most convenient to treat — let, of the general chemical 
constitution of the crust ; 2nd, of the minerals of which rocks mainly 
consist ; 3rd, of the methods employed for the determination of rocks ; 
4th, of the external characters, and, 5th, of the internal texture and 
structure, of rooks; 6th, of the classification of rucks; and 7th, of the 
more important rocks occurring as constituents of the earth's crust. 

§ i. General Ckemieol CongtiltUion of the Criitt. 

Direct acqiuuTitance with tlio chemical constitution of the globe must 
ohvionsly bo limited to that of the crust, though by inference we may 
eventually reach highly prohahle conclusions regarding the conBtitution 
of the interior. Chemical research his discovered that sixty- four ' 
simple or as yet unttecompoHable bodies, calleil elements,' iii variouft 
proportions and compounds, constitute the accessible part of the crust.* 
Of these, however, the great majority are comparatively of rare 
ffficnrrence. The crust, so far as we can ox.imino it, is mainly biiilt np 
■■iralwiit sixteen elements, *hich may bo arranged in the two following 
gi'ou]iB, the most abundant bodies l>eing placed first in each list: — 

Atomtc AUmk 

Weight. WMgtit 

Oxygen IS-9G AlnQiiiiiiuii 27-30 

flilicon 28-00 Calcium 39-90 

Carbon ..,.:, 11-97 I MBgneeinm ... . . 23-M 

Sulphur 31-<J8 , PotaBaium 39-04 



Hydrogen 1-00 Bodiai; 

Cbbrino ,....,, 35-37 ! Iron . 

PhoaphoniB . . . . . 30 -OC 

Flnorino 1910 



Tlio sixteen elements here mentioned form about ninety-nine partfi 
of the earth's crust ; the other elements constitute only abont'a himJEndtfi 
l>art, though they includo gold, silver, copper, tin, lead, BDd't3ie"otlier 

useful metals, iron excepted. By far the niost abundant and iu^porfiat 
<ilement is Oxygen. It forms about 23 per cent, by wei^t of l^J 88'tff 
per cent, of water, and about a half of all the rocks which compose tlio 
visible imrtiou or crust of the globe. Another metalloid, Silicon, alwayH 
united with oxygen, ranks nex.t in abundance as a TOustltnent of Hi" 
iTUst. Of the remaining metalloids, Carbon and Sn'i'iu' 
occur in the free state, but usually in combination v/\- . 
metal, (.'lilorino (save perhaps at volcanic vents) 'I 
free »iaU; but is almndaut in combination with tic nil. 

' Tliis imiuLrr Iiiiswilliin the liust fow yr,i, 
<,( no I'.n-cr t)»iii fourU.'ea iiaw metaln. Sou 
LrvTi s!i1i-fi..-(.iri]y prnVL-.l t« W new. T. B.. 




Part H, S ij THE EABT1P8 CRUST. 59 



with sodium. Fluorine is always found in combination, and has never 
yet been isolated by artificial chemical processes. It is the only element 
which has not been combined with oxygen. It chiefly occurs in union 
with Calcium as the mineral fluor-spar, and constitutes more than half 
■of the mineral cryolite ; but traces of its presence liave bec^n detected in 
other minerals, in sea-water, and in the bones, tcetli, blood and milk of 
mammalia. Hydrogen occurs chiefly in combination with oxygen as the 
oxide, water, of which it forms 11-13 per cent, by weight; also in com-. 
bination with carbon as tlio hydrocarlx>ns (mineral oils and gases), pro- 
duced by the slow decomposition of organic matter. Thosphorus occurs 
with oxygen principally in calcic phosphate. Of the metals, a few are 
found in the native state (gold, silver, copper, &c.), but those of impor- 
tance in the framework of the earth's crust have entered into combina- 
tion with metalloids or with each other. Putting the more important 
metals and metalloids together, we may compute that oxygen, silicon, 
aluminium, magnesium, calcium, potassium, sodium, iron and carbon 
form together more than 97 per cent, of the whole known crust. 

So far as accessible to observation, the outer portion of our planet 
consists mainly of metalloids. Its metallic constituents have already in 
great part entered into combination with oxygen, so that the atmosphere 
contains the residue of that gas which has not yet united itself to terres- 
trial compounds. In a broad view of the arrangement of the chemical 
elements in the external crust, the suggestive speculation of Durocher 
deserves attention.^ He regarded all rocks as referable to two layers or 
magmas co-existing in the earth's crust, the one beneath the other, 
according to their specific gravities. The upper or outer shell, which he 
termed the acid or siliceous magma, contains an excess of silica, and has 
a mean density of 2-65. The lower or inner shell, which he called the 
Imsic magma, has from six to eight times more of the earthy bases and 
iron-oxides, with a mean density of 2-96. To the former he assigned 
the early plutonic rocks, granite, felsite, (fee, with the more recent 
trachytes ; to the latter he relegated all the heavy lavas, basalts, diorites, 
&c. The ratio of silica is 7 in the acid magma to 5 in the basic. 
Though the proportion of silicic acid or of the earthy and metallic bases 
cannot be regarded as any certain evidence of the geological date of 
rocks, nor of their probable depth of origin, it is nevertheless a fact that 
(with many important exceptions) the eruptive rocks of the older geo- 
logical periods are very generally super-silica ted and of lower specific 
gravity, while those of later time are very frequently poor in silica, but 
rich in the earthy bases and in iron and manganese, with a consequi^nt 
higher specific gravity. The latter, according to Durocher, have been 
forced up from a lower zone through the lighter siliceous crust. The 
sequence of volcanic rocks as first announced by Kichthofen, has an 
interesting connection with this speculation.'^ 

The main mass of the earth's crust is composed of a few prodoniiuant 

* Ann. den M%ne»y 18r>7. Translat^xl })y Hftu^hton, 'Maniml of Gcolop;y,' 180^, p. ](>. 

* Pfnttt^. Book IJI. Part I. Section i. § 5. 



60 GEOGNOSY. [Book IL 



compounds. Of these in every respect the most abundant and impor* 
tant is Silicon-dioxide or Silica (Kieselerde) SiOg. As the fundamental 
ingredient of the mineral kingdom, it forms more than one half of the 
known crust, which it seems to bind firmly together, entering as a main 
ingredient into the composition of most crystalline and fragmental 
rocks as well as into the veins that traverse them. It occurs in the 
free state as the abundant rock-forming mineral quartz, which strongly 
resists ordinary decay, and is therefore a marked constituent of many of 
the more enduring kinds of rock. As one of the acid-forming oxides 
(H4Si04, Silicic acid, Kieselsaure) it forms combinations Avith alkaline, 
earthy, and metallic bases, which appear as the prolific and universally 
diffused family of the silicates. Moreover, it is present in solution in 
terrestrial and oceanic waters, from which it is deposited in pores and 
fissures of rocks. It is likewise secreted from these waters by abun- 
dantly diffused species of plants and animals (diatoms, radiolarians, &c.). 
It has been largely effective in replacing the organic textures of former 
organisms, and thus preserving them as fossils. 

Alumina or aluminium-oxide (Thonerde), AljOg, occurs sparingly 
as corundum, which, however, according to F. A. Genth, was the 
original condition of many now abundant complex alumiuous minerals 
and rocks. The most common condition of aluminium is in union with 
silica. In this form it constitutes the basis of the vast family of the 
aluminous silicates, of which so large a portion of the crystalline and 
fragmental rocks consists. Exposed to the atmosphere, these silicates 
lose some of their more soluble ingredients, and the remainder forms an 
e<arth or clay consisting chiefly of silicate of aluminium. 

Carbon is the fundamental element of organic life. In combination 
with hydrogen, as well as with oxygen, nitrogen and sulphur, it forrns 
the various kinds of coal, and thus takes rank as an important rook- 
forming element. As carbon-dioxide, COj, it is present in the air, in 
rain, in the sea and in ordinary terrestrial waters. This oxide is solu- 
ble in water, ^ giving rise then to a dibasic acid tenned Carbonic Acid 
(Kohlensaure), C0(0H)2 or H2CO3, which forms carbonates, its com- 
bination with calcium having been instrumental in the formation of 
vast miBusses of solid rock. Carbon-dioxide constitutes a fifth part of 
the weight of ordinary limestone. 

Sulphur (Soufre, Schwefel), S, occurs uncombined in occasional 
deposits like those of Sicily and Naples^ to be afterwards described, also 
in union with iron and other metals as sulphides ; but its principal 
condition as a rock-builder is in combination with oxygen as sulphuric 
acid (Schwefelsaure) H2SO4 which forms sulphates of lime, magnesia, &c. 

Calcium enters into the composition of many crystalline rocks in 
combination with silica and with other silicates. But its most 
abundant form is in union with carbon-dioxide, when it appears as the 
mineral, calcite (CaCOg), or the rock, limestone. Calcium-carbonate, 

' One volume of water ut 0° C. dissolves 1-7967 volumes of carbon -dioxide ; at 15° C. 
"..the amount is reduced to 1*0020 volumes. 



Paet JL I ij eOMPOaiTION OF THE CRUST. 61 



being Bolnble in water containing carbonic acid, is one of the most 
universally diffused mineral ingredients of natural waters. It suj^plies 
the varied tribes of moUusks, corals, and many other invertebrates 
with mineral substance for the secretion of their tests and skeletons. 
Such too' has been its office from remote geological periods, as is 
shown by the vast masses of organically-formed limestone, which enter 
BO coufipioQOUsly into the structure of the continents. In combination 
with sulphuric acid, calcium forms important beds of gypsum and 
anhydrite. 

Magnesium} Potassium and' Sodium play a less conspicuous but. still 
etttiential part in the composition of the earth's crust. Magnesium, 
ill combination with silica, forms a class of silicates of prime importance 
in the composition of volcanic and metamorphic rocks. As a carbonate, 
it unites with • calcium-carbonate to form the widely diffused rock, 
dolomite. In union with chlorine, it takes a prominent place among 
the salts of sea- water. Potassium or Sodium, combined with silica, 
is present in small quantity in most silicates. In union >vith chlorine, 
as common salt, sodium is the most important, mineral ingredient of 
sea- water, and can be detected in minute quantities in air, rain, and in 
terrestrial waters. In the old chemical formul» hitherto employed in 
mineralogy the metals of the alkalies and alkaline earths are re2>ro- 
Hcuted as oxides. Thus lime (calcium-monoxide), soda (sodium-mon- 
oxide), potash (potassium-monoxide), magnesia (magnesium-oxide), are 
denoted as in union with carbonic acid, sulphuric acid, silica, &c., 
forming carbonates, sulphates, silicates of lime, soda, &c. 

Iron 4ind Manganese are the two most common heavy metals, 
tKXJurring both in the form of ores, and as constituents of rocks. Iron 
is the great pigment of nature. Its peroxide or scH(|uioxide, now known 
as ferric uxide, forms large mineral manses, and together with the 
protoxide or ferrous oxide, occurs in smaller or larger proportions in 
the great majority of crystalline rocks, iron (as sulphate or in combi- 
nation with organic acids) is removed in solution in the water of springs, 
and precipitated as a hydrous peroxide;. ^langanese is commonly 
associated with iron in minute proportions in igneous rocks, and being 
similarly removed in solution in water, is thrown down as l)og manganese 
or wad. 

Silicic Acid, Carbonic Acid, and Suli)huric Acid are the three 
acids with which most of the bases that compose the earth's crust have 
been combined. \Vith these wo may connect the water which, besides 
merely percolating through rocks, or existing enclosed in the vesieles 
of minerals, has been chemically absorbed in the process of hydration, 
and which thus constitutes more than 10 or even 20 per cent, of some 
rocks (gypsum). 

Although every mineral may be made to yield data of more or less 
geological significance, only those minerals need be referred to here 
which enter as chief ingredients into the composition of rock-masses, 
or which are of frequent occurrence as accessories, and special note 



62 GEOamSY. [Book II. 



may bo taken of those of their characters which are of main interest 
from a geological point of view, such as their modes of occurrence in 
relation to the genesis of rocks, and their weathering as indicative of 
tlic nature of rock-decomposition. 

§ ii. Bock-forming Minerals, 

Minerals, as constituents of rocks, occur in four conditions, according 
to the circumstances under which they have been produced. 

1. Crystalliney as (o) more or less regularly defined crystals; (6) 
amorphous granules, aggregations or crystalloids, having an internal 
ciystalline structure in most cases easily recognisable with polarized 
light, as in the quartz of granite ; (c) " crystallites " or " microliths," 
incipient forms of crystallization, which are described on p. 1Q6. The 
crystalline condition may arise from igneous fusion, aqueous solution, 
or sublimation.* 

2. Glassy or t^Ureous, as a natural glass, usually including either 
crystals or crystallites, or both. Minerals have assumed this condition 
from a state of fusion. The glass may consist of several minerals fused 
into one homogeneous substance. Where it has assumed a lithoid or 
stony structure, these component minerals crystallize out of the glassy 
magma, and may be recognised in various stages of growth. 

3. Colloid, as a jelly-like though stony substance, deposited from 
atpieous solution. The most abundant mineral in nature whichtakes the 
colloid form is silica. Opal is a hardened colloidal condition of this sube 
stance. Chalcedony, doubtless originally colloidal silica, now unites tlie 
characters of quartz and opal, being only partially soluble in caustic potash 
and partially converted into a finely fibrous, doubly-refracfing substance. 

4. Anwrjphous, having no crystalline structure or form, and occurring 
in indefinite masses, granules, streaks, tufts, stainings, or other irregular 
modes of occurrence. 

A mineral which has replaced another and has assumed the external 
form of the mineral so replaced, is termed a Pseudonwrph, A mineral 
which encloses another has been called a Perimorph; one enclosed 
within another, an Endomorph. 

Minerals may either be essential or accessory, original or secondary 
constituents of rocks. A mineral is an essential ingredient when its 
ubseuce would so alter the character of a rock as to make it something 
fundamentally different. The quartz of granite, for example, is an 
essential constituent of tliat rock, the removal of which would alter the 
petrographical species. All essential minerals are original constituents 
of a rock, but all the original constitu(>nts are not essential. In granite, 
for example, topaz, beryl, sphene, and other minerals often occur under 
circumstanceis which show that they crystallized out of the original 
magma of the rock. But they form so trifling a proportion in the total 
mass, and their absence would so little affect the general character of that 

' For the microscopic characters of miuerals and rocks, see p. 99. 



Part II. § ii.] BOCK-FORMING MINERALS, 63 



mass, that they are regarded as accessory, though undoubtedly original 
and often important ingredients.^ Again, in rocks of eruptive origin, the 
essential ingredients cannot be traced back further than the eruption of 
the mass containing them. They are not only original, as constituents of 
the lava^ but are themselves original and non-derivative mineral^, pro- 
duced directly from the crystallization of molten minerals ejected from 
beneath the earth's crust, though, as Michel-Levy has shown, the debris 
of older ininerals may sometimes be traced amidst the later crystals of 
massive rocks.* In rocks of aqueous origin, however, there are many, 
such as conglomerates and sandstx)ne8, where the component minerals, 
though original ingredients of the rocks, are evidently of derivative 
origin. The little quartz-granules of a sandstone have formed part of 
the rock ever since it was accumulated, and are its essential constituents. 
Yet each of these once formed part of some older rock, the destruction 
of which yielded materials for the production of the sandstone. 

The same mineral may occur both as an original and as a secondary 
constituent. Quartz, for example, appears everywhere in both con- 
ditions ; indeed, it may sometimes bo found in a twofold form even in 
the same rock, though there is then usually some difference between the 
original and secondary quartz. A quartz-felsite, for instance, abounds 
in original little kernels, or in double pyramids of the mineral, often 
v'nclosing fluid cavities, while the secondary or accidental fonns occur in 
veins, reticulations, or other iiTcgular aggregates, distinguished by a 
^Kjculiar chequered structure in polarized light, and by an absence of the 
crowded cavities so characteristic in the quartz of eruptive rocks. 

Accessory minerals frequently occur in cavities where they liave 
had some room to crystallize out from the general muss. The " druHy ** 
cavities, or open spaces lined with well developed cryHtals, found iii 
some granites are good examples, for it is there that the non-essential 
minerals are chiefly to l>e recognised. The veins of segregation found 
in many cr^'stalline rocks, particularly in those of the granite series, are 
further illustrations of the original separation of mineral ingredients 
frum the general magma of a rock (see p. 139). In some cases minerals 
jisbume a concretionary shape, which may bo observed chiefly though 
not entirely in rocks formed in water. Some minerals are particularly 
prone to occur in concretions. Siderite (ferrous carbonate) is to be 
found in abundant nodules, mixed with clay and organic matter among 
c^jnsolidated muddy deposits. ( -alcito or calcium-carbonate is likewise 
nbuuilantly concretionary. Silica in the forms of chert and flint ai)pears 
in irregular concretions, in old calcareous formations, composed mainly 
of the remains of marine organisms. 

Secondary minerals have been develoi)cd as the result of subsequent 

' Some of the ** accessory *' minemld may be of groat importance as indicative of 
the conditions under which the rock was formed. 

- Bull, 8oc. Gtol. France^ 3rd scr. iii. 100. See also Fou(|iie' and IMichel-Levy, 
• Mineralogie Micrographique,* \). 180. Some eruptive rocks abound in corroded or 
ornnewhat rounded or broken crystals which obviously have belonged to some previous 
state of conaolidation. 



64 GEOGNOSY. [BooK-n. 



changes in rocks, and are almost invariably due to the chemical action of' 
percolating water, either from above or from below. Occurring under 
circumstances in which such water could act with effect, they are found 
in cracks, joints, fissures, and other divisional planes and cavities of 
rocks. These subterranean channels, frequently several feet or even 
yards wide, have been gradually filled up by the deposit of mineral 
matter on their sides (see the Section on Mineral Veins). The cavities 
formed by expanding steam in ancient lavas (amygdaloids) have offered 
abundant opportunities for deposits of this kind, and have accord* 
ingly been in large measure occupied by secondary minerals (amygr 
dules), as calcite, chalcedony, quartz and zeolites. 

In the subjoined list of the more important rock-forming minerals, 
attention is drawn mainly to those features that are of geological 
importance ; the physical and chemical characters of these minerals will 
bo found in any text-book of mineralogy. Reference is therefore made 
hero to modes of occurrence, whether original or secondary ; modes of 
origin, whether igneous, aqueous, or organic ; pseudomorphs, that is, the 
various minerals which any given mineral has replaced, while retaining 
their external forms, and likewise those which are found to have 
supplanted the mineral in question while in the same way retaining 
its form — a valuable clue to the internal chemical changes which 
rocks undergo from the action of percolating water (Book III. Part 11. 
Section ii. § 1 and 2) ; and lastly, characteristics or peculiarities of 
weathering, where any such exist that deserve special mention. 

I. Native elements are comparatively of rare occurrence, and only two of them. 
Carbon and Sulphur, occasionally play the part of noteworthy essential and accessory 
constituents of rocks. A few of the native metals, more e8i)cciany oopper and gold, now 
and then appear in sufficient quantity to constitute commercially imix)rtant ingredienti} 
of veins and rock-masses. 

Graphite is found chiefly in aucient crystalline rocks, as gneiss, mica-schist, 
jrranitc, &c. ; some of the Laurentiun limestones of Canada being so full of the diffused 
mineral as to be profitably worked for it; in rare instances coal hiis been observed 
changed into it by intrusive basalt (Ayrshire). Probably in most cases graphite results 
from the alteration of imbedded organic matter, especially remains of plants ; oc- 
casionally it is obser\'ed as a pseudomorph after calcite and pyrites, and sometimes 
enclosing sphene and other minerals.* 

Sulphur occurs Ist, as a product of volcanic action in the vents and fissures 
of active and dormant C€>ne8. Volcanic sulphur is formed from the oxidation of the 
sulphuretted hydi-ogen, 80 coj^iously emitted with the steam that issues from volcanic 
vcut«, as at the Solfatara, near Naples. It may also be pro<luced by the mutual 
docomiwsition of the same gas and anhydrous sidphuric acid. 2nd, in beds and 
layers, or diffused particles, residting from the alteration of i)revious minerals, 
particularly sidphates, or from deix)sit in water tlirough decomjiositiou of sulphuretted 
liydrogen. The frequent crystallization of suljOiur shows that the mineral must 
have been formed at ordinary temperatures, for its natural crystals melt at 238*1° Fahr. 
Its formation may be observed in progress at many sulphureous springs, where it falls 
to the bottom as a pale mud through the oxidation of the sulphuretted hydrogen 
in the water. It occurs in Sicily, Spain and elsewhere, in beds of bituminous 

> Vom Kath. SUzutigtfher. Wien, AhaiL x. p. G7 ; Sullivan in Jukes* • I^Ianual of 
Geology,' 3rd edit. (1872) p. 56. 



.Fabt IL § ii.] BOCK-FORMING MINERALS. 65 



limestone and gypsum. These strata, sometimes full of remains of fresh-water shells 
and plants, are interlaminated with sulphur, the very sholls being not infrequcntlv 
replaced by this mineral. Hero the presence of the sulphur may ho traced to the 
redaction of the calcium-sulphate to the state of sul])hirle, through the action of the 
decomposing organic matter, and the subsequent production and decomxx>8ition of 
aulphniettod hydrogen, with consequent liberation of sulphur.* The sulphur deiwsits 
of Sicily famish an excellent illustration of the alternate deposit of sulphur au<l 
limestone. They consist mainly of a marly limestone, through which the sulphur 
is partly disseminated and partly interstratified in thin laminsa and thicker layers, 
some of which are occasionally 28 feet deep. Below these dci^sits lie older Tertiary 
gypseoos formations, the decomposition of wliich has probably pnxluccd the deposits 
of sulphur in the overlying more recent lake basins.* The weathering of sulphur 
is exemplified on a considerable scale at these Sicilian deposits. The sulphur, in 
presence of limestone, oxygen, and moisture, becomes sulphuric acid, which combiniup; 
with the limestone, forms gypsum, a curious return to what was probably the original 
substance from the decomposition of which the sulphur was derived. Hence the site of tlic 
outcrop of the sulphur beds is marked at the surface by a white earthy rock, or borscale, 
which is regarded by the minors in Sicily to be a sure indication of sulphiu' underneath, 
as the gossan of Cornwall is indicative of underlying metalliferous veins.' 

Iron, the most important of all the metals, is found only sparingly in the native 
state, in blocks which have fallen as meteorites, also in grains or dust euclosetl in 
hailstones, in snow of the Alps, Sweden and Siberia, in the mud of the ocean floor at 
remoie distances from land, and in some eruptive rocks. There caw be no doubt that a 
small bat constant snpply of native iron (cosmic dust) is falling upon the earth's surface 
from outside the terrestrial atmosphere.^ This iron is alloyed with nickel, and contains 
small quantities of cobalt, copper and other ingredients. Dr. Andrews, however, 
showed in 1852 that native iron, in minuto spicules or granules, exists in some basalts 
and other volcanic rocks,' and Mr. J. Y. Buchanan has recently detected it in 
appreciable quantity in the gabbro of the west of Scotland. It occurs also in basalts 
of Bohemia and Greenland.* 

* Braun, BuU. Soc. Geol France^ Ist ser. xii. p. 171. 

' Memorie dal R. Comilato Geolagico ^Italiay i. (1871). 

' Journ. Soc. ArUy 1873, p. 170. E. Ledoux, Ann. (h:n Miiniit, 7'"« ser. vii. p. 1. The 
Hicilian sulphur beds belong to the Oeningen stage of the Upper Tertiary deposits. 
They contain numerous plants and some insects. H. T. Qeyler, PaVwntographicn, xxiii., 
U^J. 9, p. 317. Von Lasaulx, Neiies JdhrJ). 1879, p. 490. 

* 8ee Ehrenberg, Frorieps Notizen, Feb. 184G ; Nordenskiohl, Compter rendwt^ Ixxvii. 
p. 463, Ixxviii. p. 236. Tissandier, op. eit. Ixxviii. p. 821, Ixxx. p. 58, Ixxxl. p. 576, 
See Ixxv. (1872) p. 683. Yung, Rail. Soc. Vaudoise Set. Nat. (1876), xiv. p. 493. 
Ranyard, Monthly Not. Roy. Antron. Soc. xxxix. (1879) p. 161 T. L. Pliipson, 
('omplt't rend. IxxxiiL p. 364. A Committee of the British Association was appointed 
in 1880 to investigate the subject of cosmic dust. See its reports for 1881-8^. 

* RrU Agnoc Rep. 1852. 

* Nordenskiold describes fifteen blocks of iron on the island of Disco, Greenland, tho 
weight of the two largest being 21,000 and 8,000 kilogrammes (20 nnl 8 tons, 
respectively). He observed that at tho same locality, tho underlying Ixisiilt contains 
lenticular and disc-shaped blocks of precisely similar iron, and inferred that the whole of 
the blocks may belong to a meteoric shower which fell during the time (Tertiary) when 
the basalt was poured out at tho surface. He dismisses the suo^gestion that the iron could 
possibly be of telluric origin {Ged. Mag. ix. (1872; p. 462). But the microscope reveals 
in this basalt the presence of minute particles of native iron which, assr>ciAted with viridite, 
are moulded round the crystals of labradorite and augito (Fouquc' and Michel -Lc^vy, 
op. cU. p. 443). Steenstrup, Daubree, and others appear therefore to Ix) justified iu 
regarding this iron as derived from the inner metallic portions of the globe, which lie at 
depths inacoessible to our observations, but from which the vast Greenland basalt 
emptioDs have brought up traces .to the surface (K. J. T. StcenHtnip. Vid. Medd. Nat, 
Foren. Copenhagen (1875) No. 16-19, p. 284. Zeitxch. DeuUch. Geol. Ge^. xxviii. (1876) 
p. 225; Mineralog. Mag. July, 1884. F. Wohlor, NeiK^j^ Jahrb. 1879, p. 832. Daubree, 
Diteoun Acad, Sci. 1 March 1880, p. 17. W. Flight, Geol, Mag. ii. (2nd ser.) p. 152. 



66 QEOGNOBT. [Book H. 



In the great majority of cases the Qxidu occor oombined with some add. A few 
uncombined take a piomiiient place as essential constituents or frequent ingredients 
of rocks, especially the oxides of silicon and iron. 

2. Silica (SiO,) is found in three chief forms, Quartz, Tridymite, and OpaL 

Quarts is abundant as (1) an essential constituent of rooks, as in granite, 
gneiss, mica-schist, quartz-trachyte, quartz-porphyry, sandstone; (2) an acoessorj 
ingredient, wholly or partially filling veins, joints, cracks and cavities. It has been 
produced from (a) igneous action, as in yolcanio rocks ; (6) aquo-igneous or plutonio aetkm, 
as in granites, gneisseSi &c. ; (e) solution in water, as where it lines cavitiee or replaces 
other minerals. The last mode of formation is that of the crystallized quartz and 
chalcedony found as secondary ingredients in rocks. 

Tho study of the endomorphs and pseudomorphs of quartz is of great importance in 
the investigation of the history of rooks. No mineral is so conspicuous for the variety 
of other minerals enclosed within it. In some secondary quartz-ciystals, each prism 
forms a small mineralogioal cabinet enclosing a dozen or more distinct minerals, as 
rutilo, hsDmatite, limonite, pyrites, chlorite, and many others.^ Quartz may be 
observed replacing calcite, aragonite, siderite, gypsum, rock-salt, hssmatite, ftc This 
facility of replacement constitutes silica one of the most valuable petrifying agents in 
nature. Organic bodies which havo been silicified retain, often wltti the utmost 
perfection, their minutest and most delicate structures. 

Quartz may usually be identified by its external characters, and especially by its 
vitreous lustre and haziness. When in the form of minute blebs or crystals, it may be 
recognised in many rocks with a good lens. Under the microscope, it presents a 
characteristic brilliant chromatic polarization, with no trace of any alteration of its 
borders ; while chalcedony displays a minute concentric radial structure giving a black 
cross between crossed Nicols. Where it is an original and essential constituent of a 
rock, quartz very commonly contains minute rounded or irregular cavities or pores, 
partially filled with liquid. So minute are these cavities that a thousand millions of 
them may, when they are closely aggregated, lie within a cubic inch. The liquid is 
chiefly water, not uncommonly containing sodium chloride or other salt, sometimes liquid 
carbon-dioxide and hydro-carbons.* 

Bock-crystol and crystalline quartz resist atmospheric weathering with great per- 
sistence. Hence the quartz-grains may usually be easily discovered in the weathered 
crust of a quortzlferous igneous rock. But corroded quartz-crystals have been 
observed in exposed mountainous situations, with their edges rounded and eaten awaj.' 
The chalcedouic and more or less soluble forms of silica are more easily affected. Flint 
and many forms of coloured chalcedony weather with a white crust. But it is chiefly 
from the weathering of silicates (especially through the action of organic adds) that 
the soluble silica of natural waters is derived. Book IIL Port II. Section ii. § 7. 

Tridyinite« has been met with chiefly among volcanic rocks (trachytes, andesitea^ 
&c.), botii as an abundant constituent of those which have been poured out in the 
form of lava, and also in ejected blocks (Vesuvius).^ 

Opal, a hydrous condition of silica formed from solution in water, is usually 
disseminated in veins and nests through rocks. Semi-opal occasionally replaces the 
original substance of fossil wood (wood-opal). Several forms of opal are deposited by 
geysers, and are known under the general appellation of sinters. Closely allied to the 



' See Sullivan, in Jukes' * Manual of Geology,* 3rd edit. (1872), p. 61. 

« See Brewster, Tram, Boy. 8oc, Edin. x. p. 1. Sorby, QuaH. Jaum. Geol 8oc. xiv. 
p. 453. Proc, Boy. 8oo. xv. p. 153 ; xvii. p. 299. Zirkel, * Mikroskopische Beschaffenheit 
dor Mineralien und Gestoine,* p. 39. Bosenbusch, * Mikroskopische Physiographie,' 
L p. 30. Hartley, Joum. Chem. Soo, February, 1876. The occurrence of fluid-cavitieB 
in the crystals of rocks is more fully described in Part II. § iv. of this Book. 

* Both, Chem, Qtol. i. p. 94. 

« Vom Bath, Z, DeuUch. Oeol. Ges. xxv. p. 236, 1878. 



Paw n. I iL] BOCK-FORMING MINERALS. 67 



opds are iba fbmiB in which bydions silica appears iu the organic world, where it 
eonsfcitiitee the frnstnles of diatoms, and the skeletons of radiolaria, &o. Tripoli powder 
(Kiesrignhr), randanite, and other similar earths, are composed mainly or wholly of the 
remains of diatoms, Ac 

Ckimndtim, almninium-ozide, is found in crystalline rooks, particularly in certain 
• e ipe n tines and schists, gneiss, granite, dolomite, and rooks of the metamorphio series. 

8. Imv OzD>n«— Four minerals, composed mainly of iron oxides, occur abundantly as 
ewmfisl and waoeuoty ingredients of rocks. HaBmatite, Limonite, Magnetite, and 
ntanioiroo. 

Hnmtlto (Fer oligiste, Botheisen, Eisenglanz, Fe2O,=Fe70,O30) in the orys- 
talliiad Ibnu oooon in Tsina, as well as linmg canties and fissures of rocks. The 
fibfom and move oommon form (which often has portions of its mass passing into the 
erjBtalliaed eondition) lies likewise in strings or veins; also in cavities, which, when of 
large lize, have given opportunity for the deposit of great masses of hiematite, as in 
cavemoos limestones (Westmoreland). It occurs with other ores and minerals as an 
abundant oomponent of mineral veins, likewise in beds interstratifled with sedimentary 
or schistose rooks. Scales «nd specks of opaque or clear bright red hematite, of 
fteqoent oocunenoe in the crystals of rooks, give them a reddish colour or peculiar 
Instie (perthite, stilbite). Httmatito appears abundantly as a product of sublimation in 
defts of volcanic cones and lava streams ; also in veins and beds, and as the earthy 
pigment that gives a red colour to sandstones, clays and other rocks. It is probably in 
most oases a deposition from water, resulting from the alteration of some previous soluble 
eombination of the metal, such as the oxidation of the sulphate. It is found 
psendomorphous after ferrous carbonate, and this has probably been the origin of beds of 
red ochre occasionally intercalated among stmtified rooks. It likewise replaces calcite, 
dolomite^ quarts, baiytes, pyrites, magnetite, rook-salt, fluor-spar, &c. 

Iilmonite (Brown iron-ore, 2FetO,+8H20=Fe20, 85*56, H^O 14-44), occurs in 
beds among stratified formations, and may be seen in the course of deposit, through 
the action of organic acids, on marsh-land (bog-iron-ore) and lake-bottoms. (Book IV. 
Part n. Section iii.) In the form of yellow ochre, it is precipitated from the waters of 
chalybeate springs containing green vitriol derived from the oxidation of iron-sulphides.' 
It is a oommon decomposition product in rocks containing iron among their constituents. 
It is thus always a secondary or derivative substance, resulting from chemical alteration. 
The peeudomorphous forms of limonite show to what a largo exteut combinations of 
iron are carried in solution through rocks. The mineral has been found replacing 
OQloite,stderite, dolomite, haematite, magnetite, pyrite, marcasite, galena, blende, gypsum, 
borytes, fluor-spar, pyroxene, quartz, garnet, beryl, &a 

Magnetite (Fer oxydul<^, Magnetcison, Fe,04), occurs abundantly in some schists, 
in scattered octohedral crystals ; in crystalline massive rocks like granite, in diffused 
grains or minute crystals ; among some schists and gneisses, (Norway and the eastern 
states of North America) in massive beds ; in basalt and other volcanic rocks, as an 
essential constituent, in minute octohedral crystals, or in granules or crystallites. 
Likewise found as a pseudomorphous secondary product, resulting from the alteration 
of some previous mineral, as luomatite, pyrite, quartz, hornblende, augitc, garnet and 
sphene. Oocurs with haematite, &c., as a product of sublimation at volcanic foci, where 
chlorides of the metals in presence of steam are resolved into hydrochloric acid and 
anhydrous oxides. It may thus result from either aqueous or igneous operations. It 
is liable to weather by the reducing effects of decomposing organic matter, whereby it 
becomes a carbonate, and then by exposure passes into the hydrous or anhydrous 
peroxide. The magnetite grains of basalt-rocks are very generally oxidized at the 
snriace, and sometimes even for some depth inward. 

Tltanio Iron (Titaniferous Iron, Menaooonite, Bmenite, Fer titan^, Titanciscn 



Sullivan, Jukes' ' Manual of Geology,' p. G3. 

F 2 



68 GEOGNOSY. [Book II. 



I 



(Fo TO^OJjOocun in scattered grains, plates, and crystals as an abundant c(msiituent of 
many crystalline rocks (basalt-rocks, diabase, gabbro, and other igneous masses) ; also in 
veins or beds in syeniteit serpentine, and metamorpbic rocks.^ Scarcely to be distinguisbed 
from magnetite vrhen seen in small particles under the microscope, but possessing a 
brown semi-metollio lustre with reflected light; resists corrosion by acids when the 
powder of a rock containing it U exposed to their action, while magnetite is attacked 
and dissolved. Titanic iron frequently resists weathering, so that its black glossy 
granules project from a weathered surface of rock. In other cases, it is deoompoee<jl 
either by oxidation of its protoxide, when the usual brown or yellowish colour of the 
hydrous ferric oxide appears, or by removal of the iron. The latter is believed to be 
the origin of a peculiar milky white opaque substance, frequently to be observed under 
the microscope, surrounding and even replacing crystals of titanic iron, and named 
Leucoxene by Giimbel.' 

4. Manganese Oxides are frequently associated with those of iron in ordinary rock- 
forming minerals, but in such minute proportions as to have been generally neglected in 
analyses. Their presence in the rocks of a district is sometimes shown by deposits of 
the hydrous oxide in the forms of Psilomelane (HxMnOf+H^O), and Wad (MnO, 
+MnO+H.O). These deposits sometimes take place as black or dark brown 
branching, plant-like or deiulritic impressions between the divisional planes of close- 
grained rocks (limestone, felsite, &c.), sometimes as accumulations of a black or brown 
earthy substance in hollows of rocks, and occasionally as deposits in marshy places, like 
those of bog-iron-ore. 

5. Silicates. — These embrace by far the largest and most important series of rock- 
forming minerals. Tlieir chief groups are the anhydrous aluminous and magnesian 
silicates embracing the Felspars, Hornblendes, Augites, Micas, &c, and the hydrous 
silicates which include tlie Zeolites, Clays, talc, chlorite, serpentine, &c. 

The family of the Felspars forms one of the most important of all the constituents 
of rocks, seeing that its members constitute by much the largest portion of the plutonio 
and volcanic rocks, are abundantly present among many crystalline schists, and by 
their decay have supplied a great part of the clay out of which argillaceous sedimentary 
formatious have been constructed. 

The felspars are usually divided into two series. 1st, The orthoclastic or monoolinio 
felspars, consisting of two species or varieties, Orthoclase and Sanidine ; and 2nd, The 
plagioclastic or triclinic felspars, among which, as constituents of rocks, may be 
mentioned the species albite, anorthite, oligoclose, andesine, labradorite, and microcline. 

Orthoclase (K^O 16*89, Al^Os 18-43, SiOt 6468), occiurs abundantly as an original 
constituent of many crystalline rocks (granite, syenite, felsite, gneiss, &c.), likewise 
in cavities and vcininga In which it has segregated from the surrounding mass 
(])egmatite) ; seldom found in unaltered sedimentary rocks except in fragments 
derived from old crystalline masses; generally Dssociated with quartz, and often 
with hornblende, while the felspars less rich in silica more rarely accompany firee 
quartz. It is both an original constituent of plutonic and old volcanic rocks (granite, 
felsite, &c.), and a result of the mctamorphism' that has produced foliated masses 
of gneiss and various schists. A few examples have been noticed where it has 
replaced other minerals (prehnite, analcime, laumontite). Under the microscope it 
is recognisable from quartz by its characteristic cleovage, twinning, turbidity, and 
frequent alteration,* Orthoclase weathers on the whole with comparative nipidity, 

' Some of the Canadian masses of this mineral are 90 feet thick and many prds in 
length. 

. * f Pie Pal'aolitische Eruptivgesteine dcs FichtelgebirgosJ 1874, p, 29. S^ Boseu- 
busch, Mik, Physiog. ii. p. 836. De la Yallce Poussin and Bcnard, Mim, CouronnSe* 
Acad, Boy, de Belgique, 1876, xl. Plate vi. pp. 34 and 35. Fouqu^ and Michel-L^vy, 
'•Mineralogie Micrograph.* p. 426. - 

• On microscopic determination of felspars, see Fouqu<J and Michel-L^vy, op, eit. 

pp. fcUy, ££7t 



Paw n. 4 iij BOCK'I^OBMINCf MINERALS. 69 



though dnrable Tarieties are known. The alkali and some of the nlica are removed 
and the mineral passes into clay or kaolin (p. 73). 

8anidine,the clear glassy fissnred variety of orthoclase so oonspionons in the more 
silioated Tertiary and modem lavas, occurs in some trachytes in large flat tables (hence 
the name ''sanidine"}; more commonly in fine clear or grey crystals or crystalline 
graavlea; an eminently volcanic mineral. 

Fbtfl^oeUse (Triolixiio) Felapars.— While the different felspars which 
cryakalliae in the triolinio system may be moi^ or less easily distinguished in largo 
crystals or crystalline aggregates, they ace difficult to separate in the minute forms in 
wliSeli they bommonly occur as rock constituents. They have been grouped by petro- 
giaphan imder the general name Plagioclase (with oblique cleavage), proposed by 
Tsofaennak, who regards them as mixhires in vicious proportions of two fundamental 
compoonds— albite or soda- felspar, and anorthite or lime-felspar. 

They occur mostly in well developed crystals, partly in irregular crystalline 
gtains. On a fresh fracture, their crystals often appear as clear glassy strips, on which 
may usually be detected a fine parallel lineation or ruling, indicating a chamcterisiic 
polysynthetie twinning which never appears in orthodsse. A felspar striated in this 
manner can thus be at once pronounced to be a tridinic form, though the distinction is 
not invariably present Under the microscope, the fine parallel lamcllation seen with 
polarized liglit forms one of the most distinctive features of this group of felspars. Thu 
ekicf triclinic felspars are, Microcline (potash-felspar, K,Al,Si«0,fl) ; occurs in granites 
and some gneisses, &e.; Albite (soda-felspar Na^O 11*82, Al^O, lOoO, SiO^ G8G2), 
occurs in some granites, and in several volcanic rocks ; (soda-lime and lime-soilu 
felspars) Oligodase, Na,0 8-2, CaO 4*8, Al^O, 23*0, 8iO, 62*8) occurs in many granites 
and other eruptive rocks; Andcsine (Na,0 7*7, CaO 7*0, A1,0, 25*6, BiOj 60*0) occurs 
in some syenites, &c. ; Labradorite (No^O 4*6, CaO 12*4, Al^O, 30-2^ SiO, 52*9), 
an essential constituent of many lavas, &c., abundant in masses in the azoic rocks of 
Canada, &c. ; Anorthite (lime-fel^ar, CaO 2010, AljO, 36*82, SiO. 43*08) occurs in many 
vulcanic rocks, sometimes in granites and metamorphic rocks. 

The triclinic felspars have been produced sometimes directly from igneous fusion, as 
can be studied in many lavas, where one of tho first minerals to appear in the devitrifi- 
cation of the original molten glass is the labradorite or other plagioclase. In other 
cases, they have resulted from the operation of the processes to which tho formation of thu 
crystalline schists was due ; large beds as well as abundant diff'usod strings, veiuings, 
aud crystals of triclinic felspar (labradorite) form a marked feature among the ancient 
gneisses of Eastern Canada. The more highly silicatod species (albite, oligoelnsc) 
occur with orthoclase as essential constituents of many granites and other pliitonic 
rocks. The more basic forms (labradorite, anorthite) are generally absent where free 
silica is present ; but occur in the more basic igneous rocks (basalts, &c.). 

Considerable differences are presented by the triclinic felspars in regard to woatlicring. 
On an exposed face of rock they lose their glassy lustre and become white and opaque. 
This change, as in orthoclase, arises from loss of bases and silica and from hydration. 
Traces of carbonates may often be observed in weathered crystals. The original steuni 
cavities of old volcanic rocks have generally been filled with infiltrated minerals, wJiich 
in many cases have resulted from the weathering and dcoomposition of the triclinic 
felspars. Calcite, prehnite, aud the family of zeolites have been abundantly produced in 
tliis way. The student will usually observe that where these minerals abound in the 
colla and crevices of a reck, the rock itself is for tho most part proportionate]}- 
decomposed, showing tho relation that subsists between infiltration-products and the 
decomposition of the surrounding mass. Abundance of ca^lcite in veins and cavities of 
a felspathic rook affords good ground for suspecting tho presence in the latter of a 
l{me felspar.' 

. ' A valuable essay on the stages of the weathering of triclinic felspar ns revealed by 
the microscope was published by 6. Rose in 1867. ZeiUch. Deutsrh, Geol. Get. xix. p. 276. 



70 GEOGNOSY. tBooK H. 



SauBsnritey believed to be often a mixtme of plagioclaae and nnsifte, fonns with 
diallage some yarietiee of gabbro, and is abandaotly aaaociaied in others with labradorite, 
or with hornblende. Under the microBcope it presents a oonfnsed aggregate of 
crystalline needles and grannies imbedded in an amorphous glass-like matrix. 

Leuoite (K,0 21*58, Al^O, 23*50, SiO, 54*97) is a markedly yolcanio mineral, oocnr- 
ring as an abundant constituent of many ancient and modem Italian lavas^ and in 
some varieties of basalt. Under the microscope sections of this mineral are usually 
eight-sided, and very commonly contain enclosures of magnetite, Arc, conforming in 
arrangement to the external form of the crystal. 

Nepheline (Na,0 17*04, A1«0, 35*26, K,0 6*46, SiO, 41*24), essentially a volcanic 
mineral, being an abundant constituent of phonolite, of some Yesuvian lavas, and 
of some forms of basalt, presents under the microscope various six-sided and even 
four-sided forms, according to the angles at which the prisms are cut^ Under the name 
of Ekedite ore comprised the greenish or reddish, dull, greasy-lustred, compact or massive 
varieties of nepheline, which occur in some syenites and other ancient crystalline rocks. 

The Mica Family embraces a number of minerals, distinguished especially by 
their very perfect basal cleavage, whereby they can be split into remarkably thin elastic 
lominse, and by a predominant splendent pearly lustre. They consist essentially of silicates 
of alumma and potash or magnesia, usually with some oxide of iron, but little or no lime. 

Muscovite (Potash-mica, Glimmer, K,0 3*07-12*44, Na^O 0-4*10, FeO 0-1*16, 
Fe^O, 0-46-8*80, m|:0 0*37-3 08, A1,0, 28*05-38*41, SiO, 43*47-51*73, H,0 0*98- 
6*22), abundant as an original constituent of many crystaJIine rocks (granite, &o.), 
and as one of the characteristic minerals of the crystalline schists ; also in many 
sandstones, where its small parallel flakes, derived, like the surrounding quartz grains, 
from older crystalUno masses, im|>art a silvery or " micaceous " lustre and fissility to 
the stone. Under the microscope, thin plates of musoovite give bright chromatic polari- 
zation when cut parallel to the basal cleavage. But as the sections of the mineral 
displayed in a thin slice of any rock rarely coincide with the cleavage, but traverse it at 
various angles, they appear usually as narrow bands with fine parallel lines which mark 
the planes of cleavage.* The persistence of muscovite under exposure to weather is 
shown by the silvery plates of the mineral, which may be detected on a orumbliDg 
surface of granite or schist where most of the other minerals, save the quartz, have 
decayed ; also by the frequency of tho micaceous lamination of sandstones. 

Biotite (Magnesia-mica, MgO 10-30 per cent), occurs abundantly as an original 
constituent of many granites, gneisses, and schists ; also sometimes in ba&alt, trachyte^ 
and as ejected fragments and .'crystals in tuff. Its small scales, when cut transvenn to 
the dominant cleavage, may usually be detected under the microscope by their remark- 
ably strong dichroism, their fine parallel lines of cleavage, and their frequently frayed 
appearance at the ends. Under the action of the weather it assumes a pale, dull, soft 
crust, owing to removal of its bases. The mineral rvbeUan^ which occurs in hexagonal 
brown or red opaque inelastic tables in some basalts and other igneous rocks, is regarded 
OS an altered form of biotite. 

Lepidolite (Lithia-mica), occurs in some granites and crystalline schists, espedally in 
veins. Several hydrous varieties of Mica are distinguished— Damourite, merely a variety 
of muscovite, occurs among crystallino schists; Sericito, a talc-like variety of muscovite, 
occurring in soft inelastic scales in some schists;* Margarodite, a silvety, talc-like 
hydrous mica, widely diffused as a constituent of granite and other crystalline rocks ; 
Paragonite, a scaly micaceous mineral, forms the main mass of certain alpine schists. 



* On the microscopic distinction between nepheline and apatite, see Fouqu€ and 
Michel-L^vy,* Mineral. Micrograplu* p. 276. 

2 On the microecopic determmation of the micas, see Fouque and Michel-L^vy, op. ciU 
p. ooo. 

* On the occurrence of this mineral in schists, see Lossen, Zeittch. DeuUch. OeoL Ges, 
1867, pp. 546, 66L 



Paw IL S ii.] BOCK-FOBMING MINEBAL8. 71 



Horablflnde (Amphibole, CaO, 10-12, MgO 11-24, F03O, 0-10, A1,0, 5-18, 
8iO, 40—50 also nraally with some Na,0, K^O and FeO). Divided into two gronps. 
1st. Xoo-alnmiiKNis, indudiiig the white and pale green or grey fibrous varieties 
(tieiiK^te, actinolite, anthophyllito, &c.). 2nd. Aluminons, embracing the more 
abondaiit dark green, brown, or black varieties. Under the microscope, hornblende 
presents cleavage-angles of 124^ 30', the definite cleavage-planes intersecting each other 
in a wdl^marfced lattice work, sometimes with a finely fibrons character superadded. It 
also riiowB a maiked pleochioism with polarized light, which, as Tschermak first pointed 
out, umiallj dislingoisheB it fitom augite.^ The pale non-alnminous hornblendes are 
found among gnetsses, erystalUne limestones, and other metamorphio rocks. The dark 
varietiei^ though also fonnd in similar situations, sometimes even forming entire masses 
of lock (amphibolite, honblende-rock, hornblende-schist), are the common forms in 
gnnitic and volcanic rocks (syenite, diorite, homblende-andesite, &c). The former 
gronp naturally gives rise by weathering to various hydrous magncsian silicate?, notably 
to serpentine and tala In the weathering of the aluminous varieties, silica, lime. 
magnesia, and a portion of the alkalies are removed, with conversion of part of the 
earths and the iron into carbonates. The further oxidation of the ferrous carbonate is 
shown by the yellow and brown crust so commonly to be seen on the surface or 
penefarating cradm in the hornblende. The change proceeds until a mere internal 
kernel of unaltered mineral remains, or until the whole has been converted into a 
femgiDons clay. 

Smaragdito, a grass green variety of hornblende, or an aggregate of pyroxene and 
hornblende, occurs in gabbro and eclogite. 

Uralite, having the crystalline form of angite (pyroxene) and the internal cleavage 
and structure of hornblende (amphibole), is regarded as a product of the gradual 
alteration of angite into hornblende. Under the microscope a still unchanged kernel of 
angite may in some specimens be observed in the centre of a crystal surrounded by 
strongly pleochroio hornblende, with its characteristic cleavage. 

Angite (Monoclinic Pyroxene, CaO 12-27*5, MgO 3-22-5, FeO 1-34, Fe^O, 0-10, 
AI3O, 0-11; SiOj 40-57-4). Divided like hornblende into two groups. 1st. Non- 
aluminous, with a prevalent green colour (malacolite, sahlite, &c). 2ud. Aluminous, 
including generally the dark green or black varieties (common angite, fossaite). It 
would appear that the substance of hornblende and augito is dimorphous, for the expe- 
riments of Berthier, Mitscherlich and G. Rose showed that hornblende, when melted 
and allowed to cool, assumed the crystalline form of augite ; whence it has been inferred 
that hornblende is the result of slow, and augite of comparatively rapid cooling.' 
Under the microscope, augite in thin slices is only very feebly pleochroic, and presents 
cleavage lines intersecting at an angle of 87^ 5'. It is ofleu remarkable for the amount 
of extraneous materials enclosed within its cr^'stals. Like some felspars, augite may 
be found in basalt with merely an outer casing of its own substance, the core being 
composed of magnetite, of the ground-mass of the surrounding rock, or of some other 
mineral (¥1g. 7). The distribution of augite resembles that of hornblende ; the pale, 
non-aluminous varieties are more specially found among gneisses, marbles, and 
other crystalline, foliated, or metamorphio rocks ; the dark-green or black varieties 
enter as essential constituents into many igneous rocks of nil ages, from palsoozoic up 
to recent times (diabase, basalt, andesitc, &c.). Its weathering also agrees with that 
of hornblende. The aluminous varieties, containing usually some lime, give rise to 
calcareous and ferruginous carbonates, from which the fino interstices and cavities of 
the surrounding rock are eventually filled with threads and kernels of calcito and 



> Wien, Acad. May 1869. See also Fouque' and Michel-Levy, of, cit, pp. 349, 365. 
■ The same results have been obtained recently by Fouque and Michel-Levy, 
* Synthte des Mineraux et des Roches,' 1882, p. 78. 



72 GEOGKOST. [Book H. 



strings of hydrous ferric oxide. In basalt and dol^itey for examplet the leathered 
surface often acquires a rich yellow colour from the oxidation and hydration of the 
ferrous oxide. 

Ompbacitey a granular Tariety of pyroxene, grass green in odour, and oommonly 
aaaociated with red garnet in the rock known as eclogite. 

Diallage, probably only a Tariety of augite, is especiaUy a eonalitiient of gahbro. 

There are three rhombic forms of pyroxene, which occur as important oonstituents 
of some rocks, Enstatite, Bronzite and Hypersthene. Bnstatitey oecnrs in Ihersolite, 
serpentina, and other olivine rocks; also in meteorites. Biozudtey is found nnder 
similar conditions to enstatite, from which it is with difficulty sqwrable. It oocurs in 
some basalts and in serpentines ; also in meteorites. Bronzite and enstatite weather into 
dull green serpentinous products. Bastite or 8chiller-epar is a frequent product of the 
alterotion of Bronzite or Enstatite, and may be obsenred with its charaderistio pearly 
lufltre in serpentine. Hsrpentheiie, oocurs in hypersthenite and hypersthene-andesito ; 
also assodated with other magnesian minerals among the crystalline schists. 

OUvine (Peridot, MgO 32-4-50-5, FeO 6-297, SiO, 31-6-42-8X forms an essential 
ingredient of basalt, likewise the main part of various so-called oliTuic-rocks or 
peridotites (as Iherzolite and pikrite) and oocurs in many gabbros. Under the 
mlcrsscope with polarized light, gives, when fresh, bright colours, specially red and 
green, but is not perceptibly pleochroic. Its orthorhombio outlines can sometimes be 
readily observed, but it often occurs in irregularly shaped granules or in broken 
crystals, und is liable to be traversed by fine fissures, which are particularly developed 
transverse to the vertical axis. It is remarkably liable to alteration. The change 
begins on the outer surface and extends inwards and specially along the fissures, until 
the whole is converted either into a green granular or fibrous substance, which is 
probably in most cases serpentine (Fig. 26), or into a reddish yellow amorphous mass 
limonite% 

ECauyne. Occurs abundantly in Italian lavas, in basalt of the Eifel, and elsewhere. 

B'oeeaiL Under the microscope, one of the most readily recognised minerals, 
showing a hexagonal or quadrangular figure, with a characteristic broad dark border 
corresponding to the external contour of the crystal, and where weathering has not 
proceeded too far, enclosing a dear colourless centre. Occurs in minute forms in most 
phonolites, also in large crystals in some sanidine volcanic rocks. Both hanyne and 
noseftn ore volcanic minerals associated with the lavas of more recent geological periods. 

Epidote. (CaO 16-30, MgOO-4-9, Fe.O, 7-5-17-24, Al^O, 14-47-28-9, SiO, 33-81 
-57'Co). Under the microscope, appears as a constituent of rocks in yellow needles 
and threads, often divergent ; with distinct plcochroism and remarkably bright limpid 
yellow and orange polarization tints. Occurs in many crystalline, chiefly hornblende- 
Ijearing, rocks, probably as a result of the alteration of the hornblende; largely 
distributed in certain schists and qiiartzites, sometimes associated with beds of magnetite 
and haematite. 

Vesuviajiite (Idocrase, CaO 27-7-37o, M;^ 0-10-6, FeO 0-16, A1,0, 10-5-2G1, 
SiOj 35-31)-7, M..0 0-2-73). Occurs in ejected blocks of altered limestone at Somma, 
also among crystalline limestones and schist:). 

AndaluBite. (Al^O, 50-9G-62-2, Fe,0, 0-57, SiO., 35-3-4017). Found in crys- 
talline schists. The variety CJncufMite, abundant in some dark clay-slates, is dis- 
tin^iished by the rrp^ular manner in ^Vhich the dark substance of the surrounding 
matrix has Ixjcn enclosed within the niaclee, giving a cross-like tmnsverse section. 
Thf'se cryatilfl have been developed in the rock after its formation, and are regarde«l as 
prTofrt of metninoriihlsni. (Book XT. Part VIII.) 

Dichroite (Cordieritc, lolite, 3IgO &-2-20-45, FeO 0-11-58, Al-O, 28-72-3311, 
SiD, iSl-riOl, Iljfj 0-2'GC). Occurs in gneiss, sometimes in large amount (cor- 
dieritf;-gnei.s8) ; occasionally as an accessory ingredient in some granites; also in 
t:il«*-3cliist. Apt to be confounded with quartz, but usually gives marked diobroism 



Paw XL 'J U.] ROCK-FORMINO MINEBALS. 73 



wiHi one Kiool-prum, and pale grey-blno tints witli the two prisms. Undergoes 
numerous alterations. Laying been fonnd changed into pinite, chlorophyllite, mica, &c. 

Gkumet. (CaO 0-5-78, MgO 0-10-2, FcaO, 0-6*7, FeO 24-82-39-68, MnO 0- 
6*43, AljO, 15-2-21-49, SiO, 35-75-5211). The common rod and brown varieties 
oocor as essential constitnento of eclogite, garnet rock ; and as abundant accessories 
in mioarechlst, gneiss, granite, &c Under tho microscope, garnet as a constituent of 
rocks, presents three-sided, four-sided, six-sided, eight-sided (or even rounded) figures 
aocoiding to the angle at which the individual crystals are cut ; usually clear, but full 
of flaws or of cavities ; passive in polarized light. 

TaurmaUae (Schorl, CaO 0-2-2, MgO 0-14-89, Na,0 0-4-95, K,0 0-3-59, FeO 
0-12. Fe,0, 0-13-08, A1,0, 30-44-44-4, SiO, 35-2-4116, B 8-63-11-78, F 1-49-2-58) 
i^ith qnarti, forma tourmaline-rock; associated with some granites; occurs also 
diffuaed throng many gneisses, schists, crystaUine limestones, and dolomites. 
Pleochroism strongly marked. 

Ziroon. (ZrO, 63-5-6716, Fe,0, 0-2, 8iO- 32-35-26.) Occurs as a chief 
ingredient in the zircon-syenite of Southern Norway; sparingly in other syenites, 
granitesy gneisses, crystalline limestones and schists ; in eclogite ; as clear red grains 
in some basalts, and also in ejected volcanic blocks; gives bright colours between 
crossed Nicols. 

Titanite (Sphene, CaO 21-76-33, TiO, 33-43-5, SiO^ 30-35), dispersed in smaH 
crystala in many syenites, also in granite, gneiss, and in some volcanic rocks (basalt, 
trachyte, phonolite). Between crossed Xiools gives dark yellowish-brown tints. 

Zeolites. Under this name is included a characteristic family of minerals, which 
have resulted from the alteration, and particularly from the hydration, of other minerals, 
especially of felspars. Secondary products, rather than original constituents uf rocks, 
they oflcn occur in cavities both as prominent amygdules and veins, and in minute 
interstices only perceptible by the microscope. In these minute forms they very 
commonly present a finely fibrous divergent structure. As already remarked, a relation 
uiay often be traced between tho containiug rock and its enclosed zeolites. Thus among 
the basalts of the inner Hebrides, the dirty green decomposed amygdaloidal sheets are 
tlie chief repositories of zeolites, while tho firm, compact, columnar beds ore compara- 
tively free from these alteration products.^ 

S:aolin (A1,0, 38-6-40-7, CaO 0-3-5, K^O 0-1-9, SiO., 45-5-46-53, H^O 9-14-54), 
results from the alteration of potash- and soda-felspars exposed to atmospheric, in- 
fluences. Ordinary clay is impure from admixture of iron, lime, and other ingredients, 
among which the de'bris of the undccomposed constituents of tiie original rock may form 
a marked proportion. 

Talc (MgO 2319-35-4, PeO 0-45, AUO, 0-5-67, SiO^ 5G-62-64-53, H^O 0-6-65), 
occurs as an essential constituent of talc-schist, and as an ulteration product re- 
placing mica, hornblende, augite, olivine, diallage, and other minerals in crystalline 
rocks. Under the microscope appears in small scales, which, cut transverse to basal 
cleavage, show ragged edges and an internal fibrous structure, the fibres not being 
parallel as in muscovite ; is not pleochroic ; polarization colours, bright yellow and red. 

Chlorite (MgO 24-9-36, FeO 0-5-9, Fe^O, 0-11-36, Al^O, 10-5-19-9, SiO^ 30- 
38-5, H,0 11*5-16), including several vorieties or species, occurs in small green 
hexagonal tables or scaly vermicular or earthy aggregates ; is an essential ingredient 
of chlorite-schist, and occurs abundantly as an alteration product (of hornblende, &c.) 
in fine filaments, incrustations, and layers in many crystalline rocks. Under the 
microscope appears markedly radiated in thin plutcs or sphcrulites, with internal 
confused radiating fibrous structure. 

Serpentine (>IgO 28-43, FeO 1-10-8, A1,0, 0-55, SiO^ 37-5-44-3, U,0 95- 
14-C) is a product of the alteration of pre-existing minemls, and especinlly of 

» See Sullivan in Jukes' * Manual of Geology,' p. 85. 



74 GEOGNOSY. [Book H, 



oli?ine. It oocura in nests, grains, thiesds, and veins in rocks which once contained 
olivino,* (p. 72), also massive as a rook, in which it has replaced olivine, enstatite or 
some other magnesian bisilicate (p. 156). Under the microscope it presents, in very 
thin slices, a pale leek-green or blaish-groen base, showing aggregate polarization. 
Through this base runs a network of dark opaque threads and veinings. Sometimes 
among these veinings, or through the network of green serpentinous matter in the base, 
the forms of original olivine crystals may be traced (Figs. 26, 27). 

Delessite (CaO 0-45-3-7, FeO 0-1512, Fe,0, 817-17-54, A1,0, 15-47-18-25, 
SiOs 29*08-^1*07, H^O 11*55-12*99), occurs abundantly as an olive- to blackish-gieen 
decomposition product of augitio rocks, coating or filling amygdaloidal cavities or 
narrow filamentous veins. 

QUuoonite (CaO 0-4-9, MgO 0-5*9, K,0 0-12*9, Na,0 0-2*6, FeO 8-25-5, 
Fe,0, 0-281, A1,0, 1-5-13*3, SiO, 46*5-6009, H,0 0-147), found in many strati- 
fied formations, particularly among sandstones and limestones, where it envelopes 
grains of sand, or fills and coats foraminifera and other organisms, giving a general 
green tint to the rock. It is at present being formed on the sea-floor off the coasts of 
Georgia and South Carolina, where Pourtales found it filling the chambers of reoent 
polythalamia. 

6. Carbonates. This family of minerals furnishes only four which enter lai^ly 
into the formation of rocks, viz.. Carbonate of Calcium in its two forms, Cakite and 
Aragonite, Carbonate of Magnesium (and Calcium) in Dolomite, and Carbonate of Iron 
in Siderite. 

Oaloite, occurs as (1) an original constituent of many aqueous rocks (limestone, 
calcareous shale, &c.), either as a result of chemical deposition from water (calo-sinter, 
stalactites, &c.), or as a secretion by plants or animals ' ; or (2) as a secondary product 
resulting irom weathering, when it is found filling or lining cavities, or diffused through 
the capillary interstices of minerals and rocks. It probably never occurs as an original 
ingredient in the massive crystalline rocks, such as granite, felsite, and lavas. Under 
the microscope, calcite is readily distinguishable by its intersecting cleavage lines, by a 
frequent twin lamellalion (sometimes giving interference colours), strong double 
refraction, weak or inappreciable pleochroism, and characteristic iridescent polarization 
tints of grey, rose and blue. 

From the readiness with which water absorbs carbon-dioxide, from the increased 
solvent power which it thereby acquires, and from the abundance of calcium in various 
forms among minerals and rocks, it is natural that calcite should occur abundantly as 
a pseudomorph replacing other minerals. Thus, it has been observed taking the place 
of a number of silicates, as orthoclase, oligoclase, garnet, augite and several zeolites ; of 
the sulphates, anhydrite, gypsum, barytes, and oelestine ; of the carbonates, aragonite, 
dolomite, cerussite ; of the fiuoride, fluor-spar ; and of the sulphide, galena. Moreover, 
in many massive crystalline rocks (diorite, dolerite, &c,), which have been long ezpoasd 
to atmospheric influence, this mineral may be recognised by the brisk effervescence 
produced by a drop of acid, and in microscopic sections appears filling the crevices, or 
sending minute veins among the decayed mineral constituents. Calcite is likewise the 
great petrifying medium : the vast majority of the animal remains found in the rocky 
crust of the globe have been replaced by calcite, sometimes with a complete preserva- 
tion of internal organic structure, sometimes with a total substitution of crystalline 
material for that structure, the mere outer form of the organism alone surviving.' 



> Bee Tschormak, Wien, Akad. Ivi. 1867. 

^ Mr. Sorby has investigated the condition in which tho calcareous matter of tho 
harder parts of invertebrates exists. Ho finds that in foraminifera, eohinoderms, 
brachiopode, Crustacea, and some lamellibranchs and gasteropods, it occurs as calcite ; 
that in nautilus, sepia, most gasteropods, many lamellibranchs, &c, it is aragonite ; 
that in not a few cases the two forms occur together, or that the carbonate of Ume is 
hardened by an admixture of phosphate. Quart Jaum, (TeoZ. 8oc, 1879. Address, p. 61. 

' See Index wb voc, Calcite, 



Pabt IL S ii.] ROCK-FOBMIKG MINEltALS. 75 



AragonitiB, harder, heavier, and much less abnndant than calcite, which is the 
mote stable form of caksinm-carbonate ; oocnrs with beds of gypenm, also in mineral veins, 
in atrtngs nmning throngh basalt and other igneous rocks, and in the shells of many 
moUnsoa. It ia thus always a deposit from wat^**, sometimes from mineral springs, 
fWip^^wMw as the resnlt of the internal alteration of rocks, and sometimes throngh the 
action of living organisms. Being more easily soluble than calcite, it has no doubt 
in many cases disappeared from limestones originally formed mainly of aragonite shells, 
and has been replaced by the more durable calcite, with a consequent destruction of the 
tiaeee of ofganio origin. Henoe what are now thoroughly crystalline limestones may 
have been formed by a slow alteration of such shelly deposits. 

I>olomite (Bitter-spar, p. 120), occurs (1) as an original deposit in massive beds 
(magnesian limestoneX belonging to many different geological formations; (2) as a 
product of alteration, especially of ofdinaiy limestone or of aragdhite (Dolomitizatkm). 

Siderite (Brown Ironstcme, Spathic Iron, Ghalybite), oocuis orystallized in 
iiiriation with metalUo oree, also in beds and veins of many crystalline rocks, 
paitionlarly with limestones; the compact argillaceous varieties (clay-ironstone) are 
found in abundant nodules and beds in the shales of Oarboniferous and other formations 
where they have been deposited from solution in water in presence of decaying organic 
mattCT (see pp. 121, 174). 

7. SuLPHATU. Among the sulphates of the mineral kingdom, only three deserve 
notioo hero as important compounds in the constitution of rocks — viz., caldum-sulphato 
or sulphate of lime in its two forms, Anhydrite and Gypsum ; and barium-sulphate or 
sulphate of baryta in Bary tes. 

Anhydrite, occurs more especially in association with beds of gypsum and rock* 
salt (see p. 121). 

Oypsum (Selenite). Abundant as an original aqueous deposit in many sedimentary 
formations (see p. 120). 

Bavytee (Heavy Spar). Frequent in veins and especially associated with metallic 
ores as one of their chamcteristic vein-stones. 

8. Phospbatks. The phosphates which occur most conspicuously as constituents or 
accessory ingredients of rocks are the tricalcic phosphate or Apatite, and triferrous 
phosphate or Vivianite. 

Apatite, occurs in many igneous rocks (granites, basalts, &c.)) in minute hexagonal 
non-plcochroic needles, giving faint polarization tints ; also in large crystals and massive 
beds associated with metamorphic rocks. 

Vivianite (Blue iron-earth) ; occurs crystallized in metalliferous veins ; the earthy 
variety is not infrequent in peat-mosses where animal matter has decayed, and is some- 
times to be observed coating fossil fishes as a fine layer like the bloom of a plum. 

9. Fluobides. The clement fluorine, though widely diffused in nature, occurs only in 
comparatively small quantity. Its most abundant compound is with Calcium as the 
common mineral Fluorite. 

Fluorite (Fluor-spar) ; occurs generally in veins, especially in association with 
metallic ores. 

10. Chlobidbs. There is only one chloride of importance as a constituent of rocks 
^Bodium-chloridc or common salt, which, occurring chiefly in beds, is described among 
the rocks at p. 118. 

11. SuLPHTDES. Sulphur is found united with metals in the form of sulphides, many 
of which form common minerals. The sulphides of lead, silver, copper, zinc, antimony, 
4:c., are of great commercial importance. Iron-disulphide however, is the only ouu 
which merits consideration here as a rock-forming substance. It is formed at the present 
day by some thermal springs, and has been developed in many rocks as a result of the 
action of infiltrating water in presence of decomposing organic matter and iron salts. 
It occurs in two forms, Pyritc and Marcasite. 

Pyrite (Eisenkies, Schwefelkies), occurs disseminated through almost all kinds of 



76 GEOGNOSY. [Book It 

rocks, often in great abundance, as among dfahagcB and day-alales; alsD fteqnent in 
Teins or in beds. In microscopic sections of rocks, pyrite appeara in small cubical^ 
perfectly opaqne crystals, which with reflected light show the characteristie bnu^ 
lustre ik the mineral, and cannot tbns be mistaken for the isometric magnetite, of which 
the square sections exhibit a characteristic blue-black colour. Pyrite when free from 
marcasite yields but slowly to weathering. Hence its cubical crystals may be seen 
projecting still fresh from slates which have been exposed to the atmosphere for seTeral 
generations. 

Marcasite (Hepatic pyrites); occurs abundantly among sedimentary formations, 
sometimes abundantly diffused in minute particles which impart a bine-gray tint, and 
speedily weather yellow on exposure and oxidation ; sometimes segregated in layras, or 
replacing the substance of fossil plants or animals ; also in Teins through crystalline 
rocks. This form of the sulphide is especially characteristic <^ stratified fosailifeious 
rocks, and more particularly of those of Secondary and Tertiary date. It is extremely 
liable to decomposition. Hence exposure for even a short time to the air caucfea it to 
become brown ; free sulphuric acid is produced, which attacks the surrounding minerals) 
sometimes at once forming sulphates, at other times decomposing aluminous silicates 
and dissolving them in constdetable quantity. Dr. Sullivan mentions that the water 
annually pumped from one mine in Ireland carries up to the sur&oe more tlian a 
hundred tons of diwolved silicate' of alumina.' Iron-disulphide is thus an important 
agent in effecting the internal decomposition of rocks. It also plays a large part as a 
petrifying medium, replacing the organic matter of plants and animals, and leaving 
casts of their forms, often with bright metallic lustre. Such casts when exposed to the 
air decompose. 

It will be observed that great differences exist iu the relative im- 
portance of the minerals above enumeratecl as constituenta of rocks. 
Professor Kosenbuscli points out that they may be naturally arranged 
in four groups — Ist, ores and accessory ingredients (magnetite, haematite, 
ilmenite, apatite, zirkon, spinell, titanite), 2nd, magnesian and feriu- 
ginous silicates (biotite, amphibole, pyroxene, olivine), 3rd, felspathic 
constituents (felspar proper, nepheline, leucite, melilite, sodalite, hauyne), 
4th, free silica.^ 

§ iii. Determination of Bocks, 

Rocks considered as mineral substances are distinguished from each 
other by certain external characters, such as size, form, and arrange- 
ment of component particles. These characters, readily perceptible to 
the naked eye, and in the great majority of cases observable in hand 
specimens, are termed macroscopic (pp. 77, 91), to distinguish them from 
the m6re minute features \^hioh, being only visible or satisfactorily 
observable when greatly magnified, are known as microscopic (p. 99); 
The larger (geotectonic) aspects of rock-structure, which can only be 
properly examined in the field and belong to the general architecture 
of the earth's cnist, are treated of in Book IV. 

In the discrimination of rocks, it is not <)uough to specify their 
component minerals, for the same minerals may constitute very distinct 
varieties of rock. For example, quartz and mica form the massive 
crystalline rock, greisen, the foliated crystalline rock, mica-sohist, and 
the sedimentary rock, micaceous sandstone. Cha.lk, encrinal limestone, 

> Jukes' * Hantial of Geology,' p. 05. « Nene$ Jahrb. 1882 (ii.) p. 5 . 



PabtIliul] detebmination of bocks. 77 

stelagmite, tAa.taaxj marble are all composod of caleito. It is needful to 
take note of the maoroecopio and microscopic etractnre and texture, the 
state of ^gregation, colour, and other characters of the sercral masses. 

Three methods of procedure are available lu tlie examiuation and 
determination of rooks : 1st, macroscopic examination, either hj the 
rongh and ready, but often sufficient, appliances for use in the field, or 
by those for more careful work indoors j 2nd, niicrosoopio investigation ; 
3rd, chemioal analysis. 

L JUaoroseopio ZSzamination. 

Teata intba Bald. — The iiwf ruments iudispenaobJo for the investigation oE rookg in 
the field are few in nnmber, and simple in cLumcter and application. The observer will 
Tie anfflciently accoatred if he oanies with him a hammer of such form and weight as 
will enable htm U> break off clean, sharp, nuncathercd chips from the edges of rock- 




Fig. S.— Iluuvr, Shnth, inl Drll, with Usttan-cur Tor hoUllni AtLmiKh CVimpii". 
mawea, a small Ions, a pocket-knife of liard steel for determining the hardness of rocks 
anil minerals, a magnet or a magnetized knife-blade, and a small pockct-phial of dilute 
bydmchlnric acid. 

Should the objcot bo to foru a eolleclion of rocks, a hammer of at least three or four 
pnnnds in weight ghonlil be cnrried : also one or two cliisela and a smHll trimming 
liammer, wiighing alxiut \ 11>., for reducing tlie specimens tn shape. A convenient size 
bf specimen is 4x3 >tl iuchea. They should bo as nearly ns possible uniform in size, 
ic) as In bo cajiable uf onlcrl; arrangement in the drawers or slielvea of a ease or cabincl. 
Atti.'Dtion should be j^aid not only to obtain a thoroughly IVesh fracture of a rock, but 
uliio a weatbcrcil surface, wherever there is anything cliaracteristtc in tlie weathering. 
Kvcry specimen should have affixed to it a label, indicating ns exactly as possible tbe 
lirrolity from which it was taken. Tiiis iofonnation ought always to bo written down in 
llie Geld at tbe time of collecting, and should be wrapped up with the specimen, before 
it i* oonsignwl to the collecting bag. If, however, the atndent does not porpose to form 
a collection, bnt merely to obtain such chips as will enable bim to judge of the 
ctuuacten of rocks, a hammer weighing fKim 1} to 2 lbs. and of the shape indicated in 



78 GEOGSOST. CBooK n. 



Fig. 5 wfll be niffieieiil. The mhmtm^ ^ i^ immm i^k iBbb htmma m^h^ xmeA 
not onlj for breaJniig hard ttemga^ boi •!■! §m iplitlB^ open ifcalBi and other fiidle 
rockc, 10 that it unites the inea of haMiKr aad cUkL 

It ic of ooune, deniable that the leazwribMU fnlaBqpbe aoM knoirlDdge of the 
somenclatore of ro^s, br earefallj l iadjiag a eoDeetioB of eooeetly named and 
jodicioiMl J aekcted rockiniifif m teek eoOeetma maj mom be |iiiirhMiNl at amall 
eott from mineral dta l eii » or mmj be itadied m tiie wnmenm of boiI towna. HaTing 
aoeoiiomed hit eye to the oniinaiy exloBal chaadeB of woAm, and baring beeome 
familiar with their namea, he may proeecd to detfrmiae them for humelf in the ftdd. 

Finding himself bite to fiaee with a nck-maas, aad after noting its geotectonie 
charactefs (Book IV.)» the obserrer will lawccd to riaminr tke eipoaed or weatheied 
snrfaoe. The earliest lesson he has to lean, aad thai of vhidi |Wffhaps he will in 
after life meet with the most varied iIhHlratioiiB» is the extent to which weathering 
conceals the tme aspect of roeka. From what has been said in prerioas pages, the 
natnie of some of the aHerations will be vndentood, and finthar infocmation regarding 
the chemical prooeasea at woik win be fioimd in Book nL The pnetiml study of rocks 
in the field soon discloses the hei^ that while, in some eases^ the weatiiered orast ao 
completely obsonres the es s e ntial riianeter of a rode that its tme nature might not be 
suspected, in oth^ jnstannfB, it is the weathered crust fliat beat rereals the real 
structure of the mass. Sphero i dal cnato of a decomposing yellow fenuginons earthy 
substance, for eramplis would hardly be Identified as a compact dark basalt, yet, on 
penetrating within these csmAa, a eeaftml core of still undeoompoeed bssalt may not 
unfrequeutly be disoorered. Agafai, a Idock of limestone when broken open may 
present only a uniformly cryilalline structure, yet if the weathered sur&oe be examined 
it will not improbably diow many projecting firagments of shells, polyzoa, corals, 
crinoids, or other organisms. The really fossiliferous nature of an apparently 
iinfoflsiliferous rock may thus be rerealed by weathering. Many limestones also might, 
from their fresh fracture, be set down as tdJerably pure caibonato of lime; but from the 
thick crast of yellow ochre on their weathered fiioes are seen to be hi^y ferruginous. 
Among crystalline rocks, the weathered muhoe oommooly throws light upon the 
mineral oonstitution of the mass, fiur some minerals decompose more rapidly than others, 
which are thus left isolated and more easUy recognisable. In this manner, the ezistonce 
of quartz in many felqpathio rocks may be detected. Its minute blebs or crystals, which 
to the naked eye or lens are lost among the brilliant faoettes of the felspars, stand out 
amid the dull clay into which these minerals are decomposed. 

The depth to which weathering extends should be noted. The student must not be 
too confident that he has reached its limit, e?en when he comes to the solid, more or less 
hard, splintory, and apparently fresh stone. Granito sometimes decomposes into kaolin 
and sand to a depth of twenty or thirty feet Limestones hare often a mere fihn of 
crust, beoause their substance is almost entirely dissolred and removed by rain 
(Book m. Part IL Section ii. § 2). 

With some practice, the inspection of a weathered surfieuse will finequentiy suffice to 
determine the true nature and name of a rock. Should this preliminary examination, 
and a comparison of weathered and unweathered surfaces, fail to afford the information 
sought, we proceed to apply some of the simple and useful tests available for field*work. 
The lens will usually enable us to decide whether the rock is compact and appaxentiy 
structureless, or crystalline, or firagmentaL Having settled this point, we proceed to 
ascertain the hardness and colour of streak, by scratching a fresh surface of the stone. 
A drop of weak acid placed upon the scratehed surface or on the powder of the streak 
may reveal the presence of carbonic acid. By practice, considoable faciUty can be 
acquired in approximately estimating the specific gravity of rocks merely by the hand. 
The following table may be of assistance, but it must be understood at the outset that 
m knoidedge of rocks can never be gained from instructions given in books, but must 
|jg>H.wlwi Igr-Mual handling and study of the rocks themselves. 



Paw n. S iii.] DETEBMINATION OF BOOKS. 79 



L A Iraeh flnustore shows the rook to be oloae-grained, dull, with no 
distinot stmotnio.* 

a. H. 0*5 or leas up to 1 ; soft, ornmbliDg or easily scratched with the knife, if not 
with the finger-nail; emits an earthy smell when breathed upon, does not 
Qfler?esoe with aoid; is dark grey, brown, or bine, perhaps red, yellow, or 
even white = probably some day rock, such as mudstone, massive shale, or fire- 
clay (p. 163 ) ; or a decomposed felspar- rock, like a close-grained felsite or 
orthoelase porphyry. If the rook is hard and fissile it may be shale or clay- 
slale (pp. 164, 125). 

/I. H. 1*5-2. Occurs in beds or veins (perhaps fibrous), white, yellow, or reddish. 
Bp. gr. 2*2-2*4. Does not effervesce = probably gypsum (p. 120). 

y. Friable, crumbling, soils the fingers, white, or yellowish, brisk effervescence = 
chalk, marl, or some pulverulent form of limestone (pp. 118, 168> 

t. H. 3-4. Bp. gr. 2*5-2*7 ; pale to dark green or reddish, or with blotched and 
clouded mixtures of these colours. Streak white; feels soapy; no effervescence, 
splintery to suboonchoidal fracture, edges subtranslucent. Bee serpentine 
(p. 156). 

c. H. averaging 8. Bp. gr. 2*6-2*8. White^ but more frequently bluish-grey, also 
yellow, brown and black; streak white ; gives brisk effervescence = some form 
of limestone (pp. 118, 168). 

(, H. 3-5-4*5. Bp. gr. 2*8-2*95. Yellowisb, white, or pale brown. Powder 
slowly soluble in acid with feeble effervescence, which becomes brisker 
when the add is applied to the powder of the stone. Bee dolomite (pp. 75, 
120). 

1^ H. 3-4. Bp. gr. 3-8*9. Dark brown to dull black, streak yellow to brown, 
feebly soluble in acid, which becomes yellow; occurs in nodules or beds, 
usually with shale; weathers with brown or blood-red crust = brown iron-ore. 
Bee clay-ironstone (pp. 75, 121) ; and limonite (pp. 67, 121) ; if the rock is 
reddish and gives a cherry-red streak, see hasmatite (pp. 67, 121). 

0. Bp. gr. 2*55. White, grey, yellowish, or bluish, rings under the hammer, 
frequently splits into thin plates, does not effervesce, weathered crust white 
and distinct = perhaps some compact variety of phonolite (p. 145. Bee also 
porphyrite p. 149). 

4. Bp. gr. 2'9-3*2. Black or dark green, weathered crust yellow or brown = 
probably some close-grained variety of basalt (p. 152), andesite (pp. 148, 151), 
aphanite (p. 158)» or amphibolite (p. 129). 

K, H. 6-6*5, but less according to decompositiou. Bp. gr. 2'55-2*7. Can with 
difficulty be scratched with the knife when fresh; White, bluish-grey, 
yellow, lilac, browo, red ; white streak ; sometimes with well defiued white 
weathered crust, no effervescence = probably a felsitio rock (p. 142). 

A. H. 7. Bp. gr. 2'5-2'9. The knife leaves a metallic streak of steel upon the 
resisting surface. The rock is white, reddish, yellowish, to brown or black, 
very finely granular or of a homy texture, g^ves no reaction with acid= 
probably silica in the form of jasper, homstone, flint, chalcedony, halleflinta 
(pp. 66, 122, 130), adinole (p. 131). 

ii. A fresh fractiire shows the rock to be glassy. 

Leaving out of account some glass-like but crystalline minerals, such as quartz and 
rock-salt, the number of vitreous rocks is comparatively small. The true nature of the 
mass in question will probably not be difficult to determine. It must bo one of the 
Haasive volcanic rocks (p. 136, et seq,). If it occurs in association with siliceous lavas 



> In this table, H. = hardness ; Bp. gr. = specific gravity. The scale of hardness 
usually employed is 1, Talc ; 2, Rock-salt or gypsum ; 3, Calcito ; 4, Fluorite ; 5, Apatite ; 
6, Orthoelase ; 7, Quartz ; 8, Topaz ; 9, Corundum ; 10, Diamond. 



80 GEOGNOSY. CBookH. 



.(liporites, trachytes) it will probably be obsidian (p. 146), or pitehatoiie (p. Ii5); if it 
passes into one of the basalt-rocks, as so commonly liuppens along the edges of dykes 
and intrusive sheets, it is a glassy form of basalt (p. 153> 

iii. A fceBti fi:'aotare shows the rook to be crystalline. 
If the component crystals are sufficiently large for determination in the field, 
the name of the rock will readily be found. Where, however, they are too minute 
for identification even with a good lens, the observer may require to submit 
the rock to more precise investigation at home, before its true character can be 
ascertained. For the purposes of field-work, however, the following points should be 
. noted. 

a. The rock can be easily scratched with the knife. 
(a) Effervesces briskly with acid = limestone. 
Q}) Powder of streak effervesces less briskly. See dolomite. 
(a) No effervescence with acid : may be granular crystalline gypsum (alabaster) 
or anhydrite (pp. 120, 121). 
a. The rook is not easily scratched. It is almost certainly a siUoate. Its character 
should be sought among the massive crystalline rooks (p. 136). If it be heavy, 
appear to be oomi)osed of only one mineral, and have a marked greenii^ 
tint, it may be some kind of amphibolite (p. 129); if it consist of some 
white mineral (felspar) and a green mineral which gives it a distinct green colour, 
while the weathered crust shows more or less distinct effervescence, it 
may be a fine-grained diorite (p. 148), or diabase (p. 150) ; if it be grey and 
granular, with striated felspars and dark crystals (augite and magnetite), with 
a yellowish or brownisli weathered crust, it is probably a dolerite (p. 152) or 
andesite (p. 151.) ; if it be compact, finely-crystalline, scratched with difficulty, 
showing crystals of orthoclase, and with a bleached argillaceous weathered 
crust, it is probably an orthoclase-porphyry (p. 144), or quartz-porphyry (p. 141). 
The occurrence of distinct blobs or crystals of quartz in the fresh fractures 
or weathered face will suggest a place for the rock in the quartzifcrons 
crystalline series. 
iv. A firesh firacture shows the rock to have a foliated stmoture. 
The foliated rocks are for the most part easUy recognisable by the prominence of 
their component minerals (p. 123). Where the minerals are so intimately mingled as not 
to be separable by the use of the lens, the following hints may be of service : — 

a. The rock has an unctuous feel, and is easily scratched. It may be talc-schist 
(p. 130), chlorite-schist (p. 130), hydrous mica-schist (p. 131), or foliated 
serpentine (p. 157). 
/3. The rock emits an earthy smell when breathed on, is harder than thoee included 
in a, is fine-grained, dark-grey in colour, splits with a slaty fracture and contains 
perhaps scattered crystals of iron-pyrites or some other mineral. It is s<Hne 
argillaceous-schist or clay-slate, the varieties of which are named from- the 
predominant enclosed mineral, as chiastolite-slate, andalusite-schist, ottrelite- 
schist, &c (p. 126) ; if it has a silky lustre it may be phyllite. 

7. Tlie rock is composed of a mass of ray-like or fibrous crystals matted together. 

If the fibres are exceedingly fine, silky, and easily separable, it is probably 
asbestos; if they are coarser, greenish to white, glassy, and hard, it is 
probably an actinolite-schist (p. 129). Many serpentines are seamed with veins 
of the fine silky fibrous variety termed chrysotUe. 

8. The rock has a hardness of nearly 7, and splita with some difficulty along 

micaceous folia. It is probably a quartzose variety of mica-schist, quartzwschlBl, 
or gneiss (pp. 131, 132). 
c. The rock shows on its weathered surface small particles of quartz and folia of 
mica in a fine. decomposing base. It is probably a fine-grained variety of 
nuca-schist or gneiss. 



Pabt n. S iiij DETERMINATION OF BOCKS, 81 



▼. A flresh firactare shows the rock to have a firagmental (olastic) 
stmotnre. 

Where the oomponent fragments are large enough to be seen by the naked eye or 
with a lens, there is usually little difficulty in determining the true nature and proper 
name of the rock. Two characters require to be specially considered — the component 
fragments and the cementing paste. 

1. The Fragments, — ^Acoording to the shape, size, and composition of the fragments, 
different names are assigned to clastic rocks. 

a. Shape. — ^If the fragments are chiefly rounded, the rock may bo sought- in the 
sand and gravel series (p. 158), while if they are large and angular, it may be classed 
OS a breccia (p. 161). Some mineral substances, however, do not acquire rounded 
outlines, even after long-continued attrition. Mica, for example, splitsup into thin 
laminie, which may be broken into small flakes or spangles, but never become 
loanded granules. Other minerals, also, which have a ready cleavage, are apt to break 
up along their cleavage-planes, and thus to retain angular contours. Calc-spar is a 
familiar example of this tendency. Organic romains composed of this mineral (such as 
criuoids and echinoids) may often be noticed in a very fragmentary oonditiou, having 
evidently been subjected to long-continued comminution. Yet angular outlines and 
{iMtAk or little worn cleavage-surfaces may be found among them. Many limestones 
&mahsi largely of sab-angular organic debris. Angular inorganic detritus is cliaracter- 
iatic of volcanic breccias and tufis (p. 164). 

iS. Size. — ^Where the fragments are hard, rounded, or sub-angular quartzose graiutj, 
the size of a pin's head or less, the rock is probably some form of sandstone (p. 161). 
Where they range up to the size of a pea, it may be a pebbly sandstone, fine con- 
glomerate or grit ; where they vary from the size of a pea to tliat of a walnut, it is an 
ordinary conglomerate ; whero they range up to the size of a man's head or larger, it is 
a coarse conglomerate. A considerable admixture of sub-angular stones nuJces it a 
breociated conglomerate or breccia. Large angular and irregular blocks are characteristic 
of coarse volcanic agglomerates (p. 166). 

y. Composition. — In the majority of cases, the fragments are of quartz, or at 

Ifttst of some siliceous and enduring mineral. Sandstones cousiut chiefly of rounded 

quartz-grains (p. 160). Where these are unmixed with otlier ingredients, the rock is 

i$«imetimes distinguished as a quartzose sandstone. >Such a rock when indurated 

U-comes quartzite (p. 128). Among the quartz-grains, minute fragments of oUrt 

minerals may l>e observed. When any one of these is prominent, it may give a name U) 

the variety of sandstone, as felspathic, niicaceous (p. 96). Volcanic tufls and breccias 

Mf characterised by the occurrence of lapilli (very commonly ceUular) of the lavas from 

Uie explosion of which they have been fonned. Among interbedded volcanic rocks, tho 

student will meet with beds which he may be at a loss whether to class as volcanic, 

oT uti formed of ordinary sediment. They consist of an intermixture of volcanic detritus 

with sand or mud, and pass on the one side into tnie tufls, on the other into sandstones, 

*liales, limestones, &c. If the component fragments of a non-crystidlino rock give a 

•irisk effervescence with acid, tliey'nre calcareous, and the rock (most likely a limestone, 

•ir at least a calcareous formation,) should be searched for truces of fossils. 

2. The Paste. — It sometimes hapiKJiis that the comi>onent fragments of a clastic 
r»ck coherc merely from prcHsuro and without any discovenible nmirbc. This is 
uci-asionally the case with sandstone. !Most commonly however, there is some cementing 
pa»te. If a drop of weak acid produces effervescence from between the component 
non-calcareous grains of a rock, the paste is cidcareous. If the grains are coated with a 
rt* I crust which, on being bruised between white paper, gives a cherry-red |)owder, the 
cementing material is the anhydrous peroxide of iron. If the paste is yellow or brown 
it is probably in great part the hydrous jxiroxide of iron. A dark brown or black 
natrix which can be dissipated by heating is bituminous. Where the component 
grains ore so firmly cemented in an exceedingly hard matrix that they break across 





82 OEO&VOST. [Book IL 



rather than separate from each other when the stone is fmctored, the paste is probably 

siliceous. 

Determination of Speoifie Gravity.— The student will find this character of 
considerable advantage in enabling him to discriminate between rocks. He may acquire 
some dexterity in estimating, even with the hand, the probable specific gravity 
of substances ; but he should begin by determining it with a balance. Jolly's spring 
balance is a simple and serviceable instrument for this purpose. It consists of an 
upright stem having a graduated strip of mirror lot into it, in front of which hangs 
a long spiral wire, witli rests at the bottom for weighing a substance in air and iu 
water. For most purposes it is sufUoiently accurate, and a determination can bo made 
with it in the course of a few minutes.^ Another instrument has more recently been 
invented by W. N. Walker, consisting of a lever graduated into inches and tenths, 
and rusting on a knife-edge stand, on one side of which is placed a moveable weight, 
while on the long graduated side the substance to be weighed is suspended. This 
instrument is very convenient, and has the advantage of not beiog so liable to get out of 
order as other contrivances.' 

Meohanieal Analysis. — Much may be learnt regarding the composition of a rock 
by reducing it to powder. This may be roughly done by placing some pieces of the 
i-uck within folds of paper upon a surface of steel, and reducing them to powder by a 
few smart blows of a hammer. Bat a steel mortar is more serviceable. The powder 
can be sifted through sieves of varying degrees of fineness and the separate fragments 
may bo examined with a lens. If they are dark in colour they may be placed on white 
paper, if light-coloiutKl they are more readily observed upon a black pa|)er. Portions 
of this powder may l)e carefully washed and mounted with Canada balsam on glass, as 
in the way described below for microscopic slices. Furtlier assistance may be obtained by 
gently washing the powder with water on an inclined surface. As in the analogous treat- 
ment of veinstones and ores in mining, the particles arrange themselves according to their 
respective gravities, the lightest being swept away by the current. Magnetic particles 
may be extracted with a magnet, the end of which is preserved from contact with the 
powder by being covered with fine tissue-paper. An electro-magnet will at once with- 
draw the particles of minerals which contain far too little iron to be ordinarily recognised 
as magnetic ; in this way the i)articles of a ferruginous magnesian mica may in a few 
seconds be gathered out of the powder of a granite. 

Where the difference between the speoifie gravity of the component minerals of a rock 
is slight, they may be separated by means of a solution of given density. M. Thoulet 
proposed the use of a saturated solution of iodide of mercury in iodide of potassium, 
which at a temperature of 11** 0. lias a density of 2*77. The powder of a rock being 
introduced into this liquid, those particles whose specific gravity exceeds that of 
the liquid will sink to the bottom, while those which are lighter will float. This 
process allows of the separation of the felspare from each other, and at once eliminates 
the heavy minerals such as hornblende, augite, and black mica. By the addition of 
water the specific gravity may be reduced, and different solutions of given density may 
be employed for determining and isolating rock-constituents. This method of analysis 
is important in affording a ready means of separating the quartz and felspar of a rook.* 



* Jolly's spring Imlance can be obtained through any optician or mineral dealer from 
Bcrberioh, of Munich, for nine florins. In the United states it is manufactured by 
Geo. Wa<le & Co., at the'Hoboken Institute. 

' See Geol, Mag, 1883, p. 109, for a description and drawing of this instrument, 
and the manner of using it. It may be obtained of Lowden, optician, Dundee, and 
How & Co., Farringdon Street, London. 

'- Fouqud ond Michel-Levy, * Minenilogie Micrographiquc,' p. 1 17. Other solutions of 
higher density have been suggested for the isolation of heavier minerals. Klein (Compt 
rend, 1881, p. 318) proposed a solution of borotungstalo of cadmium, with a density 
of 3*28 ; while R, Breon (BuU. Soc. Min, France, iii. (1880) p. 46) proposed a solution 
of cjlilorido of lead, which, howe\'er, can only be employed at a high temperature. 



Part II. § iiL] DETEBMINATION OF ROCKS. 83 



Ghemioal Analsrsis. — ^The determination of the chemical composition of rooks 
by detailed analysSs in the wet way, demands an acquaintance with practical chemistry 
which oompaiattyely few geologists possess, and is consequently for the most \wti left 
in ttie hands of chemists, who are not geologists. But as some theoretical questions in 
geology inYoWe a considerable knowledge of chemical processes, so a satisfuctorybanalysis 
of locln is best performed by one who understands the nature of the geological problems 
on which such an analysis may be expected to throw light. As a rule, detailed 
chemical analysis lies out of the sphere of a geologists work : yet the wider his know- 
ledge of chemical laws and methods the better. He should at least bo able to employ 
with aocuracy the simpler processes of chemical research. 

Treaimeut tcith Acid, — The gcologist^s accoutrements for the field should include » 
small acM-bottle, with a glass stopper prolonged downwards into a point. Dilute 
hydrochloric acid is commonly employed. When a drop of this acid gives effervescenco 
upon a surface of itwk, the reaction is caused by the liberation of bubbles of carbon 
dioxide, as this oxide is replaced by the more powerful acid. Hence effervescence is an 
indication of the presence of carbonates, and when brisk is specially characteristic of 
calcinm-carbonate. Limestone and markedly calcareous rocks may thus at once be 
detected. By the same means, the decomposition of such rocks as dolerite may be traced 
to a considerable distance inward from the surface, the original lime-bearing silicate of 
the rock having been decomposed by infiltrating rain-water, aud partially converted 
into carbonate of lime. This carbonate being far more sensitive to the acid-test than 
the other carbonates usually to be met with among rocks, a drop of weak cold acid 
Buffioea to produce abundant effervescence even fh)m a crystalline face. But the 
effervescence becomes much more marked if we apply the acid to the powder of the 
stone. For this purpose, a scratch may be made and then touched with acid, when a 
more or less oopions discharge of carbonic acid may be obtained, where otherwise it might 
appear so feebly as perhaps even to escape observation. Some carbonates, dolomite for 
example, are hardly affected by acid until powdered. In other cases, the acid requires 
to be heated, or must be used tery strong, as with sidcrite. 

It is a convenient method of roughly estimating the purity of a limestone, to place a 
fiagment of the rock in hydrochloric acid. If there is much impurity (clay, sand, oxide 
of iron, &c.), this will remain behind as an insoluble residue, and may then be further 
tested chemically, or examined with the microscope. Of course the acid may attack 
m»me of the impurities, so that it cannot bo concluded that tlie residue absolutely 
represents everything present in the rock except the carbonate of lime ; but the proportion 
of non-calcareous matter so dissolved by the acid will usually be small. 

Hydrofluoric acid is a reagent of considerable service in separating the mineral 
constituents of rocks. The rock to be studied is reduced to jwwdcr and introduced 
gently mto a platinum capsule contiuniug the concentrated acid. During thu conse- 
quent effervescence, the mixturo is cautiously stirred with a platinum spatula. ISomc 
minerals are converted into fluorides, others into fluo&ilicatcs, while some, particularly 
the iron-magnesia species, remain undisiK^lved. The thick jelly of silica and alumina is 
removed with water, and the crystalline minerals lying at the bottom can then be dried 
and examined. By arresting the solution at different stages the different minerals mny > 
be isolated. This process is admirably adapted for collecting the pyroxene of pyroxeuic 
rocks.* 

Further diemieal proeessdi, — A thorough chemical analysis of a rock or mineral is 
indispensable for the elucidation of its composition. But there are several processes by 
which, until that complete analysis has been made, the geologist may add to his know* 
ledge of the chemical nature of the objects of his study. It is commonly the case that 
muierals about which he may be doubtful are precisely those which, from their small 
siasc, ore most difficult of separation from the rest of the rock preparatory to analytical 



Fouquc' and Michcl-L6vy, op. cit, p. 110. 

(; 2 



84 GEOGNOBT. [Book H. 



procoBBCB. The mineral apatite, for example, occurs in minute hexagonal prisms, which 
on cross-fracture might be mistaken for nepheline, or even sometimes for quartz. I^ 
howcyer, a drop of solution of molybdate of ammonia be placed upon one of these 
crystals, a yellow precipitate will appear if it be apatite. Nepheline, which is another 
hexagonal mineral likewise abundant in some rocks, gives no yellow precipitate with 
the ammonia solution, while if a drop of hydrochloric acid be put oyer it, cfysiak of 
chloride of sodium or common salt will be obtained. These reactions con be observed 
even with minute crystals, by placing them under the microscope and using an exceed- 
ingly attenuated pipette for dropping the liquid on tlio slide. 

Eecently two ingenious applications of chemical processes to the determination of 
minute fragments of minerals have been made. In one of these, devised by Boricky/ 
hydroiluosilicic acid of extreme purity is employed. Tliis acid decomposes most 
silicates, and forms from tlieir bases hydrofluosilicates. A particle about the size of a 
piii'8 head of the mineral to be examined is fixed by its base upon a thin layer of CSunada 
balsam spread upon a slip of glass, and a drop of tlie acid is placed upon it The 
preparation is then set in moist air near a saucer of water under a bell-glass for twenty- 
four hours, after which it is enclosed in dry air, with chloride of calcium. In a few 
hours the hydroiluosilicatos crystallize out upon the balsam and can be examined with 
the microscope. Those of potassium take the form of cubes, of sodiiuu hexagonal 
priums,' &c. 

The second process consists in utilizing the colorations given to the liame of a 
Biiiiben-bumer by sodium and potassium. An elongated splinter of the mineral to bo 
examined is first placed in the outer or oxidizing part of the flame near the base, and thou 
in the reducing part further up and nearer tlie centre. Tlie amount of sodium present 
in the mineral is indicated by the extent to which the flame is coloured yellow. The 
l)otaijsium is similarly estimated, but the flame is then looked at with cobalt glass, so as 
to eliminate the influence of the sodium.' 

4. Bloto-yipe Tevt*, — The chemical tests with the blow-pipe are simple, easily 
applied, and require only patience and practice to give great assistance in the deter- 
mination of minerals. If unacquainted with blow-pipe analysis, the student must refer 
to one or other of the numerous text-books on the subject, some of which are mentioned 
below.' For early practice the following apparatus will be found sufiicient :— 

1. Blow-pipe. 

*2. Thick-wicked candle, or a tin box tilled with the material of Child's night-lighta, 
and furnished with a piece of Freyberg wick in a metcdlic support. 

3. Platinum-tipped forceps. 

4. A few pieces of platinum wire in lengths of three or four inches. 

5. A few pieces of platinum foil. 
0. Some pieces of charcoal, 

7. A number of closed and open tubes of hard glass. 



' Archiv NatuncUf. LajuieMlurdtforseliuny von Bolunen^ iii. fasc 3, 1876. 
• Szabo, * Ucber cine neue Methode die Felspathe audi in Qcsteinen zu besiimmen* 
Budii-lVst, 1876. 

' The great work on the blow-pipe is Pkttnor's, of wliicli an English translation has 
boon imWishcd. Elderiiorst's * Manual of Qualitative Blow-pipc Analysis and Deter- 
minative Mineralogy,' by H. B. Xoson and C. F. Chandler (Philadelphia : N. 8. Porter 
and Ceates), is a smaller but useful volume ; while still less pretending is Scheeier^s 
' Intrfxluction to the Use of the Mouth Blow-pipe,* of whicli a third edition by H. F. 
Blandford was published in 1875 by F. Norgatc. An admirable work of reference will 
be found in Professor Brush's 'Manual of Determinative Mineralogy * (Now York: 
J. Wiley and Son). 

• The student who would pursue physical geology by original research in tho 
field and abroad may consult Bou<?, * Guide du Geologuo Voyageur,' 2 vols. 1885 ; 
Elie do Beaumont, *I.e9ons de Geologic pratique,' vol. i., 1845; Penning and Jukes- 
Browne, « Field Geology,' 2nd edit. 1880 ; A. Geikie, * Outlines of Field Geoloey,' 
3rd edit 1882. ^ 



Paw n. I iiij DETERMINATION OF BOCKS. 85 



8. Throe small stoppered bottles containing sodium-carbonate, borax, and microcosmio 
salt 

9. ICagnei 

This list oan be increased as ezperionoo is gained. The whole apparatus may easily 
be paoked into a box which will go into the corner of a portmanteau. 

ii MioroBOopio InveBtigation.' 

The Talue of the miorosoope as an aid in geological research is now everywhere 
acknowledged. Some information may here be given as to the methods of procedure 
in miorosoi^ioal inquiry. 

1. Preparation of mioroBoopie Blides of rocks and mineralB.— The 
obsenrer ought to be able to prepare his own slices, and in many cases will find it of 
edTantage to do so, or at least personally to superintend their preparation by others. It 
ii desirable that he should know at the outset that no costly or unwieldy set of 
apparatus is needful for his purpose. If he is resident in one place and can accommo- 
date a cutting machine, such as a lapidary's lath, he will find the process of preparing 
Tock««liecs greatly facilitated.' The thickness of each slice must bo mainly regulated 
by the nature of the rock, the rule being to make the slice as thin as can conveniently 
be cut, so as to save labour in grinding down afterwards. Perhaps the thickness of a 
shilling may be taken as a fiair average. The operator, however, may still further 
redooe this tiiickness by cutting and polisliing a face of the specimen, cementing that 
OQ glass in the way to be immediately described, and then cutting as close as possible 
to the cemented surfece. The thin slice thus left on the glass can then be ground 
down with comparative ease. 

Excellent rock-sections, however, may be prepared without any machine, provided 
the operator possesses ordinary neatness of hand and patience. He must procure as 
thin chips as possible. Should the rocks be accessible to him in the field, he should 
sdeei the freshest portions of them, and by a dexterous use of the hammer, break off 
from a sharp edge a number of thin splinters or chips, out of which he can choose ono 
nr more for rock-slices. These chips may be about an inch square. It is well to take 
tereral of them, as the first specimen may chance to 1)0 spoiled in the preparation. The 
geologist ought also always to carry off a piece of the same block from which his chip Ih 
taken, that he may have a specimen of the rook for future reference and comparison. 
Every such hand-specunen, as well as the chips belonging to it, ought to be wrapped 
up in paper on the spot where it is obtained, and with it should bo placed a label 
containing the name of the loc^ity and any notes that may be thought necessary. 
It can hardly bo too frequently reiterated that all such fic'ld-notes ought as far as 
possible to be written down on the ground, whon tlio actual faots are bofore the eyo 
fiir examination. 

Having obtauied his thin slices, either by Imving thorn slit witii a macliinc or by 
(h'taching with a hammer as thin splinters as possible, the operator may proeooil t^ the 

* This section is taken, with alterations and additions, from tlio anthor'H * Outlines of 
Field Oeolocy.' 

' A machine well adapted for both cutting and poliBhing was devised some years 
ign by Mr. J. B. Jordan, and may be had of Messrs. Cotton and Johnson, Grafton 
toeet, Soho, London, for £10 lOi. Another slicing and polishing machine, invented 
by Mr. F. G. Cuttell, 104 Leighton Road, Kentish Town, lx)ndon, costs £6 10^. These 
aaehioes are too unwieldy to be carried about the country by a field-geologist. Fuess 
•f Berlin supplies two small and convenient hand-instruments, one for slicing, the other 
for grinding and polishing. The slicing-machinc is not quite so satiHfactory for hard 
rocks as one of the larger, more solid forms of apparatus worko<l by a treadle. But the 
Krinding-fliaohine is useful, and might be added to a geologist's outfit without material 
ueoDvenieiioe. If a lapidary is within reacli, much of the more irksome part of tho 
work may be saved by getting him to cut off the thin slices in directions marked for 
him npoQ the specimens. 



86 GEOGNOSY. [Book H. 



preparation of them for the microfloope. For ihia purpose tlie following simple 
apparatus is all tliat is abjolotely needful, though if a grinding-machino be added it 
will save time and labour. 

List of Apparattu required in the Preparation of Thin Slices of Iiock$ and Minerali for 

Microscopical Examination, 

1. A cast-iron plate } inch thick and 9 inches square. 

2. Two pieces of plate-glass, 9 inches square. 

3. A Water of Ayr stone, 6 inches long by 2J inches broad. 

4. Coarse emery (1 lb. or so at a time), 
i). Fine or flour-emery (ditto). 

(». Putty powder (1 oz.). 

7. Canada balsam. (There is an excellent kind prepared by Bimmington, Bradford, 
gpeoially for microscopic preparations, and sold in shilling bottles.) 

8. A small forceps, and a common sewing-needle with its head fixed in a oork. 

9. Some oblong pieces of common flat window-glass ; 2 x 1 inches is a con?enient 
size. 

10. Glasses with ground edges for mounting the slices upon. They may be had at any 
chemical instrument maker's in different sizes, the commonest in this country being 3x1 
inches, though this size is rather too long for oonvenient handling on a rotating stage. 

11. Thin covering-glasses, square or round. These are sold by the ounce; \ oz. will 
bo sufficient to begin with. 

12. A small bottle of spirits of wine. 

The first part of the process consists in rubbing down and polishing one side of the 
chip or slice, if this has not already been done in cutting off a slice affixed to glass, as 
above mentioned. Wo place the chip upon the wheel of tbe grinding-machine, or, 
failing that, upon the iron plate, with a little ooorse emery and water. If the chip is 
so shaped that it can be conveniently pressed by the finger against the plate and kept 
there in regular horizontal movement, we may proceed at once to rub it down. If, how- 
ever, we find a difficulty, from its small size or otherwise, in liolding tho chip, one side 
of it may be fastened to tho end of a bobbin or other convenient bit of wood by means 
of a cement formed of three-parts of rosin and one of beeswax, which is easily softened 
by heating. A little practice will show that a slow, equable motion with a certain 
steady pressure is most effectual in producing the desired flatness of surface. When all 
the roughnesses have been removed, which can bo told after the chip has been dipped iu 
water so as to remove the mud and emery, we place the specimen upon tho square of 
plate-glass, and with flour-emery and water continue to rub it down until all the 
scratches caused by the coarse emery have been removed and a smooth polished surface 
has been produced.^ Care should be taken to wash the chip entirely free of any grains 
of coarse emery before the polishing on glass is begun. It is desirable also to resen'e 
the glass for polishing only. The emery gets finer and finer the longer it is used, so 
that by remaining on tho plate it may be used many times in succession. Of course the 
glass itself is worn down, but by using alternately every portion of its surface and on 
both sides, one plate may be made to last a considerable time. If after drying and 
examining it carefully, we find the surface of the chip to be ix)li8hed and free fiom 
scratches, we may advance to the next part of the process. But it will often happen 
that the surface is still finely scratched. In this case we may place the chip upon tho 
Water of Ayr stone and with a little water gently rub it to and fro. It should be held 



* Elxceedingly impalpable emery powder may bo obtained by stirring some of tho 
finest emery in water, and after tho coarse particles have subsided, pouring off the liquid 
and allowing the fine suspended dost gradually to subside. FiltercHl and dried, the 
residue can be kept for the more delicate parts of the polishing. 



Pam el I iiL] BETEBMINATION OF BOCKS. 87 



qnite flut The Water of Ayr sione, too, should not be allowed to get worn into a hollow, 
hoi should alao be kept quite flat, otherwise we shall lose part of the chip. Some soft 
rookai however, will not take an unscratched surface oveu with the Water of Ayr stone. 
These may be finished with putty powder, applied with a bit of woollen rag. 

The denied flatness and polish having been secured, and all trace of scratches and 
dirt having been completely remoYod, we proceed to a further stage, which consists in 
grinding down the opposite side and reducing the chip to the requisite degree of thin- 
ness. The first step is now to cement the polished surface of the chip to one of the 
pieces of common glass. A thin piece of iron (a common shovel does quite well) is 
heated over a fire, or is placed between two supports over a gas-ilamo.' On this plate 
must be Liid the piece of glass to which the slice is to be affixed, together with the slico 
Uaelt A little Canada balsam is dropped on the centre of the glass and allowed to 
remain until it has acquired the necessary consistency. To test this condition, the point 
of a knife should bo inserted into the balsam, and on being removed should be rapidly 
cooled by being pressed against some oold surface. If it soon becomes hard enough to 
resist the pressure of the finger nail, it has been sufficiently heated. Care, however, 
must be observed not to let it remain too long on the hot plate ; for it will then become 
brittle and start from the glass at some future stage, or at least will breuk away from 
the edges of the chip and leave them exposed to the risk of being frayed off. The heat 
should be kept as moderate as possible, for if it becomes too great it may injure some 
portions of the rock. Chlorite, for example, is rendered quite opaque if the heat is so 
great as to drive off its water. 

When the balsam is found to be ready, the chip, which has been warmed on the 
same plate, is lifted with the forceps, and laid gently down upon the balsam. It is 
well to let one end touch the balsam first, and then gradually to lower the other, as in 
this way the air is driven out With the point of a needle or a knife the chip should 
be moved about a little, so as to expel any bubbles of air and promote a firm cohesion 
between the glass and the stone. The glass is now removed with the forceps from the 
plate and put upon the table, and a lead weight or other small heavy object is placed 
upon the chip, so as to keep it pressed down until the balsam has cooled and hardened. 
If the operation has been successful, the slide ought to be ready for further treatment as 
soon as the balsam has become cold. If, however, the balsam is still soft, the glass must 
be again placed on the plate and gently heated, until on cooling, the balsam fultils the 
condition of resisting the pressure of the finger-nail. 

Having now produced a firm union of the chip and the glass, we proceed to rub down 
the remaining side of the stone with coarse emery on the irun plate as before. If the 
glass cannot be held in the hand or moved by the simple pressure of the fingers, which 
usually suffices, it may be fastened to the end of the bobbin with the cement as before. 
When the chip has been reduced until it is tolerably thin ; until, for example, light 
appears through it when held between tlie eye and the window, we may, as before, wash 
it clear of the coarse emery and continue the re<luction of it on the glass plate with fine 
emery. Crystalline rocks, such as granite, gneiss, diorite, dolcrite, and modern lavas, 
can be thus reduced to the required thinness on the glass plate. Softer rocks may 
require gentle treatment with the Water of Ayr stone. 

The last parts of the process are the most delicate of all. We desire to make tho 
section as thin as possible, and for that purpose continue nibbing imtil after one final 
attempt we may perhaps find to our dismay that great part of the slice has disappeared. 
The utmost caution should be used. The slide should be kept as fiat as possible, and 
looked at frequently, that the first indications of disruption may bo detected. Tho 
tliiuness desirable or attainable depends in great measure upon the nature of the rock. 



* A piece of wire-gauze placed over the fiame, with an interval of an inch or moro 
iK'tween it and the overlying thin iron plate, tends to diff'use the heat and prevent tho 
Iwlsam from being unequally heated. 



88 GEOGNOSY. [Book II. 



TrauBparcnt miucrals noed not be so much reduced as more opaque ones. Borne 
mineruls, indeed, remain nhsolntely opaque to the last, like pyrite, magnetite, and 
ilmeuite. 

The elide is now ready for the niicroBCoix>. It ought always to be examine<1 with 
tliat instrument at this stHge. AVe can thus see whether it is thin enough, and if any 
ehomical todts are requirctl they can readily be applied to the exposed surface of the 
slice. If the roek has proved to be very brittle, and we have only succeeded in 
procuring a thin slice after much labour and several fiiilures, nothing further should be 
(lone with the preparation, unlt^ss to cover it with glass, as will bo inmiediately 
explained, which not only protects it, but adds to its transparency. But where the slice 
is not so fragile, and will bear removal from its original rough scratched piece of glass, 
it should Ijc transferred to one of the glass-slides (No. 10). For this purpose, the 
prejmration is once more placed on the warm iron plate, and close alongside of it is 
put one of the pieces of glass which has been carefully cleaned, and on the middle of 
which a little Canada balsam has been drop^xnl. The heat gradually loosens the 
cohesion of the slice, which is then very gently pushed with the nee<lle or knife along 
to the contiguous clc(m slip of glass. (Considerable practice is needed in this part of 
the work, as the slice, b<.-ing so thin, is ujit to ;;o to irieccs in bt^ug transferred. A 
gentle inclination of the warm plate, so that a tendency may be given to the slice to Rli]» 
downwards of itself on to the clean glass, may l>e advantageously given. AVe must 
never attempt to lift the slice. All shifting of its position should be performcil with 
the point of the needle or other sharp instrmuent. If it goes to pieces we may yet be 
able to pilot the fragments to tlieir resting-pkco on the balsam of the new glaMt. and the 
resulting slide may be sufficient for the re<j[uired purpose. 

When the slice has been safely conducted to the centre of the glass slip, we put a 
little Canada balsam over it, and warm it as before. Then taking one of the thin cover- 
glasses with the forceps, wo allow it gradually to rest ujwn the slice by letting ilown 
first one side, and then by degrees the whole. A fuw gentle circular movements of the 
cover-glass with the ix>int of the needle or forceps may be neetled to ensure the total 
disappearance of air-bubbles. When these do not appear, and when, as before, we lind 
that the balsam has acijuired tlie proper degree of consistence, the slide containing the 
slice is remove<l, and placed on the table with a smtdl lead weight above it in the same 
way as already described. On becoming quite cold and hard the superabundant Imlsam 
round the edge of the cover-glass may be scraped off with a knife, and any which still 
ailheres to the glass may be removed with a little spirits of wine. Small labels should 
bo kept ready for affixing to tlie slides to mark localities and reference numbers. Tims 
labelled, the slide may !« put away for future study and comparison. 

The whole process seems perhajis a little tedious. But in reality much of it is so 
mechanical, that after tlie mode of manipulation has been loanit by a little experience, 
the rubbing-down may be done while the operator is reading. Thus in the evening, 
when enjoying a pleasant lxx)k after his day in the field, he may at the same time, after 
some practice, rub down his rook-chips, and thus get over the drudgery of the o]icraiion 
almost unoonsciouslv. 

Boxes, with grooved sides for carrying microscopic slides, are sold in different sizes. 
Such boxes are most convenient for a tmvelHng equipage, as they go into small space, 
and with the lielp of a little cotton- wool they hold the glass-slides Unnly without risk 
of breakage. For a final resting-place, a cose with shallow trays or drawers in wliioh 
the slides can lie flat is most convenient 

2. The HiCTOSOope.— Unless the observer proposes to enter into great detail in 
the iiivt.-:>1 iu:»ti'>ii of the minuter parts of rock-stmctare, he does not require to procure a 
lar-j^u III. 'I r xiHUriivc instrnment For nort geological purposes objectives of 1}, 1, and 
l.-iiiili r-M >!! t'ltulh with mflgmfjring powers of from 30 to 70 diameters, are sufficient. 
Will It > / In'.;, ijc.) for ipeekl wori^ meh aa the investigation of crystallites and 
i)> < : iMih! mil, to hsn aa oljeothre eapablo of magnifying np to 200 or 300 



Pabt n. S iil] DETERMINATION OF ROOKS. 89 



diometefa. An iDstroment with foirly good glasses of those powers, acoording to tho 
amngement of objoct-glasses and eye-pioces, may be had of some London makers for 
£5. Bnt for some of tho most important parts of the microscopical study of rooks a 
lotatiog stage is requisite, the presence of which necessarily adds to tho cost of tlie 
inatnuneni One of the best microscopes specially adapted for lithological research is 
that derised by Professor Bosenbnsch, of which an English modification is made by 
Watson, of Pall Mall, London, and sold at £21. It contains every apparatus required 
for ordinary work. A less complete bnt useful instrument is sold by the same maker 
for £9 10». Swift k 8on, of 81 Tottenham Court Boad, London, have oonstmcted some 
excellent forms of mioroscope for petrographical purposes, at £8, £10 10^., and £24. 

Among the indispensable adjuncts aro two Niool-prisms, one to be fitted below the 
stage, the other most adyantageously placed over the eye-piece. A quartz-plate is 
uaefid in examination with polarized light It should bo arranged between the two 
Nicol-prisms, either below the stage or in tho tube above tho objective, so as to bo 
conveniently slipped in and out of the field as required. A nose-piece for two objectives, 
screwed to the foot of the tube, saves time and trouble by enabling the observer at once 
to paaa from a low to a high power. Tlio numerous pieces of apparatus necessary fur 
physiologieal work arc not nccdt*d in the examination of rocks and minerals. 

3. Methods of Examination. — A fow hints may bo liero given for the guidance 
of the student in making his own microscopic observations, but ho must consult some of 
the special treatises, mentioned on p. 100 for full dotailp. 

Reflected Light — ^It is not infrequently desirable to observe with tho microscope 
the characters of a rock as an opaque object This cannot usually bo done with a 
broken fragment of the stone, except of course with very low lowers. Hence one of tlio 
most naefhl preliminary examinations of a prepared slice is to place it in the field, and, 
throwing tho mirror out of gear, to converge as strong a light upon it as can be hod, 
ahort of bright direct sunlight Tho advantage of this method is more particularly 
noticeable in tho case of ojiaque minenils. Tho sulphides and iron-oxides so abundant 
in rocks appear as densely black objects with transmitted light, and show only their 
external form. But by throwing a strong light upon their surface, we may often 
discover not ouly their distinctive colours, but their characteribtic internal structure. 
Titaniferons iron is an admirable example of the advantage of this method. Seen with 
transmitted light, that mineral appears in black, utterly structurelees grains or 0|)aqiie 
patches, though frequently bounded by definite lines and angles. But with reflected 
light, the cleavage and lines of growth of the mineral can then often be clearly Econ, 
and what seemed to be uniform black patches are found in many cases to enclose bright 
braay kernels of pyrite. Magnetite also presents a characteristic blue-block colour, 
which distinguishes it from the other iron-oxides. 

TniMmitted lAyht. — It is, of course, with tho lip:ht allowM to pns.s Ihrougli 
prf-pared slices tliat most of the microscopic examination of minerals and rocks is 
perforroiHL A little experience will show the learner tlmt, in viewing objects in this way. 
he may obtain somewhat different results from two sUcch of the same rock according to 
their reUiUvo thinness. In the thicker one, a certain mineral or rock, obsidian for 
example, will appear perhaps brown or almost black, while in the other what in 
evidently tho same mineral may be pale yellow, green, brown, or almost colourless. 
TriGlinic felspars seen in polarized light give only a pale milky light when extremely 
thin, bat present bright chromatic bands when somewhat thicker. 

Viiarized Light-^By means of polarized light, an exceedingly delicate method of 
invi'Migaticm is made available. We use both tho Nicol-prisms. If the object be singly- 
refiracting, such as a piece of glass, or an amorphous body, or a crystal belonging to 
some snbbtanoe which crystallizes in the isometric or cubic system (or if it be a tetragonal, 
hexagonal er ihombohedral crystal, cut perpendicular to its principal axis), the light will 
reach oar eja apparently unaffected by the intervention of the object. The field will 
remain daric wImii the axes of the two prisms are at right angles (crossed Nicols), in 



90 GEOGNOSY. [Book IL 

the same way as if no intervening object were there. Such bodies are iM^ropie. In all 
other cases, the substance is donbly-refraoiing and modifies the polarized beam of light 
On rotating one of the prisms, we perceive bands or flashes of colour, and numennu 
linos appear which before were invisible. The field no longer remains dark when the 
two Niool-prisms are crossed. Such a substance is anisotropic. 

It is evident, therefore, that we may readily toll by this means whether or not a 
rock contains any glassy constituent. If it does, then that portion of its mass will 
becomo dark when the prisms are crossed, while the crystalline parts which, in the vast 
majority of cases, do not belong to the cubic system, will remain conspicuous by their 
brightness. A thin plate of quartz makes this separation of the glassy and crystalline 
ports of a rock even more satisfactory. It is placed between the Nicol-prisms, which 
may be so adjusted with reference to it that the field of the microecopo appears 
uniformly violet The glassy portion of any rock, being singly-refracting or isotropic, 
placed on the stage will allow the violet light to pass through unchanged, but the 
crystalline portions, being doubly-re&acting or anisotropic, will alter the violet light 
into other prismatic colours. The object should bo rotated in the field, and the eye 
should be kept steadily fixed upon one portion of the slide at a time, so that any change 
may be observed. This is an extremely delicate test for the presence of glassy and 
crystalline constituents. 

In searching for the crystallographio system to which a mineral in a microsoopic 
slide should be referred, attention is given to the directions in which the mineral placed 
between crossed Nicols appears dark, or to what are called the directions of its extinc- 
tion. It is extinguished (that is, the normal darkness of the field between the orossed 
Kicols is restored) when two of its axes of elasticity for vibrations of light coincide with 
tlie principal sections of the two prisms. During a complete rotation of the slide in the 
field of the microscope the mineral becomes dark in four positions, each of which marks 
that coincidence. When, on the other hand, the prisms are placed parallel to eaoh 
other, the coincidence of their principal sections with the axes of elasticity in the 
mineral allows the maximum of light to pass through, which likewise occurs four times 
in a complete rotation of the mineraL The difierent crystallographio systems are 
distinguishable by the relation between their crystallographio axes and their axes of 
elasticity. By noting this relation in the case of any given mineral (and there are 
usually sections enough of each mineral in the same rock-slice to ftimish the required 
data) its crystalline system may be fixed. But in many cases it has been found possible 
to establish characteristic distinctions for individual mineral species, by noting the 
angle between the direction of their extinction and certain principal faces. It would be 
beyond the scope of this volume to enter into the details of this subject, which must be 
sought in some of the works already cited. The publications of Zirkel, Rosenbuach 
von Lasaulx, Fouque and Michel-Levy may especially bo consulted. 

Pleochroiitn (Dichroism). — Some minerals show a change of colour when a Niool- 
prism is rotated below them ; hornblende, for example, exhibiting a gradation ftom deep 
brown to dark yellow. A mineral presenting this change is said to be pleochroio 
(l)olyohroic, dichroic, trichroic). To ascertain the pleochroism of any mineral we may 
remove the upper polarising prism (analyzer) and leave only the lower (polarizer). If 
as we rotate the latter, no change of tint can be observed, there is no pleochroio mineral 
present, or at least none which shows pleochroism at the angle at which it has been 
bisected in the slice. But in a slice of any crystalline rock, crystals may usually 
be observed which offer a change of hue as the prism goes round. These ore examples 
of pleochroism. This behaviour may bo used to detect the mineral constituents of 
rocks. Thus the two minerals hornblende and augite, which in so many respects 
resemble each other, cannot always bo distinguished by cleavage angles, in microsoopic 
slices. But as Tschormak pointed out, augite remains passive or nearly so as the lower 
prism is rotated : it is not pleochroio, or only very feebly so ; while hornblende, on the other 
hand, especially in its dark varieties, is usually strongly pleochroio. It is to be 



Part U. f iv.] MACROSCOPIC CHARACTEB8 OF BOCKS. 91 

■ ■ — ■ --- ^ 

obwrred, howeTer, that the same mineral is not always equally pleochrolc, and that the 
§i)Beiaoe of thiB property is therefore less reliable as a negative test, than its presence is 
as a positive test. 

In his examination of rooks with the microscope, the student may find an advantage 
in propounding to himself the following questions, and referring to the pages hero 
dted. 

l8t» Is the rock entirely crystalline (p. 109) consisting solely of crystals of different 
mioerals interlaced ; and if so, what are these minerals ? 2nd, Is there any trace of a 
glassy groond-mass or base (p. 106) ? Should this be detected, the rock is certainly of 
Tolcanic origin (p. 106). 3rd, Can any evidence be found of the devitrification of what 
may have been at one time the glassy basis of the whole rock ? This devitrification 
might be shown by the appearance of numerous microscopic hairs, rods, bundles of 
feather-like irregular or granular aggregations (pp. 106, 111). 4th, In what order did 
the minerals crystallize? This may often be m&de out with a microscope, as, 
for instuice, where one mineral is enclosed within another (p. 105).* 5th, What is the 
nature of any alteration which the rock may have undergone ? In a vast number of 
fMOS the slices show abundant evidence of such alteration: felspar passing into 
giannlar kaolin, angite changing into viridite, olivine into serpentine, whUe secondary 
calcite, quartz, and zeolites run in minute veins or fill up interstices of the rock 
(p. 114). Gth, is the rock a fragmental one; and if so, what is tho nature of its com- 
ponent grains? (p. 112.) Is any trace of organic remains to be detected? (p. 112, f^ seq,) 

§ iv. — Oeneral Macroscopic Characters of Bocks? 

1. Structure.^ — The dififerent kinds of macroscopic rock-structures 
are denoted either by ordinary descriptive adjectives, or by terms 
derived from rocks in which the special structures are characteristically 
developed, such as granitoid, brecciated, shaly. It must be borne in 
mind, however, that the external character of a rock does not always 
supply us with its true internal structure, which must be gained from 
microscopic examination. This is of course more especially true of the 
close-grained kinds, where to the naked eye no definite structure is 
discernible. Some of the definitions originally founded on macroscopic 
appearance have been considerably modified by microscopic investiga- 

* It is possible, however, that a crystal enclosed within another may sometimes have 
crystallizea there out of a portion of the surrounding magma of tho rock which has 
lieen enclosed within the larger crystal. 

• The following general text-books on rocks may be referred to ; Macculloch. * A 
Geological Classification of Rocks/ &c., London, 1821. B. von. Cotta, * Hocks Classified 
and Described/ translated by Ijawrenoc, London, 186G. Zirkel, * Lehrbuch der 
Petrographie/ two vols. Bonn, 18GG. Sen ft, * Classification der Felsarten,* Breslau, 
1857 ; * Die KrystoUinischen Felsgemengtheile/ Berlin, 18G8. Kenngott, * Elemento 
der Petrographie,' Leipz. 1868. A von. Lasaulx, * Eleraente der Petrographie,* Bonn. 
1875. Bischof, 'Chemical Geology/ translated for Cavendish Society, 1854-59, and 
Mipplement, Bonn, 1871. Roth, * Allgemeino und Chemische Geologie/ Berlin, 1879. 
Other works in which the microscopical characters are more specially treated of, are 
ennmeratod on p. 100. 

» In the 3rd. edition of Jukes' * Student's Manual of Geology/ (1871), p. 93, it was 
prQixMed.to reserve the term " Structure" for large features, such as characterise rock-blockn, 
ana to nse tho term "texture" for the minuter charactc'rs, such as can be judged of 
in hand specimens. M. De Lapparcnt makes a Rimilar distinction (Traite', p. 554, note). 
Bat the practice of using the word structure as it is employed above in tho text, has 
received aoch a support from the potrographers of Germanv that though I still think 
it would be preferable to distinguish between texture and structure, I have adopted 
what has now the sanction of common usage. 



92 GEOGNOSY. [Book H. 



tion. Many compact rooks, for instance, have been shown to be wholly 
crystalline. 

According to their macroscopic characters, rocks fall naturally into 
two great series, in one of which the rocks are composed wholly or in 
part of crystals, in the other of fragments, so that the two fundamental 
macro-structures are CrystaUine and FragmentaL 

Crystalline (Phanerocrystalline), consisting wholly or 
chiefly of crystalline particles or crystals.* Where the individual 
elements of the rooks are of large size, the structure is coarse'CryataXline 
(granitic), as in many granites. When the particles are readily visible 
to the naked eye, and are tolerably uniform in size, as in marble, many 
granites and dolomites, the rock is said to be granular-crystalline. Suc- 
cessive stages in the diminution of the size of the particles may be 
traced until these are no longer recognisable with the naked eye, and the 
structure must then bo resolved with the microscope {fine-crystalline^ 
micrO'CryatalUne, crypto-crystalline). Fine-grained rocks may also be called 
compact, though this term is likewise applicable to the more close-grained 
varieties of the fragmental series. The microscopic characters of such 
rocks should always }ye ascertained where possible. 

Many crystalline rocks consist not only of crystals, but of a magma 
or paste, in which the crystalline particles are seen by the naked eye to 
1)6 embedded. It is of course impossible, except from analogy, to deter- 
mine macroscopically what may be the nature of this magma. It may 
be entirely composed of minute crystals, or may consist of various 
crystallitic products of devitrification. Its intimate structure can only 
be ascertained with the microscope. But its existence is often strikingly 
manifest even to the unassisted eye, for in what are termed " porphyries " 
it forms a large part of their mass. The term ^* ground-mass ** has 
l>een employed by Zirkel and others to denote this macroscopic matrix. 
Microscopic examination shows that a ground-mass may consist of 
minute crystals, or crystallites, or granules and filaments, or glass, or 
combinations of these in various proportions. (See p. 110.) 

Granitoid (Granitic), thoroughly crystalline, and consisting 
of crystals approximately uniform in size, as in granite. This structure 
is chaiucteristic of many eruptive rocks. Though usually distinctly 
recognisable by tlie naked eye (" macromerite " of Vogelsang ^\ it 
sometimes l)ecomes very fine (" micromerite "), and may bo only 
recognisable as thoroughly crystalline with the microscope; at other 
times it passes into a porphyritic or porphyroid character by the 
appearance of large crystals dispersed through a general ground- 
mass. 

Pegmatoid (Pegmatitic), usually more coarsely crystalline 
than granite, each of the constituents occurring in large masses with 
the characteristic inter-crystallization of pegmatite (p. 139). 

' Prof. BoBcnbuscb propoees the tcnn hcHocrystalUne for rooks in which there is no 
amorphous matcriul among the crystalline constituents. 
^ * Z, Deutseh, Geol, Ges. xxiy. p. 534. 



Paw IL I !▼.] MACBOSCOPIC CEABACTERS OF B0CK8. 93 

Segregated. — ^la granite and other crystalline massive rocks, 
▼ein-like portions, coarser (or finer) in texture tiian the rest of the mass, 
may be observed. These ** contemporaneous veins," as they have been 
called, belong to the last phase of consolidation, when segregations from 
the original molten or viscous magma took place along certain lines, 
where iitm fracture or otherwise the individual minerals could cr^'stal- 
lixe out from the general mass. They have been sometimes termed 
*'8egr^ation," or ** exudation " veins. 

Porphyritic (Porphyroid), com^Kwed of a compactor finely 
cryBtaUine ground-mass, through which distinct larger crystals, gener- 
ally of some felspar, are dispersed. This and the granitic structure are 
ihe two great structure-tyi)es of the eruptive rocks. By far the largest 
number of these rocks belong to the porphyritic type. Microscopic 
research has thrown much light on the nature of the ground-mass of 
porphyritic rocks. Vogelsang has proix)sed to classify these rocks in 
three divisions ^ : 1st, Oranopkipre^ where the ground-mass is a microscopic 
crystalline mixture of the component minerals with absence or si>aring 
development of an imperfectly individualised magma (see p. 110) ; 2nd, 
FeUopkifref having usually an imperfectly individualised or felsitic 
magma for the ground-mass (pp. 108, 110, 141); 3rd, Vitroyh^e, where 
the ground-mass is a glassy magma. The second sub-division embraces 
most of the porphyries, and a very large number of eruptive rocks of all 



Granular — a term applied to rocks composed of approximately 
equal grains, either worn fragments, as in sandstone, or crystalUuo 
particles, as in granite, and marble. This texture may become so fine 
as to pass insensibly into compact. The crypto-crystalline portions 
of some igneous rocks (diorites), where the component ingredients 
cannot be determined except with the microscope, are sometimes called 

Vitreous or glassy, having a structure like that of artificial 
glass, as in obsidian. Among the crystalline rocks thei*e is often present 
a variable amount of an amorphous ground-mass, which may increase 
until it forms the main part of the substance. The nature of this aiaur- 
phous portion is described at pp. 100, 11 1. Its most obvious macroscopic 
condition is that of a volcanic glass. Most vitreous rocks present, even 
to the nakeil eye, dispersed grains, crystals, or other enclosures. Under 
the microscoi>e, they are found to be often crowded with minute crystals 
and imperfect or incipient cr^nstalline forms (p. 107). He si nous is 
the term applied to vitreous rocks having the lustre of pitchstone, and 

' Vogekang, loc. cU. Cominre the classtficatiou into granitoid aud trarhytuidf 
po^ta, p. 137. 

^ Aooording to lloseubuiich the porphyritic iiiiuisivo rockti are tliose in which, 
(loriug the different stages of their production, the same minerals have been formetl 
more than once. Neues Jahrb, 1882 (ii.) p. 14. 

' Ab applied to massiTe (eruptive) rocks, Itoseubuscli wouhl restrict the tirni 
{rrannlar to those in which each individual constituent separated out during but ouo 
deflnite stage of the process of rock-building. Loc. cit. 



94 QE0GN08T. [Book 1L 



to others which are still less vitreous. Devitrification is the conver- 
sion of the vitreous into a crystalline or lithoid structure (pp. 106, 
108, 112). 

Streaked, arranged in streaky inconstant lines (Germ. Schlieren), 
either parallel or convergent, and often undulating. This structure, 
conspicuously shown by the lines of flow in vitreous rocks (obsidian) is 
less marked in crystalline rocks (diorite, dolerito, &c.) It can be 
seen on a minute scale, however, in many crystalline masses when 
examined with the microscope. (See Fluxion-Structure, p. 111.) 

Banded, arranged in parallel bands, distinguished from each other 
by colour, texture, or other slight difference; characteristic of many 
jaspers, flints, halleflintas and other flinty rocks. 

Spherulitic, composed of, or containing small globules or 
spherules which may be colloid and isotropic, or more or less distinctly 
crystalline, particularly with an internal fibrous divergent structure. 
This structure occurs in vitreous rocks, where it is one of the stages of 
devitrification in obsidian, pitchstone, &c.^ (See p. 146.) 

Perlitic, having the structure of the rock termed perlite, which is 
distinguished by being traversed by minute rectilinear fissures, between 
which the substance of the mass has assumed a finely globular character, 
not unlike the spheroidal structure seen in weathered basalt (Fig. 22). 

Horny, flinty, having a compact, homogeneous, dull texture, like 
that of horn or flint, as in chalcedony, jasper, flint, and many halleflintas 
and felsites. 

Cavernous (porous), containing irregular cavities due, in most 
cases, to the abstraction of some of the minerals ; but occasionally, as in 
some limestones (sinters), dolomites and lavas, forming part of the 
original structure of the rock. 

Cellular . — Many lavas, ancient and modem, have been saturated 
with steam at the time of their eruption, and in consequence of the 
segregation and expansion of this imprisoned vapour, have had spherical 
cavities developed in their mass. When this cellular structure is 
marked l)y comparatively few and small holes, it may be called 
vesicular; whore the rock consists i)artly of a roughly cellular, and 
partly of a more compact substance intermingled, as in the slag of an 
iron furnace, it is said tobeslaggy; portions where the cells occupy 
about as much space as the solid part, and vary much in size and shape, 
are called 8 c o r i a c e o u s, this being the character of the rough clinker- 
like scorioo of recent lava-streams ; when the cells are so much more 
numerous than the solid part, that the stone would almost or quite float 
on water, the structure is called p u m i c e o u s, the term pumice being 
api)licd to the froth-like part of obsidian. As the cellular structure 
can only bo develoiied while the rock is still liquid, or at least 

* Quartz ussumos iu soino rocks (e.g. baudcd eiirites) a finely globular Btructnre 
wliich ^ylva developed before the coesation of the motion that produced fluxion-structure, 
and winch, according to M. Michel Levy, may be regarded as connecting the ooHoid 
and crystallized conditions of silica. Bull Soc. Gtol. France. (3) v. p. 140. 



Pakt n. f W.2 MACROSCOPIC CUABACTERS OF ROCKS. 95 



Tucid, and as, i¥hile in this condition, the mass is often still moving 
away from its point of emission, the cells are not infreciuently elongated 
in the direction of movement. Subsequently, water infiltrating through 
the rook, deposits various mineral substances (calcite, quartz, clialcodon}-, 
zeolites, &o.) from solution, so that the flattened and elongated almond- 
shaped cells are eventually filled up. A cellular rock which has under- 
gone this change is said tobean amygdaloid, or amygdaloidal, 
and the almond-like kernels are known as a m y g d u 1 e s. 

Foliated, consisting of minerals that have crystallized in approxi- 
mately parallel, lenticular, and usually wavy layers or folia. Hocks of 
this kind commonly contain layers of mica, or of some equivalent 
readily cleavable mineral, the cleavage-planes of which coincide 
generally with the planes of foliation. Gneiss, mica-schist and talc- 
schist are characteristic examples. So distinctive, indeed, is this 
structure in schists, that it is often spoken ofasschistose. In gneiss, 
it attains its most massive form ; in chlorite-schist and some other 
schists, it becomes so fine as to pass into a kind of minutely scaly 
texture, often only perceptible with the microscope, the rock having on 
the whole a massive structure. 

Fibrous, consisting of one or more minerals composed of distinct 
fibres. Sometimes the fibres are remarkably regular and parallel, as in 
fibrous gypsum, and veins of chrysotile, fibrous aragonite or calcito 
(satin-spar) ; in other instances, they are more tufted and irregular, as 
ill asbestos and actinolite-schist. 

Massive, unstratifiod, having no arrangement in definite 
layers or strata. Lava, granite, and generally all crystalline rocks 
which have been erupted to the surface, or have solidified below from 
a state of fusion (or viscosity), are massive rocks. 

Stratified, bedded, composed of layers or beds lying parallel 
to each other, as in shale, sandstone, limestone, and other rocks which 
have been deposited in water. Successive streams of lava, poured one 
njxm another, have also a bedded arrangement. Laminated, consisting 
of fine, leaf- like strata or laminas ; this structure })eing characteristically 
exhibited in shales, is sometimes also called s h a 1 y. 

Clastic, fragmental, composed of detritus. Hocks possessing 
this character have, in the great majority of cases, been formed in water, 
and their component fragments are usually more or less rounded or 
water-worn. Different names are applied, accoi'ding to the form or size 
of the fragments. Brecciated, composed, like a breccia, of angular 
fragments, which may be of any degree of coarseness. Agglomerated, 
consisting of large, roughly roinided and tumultuously grouped blocks, as 
in the agglomerate filling old volcanic funnels. Conglomerated 
(Conglomeratic), made up of well-rounded blocks or pebbles ; rocks 
having this character have been formed by and deposited in water. 
Pebbly, containing dispersed water- worn pebbles, as in many coarse 
bundstones, which thus by degrees pass into con glomerates. P s a m m i t i c, 
or sandstone-like, composed of rounded grains, as in ordinary sandstone : 



96 QEOamST. [Book IL 

when the graius are larger (often sharp and somewhat angular) the 
rock is gritty, or a grit. Muddy (politic), having a textui^e 
like that of dried mud. Cryptoclastic or compact, where 
the grains are too minute to reveal to the naked eye the truly fragmental 
character of the rock, as in fine mudstones and other argillaceous deposits. 

Concretionary, containing, or consisting of mineral matter, which 
liHH been collected, either from the surrounding rock or from without, 
round some centre, so as to form a nodule or irregularly shaped lump. 
This aggregation of material is of frequent occurrence among water- 
formed rocks, where it may he often observed to have taken place round 
some organic centre, such as a leaf, cone, shell, fish-bone, or other 
relic of plant or animal. (Book IV. Part I.) Among the most 
frequent minerals found in concretionary forms as constituents of rocks, 
are calcite, siderite, pyrite, marcasite, and various forms of silica. In a 
true concretion, the material at the centre has been deposited first, 
and has increased by additions from without, either during the formation 
of the enclosing rock, or by subsequent concentration and aggregation. 
Where, on the other hand, cavities and fissures have been filled up by 
the deposition of materials on their walls, and gradual growth inward, 
the result is known asa secretion. Amygdules and the successive 
coatings of mineral veins are examples of the latter process. 

Oolitic, formed of spherical grains, each of which has an intei*nal 
radiating and concentric structure, and often possesses a central nucleus 
of some foreign body. This structure is specially found among 
limestones, (see p. 119). When the grains are as large as x>eus, the 
structure is termed pisolitic. 

2. Composition* — Before having recourse to chemical or micro- 
scopic analysis, the geologist cajji often pronounce as to the general 
chemical or mineralogical nature of a rock. Most of the terms which ho 
employs to express his opinion are derived from the names of minerals, 
and in almost all cases are self-explanatory. The following cxamjde^ 
may suflSce. Calcareous, consisting of or containing carbonate 
of lime. Argillaceous, consisting of or containing clay. 
F cl s p a t h i c, having some form of felspar as a main constituent. 
►Siliceous, formed of or containing silica ; usually applied to the 
chalcedonic forms of this cementing oxide. Quartzose, containing 
or consisting entirely of some form of quartz ; used more particularly 
of the ciystalliue forms of silica. C a r b o n a c e o n s, containing coaly 
]iiatter, and lieuco usually iissociated with a dark colour. Py r i t o u s, 
containing diffused disulphido of iron. Gypseous, containing 
layers, ucxlulcs, strings or crystals of calciuni-suli)hate. Saliferous, 
containing beds of, or impregnated with rock salt. Micaceous, full 
of layers of mica-flakes. 

As rocks aru not definite chemical compounds, but mixtures of 
different minerals in varying proportions, they exhibit many inter- 
mediate varieties. Transitions of this kind are denoted by such 
phrases as " granitic gneiss," that is, a gneiss in which the normal 



Part H. § ivj MICROSCOPIC CHABACTERS OF ROCKS, 97 



foliated structure is nearly merged into the massive structure of 
granite ; " argillaceous limestone " — a rock in which the limestone 
is mixed with clay; "calcareous shale" — a fissile rock, consisting 
of clay with a proportion of lime. It is evident that such rocks 
may graduate so insensibly into each other, that no sharp line can bo 
drawn between them either in the field or in their terminology. 

3. State of Aggregation. — The hardness or softness of a rock, 

in other words, its induration, friability, or degree of aggregation 

of its particles, may be either original or acquired. Some rocks 

^sinters, for example) are soft at first and harden by degrees ; the 

^neral effect of exposure, however, is to loosen the cohesion of tlio 

particles of rocks. A rock which can easily be scratched with the 

sail is almost always much decomposed, though some chloritic and 

talcose schists are soft enough to be thus affected. Compact rockb 

-which can easily be scratched with the knife, and are apparently 

xiot decomposed, may be fine-grained limestones, dolomites, ironstones, 

miudstones, or some other simple rocks. Crystalline rocks, as a rule, 

cannot be scratched w^ith the knife unless considerable force be used. 

"They are chiefly composed of hard silicates, so that when an instance 

occurs where a fresh specimen can l>e easily scratched, it will often 

l>e found to be a limestone (see p. 80). The ease with which a rock 

may Ijo broken is the measure of its frangibility. Most rocks break 

nioBt easily in one direction ; attention to this point will sometimes 

throw light ui)on their internal structure. 

Fracture is the surface produced when a rock is split or broken, 
and depends for its character upon the texture of the mass. Finely 
granular, compact rocks are apt to break with a splintery fracture 
where wedge-shaped plates adhere by their thicker ends to, and lie 
parallel, with the general surface. When the rock breaks off into 
concave and c(mvex rounded shell-like surfaces, the fracture is said to 
W n (• li o i d a 1 , as may l)e seen in obsidian and other vitreous rocks, 
ami in exceedingly com2)act limestones. The fracture may also ho 
t ol i a t e d , s 1 a t y , or r h a 1 y , according to the structure of the rock. 
Many opaque, compact rocks are translucent on the thin edges of fracture, 
and affi>rd there, with the aid of a lens, a gliin[se of their internal 
c»mj)08ition. A rock is f^aid to be flinty, when it is hard, closc- 
IjTained, and breaks with a smooth or conchoidal fracture like flint ; 
f ria b 1 e , when it crumbles down like dried clay or chalk ; plastic, 
when, like moist clay, it can be worked into shapes ; pulverulent, 
when it falls readily to ]>owder; earthy, when it is decomposed into 
loaiii or earth ; incoherent or loose, when its particles are quite 
•separate, as in dry blown sand. 

4. Colour and Lustre. — These characters vary so much, even 
ill the same nw^k, according to the freshness of the surface examined, 
^lut they possess but a subordinate value. Nevertheless, when 
cautiously used, colour may bo made to afford valuable indications as 
tjthe probable nature an<l composition of rocks. It is, in this respect, 



08 0E0GN08Y. [Book H. 



always desirable to compare a freshly-broken with a weathered piece of 
the rock.* 

WTiite indicates usually the absence or comparatively small amount 
of the heavy metallic oxides, especially iron. It may either be the 
original colour, as in chalk and calc-sinter, or may be developed by 
weathering, as in the white crust on flints and on many porphyries. 
Grey is a frequent colour of rocks which, if quite pure, would be white, 
but which acquire a greyish tint by admixture of dark silicates, organic 
matter, diffused pyrites, &c. Blue, or hluhh-grey is a character istictint 
of rocks through which iron-disulphide is diffused in extremely minute 
subdivision. But as a rule it rapidly disappears from such rocks on 
exposure, especially where they contain organic matter also. The stiff 
blue clay of the sea-bottom which is coloured by iron-disulphide becomes 
reddish-brown when dried, and then shows no trace of sulphide.* 
Bl€u:k may be due either to the presence of carbon (when weathering 
will not change it much), or to some iron-oxide (magnetite chiefly), 
or some silicate rich in iron (as hornblende and augite). Many rocks 
(basalts and melaphyres particularly) which look quite black on a fresh 
surface, become red, brown or yellow on exposure, black being com- 
paratively seldom a weathered colour. Yellow, as a dull earthy 
colouring matter, almost always indicates the presence of hydrated 
peroxide of iron. In modem volcanic districts it may be due to 
iron-chloride, sulphur, &c. Bright, metallic, gold-like yellow is usually 
that of iron-disulphide. Brmenyin the normal colour of some carbon- 
aceous recks (lignite), and ferruginous beds (bog-iron-ore, clay-ironstono, 
&c.). It very generally, on weathered surfaces, points to the oxidation 
and hydration of minerals containing iron. Bed, in the vast majority of 
cases, is duo to the presence of granular anhydrous peroxide of iron. 
This mineral gives dark blood -red to pale flesh-red tints. As it is liable, 
however, to hydration, these hues are often mixed with the brown and 
yellow colours of limonite. Green, as the prevailing tint of rocks, 
occurs amongst schists, when its presence is usually due to some of the 
hydrous magnesian silicates (chlorite, talc, serjTentine). It appears also 
among massive rocks, especially those of older geological formations, 
where hornblende, olivine, or other silicates have been altered. Among 
the sedimentary rocks, it is principally due to ferrous silicate (as in 
glauconite). Carbonate of copper colours some rocks emerald- or verdi- 
gris-green. The mottled character so common among many stratified 
rocks is frequently traceable to unequal weathering, some portions of 
the iron l>eing more oxidized than others; while some, on the other 
hand, become deoxidized from the reducing action of decaying organic 
matter, as in the circular green spots so often found among red strata. 

Lustre, as an external character of rocks, doe« not possess the 
value which it has among minerals. In most rocks, the granular 

* AltorcitionB of the oolourfl of rainoralH and rocks are effected by boat and even by 
Bunlight Sec Jauottaz, BuU. Sor. Geol. xxix. (1872) p. 300. 
« J. V. Hnclianan, Bnt. Amoc. 1881, p. 584. 



Part IL S v.] MICBOSCOPIO CnAItACTERS OF ROCKS. 99 

texture prevents the appearance of any distinct lustre. A completely 
vitreous lustre without a granular texture, is characteristic of volcanic 
glass. A splendent semi-metallic lustre may often be observed upon 
the foliation planes of schistose rocks and upon the lamina) of micaceous 
sandstones. As this silvery lustre i^ almost invariably due to the 
presence of mica,' it is commonly called distinctively micaceous, A 
metallic lustre is met with sometimes in beds of anthracite; more 
usually its occurrence among rocks indicates the presence of metallic 
oxides or sulphides. 

5. Feel and Smell. — These minor characters are occasionally 
useful. By the feel of a mineral or rock is meant the sensation 
experienced when the fingers are passed across its surface. Thus the 
hydrous magnesian silicates have a marked soapy or greasy feel. Some 
hydrous mica-schists containing margarodito or an allied mica, likewise 
exhibit the same character. Some rocks adhere to the tongue, a quality 
indicative of their tendency to absorb water. 

Smell. — Many rocks, when freshly broken, emit distinctive odours. 
Those containing volatile hydrocarbons give sometimes an appreciable 
hituminous odour, as is the case with some of the eruptive rocks, which in 
central Scotland have been intruded through coal-seams and carbon- 
aceous shales. Limestones have often a fetid odour; rocks full of 
decomposing sulphides are apt to give a snlphnroiis odour ; those which 
are highly siliceous yield, on being struck, an empyreumatic odour. It 
is characteristic of argillaceous rocks to emit a strong earthy smell when 
breathed, upon. 

6. Specific Gravity. — This is an imix)rtant character among rocks 
as well as among minerals. It varies from 0*6 among the hydrocarbon 
com]x>unds to 3-1 among the basalts. As already stated, the average 
siMjcific gravity of the rocks of the earth's crust may bo taken to l)e 
about 2-5, or from that to 3-0. Instruments for taking the Ri)ecific 
gnivity of rocks have l)een already (p. 82) referred to. 

7. Magnetism is so strongly exhibited by some crystalline rocks 
as powerfully to affect the magnetic needle, and to vitiate observations 
with this instrument. It is due to the presence of magnetic iron, the 
existence of which may be shown by pulverising the rock in an agate 
mortar, washing carefully the triturated powder, and drying the heavy 
residue, from which grains of magnetite or of titaniferous magnetic 
iron may l>e extracted with a magnet. This may be done with any 
basalt (p. 82). A freely swinging magnetic needle is of service, as by its 
attraction or repulsion, it affords a delicate test for the presence of even 
a small quantity of magnetic iron. 

§ V. — Microscopic Characters of Rochs. 

No department of Geology has been more advanced in recent years 
than Lithology, and this has been mainly due to the introduction of the 
microscope as an instrument for investigating niinutc internal structure. 
As far back as the year 1827, a method of making thin transparont 

n 2 



100 GEOGNOSY. [Book II. 



Hections of fossil wood, and mouiitiug them on glass with Canada 
balsam, had been devised by William Nicol of Edinburgh, and was 
employed by Henry Witham in his * History of Fossil Vegetables.' ^ 

It was not, however, until 1856 that ]\Ir. II. C. Sorby, applying this 
method to the investigation of minerals and rocks, showed how many and 
important were ihe geological questions on which it was calculated to 
shod light.^ Koference will be made in suljsequent pages to the remark- 
aide results then announced by him. To the publication of his memoir 
the Hul)se<|uent rapid develoj>ment of the microscopic study of rocks 
may be distinctly traced. The microscopic method of analysis is 
now in use in every country where attention is paid to the history of 
rocks.^ 

In § iii. p. 85 information has been given regarding the preparation of 
sections of rocks for microscopical examination, the methods of procedure 
in the practice of this part of geological research, and some of the terms 
employed in the following pages. 

1. Mioroscopic Elements of Rocks. 

liocks when examined in thin sections with the microscope are found 
to Ije composed of or to contain various elements, of which the more 
im2)ortant are, 1st, crystals, or crystalline substances; 2nd, glass ; 3rd, 
crystallites ; 4th, detritus. 

A. Crystals or Crystalline Substances. — Rock-foiining minerals, 
when not amorphous, may be either crystallized in their proper crystal- 
lographic forms, or while possessing a crj'stalline internal structure, 
may present no definite external geometrical form. The latter condition 
is more prevalent, seeing tliat minerals have usually been developed 
round and against each other, thus mutually hindering the assumption 
of determinate crystallographic contours. Other causes of imperfection 
are fracture by movement in the original magma of the rock, and 
partial solution in that magma (fig. 7), as in the corroded quartz of 

* Small 4to, Edinburgh, 1831. Thid work, tliough dedicated to Nicol, does not 
dlHtinctly recognise him iis the actiiul inventor of the process "of slicing mineral 
HubstanocB for microscopic iuTcstigution. All that was original in WiUiam's researches 
he owed either dirt^ctly or indirectly to Nicol. 

^ Brit, Assoc. 185G, Sect. p. 78. Quart, Journ, Geol. Soc. xiv. 1858. Micr. Jouru. 
xvii. (1877) p. 113. 

* Among the best text-books on this subject the followuig may be mentioned :— 

* Mikroskopische Beschaffenheit der MineraUen und Gesleine,* F. Zirkel, 1 voL 1873. 

* Mikroskopische Physiographio der MineraUen und Gesteine/ II. Rosenbusch, 2 vols. 
1873-7. 'Elemente der Petrographie,' A. von Lasaulx, 1875. 'Mineralogie micro- 
grapliique : roches eruptives francalses,* Fouqn<5 and Michel-Levy, 2 vols. 4to. Paris, 1879. 

* Microscopicid Petrography,' Zirkel, being vol. vi. of the Geol Explor, of iQth ParaUely 
Washington, 1876. The volumes for the last ten or fifteen yeurs of the Qtiarterlif 
Journal of file Geological Society, Geological Magazine, Neues Jahrhuch fiir Minentiogie, 
<tc., Zeitschrift der JJeutscJten GeologischeH Gemlechaft, Bullttin de la SocieU ghAogique 
de. France, Jahrhuch der K. K. Gfologvfchen Reichgankalt (F/e«««), contain numerous 
papers on the microscopic structure of rocks. Kutley's * Study of Rocks,' London, 1879, 
18 a convenient little book. The manual of Rosenbusch and the work of Fouque' and 
Michel-Levy, contain a tolerably ample bibliography of the subject, to which the 
student is referred. The titles of some of the more important memoirs which have 
recently appeared will be given in footnotes. 



Pabt n. S v.] MICROSCOPIC CHARACTERS OF ROCKS. 101 



quartz-porphyries and rhyolites. In some rocks, such as granite, the 
thoroughly crystalline character of the component ingredients is well 
marked, yet they less frequently present the definite isolated crystals so 
often to be observed in poqihyries and in many old and modem volcanic 
rocks. Among thoroughly crystalline rocks, good crj^stals of the com- 
]K)nent minerals may be obtained from fissures and cavities in which 
there has been room for their formation. It is in the " dnisy " cavities 
of granite, for example, that the wellnlefined prisms of felspar, quartz, 
mica, topaz, beryl and other minerals are found. Successive stages in 
order of appearance or development can readily bo observed among the 
crystals of rocks. Some ap][)ear as large, but frequently broken, or 
corroded forms. These have evidently l;)een formed first. Others are 
smaller but abundant, usually unbroken, and often disposed in lines. 
Others have been developed by subsequent alteration within the 
rock.^ 

A study of the internal stnicture of crystals throws light not merely 
on their own genesis, but on that of the rocks of which they consist, and 
is therefore well worthy of the attention of the geologist. That many 
apparently simple crystals are in reality compound, may not in- 
frwjuently l)e detected by the different condition of weathering in the 
two opposite parts of a twin on an exposed face of rock. The internal 
stnicture of a crystal modifies the action of solvents on its exterior (^e.g. 
weathered surfaces of calcite, aragonite and felspars). Crystals may 
occasionally be observed built up of rudimentary " microliths," as if these 
were the simplest forms in which the molecules of a mineral begin 
to appear (p. 107). 

Crystalline minerals are seldom free from extraneous inclusions. 
The.<»e are occasionally large enough t<» bo readily seen by the naked eye. 
I*iit the microscope reveals them in many minerals in almost incredible 
quantity. They are, a, gas cavities ; ^, vesicles containing liquid ; y, 
glnbules of glass or of some lithoid substance; 8, crystals; c, filaments, 
or other indefinitely-shaped pieces, patches, or streaks of mineral 
matter. 

a. Gas-filled cavities — are most frequently globular or 

elliptical, and appear to l>e due to the presence of gas or steam in 

the cr}'8tal at the time of consolidation. Zirkel osti mates them at 

Hr/»/M'j<»,o<)0 in a cubic millimetre of the liauyuo from Melfi.'- In some 

iuBlanoos the Ciivity has a geometric form belonging to the crystiiUinc 

Byntein of the enclosing mineral. Such a sjmce defined bj'^ ciystallo- 

gtaphic contours in a negative crystal, A cavity filled with gas contains 

^0 W)ble, and its margin is marked by a broad dark band. The usual 

^to 18 nitrogen, with traces of oxygen and carbon dioxide ; sometimes 

^t is entirely carlx)n -dioxide or hydrogen and hydrocarbons. 

p. V e B i 1 e s containing 1 i (j[ n i d (and gas). — As far back as 
1 th(3 year 1823, Brewster studied the nature of certain fluid-bearing 

* Fouqu^and Micliel-Levy, 'Min. Mioro^aph.' p. IHl. 
« * Mik. Beschafl? p. HH 



102 GEOGNOSY. [Book U. 



cavities in diflferent minerals.^ The first observer who showed their 
important bearing on geological researches into the origin of crystalline 
rocks was Mr. Sorby, in whose paper, already cited, they occupy a 
prominent place. Vesicles entirely fille*! with liquid are distinguished 
by their sharply-defined and narrow black borders. Vesicular spaces 
containing fluid may be noticed in many artificial crystals formed from 
aqueous solutions (crystals of common salt show them well) and in many 
minerals of crystalline rocks. They are exceedingly various in form, 
being branching, curved, oval, or spherical, and sometimes assuming as 
negative crystals a geometric form, like tliat characteristic of the 
mineral in which they occur, as cubic in rock salt and hexagonal in 
(quartz. They also vary greatly in size. Occasionally in quartz, 
sapphire and other minerals, large cavities are readily observable with 
the naked eye. But they may be traced with high magnifj4ng powers 
down to less than jijJoiy ^^ *^ ^^^^ ^^ diameter. Their proportion in 
any one crystal ranges within such wide limits, that whereas in some 






I 



Fig. 6.— CaviiicH in CrvHtald, highly magnifiod ; a, Liquid Iiiclusloiu) ; b, Glam InclnsioiiA ; c. Cavities 
showing the devitriflc&tion of the origioal gloM by the appearance of crystals, &c., until in the lowest 
ligure a stony ur lithoid product is formed. 

crystals of quartz few may be observed, in others they are so minute 
and abundant that many millions must be contained in a cubic inch. 
The fluid present is usually water, frequently with saline solutions, 
particularly chloride of sodium or of potash, or sulphates of 2H)tash, 
soda, or lime. Carbon-dioxide may be present in the water ; sometimes 
the cavities are partially occupied with it in liquid form, and the two 
fluids, as originally observed by Brewster, may be seen in the same 
cavity unmingled, the carbon-dioxide remaining as a freely moving 
globule within the carbonated water. Cubic crystals of chloride of 
Boilium may be occasionally observed in the fluid, which must in such 
cases be a saturated solution of this salt (Fig. C, lowest figure in 
( -olumn A). Usually each cavity contains a small globule or bubble, 
sometimes stationary, sometimes movable from one side or end of the 

* Edin. Fhil, Journ. ix. p. 94. Tratis. hoy. Soc, Edin. x. p. 1. See also W. KicoJ, 
Edin. New PhiL Journ. (1828) v. p. 94; Do la Vallee Pousdin and Beiiard, Aead. 
Hoy. Belg. 187tJ. p. 41 ; Hartley, Journ. Chem. Soc. ser. 2, xiv. 137 ; Bcr. 3, ii. p. 241 ; 
Microscop, Journ, xv. p. 170 ; Brit, Awoc, 1877, Sect. p. 232. 



Pabt II. § v.] MICROSCOPIC CHARACTERS OF ROCKS. 103 

cavity to the other, as the specimen is turned. With a high magnifying 
power, the minuter bubbles may be observed to be in motion, sometimes 
slowly pulsating from side to side, or rapidly vibrating like a living 
organism. The cause of this trepidation, which resembles the so-called 
** Brownian movements," has been plausibly explained by the incessant 
interchange of the molecules from the liquid to the vaporous condition 
along the surface where vapours and liquid meet — an interchange which, 
though not visible on the large bubbles, makes itself apparent in the 
minute examples, of which the dimensions are comparable to those of 
the intermoleoular spaces.^ The bubble may be made to disappear by 
the application of heat. 

With regard to the origin of the bubble, Sorby pointed out that it can 
be imitated in artificial crystals, in which he explained its existence by 
diminution of volume of the liquid owing to a lowering of temperature 
after its enclosure. By a scries of experiments he ascertained the rate of 
expansion of water and saline solutions up to a temperature of 200^ 0. 
(392^ Fahr.), and calculated from them the temperature at which the liquid 
in crystals would entirely fill its enclosing cavities. Thus, in the nepheline 
of the ejected blocks of Monte Somma, he found that the relative size of 
the vacuities was about *28 of the fluid, and assuming the pressure under 
which the crystals were formed to have been not much greater than 
sufficient to counteract the elastic force of the vapour, he concluded that 
the nepheline may have been formed at a temperature of about 340° C. 
(644° Fahr.), or a very dull red heat, only just visible in the dark. He 
estimated also from the fluid cavities in the quartz of granite that this 
rock has probably consolidated at somewhat similar temi)eratures, under 
a pressure sometimes equal to that of 7G,000 feet of rock.^ Zirkol, 
however, has pointed out that even in contiguous cavities, where there 
is no evidence of leakage through fine fissures, the relative size of the 
vacuole varies within very wide limits, and in such a manner as to indi- 
cate no relation whatever to the dimensions of the enclosing cavities. Had 
the vacuole besn due merely to the contraction of the liquid on cooling, 
it ought to have always been proportionate to the size of the cavity.^ 

MM. De la Valine Poussin and Eeuard, attacking the question from 
another side, measured the relative dimensions of the vesicle and of it« 
enclosed water and cube of rock-salt, as contained in the quartziferous 
diorite of Quenast in Belgium. The temperature at which the ascer- 
tained volume of water in the cavity would dissolve its salt was found 
by calculation to be. 307'' C. (520^ Fahr.). But as the law of the 
solubility of common salt has not been experimentally detenuined for 
high temperatures, this figure can only be accepted provisionally, 
though other considerations go to indicate tliat it is probably not far 
from the truth. Assuming then that this was the temperature at which 

» Charbonolle and Tliirion, Rev. Quest. Scienti/. vii (1880) 43. On tlie critical point 
of water, Ac, in these cavities see Hartley, Joum. Cfiem. Soc. feor. 3, vol ii. p. 24 J. rop. 
Set. Rev. new scr. i. p. H9. 
» Sorby, Q. J. Ged. Soc. xiv. pp. 480, 493. ^ *Mik. Leschaff/ p. 40. 



104 GEOGNOSY. [Book H. 



the vesicle was foraied, these authors proceed to determiDO the pressure 
necessary to prevent the complete vaporization of the water at that 
temperature, and ohtain, as the result, a pressure of 87 atmospheres, 
t?qual to 84 tons per square foot of surface.' Tliat many rocks were 
formed under great pressure is well sliown by the liquid carbon- 
dioxide in the pores of their crystals. 

Although, in almost all cases, the liquid inclusions are to be referred 
to the conditions under which the mineral crystallized out of the original 
magma, they may be exceptionally develoi>ed long subsequently, either 
in one of the original minerals during decomposition, or in a mineral 
of secondary' origin, such as quartz of subsequent introduction.^ 

Liquid inclusions may be dispersed at random through a crystal, or 
us in the quartz of granite, gathered in intersecting planes (which look 
like fine fissures and which may sometimes have become real fissures, 
owing to the line of weakness caused by the crowding of the cavities), 
or disposed regularly in reference to the contour of the crystal. In the 
last case they are sometimes confined to the centre, sometimes arranged 
in zones along the lines of growth of the crystal.^ They are specially 
conspicuous in the quartz of granite and other massive rocks, as well as 
of gneiss and mica-schist ; also in felspars, topaz, beryl, augite, nepheline, 
olivine, leucite and other minerals. 

y. Inclusions of glass or of some lithoid substance. 
— In many rocks which have consolidated from fusion, the component 
crystals contain globules or irregularly shaped enclosures of a vitreous 
nature (Fig. 6, Column B). These enclosures are analogous to the 
fluid-inclusions just described. They are portions of the original glassy 
magma out of which the minerals of tlio rock crj'^stallised, as portions of 
the mother-liquor are enclosed in artificially formed crystals of common 
salt. That magma is in reality a liquid at high temperatures, though 
at ordinary temperatures it becomes a solid. At first, these glass-vesicles 
may be confounded with the true liquid-cavities, which in some respects 
tliey closely resemble. But they may l)e distinguished by the im- 
mobility of their bubbles, of which several are sometimes present in the 
same cavity ; by the absence of any diminution of the bubbles when 
heat is ai)pliod ; by the elongated shape of many of the bubbles ; by the 
occasional extrusion of a bubble almost l)eyond the walls of the vesicle ; 
by the usual pale greenish or brownish tint of the substance filling the 
vesicle, and its identity with that forming the surrounding base or 
ground-mass in which the crystals are imbedded; and by the complete 
passivity of the substance in polarized light. (See p. 90.) 

' * Memoire sur les Roches dites PlutonicnncB de la Belgiquo,' De la Vnllde Poussin 
and A. Renard. Acad. Botj, Belg. 1876, p. 41. 8eo also Ward, Q. J, Geol, 8oe, xxxi. 
p. 568, who believed that the granites of Cumberland consolidated at a maximum depth 
of 22,000 to 30,000 feet. 

' See Wliitman Cross on the development of liquid inclusions in plagioclase 
during the decomposition of tlie gneiss of Brittany. Tschermak's Jifin. MUtheih 1880. 
|>. 869 ; also G. F. Becker, ' Geology of Comstock Lode.' U. 8. Geol. Surv. 1882, p. 371. 

• The way in which vesicles, enclosed crystals, &c., are grouped along the zones of 
growth of crystals is illustrated in Fig. 7. 



Pabt 11. S v.] MIOBOSCROPIC CHABAOTEBS OF BOCKS. 105 

Glass inclusions ooonr abundantly in boiuo minerals, aggrogatod in 
the centre of a crystal or ranged along its zones of growth with 
tdngalar regularity. They appear in felspars, quartii, loucite, and other 
crystalline ingredients of volcanic rocks, and of course prove that in 
such positions these minerals, even the refractory quartz, liave nn- 
dunbtedly crystallized out of molten solutiouH. 

In inclosions of a truly vitreous nature, traces of devitrification may 
not iufreqnoiitly he seen. In particular, inicroHCopiccryHtalliteB (p. lOfi) 
make their appearance, like those in the pround-niawi of the rock. 
Sometimes the inclusions, like the general gMund-mass, have an entirely 
stony character. (Fig 6, C.) This may lie well oliservcd in those which 
have not been entirely separated from the surrounding grouud-iuaHH, 
but are connootod with it by a narrow neck at the j)cripherj' of the 
enclosing crystal. In some granites and in elvans, the qu.artz by 
irregular contraction, while still in a pla«tic state, appeal's to liavo 




lirawn into iti sulmtance portions of the surrounding nh-oady lithoid 
'«»;' but this appearance may somctimcH Ix; duo to irregular cornision 
if Die ciy«tals by the magma.' 

i f'rystals and frystallino bcidics. — Many ciinijwment 
Miuenlsof rocks contain other minernls (Fig. 7). Theno owur smuu- 
>iiBes as perfect crystals, m(>n> usuaily &» what are ternii-d miin'litliK 
fp. 107). Like the glaf^-inclusions, they tend to range thciiiselv.M iji 
)■>«• along the successive zones of growth in the enclotung iiii]»i;il. 
Um^iths are of frequent occurrence in leucife, garnet, luigili'. Ii-m 
Ifriide. ciih'it.-, fluorite, &c. From the fact that mierolitliK cif tl..- .asiiy 
In-il-l. ^iifjite are, in the Vesuvian lavas, enclosed witliin tli' '■xi].iiiil\ 
i'-l'nttt.iry lencit«, it was supposed that the rclutiv.; oidij .-t lu.-ir/iln 
>■ nut alvAfs followed in the microIithH and fnv>'l<ij'iii;' '-t vmh |i<ii 



106 GEOGNOSY. [Book n. 



this has been satiBfactorily explained by Fouquo and Michel-L6vy, who 
have shown experimentally that leucite, when crystallizing from fusioDy 
tends to catch up inclusions of the surrounding glass, which, should the 
glass be pyroxenic, may assume the form of augite.^ 

€. Filaments, streaks, patches, discolo rat ions. — 
Besides tlie enclosures already enumerated, crystals likewise frequently 
enclose irregular portions of mineral matter, due to alteration of the 
original substance of the minerals or rocks. Thus tufts and vermicular 
aggregates of certain green ferruginous silicates are of common 
occurrence among the crystals and cavities of old pyroxenic volcanic 
rocks. Orthoclase crystals are often mottled with patches of a granular 
nature, due to partial conversion of the mineral into kaolin. The 
magnetite, so frequently enclosed within minerals, is abundantly 
oxidized, and has given rise to brown and yellow patches and dis- 
coloration s. Care must be taken not to confound these results of 
infiltrating water with the original characters of a rock. Practice will 
give the student confidence in distinguishing them, if he familiarises 
his eye with decomposition products by studying slices of the weathered 
parts of rocks. 

B. Glass. — Even to the unassisted eye, many volcanic rocks consist 
obviously in whole or in great measure of glass.^ This substance in 
muss is usually black or dark green, but when examined in thin sections 
luider the microsco})e, it pi'esents for the most part a pale brown tint, or 
is nearly colourless. In its purest condition, it is (|uite structureless, 
that is, it contains no crystals, crystallites, or other distinguishable 
individualised bodies. But even in this state it may sometimes be 
observed to be marked by clot-like patches or streaks of darker and 
lighter tint, arranged in lines or eddy-like curves, indicative of the flow 
of the original fiuid mass. Eotated in the dark field of crossed Nicol- 
prisms, such a natural glass remains dark, as it is perfectly inert in 
polarized light. Being thus isotropic, it may readily be distinguished 
from any enclosed crystals which, acting on the light, are anisotropic 
(p. 90). Perfectly homogeneous structureless glass, without enclosures 
of any kind, occurs for the most part only in limited patches, even 
in the most thoroughly vitreous rocks. Originally the structure of all 
glassy rocks, at the time of most complete fusion, may have been that of 
perfectly unindividualised glass. But as these masses tended towards a 
solid form, devitrification of their glass set in. Many forms of incipient 
or imperfect crystallization, as well as perfect crystals, were developed 
in the still fluid and moving mass, and were drawn out in the direc- 
tion of motion. In some cases, devitrification has proceeded so far that 
no trace remains of any glass. 

C. Crystallites.^ — Under this name may be included minute 

> *SyDthe8e des Mine'rauic,* 1882, p. 155. 

* Seo E. Cohen on glossy Rocks. Neues Jahrh. 1880 (ii.) p. 23. 

' This word was first used by Sir James Hall to denote the lithoid substance obtained 
by him alter fusing and then slowly cooling various *' whinstones '' (diabases, Ac.}, 
Binoo its reTival in Uihology it has been applied to the minuter bodies above described. 



Part II. § v.] MICROSCOPIC CIIARACTERS OF ROCKS, 107 



inorganic bodies poesessing a more or less dofiiiito form, but generally 
without the geometrical characters of crystals. They occur most 
commonly in rocks which have been formed from igneous fusion, 
but are found also in others which have resulted from, or have been 
altered by, aqueous solutions. They seem to be early or peculiar forms 
of crystallization. They are abundantly doveloj)ed in artificial slags, 
and appear in many modem and ancient vitreous rocks, but the 
conditions under which they are produced are not yet well understood.^ 
The simplest are extremely minute drop-like bodies or globtUites. 
Quite isotropic, they are sometimes crowded confusedly through the 
glass, giving it a dull or somewhat granular character, while in other 
cases they are arranged in lines or groups. Gradations can be traced 
from spherical or spheroidal globulites into other forms more elliptical 
in shape, but still having a rounded outline and sometimes sharp ends. 
These were termed by Vogelsang LonguUtes, There does not appear to 
be any essential distinction, save in degree of development, between 
these forms and the long rod-like or needle-shaped bodies which have 
been termed microliths (Beloniies), Existing sometimes as mere simple 
needles or rods, these microliths may be traced into more complex 
forms, sometimes pointed, sometimes toothed at the end, straight, 
carved or coiled, smooth or striated, at one time solitary, at another in 
groups. It is sometimes possible, from their association or optical 
characters, to determine to what minerals microliths belong. Augite, 
hornblende, apatite and felspars all occur in these rudimentary forms. 
In most cases, the microliths are transparent and colourless, or slightly 
tinted, but sometimes they are black and opaque, from a coating of 
ferruginous oxide, or only appear so as an optical delusion from their 
position. Black, seemingly opac|ue, hair-like, twisted and curved micro- 
liths, termed trichites, occur abundantly in obsidian. Good illustrations 
of the general characters and grouping of crystallites are shown in 
some vitreous basalts. In Fig. 8, for example, the outer portion of the 
field displays crowded globulites and longulites, as well iis here and 
there a few belonites and some curved and coiled microliths. Kound the 
rude augite crystal, these various l)odies have been drawn together out of 
the surrounding glass. Numerous rod-like microliths diverge from the 
crystal, and these are more or less thickly crusted with the simpler 
and smaller forms.^ In Fig. 9, the remarkably beautiful structure of an 

aud a distiuction has been drawn between crystallitos and microlitliH. It seems to me 
mo^t Oinveniont to retain the term orysiallites &» the pfeueral designation of all the 
indefinitely crystidliiio or incipient fonuB of individualisatioii aiiinng minerals, uiid to 
subdivide thc^se by the employment of such names as Vogelsiing's (Hobalites^ LoiujuliteH^ 
Microlithty &v.. The student should consult this author's *Philo8ophie der Geologic,* 
p. 13'.) ; ' Krystalliten/ Bonn, 8vo. 1875 ; also his descriptions in Aychtces Nttrhnidaieeg. 
V. 1870, vi. 1871. Sorby. Brit. Assoc. 1880. 

' They are well exhibited also in ordinary blow-pii>e beads. See Sorby, Brit. Aftttoa. 
1880. or Geol. Mmj. 1880, p. 408. Tliey have been produced experimentally in the 
artificial rocks fused by lilessrs. Fouque' and Michel Lt'vy. 

« Cieikie, iVw. Roy. Fhys. Sor. Kdin. v. p. 216, Plate v. Fig. 5. J. J. II. Tcall, 
Q. J. GtoL Soc, id. p. 221. Plato xii. Fig. 2a. 



108 



OEOGNOSY. 



[Book II. 



AiTan pitchstonc \a shown ; tbe glassy base being crowded with minute 
iui(!riilit1is which are grouped in a fine Lrnsh-like arrangement round 
tajiering rods. In tliis case, also, we two that the glassy base has been 
cUrified round t}ie larger individuals by the abstraction of the crowded 
Hinallcr microlitht). Hy the progressivo development of cr^'stallites or 
ciystals during the cooling and consolidation of a molten rock, a glam 
loses itB vitroouB character and booomea lithoid ; in other words, undergoes 
devitrification 

^\itli the cr>»taniteB may bo groniiotl the characteristic amorphona 
ur indt.huitely gtanular and fibrous or scaly matter, constituting the 
micniHcopic baNo m which the definite crystals of f«]mt«8 and porphyries 
uie imliedded (pp 03, 110 141) The tme nature of this Bubstance is 
not J et understood Betw een crossed Nicol-prisms it sometimes behavea 
iBotropically like a gl iss but in other cases allows a mottled glimmering 
light to pass through It is a prmlnct nf devitrification where, though 




tho vitreous cliaracter has disappeared, its place has not been taken by 
i-ecognisable crj-stala or crystalline particles,' 

Every gradation in tho relative abundance of ciystellites may be 
traced. In some olisidiaus and other vilreous rocks, [lortiuns of the 
glass can be obtained with comparatively few of thorn ; but in the same 
rucks we may not infrequently olserve adjacent parts where they have 
Ixjen so largely developed as to iisuqj the iilace of the original glass, and 
give the rock in consequence a lithoid aspect (]). 146). 

D. DuTniTUS. — Many rocks are composed of the detritus of pre-existing 
materials. In the great majority of cases this can ])o loadUy detected, 
even with the naked eye. But where the texture of such detrital or 
fragmental (clastic) rocks becomes exceedingly fine, their true nature 
may require elucidation with the microscope. An obvious distinction 
can be drawn between a maae of compact detritus and a crystalline or 

■ tjoo Ziikol, • Mik. Ikwhaff.' p. 280. BoeoubuBih, toI. ii. p. 60. 



Part EL f v.] MICBOSCOFIC CHARACTEBS OF BOCKS, 109 



vitreous rock. The detrital materials are found to consist of various 
and irregularly shaped grains, witli more or less of an amorphous and 
generally granular paste. In some cases, the grains arc broken and 
angular, in others they are rounded or water worn (p. IGO). They may 
consifit of minerals (quartz, chert, felspars, mica, &c.), or of rocks (slate, 
limestone, basalt, &c.), or of the remains of plants or nnimals (spores of 
lycoixxls, fragments of shells, crinoids, <fec.). It is evident therefore 
that though some of them may be crystalline, the rock of which they 
now form j>art is a non-crystalline comi)ound. Water, with carbonate 
of lime or other mineral matter in solution, ]>ermeating a detrital rock, 
has sometimes allowed its dissolved materials to crystallize among the 
interstices of the detritus, thus producing a more or less distinctly 
crystalline structure. But the fundamentally secondary or derivative 
nature ef the mass is not always thereby effaced. 

2. MicroBcopie Structures of Books. 

We have next to consider the manner in which the foregoing 
luicroficopio elements are associated in rocks. Hiis inquiry brings before 
us the minute structure or texture of rocks, and throws great light upon 
their origin and history.^ 

Four types of rock-structure are revealed by the microscope. A, 
wholly crystalline ; B, semi-crystalline ; C, glassy ; D, clastic. 

A. Wholly Crystallink, consisting entirely of crj'stals or crystalline 
individuals, whether visible to the naked eye, or requiring the aid of a 
microscope, imlwdded in each other without any intervening amorphous 
substance. Rocks of this type are exemplified by granite (Fig. 10) and 
by other igneoiuj rocks. But they occur also among the ciy stall ino 
limestones and scliists, as in statuary juarble, which consists entirely of 
crj'stalline granules of calcite (Fig. IG). 

The holocrystalline eruptive rocks (p. 92) are typically represented 
by granite, hence the term yranitoid has been adopted to exi)ress their 
microscopic stnicture. Varieties of this stnicturo are designated 
according to the relations of the component niincralH. Where no one 
mineral greatly preponderates, but where they are all confusedly and 
tolerably equally distributed in individuals readily observable by the 
naked eye, as ordinary granite, the structure is (jranitic, (See granular, 
p. 93.) Where a similar structure is so fine that it can only be re- 
cognised with the microscope, it has been called micrograniiic or cur i tic. 
Where the minerals are grouped in small, isolated, grain-like individuals, 
each having its own independent crystalline structure, so that under 
the microscope in polarized light, the rock presents the appearance of a 
brilliant mosaic, the structure has been named by French petrographers 
tjranalUic, or where only discernible by the aid of the microscope, micrO' 
granuUtic, Where the quartz and felsi)ar of a granitic rock have crystal- 

* The tirut broad clasBiticiitioii of the luicroacopic structure of rockn was that pro- 
IXMed by Zirkel, which, with slight modiflctitiou, is here udopteil. 'Mik. Beschufl'/ 
p. 266. • Basaltgesteinc,' p. 88. See ftlso Rosenbu»ch*B suggestive paper already cited, 
A'eii^a Jahrh, 1882 (ii.), p. 1 . 



no QEO0S08Y. [Book U. 

lized ill one common <]ji-ection, one within the other, the etructuro if> 
pegmatilic whore TieiLlo to the naked eye, and micropetfmalitie where the 
help of a microscope is noedod.' 

B. 8emi-crystallihb.' — This division probably comprehends the 
majority of the maseivo omptive or igneons rooks. It is dietingniBheit 
by tlie occurrence of what appears to the naked eye as a compact or 
finely granular groiind-maBs, throngh which more or lew recognisable 
cryhtals are Bcattored, Examined with the microscope, this ground-mass 
is found to preflont considerable diTemity. It may be (1) wholly a glass, 
as in some l)aBalt8, traohytca, and other volcanic products ; (2) partly 
devitrified throngh separation of peculiar little granules and needles 
which appear in a vitreouH base ; (3) still further devitrified, until it 
becomes an aggregation of such little granules, needles, and hairs, between 




Fig. le. 

ng. 10,— Wbolly CrjTiUlllM Rl 
Btripf^ pKftA tVUpiT. tbp Ion 
Fig. 11.— Sunl-CTTJUIllne Slnicl— . — ~ , . — b "■ - 

brudcr moiiDclEnlc rnmui. ■Kghllf ebaded In tbe drawing, are Auglt 
th« ruvdlc^apnl rnrma are Apalite. (5n p. IM.), 

which little or no gliVfis-baso appears ( microcry stall itic) ; or (4) "mi- 
crofelsitio," closely i-olal-od to the two previous groups, and consisting 
of ft nearly stmcturelcBs mass, marked uPiially witli indefinite or lialf 
effaced granules and filaments, but behaving like a tiingly-refracting, 
amorphous body (p. 108). 

In rocka belonging to this tyi>e, a spherulllie structure has sometimes 
lioen jiroducoil by iho apppnranco of globular bodies comixvsed of a 
crj-stalline intcrin.lly radiating sulistanee, sometimes with concentric 
shtlls of amorphous material. In many caws, spborulites aro only 
recogniMuble with Iho microHCope, when tlicy each present a black croes 

• Foiiqiii' ami Micbf 1-I,tvy, ' Min. MiiTOKrapli.' Do Lapparctit, ' GtoloRif,' p. SSS. 

' Fur lliis xtnictiire tlie lonii "niiciHl" liiia Ix'pn proposed, an Wine >> nixlurc 
nf tlic cryKtBllino iim) tuunrplioiiH (ulaBcy) Blriirturps. It lia* U-ch clcBiEnnted by 
Fnmiur nnd Michel-Lery " tiwliylowl," aa being lypically ilevelopeil nmonf; tli'c 

lnulivtiK(j»«(m, p. 137). 



Paw n. § v.] MICJtOSCOPIO CnAJtAOTERFi OF ItOCKS. Ill 

Itetween crossed Nicol-prieniB, and thoroby cliaracteriatically reveal the 
wimtpkenlilie atmctnre." 

C. Glassy. — Compoeed of a volcanic glass soch as has already been 
de«oril)6<I. It seldom happens, however, th»t rocks which seom to the eye 
to bo tolerably homogeneous glass do not contain abundant crj'atallitcs 
and niinnte crystals. Hence truly vitvcons rocks tend to graduate into 
the seooad or aomi-crystallino type. Thin gradation and the abundant 
evidence of traces of a devitriSed base or magma between the crystals of 
a vast nnmbor of eraptivo rocks, lead to tho belief that the glassy type 
was the original condition of most if not all of these rocks. Erupted 
an molten masses, their mobility would depend upon tho fluidity of tlio 
glass. Yet oven while still deep within tho earth's cniBt, some of their 
constituent minerals (felspars, leucito, magnetite, ikc.) wore often already 
CTystalli?^, and suffered fracture and corrosion by sulffictjuont action of 
the enclosing magma. This is well shown by what ik termed tho 
fi»n<M-tar»rtHTe. Crystals and crystallit^t are ranged in ciirrent-like 




linM, with their long axes in tho direction of these lines. Where ii 
Wi;b older crj-flt«l occurs, the train of minuter individuals is found to 
n'efiprrmnd it and to ronnito on the further side, or to Iks divertoil iu 
»n widy-liko course '(Fig. 12). So thoroughly is this nriangenient 
tUractcristic of tho motion of a somewhat viscid li<jnid, that thiTu 
•iiTinot lie any doubt that such was the condition of those masses lieforc 
lh"ir nmsoli<lalion. Tho flnxiim-structurc may Iw detected in many 
'■■fii|itivf rocks, from tliorougldy vitreous comptmnds liko obsidian, on 
llii; uiie hund, to complnt^ily cryHtalHuo masses liko some diilcrites, on tho 
'itliLT. It occurs not only in what arc usually regarded as vulcanic rocks, 
lint also in jdutonic or dcop-scatcil niassus whicb, there is rcnstm to 
'''lif.'ve, cunsolidatcil deep 1>eneath the surface, .is for instance in the liodo 
Teiu of the llari! and among qiiartx-porpbyries associated with granites 
in Alerdeen shire. Tho structure, therefore, cannot !« regarded oh 

' Fnui]iii' and Miulu'l-I-evy, 'M 
•-lamplm nf niicniH]i)iiTiilitic *>lrii''lilrc 
\-\'t i.iiiuliriaii t.iffH lit -St. Kiiviil'-.. (j. J. fleot. fioc. x^\i\. \i. :^la. 



1 ta* ■^EO'jyoftr. [Book U. 

'^rtratinly iuiKiratitig dutt tlui nwk in whiidi it i» Ibimd ctct flowed ont at 

^mu idsMST miJia, iu <;nolii]^ Mul oonnotiilittdiig, kMve bad tl«vel(^>e(l 
ill tlmiti h; i^Diitrai^ciitii tiie •itniutu ^rstemufn:tii:iiJataiiuid spiral cracki 
Inuiwu ait cite ptrlitU: itmutai^. ' p. I4fi i. 

TIiu tinal .4d£:umj^ ot:' « vittcooH aiiwit iiib> wdihl titune has resulted 
I Ut; from iuer<.- -*jU<titicaUuii ut' tiuj ^liuw : thiit » well ueeu at the edgo 
of ftyk^n 4iiil iiiuiinive atLeeu-iif iliffiireiit biwalt-cucks, where the igneorix 
itiaait, iiiiviu^ Inicii aailiieoly imni^ealiid idtfiif; ibi line uf cmitMct with tho 
»iiFT'>nii>liD;^ nji.'ka, renuuiut there in. the cuoititiiai of gla^ thcmgh only 
aa iD<:h fnrtEuir inwiini fpjiu tiie «<I{$e tine vitreoiu magmu has dis- 
Appeare-l, mt represeated in Fig. 25; (,2iiil) frjm the deritritication of 
the glaiM by the abonihuit development ijf sucTofeliitic granules and 
tilameDtK, aa in 4aartz-p>jrphyn>', ot ii cryvtallTtes axtd cTTBtala, as in 
■Qcb gla>w7 locks as obaiJian and tachjiite ; or i,-}nl^ &<Ma the complete 




cryBtallizatiou of the whole of the original glassy base, as may bo 
ut>8erved iii some doloritoe aiul liasalts. 

D. ('l:Aflric.— ('omiKiKod of dotrital matoriale, such as have been already 
deBcriltcd (]i. 108). Where these niatcrialB conxist of grains of quai-tz-sand, 
they withstand almost any subKoi{ueut change, and hence can be rec(^- 
nisedcTon among the most highly metamorpboaod series of rooks (Fig. 13). 
Qnartzite from snob it scries can somctiiucs be scarcely distinguished iinder 
the microscope fruni iinaltercd <|nartzoBO sandetonc. Where tho detritus 
bus rosidtcd front tlio dostrnction of alaminons or niagnesiau silicates, 
it in more snt^wi'tiblo of alteration. Hence it can bo traced in regions 
of liical n]ctanioii>bisn), l^cconiing more and luoro crystalline, until tho 
n>i'kM fiirmed of or contnining it jwibk into tnie crystalline schists. 

IVtrituB di-rivi.'d fruni the comniiuntion or decay of organic I'cmaius 
]iivst.<nls very different imd ehuractcrintic structuren.' (Fig. 14.) 

' Till' "tihlciit wlin winilil liirlln'r iiivi'Hiipilo lliin i-ul'jii't, will fiiiil b suppirtirc Hod 
liiii.iu.ni- .•!.»«> Hp.>ii it hy \\r. _S..rbyjii n rwi'iit prrMdimtiiil iiHdrm> to the GeologicHl 



Pabt n. § v.] MTCROSCOPIC OnARACTERS OF ROCKS. 113 



Sometimefi it is of a siliceous nature, as where it lias been derived from 
diatoms and radiolarians. But most of the organically derived detrital 
rocks are calcar€k)w^, formed from the remains of foraminifera, corals, 
echinodemiB, polyzoa, cirripedes, annelidcs, moUusks, Crustacea and 
other invertebrates, with occasional traces of fishes or even of higher 
vertebrates. Distinct differences of microscopic structure can be 
detected in the hard parts of some of the living representatives of 
these forms, and similar differences have been detected in beds of lime- 
stone of all ages. Mr. Sorby, in the paper cited below, has shown how 
characteristic and persistent are some of these distinctions, and how 
they may be made to indicate the origin of the rock in which they 
occur. There is an important difference between the two forms, in 
which carbonate of lime is made use of by invertebrate animals ; aragonito 
being much less durable than calcite (pp. 74, 75). Hence while shells 
or other organisms, formed largely or wholly of aragonite, crumble down 
into mere amorphous mud, pass into crystalline calcite, or disapj^ear, the 
fragments of those consisting of calcite may remain quite recognisable. 

It is evident, therefore, that the aljsence of all trace of organic 
structure in a limestone need not invalidate an inference from other 
evidence that the rock has been formed from the remains of organisms. 
The calcareous organic debris of a sea-bottom may be disintegrated, and 
reduced to amorphous detritus, by the mechanical action of waves and 
currentB, by the solvent chemical action of the water, by the decay of 
the binding material, such as the organic matter of shells, or by being 
swallowed and digested by other animals.^ 

Moreover, in clastic calcareous rocks, owing to their liability to alter- 
ation by infiltrating water, there is a tendency to acquire an internal 
crystalline texture. At the tiiuo of formiition, little empty spaces lie 
Vetween the component granules and fragments, and according to Mr. 
•Sorby, these interspaces may amount to about a quarter of the whole 
mass of the rock. They have very commonly been filled up by calcite 
iutr<>luccd in solution. This infiltrated calcite acquires a crystalline 
•^tnicture, like that of ordinary mineral- veins. But the original com- 
iKjnent organic granules also themselves become crystalline, and, save 
in 80 far as their external contour may reveal their original organic 
8«Jttrcv, they cannot be distinguished from mere mineral grains. In this 
''^ay, a cycle of geological change is completed. The calcium-carbonate 
originally dissolved out of rocks by infiltrating water, and carried into 
the sea, is secreted from the oceanic waters by corals, foraminifera, 
echiu<jderms, mollusks and other invertebrates. The remains of these 
creatures collected on the sea-bottom slowly accmmulate into beds of 
detritus, which in after times are upheaved into land. Water once 
more i>eroolating through the calcareous mass, gradually imparts to it a 
cnt'stalline structure, and eventually all trace of organic forms may be 
effaced. But at the same time, the rock, once exposed to meteoric 
influences, is attacked by carbonated water, its molecules are carried in 

* Sorby, rresidentiul Address, Q, J. GeoL Soc, 1879. 



114 GE0GN08T. [Book H. 

solution into the sea, where they will again be built up into the frame- 
work of marine organisms. 

E. Alteration of Rocks by Meteoric Water. — An important revela- 
tion of the microscope is the extent to which rocks suffer from the 
influence of infiltrating water. The nature of some of these changes is 
described in subsequent pages. (Book III. Part II.) It may be sufficient 
to note here a few of the more obvious proofs of alteration. Threads and 
kernels of calcite running through an eruptive rock, such as diabase, 
dolerite, or andesite, are a good index of internal decomposition. They 
usually point to the decay of some lime-bearing mineral in the rook. 
Some other minerals are likewise frequent signs of alteration, such as 
serpentine (often resulting from the alteration of olivine, see Fig. 26), 
chlorite, epidote, limonite. In many cases, however, the decomposition 
products are so indefinite in form and so minute in quantity, as not to 
permit of their being satisfactorily referred to any known species of 
mineral. For these indeterminate, but frequently abundant, substances, 
the following convenient short names have been proposed by Vogelsang 
to save periphrasis, until the true nature of the substance is ascertained. 
Viridite — green transparent or translucent patches, often in scaly or 
fibrous aggregations, of common occurrence in more or less decomposed 
rocks containing hornblende, augite, or olivine : probably in many oases 
serpentine, in others chlorite or delessite. Ferrite — ^yellowish, reddish, 
or brownish amorphous substances, probably consisting of peroxide of iron, 
either hydrous or anhydrous, but not certainly referable to any mineral, 
though sometimes pscudomorphous after ferruginous minerals. OpacUe 
— black, opaque grains and scales of amorphous earthy matter, which 
may in different cases be magnetite, or some other metallic oxide, earthy 
silicates, graphite, &c. ^ 

§ vi. — Classification of Rocks, 

It is evident that Lithology may be approached from two very 
different sides. We may, on the one hand, regard rocks as so many 
masses of mineral matter, presenting great variety of chemical com- 
position and marvellous diversity of microscopic structure. Or, on the 
other hand, passing from the details of their chemical and mineralogioal 
characters, wo may look at them as the records of ancient terres- 
trial changes. In the former aspect, they present for consideration 
problems of the highest interest in inorganic chemistry and mineralogy ; 
in the latter view, they invite attention to the great geological revolu- 
tions through which the planet has passed. It is evident, therefore, that 
two distinct systems of classification may be followed, the one based 
on chemical and mineralogioal, the other on geological considerations. 

From a chemical point of view, rocks may be grouped according to 
their composition ; as Oxides, exemplified by formations of quartz, 
heematite, or magnetite ; Carbonates, including the limestones and 

» Vogelsang:, Z. Deutsch Geol. Get. xxiv. (1872) p. 521). Zirkel, Geol. Expl, 4:0th 
rarallil, yoh vi. p. 12. 



Pabt n. § vij CLASSIFICATION OF ROCKS. 115 



cLty-ironstoneB ; Silicates, embracing the vast majority of rocks, 
whether composed of a single mineral, or of more than one ; Phosphates, 
such as gnano and the older bone-beds and coprolitic deposits. A 
classification of this kind, however, pays no regard to the mode of origin 
or conditions of occurrence of the rocks, and is not well suited for the 
purposes of the geologist.^ 

From the mineralogical side, rocks may bo classified with reference 
to their prevailing mineral constituent. Thus such subdivisions as 
Calcareous rocks, Quartzose rocks, Orthoclase rocks, Plagioclaso rocks, 
Pyroxenic rocks, Homblendic rocks, &c., may be adopted; but these 
terms are hardly less objectionable to the geologist, and are in fact 
Baited rather for the arrangement of hand-specimens in a museum, than 
for the investigation of rocks in situ. 

From the standpoint of geological inquiry, rocks have been classified 
according to their mode of origin. In one system they are arranged 
nnder three great divisions : 1st, Igneous^ embracing all which have been 
erupted from the heated interior of the earth ; 2nd, Aqueo^is or Sedtmen- 
inry, including all which have been laid down as mechanical or chemical 
deposits from water or air, and all which have resulted from the growth 
and decay of plants or animals; 3rd, Meiamorphic, those which haye 
undergone subsequent change within the crust of the earth, whereby 
their original character has been so modified as to bo sometimes quite 
indeterminable. Another geological arrangement is based upon the 
general structure of the rocks, and consists of two divisions, Ist, 
Siratifiedy embracing all the aqueous and sedimentary, with part of the 
less altered metamorphic rocks ; 2nd, Vmiratijied, nearly conterminous 
with the term igneous, since it includes all the eruptive rocks. Further 
subdivisions of this series have been proposed according to differences of 
structure or texture, as porphyritic, granitic, <fec. Those geological sub- 
divisions, however, ignore the chemical and mineralogical charticters of 
the rocks, and are based on deductions which may not always be sound. 
Thus, rocks may be included in the igneous series, which further research 
may show not to be of igneous origin ; others may be classed as meta- 
mori)hic, regarding the tnie origin of which there may be considerable 
uncertainty. 

A. further system of classification, based upon relative age, has been 

applied to the arrangement of the eruptive rocks, those masses which 

were erupted prior to Secondary time being classed as "older," and 

thone of Tertiary and later date as " younger." This system has recently 

>)een elaborated in great detail by Michel-Levy, who maintains that the 

^^nie types have been reproduced nearly in the same order in the two 

s^-ncH, though Iwisic rocks, often with vitreous characters, rather pre- 

' ^"^*^te in thelater.2 It must, indeed, be admitted that certain broad 

claARift^j. ®^P*^^e ^^^^ ^^^ fiuflccptililo of a convoniont and important chemical 

* SeT**" '^^^ ""'^ *"^ '^"^ (SCO p. 13(5). 
*''>»K-l!L** *'* '«'*'i<*^ •^- !>• Dana, Amer, J. ScL xvi. 1878, p. 33G. Compare also 
^"JfBuH. Soc. 0(vl. France, iii. 3rtl sor. p. 199 vi. p. 173. Foiique and Miclitl- 

I 2 



116 GEOGNOSY. [Book H. 

distinctions between the older and the later eruptive rocks have been 
well ascertained, and appear to hold generally over the world. Among 
these distinctions may be mentioned as characteristic of the palaeozoic 
rocks the presence of microcline, turbid orthoclase in Carlsbad twins, 
niuscovite, enstatite, bronzite, diallage, tourmaline, anatase, rutile, 
cordiorite, and in the younger rocks the presence of sanidino, tridymite, 
leucite, nosean, hauyne, and zeolites. Even where the same mineral 
occurs in both series, it often presents a somewhat different aspect in 
each, as in the case of the plagioclaso and augite, which in the younger 
series are distinguished Ijy the occurrence in them of vitreous and 
gaseous inclusions which are absent from those of the older series.^ 
Throughout the younger eruptive rocks, the vitreous condition is much 
more frequent and perfectly developed than in the older group, where, 
on the other hand, the granitic structure is characteristically displayed. 
Still, it may be doubted whether enough of positively ascertained data 
have been collected regarding the relative ages of eruptive rocks to 
warrant the adoption of any system of classification upon a chrono- 
logical basis. 

ITiough no classification which can at present be proposed is wholly 
satisfactory, one which shall do least violence, at once to geological and 
miueralogical relationships, is to be preferred. Avoiding, therefore, all 
theoretical considerations based on deductions (often erroneous) as to the 
origin or age of rocks, we may conveniently make use of the broad distinc- 
tion between Crystalline (including vitreous) and Clastic or Fragmental 
rocks. The former are, 1st, stratified, including chiefly chemical depoeits, 
sucli as limestones, dolomites, sinters, &c. ; 2nd, schistose, embracing 
most of the so-called metamorphic rocks ; 3rd, massive : this series is 
nearly coincident with the old division of Igneous Ilocks. The Clastic or 
Fragmental rocks are formed either of the debris of older rocks, or of the 
aggregated remains of plants or animals. In some cases, as for example, 
in limestones of organic origin, sul)sequent alteration gradually effaces 
the fragmental structure, and superinduces a true crystalline internal 
arrangement. Hence, along certain lines, fragmental rocks pass gradu- 
ally into the stratified crystalline series. 

It must be kept in view that in this proposed system of classification, 
and in the following detailed description of rocks, many questions 
regarding the origin and decomposition of these mineral masses must 
necessarily be alluded to. The student, however, will find these ques- 
tions discussed in later pages, and will probably recognise a distinct 
advantage in this unavoidable preliminary reference to them in connec- 
tion with the rocks by which they are suggested. 



lAwy^op.cit p. 150. Roscubutich, 'Mik. Phy«iog.* ii. Reyer,* PhyBik dor Eruptionon,' 1877, 




J. MiUTuy and A. Roiuird, Proc. Boy. Soc. Edin, xi. p. GOU. 



Part n. S vii.] STRATIFIED CRYfiTALLINE HOOKS. 117 



§ vii. — A Deseripilon of the moi-e Imporianl Bods of the 

EartKs Crust. 

Full detailH reganling tlio composition, microscopic structure, and 
other characters of rocks must he sought in such general treatises and 
R]>ecial memoirs as those already cited (pp. 91, 100). The purposes of 
the present text-hook will he served hy a succinct account of the moro 
common or important rocks which enter into the compositicm of the 
crust of the earth. 

A. Crystalline and Vitreous. 

1. Stratified. 

This division consists mainly of chemical deposits, but includes 
alno some which, originally formed of organic calcareous dohris, have 
acquired a crystalline structure. The rocks included in it occur as 
laniin«3 and beds, usually intercalated among clastic formations, such 
as sandstone and shale. Sometimes they attain a thickness of many 
thousand feet, with hardly any interstratification of mechanically 
derived sediment. They are being formed abundantly at the present 
time by mineral springs and on the floor of inland seas ; while on the 
bottom of lakes and of the main ooean^ calcareous organic accumulations 
are in progress, which will doubtless eventually ac^juire a thoroughly 
crystalline structure like that of many limestones. 

loe. — So large an area of the earth's surface is covered with ice, that this substance 
ileaerves notice among geological formations. Ice is commonly and conveniently 
clarified in two divisions, snow -ice and water-ioe, according as it results from the 
r«-,mpre«Bion and altemnto melting and freezing of fallen snow, or from the freezing 
r>f the surface or bottom of sheets of water. 

Bnow-ice (see Book III. Part II. Sect. ii. § 5) is of two kindri. Ist, Fallen snow 
on mountain slopes above the snow-lino gradually assumes a granular structure. The 
little crystalline neecUes and stars of ice are melted and frozen into rounded grannies 
vhirh form a more or less compact moss known in Switzerland as Neve or Fim. 
2ndf When the granular ne've slowly slides down into the valleys, it acquires a more 
compact crystalline structure and becomes g1acier-irt\ According to the researches of 
V. Klocke, gkcier-ice is, throughout its mass, an irregular aggregate of distinct crystalline 
gTAiiw, the bomidarics of which form the minute capillary fissures so often described. ' Its 
•tmctore thiu ckMely corresponds to that of marble (p. 110). Glacier-ice in small 
fiBgiiients ifl white or colourless, and oft£*u shows innumerable fine bubbles of air, some- 
time aln fine particles of mud. In larger masses, it has a blue or green-blue tint, and 
displays n vc^ineil structure, consisting of parallel vertical veinings of white ice full o( 
*iir-bu>jbli ^. ami of blue clear ice without air-bubbles. Snow-ice is formed alK)vn the 
-!f'A- liii". but may descend in glaciers far below it. It covers large areas of the more 
I'lfiy luountainii of the globe, even in tropical regions. Towards the ])oles it dcscendrt 
tu tkui aen-Uxtlf whete laige pieces of it break off and float away as icebergs. 

Water-ice (tee Book IIL Part II. Sect. li. § 5) is formed, Ist, by the froczin«r of 
iUf vurfure of freshwater (river-ioe, lake-ice), or of the sea (ice- foot, flrx^ioe, ]Mck-i(X'. ; 

- i' a rompaett elear, white or greenish ice. 2nd, by the freezing of the layer of wuter 
■ : .r .,11 iiie bottom of riven, or the sea (bottom-ice, ground-io^', anehor-ie/) ; tiiir 
' li ■ i> is more spongy, and often encloees mud, sand and stones. 

» NtuesJaJtrb. 1881 (i.) p. 2X 



118 GEOONOSY. [Book H. 



Book-Salt (Sel gemme, Steinsalz) oocurs in layers or beds from less than an 
inch to many hundred feet in thickness. The salt deposits at Stassfort, for 
example, are 1197 feet thick, of which the lowest beds comprise 685 feet of pure rook 
salt, with thin layers of anhydrite }-inch thick dividing the salt at intervals of from 
one to eight inches. Still more massive are the accumulations of Sperenherg near 
Berlin, which have been bored through to a depth of 4200 feet, and those of Wicliczka 
in Gallicia which are hero and there more than 4600 feet thick. 

The more insoluble salts are apt to appear in the lower parts of a salifcrous series and 
to disappear towards the top. When purest, rock salt is clear and colourless, but usually 
is coloured red (peroxide of iron), sometimes green, or blue (chloride or silicate of copper). 
It varies in structure, being sometimes beautifully crystalline and giving a cubical 
cleavage ; laminated, granular, or less frequently fibrous. It usually contains some 
admixture of clay, sand, anhydrite, bitumen, &c, and is oAen mixed with chlorides of 
magnesium, calcium, &c. In some places it is full of vesicles (not infrequently of 
cubic form) containing saline water; or it abounds with minute cavities filled with 
hydrogen, nitrogen, carbon-dioxide, or with some hydrocarbon gas. Occasionally remains 
of minute forms of vegetable and animal life, bituminous wood, corals, shells, 
crustaceans, and fish teeth are met with in it. Owing to its ready solubility, it is not 
found at the surface in moist climates. It has been formed by the evaporation of very 
saline water in enclosed basins — a process going on now in many salt-lakes (Great 
Salt Lake of Utah, Dead Sea), and on the surface of some deserts (Kirgis Steppe). 
In different parts of the world, deposits of salt have probably always been in progress 
from very early geological times. Saliferous formations of Tertiary and Secondary 
age are abundant in Europe, while in America they occur even in rocks as old 
as the Upper Silurian period, and among the Punjab Hills in still more ancient strata.^ 
Iitmestone (Galcaire, Kalkstein),— essentially a mass of calcium-carbonato, some- 
times nearly pure, and entirely or almost entirely soluble in hydrochloric acid, somo- 
times loaded with sand, clay, or other intermixture. Few rocks vary more in 
texture and composition. It may be a hard, flinty, close-grained mass, breaking 
with a splintery or conchoidal fracture ; or a crystalline rock built up of fine crystalline 
grains of calcite, and resembling loaf sugar in colour and texture ; or a dull earthy 
friable chalk-like deposit; or a compact, massive, finely-granular rock resembling a 
close-grained sandstone or freestone. The colours, too, vary extensively, the most 
common being shades of bluc-groy and cream-colour passing into white. Some lime- 
stones are highly siliceous, the calcareous matter having been accompanied ¥rith silica 
in the act of deposition; others are argillaceous, sandy, ferruginous, dolomitic, or 
bituminous. By far the larger number of limestones are of organic origin; though 
owing to internal re-arrangcmcnt, their original clastic character has frequently been 
changed into a crystalline one. Under the present subdivision are placed all those lime- 
stones whicli have had a distinctly chemical origin, and also those which though doubt- 
less, in many cases, originally formed of organic di^bris, have lost their fragmental, and 
have assumed instead a crystalline structure. (For the organic limestones see p. 167.) 

Compact, common limeston e, — a fine-grained crystalline-granular aggre- 
gate, occnrring in lK?ds or laminaj intcrstratified with other aqueous deposits. When 
purest it is readily soluble in acid with effervescence, leaving little or no residue. 
ISIaiiy variotiea occur, to some of which separate names are given. Hydraulic Umestone 
contains 10 per coat, or more of silica (and usually alumina) and, when burnt and 
HubsiMinrntly luixod with water, forms a cement or mortar, which has the pro}ierty of 
".sitting" or hardening under water. Limestones containing perhaps as much as 
25 jRT cent, of silica, alumina, iron, &c., which in themselves would be imsuitable for 
many of the ordinary purpmscs for which limestones are used, can be employed for making 



• On wilt dei>u8its of various ages, see A. C. Ramsay, BriL Assoc. Hep. 1880, p. 10; 

alho Index, ml voc. *' Salt DcposiUj." 



ik 



Pabt IL § viL] STRATIFIED CRYSTALLINE ROCKS. 119 



hydniilio mortar. Theso limestones occur in bods liko those in tlio Lias of Lyme 
Regis* or in nodules liko those of Shoppcy, from which Roman cement is made. 
CemenUUme is the name given to many pale dull ferruginous limestones, which 
contain an admixture of olay, and some of which can be profitably used for makiug 
hydranlic mortar or cement. Feiid limestone (ttinhiiein^ smnestcne) gives off a fetid 
sntell (sulphuretted hydrogen gas), when struck with a hammer. In some cases, the 
rock seoms to have been deposited by volcanic springs containing decomposable 
sulphides as well as lime. In other instances, the odour may be connected witli the 
decomposition of imbedded organic matter. In some quarries in the Carboniferous 
Limestone of Ireland, as mentioned by Jukes, the frcshly-brokcu rock may Ijo 
smelt at a distance of a hundred yards when the men arc at work, and occasionally 
the stench becomes so strong that the workmen arc sickened by it, and require to 
k-ave off work for a time. Cornsione is an arenaceous or siliceous limcstono 
particularly characteristic of some of the Palieozoic red sandstouo formations. Rotten- 
done is a decomposed siliceous limestone from which most or all of the lime has 
liecn removed, leaving a siliceous skeleton of the rock. A similar decomposition hikes 
place in some ferruginous limestones, with the result of leaving a yellow skeleton of 
ochre. Common limestone, having been deposited in water usually containing other 
■abiftances in suspension or solution, is almost always mixed with impurities, and whore 
the mixture is sufficiently distinct it receives a special name, such as siliceous lime- 
stone, sandy limestone, argillaceous limestone, bituminous limestone, dolomitic limestone. 
Travertine (calcareous tufa, calc-sinter) is the porous material deposited by cal- 
careous springs, usually white or yellowish, varying in texture from a soft chalk-like 
or marly substance to a compact building-stone. (See Book III. Part II. Sect. iii. § 3, 5.) 
SUU4ietiU is the name given to tlie calcareous pendant deposit formed on the roofs of 
Hmestoiic-cavems, vaults, bridges, &c. ; while the water, from which the hanging lime- 
icicles are derived, drips to the floor, and on further evaporation there, gives rise to the 
Grosi-liko deposit known as stalagmite, Mr. Sorby has shown that in the calcareous 
df'posits from fresh water there is a constant tendency towards the production of calcito 
crystals with the principal axis perpendicular to the surface of deposit. Where that 
rarfiioc is curved, there is a radiation or convergence of the fibre-like crystals. This is 
well seen in sections of stalactites and of some calcareous tufas (Fig. 100). 

Oolite, — a linjcstone formed wholly or in part of more or less perfectly spherical 
icraius, and having somewhat the aspect of fish-roe. Each grain consists of successive 
couccntric shells of carbonate of lime, frequently with an internal radiating fibrous 
itracture, and was formed round some minute particle of sand or other foreign body which 
f&s kept in motion, so that all sides could in turn become encrusted. Oolitic grains of 
this cluiracter are now forming in the springs of Carlsbad (Sprudclstein) ; but they may 
00 doubt also be produced where gentle currents in lakes, or in partially enclosed areas of 
the sea, keep grains of sand or fragments of shells drifting along in water, which is so 
cliarged with lime as to be ready to deposit it ui)on any suitable surface. An oolitic 
limestone may contain much impurity. Where the calcareous granules are cemented in 
» tumewhat argillaceous matrix the rock is known in Germany as Rogenstoin. Where 
the individual grains of an oolitic limestone are as large as peas, the rock is called a 
pisolite. The granules sometimes consist of aragonite. Oolitic structure is found 
in limestones of all ages from Pala)ozoic down to recent times. 

Marble (granular limestone), — a crystalline-granular aggregate composed of 
''nAUilline calcite-granulcs of remarkably unifonn size, each of which has its own 
iiHlepeudent twin lamellffl (often giving interference colours) and cleavage lines. This 
cUritctcristic stnicture is well displayed when a thin slice of ordinary statuary marble 
ijsplarwl und(*r the microscope (Fig. 10). Typical marble is white, but the rock is also 
Jillow, grey, blue, green, red, black, or streaked and mottled, as may be seen in the 
mmuroiis kinds used for ornamental purposes. Its granular btructuro giv(*s it a roscm- 
^Unce to loaf-sugar, whence the term ** saccharoid " applied to it. Fine silvery scales 
"f laica or talc may often be noticed even in the purest marble. Some crystalline lime- 



rT51&;.L!T*lilllillC. 




•' ■ ■■■' ' >:■■' i. J M' li.-i iU- '..Milii.r) f r..^ i.:X-^^i-.^ ^:i-i.-- 

'■■ ■ I''l M ■ii.,-...flli. K...l.rnM|.. 

"h-"""! ■ 'I il.h I i..|,.,..|. *. I i m.r.u. or *ii-..TT ^.c^^^:■..^■ ■■! 

*'"■'■'"■■"' ■ 'I ' ■" |. .■.r.i..-..i I -. 'Hli.1. f..ri'^r:.ifl.-i«:;Lii.ii.:.iI. 

•'*""'"■■' ' '■ "■■ '■■■■■ • ■li'< ,li.ii..,-....J..1.l.. tr-.ti. ![[„.,)..:,(.. iri.i.-l, it 
V>'<i4>'l>'W^ttt ,, , „, „,,i, ,. , ,., ..,.|.,„r„Uwy.rin..«ii 1.j;.n 



thv v 



■ /■.. 



Part II. § viij STIfATJFIED CRTSTALLIXK ROCKS. 121 



a«lmixtiire of clay or bitameii, or yellow and red by k'inp: Riuinc<l with iron-oxide. It 
occum in bedft, lenticalar intercnlationH and Btrinrr», nnuidly nHmioiatcfl with beds of rod 
clay, rook-tfalt, or anhydrite, in forinationfl of nmny variouH pfoological periods from tlie 
Silurian (New York) down to recent tiniCM. The Triosaic fi^ypHnni depoHits of Tlmrinj^io, 
Hanover and the Hans have \<mf^ been fainoas. One of them runH along the south flank 
of the Han Mountains as a great band six miles long nnd reaching a height of some- 
times 430 feet. 

Gypsum fnroishes a good illustration of the many different ways in which somo 
minend sabstanoes can originate. Thus it may be produce<l, Ist, as a chemical 
precipitate ftom solution in water, as when sea-water is evaporated ; 2nd, tlirough the 
decomposition of sulphides and the action of the resultant sulphuric acid upon lime- 
stone ; Sid, through the mutual decomposition of carbonate of lime and sulphates of iron, 
copper, magnesia, &c ; 4th, through the hydration of anhydrite ; 5th, through the action 
of the sulphurous vapours and solutions of volcanic orifices upon limestone and cal- 
careous rocks.* It is in the first of these ways that the thick beds of gypsum associated 
with rock-salt in many geological formations have been formed. The first minerul to 
appear in the evaporation of sea-water being gypsum, it has been precipitated on the 
floors of inland seas and saline lakes before the more soluble salts. 

Anhydrite, — the anhydrous variety of calcium-sulphate, occurs as a compact or 
Iptmnlar, white, grey, bluish or reddish aggregate in saliferous deposits. It is less 
frequent than gypsum, from which it is distinguished by its much greater hardness 
(3-8*5) aad into which it readily passes by taking up 0*2625 of its weight of water.' It 
often occurs in thin seams or partings in rock-salt; but it also forms large hill-like 
maasea, of which the external parts have been converted into gypsum. 

Ironstone. — Under this general term are included various iron-ores in which 
the peroxide, protoxide, carbonate, &c., are mingled with clay and other impurities. 
They have generally been deposited as chemical precipitates on the bottoms of lakes, 
under marshy gromid, or within fissures and cavities of rooks. Some of the iron-ores 
are aModated with the schistose rocks ; others are found with sandstones, shales, lime- 
stones and coals ; while some occur in the form of mineral veins. Those which have 
resulted from the co-operation of organic agencies arc described at p. 174. 

Hiematite (red iron-ore), a compact, fine-grained, earthy, or fibrous rock of a 
blood-red to brown-red colour, but where most crystalline, steel-grey and splendent, 
with a distinct cherry-red streak. Consists of aniiydrous ferric oxide, but usually is 
n)ixe<l with clay, sand, or otlier ingredient, in such varying proportions as to pass, l)y 
insensible gradations, into ferruginous clays, sands, quartz, or jasper. Occurs as IhxIh, 
hugv* concretionary masses, and veins traversing crystalline rocks; sonietimes, as in 
W(.';ftmoreland, filling up cavernous spaces in limestone. Is found occasionally in l)ed8 
of an wditic structure among stratified formations. 

L i ni o n 1 1 e (brown iron-ore), an earthy or ochreous, compact, fino-grained or fibrous 
rock, of an ochre-yellow to a dark-brown colour, distinguishable from haematite by being 
hy«lioufl and giving a yellow strt'ak. Occurs in beds and veins, sometimes as the result 
of the oxidation of ferrous carlx)nate ; abundant on the fioors of some lakes ; commonly 
fnund under marshy soil, where it forms a hard brown crust upon the impervious subsoil 
•9ffitj irftn-itre). Found likewist^ in oolitic concretions sometimes as largo as walnntn, 
niiiiiisting of concentric layers of impure limonite with sand and clay {Bohnerz). See 
p. 175 and Book III. Tart II. Sect iii. 

8pathicIron-ore, a coarse or fine crystalline aggregate of the mineral sidcrito or 
ferrous carbonate, usually with carbonates of calcium, manganese and magnesium ; has 
A prevalent yellowish or brownish colour, and when fresh, its rhombohedral cleavage- 
faces show a pearly lustre, which soon disappears as the surface is oxidised into limonite. 

* Roth. CUin, Gaol. i. p. 553. 

• See G. Rose on formation of this rock in presence of a solution of chloride of 
Nidium. Netten JaJuh. Min. 1871, p. 932. Also Bischof, ' Chem. und Phys. («eol.* Suppl. 
(l«71)p. 188. 



122 GEOGNOSY. [Book IL 



Occurs iu beds and veins, especially among older geological formations. The colossal 
Erzberg at Kiseuerz in Styritv, wliich rises more tlian 2700 feet aboTO the valley, consists 
almost wholly of siderito.* 

Clay-ironstone (Sphaerosiderite), a dull l>rown or black, compact form of siderito, 
with a variable mixture of clay, and usually also ef organic matter. Occurs in the 
Carboniferous and other formations, in the form either of nodules, where it has usually 
been deposited round some organic centre, or of beds interstratified with shales and coals. 
It is more properly described at p. 175, with the organically derived rocks. 

Magnetic iron-ore, a granular to compact aggregate of magnetite, of a black 
colour and streak, more or less perfect metallic lustre, and strong magnetism. Commonly 
contains admixtures of other minerals, notivbly of liAmatite, chrome-iron, titanic-irou, 
pyrites, chlorite, quartz, hornblende, garnet, epidoto, felspar. Occurs in beds and 
enormous lenticular masses (Stockc) among crystalline schists. Thus among the 
Scandinavian gneisses lies the iron mountain of Gellivara in Lulea-Lappmark, 17,000 
feet long, 8500 feet broad, and 525 feet high. 

Siliceous Sinter (Geyserite, Kieselsinter), the siliceous deposit ma<le by hot springs, 
including varieties that are crumbling and earthy, compact and flinty, finely laminated 
and shaly, sometimes dull and opaque, sometimes translucent, with pearly or waxy 
lustre. The deposit may occur as an incrustation round the orifices of eruption, rising 
into dome-shaped, botryoidal, coralloid, or colmunar elevations, or investing leaves and 
stems of plants, shells, insects, &c., or hanging in pendent stalactites from cavernous 
sptices which are from time to time reached by the hot water. When purest, it is of 
snowy whiteness, but is often tinted yellow or flesh colour. It consists of silica 81 to 
01 per cent., with small proportions of alumina, ferric oxide, lime, magnesia, and alkali, 
and from 5 to 8 per cent, of water. 

Flint (Silex, Feuerstein), — ^a grey or black, excessively compact rock, with the hard- 
ni58s of quartz and a perfect conchoidal fracture, its splinters being translucent on 
the edges. Consists of an intimate mixture of crystalline insoluble silica and of 
umorphous silica soluble in caustic potass. Its dark colour, which can be destroyed by 
heat, arises chiefly from the presence of carbonaceous matter. Flint occurs principally 
as nodules, dispersed in layers through the upper chalk of England and the north-west 
of Europe. It frequently encloses organisms such as sponges, echini and braohiopods. 
It bus been deposited from sea- water, at first through organic agency, and subsequently 
by direct chemical precipitation round the already deposited silica. (Book III. Part IL 
Si'ct. iii.) Chert (phtanite) is a name applied to impure calcareous varieties of flint, 
in layers and nodules which are found among the palieozoio and later limestones. 
Horn stone, an excessively compact siliceous rode, usually of some dull dark tint, 
occurs in nodular masses or irregular bands and veins. Vein-Quartz may be 
alluded to here as a substance which sometimes occurs in large masses. It is a massive 
form of quartz found filling veins (sometimes many yards broad) in crystalline and 
clastic rocks ; more especially in metamorphio areas. (See Quartz-Bocks, p. 127, and 
Felsitoid-liocks, p. 130). 

i$ome of the other varieties of silica occurring in large masses may be classed as rooks. 
Such lire joaper, and ferruginous quartz. These, as well as common vein-quartz, occur as 
veins traversing both stratified and unstratified rocks ; also as beds associated with the 
crystalline schists. With them may be grouped Lydian-Stone (Lydite, KieseUehiefer), a 
blftck or dark-coloured, excessively compact, hard, infusible rock, with splintery fracture, 
occurring in thin, sharply defined bands, split by cross joints into polygonal fragments, 
whicli are sometimes cemented by fine layers of quartz. It consists of an intimate 
mixture of silica with alumina, c^urbonaceous materials, and oxide of iron, and under the 
microscope shows minute'quartz-granulcs with dark amorphous matter. It occurs in thin 
layers or bands'iu the Silurian and later PuliDozoio formations interstratified with ordinary 
sandy and argillaceous strata. As these rocks have not been materially altered, the bands 

* Ziikel, Lthrb, i. p. 345. 



Paw II. $ rti.] BOniSTOSE OSYSTALLINE BOCKS. 123 

of Lydian'stoDe may be of origiiiEJ formation, Ihough tlie citont to irbicli tboy arc oHen 
veined with qnutz ahows that thoy have, in luany c&sos Ijcen pcnueuteil by nliccuus 
watei liDOa tbeir deposit Tbe siliccouB rocks due to tlio ojicriitions of plant ond 
animal life are dsKribod on p. 169, also iu Book III. Tart II. Hcct. iii. § 3. 

Hnne originally clastic siliceonB rockd hare ooquited a inoro or lees cryslalUiie 
atmetuie from the action of thermal wnln or otlicmise. One of the mont marked 
varietica has been termed CrytlulUtte Sandilone (sec p. 162). Anotlicr variety, known 
as Qaarlnte, ia a granular and compact aggrogete of iiuartz, nliicli nill bo deBcribud in 
connection with the BChutoeo rocka among wUicb it generally ocoura (p. 128). 

2. Sohiatoae or Foliated. 
The Cryetallino Schists form a remarkably well-dofiuod aories of rocks. 
They are mainly composed ef silicates. Their structure ia cryBtallino, 




Fls-l''— PioUeoF 



l)ut is diBtinguished from that of the Massive roclca by its more or less 
closely parallel layers or folia, couBistin)^ of mat«rialB which have aasilnicd 
a crystalline character along these layers. The folia may lie composed 
of only one mineral, but usually consist of two or more, which occur 
either in distinct, often alternate lamina;, or intermingled in the same 
layer. In some respects, this stnicturo resembles that of the stratified 
rocks, l>nt is differentiated (1 ) by a prevalent striking want of continuity 
in Ihu folia which, nsn rule, arc conspieuimsly lenticular, thickening out 
and then dying away, and reappearing aftor an interval on the samo or 
a difleront piano (Fig, 17) ; (2) by a (Kjculiar and very chiiract«ristic 



124 



GEoamonr. 



weldiug of the fi>lia into each other, the crj'Htalline particles of one laj 
bL'in)<; Ro iiitvniinigltHi with tUnno of the layors a1)ove and helon- it tl 
tho niiolo cohercH qb a tuugL, not easily liMiile mass ; (3) by a frequE 
remarkable ami eminently distinctive puckering or crumpling (wi 
frequent minute faulting) of the folia which becomes sometimea so fi 
as to be discernible only under the miorosGope ' (Fig. 1(>), but is oft 
prcBont conspicuously in hand-specimens (Fig. 18), and can bo trac 
in increaaing diiuenaions, till it connects itself with gigantic curvatm 
of the strata, which embrace whole mountains in their sweep. Tfat 




KLg. la— Vlnrofsliand-tpKli 



characters are sufficient to indicate a great difference between e 
riicks and ordimtry stratified fomiatioua, in which the strata lie 
tinuous flat, pamllcl, and more or less easily separal3]e layers. 

A rock poHBCBsing tliis cryBtalline arrangement into tepMral 
in English termed a "schist."* Tfais word, tlunigli oopk 



(Jro/. Soe. sxxii. p. 407. &itbj, 'op. ctt. xxzTJ. p. 8_, ... _ , 

r <I. £iitBtt.'hDiig d. Altkr^it. BehieCn,' Bonn, IBU; and otbec n 



BcWMa and to shales. In Qmnan alw 
ncliiots, bat is also emplojed fbr 
schieFtiitbon = iliale. 




Part H. § vii.] SCHISTOSE CRYSTALLINE BOCKS, 125 



general designation to descrilx) the stinicturo of all tnily foliated rocks, 
18 also made nse of as a Huffix to the names of the minerals of which 
some of the foliated rocks largely consist. Thus wo have " mica-schist," 
'* chlorite-schist," "hornblende-schist." If the mass loses its fissile 
tendency, owing to the felting together of the component mineral into a 
tough coherent whole, the word rock is usually suKstituted for schist, as 
in '* hornblende-rock," " actinolite-rock," and so on. The student must 
bear in mind that while the possession of a foliated structure is the 
distinctive cliaiacter of the crystalline sc^hists, it is not always present 
in every individual bed or mass associated with these rcKjks. Yet the 
non-schistose portions are so obviously integral })arts of the schistose 
series that they cannot, without great violation of natural affinities, be 
«je]>arHtcd from them. Hence in the following enumeration, they are 
included as common accom])animents of the schists. For the same reason, 
quartzite is placed in this subdivision, though in its typical condition 
it shows no schistose structure. 

The origin of the crystalline schists has lxH>n the subject of long 
discussion among geologists. Werner held that, like other nK^ks of 
liigh antiquity, they were chemical precipitates from a universal ocean. 
Hut ton and his followers maintaineil that they were mechanical aqueous 
sediments altered by subterranean heat. These two doctrines, in various 
modifications, are still maintained by op|)Osite schools. Some schists are 
undoubtedly altered sedimentary rock«, and may i)roperly be termed 
"metamorphi c." AVliether this has also been the origin of certain 
ancient gneisses and schists underlying the oldest fossiliferous forma- 
tions is less easily determined. (See Book IV. Sect, viii.) 

Some minerals are specially characteristic of the schists. Among 
these, reference may more particularly l)e made to those which have 
crystiiUised in a r<K'k originally composed of mere clastic detritus, 
such as shale or slate, and which therefore illustrate the process of 
metamorphism. Such are chiastolite, andalusite, sUiurolitc, garnet, 
vesuvianite, epidote, tourmaline, nitile, kc. 

In the following enumeration the rocks are arranged according to 
their chief constituents. It will be understood, however, that in almost 
all cases, other minerals are mingled witli them in varying proportions. 

1. AmiiLLiTEH — Clay-slate, argiUaceous-Bchist. (Pliyllitc, Phyllade, Schiste 
ordoise, Thonschiefer, Tlionglimmerschiefer). Under thcBe names are included a^rtain 
hard fissile argillaceous masses, composed primarily of compact clny, with macroscopic 
and mioToeoopic scales of one or more micaceous minerals, granules of (|uartz and cubes 
or concretions of pyrites, as well as veins of quartz and calcite. Tlie fissile struc- 
ture is specially characteristic. In some cases this structure is merely thut of original 
deposit, as is proved by the alternation of fissile bods with bands of hardened sandstone, 
conglomerate or foesilferous limestone. Such arc tlic argillaceous schists of the Scottish 
Highlands. But in certain regions, where the rocks have been much compressed, the 
fissile structure of the argillaceous bauds is indepeudent of stratification, and can be seen 
traversing it. Sorby has shown tliat this superinduced fissility or '* cleavage ** has resulted 
from an internal rearrangement of the particles in planes perpendicular to the direction 
in which the rocks have been compressed (See Book II. Section iv. § iii). In England 
the tenn " slate '* or ** clay-slato " is given to argillaceous, not obviously crystalline 



126 GEOGNOSY. [Book H. 

rocks possessing this cloayage-stnioiurc. Those where the fissility corresponds with 
the original sedimentation may ho called argillaceous schists. Where the micaoeoos 
lustre of the fiuoly disseminated superinduced mica is prominent, the rocks are phyllites. 

Microscopic examination shows that while some argillaceous rocks consist mainly 
of granular debris, many cleaved clay-slatos contaiu a largo proportion of a micaoeons 
mineral in extremely minute flakes, wliich in the best Welsh slates have an average siie 
of 3^ of an inch in breadth, and ^ of an inch in thickness, together with very fine 
black hairs which may bo magnetite.* Moreover, many clay-slates, though to outwaid 
appearance thoroughly noncrystalline, and evidently of fi-agmental composition and sedi- 
mentary origin, yet contain, sometimes in remarkable abundance, microscopic mioroliibs 
and crystals of different minerals placed with their long axes pckrallel with the pianos 
of fissility. These minute bodies consist of yellowish-brown needles of rutile, greenish or 
yellowish flakes of mica, scales of calcite, and probably other minerals.* Small grannies 
of quartz containing fluid-cavities, show on their surfaces a distinct blending with the 
substance of the surrounding rock. M. Renard has found that the Belgian whet-slate 
is full of minute crystals of garnet.' Some of the more crystalline varieties (phyllite)arc 
almost wholly composed of minute crystalline particles of mica, quartz, felspar, chlorite, 
and rutile, and form an intermediate stage between ordinary olay-slate and mica-schist 

A distinction has been drawn by some petrographers between certain rocks (phyllite, 
urthonschiefer) which occur in Archaean regions or in groups probably of high antiquity, 
and others (ardoise, thonschiefer) which are found in Palaeozoic and later formations. But 
there does not ap])ear to be adequate justification for this grouping, which has probably 
been suggested rather by theoretical exigencies than by any essential differences between 
the rocks themselves. That the whole of the series of argillaceous rocks, beginning with 
clay and passing through shalo into slate, argillaceous schist and phyllite, is of sedimen- 
tary origin is indicated by the organic remains, false bedding, ripple-mark, &c., found in 
those at one end of the series, and by the insensible gradation of the mineralogical cha- 
racters through increasing stages of metamorphism to the other end. Some microecopio 
crystals may poisibly have been originally formed among the muddy sediment on the sea- 
floor. But more probably they have been subsequently developed within the rook, and repre- 
sent early stages of the process which has culminated in the production of crystallino 
schists. The development of crystals of chiastolite and other minerals in clay-slate is 
frequently to be observed round bosses of granite, as one of the phases of contact-mota- 
morphism. (See Book IV. Part VIII.) 

A number of varieties of Clay-slate are recognised. Roofing slate (Dachschiefer) 
includes the finest, most compact, homogeneous and durable kinds, suitable for roofing 
houses or the manufacture of tables, ohimnoy-pieces, writing- slates, &c. ; it occurs in the 
Silurian and Devonian formations of Central and Western £urope. Anthraoitic- 
slate, (anthracite-phyllite, alum-slate) dark carbonaceous slate with much Iron 
disulphide. Bands of this nature sometimes run through a clay-slate region. Tho 
carbonaceous material arises from tho alteration of the remains of plants (fucoids) or 
animals (frequently graptolites). The morcasite so abundantly associated with these 
organisms decomposes on exposure, and the sulphuric acid produced, uniting with tho 
alumina, potash, and other bases of tho suirounding rocks, gives rise to an efflorescence 
of alum, or the decomposition produces sulphurous springs like those of Moffat. The 
name Greywacke-slato has been applied to extremely fine-grained, hard, shaly, more 
or loss micaceous and sandy bands, associated with grey wacke among the older Palieozoio 

* Sorby, Q. J. Oe/)l. Soc. xxvi. p. 68. 

* These "clay-slato needles" were not crystallized contemporaneously with the 
deposit of the original rock, but have been developed by subsequent actions. They 
indicate one of the early pliasos of metamorphism rSee Book R^ Part iii.) For their 
character sec Zirkel, *Mik. Be»chaff.' p. 490. Kalkowsky, N. Jahrh. 1879 p, 382; 
A. Cathrein, op. cit. 1882 (i) p. 169. F. Peiick. Sitzh. liuyir. Ahad, Math, Vhyt^. 1880. 
p. 461. A. Wichmaun, Q. J. Of oh Sor. xxxv. j). ir)6. 

» Acad. Itoy. Jidgitjur, xli. (1877). 



Pabt IL I viL] 8CEI8T0SE CRYSTALLINE ROCKS. 127 



toAa, Whet-fllato, Novaculite, Hone-stone, is an exceedingly hard fine- 
grained siliceons rock, some varieties of which derive their economic valae from the 
presence of microscopic crystals of garnet. Chiastolite-slato (schiste macl6), a 
clay-slate in which crystals of chiastolite have been developed, oven sometimes side 1)y side 
with still distinctly preserved graptolites or other organic remains ; ^ (Skiddnw, Abc^rdecn- 
shire, Brittany, tiie Pyrenees, Saxony, Norway, Massachusetts, &c.) Staurolitc- 
8 late, a micaceous clay-slate with crystals of staurolifo (Banfifshire, Pyrenees). 
Ottrolite-slate.a day-slate marked by minute, six-sided, greyish or blackish green 
lamelUo of ottrclite (Ardennes, where it is said to contain remains of trilobites, Bavaria, 
Kew England). Dipyre-slate is full of small crystals of dipyro. Sericito- 
phylliteisa name proposed by Lossen for those compact, greenish, reddish or violet 
seridte-flohists in which the naked eye can no longer distingaish the component minerals. 
Hioa-phyllite (phyUade gris feuillete of Dumont) a silky, usually very fissile slate, 
with minute scales of mica. German petrographers have distinguished by name some 
other varieties found in metamorphic areas and characterised by different kinds of 
concretions, but to which no special designations have been given in English. 
KnotenBchicfor (Knotted schist) contains little knots or concretions of a dark- 
^rcen or brown, fine-granular, faintly glimmering substance, of a talcosc or mica- 
ceous nature, imbedded in a finely laminated matrix of a talc-like or mica-liko mineral.' 
In Fruohtschiefer these concretions are like grains of com; in Garben- 
flchicfer, like caraway seeds; inFleckschiefer, like flecks or spots. Some of those 
locks might be included with the mica-schists, into varieties of which they seem to pass. 
Hoand some of the eruptive diabase of the Harz, the clay-slates have been altered 
into various crystalline masses, to which names have been attached. Thus Spilosite 
is a g^reenish, schistose rock, composed of finely granular or compact felspathic material, 
'With small chlorite concretions or scales. Desmositeisa schistose mass in which 
Himilar materials are disposed in more distinct alternations.' 

2. QuABTZ ROCKS.* Qoajrtz-scliist (schistose quarizite) an aggregate of granular 
<iuartz with a sufficient development of iiuo folia of mica to impart a more or Iors 
definitely schistose structure to the rock. The disappearance of the mica gives quartzito, 
stud the greater prominence of this mineral affords gradations into mica-schist. Such 
gradations are quite analogous to those among recent sedimentary materials, from pure 
sand, through muddy sand, and sandy mud, into mud or cluy, and between sandstones 
find shales. The Highlands of Scotland, for instance, embrace large tracts of quartz- 
schists — rocks which are not properly cither mica-schist or ordinary quartzito. 
Consisting of granular quartz, with fine parallel lamina) of mica, and capable of 
being split into thick or thin flagstones, they were evidently at first stmdstones, witli 
interleaved seams of fine mud. The sand has been converted into quartzito, and the 
argillaceous layers into various micaceous minerals. Endless varieties in the relntivc 
proportions of these ingredients may be obser^'ed. Interstratified pebbly varieties ooonr. 

Itacolumit e — a schistose quartzito, in which the quartz-gnmules are separated by 
fine scales of mica, talc, chlorite, and serieite. Occasionally these pliable scales are so 
arranged as to give a ceriain fiexibility to the stone (flexible sandstone). This ruck 
occurs in tho south-eastern states of North America, also in Brazil, as the matrix in 
which diamonds are found. 

Hiliceous schist (Lydian stone, Lydite, Kiosolschiefer), has already been 



* A good illustration of this association is figured by Kjerulf in his ^ Goologie des 
dlichen und Mittleron Norwegen,* Piute xiv. fig. 240. Sec also Broggcr*H memoir on 
'pper Silurian fosaili* among the crystalline rocks of Bcrgeu. Cliristiania, 1882. 

P 




]>mMt. Geo. Get. xix. (1807) p. 500. xxi. p. 21)1. xxiv. p. 701. Kaysor, op. cit. xxii. p. J 03. 
* J. Maccullo<«h, Trnnt. Gtol. Sor. Ist. sor. ii. (ISII), p. 4r)0, iv. (1817). p. 204 ; 
2im1 8or. i. (181I>), p. 53. Lohsom, Zeitxrh. Dviihrh. (Uol. Gen. xix. (18G7), pp. G15— <;,H1. 



128 QEOaNOSY. EBooit II. 

doMribed (p. 122) mnong tho atnitified racks; but it ma; be enamcmted aim hero u 
occurring likeviio in bands among' tbo crjatalline achiata. 

QuarUlte (QuartZ'rock), tbougli not praperl; a icbiitoee rock, muy be moitt eon- 
ven[cnt!y coueidered here, as it iaao constEuitsu acoompantment oFthe Hcliisls, and, like 
them, can often bo directly traced to tbc alteration of former sedimcntery fomutiont. 
It ia a gmnnlar to compact mass of quartz, geuerallj wliiCe, sometimes yellow or red, 
with a cbaractoriatic liutrona fracture. It occurs in tbia and tbick beds in asiocUtkin 
trith Bchistii, Bouetimee in continuoaa luaagca seycrol thousand feet thick. Id SootUud 
it forma rangca uf mountaiuH, and is there frequently Hooompanied with Huboidinate 
beds of limodtone, wliicb in SallicrlaudBhire contain Lower Silurian faesiJB.' 

Even to the naked oye, the finely granular or areneceoiiB Btractnre of quartiite is 
diatiuctly Tiaiblo. MicroBcopic ciiuuination ahowa thia atmcturc itill more clearty, Euid 
IcavoB no doubt that the rock origiaally conBieted of a tolerably pnre quarti-und, which 
hae been metaniorphoacd by pressure and tlie tmnafunon of a ailiccoua cement into an 
cxceodiugl; hard mass. Tbia cement waa probably produced by the Bolvcut action of 
heated water upon the quartz graiuB, which Becni to shade off into each utber, or intu 
the intervening aillco. It ia owing, no doubt, to the purely eihccona character of the 
gruina that the blending of thc«e with (bo Burroauding cement ia ao intimate as oRcii 





Flff. M.^MLcTOKOpIc 



to give the rock an ahuost filnly homogeneous texture. Tliat qtiartzite, oi here 
■leecnbcd ib an ongmal acdimenlnry rook, and net a chemical deposit, ia cbonn unt 
only by its <n'anuliir (extun but by the exact rcsenblauce of ilII ita leading featarai to 
ordinary Banilatoue— CiilBe beddiup, alternation of ooaraer and finer layer*, worm-burrows, 
iiud fucoid coats Ihc lUBtrous fracture that distinguishes thia rock from saodatonc, 
IB due to the exceedingly firm cohesion of the coiupoueut graina which break acTOM 
rather ihan scpamte, aud lo the consequent production of innumerable minnte cleu 
vitreous snrfncoa of quartz. A aaudalone, on the other bnnd, boa its grnina so loosely 
coherent that when the rock is bmhon, the fracture poxsei between them, and the new 
aur&ce obliiincd prtisenls innumerable dull romidcd gmina. 

Besides occurring in alternation with achista, qnatzKc is also met with locally as an 
altered foriii of saiuUstone, which when traversed by igneous dykes, ia indamted for a 
ilislunco of a few inches or feet from the intrusive mass. These locol prodnctiooa of 
quiirt/ilo show Ihcohuroetoriatic lustrous fracture, and have not yet bc<'n diatiuguiahcd 
by the microBcopo from the quartz-rock of wide melamorpliic regions. There ia yet 
another condition under which this rock, or one of analogous atruoture. may bo Been. 



Pakt II. § vii.] SCHISTOSE CRYSTALLINIC HOCKS. 129 

Highly silicated bands, having a lustrous tispoct, fine grain, and great hardness, oecur 
among the unaltered shales and other strata of the Carboniferous system. In such cases 
tlio supposition of any gencnd mctamorplusm being inadmissible, we may infer either 
that these quartzose bands have been indunited, for example, by tho.passtige through 
them of thermal silica ted water, or that they are an original formation. 

3. PriioxENE-RocKS. — ^Augite-BChist— a finegrained schistose aggregate of pale or 
dark-green augite, with sometimes quartz, plngioclaso, magnetite or chlorite ; found rarely 
among the crystalline schists. Among the schistose rocks of the Taunus, Lessen has 
dfscribed some interesting varieties under the name of Augite-schist (Augitschicfer). 
They are green, compact, sometimes soft and yielding to the iinger-nail, usually distinctly 
schistose, and interbedded with the gneisses and schists. They are composed of a 
line dull diabase-like ground-mass, tlirough which are dispersed crystals of augite, 1 to 
2 trim, in length, which in the typical varieties are the only components distinctly 
recognisable by the naked eye.* Augite-rock— a granular aggregate of augite (with 
tonmialine, sphene, scapolite, &c.), found in beds in the Laureutiau limestone of Cantula. 
3[alacolite-rook is a pale granular to compact, or even fibrous aggregate of 
malaoolito found in beds in crystalline limestone (Iliesengebirge). 

Schistose Gabbro — a granular to schistose aggregate of plagioclase and diallage, 
occurs in lenticular bands among the amphibolites and granulites of the crystalline 
achists. The diallage may occur in conspicuous crystals, and is sometimes associated 
with abundant olivine, as in ordinary gabbro (p. 154).- 

4. Hoi»BLEin>E-IlocKs.~Ainphibolite8— a name applied to a group of rocks, 

composed mainly of hornblende, sometimes schistose, sometimes thick-bedded. Besides 

the hornblende, numerous other minerals, such as are common among the schists, 

likewise occur,— orthoclase, plagioclase, quartz, augite and varieties, garnet, zoisitc, mica, 

ratile, &c VThere the rock is schistose, it becomes an amphibolite-schist or horn- 

Uande-sohist ; or if the hornblende takes the form of actinolite, actinolite-sehist. 

Where the rock is not schistose, it used to be termed hornblende-rock. Nephrite 

(Jade) 18 a compact extremely finely fibrous variety. The presence of other minerals in 

noticeable quantity may serve to furnish names for varieties. Thus, where plagioclase 

(and 0omc orthoclase) occurs, the rock becomes afelspar-amphibolite, dioritic 

amphibolite, or diori te-s chist.' Amphilwlites occur as bands associated 

with gneiHs and other schistcse formations. It was suggested by Jukes that they may 

possibly represent former beds of bomblendic or augitic lava and tuff, which have 

been metamorphosed together with the strata among which they were intercalated. 

This suggestion has recently received confirmation from the researches of the Geologicul 

Survey in the north of Scotland, where diorites erupted across the schists, apparently 

prior to the nietamorphism of the region, have partially assumed a foliated structiu*e, 

l^swing into amphibolite-schists and serpentine, while their felspar has aggregated into 

lUMstij of whiit may be termed Labradorite-rock. The connection of some schists with 

original niaK^os of diorite, gabbro and diabase is likewise pointed out by Lehmann.* 

T). (;aknkt-Ko<ks. — Ecloglte, one of the most beautiful meml>ers of the crystalline- 
bchist series, is a granular aggregate of grass-green omphacite (pyroxene) and red 
ginitt, through which are frequently dispersed bluish kyanite, and white mica. It 
'^"curs in bands in the Archajan gneiss and mica-schist. To those varieties where 
^^': kyanite becomes predominant, the name of Kyanite-rock has been given, 
^•arnet-rock in a oryt»talline-granular rov'k composed mainly of garnet, with horn- 
^knUv and magnetite; by the diminution of the garnet it passes into an amphibolite 

• I/)«M.'n, Z*i't$ch. VmUch. (hd. Oim. xix. (I8C7), p. 598. 

■ Kocks of this character occur in the Saxon " (irnnulitgebirgc " and also in I-.owtr 
An^tria. F. Becke, Trtchermak's Min. Miith. IV. p. 352, J. Lehraann's * Vntersuchungen 
"i^J^TiUo Kntsteliung der Altkrystallinischeu Sehiefergesteiue,' Bonn, 1884, p. 190. 

/ t^iv F. becke, Tschermak^s MuuMittli. IV. p. t'S^, This author likewise distin- 
^'iiUlios «lin1lafjeHnnphibofUff (larnct-amphibolitct miite-amphiboIUt\ zoigite-amphiboUte. 

' ' Untersuchungen Ulx'r ^lie Kntstehung der Altkrystall. Schief.* .See Hk. I V. pt, viii. 



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XTMUular vil»t iM' vt3. numrntoi. Hpnzs s. jgmKNiui7 triuaiw ftoni nt Dmi II oantun, 
ut«jr NvImvu. Nrv £««aiii>£ 21 NicCL \s7/^tt j: 3» inmtd il bade vitli lamiiiated 
^1nK■turf iiAfSTttCkKH*! It ^injiutnifif-^aifflMi Xli 7 f^ laamt roAt pM» into Serpen* 
tiiJt-. vlikAi luTMH lA^tt*- W ^ ^ lAwat ' t m- in»r re tin iviLisxflK w vdl a« one of the 
fni|.4.ivf Mnr». i!<«bt £«Buukhi^ nr^MinK 4«c7«a>tziwf ficnir istefbedded unong 
plijUiin'. iii¥niHriai«* and ImmAiVrtY h. Rsc&dJn. 

10. Kn>n<u>ii>-B^>.^'. — Tsi»t« an ibCrLrs'ii&*«fl ^7 ui exof^Mtiiijdj eompact felutc- 
like uiatHx. Ti>«7 uc^.'v iz Ued* \y l*;«>-like iuw«9i%. iK-oiKlaiDC^ in diftxic4t of ooiitaet 
UMftiuiAoq<Li*«ui. vjiiKiixM* a«f!».iriaVi«i wiik ^hif% zziikARC:f of M-kiAft. 

HillfBllinta.— arfi fXfrt<^ii:iHT c»?K;«n. b>:TnAcoe4ik«, felsitie, grey, yeUowish, 
groeulifh, r^lditb. >^r^vni4i, '.>r I4ack Fix-k. ocvufOHd of ftn intimAte mixtore of micro- 
Myopic iwrticli-ft <:«f ft* Upar and qiiartz. with t^^ vak» of mica and dklorite. It breaks 
with a vpMuttry or cnurlioijtil fra«1nre, pK-senisnndertbemirmMctpeaflnely-cryetallinti 
»triK'tiir<', o<*caiUoiia11y with ijc^t^ of qimnz. and i« only fiisil>le in fine splinters brfore 
thi; l>low-pt|i('. Thotigh externally prewnting a Tv«(iul*]anci' to fclAite (p. 142^ it oocurs 
ill )m*U H» iiitniittt<;Iy atiKKjciatctl with the «n)i'iMe» of Norway, that it has proljably been 
|itiHlii<'«-'| l»y Ihc bJiiiu- H<rk*H of <'ha!i§es that gave rij^e to the cr^'stalline E<^i«ti*.' 



' Kill Wicliiiiftiiij iiti UhcUh of Timor, ^Bcitriigt' zur Ciculogic tJtfi-Asiuiia uud 
\ii'.liMMi iia ' II. purl 2, j». 117. Lcydcii, 1884. 

• K • r»lM iiimk., Af/7«//. /4//W/. IKmk/m., Vienna, hi. (1867). F. Ikckt-, IVhcnunk'e 
Mh, MiUh I V. (IMM2) p. :i22. E. Dathe. Neutt Jakrb. 1876, 255-337. 

• l''#i ■uiitl)«M»iH'i' If. Kiiiit('SH'inf'*Kcniii«ka Bergwirtnnnlyser," 8vo.. }:>toi*kholnj, 1877. 



Part II. f viij SCUHSTOSE VltYSTALLlNE JIOCKS, 131 

Adinole (Adinole-schUt),— arockcztemally reueiublin*; tlio laat, but (li«tiiigui»bc(l 
fpwu it by its greater fusibility. It is an intimate mixture of quartz and albitc, 
oontaiuing about 10 per cent of soda. It is a product of alteration, being found among 
the altered Carboniferous shales around the eruptive diabases of the Harz; in tlie altered 
DeTOulan rocks of the Tannun, and in the altered Cambrian rock^ of Soutli Wales.' 

Forphyroid — a name bestowed upon certain rocks comjiosed of a felsite-liku 
Kronud-niMS which has assumed a more or less schistose structure, from thu devclop- 
iiivut of micaceous scales, and which ctnitaiiis porphyritically scuttt'rcl crystals of 
feller and quartz. The felspar is either orthoclaso or albite, and may be obtaine«l 
in tolerably perfect crystals. The quartz occasionally presents doubly termiuated 
|iymmids. The micaceous mineral may bo ptmigonito or serioitc. Porphyroid occurs 
among the achistoso rocks of Saxony,* in tho palasozoic area of the Ardeiuies.^ as well as 
iu Westphalia and other parts of Europe.* 

11. QiARTZ- AHD Tui'RMALiNK-KocKs.— Tourmaline-schist (Sdiorl-schist), a 
bla?kish, fiuely granular, quartzoso rock with abundant granules and neudles of black 
tuurmalino (schorl), which occurs as one of tho products of contact-metamorphism 
iu the neighbourhood of somo granites (Cornwall). 

12. Quartz- and Mica-KcxivS.— Mica-schist (Mica-8lat(% Glimmerschiefer), a 
si'lustose oggregateof quartz and mica, tho relative proportions of the two minerals Tary- 
iiig widely even in the same mass of rock. Each is arranged in lenticular wavy lamiute. 
The quartz shows great inconstancy in the number and thickness of its folia. It often 
retains a granular oharaoter, like that of quartz-rock, no doubt indicative of its original 
sedimentary origin. Tho mica lies in thin plates, sometimes so dovetailed into each 
other as to form long oontinuous irregular crumpled folia, separating the quartz layers, 
and often in tho form of thin spangles and membranes running in the quartz. (Figs. 18 
and 19.) As the rock splits open along its micaceous folia, the quartz is not readily 
seen save in a cross fracture. 

Bliuconte is the usual mica in typical mica-schist ; but it is Hometinics replaced by 
biotite or by paragonite. In many lustrous, unctuous schists which nro now found to have 
a wide eztent, the silvery foliated mineral is ascertained to be a hydrous mica (margaro- 
ditc, damoiuitc, &c.), and not talc, as was once 8Upix>8e(l. Tiicse, us already stateil, have 
Ixfu named hydro-mica-schists. Among the acceesory minerals, garnet (sjiecially charac- 
tfristic), schorl, felspar, hornblende, kyanite, staurolitc, chlorite, and talc may be men- 
tioned. Mica-schist readily passes into other members of the schistose family. By addition 
of felspar, it merges into gneiss. By loss of quartz and increase of chlorite, it passes into 
schitit, and by loss of mica, into quartz-schist ond quartzito. Mica-schist varies in 
c<»lonr mainly according to the hue of its mica. 

Mr. Sorby has pointed out that thin slices of true mica-schist, when examined under 
the microscoi)e, show traces of tho original grains of quartz-sand and other sedimentary 
particles of which the rock at first consisted. He has also found indications of tho 
current-bedding or ripple-drift seen in many fine sedimentary deposits, and concludes 
that mica-schist is a crystalline metamorphosed sedimentary rock.^ In some cases, 
however, tho foliation does not correspond with original bedding, but with structural 
planes (cleavage, faulting) suporindu(^ by pressure, tension or otherwise, ujwn rocks 
which may not always have been of sedimentary origin. 

Among the varieties of mica-schist may be mentioned, Scricit(; -schist (whieh 
may be also included among the phyllites) composed of an aggregate of line folia uf 
the silky micaceous mineral, sericite, in a compact honestone-likc (luiirtz ; Paragonite 



' Losscu. ZeiUcJt. Utntttdt, Gtol. (tvH'I. xix. (1807) p. oTo. See also Quaii. Jouni. (Jio!, 
tio>\ xxxiz. (1883) pp. 302, 320. 

' \Xoi\\p\iiiz<, OhpI. Surrey Snxoittjf Kxplanation of Section Um^hlitz. 

^ Do la Valleo Poussin and Kenard, Mnu. CouronntiH Acad, lluij. Bvhj. IS7G, i). 8.">. 

• Ixwsen. Sitz. GeiteUKch. NiUur/. Freumli, 1883, No. 9. 

* Q. /. Otol JSoc. '^ISOS), p. -iOl, and Ids address in vol. xxxvi. (1880). j). 8."). 



132 ajjoos-osy. [Vh^^k ii. 

scliist wIktc llir mica In the liyilnms s^xlii variety, purii^onitc ; Mar jraroili t c- 
f'cli ist \\liire iho inira is llu' Iiydrouts ibnii, mnrpirodite. (incish-mica-scb ist. 
containing dispersed konifls of ortboclaao. Soiiie of tliciix? rockt^ contain little or no 
fjiiart/. the place of whicli is taken by felsimr. 

Xnrnial niica-ecliist, together with olhor schintose n>cks. fonns cxtenKivc regions iu 
Nurway, Scotland, the Alps, and other parts of Europe, and vast tractn of the Arclweau 
r« L'ions I if North America. Sonic of it?* varieties arc also found cncircHn<; granite 
m.i.^>es (Scntlaml, Ireland, ^'C.) as a niclaniorphic zone a mile or so broiul, which sliade.'j 
away inlo unalteriil jrreywacke or slate out>«idc. In these ca8Ct>, it is nn<|nestiona1ily a 
iiietamorphosifl condition of ordinary HCilimcntary strata, the change being connc^tctl 
with the extravasitioii of granite. (IVkA IV. Part VIII.) 

Though the jvohsession of a lissilc structure, showing abumlant divisional surfaces 
covcrcil with gh^ttning mica, is characteristic of micn-schi^f, we must di.stiiiguish 
between lliis htrncturc and that of many micaceous smdstonos which C4in bu split into 
liiiu seams, cnch bph^nd<Mit with the sheen of its mica-flukes. A little cNamination will 
show that in th«* latter casj the mica has not crystdlizcil in ^'/7r/, but exists meridy in 
the form of detaf'hed worn scale.^, which, though lying on the same general plane, 
are not welded into Ciich other as in a schist; also that the quartz does not exist in folia 
but in rounded separate grains. 

1:^. QrAKTZ- AND Felspar-II(x;ks. — The rcplucemeut of the mica of a mica-schist 
by feL^]>iir, or the disn]>peurance of the mica from a gneiss, gives rise to au aggregate of 
fels]rfir and quartz. Such a rock uiay lie observed iu thin bands or courses, oltcruatiug 
witli the surrounding mass. In mineral composition, it may be compared to the quartz- 
l>'irphyrics or granite-iwrphyries of the massive rocks, but it is usually Tcadily diHtin- 
guishflble by a more or less developed foliated structure, and by the absence of any 
felsitic ground-mass. 

14. (^lAKr/-, Felm'aij- and Mioa-Kocks. — Gneiss, a schistose aggregate of orllu>- 
ela>.e (gomctimcs microclino or a ]dagiochtstic felspar, either seimrato or crystuliizc<l 
togi'ther), quartz, and mica.^ It differs from granite chiefly in the foliated arrangement 
of the mini'fals. Tlie quartz sometimes contains abundant liquid inclusions, in which 
liquid carlx)n -dioxide has Ixjen <letccted. The R-lative pr«»iM)rtions of the niiucraK and the 
manner in which tlicy are grouped with Ciwh other, prcs<int groiit variations. As a rule, 
the folia are coarser, and the schist«.»sc chanu^tcr less perfect than in mica-schist. Sonie- 
tiines the quartz lies in tolerably ]iure 1»andi<, a foot or even more in thickness, with 
plates of mica scattered through it. Thest? quartz layers may be replacetl by a 
cryctallino mixturt> of quartz and felspar, or the felspar will take the form of imlepcn- 
dent lenticular folia, while the lamina} <tf mica which lie so abundantly in the rock, 
give it its fissile structure. The felspar of many gneisses presents under the microscope 
a remarkable flbrous structure, due to the crystallization of fine lamellte of some plagio- 
clasc (albite or oligoolasc-albite) in the main mass of orthoclase or microcline.' 
Among the accessory minerals, garnet, tourmaline or schorl, hornblende, a|>atite, 
graphite, pyrites, and magnetite may be enumerated. 

3Iany varieties of gneiss occur. Some are distinguished by peculiarities f»f 
structure, as (* rani tc-gue iss, where the schistose arrangement is so et>arso as Iu 
Imj nnrecognisiible, save in a large mass of the rock; I*orphyritic gncibs or 
A n gc u g n e i s s, in which largo eyc-liku kernels of orthoclase are dis|»orscil. Otlu-r 
varieties arc named from the occurrence in them of one or more distinguishing 
minerals, as 11 o r n b 1 o ii d e - g n e i s s (syenitio gneiss), in which homblondo occurs 
iupjead "f (tr in adtlition to mini; I' rotogine-gnoiss where the onlinary mica 



' S'X> Kalltowsky'h - fincissformation des Eulengebirgcs,' I^'ipzig 187S; also a 
recent il.i(>rir.\<i- pa|K:r by F. Bccko, Tschermak's Min. MHth. 1882, p. ItH, and unothcr 
bv r W. K. ! ..,. ...I. 1S8I, p. 1. 

? - < If rrnok*! Jtfin. MiUh, 1882 r.iv.) p. 198) descril>cs this structure and 



m-J^ 



Part IT. { vii.] SCIflSTOSE CHYSTALUNE JiOCKS, 133 

Rives place to talc, or somo talcoso mineral which macroscopically passes fi»r talc. 
Chlorite-gneisB, wherein the mica is replaced by chlorite; Soricite-gneisB, 
a schitftoee aggregate of Berioite, albite, quartz, with less frequently wliite an<l black 
mica and a chloritic mineral *; Augite-gnoiss, containing an augitic mineral (not 
of the diallage group) and potash-felspar or potash-soda-felspar or scapolite, with 
hornblende (which has often crystallized pamllel with the augite), brown mica, more 
or less quartz, and also frequently with garnet, calcite, titanite, &c.'; Plagioclase- 
gneiss, with plagioolase more abimdant than the more usual orthoclase, sometimes 
containing hornblende, sometimes augite. 

The UKMit typical gneisses occur among the Archaean rocks, of which they form the 
leading type. (See lk)ok YI. Part L) They cover considerable areas in Scandinavia, N.W. 
Scotland, Bohemia, Bavaria, Krzgebirge, Moravia, Central Alps, Canada, &c. But rocks 
tn which the name of gneiss cannot be refused appear also among the products of the 
metamorphism of various stratified formations. Such are tlie gneisses associated with 
many other crystalline schists among the altered Silurian recks of Norway, Scotland, 
ami New England, tlie altered Devonian rocks of the Taunus, and other regionn, whicli 
will be described in Book lY. Part YIII. 

15, QuABTz-, FELsrAR- AND Garnet-Kocks.— GranuUte ' (Eurlte-schistoidc, T^pty- 
nite of French authors, Weiss-stein)— an aggregate of pale reddish, yellowish, or wliito 
fvltfjiar with quartz and small rod garnets, occasionally with kyanile, biotito and 
luioroseopic rutile and tourmaline. The felspar, whicli is the predominant constituent, 
jin^flcnts the peculiar fibrous structure referred to in the foregoing description of gneiKS 
(microperthito, microcline), and appears seldom to be true orthoclase. The quartz irt 
eonspicuons in thin partings between thicker more fcl8))athic bands, giving a distinctly 
fiiiidle bedded character to the mass. A dark variety, interstratilied with the normal 
rock, ii distinguished by tho presence of microscopic augite or diallage (Augitgranulitc 
of Saiony). Granulite occurs in bands among the gneiss and other members of tlic crystal- 
line schist scries in Saxony, Bohemia, Lower Austria, the Yosges and Central Franco. 

]G. Felspar- awd Mioa-Kocks. — Bocks composed essentially of a schistose aggre- 
irate of minutely scaly mica with some felspar, quartz, andalusito or oth<'r mineral, 
occur in regions of metamorphism. Cornubianite was a name proix)ded by Boaso 
for a rock composed of a felspar base, with abundant niica.^ It is found around tho 
granite of Cornwall, of which it is a mctaniorphic proiluct. By some writers this rock 
has licen associated with the gncLfsos, but from these it is distinguished by the scarcity 
or absence of quartz. 

17. BaiiSTOSE CoxtiLOMERATK IlocKS. — In some regions of gneiss and schist, 
pebbly or conglomeratic l>ands occur, in which jXibblcH of t^nartz and other nuiterials 
Crom lees than an inch to more than u foot in diameter arc iml)e(lded in a foliated 
matrix, which may be phyllito, mica-schist, gnoisd, quartzite, &c.* Exainiples of this 



' K.>. Lessen, ZeiUch. JhuMi. (Uol. Ga. XIX. (18t;7). p. 505. 

' The oocnrronco of augite as an abundant constituent of some gneisses has been 
made known by microscopic research. Rocks of this nature occur in Sweden (A. 
!4telzner, N. Jahih. 1880 (ii.), p. 103), and have been fully chrscrihed from l^mer Austriu 
(F. Becke, Tschermak's Min, MiUh., 1882 (iv.) pp. 21*J-3«5). Thoy arc likewise well 
Ueveloped among the Archaean gneisses of the north-west of Sutherland in Scf)tland. 

■ Michel-Levy lias proposed to reserve tho names ** Leptynite " for schistose and 
*-r.rauulite" for eruptive it>cks. Bull iSoc. dtoL France. :Wi\ ser. ii. pp. 177, 189, iii. 
\i 2?<7, iv. p. 730, vii. p. 7G0; I/Ory, op. cit. viii. p. 14. Schccrer, A7m<?** Jahrb. 1873, 
1.. «7H. Ihuhe, .V. Jahrh. 1876, p. 225; /. Dtuti^h. (ifol. Or*. 1877, p. 274. l^tetails 
r L^irding the great development of the granulite of Saxony ((iranulitgebirge) will 
1- found in the explanatory pamphlets publishe*! with the sheets of the G(K)logicid 
Sitfvty of Saxony, especially those of sections Uochlitz, Geringswaldi*, and Waldheim. 
The history of the origin of granulite is discusw^d by J. Lehmann, " Unterrfuchungen 
iiU-r die Entstehang der Altkiystall. Schiefergestcine.'* 

• * GcolosT of Cornwall ' (1832), pp. 22(5, 230. 

• Prof. Svlchmann describes B<>nie curious exami»hs of »*<ri>fntine ermglonierateH. S^t 



134 OE0GN08Y. [Book IL 



kiud are found in the pass of the Tdte Noire between Martigpiy and Gliamouni, in the 
Saxon granulite region, in Norway, in the north-west of France, in north-west Ireland, in 
the islands of Bute, Islay, Garvelloch, and different parts of Argyllshire. The pebbles 
are not to bo distinguished from the water- worn blocks of ordinaiy conglomerates ; but 
the original matrix which encloses them has been so altered as to aoquiie a micaceous 
foliated structure, and to wrap the pebbles round as with a kind of glaze. These facts, 
like those already referred to in the microscopic structure of mica-schist, are of con- 
siderable value iu regard to ilie theory of the origin of the crystalline schists, 

IJeforo pasHing from tlio Scliistose series of rocks, the student will 
oljservo that some interesting analogies may l>e traced in it, to Massive 
(»niptivo rocks on the one hand, and to Sedimentary rocks on the other. 
Some of the massive nxjks liave their foliateil c(mnterparts among the 
schists. Granite, for example, is representee! there b}' gneiss; diorite hy 
amphil)olite-schist8 ; gabbro by schistose gabbro ; felsite by halleflinta, and 
tnifs by porphyroid. How far this analogy points to original identity, 
or to assimilation by metamorphic change, is a complex problem. In some 
cases, indeed, it is perhaps conceivable that a process, such as irregular 
internal motion of the mass, that could change the schistose structure 
of gneiss into the massive structure of granite, would give rise to a rock 
which, whatever its previous history might have been,might not be distin- 
guishable from granite. On the other hand, any internal reaiTangement 
by pressure, tension, minute faulting or otherwise, which could induce 
a foliated structure within a mass of granite, would present a rock that 
would deserve the name of gneiss. That such internal transformations 
have taken place among the crystalline schists and some granites and 
other eruptive rocks can hardly be doubted. (See Book IV. Part VIII.) 

Still clearer are the relations of the Schistose to Sedimentary rocks. 
Many clay-slates are obviously only altered marine clays, and still retain 
their recognisable fossils. From such rocks, gradations can be followeil 
into chiastolite-schist, mica-schist and fine p^ieiss. Quartzites and 
quartz-schists often still retain the false-bedding of the original sandy 
sediment of which they are composed. The j>ebl)ly and conglomeratic 
Iwinds associated with some schists afford convincing proof of their 
original elastic nature. Thus, at the one end of the schistose series 
we find rocks in which an original sedimentary character remains 
unmistakaljle; while, at the other, after many intermediate stages 
of progressively augmenting crystallization, wo encounter thoroughly 
crystalline amorphous masses like granite and syenite, which should he 
placed among the massive rocks. This arrangement no doubt correctly 
represents what has l)een a real cycle of alteration among rocks. 
Sedimentary deposits have been gradually changed and crystallized. 
These m(»tamorphosed products, by upheaval and exposure at the surface, 
have again l)cen reduced to sediment, perhaps once more to pass through 
the same succession of alterations and to become yet again crystalline. 



his paper in '^Beithl^e zur Geologic Ost-Asiens und Australiens," ii. pp. 35, 111. On 
the conglomerate-schists of Saxony, see A. Saner, * Geol. Specialkarte Sachsen,' Sect 
" Elterlein,*' also Ijelimann's * Altkryet. Schiefergesteine,' p. 124. Iteusch, * Silurfossiler 
og Pressoilo Konglomorater,* ( *hristiania, 1882. mrroin, Ami. Sor. GtoJ, Nord, xi. 1884. 



pam n. f th.] schistose crystalline bocks. 



135 






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186 OEOQNOBY. [Book II. 



3. Massive. 

I'hiH important sub-iUvision is nearly coincidont with what is 
embraced by the old and useful terms Igneous or Eruptive Bocks. 
Almost the whole of its members have been produced from within the 
crust of the earth, in a molten or at least in a pasty condition. Nearly 
all consist of two or more minerals. Considered from a chemical point 
of view, they may be described as mixtures, in different proportions, of 
silicate? of alumina, magnesia, lime, potash, and soda, usually with 
magnetic iron and phosphato of lime. In one scries, the silicic acid has 
not been more than enough to com])ine with the different bases; in 
another, it occurs in excess as free quartz. Taking this feature as a 
basis of arrangement, some i>otrographers have proposed to divide the 
rocks into an acid group, including such rocks as granite, quartz- 
porphyry and quartz-trachyte, where the jxsrcentage of silica ranges from 
GO to 75 or more, and a >>asic group, typified by such rocks as Imsalt, 
where the proportion of silica is only about 50 per cent. 

In the vast majority of igneous rocks, the chief silicate is a felspar — 
the number of rocks where the felspar is represented by another silicate 
(as leucite or nepheline) being comparatively few and unimportant. As 
the felspars group themselves into two divisions, the monoclinic or 
orthoclase, and the triclinic or plagioclase, the former with, on the 
whole, a preponderance of silica; and as these minerals occur under 
tolerably distinct and definite conditions, it is customary to divide the 
felspar-bearing Massive rocks into two series : (1) the Orthoclase rocks, 
having orthoclase as their chief silicate, and often ivith free silica in 
excess, and (2) the Plagioclase rocks, where the chief silicate is some 
sjiecies of triclinic felspar. Tlie foniier seiies corresponds generally to 
the acid group above mentioned, while the plagioclase rocks are on the 
wliole decidedly basic. It has l)een objected to this arrangement that the 
so-called plagioclase felspars are in reality very distinct minerals, with 
proiwrtions of silica, ranging from 43 to 69 per cent. ; soda from to 12 ; 
and lime from to 20.^ But the state of minute sulxlivision in which 
the minerals occur in most massive rocks, makes the determination 
of the speeies of felspar so difficult that the term plagioclase is of great 
Bcrvice, as at least a provisional term under which to unite the fels}>ars 
that crystallize in triclinic forms. In addition to the felspar- rocks, 
there nnist be noted those in which felspar is either wholly alwent or 
si>aringly present, and where the chief i>art in rock-making has been 
taken l)y nepheline, leucite, olivine, or serpentine. 

From the jxdiit of view of internal structure, a classification based 
upon microscopic rcHcarch has l)een adoi)ted l)y some petrographers, 
\\\\i\ rec(»gnise tlirec leading types of micro- structure — Granular^ Par- 
phi/riiic and Glassy, A inore elaborate system has ])een proposed by 

' Dana, Am^r. Jour. 8ci. 1878, p. 432. See, on tlie Piibject of the retention of tho 
ii'YWi »• plrtiriocliise," Bonnoy, f,Vo/. Marf. 1879, p. 200. TIjo modem methoils of 
Hi'par.Uini; tin* f<l.spar8 riMnove some of the difticnlty nbove rt»ferretl to. 



Pabt n. S vii.] MASSIVE liOCKS. 137 



MM. Fouque and Michel-Levy, who pointing out that most eruptive rocks 
are the result of Buocessive stages of crystallization, each recognisable 
by its own characters, affirm that two phases of consolidation are 
specially to be observed, the first marked by the formation of large 
crystals which were often broken and corroded by meclianical and 
chemical action within the still unsolidified magma ; the second by the 
formation of smaller crystals, crystallites, &c., which are moulded 
round the older series. In some rocks the former, in others the latter of 
these two phases is alone present. Two leading types of structure are 
Tccognisod by these authors among the eruptive rocks. 1. Granitoid, 
where the constituents are mainly those of the second epoch of consoli- 
dation, but where neither amori)hous magma, nor cr^^stallites are to l)e 
seen. This structure includes three varieties, (a) the granitoid proper. 
Laving crystals of approximately equal size ; (6) pegmatoid^ where there 
lias been a simultaneous crystallization and regular arrangement of two 
constituents; (c) aphitic^ in which the felspars are ranged parallel to one 
of their crystalline faces, forming a kind of transition into microlithic 
Tocks. 2. Trachytoid, distinguished by a more marked contrast 
Itetween the crystals of the first and second consolidation, the usual 
j^reaence of an amorj)hous magma, and the fluxion structure. Three 
varieties are named : (a) peirosiliceow, with trains and spherulites of a 
finely clouded substance characteristic of the more acid rocks ; (h) micro- 
Jithicy characterised by the abundance of microliths of felspars and other 
minerals ; (c) vitreouSj derived from the two foregoing varieties by the 
predominance of the amorphous paste. ^ 

It is common to introduce a chronological element into the classifica- 
tion of the massive rocks and to divide them into an ancient (Paleozoic 
and Mesozoic) and modem (Tertiary and recent) series. Certain broad 
distinctions can doubtless be made between many ancient and luodeni 
eruptive rocks ; but, for reasons already stated, it seems inexj^edient, in 
the present state of our knowledge, to employ relative antiquity (wliich 
must be determined by a totally distinct branch of geological inquiry, and 
may be erroneously determined) as a basis of petrographical arrange- 
ment.* 

L Felspar-bearing Series. 

a. Orthooloso-Rocks. 

1. QcABTziFEROCs. — In tliis fumily the silicic acid has been in such excess as to 
separate out abundantly in tbo form of free quartz. Sometimes, as in granite, it has 
not assumed a definitely crystallized form, but is moulde<l round the other crystals 
as a later stage of consolidation. In other rocks (quartz-porphyry, A'c.) it occurs as a 
product of earlier consolidation. It often assumes perfect crystullographic contours, 
occurring even in double pyramids. The texture of the rocks is (1) crystalline-granular 
(granitoid) aa typically developed in granite; (2) porphyritic (trachytoid), as in quartz- 
porphyry or felsite ; (3) vitreous, as in pitchstone. 



' * Mineralogie Micrographique,' p. 150. 

* For a recent tabular arrangement of tlio massive ((TUptive) rooks and critirul 
remarks on their classification, see lioscnbuaeli, Ntm-H Jalirh. 1SS2, ii. p. 1. 



'M ^TSC'rTOi^T, [Book H. 




Gkamafe —X -iuaBtmifaLY T?ntnilinn ■nanaiT unwiai i nt 4f UifK. aioA, ftnd 
rti»rtft, n j*rriri#* ti* :itif*Biiu9^ -mifinni «bel T!ie iihyii » dttd^ vUle or pink 
irrlinefjMi^. vu iTrJinia :Himm •lu^nr'iiw loi libiib, mgy »nifaL W liii lul in ananer 
iiiftntiKv. 'mifi#»nrt<* fimnemiiffUHiie ly :iM!ir ±i» ittzBOint ■■£ 
.i4>«-7Ti^Hni' IT Mir iiAPfiinaric w ^^v^L 3» 'iut lmMiiiiMi n lIli>1inr 'if 
'i^*ftiunn\ Thp Aim saf ^ -tidusr diu MOifc. fsniBivte^ 
T'iir»* «i'.*4*r* iHOM^ . IF mtv 'ifRJanirn liiiiifB {matsmBMB, ainsaV< 
r M •.'trniumii? turfe 'imvvn ir luiek. Tbii '^nrci mv' hn 'liwuiiijii ft» 
)fiwr*i ^r Aiupui 'mooini^ tnimil riit* ifiuir aiezMam. Only in 
'ri -.!•* •nmiyAfUMu: aiiu*s?ui» irrnr ia isiienimbaifi ii*>nrd»Esn0icmteifwaal tbetv^ too tlie 
v't^-MA*^ .tiin<»mii« ''vrTjL jiiwz, ^TriH" £iRU?r. ^>. apt •fftortfr 6«mI. 

^.r\ni X wuKmmwf^ui ■fTjwimacon -if smnttf, is wm tfiBnarihr ■ftnni tkni the reek 
m(# ^ !duuwi*|*i '.f trfHttnini^ -tcnetinB?. wisft. vi - UMwifM a ^m cnaki-BnaL ww microiropic 
■4iki>» ii* ;ia7 k^mi AR£w*en cw ii jif tfy 'ir •'rv--fttiiixatt infiTiinilk Xcw recent and 
»t.iaftv«eiT4 ««iif^ -f^ zhi^. ^nfrMWf. ofiwtv!*!:. Bw- inl »*• t&« «HM£mMi tliAt though 
irAuti^ l.^ * ttaRribt. ''.r <vi>it povp&TRCn*. 2mii»£-anv «nia b« «icte«ted. there \» yet 
w;!iii^^«i^M "^iM'i^m.i-v^^ «& xiM£a^i«ft kiiiil if fOBBefir •rrT'fCiIEEsp OBasvA. in vhidi the 
«77«Q«4U '*p •tri^ArfLTU 'i^jrji -.f A« nek we CBCieliM. aj»i 12 vhieh they aie partially 
<4.iiiw/rjtyj Wn'stkr n^pad tf» Ihe icftuiaBi Wf wn ttu ma^^mi^ and its enclosed 
Mi^«^fccii»« M. ]f mM-I>t7 ho* ^i li tn g J tka MBemwsfir nwinntioo fioints to a 
4 «ehieirA 1s«ii<9Mn i^nailea in vkiih tbe <|aaiiz it iMie icccat than the other oon- 
•ciiMib^j* aorl kM erjflMiidalai at oaee. a»d Aooe in which tiboe aie lyaina of earlier 
U'%fjfMmvhd 'inartz. He dirt^nsnaihf* iheae two «fiei na (A) Anrieni gmniteit, 
»tHt$yf*M *A }A»itk Lkka, hrjfnblende. digqelnae. sbnI oithoelaae. fonaing a ery«taIlino 
iU'i^'tM «^0AfUfl in a more ree«nt ajvtalCne masoM of ofthodaae and qaartz. (B) 
l'*fr\AijTffA $rrvu'iUA, geoerallj finer in crain than the pffeeedii^. and farther difttin- 
rnhAiM \/y the oeenrrence of U-pjiamidal eryatals of qaartz (vhieh made their appear- 
utni^i }Mw«i^m the old felqiar and the leeeat offthorlaMX and of a notable quantity of 
wUlUt utUim Crare among the ancient gianitei) po4erwr in ailvent ervn to the more 
r<^^<t quartz,' 

Atwntu the comprment minerals of granite, the qnarti piesents special iuterest 
tfr*/l«f tb<; rniTONcofie. It \* often found to be foil of caritiea containing liquid, 
>^fiit*^in»iTn \n Hncli nnmbenf as to amount to a thousand millions in a cabic inch and to 
i(tvtt a to f Iky turbid aspect to the mineral. The liquid in these cavities appears usually 
t/f )t*i wtiUif ifvintaifiiiig sodium and potassium chlorides, with sulphates of tliese metals 
Mild fif f'flldum (p. 101). 

TIk; m«,'an of alevan analyses of granites made by Dr. Haugbton ga?e the following 
fivrniK<! r/itnpr>fiition : silicA, 72 '07; alumina, 14-81; peroxide of iron, 2*22; potash, 
Ti'll ; n^irbi, 2'7{l; lime, l'«3; raagneBia, 0-33; los?* by ignition, 109; total, 10005, 
with tt mean NjKtriflc gmfity of 2*6G. 

Most largo utnnHL'M of granite prenciit difiercnceti of texture in different parts of their 
tirfu. In imrlUMiIiir, they are apt to bo traversed by veinsr, sometimes due to a segregn- 
ii'iti of till! Nurmiinding mihoralH in rents of the original pasty magma, Bometimes to a 
|iro(rUHioii of a Ichh cosrHcIy crystalline (felHitie) part of the gnranitio mass into fissures 
of tlip niiin ntck (Fig. 21). Home of the more important of these varieties aredistin- 
KiiUhnrl l>y f«|N>cial nnmee. Thus where the component minerals assume largo pro- 
iHirtlofiN, with atondonoy to an orientation of their longer axes in ouo general direction, 



' Oil thn Ntriirturo of gmnito, sco the manuals of Zirkcl and Roscnbusch and the 
inotiioirs tlirro oitinl; qIko ZirkePs 'Microscop. Petrography,* 1876, p. 39; PhiUi|is, 
(/ J. (irttt. StH*. xxxi. p. JWW: xxxvi. p. 1. J. C. Ward, op, cit. p. 509 ; and xxxii. p. 1. 
Klim'n •HystiMiuitlr (irology,' (vol. i. of Explor. 40th Parallel), p. Ill, et wy. Micliol- 
l.ivy^ lUil). Siir, (M)/. Fmtief, Snl scr. iii. p. 190. Rosenbuach, Z«<«*. Deuftirh, Qi^, 
a*^U, xxvlll. (!H7«5). |». JW9. H. Mohl. Nyt. Mag. Nat. xxiii. p. 1, et /»«?. J. I^hmnnn, 
Allkrynt. H«'hloforgtwt<'ino; 1884, p. 3. 

' null N.i»'. <M»/. Frfiwrr'. 3r.l sor. iii. (187r>), p. li»0. 



Paot ir. I TiiJ MASSIVE ROCKS — QRANITE. 



139 



SBtheyBTeqmouUjapt todoinBetTegRtion-veii)B,the rock ia termed Pegmatite, Uie 
qnutt and felnp&r tuinng cijatallizeil together in maasei often larger than a man's 
hemd, tlio mien tXao uiiuuing the ihape of plates scTerol incbcBor even feet in diameter. 
Such coMwe gniued varietici may be foand hero and there, in tcdous or caveraoue 
■parcel^ in the heart at maaj ordinorj granites. One of the most interesting structnral 
Tarietietii* theformofpcgiiMtite termed Graphic Grnnite, in which tlie orientation 
of theqaartiand felqar ia ungnlarlj irell developed. Thoqnartzlms assumed the t^kpe 
of long imiwrfwt oolmnnar sbelU, plaooil parallel to eaph other and enclosed within tho 
nrtboclate, so th»t % truufsru section beaie Bome resemblance to Hebrew writing. 
The two minends hkTc ct^stAlIiEed together, with their piineipal axes pnmllei. ThiH 
fltrurtnre, whioh af^ieani usually to Iihto tnken phiee iu veinn, seems (o show that (hero 
onnbl \\ntf be™ little or no inl^mnl mnvonient of theso \e\n* wlien the eoiii|)oncnt 




'Uiiiii'Ci 



minemlii oiuiiimeil their cryrrtnlline foniif. Here and tiiero, nii Piample may be fnnnil of 
n Kntiiit*> bcoiiiiuft line-gmiiied, bnt containing larfn: Hmtlrre^l Iclspar eryntiil''. Sneli 
n rock maj Ix! termed a jnrphijrilir grnuile (nee Gmnophyre). Shiic jtrauileH abonnd ill 
• ncliuied cryiitalline eoneretions or fragniimts. Thew ere RomctinvM mere I'ei.'rogalioiMor 
ih>^ iiiatcriMLi of llic Rninite, when they are nsiially ovoid in form and porpliyritie in 
■•trnniiri' ; in other cawK, tliey nrc fmgiiienln of other roi-ka, and nre then eommonty 
FK-biniiw in stnietiiR and irregular in form.' In the crnlrc, es well tw roirnil the eilj,Ts 
•if larKC IvnnicH of granite, tliu uiineralu oeeastonally osatimc a more or lens ix^rfeclly 
M'hiat'ne arranf^ienient. When this takes plnco, tlie roek is nilled gneiHSooe or 
^'nei«s ;rrftnitc. (Se« Book IV. Piirt VIl.J 

Differences intheproportionsornntnreof the coni|Minont niineralrt have likewise kuj- 
ifv-tiildistinotiveniunci'. Of them? the following nre the more imporintit ; Granit ile. 



I J. A. Pliilliiw, Q. J. I 



I [., I- 



140 GEOGNOSY. [Book H. 



— a mixture of pink ortlioclaao and abandant oligoclasc, with a little qnartz, iome 
blackirih green magnesia-mica and occasionally with hornblende or angite; Proto- 
gine-grauit e— consisting of ortlioolase, oligoclase, hexagonal tables of a daik green 
mica, and a pale green diffused mica (or chlorite) occurs among the crystalline rocks 
of the Alps; Syenite-granite (hornblende-granite)— a rock with horableiide 
added to the other normal constituents of granite, and usually poorer in quarts than 
normal granite, derives its name from 8yene in Upper Eg3rpt, whence it was 
obtained anciently in large blocks for obelisks and other architectural works. The 
well-known Egyptian monoliths are made of it Syenite-granite is found in the 
Yosgcs, in Bohemia, in the Pyrenees, and in different parts of Scotland, notably 
ill masses of tertiary age which have invadc<l and altered the Lias rooks of Skye 
and Itaasay. It there sometimes assumes a porphyry-structure. Tourmaline 
granite — a granitito with disseminated tourmaline. Oreisen — a granitic rock 
from which the felspar has disappeared, found in some granite districts, especially in 
those wherein mineral-veins occur. Under the name GranuUte M. Michel-Ldvy 
includes certain fine-grained granites with white mica, which to the naked eye i^pear 
to be composed entirely of felspar and quartz, or of felspar alone, though both miea 
and quartz appeor in abundance when the rocks are microscopically examined. He 
includes in this category most of the rocks of the Alps described as ** protogine." 

Surrounding large masses of granite there are usually numerous veins, which consist 
BometimoB of granite, and sometimes of varieties of quartz-poiphyry. There can be no 
doubt that these porphyritio protrusions really proceed from the crystalline granite mass 
LoHsen has shown that the Bode vem in the Harz has a granitoid centre, with compact 
I)orphyry sides, in which ho found with the microscope a true glassy base.' Sometimes 
tiio rocks associated in this way with granite differ in composition from the main granite. 
Tourmaline is one of the characteristic minerals of granite-veins, though less observable 
in the main body of the rock ; with quartz, it forms Schorl-rook. 

Granite weathers chiefly by the decay of its felspars. These are converted into 
kar»lin, the mica becomes yellow and soft, while the quartz stands out soarcely affected. 
Tlio granite of the south-west of England has weathered to a depth of 50 feet and upwards, 
so that it can be dug out with a spade, and is largely used as a source of porcelain-day. 

Granite occurs (1) as an eruptive rock, forming huge bosses, which rise through 
other formations both stratified and unstratifled, and sending out veins into the surroimd- 
ing and overlying rocks, which usually show evidence of much alteration as they 
approach the granite ; (2) connected with true volcanic rocks (as in the case in Skye 
just cited) and forming, perhaps, the lower portions of masses which fiowed out at the 
surface as lavas ; and (3) in the heart of mountain-chains and elsewhere, interbedded 
with gneiss and other metamorpliio rocks in such a manner as to suggest that it is itself 
a final stage of metamorphism. Granite is thus a decidedly pluUmic rock ; that is, it 
has consolidated at some depth beneath the surface, and in this respect differs from the 
8ui)erficial volcanic rocks, such as lavas, which have flowed out above ground from 
volcanic orifices. 

Granophyre (Granite-porphyry)— a rook composed of a compact, but thoroughly 
crystalline (microgmnitic) base, tlirough which are porphyritically dispersed crystals of 
felspar, mica, and quartz (often doubly terminated). Among the constituents of tho 
finely granular base, hornblende and augite occur, and as they are commonly mom ot 
loss decomposed (chlorite) they impart a characteristic dirty green tint to the rook. 
This rock forms a link between the granites and quartz-porphyries. Its quartz and 
orthoclase contain gloss inclusions, though nothing of a vitreous nature occurs in the 
crystalline base.' Elvan (jehanite) is a Cornish term for a crystalline-granular mixture 



* ZeHfch. Dentach. Gtol. Gtf. xxvi. (1«74) p. 850. 

* Zirkel, * Mic. Petrography,' p. 60. Kosenbusch, Ztifach. Deutttch. Gfoi. Ges. xxviii. 
(1876) p. 387. Kalkowsky, Neues Jahrh. (1878) p. 276. Michd-lA'vy. Bull Sor, GM. 
Franre, iii. 3rd sen p. 205. 



Part 1L f viij MASSIVE ItOCKS—QUAnTZ-POUPHYltY. 141 



of quartz and orthoclase, forming veins which ])rocccd from granite, or occur only in its 
neighbourbood, and are evidently associated with it.^ 

QiiAl*tB-Forphyi7(Quartz-felsite, Euritc).^— Under this title arc included several 
▼arieties of rook whicli agree in consisting fundamentally of a very fine-grained fdsitic 
giDund-maas, oompoBed mainly of orthochuse and quartz. Whore the minerals ore partly 
crystallized in conspicuous porphyritic forms tlie rock is a qmtrtzjwrjyhyry (feUHe- 
porpkjfrtf, euriie); where the whole mass is more homo>gcneous and flinty in texture it is 
%feUUe fxtfdtUme. 

Qoartz-porphyry is oompoeod of a compact ground-masH, through which arc diriporscd 
erytftala or crystalline blebs of quartz, and crysttvls of orthochisc, sometimes of a tricliuic 
felsimr, mica or hornblende. Though to the eye, in fresh 8i>ccimen8, the ground-mass 
often appean homogeneous and almost flinty in texture, it generally presents under tlio 
mkrvoeoopo the microfelsitic structure already described (p. 110). Sometimes the base 
is found to be distinctly glossy ; in other cases it appears partly glassy and partly micro- 
felsitic ; while occasionally, it ossimies a microcrystalline character, even sometimes 
recalling the structure of a fine grained granite (Granophyrc). Itcautifid examples of 
spberulitic structiuro are occasionally to be oliser\'cd. In some ciMcs, the stnicture 
oonaista of true spherulites composed of a homogeneous substance with internal fibrous 
radiating arrangement, and presenting a black cross between crossed Nicol-prisms. In 
other cases, the substance of the concretions is of a microfelsitic nature with less definite 
external forms (felsospherulitesX or it is composed of miorocrystuUine material (grano- 
i^hemlites).' Fluxion-structure is well developed among some quartz-porphyries, and 
may oocasiooally be observed in the same rock with the spherulites. Where this 
stmcture has been highly developed, the rock has sometimes acquired a kind of streaky 
and even fissile character. 

The quartz occurs in imperfect, occasionally corroded, crystals or blebs, but some- 
times in ]ierfcct doubly-terminated pyramids, varying in size from minute forms only 
disoeniible with the microscope, up to crystals as large as a bean. It abounds with 
liquid inclusions. The orthoclase takes the form of more or less complete crystals, 
not seldom twinned ; the contour which its cross sections present to the eye, depending 
U|)on the angle at which the individual crystals are bisected. It is chiefly the dispersed 
orthoclase which gives the distinctively porphyritic aspect to the rock. Triclinic felspar 
(believed to be usually oHgoclase) also takes a place, distinguishable when fresh, by its 
fine lineation, but apt to become dull and kaolinized by weathering. Mica and horn- 
blende are among the most common of the minerals which accompany the two essential 
constituents, while apatite, map^etito, and pyrite are not infre(|uent accessories. 

The flesh-red quartz-porphyry of Dobritz, near Meissen, in Saxony, was found by 
Rentzsch to have the following chemical composition: — Silica, 76-1)2; alumina, 12-89; 
potash, 4 • 27 ; sotla, • 68 ; lime, - 08 ; magnesia, • 98 ; oxide of iron, 115; water, 1 - 97 ; 
total, 99 -a*, — specific gravity, 2*49. 

The colours of (iuartz-]K)rphyry depend chiefly upon those of the felspar, — flesh-reil, 
nddish-brown, purple, yellow, bluish or slnte-grey, and even white, being in different 
places characteristic. The presence of much niica or hornblende gives dark grey, 
brown, or greenish tints. It will be observeil in this, as in other rocks containing much 
felspar, that the colour, besides dei)ending on the hue of that rainend, is greatly regu- 
lated by the nature and stage of decom]K)sition. A rock, weathering externally with a 
pole yellow or white crust, may be found to be quite dark in the C4jntral undecaved 
portion. Besides ihoite difit'rcnces of oHi>eot, arising from varieties of colour, grouud- 

* J. A. Phillips, Q. J. Gvol. Soc. xxxi. p. 331. Michel-I^vy, BnU. Soc. (u'lH. France, iii. 
:5nl8cr. p. 201. 

' Zirkel, "Microscop. Pctrog.*' p. 71. See jjarticularly Hosenbuscli, ** 3Iik. rhvg.' ii. 
p. 50. 

* Lessen, ZeiUrh. Deuhch, GvuK Oatf. xxviii. p. 409. See also Quart. Journ. Gtol, 
•Smt, xxxix. p. 315. 



142 (fKOGNftsr. [Book U. 



inaBU, &c., diBtinctioDS are to be obsenred according to the relative abundance and dzo of 
the felspar crystals, and tlie preseucc of micti (niicdceous quartz-porphyry), hornblende 
(ttornhlendie quartz-porphyry)^ pyroxene (pyroxenic qnartz-porpliyry)^ or other acceAwry 
ingredient. When the We is very compact, and the felspar-crystals well-defined and 
of a different colour from the biisC) the rock, as it takes a good polish, may be 
used with effect as an ornamental stone. In popular language, such a stone is daaaed 
with the •* marbles,'' under the name of ** porphyry." 

F e 1 tf i t e (Felstono, Petrosilex), a hard an<l excetwively comiMict fliuiy-like rock, 
comiK)sed of an intimate mixture of cjuartz and orthoclase. The ground-mass presents 
under the ndcroscropc a stnicture like that of quartz-porphyry, into which felsile 
naturally pusses by the api>oaninco of the iwrpbyritic minerals. 

The Quartz-i)orphyrie8 and Felsites occur (I) with pliitonic rocks, as eruptive bosses 
or veinn, often iissoeiated with granite, from which, indeed, as above stated, they nuiy be 
wen to proceed directly; of fre<|uent occurrence also as veins and irreg^arly intruded 
niatteK?8 among highly eonvolut<Ml rocks, csiieciully when these have been mere or less 
uietamorphosed ; (2) in the chimnc?ys of old volcanic orifices, forming there the " nock " 
or plug by which a vent is filled up ; an<l (.S) as truly volcanic rocks which have been 
erupted at the surface in the form of flows of lava, either (a) submarine, as in tho Lower 
Silurian felstones of Wales,* and the south-c>tist of Ireland, or Qi) subaerial, as probably 
in the quartz-porphyry of the Isle of Arran, and perhaps in the series of •* green-slates 
and porphyries " of the Silurian system in Cumberland,' which Professor Kamsay has 
conjectured to be the products of a subaerial volcano. These eruptive rocks are 
abundant in Britain among formations of Lower Silurian, Old Bed Sandstone and Lower 
Carboniferous age. In the Inner Hebrides they overlie and alter the Jiunssio rocks. 
Thoy were jtoured out on a great scale during Permian and early Triassto times in 
Westphalia and the Thuruigor Wald. 

Liparite — (Rhyolite, Quartz-trachyte), a rock composed of a comi^act or fino-graineil 
ground-mass, containing crystals of sanidine, quartz, black mica, and hornblende, with 
fre<£uently triclinic felspar, tridymite, augito, apatite, or magnetite. This rock has a 
clo^e relationship to the quartz-i)orphyries. Considerable diversity exists in its texture. 
Frequently it is finely cavernous, the cavities being lined with chalcedony, quartz, 
aunethyst, jasi>or, &e. Some varieties arc coarse and gninitoid in character. Inter- 
mediate varieties may be obtained like the quartz-porphyries, and these ixass by degrees 
into more or loss distinctly vitreous rocks. Throughout these gradations, however, 
which uuiy represent different stages in the crystallization of an original molten glass, 
a characteristic groimd-mass can be seen under the microscope having a glassy, enamel- 
like, i)orcellauous, microfelsitic, or sometimes even a microgram tic character, with 
characteristic spherulitic an<l fluxiou stnicture. In the quartz, glass-inclusions, having 
11 dihexahedral form, may often be detected ; but liquid inclusions are absent. An 
analysis by Vom Kath of a rhyolite from the £uganeau Hills gave — silica, 76 '03; 
alumina, 13-32; soda, 5*29; potash, 3*83; protoxide of iron, 1*74; magnesia, 0*80; 
lime, 0-85; loss, 0*32; total, 101* 68,— specific gravity, 2 553. 

Liparite is an acid rock of volcanic origin and late geological date, which in more 
recent times, has played a part similar to that of the granitic and felsitio rocks of older 
IK.'riods, though it has not been yet observed as a product of any still active volcano. It 
forms enormous masses in the heart of extinct volcanic districts in Europe (Hungary, 
Euganean Hills, Iceland, Lipari), and in North America (Wyoming, Utah, Idaho, 
Oregon, California).' 

N e V a d i t e — a variety of rhyolite nameil by Bichthofeu from its development in 
Nevada, and characterise<l by its resemblance to granite, owing to the abundance of its 



* J. C. Ward, Q. J. (Jcul. Nw,*. xxxi. p. 309. The felbite of Aran Mowddwy cuutuius 
83*8 i)cr cent of silica. - J. C. Wartl, op. cit. p. 400. 

» On liparite or rhyolite sec Zirkel, * Micro. Petrog.* p. 103. King, * Explor. 40th 
l*arallel,' vol. i. p. 006. 



Takt II. § vii.] MASSIVE HOCKS — SYENITE. 143 



purphyritac oiy<ta]«, and the relatively Hmall aiuouut of ground-maKH iu wliich they are 
imbedded. The granitoid aspect is external only, as the ground-muHts is distinct, and 
varies fhnu a holo-ciystalline character to one with uhaudaut glutw, and the texture 
ranges from dense to porons.* 

Among the qnartauferons rocks above enumerated a distinct gradation can Home- 
times be traced from a ci3r»ta11ine granitoid structure into a |)orphyritio mass with 
charaoteristic ground -mass. Among tlic porphyritic varieties also, trao^s can be detected 
of a TitreooB base, indicative of the rocks having once exi8tc<l as glass. The vitreous 
<.*omiM>undii are placed together at the end of iho non-<|uartziferous group (pp. 115-1 47)* 
2. QuAKTZLSSS, OB FOOR IN QuARTZ. — In this group, frci^ quartz is not found as a 
marked constituent, altiiough occasionally it occurs iu some quantity, as microscopic 
osiuuinatioa has shown in the ease even of some rocks where tlic mineral was formerly 
lnslicvctl to be absent. A range of structure is displayed similar to that in the quartzi- 
fftroiis seri<rs. The thoroughly crystalline varieties are typil!(}<l by syenite, which 
ivpreg^ents the granites of the quartziferous ro<*ks, those Avliich i»os8eHM a jwrphyritic 
^^nnd-mass by orthociase-pori)hyry and trachyti', answering to quartz-i>orphyry and 
liparite, while the vitreous orthoclase-rocks may represent the glassy condition of both 
tlM ((osftziferons and qnartzless group. 

Syenite. — ^This name, formerly given iu England to a granite with hornblende 
Ti'placing mica, is now restricted to a rock consisting essentially of a crystalline-granular 
luixtnre of orthoclase and hornblende, to which plagioclase, biotite, augito, or magnetite 
may be added. The word, first used by Pliny in reference to the rock of Syene, was 
introdaced by Werner as a scientific designation, and a]>plied to the rock of the Plauen- 
scher-Onmd, Dresden. Werner ofterwards, however, made that rock n greenstone. The 
liase of all syenites, like that of granites, is thoroughly crystalline, without an amorphous 
ground-mass. 

' The typical syenite of the Plauenscher-Grund, formerly described us a coarse-grained 
mixiore of flesh-coloured orthoclase and black hornblende, containing no quartz, and 
with no indication of plagioclase, was regarded as a normal orthoclase-homblende rock. 
Microscopical research has, however, shown that well-striated triclinio felspars, as well 
as quartz, occur in it. Its composition is: — silica, 59*83 ; alumina, IG'85 ; protoxide of 
Iron, 7-01; lime, 4 "43; magnesia, 2*61; potash, G-57; soda, 2*44; water, &c., 1 • 21) ; 
total, 101 • 03. Average specific gravity 2 • 75 to 2 • IH). 

8yenite, while always thoroughly gninitio in stnioture, varies in texture from coarse 
granular, where the individual minerals can readily be distinguished by the naked eye, 
to compact. Among its accessory minerals of common occorrence may be mentioned 
titanite (sphene), quartz, apatite, epidote, orthite, magnetite, pyrite, zircon. U'he pre- 
dominance of one or more of tlie ingredients has g^ven rise to the separation of a few 
varieties under distinctive names. In the typical syenite, the dark silicate is almost 
wliolly hornblende; where this mineral is replaced by augite, as iu the well-known 
rock of Monzoni, the rock is termed Augite-syenite or Monzonite; where 
brown mica predominates it gives rise toMica-syenite or Minette. 

Syenite occurs of many difierent ages from early Puloiozoio up to Miocene, under 
conditions similar to those in which granite is found ; it has been erupted in large 
irregular masses, espacially among metamorphic rocks, as well as in smaller bosses and 
veins. It is likewise sometimes associated with syenitic granite, quartz-porphyry, and 
other orthoclase rocks at the roots of volcanic hills, as in Kaasay, Skye and Antrim, 
where it has penetrated Jurassic rocks, and is itself probably of older Tertiary age. 
Elaeolite-sycnite (Nepheline't^yenitt) is a granitoid rock, characterised by the 



' Hague and Iddingn, Aimr. Jouni, jSci. xxvii. (1884) p. IGl. Tliese authors iVia- 
tiugiiish between Ncvadite and Lii>arile, the latti'r being chanicterise<l by the suiuU 
uuQiber of porphyritic crystals embedded in a relatively large amount of ground- nitihH. 
which, as in Nevadite, may l>o holo-cryst^illine or glas8y. They also distinguibli 
lUhoiial RhytjlHe and Hyaline HhyoUte as additional varieties. 



144 GEOGNOSY. [B4K)K II. 



asHOciation, of tlio variety of ncphcline known as elaeolite with orthoclatic, and wiUi 
minor proportioiiM of plagioclase, miorocline, borublende, angitc, biotitc, sodalite, zircon, 
and spheue. It is distinguished by the mre mincrabt, upwards of fifty in number, which 
it contains, and in which some of the rarer elements are combined, such as thorium, 
yttrium, cerium, lanthanum, ttintalum, niobium, zirconium, &c. It is typically 
developed in Southern Norway (Brevig, Laurvig). Where zircon enters as an abundant 
constituent the rock is known as Zircon-syenite. Foyaitois the name given to 
a hornblendic variety found at Mount Foya, Portugal; Miascite is a variety with 
abundant mica, found at Miask ; D i t r o i t e, containing socialite, spinel, &c., occurs at 
Ditro in Transylvania. 

Orthoclase-Forphyry ((^uartzless-porphyry, Orthophyre) Btinds to the syenites 
ill the same relation that quaHz-porphyry does to the granites. It is composed of a 
compact porphyritio ground-mass, with little or no free quartz, but through which 
arc usually saittered numerous crystals of orthochise, sometimes also a triclinic felspar, 
black hornblende and glancing scales of dark biotitc. It contains from 55 to 65 per 
cent, of silica, thus differing from quartz-poq)hyry and felsito in its smaller proportion 
of this acid. Except by chemical or microscopical analysis, however, the distinction must 
often be difficult to establish between the fine compact felsites and tlie orthoclaso 
porphyries, especially when the latter (as the microscope shows) contain free quartz. 
The term *' syenite-porphyry,*' sometimes given to this rock, should be retaincMl for 
the microgranitoid varieties, which, among the quartzless orthoclaso-rocks, would thus 
represent granite-porphyry in the quartziferous series. Orthoclase-porphyry occurs 
in veins, dykes, and intrusive sheets. Probably many sonralled felstones, whether 
occurring as lavas or as intrusive masses, among the older Palffiozoio fonuations are 
really orthoolase-porphjTries. Homo highly micaceous varieties have been called 
1^1 i c u - 1 r a p — a terra under wliich have also been included Minettes, Micaceous Quartz- 
iwrjihyrics, &c. (See Diorite, p. 148, and Kersantite, p. 150.) 

The orthocla6e-pori)hyry of Pieve in the Vicenlin was found by Von I.4isau1x to have 
Iho following composition. Silica, 01*07; alumina, 18*5G; peroxides of iron and 
nmnganeso, 2 -GO; potash, G'83; soda, 3*18; lime, 2 '80; magnesia, 1*18; carbonic 
acid, 1*30; loss, 2* 13— specific gravity, 2 •59.* 

Orthoclase-porphyry is largely developed among the later I'alieozoic fonuations of 
Thuringia, the Harz, Saxony, and the Vosges, occurring both intrusively in dykes, and 
intercalated in large beds. The orthophyres of Puy de Dome and the Loire are acoom- 
panied by tuffs and breccias. 

Trachsrte- — a term originally applied to modem volcanic rocks possessing a 
characteristic roughness (t/kix^s) under the finger, is now restricted to a compact 
l>orphyritic rock consisting essentially of sanidine, with more or less triclinic felsjiar, 
usually with hornblende, bioiite, and magnetite, and sometimes with angitc, apatite, 
and tridyiuite. It is thus distinguished macroscopically from liparite or quartz-trachyto 
by the absence of quartz. Microscopically it is to be discriminated from that rock by 
the absence or feeble development of the microfclsitic substiince, so abundant in liparite, 
and by the i>rcix)ndemting aggregate which it presents of minute colourless felspar- 
microliths, with usually needles and granules of greenish hornblende and much ditTusetl 
nia^rnctito <lust. The sanidine crystals present abundant steam-pores and glass-iiicln- 
siony, as well as homblende-niicroliths and magnetite. In some varieties, the ground- 
mass appears to bo entirely composed of niicroliths ; in others, minor degrees of devitrifi- 
cation can be trawMl, until the groun<l-mass passes into a perfect glass (obsidian). The 



' Zt:H>*ch. DttitMch. (icol. Ges. xxv. p. 320. On *' mica -traps,*' sec Bonuey, Q. J. 
(ifol. Soc. XXXV. p. 1(>5. 

- On trachyte, sec Ziikel, 'Micro. Petrog.* p. 14u. King in vol. i. of *Explor. lOth 
Pandlel,' p. 578. On the relative ago and classiticatiun of Hungarian trachytes, Sztib«s 
Zrifitrh. J)en(it('h. (noi (icf. xxix. p. (535, oud * Coniptc reud. Congrcs internntionalc de 
(;e()lngi.' ' (1,S7S), Pari.s, ISSO. 



p^ 



Part IL 5 vii] MASSIVE SOCKS — PHONOLITE. 145 



traehytes of HnDgary ha?e been grouped as AugUe-tradhyte^ Amphihole-trachyU^ and 
Biotitetraehifte, Another clasdfioation, proceeding on tlio nature of their predominant 
feUpaihie constiluent, ranges them as Anorthite-iraehyte, Labrador-trachyte, Oligoclase- 
Irtuhjfie, and Orthoge-trachyte. Average composition of Trachyte: — silica, 00*0-64*0; 
alumina, 17*0; protoxide and peroxide of iron, 6* 0-8*0; magnesia, 1*0; lime, 3*5; 
soda, 4*0; potash, 2*0-2*5. Average specific gravity, 2 * 65. 

Trachyte is an abundantly diffused lava of Tertiary and Po8t«tertiary date. It 
oocora in moit of the volcauic districts of Europe (Siebengebirge, Nassau, Transylvania, 
Bay of Naples, Euganean Hills) ; and in the Western Territories of the United States.' 
It ooctUB also in New Zealand. 

Domiie (so name<l from the Puy-de-D6me) is a porous loosely aggregated traoliyte, 
having a miciolithic ground-moss, through which are dispersed tridymite, sanidine, 
plagioclase, hornblende, magnetite, biotite and s^iecular iron. 

Fhonolite (Clinkstone) * — a term suggested by the metallic ringing sound emitted 
by the fresh compact varieties when struck, is applied to a compact, grey or brown, 
quartzleas mixture of sanidine and nepheline, with hornblende and usually nosean. 
Under the microscope, the ground-mass is not vitreous or half-dcvitrified, but appears as 
a crystalline aggregate of plates of sanidine and hexagonal prisms of nepheline, with 
Ittfl frequent crystals of leucite, hornblende, augite, magnetite and hauyne. The rock is 
rather subject to decomposition, hence its fissures and cavities are frequently filled with 
zeolites. An average specimen gave on analysis — silic^^ 57*7; alumina, 20*G ; potash, 
6-0; soda, 7*0; lime, 1*5 ; magnesia, 0*5 ; oxides of iron and manganese, 3*5 ; loss by 
ignition, 3*2 per cent. The specific gravity may be taken aa about 2*58. Phono! ite is 
sometimes found splitting into thin slabs which can be used for roofing purposes. 
Occasionally it assumes a porphyritic texture from the presence of large crystals of 
sanidine or of hornblende. When the rock is partly decomposed and takes a somewhat 
porous texture, it resembles trachyte in appearance. 

Like trachyte, phonolite is a thoroughly volcanic rock, and of Tertiary date. It 
occurs sometimes filling the pipes of volcanic orifices, sometimes as sheets which have 
been poured out in the form of lava-streams, and sometimes in dykes ond veins, as in 
Bohemia and Auvergnc. 

3. Ctla86Y Obthoclase Rocks. — These arc found associated with iwrphyrios, 
trachytes and phonolites, and represent the vitreous condition of eruptive rocks belonging 
probably to both the quartziferous and qunrtzless groups. 

PitchBtone (Rotinite) — a vitreous, pitch-like rock, easily frangible, tmnslnceut on 
thin edges, having usually a black or dark green colour, that ranges through shades of 
*;reen, brown, and yellow to nearly white. It is essentially an orthoclose rock, and may 
be regarded as the natural glass resulting from the rapid cooling of some of the more 
^mmular or crystalline orthoolaiio-rocks, such as the quartz*porphyric8 or felsites. Ex- 
aiainod microscopically, it is found to consist of glass in which are diffused, in greater 
or less abundance, hair-like microliths, angular or irregular grains, or more definitely 
formed crystals of orthoclose, plagioclase, quartz, hornblende, augite, magnetite, &c. The 
pitchstone of Corriegills, in the island of Arran, presents abundant green, feathery, and 
(lendritio microliths of honiblende (Fig. 9).' Occasionally, as in Arran, pitohstono 
a«sumes a sphcrulitic or })erlitic structure. Sometimes it becomes porphyritic, by 
the development of abundant sanidine crystals (Isle of Eigg). Some petrographcrs 



' It woulil appi'or that much of what has been regarded as trachyte in Western 
America is andesito, consisting essentially of plagioolase, and not of sanidine. Tho 
D(>rmal trachvtes are now described as homblende-miea-audesitos, and tho augitc- 
tnichytes are bypersthcnc-augite-andesites, most of tho rest being docites, and some of 
thciu'rhyoliteH.' Hague and Iddings, Amer. Joum. 8ci xxvii. (1884) p. 456. 

' Boricky, * Petro*n«ph. Stud. Phonolitgestein. B<">hniens.' — Arrhir Landettdurch- 
ptTHihing BOhnien, 1874. G. F. Ftihr, "Die Phonolite des Hegau*8,*' Verh. Phys. Med. 
Ge.. Wurzhurg, xviii. (1888). 

' Sec F. A. Gooch, Min. MUiheil 1876, p. 185. Allport, Ocol Mag. 1881, p. 438. 



146 GEOGNOSY. [Book U. 



disfciDguish tlieorctically between feUite-pUduftone and traehyte^iehsUmej but the dis- 
tinction rests upon no essential peirographical differences. An average spocimen of 
pitcbstone gave on analysis— silica, 70 -G: alumina, 15-0; ])otasli, I'G; soda,2'4; lime, 
1*2; magnesia, O-G; oxides of iron and manganese, 2*0; 1oj<s by ignition (partly 
waterX 6*0. 3fean speciiic gravity, 2*34. 

Fitclistonc is found as (1) iutnisive dykes, veins, or bosses, probably in dose con- 
nection with former volcanic activity, as in the case of the dykes which in Airan 
traverse Lower Carboniferous rocks, but are probably of Miocene age, and those which 
in Meissen send veins through and overspread the younger Pulfsozoic felsite-por^^yries ; 
(2) slieets which have flowed at the surface, as in the remarkable moss forming the 
Scuir of Eigg, which has filled up a river-channel of Miocene age.* 

Obsidian — a volcanic glass, representing the vitreous condition of a sauidine-roek 
(trachyte, liparite, or phonolite). Tt externally resembles botUe glass, having a perfect 
conchoidal fnicture, and breaking into sbar]) Hplinters, somi-transiHurcnt or trandncent 
at the edges. Its colours arc black, brown, or greyish-green, rarely yellow, blue, or ted, 
but not infrequently streaked or banded with i)aler and darker hues. A thin slice of 
obsidian prei)ared for the microscope is found to be very palo yellow, brown, grey, or 
nearly colourless, and on being magnified, shows that the usual dark colours are almost 
always produced by the ijrescnce of minute opaque crystallites, which present them* 
selves as black opaque IrichiteH, sometinU'S beautifully arranged in eddy-like lines show- 
ing the original tluid movement of the rock (Fig. 12); also as rod-like transparent 
microliths. They occasionally so increase in abundance as to make the rock lose the 
aspect of a glass and assume that of a dull flint-like or enamel-like stone. This deritri- 
iication can only be properly studied witli the microscoiie. Again, spherulites of a dull 
grey enamel appear in some parts of the rock (^Spherulitic OMdian) so abundantly as to 
convert it into pearlstone. These spherulitic enclosures may be observed in Lipari in 
great abundance, drawn out into layers so as to give the rook a fissile structure, while 
steam- or gas-cavities likewise occur, sometimes so hirge and abundant as to impart a 
cellular aspect. The occurrence of abundant sanidine crystals gives rise to Porplkyritie 
Obsidian, Many obsidians arc coarsely cellular, from the presence of large steam- 
vesicles, and pas8 into pumice. Now and tlien, tlic sieam-pores arc found in enormous 
numbers, of extremely minute size, as in an obsidian from Iceland, a plane of which, 
about one square millimetre in size, has been estimated to inclnde 800,000 pores. The 
average chemical composition of obsidian is — silica, 71*0; alumina, 18*8; potash, 4*0; 
soda, 5*2; lime, I'l; magnesia, 0*0; oxides of iron and manganese, 8*7; loss, O'G 
(little or no water). Mean specific gravity, 2-40. Obsidian occurs as a produdt of the 
volcanoes of late geological periods. It is found in liipari, Iceland, and Teneriffe ; 
in North America, it has been erupted from many points among the Western 
Territories ; it is met with also in New Zealand.' 

Ferlite (Pearlstone), is not so much a distinct rock-species as a peculiar condition 
of some originally vitreous rocks, especially of obsidian and pitcbstone. It consists, as 
its nnmo indicates, of enamel-like or vitreous globules, occasionally assiuning polygonal 
forms by mutual pressure. These globules sometimes constitute the entire rock, their 
outer portions shading off into each other, so as to form a compact moss ; in other oases, 
they are separated by and cemented in a compact glass or enamel. They consist of sac- 
cessive very thin shells, which, in a transverse section, are seen as concentric rings, usoally 
full of the same kind of hair-like crystallites and crystals as in obsidian. (Fig. 12.) As 
these bodies both singly and in fluxion-streams traverse the globules, the latter may 
])e conjectured to be a stmcture developed by contraction in the rock, duiing its con- 
solidation, analogous to the concentric spheroidal structure seen in weathered basalt 
(Fig. 22.) Occasionally among these concentrically laminated globules are found true 



» Quart. Joum. Geol Soc. 1871, p. 808. 
* On Obsidian, see Zirkel, * Micro. l*etrog.' 



Pabt IL 5 vU.] 



MASSIVE SOCKS— PVMIOE. 



147 



■pbernliU* where tlie intenud itraoture it lodiating tlbtoas. A prcdaniinanco of thcM 
bodies foraij^pAmJAK; PtirliYe or Bphorulite-iook. (Fig. 23.) 

The rock to which the name PerKte lias been mote specially applied ia Dmrkedl]' 
acid, ita peroentage of lilica nnKiDg between TO 6 and 82-8, nud its averoga specitii- 
tnarity betwMO 2'37 and 2'4G. It occurs rooet ronejiiaaoaslj in Haogatj, where 
it takes the form of lava Btreaioa proceeding- from old trachyte Tolcnnoes; nbn 
among the Gnf^nean Hilli, Ponza iHlandB and Aaceneion.' 

Pamloe (Ponca, Bimatein) — a general term for tho looae, spongy, cellalar, 
fllamentona or froth-Hko parfs of lavaa. Ho distinctive is this atrurtare, tliat the tenn 
pKaKcoMbascomeiDtogeneralueo todeseribeit. There can be no doubt that lhiaA«tli- 
like rock owe« its poooltarity to llie abundant escape of etoam or gaa through iU moss 
while itill in a itale of foaion. Microscopic examioadon revealB a gloss crowded with 
raonnoD* numbers of miunto gas- or Tapour-cavitios, usually drawn out in one direction, 
alao abnndant crystallites like those of obsidian. In the great inujority of caaos, pumice 
ia a form of the obiidians, possessing a percentage of silica from ^S to 74, and a apeeiflo 
^nvity of 2'0 to 2->'i3, though, owing to its porous nature, it possesses grent buo}-ancy 
•od readily Hoata on water, drifting on the ocean to ilislAncc:* of many hundreds of miit-H 
fmoi land, until the cells are gradually flilo'l with water, nben the floating mnsscR 




sink In the bottom.' Abundant mundcd blocks of pomice were drcilged np by tbo 
OutUrngrr ftoni the floor of the Atkntiu and Pacirte Ocenns. At Hiiwnii, some of tlio 
lja.tic pyroieiiic or olivine lavas give rise to n pumiceouH frolb. 

b Plogioelnso-Tlorks. 
The rrx'kH of liii^ division arc of all a<>es up to tlie present time. They consist 
>«iFDlinlly of name Iriclinic felspar tii whicli one. more usually scveriil, other silicatoa 
ut- aildi^l. Ah a rule, tlicy are basic compounds, tliough in a few nf them free quartz, 
ts au nriginol conslituent, can he dotcctod with or wit)iout the mLcroscoi>e. lu 
ilTQcture. Ihey prus<-nt a range similar to that of tlie ortlioelase rocks. Some of llicni 
ire IhorrnigblT cry!it;dlino {dioritc), Ihougli tlicy never attain (ho coarsenens of toxtnre 
■hicli la often reoclicl by granite. Many of thorn ate characteriHtically porphyritic 
(porphjritoX while in tome coses, tbey assume a completely vitreous texture (laehylile). 



IW I»w<T Silurian laTBH of North Wales. Op. c 

* <>n porosity, hyilraliou, and flotation of pnmiec 
wppl. (1871) p. 177. 



148 QEOQNOSY. [Book IL 

Tbcy mBy bo amiTigixl in Rroiipe, Bo«>nliag «a the pi«domiD»nt minoTBl aflsr the 
fclipar is liornblcndc, mien, Rugite, or dioUa'.'e. 

1. PLiQioc'LAaK-lIoiiSBLmiiK-(oBMic?A)BocKB, — Slorito (GrcenBtonp in jitut)' — 
a crvBtollinO'gTKuiildT nggrcg&tc of » tridinic felspar anil hornblende, aaually with 
ina^ctito aud apatite, soinetimtB with augite or mien. The proportiona bttwt.'en tlie 
fcUp^ir and liomblcndo vary so ^eaiXj na tn f^iTC rise to conBiilerablo differrnm in the 
colour and conipiwition of the roc'k. Tho fuUpnr whon fresh i<howB its twin lanclla- 
tions, but is rrtqucntly tinted grran (from dnmupMition of tbo horoblenilo), and more 
or lcB« (tocsycd. The hornblende \i dark fjcea or black, with vitrcoua Instrr on the 
rlravago plitnpB when fresh, bitt apt to deeompoao nnd to give rise to Mcondary 
prwluctg, Bueh Rs epiJote and ehlorite. The apatite ocenrs in fine noedlei, neually onlr 
iliBcomible under the luierosrope. Tlii^re is rommmily no trace of an; bMe Wtween tlte 
iiigrt'dionb) of tho rock, whieh thus presents a thoroughly rry.taUine or graiiituid atnic- 
tore. AvernRo cliomical composition: silica, TA; aluuiiin, IG'O-lSj potanh, 1-5-2-9: 
soda, 2-3 ; limn, G-7'5; raagnosia, G'O; oxides of iron and manganese, 10-11 ; mmn 
specific gravity about 205. 

Among tho TDricties of Diorite the following may bo enumerated. Qnarti- 
diorite, containing free quartz, usually only to be detects by mieronoopic einmina' 
t'lun, Aplinnite (Aphnnitie diorite) an cxeeedingly eompaet roi-k, in whieli 
the component minerals aro not mncroscopienlly dintinguishablc. A rariety mntoin- 
ing dispersed rrystala of felupar or liomlilende in tenreil Diorito-porphyry. 
Corsito is n granitoid mixtoreof greyish-white anortliile, blackiidi-greon hornblende 
and Home qnortz, which here and there hare grouped thamselTCS into globular aggre* 
gations(Ujb;cular diorite, Kngeldinrit, Kapoleonite). Micadiorite 
contains abundant dork inieo, whiob uiny even locally replace the hornblende 
(soo KrrMnfiU, p. 150} ; Epidiorito, with fibrous homhieade, augite, titanifetouH 
iron, msgnetitc, and pyrite; Tonalite, a variety eontaining quarts, hornblende and 

Diorite occurs aa an eruptive rock under oonditions aimilnr to those of qnnrti* 
porphyry aud syonito. It is found among Palsjozoievolconic regions, oa in North Wales, 
ill " neck "-liko masses whieh may mark the position of somo of the voleanir oriDces nf 
eruption. It occurs also in a&jociatiou with granite and the crystnlline sehiiits, in suck a 
manner as aomctimes to si^ggest that it waa erupted previous to the process by whieh 
liic surrounding strata were metamorphised into eohists (sec p. 120). 

Sooite — eouipoHed mainly of plngioclaw. qiutrtz and mica, with a varying amount 
of aanidine as an aeccgsory ooustitncnt, and. by addition of hornhlendo and pyroxene, 
graduating into homhlendc'amleaito. The ground-nia»i has n fclaitie, sometimea 
■phcruhtic, glasKv, or finely grannlar base. CompoKitiun: silica, GO-SG; alumiuai, IG-S3; 
iron oxides, 24I: lime, 317; magnesin, IM; alkalies, 708; water, 0-45. Hean 
spi-eifle gravity, -2-GO. Tliis roek i« extensively deveteiied in the Gteat Basin and other 
tracts of we»tem Xorth America among Tertuir>- and recent volcanlo ontbnrala, 

Horablende-AndeBite'consJals of a triclinic felspar and hornblende, often wtth 
a liltle annidiiic. The ground-mass ig frequently quite erystalline, or ahmn a w^H 
proportion of a felailio natun>, witli wicroliths and gtannle*. Sometimea dtstinoUr 
e-ry stall ill e, sometiiiies eitrenicly eompoct, almost vitrconi^ It eontabiR rrvrilali nf 
lilagioeloB.' (andcsine), hornblende, augite, and rarely Muidine, with not ivr'.- ,>!..i,ily 

' On iliorite, its slroctuii^ and geological relations, eoii«>)li li 
plulonie rocks bv lie la Vultee Poussin aud A. Beiiard, JU,in. .laur 
llehretis, X, net J.tlirl.. Mill. IS 71, p. 460; Zirkel. ' MicrosnTT 
riiillijw, Q. J. G"'t. S-ir. xxxii. p. 15.1, and xxiiv, p. i71- * 
the constitution ofsnnic of the " greonstouei " of tin- ■■'■i 
out. Many of tliese aneient rocliB ore then shniv^ '' 
<'hanjn: of thiir original augite into honhlen'* 

= t<oo /irkel, ' MirroBCOpienl Petr"- 
ParaJle!; p. :.Ci. Hague and Iddiu- 




Past II § viij MASSIVE HOCKS— ANDESITR 149 



biotite, apiUite, and tridymite, imbedded in a batio composed of an aggregutc of 
oolonrleas felspar-microliths and grains of magnetite. Composition, silica, 61*12; 
alomina, 11*61; oxides of iron, 11*64; lime, 4*33; magnesia, 0*61; potash, 3*52; 
soda, 8*85 ; ignition, 4*35. 

Homblende-andesite is a volcanic rock of Tertiary and post-Tertiary date found in 
Unngaij, Tiansylyania, Siebengebirge, and in some of the Western Territories of the 
United States. According to recent researches by Messrs. Hague aud Iddings, 
gradations from this rock into basalt and hypersthene-andesite can be traced in 
Oallfomia, Oregon and Washington. These rocks, therefore, cannot be said to have 
sharply defined and distinct forms.^ 

Hornblendo-Mioa-Andesite. — ^Under this name is now classed by 
American petrographers a frequent variety of rock throughout the Great Basin, 
characterised by the vitreous appearance of its felspar, its rough porous trachyte-like 
groond-mass, and the presence of mica as an essential constituent This term will 
iuclade a large |»roportion of the rooks hitherto classed as trachytes, but in which the 
felspar proves to be plagioclase and not sanidine.' 

Propylite— a name given by Richthofen to certain Tertiary volcanic rocks of 
Hungary, Transylvania and the Western Territories of the United States, consisting of 
a triclinic felspar and hornblende in a fine-grained non-vitreous ground-mass, and 
closely related to the Homblende-andesites. They are subject to considerable altera- 
tion, the hornblende being converted into epidote. Some quartziferous propylites have 
been described by Zirkel from Nevada, wherein the quartz abounds in liquid inclusions 
containing briskly-moving bobbles, and sometimes double enclosures with an interior of 
liquid carbon-dioxide.' A specimen from Storm Canon, Fish Creek Mountains, gave 
silica, 60*58; ainmina, 17*52; ferric oxide, 2*77; ferrous oxide, 2 * 53 ; manganese, a 
trace; lime, 3*78; magnesia, 2*76; soda, 3*30; potash, 4*46; carbonic acid, a trace. 
IjOss by ignition, 2*25; specific gravity, 2*6-2*7. More recent investigation by the 
geologists of the Geological Survey of the United States has led them to conclude 
that the rocks included under the term *' propylite " in the western parts of America 
arc varieties of other species in various stages of decomposition — granular diorite, 
pori)liyritic diorite, diabase, quartz-porphjTy, hombleude-audesite and augite-audesite. 
Should this prove to be the case elsewhere also, the nume^iyill bo distised.^ 

Porphyrite, — This term may be used as the designation of rocks which, consisting 
C:jsentially of some triclinic felspar, show a true porphyry ground-mass, containing 
crystals of plagioclase, with hornblende, magnetite or titaniferous iron, augito, or mica. 
Thus defined, these rocks correspond, in the plagioclase series, to the orthoclase- 
porphyries and felsites of the orthoclase series. Their texture varies from coarso 
crystalline-granular to exceedingly close-grained, and passes occasionally even into 
vitreous. Porphyrite is a volcanic rock very characteristic of the later Palseozoic 
formations, occurring there as intcrstratified lava-beds, and in eruptive sheets, dykes, 
veins, and irregular bosses. In Scotland it forms masses, several thousand feet thick, 
erupted in the time of the Lower Old Red Sandstone, and others of wide extent, and 
several hundred feet in depth, belonging to the Lower Carboniferous period. In 
Germany it appears also at numerous points, where it is referred to later Pal«eozoic 
times. 



* Amer. Journ. ScL Sept. 1883, p. 28^. 

« Hagne and Iddings, Amer. Journ, 8ci. xxvii. (1884), p. 4G0. 

» Zirkel's * Microscopical Petrography,' p. 110. King, •» Explanation of 40tli Paral- 
lei," vol. i. p. 545. C. E. Button's '^High Plateaux of Utah" {U.S. Oeographicaland 
Gtdogieal Survey of the Uoclcy Monniainti), chaps, ill. and iv. Hague and Iddings, 
Amer, Journ. Set. 1883. 

* G. F. Becker on the Couisiock Lodo. licjioriA of U.S. Geological Survey 1880-81, 
and his full memoir in vol. iii. of the Monograph of U.S. Oeol Survey (1882). Hague 
and Iddinga, Amfr. Journ. Set, xxvii. (1884), p. 454. 



150 GEOGNOSY. [Book II. 

Turphyritc foniis a connocting liuk botweuu tliu plagioclasc-honibleDdo rocks and 
thu plagioclaac-uugiU* »erii>:3.' 

2. rLA(ii<KLAs«E-3iioA liucKft?. — Kersantite— an aggregate of a plugioclaso felspar 
and black mien witli apatite, augito, lioinblcude, ortliocla^c and occatiioually quartz. 
The toxtiiR' id compact tu porpliyritic. This rock occurs in dykes and veint:, especially 
among the Palicozoic formations, its typical locality being Kersauton in lirittany, where 
it is a dark-grcL-n remarkably durable rock. A siugnlar vein of kersantite, 3 to GJ feet 
bioa<], lias 1k?cu traced for nearly live miles in the Ilarz.' 

3. PLAt..i«.M.:LASK-Ar(iiTK Ku('K8. — Diabaso.^ — This name has Ijceu given to certain 
dark green or bhick orui)tive locks found in the older gcrdogical fonnations, and ciuibist- 
iug CMientially of triclinic felsjiar, augit4>, magnetite or titauiferous iron, apatite, sunic- 
timcs olivine, usually with more or less of difl'used grci^nish chloritic substances (viridite) 
whicli have resuUwl frrnu the idteration of the augite or olivine. The microMMpic struc- 
ture Is quite crystal I inc. The average eomiiositlon of typical diabase may be Uikeii to be 
Kiliai,48-r»0;aluminn, 100; protoxide of iron, 12- 15; lime, 5-11 : magnesia, 4 il; ]intab]i, 
OH-1-5; so<lii, :; 1-5; wat<r, 1*5-2. Specific gravity alxait 2 '9. There is generally 
c^irbonic acid pa^sent, luiited with soujo of the lime as a decomi)osition pro<luct. 

Diabase is occasionally coarse and even granitoid in texture.^ From this cuu- 
dilion gmdations may be tmcc<l into exceedingly fine-grained and conipnct varieticb 
(I)iabaso-aphanite), which sometimes absnme a fissile character (Diubus- 
Hch ief er). Some kinds present a iNjrphyritic structure, and &how dispersed crystals 
of the component minerals (l)i aba so -porphyry, Labrador-porpliy ry, 
Augite-jjorphyry); or, as in some varieties of diorite, a concretionary urrange- 
meut is i>roduced by the a}>poarance of abundant pea-like bodies of a eoiniiact felsitic 
nniterial, imbeddeil in a conqiact or finely crystalline ground-mass (Variolite). 
When the green compact ground-mass contains small kernels of carbonate of lime, some- 
times in gre.it numbers, it is calleil i;alcareuU8 aphanitc (tr Calcapliunite. 
Smietiuies the rock is abundantly amygdaloidal. Though, as a rule, free bilica 
docs not occur in it, some varieties have been found to contain this mineral, and are 
distinguished as Quartz-diabase. The presence of olivine Imis suggesto<l the 
name Olivine-diabase as distinguished from the normal kinds in which this 
mineral is absent. A variety containing hornblende is termed Protcrobase. 
O p h i t e, a variety occurring in the Pyrenees, contains diallagc and epidote. 

Diabase occurs both in contemporaneous beds and in intrusive dykes and sheets. It 
was formerly supposed to be confined to the older gi'ological formations, while its place 
in Tertiary and recent times was taken by basalt. But some of the Miocene volcanic 
rocks of the west of Scotland are as good diabase as any among the Palaeozoic formations : 
while, on the other hand, many of tho dark heavy eruptive rocks 1)elouging to (lie 
('arbonifcrous system in the basin of the Firth of Forth are nnqnestionablc basalts. 
The main difiVrence Ix^tween diabase and basalt appears to bo that the rocks included 
under the former name have undergone more internal alteration, in particular acquiring 
the diffused *' viridite '* so characteristic of them. 

Melaphyre.— This t(»rm has been so variously defined that tho sense in which it is 
used requires to be explained. Senft* described melaphyre as an indistinctly mixed 
r(K?k, dirty groonish-brown, or reddish-grey, or greenish black-brow^n to black ; hard and 



' Sec an analysis of a porphyrito from the Vicentin, Von Lasaulx, Z. Vvut^th. Gtol. 
ftVx. XXV. p. .-^23. On microscopic structure of iwrphyrite of llfeM, see A. Streng, Keutt 
Jdhrh. 1S75, p. 785. 

- l.o8s>fii, Zcitsrh. JMvtHcJi. (Uul (i*:.<. xxsii. (1880), p. 445. Jiihrb, Prewf, Gtd, 
TAin(l(*nnift. IHiSO. A. von (Jroildeck, nn. rit. l.S8:i. 

» The student will fijid in the X^-iMin/t DtuUch, Geol. Gei. 1874, p. 1, an importoiit 
memoir by Dathe on tho com]iof>ition and Btructnre of diabaae. See also Zirkel's 
*^Iicroscop. IVtrog.' p. 07. 

* ^[ichel-lievy, Buli Soe. Irivl Fraui»^ &nl MC xi. p. 282. Geikie, Tnw, Jfov. Soe, 
Eilln. xxix. p. 487. • • Claisifioatiim 4er PdaorteD,' 1857. p. 263. 




Part IT. § vii.] MASSIVE nOCKS. — MELAPnrnE, 101 

tjii^h wLen frosh (but also often witli a pitcLstonc-likc greasy lustre, (^r like biisalt), and 
feLowiu*; cry&talb of reddisli-grey lubrudorite, with magnetic titaniterouis iron, and usually 
with uarbonatea of lime and iron, and femiginous chlorite (delesuite), and a cryvtallinc- 
gr.iuiilar or compact, earthy, porphyritic or amygdaloidal texture. Nuuinanu dcftneii melu- 
phyre as u gree&iah, brownish or roddish-bhick micro-crystalline or crypto-crydtallinc, 
fit'ldom slightly granular rock, with conspicuous dispersed crystals of labnidorite, and less 
frt-queut and diiitiuct crystals of pyroxene, not uncommonly rubellan or mica, but no 
i|uartz.' Zirkel in his first work called it a generally cryptocrybtalline, sometimes 
pirphyrllic, very often amygdaloidal mixtui-e, consisting essentially of oligoelase and 
aiigitu with magnetic iron.- In his more recent synopsis of the microscopic eharucters 
ffroi'ktt he admits the great diversity that has prevailed in the use of the term melu- 
phyre, und the wide range of structure of the rocks that have 1x)en includeil under it. 
Hi- rt'ganls the melaphyres as early i>recursors of (ho felspar-basalts, with but a rare 
devi-Iupmcnt of a })urely crystalline structure, and on the contrary a prominent non- 
iniUvldiialisctl substance which may either be abundantly develoiieil as a base or appear 
only sparingly ix.'t\veeu the crystals, and mny 1)0 sometimes purely glassy, sometimes 
li df-glassy, and sometimes completely dcvitritied.^ 

Kosenbus<.*h, aft<T a review of all the previous literature of the subject, proiwses that 
the term nielaphyro sliould 1)0 restricted to an older massive rock, consisting esiientially 
i«f plagioclase, augite, olivine, with free iron-oxid«;s and a ixjrphyry-bjisc of any btrneturu 
a':d in variable itroiKirtions, and belonging for tlie most part to the age of the Carlxjni- 
r. rolls or older rcrmian, less fret^uently of the Triaasic formations/ According to his 
iirraugt'meut, the old phigioclnsc-augite r(.K;ks arc groui»ed in three sections; Iht, the 
granular i-ection, ineludiug (a) JJialMun', comjiosed essentially of plagioclase and augito, 
a!id('') OHrintr-diahtitet composed of plagioclase, augite and olivine; 2nd, the {lorphy- 
ritic ;«cction (with a ground-mass), comprising (a) JJtahufc-itorphi/rifc — '.i diabase having 
a jirirphyry ground-mass, (h) Melaphyrey containing olivine in addition to the idtigiwlase 
ami augite : 3rd, the vitreous section, in which the subordinate glassy varieties of the 
di:il«fle-p Tphy rites are embraced.^ 

The attempt to base a classification of eruptive rocks uihiu chronological 1 eonsidrra- 
tiiiL-s lia3 been fruitful of mistakes by lca<Ung to false assumption regarding the age of 
\jut.fm6 ny^ks. Tl»c *>called melnphyre:^, like the dialjases, do not tlilfcr in any 
i:?-K:ntial f«.'alure of stnu-ture or romposition from the Ixi.saltH. So fntin-ly is tliis 
liif r.\si\ that, as aljove remarked, rocks now known t^) Ikj of Tertiary date, have 
h'.*»-n di.'S'^rilK:'*! as melaphyrcs, while others of liower Carboniferous ago hav»,* be«n 
Tiiiheiritutinirly referred to as Ixisalts.* 

Au^te-Andesite is the name given to certain dark ernptive rocks of Tertiary and 
ln.'at-Ttniary ilate, which consist of a triclinic felspar (olig<iclase, or some s]>eeii.'M nitln;r 
riirh»-r in silica than labradorite) and angite, with sometimes sanidine, hornblcnchr, 
biotiti*. magnetite, or ajHitite, and in wmio varieties quartz. The ground-mass is rc- 
H'lvable under the microscope, sometim(.-3 into a gla^^sy, sometimes into a more or lebs 
(•illy dev.trific-<l base, in which grains or crystals of plagioclax*. augih', sunitline, horn- 
Irleudv, magnetite, biotito and even quartz may occur. Tin- quartz-l>earing varioti< .s 
contain frkm 01) to C7 per c^'Ut. of silica, and in this resiK.'<'t, as well as in the failure of 
olivine, are dt^tiuguisheil from thf* bas3lt<<. The average eoi»ipo>ition of the quartzle.-s 
v;iri».-ti<:-s Cwhicli are likewise more acid than t'.ie basidts) may Ik.- thus given: silica. 
riTl.'i; oluaiina, I'MO; prot^)xide of iron, i:JO; lime, TrTo; magnesiii, 2'21 ; pota«h. 
I'^l ; >*Oila. :J S"* : ni'.-an specific gravity, 2-75 '2- Ho. 

A'!- • A !— it'^ orvurs in dyke««, lava-streams, plateaux, sheets and ncck-liki. 
■ iiS of extinct and active voleatiora, af? in 'J'nni.-vlva!iia an<l Ilniiirarv. 

it'oiii'li d. ( reognoiiic,* i. p. 5S7. " * IVtri»L'raphi»/ ii. p. '.'»J. 

>lii. Hirsebaflf.' p. 411. « *Mik. Phy.-iog.' p. :;v*2. - Op. rit. p. Ml. 
! .■-, pp. 115, 137, 150. S<»e als«» Tn'tn*. //o»/. >'*-■. VAhi. vol. \\\\. p. r.»:». On 

: iiielaphyroa see Boricky, Are-hir \>tt. L^mi. Lah.n. id. pt. 2, h' fi -'. j.. J. 



152 QE0OS08T. [Book II. 

Hastorin, loeland, Tcacrifle, the Weateni Territories oT Noth America, the Andes, 
Mew Zealand, &c. 

Bas&lt-Booka (FoUpor-bnealls).'— Under this title ia eubnicod as important and 
widespread series of volcanic rocks, which eontist usentiallj of some trielinic fek]jar, 
angite, oliriiie, mi^Detito or titanifennu iron. Frequently with apatite, aometimea with 
aanidineornephelinc. Four Tarioties are distinguished accordiag to texture: dohirite, 
anamesile, biualt, Bcd Titrcous bamlt 

Dolcrite igreemtont, in pnit, ofoldurauthon). This indndee all the larger-gr^ned 
kinds in wluch Ihecomponent crjsUlscan be readil; diatin^^hed with the naked eye- 
Tbc felspar, which among the baBalt-rocks is protiablf often a mote lilicated form than 
labradurite, is usually the most oonspicnotu ingredient ; the dark prisms of aagite, and 
llicdnsty ormiuQlcly octahedrolmagnetite^giTethe greyer blackhneto the rock. The 
toicruscopic atrnctare is crystalline, thongh a Bmall quantity of an amorplious base may 
hero and there bo traccil. 

Anameiito iucludea (hose kiods of which the texture is so fine that tbe naked eye 
can obaervo only thnt tbe masa ia a finoly crystallized grannlar aggregate. Under the 
microscope more of nn umorphoni base with mleroliths is seen than in dolcrite. 




yig. M.— MitTWUaipIc Stmctiireof Ruull.— Thgiarpi Bbodfd crjiiUla s 

right of lh« (EBlro at Ihe drawing, ire iggregsttd Into ■ li 
specks Arc Magnetile, 

Fig. SS.'-Junnliin of IntrniilTC IHibiM vith SijHMon«. Salisbury Crag. FjlInlniTsfa. llaKnlfied 10 
PUm'^rH.— The Rruihlar pitrllon hi the iHrtiom or tbo drawing ii Sandatonp, ■ part of wbtcli ii 
iiivulvtd In thp IMabaiv Ihst nrcuplRi III" rviil of Ihp M\\e. Tin duiker p;inlnn nrxl the SaiuMwie 
n» Wn HTiKiitiniifd. It roirtntas rrj-Malu o( PUglorUsi! and vaponr 
~ bove tho darker part Ihe glaiwy coudUkin rapVdlr t^bh 
. . iconsldfrabljralured.cnldtt occuprlng 

Basalt.— This name, when used hm thcdcsi^etionoCn particular rock, ia applied to 
hinrk, cilrcmely compact, oppnrcntly homogeneous varieties which break with a splintery 
or conctioiilal fracture, and in which the component mincmln mn only be obserred 
with the microecoiH', un1ci« where they are scattered porphyriticallj through the masi. 
Tilt minerals cimiiist of tlioae nbote menlioncd, imboddol in a lia!C which is sometime* 
11 glaijs, but is ortcn psirtially dcvilriiird by tlic nppoamnrc of rarioiu cryatallitcs, 
()iat somttimcs iticrcasc till llie glass disappeora, and ila place is taken hy an 

' On basalt roeka sec Zirkel's 'BaBoltgrsteinp,' 1870. IJoricky's ' Petrographische 
Stnilinii an di'n Bii»ltKi-'sl«inen Bulimcns," in .^liWiiV fiir Natunvit. T^»d^dHreh~ 
/■•r^hm-j TOM llrj„M„, ii. lS7;i. Allpnrf, Q. J. Oeol. S>«: \xx. p. 329. Gcikie, Tram, 
lliy. S<-r. Killn. xxii. Miihi, Nov. Art. Anid. Lrop. Carol, xxxvi. (1873), p. 74; Nttut 
Jnhrh. 1873. pp. 44!). 894. F. Eichsdiilt on HsRalts of Soinia, Srerigr* Giol. Umlmilb, 
scr. !■. X'.. .11, 18H2. E. Syedninrk, op. eil. No. GO, 1«8I(. 



Pabt it. § vii.] 



MASSIVE BOCKS— BASALT. 



153 



•SCP^egaie of minote granules, hairs, needles and crystals. The proportion of this base 
▼vies within wide limits, insomnch that while in some basalts, it so preponderates that the 
indiyidoAl crjrstals are scattered widely through it, or are drawn out into beautiful streaks 
ftnd eddies of flnxion structure, in others, it almost or wholly disappears, and the rock 
then i^pears as a nearly or quite crystalline mass. The component minerals frequently 
appear porphyritically dispersed, especially tlie olivine, the pale yellow grains of which 
are characteriatia Two types of basalt have been recognised in the great basaltic 
outbursts of Western America: (1) tlie porphyritic, consisting of a glassy and micro- 
lithic or microcrystalline ground-mass, bearing relatively large crystals of olivine, felspnr, 
and occasionally augite, a structure showing close relations to that of many andesitcs ; 
(2) the granular (in the sense in which tliat term is used by liosenbusch, ante, p. 93) 
— an aggregate of quite uniform grains, composed of well-developed plagioclase and 
olivine crystals, with ill-defined patches of augite, and frc([uontly witli a considerable 
amount of glass-base. By diminution of olivine and augmeniiition of silica, and the 
appeaninoe of hypersthene, gradations can be traced from true olivino-basalts into 
normal andesites.^ Many years ago, Andrews detected native iron in the barnilt of 
Antrim. More recently Nordenskiold found this substance abundantly diffused in the 
baaalt of Disco Island, occurring even in large blocks like meteorites (ante, p. 65). 

Basalt occurs in amorphous and columnar sheets, wliich may alternate with each 
other or with associated tuffs. It also fonns abundant dykes, veins, and Intrusive bosses. 
It frequently assumes a cellular structure, which becomes ninygdaloidal by the 
deposit c^ calcite, zeolites or other minerals in the Ycsicles. A relation may bo trace<l 
between the development of the amygdules and the state of the rock ; the more 
amjgdaloidal the rock, the more is it decomposed, sliowing that the materials of tlie 
•mygdulet hare probably in large measure been derived by inHItrating water from the 
boMlt itaelf. 

Titreoua Basalt (Basalt-glass).' — In some cases, basnlt imsses into a condition 
which, even to the naked eye, is recognisable as that of a true glass. This more especially 
takers place along the edges of dykes and intrusive sheets. Where an external skin of 
the original molten rock has rapidly cooled and consolidated, in contact with the rocks 
through which the eruption took place, a transition can bo traced within the space of 
le»s than a quarter of an inch from a crystalline dolcrite, anamesite, basalt, or andositc into 
a black glass, which, under the microscope, assumes a pale brown or yellowidh colour, and 
is isotropic, but generally contains abundant microliths, sometimes with a globular or 
fcphenilitic concretionary structure. In such cases it seems indisputable that this glass 
represents what was tlie general condition of the whole molten mass at the time of 
eruption, and that the present crystalline structure of the rock was developed during 
cooling and consolidation. Some varieties contain a good deal of water {UydrotachijUte, 
ralwjoniU) ; others have little or none (Tarhylite, Ilyalomelan). It is worthy of remark 
tiiat in the analyses of vitreous basalts, the percentage of silica rises usually above, while 
Ihfir s|»ecii{c gravity falls below,' that of ordinary crystalline basalt. The average 
composition of tlie basalt rocks is shown in the subjoined Table : 





HiUca. 


Alu- 
min*. 


Lime. 


Magnofda. 


OxldcH of 
Iron and 
Mangan- 
ese. 


1 I»ss l>y 
Polaxh Socla ! 'PlHioil 


.Specific 
gravitj'. 


Ati«in«?9ite . . 
Ha^lt . . . 
VitrnoQt B«f«tt 


45-55 
46-53 
45-55 

4i$-58 


12-16 
12-15 
10-18 
12-17 


7-13 
8'8-13 

7-14 
6-5-9-5 


3-9 
1-6-9-5 

3-10 
0-5-6 


9-18 

10-15 

9-16 

8-20 


0-1 
0-6-1 
0-6-3 
0-6-9'5 


2-5 

2-3 

2-5 

2-5-5 


0-5-3 

1-3 

1-5 
0-5-3-6 


2«75-2-06 
2-7-3-8 

2-85-3-10 
2-6-2-7 



' Ilaguo and Iddings, Amer. Journ, Set. xxvii. (1881), p. 450. 

* See Judd & Cole, Q. J. GeoL Soc, xxxix. (1883), p. 444. On the glassy basalt- 
l:\v:u» of Sandwich iMlands, Cohen, New>n Jnhrh. 1870, p. 744 ; 1880 (vol. ii.), p. 2X 



154 GE0ON08T. [Book IL 

Tho basalt-rocks are thorouglily volcanic rocks, appearing in lava-tftreams, sbiects. 
plateaux, dykes, and veins. The columnar structure is so eommon among the finer- 
grained varieties that the term ^^ basaltic *' bus been popularly used to denote it. As 
already stated, it Las been assumed by some writers that basalt did not begin to be 
erupted until tho Tertiary period. But true basalt occurs abundantly in Scotland, as a 
product of Lower Carboniferous volcanoes. There seems, however, to be no doubt that, 
us liiclithofeu first pointed out, iu the order of appearauce at any given volcanio foons, 
basalt usually comes up after the r)iyolitic and trachytic eruptions have ceased. (See 
Book III. Tart I. Section i. § 5.) 

4. rLAciui'LASE-DiALLAiiE KocKS. — Qabbro ' (Diallagc-rock) is a thuroughlj 
crystalline granitoid aggregate of a triclinio felspar (sometimes, however, saussaritc) 
and diallago or smaragdite. The felspar (lubradorite or anorthite) occurs in distinct 
crysUds or crystalline aggregates of grey, white, or violet tint, and under tho microsco]K3 
ib sometimes found to be crowded with crystallites. The saussurito is likewise light* 
coloured, while the diallage is distinguishable by its dirty-green or brown tint, the 
metalloidal or pearly lustre on its cleavage planes, and the frequent presence of layers of 
microscopic dark brown or black lamella). Some varieties contain abundant olivine 
(0 1 i V i n - g a b b r o). Average composition — silica, 49 ; alumina, 1 5 ; lime, 9*5; mag- 
nesia, 9*7; oxides of iron and manganese, 11*5; potash, 0*3; soda, 2*5. Loss by 
ignition 2*5; specific gravity, 2 * 85-3 * 10. 

A variety of gabbro abounding in olivine, but with little or no diallage, is known by 
Cierman petrographers under the name of Forellenstein, its olivine being usually 
sorpcntinized and its felspathic constituent (anorthite) being also often much docom* 
jjoscd. 

Gabbro occurs (1) in association with granite, gneiss, and other crystalline rocks as 
large irregular bosses (Saxony, Silesia, the Harz, &c.), and (2) in large sheets and bouses 
associated with volcanic eruptive rocks. In the latter case it occurs in Skye and Mull 
connected with Tertiary volcanic outflows. 

5. PLAttiocLASE-mioMDic-PYBOxKNE HocKs. — Hyperstheiie- Andesito. — Under 
this name, certain Tertiary or recent rocks, stretching over vast areas in Western America, 
have been described as associated with other andesites and basalts. They are black 
to grey, or retldish-grey, in colour, and vary in texture from dense, thoroughly crystalliuo 
forms, to otliers opproaching white glassy pumice, the base under the microscope ranging 
from a brown glass to a holo-crystalline structure. The magnesian silicate is pyroxene, 
chiefly in tiic orthorhombic form as hypersthenc, but partly also aug^te. An analyBis 
of the puniiceous form of the rock gave 62 per cent of silica, while the percentage of the 
iiumo constituent in Uie glass of the base was found to rise to 69*94.' 

Norite. — Under this name Bosenbuscii has proposed to group all the older gabbro- 
like rocks in which any rhombic pyroxene (Enstatite, Bronzite, Hypersthene) is con- 
joined with a i)lugioclaso felspar. The term was proposed by Esmark for certain 
Norwegian rocks which appear to be for the most part varieties of diorite, and was applied 
by Schecrcr to some Norwegian compounds of plagioclase (and orthoclase) with diallagc 
or hyixirsthene and usually some quartz. 

Hypersthenite — a granitoid aggregate of lubradorite and hypersthene found in 
K'<ls, veins, and bosses, among Archaean rocks (St. Paul's Island, Labrador ; Greenland, 
and Norway). 

Schiller-spar Rock (Schilkrfels, Protobastilfel^)— an aggregate of anorthite and a 
rliombic pyroxene (cnstatito), the latter mineral being usually altered into schiller-spar 
nr aorpcntiiie (protolmstite), with the development also of chromite and magnetite. ITic 
tVlspar uppojirs somewhat like the S4iU83uritc of gabbros, the em(t4&tite shows a pearly 



'On Gabbro, sec Loosen, /. Vt^itsrJt. (iroL <ics. xix. p. 6ol, Lang, op, cU. xxxi. p. 484. 
/iikel on (iabbros of Scrtland, oj). n't. xxiii. 1871. 

^ M'hitninn Cross, null. V.S. Grol Surrey, 1883, No. 1. Hague and Iddings, Amer. 
Joniit. Sri. xxvi. (1883), p. 226; xxvii. (1884), p. 457. 



Paot n. § viL] MASSIVE BOCKS. 155 



lasire on its daovage faces, bat this is commonly replaced by the metallic lustre and 
green serpeutinous aapeci of the more or loss decomposed schiller-spur.* 

ii. Nepheline-Bocks. 

Zirkel proved that basalt-rocks sometimes contain little or no felspar, the i>urt of 
tliat mineral being taken in some by nepheline, in others by leiicite.^ Under the numo 
of Xopheline-rocks is grouped a series of distinctly crystulline and also compact, dark 
rucks, composed of nepheline, angite, and magnetite, often with olivine, Eometimes with 
a little iriclinic felsi>ar. They are thus distinguisheil by the fact that iu tlicm, the imrt 
taken by felspar iu the rocks already enumerated, is 8up})lied by nepheline. They are 
nhually divided into Nepheline-Dolerite, a crystalline granular aggrcgtite closely 
resembling in general character true dolerito; and Nepheline-Basalt, a black, heavy 
cum^Kict rock, not to bo outwardly dibtinguished from oiilinary felnpar-baualt. Whero 
olivine is absent, Kosenbusch has proposed to call the rock Nephcltnite, The nepheline 
rucks are volcanic masses of late Tertiary age, but occur much more sparingly than tliu 
true basalts. They are found in tlic Odenwald, Thuringcr Wald, Erzgcbirge, Baden, kv. 
The mean rom|K>8ition of Nejiheline-basnlt may be taken to be— silica, 45*52; alumina, 
10 '50; ferric and ferrous oxides, 11*20; lime, 10*62; magnesia, 4*35; jiotash, 1*05; 
» ida, 5 ' 40 ; water, 2 * G8. Mean specific gni vity, 2 * 0-3 • 1 . Nepheline-Tephrite— a 
dark uouijiact aggregate of nepheline and plagioclasu with hornblende, augite, mica, 
KUiidine, olivine, ajiatito, sphene, magnetite, or titaniferous iron (Canary Islands, 
Bohemia, Sweden). 

iii Leuoite-Books. 

This division includes certain grey or black crystalline or compact volcanic rocks 
resembling some of the basalt series, but distinguished from them by the predominance 
of leocite. The more crystalline-granular varieties, named Leudtophyre or Ijeucite- 
porphyry, are composed of a characteristically dull grey aggregate of leucite, augite, and 
magnetite, with sometimes a little nepheline, olivine, or mica. The leucite occurs iu 
well-defined garuet-like crystals of a dull white colour, sometimes an inch in diameter, 
not infrequently broken and with fissures interx>enetrated by the surrounding ground- 
iiLiM. The rock is one of tlie products of the extinct volcanoes of Southern Italy. 
Leucite-Basalt is to outward appearance quite like true basalt, and occurs under similar 
conditions, but is less widely distributed than even nephcline-basalt. Under the micro- 
soojie, it presents a finely crystalline structure with little trace of any amorphous 
base, and abundant minute sections of the characteristic leucite, with augite, olivhiu, 
magnetite, and nepheline. This rock occurs among the extinct volcanic cones of the 
Kifel, in the Thuringer Wald, and in the Italian volcanic districts (Albano, Capo di 
Bove). l-icucite-rocks, so far as known, occur only among later Tertiary and recent 
Vftlcanic products, llosenbusch has proposed to separate the varieties containing no 
"livinc as a distinct group under the name of Leucitite. The modern lavas of Vesuvius 
tiiough closely relate.1 to leucite-basalt, contain sanidine and plagioclase, in addition to 
tlic onlinar}' constituents of that rr)ck, and i)08sess a more vitreous base. Ijeucite- 
Tephrite — a rock resembling ncphcline-tephrite, but with leucite replacing the nepheline 
(Uocca Monfina). 

Iv. Melilite-Bocks. 

In continuation of ZirkeFs research, A. Stelzner has shown that in some basalts the pari 
"f fclsjjar and nepheline is played by melilite.' In outer appearance, the rocks jwssessing 
tliia com[)O0ition, and to which the mime of Melilite-Basalt has been given, cannot be 
•listinguished from ordinary basalt. Under the microscope, the ground-mass appears to 
U mainly composetl of transparent sections of mclilite, either disposed without order, or 

^ Stn.ng, New:s Jahrb. 1862, p. 513 ; 1864, p. 257. « Zirkels * Basaltgesteine.' 
» Netw^ Jahrh. (IJeihigeband). 1883, p. 360-439. 



mnc^ in iliixiiin litlM rntinil Uis iai^ nliTUU ui 



TIiiH 'liviKini) '■■nhrnm \ !i«hKi "I' i.TT«tKilinB nda <vmpiimd \aaBatiallj iti oUna*, 
vjth iiimitllr nw or two other m 

Pllrrfto iT^Mimibntd. Prkme-nnTphiTT; — & rDcktieli in. olivine, i 
l^an >i>>Tp«ntini»vl. i»itli :in5itp. :dagnelite. ■■ ' 

rpl«»<'H tn ih" Unhiam into irhieh iif tfaeaiiditiDnif pi 

;n ,1 liAiw mnr'- or Iphh ^trmna. Dnmilft Uttr^ttiir ami irtfaer aIivii»*rDckA daml f^*iTg 
'hn >-r)'KWIiTiP nrtiiiiti fp. 1:10) miebtiljiD be induded Eieip. 

On« 'if thn iniuil remarkiihie tcBtnm ;tiKiin theM rocks ui thtJr fretintat MMxiatkni 
with vTp»ntlrH> aiiH tbHr '^'iuIi^ft to ]mt» into tliat imk. Tben l-sii imfeed be no 
<t>>>iM DihC, m T'j-iierDuili drat [minted out, maoj deipaitulea vera once oliiute mcki. 




Vl«. K^SUSM m Hi*«li*i»lloiio[(Jl.*lM. *. Ow DMTlj&Trfi trjitil i ». tW lUetUim UtU oonnJtMa 



vi. Berpoitine-Boclcs. 

VUAei tlhU nnme nwj be included iock» whkh, wtulerer may havo been tliei/ 
'irlKln"! flmrantiT nml enntpodtiiin, now conaiat moinlj or wliollf of Mrpentine. As 
ti\vw\y M«ln1, oliTlnn ri«dily pwuei into the condition of aerpentine, while the other 
nilnptnlfi mnf itimnlri nrnrty nnaffoctcd, m ia ailmimbly seen in «ome pifcrit^a. Miiny 
norjiPiillrKi-rnnli* originnlly conHiateil principelly nf olivine. Dioiite, gnbbro and other 
tnoliri.mnaiBtinKliW'ly ofmftpiraianailicatca, likewise poEB into serpentine. If Tariclies 
i1iti> bi iIlfTiTont plHiW'M »f alteration were judged worthy of acpnTate dcb-ignBtiiHi, each 
iwmlmr <if Hip ullrino-MckH miglit of course htive a conceiioblc or actnal rcpreBentali«e 
iHiHMijt Hip i«'r|ii<iitliiii aoriea. Itiil, without ntterapting thia minutcnew of claaai&cation, 
wi' may with i«l*ntilnKi' trwit by ituclf, aa dcsorring apcoiol notice, the maaBiTC form of 
l.lip tiilitPml Hi<T|H'ntliiP t<i wlintMiever ciiuso ita mode of formation may be assigned. 

Merptmtlno * a rniii]iiii<l iir lliiely graniilnr, fidntly glinunering, or doll mck, eaaily 
vul iir M-mli'lii'd, liiivliiK n jircvnilinf; dirty-green nilonr, Eonii-tinics varionaly atreoked 

' IMiiiIk-I, 'IIIi' 1\iln<H>lilhiK'bo]i KruntiYeiMtviue d«a I'ichtolgebirgca ' : Munich 
IHTt 7>...i., ((..«. W-. Wl.«. xxix. p. tm. 

•-■r-br.nimk. SiU. Ahvl. ll'iVn. Ivi. .Inly, ISfiT ; Bonnoy. p. J. C.fol. Sw. sjsiiL p. 



\....« ,l.ik,f ls:«, mv S.'Hi.'llllT. 
,l.....iit., .1 ^M„ H„. !l«v« BTiniili 



Paw n. I viL] XA88T7E BOCKSSEBPENTINE. 157 

or flecked witb brown, yellov, or red. It is a mnatiiTe fono of tho mineral acrpentiiie, 
bnt treqnenU]' eoDtaiits other miaeralo. One of ita commonest tuxompauimenta is 
duTMtile or flbiona serpentine, whioh in vaininga of a ailkj lustra, often rtimiflca 
throogli the took in all directions. Other common enclosnres are olivine, bronzite, 
enatatite, inagnetile, and obiomio inm. 

Serpentine oconn in two distinct forms ; flret in beds or indefinitely-ibaped bosses, 
interoUated among sobistoBo rooks, and associated especially witb crystalline limestoues ; 
Koood Id djkea or veins tnversing other rocks. 

Aa to its mode of origin, there can be no donbt that in somo cases it was originally an 
etoptiveroek. In the Old Bed Sandstone of ForfiuBhirc aod Kincardinoshire it is found 
in dykM travetdng the sandstones and oonglooicrslas. Tbe frequent oconrcenco of 
recognlnble olivine crystals, or of their stili remaining contours, in tbe midst of tlie ser- 
pentine-matrix, affords likewise good gionnds for assigning an eruptive origin to many 
serpentines wbiob have no distinctly eruptivo oitemal form. Tbe rock cannot, of oonrse, 
have been ejected as the hydrous magnetdan silicate serpontiDo, but it may have been 
nijginaUy an eruptive olivine rook, or a highly homhiendic diorito, or an olivine-gabbro, 
injectod in the form of sheets or dykes. But, on the other band, the intercalation oi 
bed* of serpentine among schistose rocks, and i)articn1arly the froqnent occurrence of 
■npcnline in oonneotioQ with more or less altered limestoncs(West of Ireland, Highlands 




of SootUnd, Ayrshire), snggests another mode of origin in tlieso cases. Some writers have 
contended thnt such serpentiucK are products of the alteration of dolomite, tho magnesia 
having been taki^n up by silica, leaving the carbonate of lime liehind nsbeds of limestone. 
It is conceivable, however, that in some eases the original roclo, from which tho scr- 
pratines were derived, wero a deposit from ocennic water, aa has been suggested by Sterry 
Hunt in tho ease of those associated with the crystalline schists.' Bods of serpentine 
uilcrcolateil with limestone might conceivably have been due to the elimination of 
■oagTKsian ellicnti^s from sea-waterby organic agency, like the ghtuconitonow found filling 
the chamtiers of /ommiaifera, the cavities of corals, the cnnals in shells and sGA-ureliiri 
■pines and other organisms on the floor of the present sco.* Among tho llucitones and 
rryi^alline nchistisof Banffshire, as already slnted (niitf, p. 130) serpentine occurs in thick 
Imtii-uUr liods which possess a schigtoee crumplol atructuro and agree in dip witli tlio 
fonnunding rocks. They mny have been deposits of contemporaneous origin with the 
limettimes and schists amoni; which they occur, and in association with which they have 
undergone the eharacteristie schistose puckering and crumpling. 

' 'Chemical B«says,' p. 123. 

' According to Berthier, one of the glauconitic deposits in a tertiary limestone is a 
iTDt serpentine. See Sterry Hunt, ' Chem. Essays.' p. 30'J. 



158 GEOGNOSY. [Book II. 

B. F R A G M E N T A L (0 L A S T I c). 

TliiB great Beries cniLraees all rocks of a secondary or derivativo 
origin ; in other woixls, all formed of fragmentary materials which have 
previously existed on or beneath the surface of tlio earth in another fonn, 
and the accumulation and consolidation of wliich gives rise to new 
compounds. Some of these materials have l)een produced hy the 
mechanical acticm of wind, as in the sand-hills of sea-coast« and inland 
deserts (-^TColian rocks) ; otliers hy the operation of moving water, as the 
gravel, sand and mud of shores and river-l)eds (Aqueous sedimentarj' 
rocks) ; otliers hy the accumulation of the entire or fragmentary remains 
of once living plants and animals (Organically-formed rooks); while 
yet another series has arisen from the gathering together of the loose 
dc'hris thrown out hy volcant)eK (Volcanic tuffs). It is evident that in 
dealing with these various detrital formations, the degree of consolidation 
is of secondary importance. The soft sand and mud of a modem lake- 
bottom differ in no essential re8i>ect from ancient lacustrine strata, and 
may tell their geological storj' equally well. No line is to l)e drai^Ti 
between what is popularly termed rock and the loose, as yet unconi- 
pacted, debris out of which solid rocks may eventually l)e formed. Hence 
in the following aiTaugement, the modem and the ancient, Ixjing one in 
structure and mode of formation, are classed together. 

It will bo ol)8erv'ed that, in several directions, we arc led by the frag- 
mental rocks back to those stratified deposits with which we began at 
p. 117. Both series of deposits arc accumulated simultaneously and are 
often interstratified ; and, as wo have seen, the calcareous organic frag- 
mental rocks (p. 118) actually undergo a gradual internal change which 
more or less effaces their detrital origin, and gives them such a crystal- 
lino character as to jontitlc them to be ranked among the crystalline lime- 
stones. 

1. Gravel and Sand Books (Psammites). 

Afi the deposits included in this subdiyision are produced by the diBlntegration and 
removal of rocks by the action of the atmosphere, rain, rivers, frost, the sea, and other 
superficial agencies, they are mere mechanical accumulations, and necessarily vary 
indefinitely in composition, according to the nature of the sources from which they are 
derived. As a rule, they consist of the detritus of siliceous rocks, these being among 
the most durable materials. Qnartz, in particular, enters largely int-o the oompositioQ 
of sandy and gravelly detritus. Fragmentary materials tend to group themaelTes 
according to their size and relative density. Hence they are apt to occur in layers, and 
to show the characteristic stratified arrangement of sedimeiifury rocks. They may 
enclose the remains of any plants or animals entombed on the same sea-floor, river-bed, 
or lake-bottom. 

In Uie majority of these rocks, their general mineral composition is obvious to the 
naked eye. But the application of the microscope to their investigation has thrown 
considerable light upon their composition, formation, and subsequent mutations. Their 
component materials are thus ascertained to bo divisible into — 1 st, derived fragments, of 
which the most abundant are quartz, after which come felspar, mica, iron-ores, zircon, 
rutile, apatite, tourmaline, garnet, sphenc, augite, hornblende, fhigments of various rocks, 
and clastic dust; 2nd, constituents which have been deposited between the particles, 



Part IL § viL] FRAQMENTAL ROCKS. 159 



and which in many oases serve as the cementing material of the rock. Among the more 
important of these are sQicio acid in the form of qnartz, chalcedony and opal ; oarbonateB 
of lime^ iron or magnesia ; hiematite, limonltc ; pyrite ; glanconite ; mica ; rutile.* 

ClifT-IMbris, Koraine Stuff— angular nibhish disengagf^] by frost and ordinary 
atmospheric waste from cliffs, crags, and steep slopes. It slides down the declivities of 
hilly regions, and aocnmnlates at their base, until washed away by rain or by brooks. 
It forms talos^opes of as much as 40% though for short distances, if the blocks are laige, 
the general angle of sbpe may bo much steeper. It naturally depends for its oom- 
positioa npon the nature of the solid rocks from which it is derived. Where cliff-debris 
falls npon and is borne along by glaciers it is called *' Bf oraine-stuff," which may be 
deposited near its source, or may be transported for many miles on the surface of 
theioe. 

Pwrched Blocks, Erratio Blocks-— large masses of rock, often as big as a house 
which liave been transix>rtod by glacier-ice, and have been lodged in a prominent position 
in glacier valleys or have been scattered over hills and plains. An examination of their 
mineralogical character leads to the identification of their source and, consequently, to 
the patli taken by the tmnsporting ice. (See Book III. Part II. Section ii. § 5.) 

Bain-^vmah — a loam or earth which accumulates on the lower parts of slopes or at 
their base, and is due to tlio g^dual descent of the finest particles of disintegrated rocks 
by the transporting action of rain. Brio k-e a r t h is the name given in the Eouth-east 
of England to thick masses of such loam, which arc extensively used for making 
bricks. 

Soil — the product of the snbaorial decomposition of rocks and of the decay of plants 
and animals. Primarily the character of the soil is determined by that of the subsoil, 
of which indeed it is merely a furthnr disintegration. The formation of Eoil is treated of 
in Book UI. Part II. Section ii. § 1. 

Subsoil — ^the brokcn-up part of the rocks immediately under the soil. Its character, 
of course, is determined by that of tho rock out of which it is formed by subacrial 
disintegmtion. (Book III. Part II. Section ii. § 1.) 

Blown Sand — loose sand usually arranged in Hues of duiies, fronting a sandy 
beach or in tho arid interior of a conliiient. It is piled up by the driving action of 
wind. (Book III. Part II. Section i.) It varies in composition, being sometimes entirely 
liliceous, as npon shores where siliceous rocks arc exposed ; sometimes calcareous, where 
derived from triturated shells, nullipores, or other calcareous organisms. Layers of 
finer and coarser particles often alternate, as in water-formed sandstone. On many 
coant-lines in Europe, grasses and other plants bind the surface of the sliifting sand. 
TiM»e layers of vegetation are apt to be covered by fresh encroachments of the looso 
material, and then by their decay to give rise to dark peaty seams in the sand. Cal- 
careous blown sand is compacted into hard stone by the action of rain-water, which 
alternately dissolves a little of the lime, and re-deposits it on evaporation as a thin crust 
cementing the g^ins of sand together. In tho Bahamas and Bermudas, extensive 
masses of calcareous blown sand have been cemented in this way into solid stone, 
which weathers into picturesque crags and caves like a lin)estone of older geological 
date.' At Newquay, Cornwall, blown sand has been solidified by the decay of 
abundant land shells. 

Biver-sancU Sea-sand. When the rounded water-worn detritus is finer than 
that to whieh the term gravel would be applied, it is called sand, though there is 
obviously no line to be <lrawn l)etween tlio two kinds of deposit, which necessarily 



» G. Klemm, Zeitsrli. Deiitsch. Geol Get*, xxxiv. (1882) p. 771. II. C. Sorhv, Quart, 
Jmrn. Geol. Soc. xxxvi. (1880). J. A. Phillips, op. cU. xxxvii. (1881), p. G. 

* For interesting accounts of the -^k)lian dei)osit3 of the Bahamas and Bennudns, 
Me Nelson, Q. J. Geol. Soc, ix.p. 200, Sir Wyville Thomson's "Atlantic,'' vol. i. ; also 
J. J. Rein, Sencketih. NtU, GeMl^ch. Bericht. 1869-70, p. 140, 1872-3, p. 131 ; on tho Red 
Sands of the Arabian Desert, see J. A. Phillips, Q. J. Geol Soc. xxxviii. (1882) p. 110. 



160 GEOON087. [Book II. 

^riuliiatc iuto c-och other. Tlio particles of sand range down to suoh minute forms 
as can only be ilistinctly discerned witU a microscope. TLe smaller forms are 
<:cncrally loss well roumk-il than those of greater dimensions, no doubt because their 
iliminutivc size ullows tliem to remain suspended in agitate<l water, and thus to eficajH) 
the niutuni attrition to which the larger and heavier grains are exposed upon the 
bottom. (Book III. Part II. Section ii.) So far as experience has yet gone, there is no 
method by which inorganic sea-sand can be distinguished from that of rivers or lake?. 
As a rule, >«and consists largely (often wholly) of quartz-grains. The presence of 
fi-agments of marine shells will of course betray its salt-water origin; but in tlie 
trituration to which sand is exj)osed on a coast-line, the shell-fragments arc in great 
measure grouml into calcanK)us mud and removed. 

^Ir. Sorby has recently shown that, by microscopic investigation, much information 
may bo obtaiudl regarding the history and source of sedimentary materials. He has 
studic<l the minute structure of mo<lem sand, and, finds that sand-grains present the 
following five distinct tyims, which, however, gradiuite into each other. 

1. Normal, angular, fresh-formed sand, such as lias l)ecn derivetl almost directly 
from the breaking up of granitic or schistose rocks. 

2. Wcll-wom sand in rounded grains, the original angles being completely lost, and 
th(? surfaces looking like iinc ground glass. 

3. Sand mechanically broken into sharp angidar chips, showing a glassy fracture. 

4. Sand having the grains chemically corroded, so as to produce a peculiar teztnro 
*>f the j*nrface, tliffering from that of worn grains or crystals. 

5. Sand in which the groins have a i>erfectly crystalline outline, in some rases 
undoubtctlly due to the deposition of quartz upon rounde<l or angular nuclei of onlinaiy 
uon-or}'6talline sand.* 

Tho same acute observer points out that, as in the familiar case of conglomerate 
lobbies, which have sometimes been use<l over again in conglomerates of very ditTerc'nt 
ages, so with the much more minute grains of sand, we must distinguish between the 
ugi' of the grains ami tlio ago of the deposit formed of them. An ancient sandstone 
may consist of grains that htul hardly been worn before they were fmally brought to 
rest, while the sand of a modern beach may have been ground down by the waves of 
many successive geological periods. 

Sand taken by Mr. Sorby from the old gravel terraces of the River Tay, was found to 
be almost wholly angidar, indicating how little wear and tear there may be among 
particles of quartz 4u ^^^ ^^ "^^^ ^° diameter, even though exposed to the drifting action 
cif a rapid river.' Sand from the boulder clay at Scarborough was likewise ascertaincil 
to bo almoAt entirely fresh and angular. On the other hand, in geological formations, 
which can l)c tracetl in a given direction for several hundred miles, a progressively 
large pn>[x)rtion of rounde<l particles may be detected in the sandy bcils, as "Sir, Sorby 
has found in following the Grcensand from Devonshire to Kent. 

Varieties of river or sea-sand may be distinguished by names referring to some 
remarkable constituent, e.g., magnetic sand, iron-sand, gold-sand, auriferous sand, &c. 

Gravel, Shingle — names applied to the coarser kinds of rounded water-worn 
detritus. In (travel, the average size of the component pebbles ranges from that of 
a small pea up to al>out that of a walnut, though of course many includiHl fragments 
will l)C ol>servrd which exceed these limits. In Shingle, the stones are courser, ranging 
u]> to blocks as big as a man's head or larger, (rerman geologists distinguish as 
" schotter/' a .-liingle containing dispersi-d lioulders, and *' sehotter-oon glomerate,** a rock 
wlicrcin thrse nmtorials havt^ Ix^couie consolidated.' All these names arc applied quite 
irrespec'tivo of the comjioaition of tlio fragments, which varies greatly from point to 

* Addzt^ Q. J. Gedf, Soe, xxxtL (1880) pi 58. 
' See Book UL fui n. Section U. § ilL 

^ See, fur I'sample, an account of toa nhotler^ongiomerates of Northern Portia by 
v.. Ti<!tze, Jahrb. CM. lUMimmL ViiU, 1881, p. 



%., 



Part II. § viL] FRAOMENTAL ROCKS. 161 



point. As a nile, the stooes oonflUt of hard crystalline rooks, since these are best 
fitted to withstand the powerftil grinding action to which they are exposed. 

Conglomerate (Poddiogstone) — a rock formed of consolidated gravel or shingle. 
The oomponent pebbles are rounded and water- worn. They may consist of any kind 
of lock, thoogh usually of some hard and durable sort, such as quartz or quartzitc. 
A qieoial name may be given according to the nature of the pebbles, as quartz 
eonglomerate, limcstoue-oonglomerate, granite-conglomerate, &c., or according to that 
of the paste or cementing matrix, which may consist of a hardened sand or clay, and 
may be siliceous, calcareous, argillaceous, or ferruginous. In the coarser conglomeiates, 
where the blocks may exceed six feet in length, there is often very little indication 
of stratification. Except where the flatter stones show by their general parallelism the 
mde lines of deposit, it may be only when the mass of conglomerate is taken as a 
whole, in its relation to the rocks below and above it, that its claim to be considered 
a bedded rock will be conceded. The occurrence of occasional bands of conglomerate 
in a scries of arenao^eous strata is analogous prolmbly to that of a shingle-bank or 
gravel-beach on a modem coast-line. But it is not easy to understand the cir- 
cnmstanocs under which some ancient conglomerates accumulated, such as that of 
the Old Ked Sandstone of Central Scotland, which attains a thickness of many 
thousa nd feet, and consists of well rounded and smoothed l^locks often several feet in 
diameter. 

In many old conglomerates (and even in those of Miocene age in Switzerland) the 
component pebbles may bo observed to have indented each other. In such cases also 
they may be found elongated, distorted or split and recemcnted ; sometimes the same 
pebble has been crushed into a number of pieces, which are held together by a retaining 
cement Those phenomena point to great pressure, and some internal relative movement 
m tho rocks. (Book III. Fart I. Section iv. § 3.) 

Breooia — a rock composed of angular, instead of rounded, fragments. It commonly 
presents less trace of stratification than conglomerate. Intermediate stages between 
these two rocks, where the stones are partly angular and partly subangular and 
rounded, are known as hreceiated conghmerafe, ConHidcred us a dctritul deposit formed 
by superficial waste, breccia points to the dit<inti>gration of rocks by the utmosplicre, 
and the accumulation of their fragments with little or no intorv'cutiou of running 
waiter. Thus it may be fonued at the base of a clifl*, either sulmeriully, or whore tho 
dehris of the cliff fulls at once into a lake or into deep sea-water. 

The term Breccia has, however, )>een applied to rocks formed in a totally different 
manner. Angnlar blocks of all sizes and shapes liave been discliarged from volcanic 
orifices, and, falling back, have consolidated there into brccciutod masses of rock. In- 
tnuive igneous eruptions have sometimes torn off fragments of the rocks through which 
they have ascended, and tlicHC angular fragments have been enclosed in tho h'quid 
or pasty mass. Or the intrui<ivu rock has cooled and solidified externally while still 
motnle within, and in its ascent has caught up and involved some of these consoli- 
dated parts of its own substance. Again, where solid masses of rook witliin the crust 
of the earth have ground against each other, as in dislocations, angular fragmentary 
mbbish has been produced, which has subsequently been consolidated by some 
iofillimting cement (Fault-rock). It is (>videut, however, that breccia formed in one 
or other of these hypogeno ways will not, as a rule, bo apt to ]>i) mistaken for the true 
brfi-io.-, itriislug from superficial disintegration. 

Bandatone (Gres)' — a rock composed of consolidated sand. As in ordinary 
mrfbrii Hiimly the integral grains of sandstone are oliiofly (piartz, which must hero 
if re;o^rde<l ut the residue loft after all tho less durable minerals of tho original 
Mrk^ havo been carried away in solution or in suspension as fine mud. The colourri 
nf iuiniUtoQCs arise, not so much from that of the quariz, which is commonly white or 



' For recent analyses of some Brituih sandstones useil a^ building stones, see Wallace, 
r,i^, PMK 8oe. GUu^, xiv. (1883), p. 22. 



162 GEOGNOSY. [Book H. 



grey, as from the film or criwt which often coats tho grains and holds them together as 
a cement. Iron, tlio great colouring ingredient of rocks, gives rise to red, brown, 
yellow, and green hues, according to its degree of oxidation and hydration. 

Like conglomerates, sandstones differ in the nature of their component grains, and 
in that of the cementing matrix. Though consisting for the most part of siliceous 
grains, they include others of clay, felspar, mica, or other mineral ; and these may 
increase in numher so as to give a special character to the rock. Thua, sandstones may 
be argillaceous, felspathic, micaceous, calcareous, &c. By an increase in tho argillaceous 
constituents, a sandstone may pass into one of the clay-rocks, just as modem sand on 
the sea-floor shades imperceptibly into mud. On the other hand, by an augmentation 
in the size and sharpness of the grainR, a sandstone may become a grit, and by an 
increase in the size and number of pebbles, may pass into a pebbly or conglomeratic 
sandstone, and tlience into a fine conglomerate. A piece of fine-grained sandstone, seen 
under the microscope, looks like a coarse conglomerate, so that the difference between 
tho two rocks is litUe more than one of relative size of particles. 

The cementing material of sandstones may be ferruginotu, as iu most ordinary re<l 
and yellow sandstones, where the anhydrous or hydrous iron-oxide is mixed with elay 
or other impurity — ^in red sandstones the grains are held together by a hiematitic, in 
yellow sandstones by a limonitic cement; argtllaeeouSf where the grains are united 
by a base of clay, recognisable by the earthy smell when breathed upon ; oa/carw>f», 
where carbonate of lime occurs either as an amorphous paste or as a crystalline cement 
between the groins; siliceotu, where the component particles are bound together by 
a flinty substance, as in the exposed blocks of Eocene sandstone known as "grey- 
weathers " in Wiltshire, and which occur also over the north of France towards the 
Ardennes. 

Among the varieties of sandstone the following may here bo mentioned. Flag- 
stone—a thin-bedded sandstone, capable of being split along the lines of stetification 
into thin beds or flags ; Micaceous sandstone (miea-pMmmits) — a rock so full of 
mica-flakes that it splits readily into thin lamintc, each of which has a lustrous surftMse 
from the quantity of silvery mica. This rock is called "fakes" in Scotland. Free- 
st o n e— a sandstone (tlie term being applied sometimes also to limestone) which can 
lx» cut into blocks in any direction, without a marked tendency to split in any one plane 
more than in another. Though this rock occurs in beds, each bed U not divided into 
laminae, and it is tlio absence of this minor stratification which makes the stone so 
useful for architectural purposes (Craigleith and other sandntones at Edinburgh, some 
of which contain 98 per cent, of silica). Glauconitic sandstone (greon-eand)— 
a pfludstono containing kernels and dusty grains of glauconite, which imparts a genersl 
greenish hue to the rock. The glauconite has probably been deposited in association 
with decaying orpranic matter, as where it fills echinus-spines, foraminifera, shells 
and corals on the floor of tho present ocean.* Buhrstone— a highly siliceous, 
exceedingly compact^ though cellular rock (with Chara seeds, &c.), found alternating 
with unaltered Tertiary Btrata in the Taris basin, and forming from its hardness and 
rouglmeFB, an excelhmt material for tho grindstones of flour mills, may be mentioned 
here, though it probably has been formed by the precipitation of silica through the action 
of orfraninms. A r k o s e (granitic sandstorn')—Vk rock composed of disinte>grated granite, 
and foiiTul in geolofrical formations of different ages, which have been derived from 
gran i li o rockn. Crystallized e a n d a t o n o— an arenaceous rock in which a deposit 
of cr\\stalline qnartz has taken place upon the individual grains, each of which becomes the 
nucleus of a more or less perfect quartz crystal. Mr. Sorby has observed such crystallize<l 
sand ill deposits of various ages from tho Oolites down to the Old Red Sandstone.* 

Qreywacke— a compact aggregate of rounded or stibangular grains of quartz, 

I Ante, pj). 74, 157 : BoUan, Gt-ol. Mag. iii. 2nd sor. p. 530. 

" Q.J. (icol. Sor. xxxvi. p. G3. See Daubree, Ann. des Mines, 2ud scr. i. p. 200. 
A. A. A ouug, Amer. Jonrn. Set. 3rd. ser. xxiii. 257 ; xxiv. 47. 



Pabt IL § vii.] FBAGMENTAL BOCKS. 163 



felipor* alaie, or other minerals or rocks, oementcd by a paste which is usually siliceous, 
bot may be argillaceous, felspaUiic, calcareous, or anthracitic. (Fig. 13.) Grey, as its 
name denotes, is the prevailing colour : but it passes into brown, brownish-purple, and 
■ometimes, where anthracite predominates, into bluck. The rock is distinguished from 
ordinary san^lstone by its darker hue, its hardness, the variety of its component grains, 
and, above all, by the compact cement in which the grains are imbedded. In many 
Tarieties,80 pervaded is the rock by the siliceous paste, that it possesses great toughness, 
and its grains seem to graduate into each other as well as into the surrounding matrix. 
Sneh rocks when fine-grained, can hardly, at first sight or with the unaided eye, ]>o 
distingaiBhed £rom some compact igneous rocks, though a microscopic examination at 
ones reveals their fragmental character. In other cases, where the grey wacke has been 
fomied mainly oat of the debris of granite, quartz-porphyry, or otlicr felspathic masses, 
the grains consist so largely of felspar, and the paste also is so felspathic, that the rock 
might be mistaken for some close-grained granular porpliyry. Greywacko occurs ex- 
tensively among the Palieozoic formations, in beds alternating with shales and conglo- 
merates. It represents the muddy sand of some of the Palsaozoic sea-floors, retaining 
oflea its rii^le marks and snn-cracks. The metamorphism it has undergone has 
generally not been great, and for the most part is limited to induration, partly by pressure 
and partly by permeation of a siliceous cement. But whore felspathic ingredients proyail, 
the rock has offered facilities for alteration, and has been here and there changed into 
gneiss, and even into rocks that assume a granitoid texture. Some varieties have a 
crystalline base, in which have been developed muscovitc, biotite, quartz, rutile, tour- 
maline, &c. 

The more fissile finc-giainod varieties of this rock have been termed greywacke-slato 
(p. 12G). In these, as well as in grey wacke, organic remains occur among the Silurian and 
Devonian formations. Sometimes in the Lower Silurian rocks of Scotland, these stratti 
become black with carbonaceous matter, among which vast numbers of groptolites may 
be observed. Gradations into sandstone are termed Greywacke-sandstono. 
In Norway the reddish felspathic grey wacke or sandstone of the Primordial rocks, is 
called Sparagmite. 

2. Clay Bocks (Pclitcs). 

These are composed of the finer argillaceous sediment or mud, derived from the 
waste of rocks. Perfectly pure clay or kaolin, hydrated silicato of alumina, may bo 
obtained where granites and other felspar-bearing rocks decompose. But, as a rule, 
tlio argillaceous materials are mixed with various impurities. 

Clay, Mud. — ^The decomposition of felspars and allied minerals gives rise to tho 
formation of hydrous aluminous silicates, which occurring usually in a state of very fine 
subdivision, are capable of being held in suspension in water, and of being transported 
to great distances. These substances, differing much in composition, are embraced 
under the general term Clay, which may be defined as a white, grey, brown, red, or 
bluish substance, which when dry is soft and friable, adheres to the tongue, and shaken 
in water makes it mechanically tiu'bid ; when moist is plastic, when mixed with much 
water becomes mud. It is evident that a wide range is possible for varieties of this 
substance. The following are the more important. 

ir«/>l-iT% (Porcelain-clay, China-clay) has been already noticed (p. 73). 

Pipe-Clay — white, nearly pure, and free from iron. 

Fiie-Clay — lar^^cly found in connection with coal-seams, contains little iron, 
and is nearly free from lime and alkalies. Some of tho most typical fire-clays are those 
long used at Stourbridge, Worcestershire, for the manufacture of pottery. Tho best glass- 
bouse pot-day, that is, tho most refractory, and therefore used for the construction of 
pots which have to stand the intense heat of a glass-house, has tho following composi- 
tion: silica, 73*82; alumina, 15*88; protoxide of iron, 2*95; lime, trace; magnesia, 
trace; alkalies, '00; sulphuric acid, trace; chlorine, trace; water, 6-45; specific 

gravity, 2*51. 

M 2 



M4 'TEO^^ynsT. [Book n. 



Qiiiiiifi — t -Tf-rr .il-ep*'iiu 'U MK - eiiua ed TBxecr. fb^ii is tbe Lflwer God 
ri^tmma ^f :S.p N'ovtli -f Rn?tanii. iii«i 3ow '.Mapri^ ifsovBi Jdwh m§ % mmkaml te the 

Brtek-^lfly— fir-fyriv scfacr m juinnsal dan & zeoloanl fiBfm, snee it if 
ytfiiitifi v> in]r *!ar. ItmaL. ir 3aith. ^hm wfaicfa. vxeka t •squib porteiy on Bade. It ii 
in impnrr- 'Ur. v^ntainxn? i *ni>i ieai if iron, ^njdi nber in^recBentK. An aaaXp^ 
jar^ "h^ ^ilowrnir Ytmimadon >[ i hridc-dsr: siiLea. 4D-4^: jbrnoBm^^'^; «■%«• 
-Tif^ '^.rrn. T-Tt; iime: I *fi : aaeaoB. 3'!^: ^fastXL I*^ 

Fuller's Earth Tm- & fcmnn. Waikefde)— «& *reesiiih 'W hmviiBi^ eKtbT^nft, 
y'lnrvfaAC -mrmnnm ifi^jnnufrr^, -rth ti ^jiixiizi^ mnaiL, whiA 'hxa not Lilouum pliflif 
-r.rli ^nu-r. Snt <:ninbi» ioim juo stud. £c ia a IiTdnma almiinaaa irilif tw vikh 
^TiM ma^neAia^ imn-oxide jnd ^wda. Tlie 5«iQow iniUes'i eardL of Eftgatt cnntiiM 
SiiuA H. alamina IL nxide if iron la oaeziBia t. lime X aula J.^ £i Espiaiid fidkrli 
'iArth 'wnn in Vfb amixie tiu* riiiaaiii jnd CifcttceoiB fririunMiiML £i Saxony it ii 
fonivi M ft naoit •^f die •ieeompoosioii ai ifiaoaae and ptbhto. 

WftdEa— A 'iirtj-^mtrn ai brownish-biack. eaitiif « ocmpact. but tender aad 
ftptpan»tlj hmno^neaiu daf. which uiMa aa die nlomase itege of the deeampostioD 
of haMlfrmid m n'iif . 

Tin, Boaldcr-day — a adiT aamir ami jbhit oiar. Tarrmg in eoloar and eoM- 
p^Qon, a cpi plin g to the efaanrter of die zocIb of die diatiKt in wiiich it liea^ It ii 
fnll of vora Mooea of ill siea. up to blorki wdsdune Kveral toBa» and often veD- 
iiTikyjdMd and atriateiL It ia a g;iaeifll depngt, aod wifl be dcKribed among die frnnatinBa 
of di« Gladal Pehnd. 

Modatona a due, oanall j mure or les aandr, arzillaeeooi lodc haTin^ no fiaile 
rfaancter, and of iomewtias zreater hardneB than aaj form of dar. Tiie term C 1 a j- 
r o e k has bom applied hj Mme writers to an indncUed elav reqniring to be groimd 
and mix^ with water befire it aerioires plastieitr. 

Shnle (Mtiitft, SehiefefthoQ>-« zenaal tera Uy di»cribe clay that haa aaramed a 
thmlj straktiflerl or fLaAUe jtmctnre. Under thii term are inclaled laminated and aone- 
what harrleoffi arsiUaeeijTu rocki, which are capable of bein^ split along tbe lines of 
*\f^m\t into thin Iraves. They present almost endless varieties of textnze and eom- 
fiTiftition, {Msain;^, m the one hanil, into clays, or, whei^ mnch indurated, into slates and 
urj^UH^fjju schists, on the other, into dagstonea and sandstones, or again, through 
f^ilrarr-onii (pudatiooa into limestone, or thion;;h fermginons varieties into olay-ironstone, 
nij'I tiiron^h bitnminon:! kinds into coal. Some of the altere^l kinds of day-rocks bsTO 
fihftulj heen dr-scriljed (p. 12r>> 

liOam — an f:artby mixtnre of clay and sand with more or less organic matter. The 
Murk soils of Knssia, India, See, (Tchemosem, Regnr <, are dark deposits of loam rich in 
or<^nic inatt<>r, and sometimes upwards of twenty feet deep. 

IfOesa — a pale, somewhat calcoreons clay, probably of wind-drift origin, found in some 
river-valleys (Bhine, Danul>e, 5fi»iBHippi, &c.), and over wide regions in China aad 
ilHfwlicre. It is described in Book III, Port ii. Sect. i. § 1. 

Xiaterita — a ccUulnr, reddish, femiginous clay, found in some trojncal countries as 
ttio rosiilt of the subacrial dccom|X)6itiou uf rocks ; ii acquires grcnt hardness after being 
'Iunrri(Ml out and dried. 

8. Volcanic Frag^ental Rocks— Tiiflfk 

This HrTfioii compri»c8 nil deposits which have resulted from tho comminution of 
volraiilr nwks. Thry thus include (1; those wliioh consist of tho fragmentary materials 
fjpotiHl fr(»m vulraiiio foci, or tho true nshcs and tufts; and (2) some rooks derived frooi 
tho HU|>rriioiiil disiutcgration of already criiptod niid consolidated volcanio masses 
i >bvioUHly the Hooond Hcries ought properly to be classed with tho sandy or clayey rocka 
idMivc doserilH d, sinco they havo been formed in tho same way. In practice, however, 

' lire's Jh'rL .-Ir/x, Ac. ii. p. 142. 



Part H. § vii.] FRAOMENTAL BOCRS— VOLCANIC. 165 

thfise dctrital reoonstnteted rooks cannot always be certainly distiuguisbcd from those 
which have been formed by the consolidation of true volcanic dust and sand. Their 
chemical and lithological characters, both macroscopic and microscopic, are occasionally 
■0 similar, that their respective modes of origin have to be decided by other considora- 
taonB, snch as the oocarrence of lapilli, bombs, or slags in the truly volcanic series, and of 
well water-worn pebbles of volcanic rocks in the other. Attention to these^ features, 
however, usually enables the geologist to make the distinction, and to perceive that the 
nnmber of instances where he may be in doubt is less than might be supposed. Only u 
oomparaiively small number of the rocks classed here are not true volcanic ejections. 

Beferring to the account of volcanic action in Book III. Part I., we may here merely 
dg&o» the use of the names by which the di£ferent kinds of ejected volcanic materials 
m known. 

Voloanio Blocks — angular, sub-angular, round, or irregularly-shaped masses of 
lavm, several feet in diameter, sometimes of uniform texture throughout, as if they were 
large fragments dislodged by explosion from a previously consolidated rock, sometimes 
Dompaet in the interior and cellular or slaggy outside. 

Sombe — ^round, elliptical, or discoidal pieces of lava from a few inches up to one 
or more feet in diameter. They are frequently cellular internally, while the outer parts 
ue fine grained. Occasionally they consist of a mere shell of lava with a hollow 
interior like a bomb-shell, or of a casing of lava enclosing a fragment of rock. Their 
nude of origin is explained in Book UI. Part I. Sect. i. § 1. 

IiapiUi (rapilli)— ejected fragments of lava, round, angular, or indeflnite in shape, 
niying in size from a pea to a walnut. Their mineralogical composition depends upou 
ihmi of the lava from which they have been thrown up. UsuaUy they are porous or 
inely vesicular in texture. 

Volcanic Band, Voloanio Ash— the finer detritus erupted from volcanic 
crifiees, consisting partly of rounded and angular fragments up to about the size of a 
pea, derived from the explosion of lava within eruptive vents, partly of vast quantities 
of microliths and crystals of some of the minerals of the lava. The finest dust is in a 
lUte of extremely minute subdivision. When examined under the microscope, it is 
tometimes found to consist not only of minute crystals and microliths, but of volcanic 
giftfls, which may be observed adhering to the microliths or crystals round which it 
flowed when still part of the fluid lava. The presence of minutely cellular fragments is 
ehsracteristic of most volcanic fragmental rocks, and this structure may commonly be 
obaerved in the microscopic fragments and filaments of glass. 

When these various materials arc allowed to accumulate, they become consolidated 
and receive distinctive names. In cases where they fall into the sea or into lakes, they 
are liable at the outer margin of their area to bo mingled with, and insensibly to pofes 
iuto ordinary non-volcanic sediment. Hence wo may expect to find transitional varieties 
between rocks formed directly from the results of volcanic explosion and ordinary st di- 

Bieotary deposits. 

Volcanic Conglomerate — a rock composed mainly or wholly of rounded or 
iob-angular fragments, chiefly or wholly of volcanic rocks, in a paste derived from 
;he same materials, usually exhibiting a stratified arrangement, and often found 
atercalated between successive sheets of lava. Conglomerates of this kind may have 
icen formed by the accumulation of rounded materials ejected from volcanic vents ; or 
iS the result of the aqueous erosion of previously solidified lavas, or by a combination 
JL both these processes. Well-rounded and smoothed stones almost certainly indicate 
ong-continued water-action, rather than trituration in a volcanic vent. lu the Western 
r«rritories of the United States vast tracts of country aro covered with masses of such 
.Mntglomerate, sopiotimcs 2000 feet thick. Captain Duttou has recently shown that 
omilar deposits are in course of formation there now, merely by the influence of dis- 
integration upon exposed lavas.* 

» ' High Plateaux of Utah,' p. 77. 



Volc^nio coDglomoraks recoiv<i different n&mea nccnrding to the nature of the onm- 

poiioiit fmBiQontB ; iliiis we have hae<Ut-(oni}loiiKraU>, whcro these fragment* are 
i\]io]ly or mninlj of basnlt, trai^hyte-conglomerattt, porphyrile-eonglomtTalfi, ^lonolife- 

Volcanic Breccia n'scmblcs Volcunio Coni^lomerate, osoept thnt the itenec are 
angular. This ao^'ularit; indicstCE an absence of aqnoons oiadoD, and, nnder the 
circumBtances iu whiob it Is foond, nanally pointa to immediately adjeoent voleanio 
L-ipIosionB. There is a great vailetjr of breoeias, ee batall-bneda, didbaie-ireeeia, to.- 

Tolconic Agglomerate— a tnmnltaoiu nssemblago of blocks of all rixtit up ta 
uuuBoa BCToml yards in diameter, met with iti the " necks " or [Hpes of old Toloajiie 
orifloes. The stoues and paste are oommonly of one or mora voleanio Todu, soch m 
basuU or porpbyrile, but they inclnde also frngwcnla of the sntrotinding rooks, irhatoTet 
tbcBoma; bo, throngh vbloli tlio yolcanioori&ce has been drilled. Asan]1e,eggIometate 
is devoid ofatratiflcation; but sometimes. it includes portions which have a mran orlen 
distinct Bimngement into beds of coarser and finer detritus, often placed on end, or 
inclined in diflcrent directions at high onglet, aa described in Book IV. Port VU. 

Voloonic Tuff. — This gcuoral term may ho made to iitclado all the fluct klads of 
volcanic detritus, miging, on tlio one hand, thiongh cettrse gravelly deposits into cou- 
glomcrates, and on the other, into cioecdingly compact fino'grained loclu, formed of the 
finest and most impalpable kind of volcanic dust. Some modem tuflHi are full of 
microliths, derived frran the lava wliich nas blown into dust. Others are formsd of 
small rounded or anguUr grains of different lavas, with fragments of various locka 
through which the voIcaDto funnels have been drilled. The tuffs of earlier gcohigkal 
periods Iiave often been so much altered, that it is difficult to slate what may have been 
their original oouditioo. Tbe absence of microliths and glass In them ia no proof that 
they are not tnic tuffs ; for the presence of tliesc bodies depends upon the natnre of tho 
lavas. If the latter were not vilreoai and miorolithie, neither would bo the tnffii 
derived f>om them. In the CWboniferous ToIc»nio area of Central Soollond, Uio tuflk ua 
madeup of debris and blocks of tbe basaltic Istu, 
and, liko these, are not microlithio, though in 
Botne places they abound In frsgments of pala* 
gonile. (Fig. 28.) 

Tuffs have eoneolidated sometimes under 
water, sometimes on dry land. As a rule, they 
aro distinctly stratified. Near the original Tenfai 
of eruption they commonly preeent rapid altet^ 
nations of finer and ooarscr detritus, indieatiTO 
of successive phases of voleanio activity. They 
neneflarily shade off into tbe sedimentary for- 
mations with which they were oontemporaneomL 
Thus, ne have tuffs passing giadually into shale, 
limestone, sandstone, ix. The intermediate 
varieties have been called aAy $halt, tufaeeoM 
' thole, or thalty tnf, &c. From the dmunutanooi 
of their formation, tuffs frniuently preseire the 
remains of plants and animals, both terrestrial and aquatic. Tboee of Ifonte Somma 
oontain fragments of land-plants and shells. Some of those of Carboniferous ago in 
Central Scothiod have yielded crinoids, brachiopods, and other marine shells. Like the 
other fragmentary voleanio rocks, the tuffs may he subdivided accoiding to tbe nature 
of the lava from tho disintegration of which they have been formed. Thus we bave 
fthiU-lnfi, Iraehyle-tufft, bamti-tufi, pumiee-lafe, porpliyHle-tvffM, fto. A few varietin 
with special oharactcriatirs may bo mentioned here.' 

' Oh tJio occurrence and Btructurc of tufla, see J. C. Ward, Q. J, (,'roi Soe. ixxi 
p. iiS8; Beyer, Jahrb. Qtol Jfcicftwnif . 1881, p. 57 ; Oeikie, Traai. Soy. Soe. Edin. xiix.; 




Part 11. § viL] FRAOMENTAL ROCKS. 167 



TrmSB-— a iMile yollow or grey rook, rough to the feel, oompoBed of an earthy or 
eo m pae t pumioooiis duaft, in which fragments of pumioc, trachyte, grey wackc, basalt, 
curfaanixed wood, &o^ are imbedded. It has filled np some of the valleys of the Kifel, 
where it is largely quarried as a hydraulic mortar. 

Fdpaorino — a dark-brown, earthy or granular tuff, found in conHidemble quantity 
among the Alban Hills near Rome, and containing abundant crystals of augite, 
mioa, lencite, magnetite, and fragments of crystalllno limestone, basalt, aud Icucite- 
lafm. 

Palagonite-Tuff—- a bedded aggregate of dust and fragments of basaltic lava, 
among which are couspiouous angular pieces and minute granules of the pale yellow, 
greeOt red, or brown basic glass called palagonite. This vitreous substance is intimately 
lelatod to the basalts. It appeara to have gathered within volcanic vents and to have 
beeu emptied thence, not in streams, but by successive aeriform explosions, aud to 
have been subser^uently more or less altered. The itercentage composition of a 
specimen from the typical locality, Palagonia, in the Yal di Noto, Sicily, was estimated 
by Sartorius von Waltershausen to be silica, 41*26; alumina, 800 ; ferric oxidc^ 2532 ; 
lime, 5'59 ; magnesia, 4*84 ; potasli, 0*54 ; soda, 1*06 ; water, 12*79. This rook is largi^ly 
developed among the products of the Icelandic and Sicilian volcanoes ; it occurs aUo in 
the Eifel and in Nassau. It has recently been found to bo one of the chamcteristio 
features of tuffs of Carboniferous ago in Central Scotland.' (Fig. 28.) 

Sohalatein* — ^Under this name, German petrographers have placed a variety of 
rucks which consist of a green, grey, red, or mottled diabase-tuff, iuiprcgimte<l with 
carbonate of lime, and mixed with calcareous and argillaceous mud. Tbey are inter- 
stratified with the Devonian formations of Nassau, the Harz and Devonsbire, and with 
the Silurian rocks of Bohemia. They sometimes contain fragments of clay-slate, and 
are occasionaUy fossiliferous. They present amygdaloidal and porphyritic, as well as 
perfectly laminated stmctures. Probably they are in most cases true tuffs, but 
£r*nietimes they may be forms of diabase-lavas, which, like the stratified fonnations iu 
which they lie, have undergone alteration, and in particular have acquired a more or 
less distinctly fissile structure.' 



4. Fragmeutal Bocks of Organic Origin. 

This series includes deposits formed either by the growth and decay of organisms 
««i isita^ or by the transport and sulisecjuent accumulation of their remains. These may 
1>i> couveniently grouped, according to their predominant chemical ingredient, into 
Calcareous, Siliceous, l*hosphatic. Carbonaceous, and Ferruginous. 

1. Calcareouh. — Besides the calcareous formations above described (p. 118) among the 
stratified cr}'stalline rocks as resulting from the deposition of chemical precipitates, a 
still more important series is derived from the remains of living organisms, either by 
growth on the spot or by transport and accumulation as mechanical sediment. To by far 
the larger j>art of the limestones intercalated in the rocky framework of our continents, 
an organic origin may with probability be assigned. It is true, as has been above 
mentioned, that limestone, formed of the remains of animals or plants, is liable to an 
internal crystalline rearrangement, the effect of which is to obliterate the organic 
structure. Hence in many of the older limestones, no trace of any fossils can 1x3 




metamorpl , __„ 

(U. 8. Geograpb: and Geol. Survey of llocky Mounts.), 1880, p. 79. 

* Tran». Roy. Soc. Edin. xxix. p. 514. 

« C. Koch, Jaiirb. Ver, Nat Na$mu, xui. (1858) 21G, 238. J. A. Philli])s, (^. J, Otul, 
ike. xxxii, p. 155, xxxiv. p. 471. 



168 GEOGNOSY. [Book H. 

detccl«il, au<1 yot these rocka irero olmoat certainly fonued of orgtmio lemaini. An 
nttuDlive microscopio atndy of orgiinic cnl«aroons gtrncturoa, and oT the mode of their 
leiilucement by cryriulline calritc, HfTordH. however, indications of former oigknianu, 
uvcD in tlic midst of thntoughly cryetalliDO materialB.' 

Limestone, onnpoHed of the Temaina of calourcoiu nrf^nisma, is Ibond in lajen 
wlitcli range from mL-re tbin lamioK) ap to uutasiTe bods, sevend feet or eren yards in 
thickness. In somo ii)8tann»i snch m that of the UarbonifetouB or Honntsin Umeatooa 
of England nnd Ireland, iind that of the Coal-mcoanrca in Wyoming and Utah, tt oocmi 
in coDtinoouB superpoaod bcdx to a united thicknesaof several thousand feet, and extenda 
for hundreds of square miles, forming u ro^k out of vhich picturesque gorges, hills, 
and tablelands have been excavated. 

Limestones of orgiiuio origin present ever; gradation of texturo and ttmetiue, from 
mere soft calcareons mud or earth, evidently composed of entire or cnimbled orguiisitii 




lip In bolid compact eri-slaUine rock, in whieh indioutions of an orgaoio source can 
hardly bo perceived. Mr. Sorby, in tho dddrass already cited, called renewed attentioii 
to tho importance of the Form in which carbonate of lime is built up into uimal 
structures. Quoting tho opinion of Hose esprcaacd in 1858, thet llio divBraily in the 
stole of preeervaljon of different eliells might be due to the fact ttiat some of thorn had 
llieir limo as calcite, otlierB as aragonite, he showed tliat tliis opinion is amply supported 
Ijy niicroDCOpie examination. Even in the shells of a recent raised beach, ho observed 
tliiit the inner aragouite layer of the common niUBscl hnd been completely removed, 
thimsh tlie outer layer of euleite was well preservi-d. In some shellj limestones con- 
taining casU, the ursgonllo sliolls have alone disuppearai, and wliero those still remain 



' Sorby, Addrett to Geol. Si-oietij, February, 1870. 



Paw n. S viL] FRAQMENTAL BOOKS. 169 



repraieiited by a oaloareous layer, tUis lias no longer the original structure, but is more 
or less ooaisely orjrstalline, being in fact a pseudomorph of calcito after aragonite, and 
quite unlike oontignoiis oaldte shells, which retain their original microscopical and 
opika] ofaaracten.1 

The following list comprises some of the more distinctive and important forms of 
dganically-derifed limestones. 

8 hell- Marl— a soft, white, earthy, or crumbling deposit, formed in lakes and 
poods by the aocamnlatitni of the remains of shells and Entomosiraca on tho bottom. 
Whoi such caksaieous deposits become solid compact stone they are known as fresh-water 
(faauinme) UmmUmei, Those axe generally of a smooth* texture, and either dull white 
or pale grey, their fxaoture slightly conohoidal, rarely splintery. 

Lnmachelle— a oompaot, dark-grey or brown limestone, charged with ammonites 
or other fossil shells^ which are sometimes iridescent, giving bright green, blue, orange, 
and dark red tints (fire-marble). 

Calcareous (Foraminiforal) Ooze— a white or grey calcareous mud, of 
organic origin, found covering vast areas of the floor of the Atlantic and other oceans, 
and formed mostly of the remains of Faramini/era^ particularly of forms of tlio genus 
Ohbigerima. (Fig. 29.) Further account of this and otlier organic deep-sea deposits is 
given in Book Uh Fit U. Section iiL 

8 hell -8 and— a deposit composed in great measure or wholly of comminuted 
shells, found commonly on a low shelving coast exposed to prevalent on-shore wintls. 
When thrown above the reach of the waves and often wetted by rain, or by trickling 
nioDels of water, it is apt to become consolidated into a mass, owing to the solution and 
redepoeit of Kme round the grains of shell (p. 159). 

Coral-rook — a limestone formed by the continuous growth of coral-building 
polyps. This substance aifords an excellent illustration of the way in which organic 
stnietuxe may be eflaoed from a limestone entirely formed from the remains of once 
living animals. Though the skeletons of the reef-building corals remain distinct on 
tiio upper surfiaoe, those of their predecessors beneath them are gradually obliterated by 
the passage through them of percolating water, dissolving and redcpositing calcium 
carbonate. We oan thus understand how a mass of crystalline limestone may have 
been produced from one formed out of organic remains, without tho action of any 
subterranean heat, but merely by the permeation of water from tlic surface.' 

Chalk — a white soft rock, meagre to tho touch, soiling tho fingers, formed of a fine 
calcareous flour derived from the remains of Foraminifenij echinoderms, mollosks, and 
other marine organisms. By making thin slices of the rock and examining them under 
the microscope, 8orby has found that Foraminifera, particularly Globigerina, and 
single detached cells of comparatively shallow-water forms, probably constitute less 
than half of the rock by bulk (Fig. 14), the remainder consisting of detached prisms of 
the outer calcareous layer of Inoeeramus, fragments of Ontrea^ Peclen, echinoderms, 
spicules of sponges, &c It is not quite like any known modem deep-sea deposit. 

Crinoidal (Encrinite) Limestone — a rock composed in great part of 
crystalline joints of encrinites, with Foraminifera^ corals, and mollusks. It varies in 
colour from white or pale grey, through sliadcs of bluish-grey (sometimes yellow or 
brown, less commonly red) to a dark-grey or even black colour. It is abundant among 
Paheozoic formations, being in Western Europe especially characteristic of the lower 
part of the Carboniferous system. 

2. 8iLicEors. — Silica is directly eliminated from both fresh and salt water by the 
vital growth of plants and animals. (Book III. Part II. Section iii.) 

' The student will find the address from which these citations are mode full of 
toUggestive matter in regard to the origin and subsequent history of limestones. 

' Sec Dana's ' Coral and Coral Islands,' p. 354 ; also tho account of the Devonian 
and Carboniferous limestones in tlie present voliuuc. Dui)ont has shown that many of 
tho massive limestones of Belgium have been formed by reef-like masses of Stromatopora 
or allied organisms. 



170 OHOiiSoSY. [Bu.»K ir. 

Diatom-earth, Tripolite (lufiuoriul curth, Kittti'lguhry-^ 8iliceoiudc|)CMit fornieil 
chicHy of tho fruutules oi'dialomis, laid down both in salt and in fresh wuter. Wide aretu 
(tf it arc now iKrinp: dciKwited un the bed of tho South Pacific {Diatomooz*:, Fi<;. 173). lu 
Vir;::inia, Uuitod Stat(.>s, nn cxtoiitiivo tntc^t oc^nird i>ovcrcd with diatom-earth to a depth 
of 4U feet It likfwiHO underlicB poat-mos8C8, probably as an original hiko-depnsit. It 
irt UHcd Oi* Tn'ifoli powtlcr for i)oli»hin« pur])OHCtf. 

Radiolarian-ooze — an abyduuil murine tlqiosit oonBitftiuj^ mainly of the remains 
of bilJL'cous r.i<liohiriand and diatoms. It is further referred to in Book III. Part II. 
iSection iii. 

Flint (Uliert, Phtiinite,) haH V)ecn already (p. 122) dcscribetl, but should iiud a plai-u 
also h<*rr from its evident connecHon \iith orpinic agency. It frc<|ucntly encloses 
si^jngos, ccliini, shells, &c., and has evidently formed round these on tho sea-floor, and has 
repla(*i.Hl their original culcium-carlionate. In some cases, as in the spicules of apouges, 
it had had a directly organic origin, having l)een secreted from sea-water by the living 
orgiinismH; in other oasi^s, when' for exam])lo we lind a calcareous hhelK or echiniw, ur 
coral, convertetl into nilira, it would seem that tlio suI>stitntion of sili<*a for calciimi-car- 
bonate has been eftecteil by a process of chemical pseudomorphism, cither after orduriu;; 
the formation of the limestone. Tho vertical ramifying masses of flint in chalk shuw 
that the calcareous ooze had to some extent accumulated before the segregivliou of thcM^ 
massi's.* (.'hert (I*htanite) al>ounds in tho Carboniferous Limestone o( Western Europi-. 

3. IMiosruATic. — A few invertebrata contain phosphate of lime. Am<mg these may l*e 
mentioned the bntchiopods Lingula and Orldenliit^ also Conularia^ Serpulite*, anil some 
recent and fotisil Crustacea. Tho shell of the recent Lingula ovalU was found by Hunt t'l 
contain, after calcination, <31 per cent, of fixed residue, which consisted of 85*70 per cent, of 
pliosphate of lime ; 11*75 carbonate of lime, and 2*80 magnesia. The bones of vertebrate 
animals likewine contain a1x)ut 60 per cent, of phosphate of lime, while their excrement 
sometimes abounds in the same substance. Uence deposits rich in phosphate of limo 
have resulted from the accumulation of animal remains from Silurian times up to the 
present day. Associated with the Bida limestone, in tho Lower Silurian scries of 
North Wales is a band composed of concretions cemented in a black, graphitic, slightly 
lihosphatic matrix, and containing usually (H i)er cent, of phosphate of limi 
(phosphorite).' Tlu* tests of the trilobites and other organisms among the Cambriar 
rocks of Wales also contain phosphate of lime, sometimes to the extent of 20 pf 
cent.* Phosplialic, though certainly far inferior in cxtt'ut and importance to calcan>or 
and even to siliceous, formations, are often of singuhir gef»logical interest. The followi 
examples may serve as illustnitions. 

Guaiio— a dejxisit consisting mainly of the ilroppings of sea-fowl, formed 
islanils in niinloss tracts oft" the western coasts of South America and of Africa. 
is a brown, light, powdery substance with a |>eculiar ammoniacal odour. Anal 
of American guano give — combustibh; organic matter and acids, 11*3; amir 
(<Nirbonate, urate, l^c), •»1'7 ; fixed alkaline salts, sulphates, phosphates, chlor 
&e., SI ; pho.s2)hatee (»f lime and magnesia, 22*5 ; oxalate of lime, 2*6; sand aude 
matter, I'G; water, 22*2. This remarkable substance is highly valuable as a sov 
artiticial manures. (Book III. Part II. Section iii.) 

Bone-Breccia — a deposit consisting largely of fragmentary bones of lb 
ext iuct species of mammalia, found sometimes under stalagmite on the iioom f 
hton(> caverns, more <^r less mixcil with earth, sand, or lime. lu some older ^ 
tonnations, bone -beds occur, formed largely of the rcmuinB of leptiJcft 
U!j the *'Lias bone-bed,"* and the "Lmllow bone-bed." 



* ( >n formation of clialk-flixiis, siw Book UL BiMin. fleetlon Ul f 8 
'r^tL^TryHaai,Amer,J(mrn,8oe.%iU»(JMI^j^9iWM. Logan's ' Geoli^^ 
1 s<j:t, p. 461. • - r ifJBf '-* ^ 

' D. C. DavicB, Q. J. (7s ^^^^itOtl^mUjit. AUUkM, cp, o^ 



Part 11. S viij FUACf MENTAL JiOCKS. 171 

Ckyprolltio noduleB Hud beds > — aro fonnetl of the accuiuulaU'd excremcut of 
▼ertebrated animals. Among tho Oar1>onifcroa8 shules of the btusiii of the Firth of 
Forth, ooprolitio nodules aw abundant, together witli the bones and scales of tlio 
larger ganoid fishes which voided them ; abundance; of broken Hcales and bones of the 
smaller ganoids can nsnally bo o1)serviMl in the eoprolites. Among tho Jjower Sihiriau 
rocks of Canada, nmnerous phosphatic nodules, supi)OB<>d to be of coprolitic origin, 
occur.* The phosphatic beds of the Gumbridgeshiro Cretaceous rocks are now largely 
worked as a source of artificial manure. 

4. Cabbonacbous.— The formations here included have almost always resulted from 
tho decay and entombment of vegetation on the spot where it grew, sometimes by the 
drifting of the plants to a distance and their consolidation there. (Sea Book III. l*art II. 
Section ill., Lov.) In the latter case, they may bo mingled with inorganic sediment, 
ao as to pass into carbonaceous shale. 

Ftoat — ^vegetable matter, more or less decomposed and chemically altered, found 
throughout temperate climates in boggy places where marshy plants grow and decay. 
It varies from a pale yellow or brown fibrous substance, like turf or compressed 
hay, in which the plant-remaius are abundant and conspicuous, to a compact dark- 
brown or black material, resembling black clay when wot, and some varieties of lignite 
when dried. The nature and pro|)ortions of tho constituent elements of peat, after 
being dried at 100^ C, are illustrated by the analysis of an Irish example which gave-^ 
carbon, G0'4S ; hydrogen, 6*10 ; oxygen, 32-55 ; nitrogen, 0*88 ; while the ash was 3*30. 
There is always a large proportion of water which cannot be driven ofi* even by 
drying tho peat In the manufacture of compressed peat for fuel this constituent, 
which of course lessens the value of the peat as compared with an equal weight of 
coal, is driven off to a great extent by chopping the peat into fine pieces, and thereby 
exposing a large surface to ovi^ration. The ash varies in amount from less than 
1-00 to more tlian 65 per cent , and consists of sand, cby, ferric oxide, sulphuric acid, 
and minute proportions of lime, soda, potash, and magnesia.' Under a pressure of 
(3000 atmospheres peat is convertcnl into a hard, black, brilliant substance having the 
physical aspect of coal, and showing no trace of organic structure.* 

Ijignite (Brown Coal) — compact or earihy, compressed and chemically altered 
vegetable matter, often retaining a lamellar or ligneous texture, with stems showing 
woody fibre crossing each other in all directions. It varies from pale-bro\sTi or yellow 
to deep-brown or black. Some shade of brown is tho usual colour, whence the name 
Br*)trn coal, by which it is often known. It contains from 55 to 75 per cent, of carlwn, 
has a specific gravity of 0*5 to 1-5, burns easily to a light ash with a sooty flame 
and a strong burnt smell. It occurs in beds chiefly among the Tertiary strata, under 
conditions similar to those in which coal is found in older formations. It may be 
regarded as a stage in the alteration and mineralization of vegetable matter, inter- 
mediate between peat and true coal. 

Ck>al — a compact, usually brittle, velvet-black to pitch-black, iron-black, or 
dull, sometimes brownish rock, with a greyish-black or brown streak, and in some 
varieties a distinctly cubical cleavage, in otliers a conchoidal fracture. It contains 
from 75 to 90 per cent, of carbon, and a small percentage of sulphur, generally in 
the form of iron-disulphide. It has a speciOc gravity of 1*2-1 '35, and bums with com- 
parative readiness, giving a clear flame, a strong aromatic or bituminous smell, some 
varieties fusing and caking into cinder, others burning away to a mere white or red 
ash. Though it consists of compressed vegetation, no trace of organic structure 



* On tho origin of phosphatic nodules and beds, «i'(^ (iruner, Bull, Soc, Otol. France^ 
xxviii. (*2nd ser.) p. 02. Martin, op. cit. iii. (3rd scr.) p. 273. 

' Logan's 'Geology of Canada, p. 401. 

» Seo 8enfl*s * Humus-, Marsch-, Torf- und Limonit-bilduugen,' Leipzig, 1802. J. J. 
Friih ' Ueber Torf und Dopplerit,' Zurich, 1883. 

* Spring, BvUL Acml Boy, Bru^dles, xlix. (1880), p. 307. 



172 



OEOONOSr. 



[BookU. 



ia uauaJly apparent.' An ntteiitivo ciaminatioD, hoirovcr, will often ilbcloao porlknu 
of iiteinB, leaves, jcc, or at luust of oarbonizod woody fibre. Same kindD are aliooft 
wholly miulu up of tlic Bpotc-ctnea of lycopodinccona plantB. There Is reasoD lo believe 
that different varieties of cual may havo arisuu from original divondtiot in the nature of 
tlio vegetation out of wbicli they wero formed. Tbo sccomponying table abowa the 
chemical gradation between unaltered vegetation and the more highly mineralized forms 
of coul. 

Table bhowing tiib gbaddal Cuange in Couposmox fhou Wood to Ciuscoal.' 



1 

SutataDce. 1 CMlxin. 


1 


whMu'rtqalndto 


1. Wood (mcBU of several unalyBCs) . 1 100 

2. Peat ( } . , 100 

3. Lignite (muonof 15 vuiieUos). . 100 

i. Ten-jtttd cool of S. Staffordahirel. ,-„ 

5. Blcam ci,al from the Tyne . . . t 100 

6. PentrcfelincoalofS. Wales . . 100 

7. ADthiaciterromPeuni(ftvania,U.». 1 100 


I218 88-07 
0-85 '■ 55-67 
8-87 12*2 
612 21-28 
501 18-82 
*-75 1 5-28 
2-81 1-74 


1-80 
2-89 
807 
3-47 
8-62 
4-09 
2-ti3 



Coal occurs iu scams or beds intercalated between straU of Bandifone, shtdc, flreckj, 
Ac, in geological formations of Palteozoic, BecoDdary, and Tertiaiy ago. It Bhoold be 



w 



Fig. 30.— Mlcnwopli: Structure of rialkrith Oml. iliowliig I^nqiodUoeoM ^oo^^ 
l,inigullkd ion Dluiift^n). 

rrmcmben-d Ibat the word conl U rather a popular than a ■ 

indiiieriininnti'ly apjilied to any niineml substance oapabler-<i(''|iii|^.;i| 

SIrirlly employed, it ought only to be used with rehroMO to' IMs i 

vi';,-t.'liition, the rcsnlt either of the growth of plant* OH Ua mlol Vt Ute.itttUat^^. 

thm thither. -^t^-wt, . .^...^■■,j^ 



' Ou the inllucucc of pressure on th 
20 May, IH7V. t^t'nug, BuU. Acad. B 
' I'en-y's ' Metallurgy,' voL 1. p> II 



Pabt n. I TiL] 



FRAGMENTAL BOOKS. 



173 



The fioUowing onalyaeB show the chemical oomposition of peat, lignite, and some of 
the prineipal farieties of coal ^ : — 



Caihoii 

Hydrogen 

Oxygon 

Nitrogen 

Sulphur 

Ash 

Spedflo gravity . . 



Peat, 

DeYoii< 
ahlre. 



54*02 
5-21 

28*18 
2*80 
0*56 
9-73 



0-850 



Lianile, 

Bovey 

Trao^, 

Devou. 



66 
5 

22 

2 
2 



31 
63 
86 
67 
86 
27 



1129 



Caking 
Coai, 

Northum- 
berland. 



78-69 
600 

1007 
2-37 



1 
1 



51 
36 



1*269 



yon-Cak- 
ing Coal, 

S. Stafford- 
■hire. 



78 
5 

12 
1 

1 



67 
29 
88 
84 
39 
03 



1-278 



Cdnnd 
Oodl, 

Wigan. 



8007 
5*58 

8*08 
2-12 
-60 
'70 



1 

2 



1276 



Antkra- 
cite, 

S. Wales. 



90 
3 
2 


1 



89 
28 
98 
88 
91 
61 



1-892 



Tbeae analyses are exdoBive of water, which in the peat amounted to 25*56, and in 
the lignite to 34-66 per oent. 

Anthraolte — ^the most highly mineralized form of yegetation — is an iron-black to 
TelTet>blaok substance, with a strong metalloidal to vitreous lustre, hard and brittle, 
eaniaiiung over 90 per cent, of carbon, with a specific gravity of 1-35-1 '7. It kindles 
with difficulty, and in a strong draught bums without fusing, smoking, or smelling, but 
giving out a great heat It is a coal from which the bituminous parts have been 
eliminated. It occurs in beds like ordinary coal, but in positions where probably it 
hat been subjected to some change whereby its volatile constituents have been 
cxiwlled. It is found largely in South Wales, and sparingly in the Scottish coal-fields, 
where the ordinary coal-seams have been approached by intrusive masses of igneous 
rock. It is largely developed in the great coal-field of Pennsylvania. Some Lower 
Silurian shales are black from diffused anthracite, and have in consequence led to 
fruitless searches for coal. 

Oil-Bhale {Brandtchie/er) — shale containing such a proportion of hydrocarbons as 
to be capable of yielding mineral oil on slow distillation. This substance occurs as 
ordinary shales do, in layers or beds, intcrstratified with other aqueous deposits, as in 
the Scottish coal-fields. It is in a geological sense true shale, and owes its peculiarity 
to the quantity of vegetable (or animal) matter which has been preserved among its 
inorganic constituents. It consists of fissile argillaceous layers, highly impregnated 
with bituminous matter, passing on one side into common shale, on the other into 
cannel or parrot ooaL The richer varieties yield from 30 to 40 gallons of crude oil to 
the ton of shale. They may be distinguished from non-bituminous or feebly bituminous 
shales (throughout the shale districts of Scotland), by the peculiarity that a thin paring 
ourls up in front of the knife, and shows a brown lustrous streak. Some of the oil- 
ehales in the Lothians are crowded with the valves of ostracod crustaceans, besides 
scales, ooproUtes, &c., of ganoid fishes. It is possible that the bituminous matter may 
in some cases have resulted from auiuLal organisms, though the abundance of plant 
remains indicates that it is probably in most cases of vegetable origin. Under tlio 
name " pyroschists " Sterry Hunt classes the clays or shales (of all geological ages) 
which are hydrocar])onaoeou8, and yield by distillation volatile hydrocarbons, in- 
flammable gas, &c. 

Fetroleunit a general term, under which is included a series of natural mineml 
oils. These are fluid hydrocarbon compounds, varying from a thin, colourless, watery 
liquidity to a black, opaque, tar-like viscidity, and in specific gravity from 0*8 to 1*1. 
The paler, more limpid varieties are generally calle<l naphtha, the darker, more 

* From Percy's * Metallurgy,* vol. i. 



174 GEOGNOSY. [Book IT. 



viscid kinds mineral tar, whilo the name petroleum, or look-oil, has been 
more generally applied to the intermediate kinds. Petroleum occurs sparingly 
in Europe. A few localities for it are known in Britain. It is found in large 
quantity along the country stretching from the Carpathians, through Gallicia and 
Xoldavia, also at Baku on the Caspian.^ The most remarkable and abundant 
display of the substance, however, is in the so-called oil-regions of North America, 
I)articularly in Western Canada and Northern Pennsylvania, where vast quantities 
of it have been obtained in recent years. In Pennsylvania it is found especially 
in certain porous beds of sandstone or " sand-rocks," which occur as low down as the 
Old Reil Sandstone, or even as the top of the Silurian system. In Canada it is 
largely present in still low^er strata. Its origin in these ancient formations, wbeie it 
cannot be satisfactorily connected with any destructive distillation of coal, is still an 
unsolved problem. 

Asphalt — a smooth, brittle, pitch-like, black or brownisli-block mineral, having a 
resinous lustre and conchoidal fracture, streak paler than surface of fracture, and 
specific gravity of 1*0 to 1*C8. It melts at about the temperature of boiling water, 
and can be easily kindled, burning with a bituminous odour and a bright but smoky 
flame. It is composed chiefly of hydrocarbons, with variable admixture of oxygoi and 
nitrogen. It occurs sometimes in association with petroleiun, of which it may be 
considered a liardened oxidized form, sometimes as an impregnation filling the pons or 
chinks of rocks, sometimes in independent beds. In Britain it appears as a produet of 
tlie destructive distillation of coals and carbonaceous shales by intrusive igneous rocks, 
as at Binny Quarry, Linlithgowshire, but also in a number of places where its origin Is 
not evident, as in the Cornish and Derbyshire mining districts, and among the daik 
flagstones of Caithness and Orkney, which are laden with fossil fishes. At Seyisel 
(D^partcmont do TAin) it forms a deposit 2500 feet long and 800 feet broad, whidi 
yields 1500 tons annually. It exudes in a liquid form from the ground round the 
borders of the Dead Sea. In Trinidad it forms a lake 1} mile in circumference, which is 
cool and solid near the shore, but increases in temperature and softness towards the 
centre. 

G-raphite. — This mineral occurs in masses 'of sufficient size and importance to 
deserve a place in the enumeration of carbonaceous rocks. Its mineralogical oharacten 
liave already (p. 04) been given. It occurs in distinct lenticular beds, and also diffused 
in minute scales, through slates, scliists, and limestones of tlie older geological forma- 
tions, as in Cumberland, Scotland, Canada, and Bohemia. It is likewise found 
occasionally as the result of the alteration of a coal secun by intrusive basalt, as at New 
Ciunnock in Ayrshire. 

5. Ferruginous. — The decomposition of vegetable matter in marshy places and 
shallow lakes gives rise to certain organic acids, which, together with the carbcmlc 
acid so generally also present, decompose the ferruginous minerals of rocks and carry 
away soluble salts of iron. Exposure to the air leads to the rapid decomposition and 
oxidation of those solutions, which consequently give rise to precipitates, consisting 
partly of insoluble basic salts and partly of the hydrated ferric oxide. These precipitates, 
mingled with clay, sand, or other mechanical impurity, and also with dead and decay- 
ing organisms, form deposits of iron-ore. Operations of this kind appear to have 
been in progress from a remote geological antiquity. Hence ironstones with traces 
of associated organic remains belong to many different geological formations, and are 
being formeil still.* 

Bog Iron-Ore (Lake-ore, minerai des marais, Sumpferz)— ii dark-brown to bladr, 
earthy, but flonietinies compact mixture of hydrated peroxide of iron, phosphate of iron, 



' Abioh, ./ri/iW>. Ginil, Rficlmmst. xxix. (1879), p. 105. Trautschold, ZeiUeh, Dentsfh. 
(Irol. (h^. xxvi. (1874) p. 257. See poatea. Book III. Part I. Sect. i. § 2, where other 
uullioritieH uro citcil. 

• See SenftV work already (p. 171) cited, p. 108 ; also poftea^ Book III. Part II. Sect iii. 



Pam n. S Tit] FHAOMENTAL liOCKS. 175 

■ad hydnted oxide of nuiDguiefle, freqnenlly with ciaj, sand, and organio malltr. An 
ocdiiHTy spedoMii yielded, peroxide of iron, 62-59; osido of manganeso, 8-52; snud, 
lt-37 ; phoqiboric acid, 1-30; snlpliaric acid, traces; vuter and organic matter. 
1&02=10(H)0. Bog; iron-ore may either be formod I'lt titu from still vater, or may be 
laid down by currents in lakes. Of the former mode of formation, a familiar illustration 
i* fiuiiidwd by the"n»oor-bfttidpftn" or hard ferruginous cruat, which in boggy places 
titd on aome ill-drained land, forma at the bottom of the soil, on tho top of a, stiff and 
telentblj imperrionsgulMoil. Abundant bog-iron or lake-ore is obtained from the botlomB 
of aome lake* in Norway and Sweden. It forma everywhere on tho shallower slopes ' 
neir bonka of reeds, where there is no strong current of water, occurring in granulnr 
concretions (Bohnorz) that vary from the bi/o of graina of coarse gunpowder up to 
■udulea 6 inches in diameter, and forming layoia 10 In 200 yards long, 5 to 15 yanU 
broul, and 6 to 30 inches tliick. Theae ileposits are worked during winter by inserting 
perfoiated iron shovels through holes cut in the ice; and ho rapiill) do theynccumnInU', 
that inataiKca are linown where, after having been eumjiletely removed, the oro at the 
end of twcnty-aii years was found to have gathered agaiu to a, thickness of «cvernl 
iurhes. A layer of loose earthy ochic 10 feet thick ie believed to have formed in COO 
years on the floor of the Tioke Tisken near the oUl copper mine oF Falun in Swcilen .' 
Aeoirdiiig to Ehrenberg, the formation of bog-ore in due. not merely to the chemieni 
actiona arising from the decay of orgauiu matter, but to a power possessed by diatoms of 
•epanitiuft iron from water and depositing it as hyilroua pcToxido within their siliceoua 
fnniework. 

Alnminoae Tellow Iron-dre is closely related to tlie foregoing. It is a 
■listiire of yellow or pale brown, hydralod peroxide of iron, with clay and sand, aome- 
tinea with siLcate of iron, hydratod oxide of manganoae, and carbonate of lime, and 
<esan in dull, usually pulveruleDt grains and nodules. Occasionally theso nodules ma; 
lie obserred to consist of a shell of harder material, within whicli the yellow oxide 
iMomes progressively softer towards the centre, which is aometimus quite empty. Such 
emcretions arc known as (etites or ef^lc-atonea. This oro occurs in tlie Coal-measures 
"T^ony and Silesia, abw in the Ilarz, Baden, llavaria, &c., and among the Jurassic 
nrk* in Eugknd. 

Clay-Ironstone (Sphrerosidorite) has been already (pp. 7 
'wor* abimdaiitly in nodules and beds in tho Cariwuiifc reus »; 
KoTiipc. The nodules are generally oval nnd flattened in form, 
iitjing in size from a small beau up to concretions a foot or 
""re in diameter. In many cases, tiiey eoutnin iu the centre 
"mi' organic snbritauec, aueh as a cnprulite, fern, coue. shell, 
"rlii>1i, that has served as a surface round Vfhicli the iron in 
I'll' aatcr an<l tliu surrounding mud could be precipitated. 
Keami of chiy -ironstone vary in thickness from mere pnper-likc 
Mings up to bcU several feet deep. The ClevclaDd seam . — 

in tlie Middle Lina of Yorkshire ia about 20 f,ct thick. In ^''' J'^'^^.l^a^ar^"^'"" 
'he CarlHinifcmus svsUm of ScotUiiid certain seams known as 

Khrtlnaul contain from 10 to '>2 |icr cent of coaly matter, and admit of being ealcineil 
titli the addition r.f little or no fuel They are ai>metim(S crowded wjtli oi^nic 
Itniainf, eviieeiully lamLllibrunLhs ( lulliriieiiem, inlhrai'oia'ja, &c ) and fishes (_Rhi!Odu>, 
UnjaU'ehlhi/i, Ac). 

A microscopic examination of some btock-band ironstones reveals n very perfect 
oilitic slmi-tiire, sliowiug that tlie iron ]ia* Ixtn prccipitattd m «alcr having such a 
K^tlc movement as to keep the granules quii tU rolling along, whilo their s 
fiiniintric laycra of cnrlionate were being diposited Mr Sorby has obscrveif in i 
U'^i-laud iroiistuiiea an ubiuiriual form of <>i>litic structnn., and remarks that i 




' A. F Tliortld, flert ti,r,» IWhamt AiocAAoJni, in p. 20. 



178 



rEO^imST, 



[Book IL 



speeimeB bote evidtooe xbmL 
carbonate^ had been 
replaeed some oi ifae trawiniiff *yi the 
The aabjainedaiMii 



of the 
M it had 




Peioxiiie rtt ima 
Procoside •<t Ima 
Pn»tt>xiile*>t 
Almiiim 
Lime . 



Caibonic acid . 
Pba^ihode :wid 
Sidphime -Jirid 
Iroo pjTiti» 
Water . 
Otsaiik mauer 



PeN«nia^ uc ixoa 




TmkakM 


I'tf 


M'l^ 


L*S 


•J-TIr 


i-TO 


±-IT 


i>-tS 


17- 3T 


»5r 


•>•» 


<M0 


l-TT 


t-|» 



( 



»-7S 



2»-li • 



0-78 

n-io 



17- 






43-08 
•'40 

5-87 

5-14 (itDc) 
5-21 

7-17 

25-50 

1-81 



0-15 

lOO-d 

35-46 



1 AddKtis to Ge<)L Sw. F«iiraar?« 1879. 

* r^t Peiv«*» -liecaUu^. yvL ik BwefaaC. -Chok uad Fhm G«oL* Mpfi 0^0 
1^.05. 



( 177 ) 



BOOK III. 

DYNAMICAL GEOLOGY. 

Dynamical Geology investigates the processes of change at prosont in 
progress upon the earth, whereby modifications are made on the 
Btnicture and composition of the crust, on the relations between the 
interior and the surface, as shown by volcanoes, earthquakes, and other 
terrestrial disturbances, on the distribution of land and sea, on the 
outlines of the land, on the form and depth of the sea-bottom, on marine 
cnrrents, and on climate. Bringing before ns, in short, the whole 
Tinge of geological activities, it leads us to precise notions regarding 
their relations to each other, and the results which they achieve. A 
howledge of this branch of the subject is thus the essential groundwork 
of a true and fruitful acquaintance with the principles of geology. The 
■tudy of the present order of nature, provides a key for the interpre- 
tation of the past. 

The operations considered by Dynamical Geology may be regarded 
as a vast cycle of change, into the investigation of wliicli the student 
may ]»reak at any point, and round which ho may travel, only to find 
HiDself ])rought back to his starting-point. It is a matter of coni- 
l»aratively small moment at wliat part of the cycle the inquiry is Ijegun. 
The changes seen in action will always bo found to have resuKed from 
•iomo that preceded, and to give place to others that follow them. 

At an early time in the earth's history, anterior to any of the periods 
of which a record remains in the visible rocks, the chief sources of 
geological energy probably lay witliin the earth itself. The planet still 
retaiiio<l much of its initial heat, and in all likelihood was the theatre 
of great chemical changes. As it cooled, and as the supei*ficial 
feurliances duo to internal heat and chemical action became less 
luarkod, the influence of the sun, which must always liave operated, 
aud which in wirly geological times may have been more effective than 
it afterwards l)ecame, would then stand out more clearly, giving rise to 
that wide circle of surface changes wherein variations of temperature 
and the circulation of air and water over the surface of the earth come 
into play. 

lu the pursuit of his inquiries into the past liistory and into the 
pretjent economy of the earth, the student must needs keej) his mind ever 
open to the reception of evidence for kinds, and especially for degrees, 

• N 



178 DYNAMICAL GEOLOGY. XBook IIL Part I. 



of action which he had not before encountered. Human experience has 
been too short to allow him to assume that all the causes and modes 
of geological change have been definitively ascertained. Besides the 
fact that both terrestrial and solar energy were once probably more 
intense than now, there may remain for future discovery evidence of 
former operations by heat, magnetism, chemical change or other agencji 
that may explain phenomena with which geology has to deal. Of the 
influences, so many and profound, which the sun exerts upon our planet, 
we can as yet only perceive a little. Nor can we tell what other ooamical 
influences may have lent their aid in the revolutions of geology. 

In the present state of knowledge, all the geological energy upon and 
within the earth must ultimately be traced back to the primeval energy 
of the imrent nebula, or sun. There is, however, a certain propriety 
and convenience in distinguishing between that part of it which Ib due 
to the survival of some of the original energy of the planet, and that 
part which arises from the present supply of energy received day by 
day from the sun. In the former case, the geologist has to deal with 
the interior of the earth and its reaction upon the surface ; in the latter, 
he is called upon to study the surface of the earth, and to some extent 
its reaction on the interior. This distinction allows of a broad treatment 
of the subject under two divisions : — 

I. Hypogene or Plutonic Actio n — the changes within the 
earth, caused by original internal heat and by chemical action. 

II. Epigene or Surface Action — the changes produced on 
the superficial parts of the earth, chiefly by the circulation of air and 
water set in motion by the sun's heat. 

Part I. Hypogene Action, 

An Liquiry iiUo the Oeological Changes in Progress benetUh the Sur/ciee 

of ihe Earth. 

In the discussion of this branch of the subject, it is useful to carry in 
the mind the conception of a globe still intensely hot within, radiating 
heat into space, and consequently contracting in bulk. Portions of 
molten rocks from inside are from time to time poured out at the surfaoe. 
Sudden shocks are generated, by which earthquakes are propagated to 
and along the surface. Wide geographical areas are upraised or 
depressed. In the midst of these movements, the rocks of the crust are 
fractured, squeezed, cnimpled, rendered crystalline, and even fused. 

Section i. Volcanoes and Volcanic Action.^ 

§ 1. Volcanic Products. 

The term volcanic action (volcanism or volcanicity) embraces all the 
phenomena connected with the expulsion of heated materials from the 

^ The siiuleut Ih roferrod to the fullowiug geuoral works on the phenomena of vol- 
canoes. Serope, 'Considerations on Volcanoes,' London, 1825; * Volcanoes,* LoodQO, 



Sbot. L I 1.] VOLCANIC PRODUCTS. 179 



interior of the earth to the surface. Amoug these pheuomeua, some 
possess an eyanescent character, while others leave permanent proofs of 
their existence. It is naturally to the latter that the geologist gives 
chief attention, for it is by their means that he can trace former phases 
of voloanio activity in regions where, for many ages, there have been no 
Tolcanio emptions. In the operations of existing volcanoes, ho can 
observe only superficial manifestations of volcanic action. But exam- 
ining the rocks of the earth's crust, he discovers that amid the many 
terrestrial revolutions which geology reveals, the very roots of foiiuer 
▼dlcanoes have been laid bare, displaying subterranean phases of 
Tolcanism which could not be studied in any modem volcano. Hence 
an acquaintance only with active volcanoes will not afford a complete 
knowledge of volcanic action. It must be supplemented and enlarged 
by an investigation of the traces of ancient volcanoes preserved in the 
crust of the earth. (Book IV. Part VII.) 

The word '* volcano " is applied to a conical hill or mountain (com- 
posed mainly or wholly of erupted materials), from the summit, and 
offcen also from the sides of which, hot vapours issue, and ashes and 
streams of molten rock are intermittently expelled. The term 
** volcanic" designates all the phenomena essentially connected with one 
of these ohannels of communication between the surface and the heated 
interior of the globe. Yet there is good reason to believe that the active 
volcanoes of the present day do not afford by any means a complete type 
of volcanic action. The first effort in the formation of a new volcano is 
to establish a fissure in the earth's crust. A volcano is only one vent or 
group of vents established along the line of such a fissure. But iu 
many parts of the earth, alike iu the Old World and the New, there have 
been periods in the earth's history when the crust was rent into innumer- 
able fissures over areas thousands of square miles in extent, and when the 
molten rock, instead of issuing, as it does at a modem volcano, in narrow 
streams from a central elevated cone, welled out from numerous points 
along the rents, and flooded enormous tracts of country without forming 
any mountain or volcano in the usual sense of those terms. Of these 



2ud edit 1872; 'Extinct Volcanoes of Central France/ London, 1858; *0u Volcanic 
CuQCii and Craters,' QvmH. Journ. Geol. 1^, 1859. Daubeny, * A Description of Active 
and Extinct Volcanoes/ 2nd edit., London, 1858. Darwin, * Geological Observations on 
Volcmnic Islands,' 2nd edit., London, 187G. A. von Humboldt, * Ueber den Ban und die 
Wiikung der Vnlkane/ Berlin, 1824. L. von Buch, * Ueber die Natur dor vulkanischen 
Erscheinongen auf den Cauarischen Insoln/ Pogqend. Annalen (1827), ix. x. ; * Ueber 
Erhebungaloratere nnd Vulkane,* Pwjgend, Annaltn (1836), xxxvii. E. A. von Hoff, 
*■ QeMhicnto der dnrcli Ueberlieferung nachgewiescnen natUrlichcn Verunderungen der 
Erdoborflache • (part ii., ** Vulkane nnd Erdbeben"), Gotha, 1824. C. W. C. Fuchs, 
* Die vulkanischen Erscheinungen der Erde/ Leipzig, 1865. R. Mallet, **0n Volcanic 
Energv/* Phil, TraM. 1873. J. Schmidt, * Vulkaustudien,' T^ipzig, 1874. Sartorius 
TOO Waiterskauflen and A. von Lasaulx, * Der Aetna/ 4to, Leipzig, 1880. E. Reyer, 
^Beitrag xnr Physik der Ernptionen/ Vienna, 1877; * Die Euganeon ; Ban und Ge- 
acbichte einos Vidkanes/ Vienna, 1877. Foiiqu^ * Sontorin et ses eruptions/ Taris, 1879. 
Jadd, 'Yoloanoes,' 1881. O. Morcalli, ' Vulcani e Fonomeni vulcauici in Italia/ Milan, 
UiS!S, Ch. Yclain, ' Jjcs Volcans/ Paris, 1884. References will be found in succeeding 
tttgcs to other and more siieoial memoirs. 



180 DYNAMICAL GEOLOGY. [Book IIL Pabt I. 



*' fiHsnre-oruptions," apart from central volcanic cones, no examples ha^e 
occurred within the times of human history, unless some of the lava- 
floods of Iceland may be so regarded. They can only be studied &om 
the remains of former convulsions. Their importance, however, has 
not yet been generally recognised in Europe, though acknowledged in 
America, where they have been largely developed. Much still remains to 
be done before their mechanism is as well understood as that of the lesser 
type to which all present volcanic action belongs. In the succeeding 
narrative an account is first presented of the ordinary and familiar 
volcano and its products ; and in § 3, ii., some details are given of the 
gcneml aspect and character of the more gigantic fissure-eruptions. 

The openings by which heated materials from the interior now 
reach the surface include volcanoes (with their various associated 
orifices) and hot-springs. 

The prevailing conical form of a volcano is that which the ejected 
materials naturally assume round the vent of eruption. The summit of 
the c o n e is truncated (Fig. 32), and presents a cup-shaped or cAuldron- 
like cavity, termed the c r a t e r, at the bottom of which is the top of the 
main funnel or pipe of communication with the heated interior. A 
volcano, when of small size, may consist merely of one cone ; when of 
the largest dimensions, it forms a huge mountain, with many subsidiary 
cones and many lateral fissures or pipes, from which the heated volcanic 
products are given out. Mount Etna (Fig. 32 j rising from the sea to a 
height of of 10,840 feet, and supporting, as it does, some 200 minor coneB» 
many of which are in themselves considerable hills, is a magnificent 
example of a colossal volcano.^ 

The materials erupted from volcanic vents may be classed as (1) 
gases and vapours, (2) water, (3) lava, (4) fragmentary substances. A 
brief summary under each of these heads may be given here ; the share 
taken by the several products in the phenomena of an active volcano is 
described in § 2. 

1. Gases and Vapours exist absorbed in the molten magma 
within the earth's crust. They play an important part in volcanic 
activity, showing themselves in the earliest stages of a volcano's history, 
and continuing to appear for centuries after all the other evidences of 
subterranean action have ceased to be manifested. By much the most 
abundant of them all is steam, which has been estimated to form 
Y\fy\jths of the whole cloud that hangs over an active volcano. In great 
eruptions, it rises in prodigious quantities, and is rapidly condensed into 
a hf^avy rainfall. M. Fouque calculated that, during 100 days, one of the 
parasitic cones on Etna had ejected vapour enough to form, if condensed, 

^ The Btnicture and history of Etna are fully described in the great work of 
Sartorius von Waltorshausen and A. yon Lasaulx cited on p. 179 — a treasure-hoiise of 
facts in volcanic geology. See also G. F. RodwcU, * Etna, a history of the mountain 
and its eruptions,* London, 1878 ; O. Silvostri, * Un Viaggio alF Etna,' 1879. Kotioes 
of recent eruptions of the mountain will bo found in Nature^ vols, xix., xx., xxi, zxii., xxv, 
(observatory on Etna, p. 394), xxrii. The work of Mercalli, cited on p. 179, giv«8 
descriptions of this and the other Italian volcanic centres. 



SitCT. L f IJ rOLCANZa OASES AND VAPOURS. 



181 



2,100,000 cubic mdtres (462,000,000 gallone) ofvatei. But even from 
TiJoanoefl which, like the Solfatara of Naples, have beeo dormant for 
oentnries, ateun sometimes Btill rises without interraiBsioa and in oon- 
Ddexftble v(dome. Jets of vapoar rush oat from olefta iu the sides and 




bottom of a crater with a noiso like that made by the steam blown off 
bf a locomotive. The number of these funnels or fumarolos is often so 
IkTge, and the amount of vapoar so abundant, that only now and then, 
vlien the wind blows the dense cloud aside, can a momentary glimpse 



X82 BTKAMICAL OSOLOOT. [Book IIL Paw L 

be hftd of a part of the bottom of tlie crater ; vhile at tbe nvme time the 
ruab and roar of tbe escaping at^am remind one of the din of some vaat 
factory. Aqueoux vapour rises likewise from rents on the ontaide of 
tbe Tulc&Dic couo. It issues so copioosly from some flowing lavas thkt 
tbe stream of rock may be almost concealed from view by tbe clond ; and 
it contiiiueR to escape from fisanres of tbe lava, far below tbe point of 
exit, for a long time after tbe rock bas solidified and come to rest. So 
saturated, as it wero, are many molten lavaa witb tbe rapoor of watar 
tbat Mr. Scru]^>o even maintained that tbeir mobility was due to this 

Probably in no caao is the steam mere pnre vapour of water, thougb 
when it comlonses into copioiiH rain, it ia fresb and not salt water. It ii 




Us. a).-Vli.it of VmovIm •« I 



ling Um dcnw ck 



associated witb other vapours and gases diticiigaged from tbe potent 
chemical laboratory undenicatli. There seems to bo alwaya a definite 
order in the appearance of these vapours, though it may vary int 
difierent volcanoes. Tlie liottcst and moHt nctivo " f uniarolcs," or 
vapoiir-vcnta, may contain all tbe gases and vapours of a volcano, 
but as the beat diminishes, the series of gaseons emanations is ledncod. 
Thus in the \'cBTivian eruption of 1855-56, tlio lava, as it cooled and 
hardened, gave out succeasively vaponi-s of hydrochloric acid, chlorides, 
I snlpbnroQS acid; then steam; and, finally, carl)On-dioxido and 
-^—'^'' ■ " More recent observations tend to corroborate the 

.OBTolc«norB'(lRaf),p. lin. 

gjppgyfc pBvUle and I*blMC, Aan. Ckim. rt r\y,. 1858. lit. p. 19 ,f •«. 
W arwWTtBa ami itii pmptimui, hrsMM the cencral worln almulf c(t«t 



Sect, i, $ L] VOLCANIC GASES AND VAPOURS, 183 



deduotiona of 0. Sainte-Claire Deville that the nature of the vapoura 

evolved depends on the temperature or degree of activity of the voloanio 

orifioe, chlorine (and fluorine) emanations indicating the most onorgetio 

phase of emptivity, sulphurous gases a diminishing condition, and 

carbonic add (with hydrocarbons) the dying out of the activity.^ A 

^ ool&tara," or vent emitting- only gaseous discharges, is believed to 

pass through these snooessive stages. Wolf observed that on Cotopaxi 

while hydrochloric add, and even free chlorine escaped from the summit 

of the cone, sulphuretted hydrogen and sulphurous acid issued from the 

middle and lower slopes.' Fouque's studies at Santorin have shown 

also that from submarine vents a similar order of appearance obtains 

among the volcanic vapours, hydrochloric and sulphurous acids being 

only found at points of emission having a temperature above 100^ 0„ 

while carbon-dioxide, sulphuretted hydrogen and nitrogen occur at all the 

fumaroles, even where the temperature is not higher than that of the 

atmosphere.^ 

The foUowiDg are the chief gases and acids evolved at volcanic f umarolea. Hydro- 
chloric aoid is abundant at Vesuvius, and probably at many other vents whence it 
has not been zeoocded. It is zecognisablo by its pungent, suffocating fumes, which mako 
approach difficult to the defbi from which it issues. Sulphuretted hydrogen 
and sulphurous acid are distinguishable by their odours. The liability of the 
former gas to decomposition leads to the deposition of a yellow crust of sulphur; 
occasionaUy, also^ the production of sulphuric acid is observed at active vents. 
From obaervatioDS made at Vesuvius in May 1878, Mr. Siemens concluded that vaet 
quantities of free hydrogen or of combustible compounds of this gas exist 
dissolved in the magma of the earth's interior, and that these, rising and exploding 
in the ftmnels of volcanoes, give rise to the detonations and clouds of steam.^ At 
the eruption of Santorin in 1860, the same gases were also dibtinctly recognised by 
Fouque, who for the fiiii timo established tbe existonco of true volcanic flames 
These were again studied spectrosoopioally in the following year by JansBen, who 
found them to arise essentially from the combustion of free hydrogen, but with traces 
of cblorine, soda, and copper. Fouque determined by analysis that, immediately over 
the focus of eruption, free hydrogen formed thirty per cent, of tbe gases emitted, but 
that the proportion of this gas rapidly diminished with distance from the active vents 
and hotter lavas, while at the same time the proportion of marsh-gas and carbon-dioxido 
rapidly increased. The gaseous emanations collected by him were found to contain 
abundant free oxygen as well as hydrogen. One analysis gave the following 



on p. 179, consult J. Phillips' 'Vesuvius,' 1869; J. Schmidt, * Die Eruption des 
Vesnv, 1855,' Vienna, 1856 ; Mercalli's • Vulcani, &c/ ; H. J. Johuston-Lavis, Q. J, 
GfoL 8oe, xL 35. A diary of the volcano's behaviour for six months is given in 

Naiurt^ xxvi. 

' He distinguished volcanic emanations according to their order of appearance as 
regards time, nearness to the vent, and temperature : viz., 1. Dry fumarolcs (without 
steam), where anhydrous chlorides ore almost the only discharge, and where the tempera- 
ture is very high (above that of melted zinc). 2. Acid fumarolcs, with sulphurous and 
bvdiochloric acids and steam. 3. Alkaline (ammoniacal) fumaroles ; temperature about 
lOO C. ; abundant steam with chloride of ammonium. 4. Cold fumaroles ; temperature 
below 100 C, with nearly pure steam, accompanied with a little carbon-dioxide, and 
sometimes sulphuretted hydrogen. 5. Mofettes; emanations of carbou-dioxido with 
nitrogen and oxygen, marking the last phase of volcanic activity. 

* Nenet Jahrh. 1878, p. 164. 

' * Santorin et ses e'ruptions,' Paris, 1879. 

« MomtA, K. Prewuf, Akad, 1878, p. 588. 



184 DYNAMICAL GEOLOGY. [Book HI. Part I. 

i^ults: carbon-dioxide 0*22, oxygen 21-11, nitrogen 21'90» liydrogen 56*70, marah 
gas 0*07,=100'00. This gaseous mixture, on coming in contact with a burning body, 
at once ignites with a sharp explosion. Fouqud infers that the water-vapour of Tolcanic 
vents may exist in a state of dissociation within the molten magma whence lavas rise.' 
Garbon-rdioxido rises chiefly (a) after an eruption haa ceased and the volcano 
relapses into quiescence } or (h) after volcanic action has otherwise become extinct. 
Of the former phase, instances are on record at Vesuvius where an eruption has been 
followed by the emission of this gas so copiously from the ground as to soffocate 
hundreds of hares, pheasants, and partridges. Of the second phase, good examples 
arc supplied by the ancient volcanic regions of the Eifel and Auvergne, where the gas 
still rises in prodigious quantities. Bischof estimated that the volume of carbonic 
acid evolved in the Brohl Thai amounts to 5,000,000 cubic feet, or 800 tons of gaa, in 
one day. Nitrogen, derived perhaps from the decomposition of atmospheric air 
dissolved in the water which penetrates into the volcanic foci, has been frequently 
detoctecl among the gaseous emanations. At Santorin it was found to form from 4 to 
88 per cent of the gas obtained from different fumaroles.' Fluorine and iodine 
have likewise been noticed. 

With these gases and vapours are associated many substances which, sublimed by 
the volcanic heat or resulting from reactions among the escaping vapours, appear as 
Sublimates along crevices wherein they roach the air and are cooled. Besides 
sulphur, there are several chlorides (particularly that of sodium, and less 
abundantly those of potassium, iron, copper, and lead) ; also free sulphuric acid, 
sal-ammoniac, specular iron, oxide of copper, boraoic acid, 
alum, sulphate of lime, felspars, pyroxene, and other substances. Car- 
l^onato of soda occurs in large quantity among the furoaroles of Etna. Sodium-chloride 
sometimes appears so abimdantly that wide spaces of a volcanic cone, as well as of the 
newly-erupted lava, are crusted with salt, which oan even be profitably removed by the 
inhabitants of the district Considerable quantities of chlorides, &c., may thus bo 
buried between successive sheets of lava, and in long subsequent times, may givo rise to 
mineral springs, as has been suggested with reference to the saline waters which issue 
from volcanic rocks of Old Red Sandstone and Carboniferous age in Scotland.' The 
iron-chloride forms a bright yellow and reddish crust on the crater walls, as well as on 
loose stones on tlie slopes of the cone. Specular iron, from the decomposition of iron- 
chloride, forms abundantly as thin lamellie in the fissures of Yesuvian lavas. In the 
spring of 1873 the author observed delicate brown filaments of tenorite (copper-oxide^ 
CuO) forming in clefts of the crater of Vesuvius. They were upheld by the upstream- 
ing current of vapour until blown off by the wind. Fouqiie has described tubular yents 
in tlie lavas of Santorin whert*in crystals of onorthite, sphene, and pyroxene have 
recently been formed by sublimation. In the lava stalactites of Hawaii needle-like 
fibres of breislakite abound. 

2. Water. — Abundant dischargee of water acoompany some volcanic 
explosions. Three sources of this water may be assigned : — (1) from 
the melting of snow by a rapid accession of temperature previous to or 
during an eruption; this takes place from time to time on Etna, in 
Iceland, and among the snowy ranges of the Andes, where the cone of 
Cotopaxi is said to have been entirely divested of its snow in a single 
night by the heating of the mountain ; (2) from the condensation of the 
vast clouds of steam which are discharged during an eruption; this 
undoubtedly is the chief source of the destructive torrents so frequently 
observed to form part of tlie phenomena of a great volcanic explosion ; 

* Fouque', * Santorin et ses eruptions,' p. 225. ' Fouqu^, loc: eiL 

• Vroe, Roy. Soc. Edin. ix. p. 367. 



Sect. i. §1.] LAVA. 185 

and (3) from the diaruption of reservoirs of water filling subterranean 
cavities, or of lakes ocoupying crater-basins; this has several times 
been observed among the South American volcanoes, where immense 
quaniities of dead fish, which inhabited the water, have been swept 
down with the escaping torrents. The volcano of Agua, in Guatemala, 
raoeived its name from the disruption of a crater-lake at its summit 
by an earthquake in 1640, whereby a vast and destructive debacle of 
water was diaoharged down the slopes of the mountain. In the 
beginning of the year 1817, an ei-uption took place at the large crater 
of Idj^n, one of the volcanoes of Java, whereby a steaming lake of hot 
acid water was discharged with frightful destruction down the slopes of 
the mountain. After the explosion, the basin filled again with water, 
bnt its temperature was no longer high. 

In many cases, the water rapidly collects volcanic dust as it 
rnshes down, and soon becomes a pasty mud; or it issues at first in 
this condition from the volcanic reservoirs after violent detonations. 
Hence arise what are termed mud-lavas, or aqueous lavas, which in 
many respects behave like true lavas. This volcanic mud eventually 
crmBolidates into one of the numerous forms of tuff, a rock which, as 
has been already stated (p 164), varies greatly in the amount of its 
coherence, in its composition, and in its internal arrangement. 
Obviously, unless where subsequently altered, it cannot possess a 
crystalline structure like that of true lava. As a rule, it betrays its 
aqueous origin by more or less distinct evidence of stratification, by 
the multifarious pebbles, stones, blocks of rock, tree-trunks, branches, 
Bhellin, 1x)ne8, skeletons, &c., which it has swept along in its courHe and 
j#re8erved within its mass. Sections of this compacted tuff may bo sc^en 
at Iforculaneum.^ The trass of the Brohl Thai and otlior valleys in the 
Eifel district, referred to on p. 167, is another example of an ancient 
vc»]eanic mud. 

.*i. Lava. — The term lava is applied generally to all the molten 
rockrt of volcanoes.^ The use of the word in this broad sense is of 
j^reat convenience in geological descriptions, by directing attenti(ni 
to the leading character of the rocks as molten products of volcanic 
action, and obviating the confusion and errors which are apt to arise 
from an ill-defined or incorrect lithological terminology. I'recise 
«le6uiticni« of the rocks, such as those alK)ve given in BcK)k II., can 
U.* abided when required. A few remarks regarding some of the 

* Mul1<rt tlioupht that the 8o-called •' mud-lavas " of Herculatioum and Pompeii were 
tot aqiicourt depoBits {Jnnrn. Roy. Geol Soc. Ireland, IV. (1876) p. 144). But there 
Beems no TvoJityn to doubt that while au enormous amount of ashes fell during the 
tmpti<»n of A.D. 79, there were likewise, especially in the late phases of eruption, copious 
tffrn.-ntjf of water that mingled with the fine ash and became "mud-lavas." Tlio 
i>harf)Q4;80 of outline, and the absence of any trace of abdominal distension in the moulds 
'»f the human liodies fouud at Pompeii, probably shows that these victims of the cu ta- 
rt niphtr were rapidly enveloped in a tirm coherent matrix which could hardly have been 
mere looee dnat. See H. J. Johnston-Lavis, Q. J. Geol. Soc. xl. p. 89. 

* " AUe« ist liava was im Vulkano lliesst und durch seine FlUasigkeit neuo 
liCigfcntatter eiumiumt" is Lt*)pohl von Bucli*s comprehensive definition. 



186 DYNAMICAL GEOLOGY. [Book IH. Part L 

general lithological eharactera of lavas may be of servioe here; the 

behaviour of the rocks in their emission from volcanic orifices will 

be described in § 2. 

While still flowing or not yet cooled, lavas differ from eaoli other in the extent to 
which they ore impregnated with gases and vapours. Some appear to be mtoiBtedy 
others contain a much smaller gaseous impregnation; and hence arise important 
distiQctions in their bchavioar (pp. 200, 207). After solidification, lavas present some 
noticeable characters, then easily ascertainable. (1) Their average specific gravity 
may be taken as ranging between 2'37 and 3*22. (2) The heavier varieties oontain 
much magnetic or titaniferons iron, with augite and olivine, their composition being 
basic, and their proportion of silica averaging about 45 to 55 or 60 per cent In thia 
group come the basalts, nepheline-lavas, and leucite-lavaa The lighter varieties 
oontain commonly a minor proportion of metallio hoses, but are rich in silica, their 
percentage of that acid ranging between 60 and 80. They are thus not basic bnt aoid 
rooks. Among their more important species, trachyte, rhyolite, obsidian, pitchstone^ 
and pumice may be enumerated. Some intermediate varieties (augite-andesite, honi* 
blende-andesite) connect the acid and basic seriea (3) They differ much in stmctnxe 
and texture, (a) Some are entirely crystalline, consisting of an interlaced mass of 
crystals and crystalline particles, as in some dolerites, and granitoid liparites. Even 
quartz, which used to be considered a non-volcanic mineral, characteristic of the older 
aud chiefly of the plutonic eruptive rocks, has been observed in large erytMB in 
modem lava (liparite and quartz-andcsite^). (6) Some show more or less of a half* 
glassy or stony (dovitrified) matrix, in which the constituent crystals are imbedded ; 
this is the most common arrangement, (e) Others are entirely vitreous, such crystals 
or crystuUino particles as occur in them being quite subordinate, and, so to wgoB^ 
accidental enclosures in the main glassy mass. Obsidian or volcanic glass is the typd 
of this group, {d) They ftirther differ in the extent to which minute pores or larger 
cellular spaces have been developed in them. According to Bischof, tlie porosity of lavas 
depends on tlicir degree of liquidity, a porous lava or slag, when reduced in his experi- 
ments to a thin-flowing consistency, hardening into a mass as compact as the densest 
lava or basalt.' The presence of interstitial steam in lavas, by expanding the still 
molten stone, produces an open cellular texture, somewhat like that of sponge or of 
bread. Such a vesicular arrangement very commonly appears on the upper surface of a 
lava current, which assumes a slaggy or ciudery aspect (4) They vary greatly in 
colour and general external aspect. The heavy ba^ic lavas are usually dark grey, or 
almost black, though, on exposure to the weather, they acquire a brown tint from the 
oxidation and hydration of their iron. Their surface is commonly rough and ragged, 
until it has been sufficiently decomposed by the atmosphere to crumble into soil which, 
under favourable circumstances, supports a luxuriant vegetation. The less dense lavas^ 
such as phonolites and trachytes, are frequently paler in colour, sometimes pale yeUoW 
or buff, and decompose into light soils ; but the obsidians present rugged black sheets 
of rock, roughened with ridges and heaps of grey froth-like pumice. Some of the most 
brilliant surfaces of colour in any rock-scenery on the globe are to bo found among 
volcanic rocks. The walls of active craters glow with endless hues of red and yellow. 
The Grand Ca&on of the Yellowstone Biver has been dug out of the most marvellously 
tinted lavas and tuffs. 

4. Fragmentary Materials.— Under this title may be included all 
the substances which, driven up into the air by volcanic explosions, 
fall in solid form to the ground — the dust, ashes, sand, cinders, and 
blocks of every kind which are projected from a volcanic orifice. 
These materials differ in composition, texture, and appearance, eveii 

» Wolf, Nem$ Jahrb. 1874, p. 377. 

* • Chem. und Phys. Qeol.' supp. (;i871). p. 144. 



\ 



Sect. i. § Ij VOLCANIC ASHES. 187 

during a single eruption, and still more in BUcooBsive explosions of 
the same volcano. For the sake of convonionce, separate names are 
applied to some of the more distinct varieties, of which the following 
may be enumerated. 

(1) Ashes and san d. — In many eruptions, vast quantities of an exceedingly fine 
light grey powder are ejected. As this substanoo greatly resembles what is left after a 
piece of wood or coal is burnt in an open fire, it has been popularly termed aatij and 
this name lias been adopted by geologists. If, however, by the word ash, the result of 
eomboBtion is implied, its employment to denote any product of volcanic action must be 
legiettedv as apt to convey a wrong impression. The fine ash-like dust ejected by a 
volcano is merely lava in an extremely fine state of comminution. So minute are the 
pazticlet that they find their way readily through the finest chinks of a closed room, 
and settle down upon floor and furniture, as onlinary dust does when a house is shut 
np. From this finest form of material, gradations may bo traced, through what is 
termed volcanic sand, into tho coarser varieties of ejected matter. In compobition, the 
ash and sand vary necessarily with the nature of the luva from which they are derived. 
Their microscopic structure, and especially their abundant microliths, crystals, and 
volcanic glass have been already referred to (p. 1G5). 

(2) Lap ill! or rapilli (p. 165} are ejected fragments ranging from the size of 
a pea to that of a walnut ; round, subangnlar, or angular in shape, and having the same 
indefinite xmnge of composition as the finer dust As a rule, the coarse fragments &11 
nearest the focus of eruption. Sometimes they are solid fhigments of lava, but more 
nanallj they have a celluhr texture, while sometimes they are so light and porous as to 
float readily on water, and, when ejected near the eea, to cover its surfoce. Well-forniod 
crystals occur in the lapilli of many volcanoes, and are also ejected separately. It lins 
been observed indeed that tlie fragmentary materials not infrequently contain finer 
crystals than the accompanying lava.' 

(3) Volcanic Blocks (p. 1G5) are larger pieces of stone, often angular in slinpo. 
In fkrine cases they appear to be fragments loosened from already solidified rocks in tho 
chimney of the volcano. Hence we find among them pii'ces of non-volcanic rocks, uh 
Well as of older tufls and lavas rccognisably belonging to early eruptions. In many 
ouaei*, they are ejected in enormous quantities during the earlier phusos of violent 
eruptiun. The great explosion from the side of Ararat in 1840 was accompanied by 
the discharge* of a vast quantity of fragments over a siwioe of many square miles around 
the mountain. Whitney has described the occurrence in C!alifonuu of )>edH of such 
fragmentary volcanic breccia, hundreds of feet thick and covering many scjuare miles 
of surface. Jnnghulm in his account of the eruption in Java in 1772, mentions that 
a valley ten miles long was filled to an average depth of fifty feet with angidar 
Tokaaic debris.' 

Among the earlier eruptions of a volcano, fragments of the rocks through which tho 

nnt baa been drilled may frequently l>c obsen-eil. These are in many cases not 

tolffanie. Blocks of schist and granitoid rocks r>ccur in the cinder-beds at the Imim' 

4 the v<)lcanic scries of Santorin. In the older tuffs of Somma, pieces of altered 

Vaiii.3i"U>- (.-mn^times measuring 200 cubic feet or more and weighing upwunls of 

i'- ^-^) are ubundant and often c(^>ntain cavities lined with the characteritotii* 

-V' ••n'ciu niincrals." * Blocks of a coarsely cr}'stalline granitoid (but really traehytie) 

UN a liavts been particularly observed both on Etna * and Vesuvius. In the year 1x70 

AIUM4 (i{ tliiit kind, weighing several tons, was to be &een lying at the foot of Vesuviuh. 



^.trturiuvon WaltenhauiCD, 'Sicilien und Island,' 1853, p. .32H. 

l^ti\ E«e tbfi niBKks already made on volcanic conglomerates, antf, p. ht5. 

>• '■ H. J. JohnsUm-Lavis, Q. J. Gtol. i^. xl. p. 75. 

» ■' i1m? (Topled blocks (AuMWiirflinge) of Etna Hoe * lli-r A'tn:i/ ii. |,|,. 2l<:, 



DYNAMICAL GEOLOGY. [Book HI. Pabt L 



within the entruioe to the Atrio del CaTollo. SimiUr blocka ooeni amoi^; the 0*r- 
boiitFeroiu volc&nic pipes of central Bootlsnd, together BometimeB with fragmenta of 
BOixLitoDc, shale, or limOBtonc, not iufttsqucntly fall of duboulferaua foaailB.* 

(4)Votouiiic liombB and b 1 a g s.—Tbeas tiavo originally formed portiona of 
llio oolumn of lava luccuding tho pipe of tho volcano, and have been detached and 
hurled into the air by tho euoceauvo eiplodcni ef steam. A bomb (Fig. S4) ia a 
round, elliptical, or pcar-ahaped, often dUcoidal man of lava, fran a few Inehea to 
BcVGial feet in diameter ; somotimea tolerably solid thronghout, more nanally ooaiMlf 
cellular inaidc. Not infrequently its interior ia hollow, and the bomb then conauta of 
a eholl which is matt cloac-grained towards the outiide, or the centre ii a block of 
Btone with an external canting of lava. There can bo no doubt that, when iora 
by cructaliona of ateam from the surface of the boiling lava, the material of 
these bombs is in as thoroughly molten a condition as tho rest of the maaa. Vtout 
the rotatorjr motion imparted by its ejection, it takes a ciipulnr form, and in pioportion 
to its rapidity of rotation and fluidity is the amonnt of its " flattening at the pole*.** 
Tiie centrifugal force within allows the expansion of the interstitial vapour, while tlio 




Fig. 34,— .Btdlon of Volcanic llMiib, oDe-tblrd 



outer surTace mpidly cools und aolidifles; hence the solid crust, and the porous or 
cavtruoua interior. Bnch bombs, varying from the sixc of an apple to that of a man's 
body, were found by Darwin abundantly strewn over tho ground in the Island Af 
Ascension ; they were also ejected in vast qnantitica during the emption of Banlorin In 
11JG6.' Among the tufTa of the EiFci region, aniall bombs, eonalsting mostlj of giaunlai 
oliviDc, ate of common occurrence, as also pieces of sanidioe or other leas fusible 
uinomla which have aegtt^ted out of the magma before ejection. In like manner, 
among tho tufia Ailing the volcanic necks in the Lower Carbonifooos rocks of FiGj, 
large worn crystals of orthoclaae. biotite,&a, ore found. When the qected fragment 
of lava has a rough irregular form and a porous Btmotttre, like the clinkei of an iron- 
furnace, it is known as a slag.' 

> Traiu. Boy. Soe. Edin. i^\\. p. J59. See fioafca. Book IV. Sect. viL $ 1. 4. 

* Darwin, ■ Oeologieal Ubservations on Volcanic Islauds,' Zad edit. p. 42. ^jnqn^ 
'Bantorin ' p. 7Q. 

' On the ratio between tiie pores and volume of tho lock iu aUga and lava^ RW 
dctcrminationa by Bischof, 'Chem. und Phya. Gcol.' snpp. (1871) p. 158. 



.Sect, i § 2.] VOLCANIC ACTION. 189 

The fragmentaiy matorialB ornpted by]a volcano and deposited around it, acquire by 
degrees more or lots oonsolidation, partly from the mere pressure of the higher upon 
the lower strata, partly from the influence of infiltrating water. It has been 
already stated (p. 164) that different names are applied to the rocks thus formed. 
The eoane, tomoltiioiis, Tinstratified accumulation of volcanic dcfbris within a crater or 
fnniiel is called Agglomerate. Wlien the debris, though still coarse, is more 
xoimded* and is airaQgcd in a stratified form on the slopes of the cone or on the plain 
beyond, it beoomes a Voloanic Conglomerate. The finer-grained varieties, 
formed of dost and lapilli, are included in the general designation of Tu ffs. These 
aio nsnally pale-yellowish, greyish, or brownish, sometimes black rocks, granular, 
poTons, and often incoherent in texture. They occur interstratified with and pass into 
ordinarf non-Tolcanio sediment 

Orgsnio remains sometimes occur in tuff. Where volcanic dc'bris has accumulated 
over the floor of a lake, or of the sea, the entombing and preserving of shells and other 
organic objects must continually take place. Examples of this kind are cited in later 
pages of this volume from older geological formations. Professor Guiscardi of Naples 
has found about 100 speoies of marine shells of living species in the old tuffs of Vesuvius. 
Marino shells have been picked up within the crater of Monte Nuovo, and have been 
frequently observed in the old or marine tuff of that district. Showers of ash, or sheets 
of Toloanie mod often preserve land-shells, insects, and vegetation living on the area 
at the tiiiie. The older tufls of Vesuvius have yieldeil many remains of the shrubs and 
trees whioh at ssocessive periods have clothed the flanks of the mountain. Fragments 
of ooniteoiu wood, whioh onoo grow on the tuff-cones of Carboniferous age in central 
Sootland, are abundant in the " necks " of that region, while the minute structure of 
•ome of the lefadodendroid plants has also been admirably preserved there in tuff.^ 

§2. Yolcanic Action. 

Yoloanic action may bo either conBtant or periodic. Stromboli, in 
the Mediterranean, so far as wo know, has been nnintorruptodly 
emitting hot stones and steam, from a basin of molten lava, since the 
earliest period of history.^ Among the Molnccas, the volcano Sioa, 
and in the Friendly Islands, that of Tofiia, have never ceased to be 
in eruption since their first discovery. The lofty cono of Sangay, 
among the Andes of Quito, is always giving off hot vapours ; 
C-otopaxi, too, is ever constantly active.^ But, though examples of 
unceasing action may thus ho cited from widely different quarters of 
the globe, they are nevertheless exceptional. Tlie general rule is 
that a volcano breaks out from time to time with varying vigour, and 
after longer or shorter intervals of quiescence. 

Active^ Dormant, and Extinct Phases.— It is usual to class 
volcanoes as active, doruiant, and fxtinrf, Tliis arrangement, liowevor, 
often presents considenible difficulty in it^ application. An active 
v<ilcan<» cannut of C4)urso he mistaken, lor oven wlicn not in eruption, 
it shows by its discharge of steam and hot vapours that it might 
break out into activity at any moment. But in many cases, it is 

' TraH$, Roy. 8oe. Edin. xxix. p. 470 ; ]H>ffra, Book IV. Part vii. Sect. ii. § 2. 

" For accouuts of StTomlK)li seo Spalluiizniii*8 * Voyafrrs duns len deux Siciles.* 
Scrope's * Volcanoes.' Judd, Geol. Mag. 1875. Mcrcalli's * Vnlcaiii, &c,' p. lar*; mid liis 
pupcr in AUt Sue. Hal ScL ML xxiv. (1881). 

■ For doscriptious of Cotupaxi, see Wolf, Neitts Jahrb. 1878; Whymper, A'a/Mrc, 
xziii. p. S28. 



190 DYNAMICAL OEOLOOT. [Book IH. Part L 



impossible to decide whether a volcauo should be called extinct or 
only dormant. The volcanoes of Silurian age in Wales, of Carboni- 
ferous age in Ireland, of Permian age in the Harz, of Miocene age 
in the Hebrides, of younger Tertiary age in the Western States and 
Teiritories of North America, are certainly all extinct. Bat the 
Miocene volcanoes of Iceland are still represented there by Skaptar- 
Jokull, Hecla, and their neighbours.^ Somma, in the first century 
of the Christian era, would have been naturally regarded as an extinct 
volcano. Its fires had never been known to have been kindled; 
its vast crater was a wilderness of wild vines and bmshwood, 
haunted, no doubt by wolf and wild boar. Yet in a few days, 
during the autumn of the year 79, the half of the crater walls was 
blown out by a terrific series of explosions, the present Vesuvius was 
then formed within the limits of the earlier crater, and since that 
time volcanic action has been intermittently exhibited up to the 
present day. Some of the intervals of quietude, however, have been 
so considerable that the mountain might then again have been daimed 
as an extinct volcano. Thus, in the 131 years between 1500 and 
1631, so completely had eruptions ceased that the crater had once 
more become choked with copsewood. A few pools and springs of 
very salt and hot water remained as memorials of the former condition 
of the mountain. But this period of quiescence closed with the 
eruption of 1631, — ^the most powerful of all the known explosions of 
Vesuvius, except the great one of 79. In the island of Ischia, Mont* 
Epomeo was last in eruption in the year 1302, its previous outburst 
having taken place, it is believed, about seventeen centuries before 
that date. From the craters of the Eifel, Auvergne, the Vivarais, and 
central Italy, though many of them look as if they had only reoentiy 
been formed, no eruption has been known to come during the times 
of human history or tradition. In the west of North America, firom 
Arizona to Oregon, numerous stupendous volcanic cones occur, bot 
even from the most perfect and fresh of them nothing but steam 
and hot vapours have yet been known to proceed. But the existence 
there of hot springs and geysers testifies to the continued existence of 
one phase of volcanic action. 

In short, no essential distinction can bo drawn between dormant 
and extinct volcanoes. Volcanic action is apt to show itself again and 
again, even at vast intervals, within the same regions and over the 
Bamo sites. The dormant or waning condition of a volcano, when only 
steam and various gases and sublimates arc given off, is sometimes 
called tlie Solfiitara phase, from the well-known dormant crater of 
that name near Naples. 

Sites of Volcanic Action. — Volcanoes may break through any 

^ On the volcanic phenomena of Iceland consult G. Mackeu2ie*B ' Travels in tbo 
Island of Iceland during the Summer of 1810.' E. Henderaou's * Iceland.' Zirkd, 'De 
^eoguostica iHlandiss confltitutione ohscrvationcs,' Bonn, 1861. Thoroddacn, QtoL 
Ma<j. 1880, p. 458 ; NaU're, Oct. 1884, and works cited on p. 238. 



facp. i S 2.] VOLCANIC ACTION. 191 



geological formation, In Anvergne, in the Miocene period, they 
barst through the granitic and gneiBsose plateau of central France. 
In Lower Old Sed Sandstone times, they pierced contorted Silurian 
rooikB in central Scotland. In late Tertiary and post-Tertiary ago;', 
they found their way through recent soft marine strata, and formed 
the huge piles of Etna, Somma and Vesuvius; while in North 
America, during the same cycle of geological time, they flooded with 
laya and tufif many of the river courses, valleys, and lakes of Nevada, 
Utahy Wyoming, Idaho and adjacent territories. On the banks of 
khe Bhine, at Bonn and elsewhere, they have peneti'ated some of the 
alder alluvia of that river. In many instances, also, newer volcanoes 
liave appeared on the sites of older ones. In Scotland, the Carboni- 
bioufl volcanoes have risen on the ruins of those of the Old Hod 
Sandstone, those of the Permian period have broken out among the 
3arlier Carboniferous eruptions, while the Miocene lavas have been 
injected into all these older volcanic masses. The newer puys of 
iuvergne were sometimes erupted through much older and already 
greatly denuded basalt-streams. Somma and Vesuvius have risen 
mt of the great Neapolitan plain of older marine tuff, while in central 
Italy, newer cones have been thrown up upon the wide Boman plain 
yf more ancient volcanic debris.^ The vast Snake Biver lava-fields of 
Idaho overlie denuded masses of earlier trachytic lavas, and similar 
iroofii of a long succession of intermittent and widely-separated volcanic 
mtlmrsts can be traced northwards into the Yellowstone Valley. 

When a volcanic vent is opened, it might be supposed always to 
ind its way to the surface along some line of fissure, valley or deep 
iepression. No doubt many, if not most, modem as well as ancient 
rents, especially those of large size, have done so. It is a curious 
act, however, that in innumerable instances minor vents have 
appeared where there was no line of dislocation to aid them. This 
las been well shown by a study of the ancient volcanic rocks of the 
)ld Red Sandstone, Carboniferous and Permian formations of Scot- 
and.' It has likemse been most impressively demonstrated by 
he way in which the minor basalt cones and craters of Utah have 
>roken out near the edges or even from the face of cliffs, rather than 
,i the bottom. Captain Button remarks that among the high plateaux 
»f Utah, where there are hundreds of basaltic craters, the least 
ommon place for them is at the base of a cliff, and that, though they 
Kxjur near faults, it is almost always on the lifted, rarely upon the 
lepresscd sidc.^ On a small scale, a similar avoidance of the valley 
jottom is shown on the Rhine and Moselle, where eruptions have taken 
place close to the edge of the plateau through which these rivers wind. 

* Aceoidiog to Profewor G. Pozzi, the principal volcanic outbursts of Italy are of 
the Glacial Period. AtU Lincei, 3d ser. vol. ii. (1878) p. 85. Stefani r^arda those of 
Totcany as partly Miocene, partly Pliocene nnd post-Pliocene. (Froc» Tosc. 8oc. Nat, 
Vita, I. p. xxi.) 

• Tran$. Itoy. Hoc.. Edin. xxix. p. 437. 

» "High Plateaux of UUli/' Geol. and Geog. Survey of Tanitoritt, 1880, p. 02. 



— m 

:' .^-..-ri. :i.*r i£r -rLanr: 4 .erT?fiBfeL2a> &uii miipCT of the 

■ .■■■_. • :..- •■/^-r-Zliit fr -rgr^ir-T-f^r- z^eXOTKu P.OIlIlW^Wl with 



■...iv lu- .'t;::-zlsa i-rirf^ r lETT ;urri---iiir flnmsim. iire still 

- :.!• r.n....*:. ■.!.•* : ..Eraa i»r.iieTL* TTP^anr!? '"vip-r "iif uxa kr :^ ^iizze. In 
..:• .o^ : ■. -'-:iAiLL-: t: — >'.l _lic- tt^hil -»ii. •^ririn. u» Sarjije p«>iDted 
. -. ■;.•> -::r;'*i.iiiT.> -.n.---mijrtui ^"rri> ^rrmm. diiL -iiis ysnnHwcv^ effect 



.7 ^ ^'^.in^^ :. "i.^- rtin::! : -iie "^itan-. -iA-'-jirnm^y, 3 claa long 

• 
vi-AT^ft;' "iif:r> .a tE "rTTonnioii I _:;^ n* r-ni'.in* LiiKuuir:^^ •:£ i^inzn and 

^-/.♦■.fr<f 'irjan n infc ■▼«=Arcifer. Hiey :nikiEa lae n ■iia !»Tie ;ii!- a w^ather- 

^ -*j<ft, r.-^ .nrr'Vkj**- if .XA icri''-ir7 jiiiiL^-^riii^r i rhtTrnzr, laii nni*- -iraiiniitiirn 

* r*i;'i:Z i--^i".n\f.ri^r. In Lik^i "naimtrr, Ztaai. ic'^nriinif c? Surti^Tcnu Ton 
TCt^rt-rwyiiif**::. .* ui;'"" M-i—'r .n. "iie "^viiiDir !m?iittU!L When we 
.* '.rcn: rn . .*t r '. 1 r -. i nn ficr*! -n- ni j 'v In* Ln: iia .1 17 ^sci is litHieiL T:-t>tw¥«»ii a more 
I* . •-. « ! .ti« ;.*> : r:: iir ^r? if n :• •- i Jim p in mine* »nt £ 1 Ii .• werinx • r-f asnoispheric 
%:-:^:;.":. Vi-r .uix^ A ;jr=rr.ar**L Tj imL % -amiliir iiufneE.':** *f acting the 
*:w,a:.-: '.t •rtr.i.M-.-s rr.'.m 'hft "ipiier 'Pircioe in r^ LkTk-^zt'jlTtsifi of a 
V;u;aL.'* . ; f.r '.". M iir.r v. ni-ir!i '.i ^e la*^ r'aielf ;&» to th?* expansive 

I7».fi ^^tTi nr.rj. •fii': r. -I r.-. r.frr whioh ^.^^k pljioe :a winter .^ii'i spring was 
Vf tK;*f. of rf.',«w; xh>K rr.kft -.n:: in 'jTmiier iJWLd aamnin as 7 to 4. 
lint t}i*iT(: rf»^y u-. ^,tt.(;7 '';aaaf.-?» r.oaiile** Acm'.ephtrio pressniv coneemeil 
in i\i<M» f\ifff:TfiTir.fA ; t.hft jTe^ioc'ierui'?e of nin dnring the winter 
firi'l fff>rioj( Tfiay V; onf; of t?i':*>':. At.^-riin^ to 3Ir. Csan, previonK to 
th^; p;r*!ti Ffaw^klAn ftrnption '^if 1^»>-S th»rre hi«l l^e^-n unusually wet 
wfAth«-r, find Uf t.hift ffi/rt. ho attril-ut^s the exceptional severity of the 
i'.firi]u\m%k«iH siuf\ volrranir: fsxplosions.^ But at most, the effects of 
viiryifij^ jitrnoH|»l»f!rif: pn;hHiin; ojin only slightly mo<lify volcanic activity. 
KniptiniiH, ViUc tfi<i j^r^Jif. on*? rif f'otopaxi in 1877, have in innumerable 
iiiHiiiiiccw UiUtu pliirn wiflioiit,, ho far an cau l>e asci-rtained, any roferenci^ 
in iitftKiHphf^rir cniiflif ioriH. 

' Kiir «ir«'»iiiiilH fir Mil' v'llf'fttiin iiliciKiinriiu of HiiwAii, eoe W. EUitii, 'Polvnesinu Rc- 
MMirolwH.' \\\\Ur» V M. Kxpl'TiiiK Kxp»M|itl<)ii, 1838-42/* (Jenlogy," by J. D.^DaDa, The 
|{< \. '\\ (Niiiii. II iiil«iuiitiniiy n iii<li<iii in IIiiWiiii\ o1>H<Tve<l tho operations of the voloanoc^d 
for iipwiirilM ot liiity y<*iirH. iiiul puliliHhnl fnmi time to lime short notices of tbeiii 
III tlu> .hiiui.iiM Jtun mil nf Sit hr,\\iAH, xiii. (IS.Vi) xiv. XV. xviii. xxi. xxii. xxiii. xxt. 
\v\ii. \\\^ii. \I. \!iil. \lvii. \li\.: 'M nrr.ii. (IS7n iv. vii. viii. xiv. xviii. xx. xxi. 
\\n ilS^lV Srriili«i(\ K PuHnn. .1mm r. ./oMni. ^^•/. xxv. (1883) p. 219; Seport I/JS. 

• :. .'. :i.M.' »<;.M%-;. ISS'.* S:l. For iiii lU'roiinl of ihi' ix'umrkablo glassy In viis ot Hawaii. 

,., r I o.t i«, \. lu .- Jithi'iK isso 01 ) i». y*^ 



Sect. i. § 2.] VOLCANIC ACTIOIf. 193 



Kluge has sought to trace a connexion "between the years of 
maximam and minimum snn-spots and those of greatest and feeblest 
volcanic activity, and has constructed lists to show that years which 
have been specially characterized by terrestrial eruptions have coincided 
with those marked by few sun-spots and diminished magnetic dis- 
turbance.^ Such a connexion cannot be regarded as having yet been 
satisfactorily established. Again, the same author has called attention 
to the frequency and vigour of volcanic explosions at or near the time 
of the August meteoric shower. But in this case, likewise, the cited 
examples can hardly yet be looked upon as more than coincidences. 

t, Periodicity of Emptions. — At many volcanic vents the erujitivo 
energy manifests itself with more or less regularity. At Stromboli, 
which is constantly in an active state, the explosions occur at intervals 
varying from three or four to ten minutes and upwards. A similar 
rhythmical movement has been often observed during the eruptions at 
other vents which are not constantly active. Volcano, for example, 
daring its eruption of September 1873, displayed a succession of ex- 
plosions which followed each other at intervals of twenty to thirty 
minutes. At Etna and Vesuvius a similar rhythmical series of con- 
vulsive efforts has often been observed during the course of an eruption.^ 
Madi more striking, however, is the case of Kilauea, in Hawaii, which 
seems to show a regular system of grand eruptive periods. Dana 
has pointed out that outbreaks of lava have taken place from that 
volcano at intervals of from eight to nine years, this being the time 
required to fill the crater up to the point of outbreak, or to a depth 
of 400 or 500 feet. But the gieat eruption of 1868 did not occur 
until after an interval of 18 years. The same author suggests that 
the missing eruption may have been submarine.^ 

General sequence of events in an Eruption. — The approach of 
an eruption is not always indicated by any premonitory symptoms, 
for many tremendous explosions are recorded to have taken placo 
in different parts of the world without perceptible warning. Much 
in this re82)ect would appear to depend upon the condition of 
liquidity of the lava, and the amount of resistance offered by it to 
the passage of the escaping vapours through its mass. In Hawaii, 
where the lavas arc remarkably liquid, vast out-pourings of them 
have taken place quietly without earthquakes during the present 
century. But even there, the great eruption of 1868 was accom- 
panied by tremendous earthquakes. 

The eruptions of Vesuvius are often preceded by failure or dimi- 
nution of wells and springs. But more frequent indications of an 
approaching outburst are conveyed by sympathetic movements of 

' Uelier Synchronismus uud AntagonismuB, 8vo, Leipzig, 1863, p. 72. A. lV>ey 
[r*m%pU» Rend. Ixxviii. (1874) p. 51) bcliovea that among tho 786 cniptious rocordetl by 
klnge, botweon 1749 and 1801, the inaxima correspond to periods of minimum in solar 
iiuf>U. See, however, pontea^ pp. 250, 259. 

* (i. Mcrcalli. Atii Soc. Ital. Sci. Nat. xxiv. (1881). 

' On the periodicity of eruptions, see Kluge, Ncues Jahib. 18C2, p. 582. 





i 



194 DYNAMICAL GEOLOGY. [Book EL Pabt I. 

tho ground. Subterranean rumblings and groanings are heard; 
slight tremors succeed, increasing in frequency and violence till they 
become distinct earthquake shocks. The vapours from • tho crater 
grow more abundant, as the lava column in the pipe or funnel of 
the volcano ascends, forced upward and kept in perpetual agita- 
tion by tho pasaago of elastic vapours through its mass. After a 
long previous -interval of quiescence, there may be much solidified 
lava towards the top of the funnel, which will restrain the ascent of 
the still molten portion underneath. A vast pressure is thus 
exercised on the sides of the cone which, if too weak to resist, will 
open in one or more rents, and the liquid lava will issue from the 
outer slope of the mountain; or the energies of the volcano will bo 
directed towards clearing the obstruction in the chief throat, until, 
with tremendous explosions, and the rise of a vast cloud of duBt and 
fragments, tho bottom and sides of the crater are finally blown out, 
and tho top of the cone disappears. The lava may now escape from 
the lowest part of the lip of the crater, while, at the same time, 
immense numbers of red hot bombs, scoriae, and stones are shot up 
into the air. The lava at first rushes down like one or more rivers 
of melted iron, but, as it cools, its rate of motion lessens. Clouds of 
steam rise from its surface, as well as from the central crater. Indeed, 
every sucoessive paroxysmal convulsion of the mountain is marked, 
even at a distance, by the rise of huge ball-like wreaths or doads of 
steam, mixed with dust and stones, forming a column whioh towers 
sometimes a couple of miles above the summit of the cone. By degrees 
these eructations diminish in frequency and intensity. The lava 
ceases to issue, the showers of stones and dust decrease, and after a 
time, which may vary from hours to days or months, even in the regime 
of the same mountain, the volcano becomes once more tranquil.^ 

In tho investigation of the subject, tho student will naturally 
devote attention specially to those aspects of volcanic action which 
have more particular geological interest from the permanent changes 
with which they are connected, or from tho way in which they 
enable us to detect and realize conditions of volcanic energy in 
former periods. 

Fissures. — Tho convulsions which culminate in the formation of 
a volcano usually split open the terrestrial crust by a more or less 
nearly rectilinear fissure, or by a system of fissures. In the sulwequent 
progress of the mountain, the ground at and around the focus of action 
is liable to be again and again rent open by other fissures. These tend 
to diverge from the focus ; but around the vent where the rocks have 
been most exposed to concussion, the fissures sometimes intersect each 
other in all directions. In the great eruption of Etna, in the year 1669, 
a series of six parallel fissures opened on the side of the mountain. One 

* See J. F. J. Schmidt's narrative of the eruption of Vesuviug in May, 1855. An 
ftccount of the groat eruption of Cotopaxi in June 1877, by Dr. Th. Wolf, will be found 
in JSeue9 Jahrb, 1878, p. 118. 



Sect, i t 2.] 



VOLCANIC FISSURES. 



195 



of these, with a width of two yards, ran for a distance of 12 miles, 
in a somewhat winding coarse, to witliin a milo of the top of the cone.' 
Similar fisearee, bnt on a smaller scale, have often been observed on 
VesnTittB ; and they are recorded from many other volcanoes,* 

Two obrions catises may be aasigncd fur the fissuring of a volcanic 
cone: — (1) the enormons expansive force of tho imprisoned vapours 
acting npon the walls of the funnel and couvnlaing the cone by 
BQCceasive explosioua ; and (2) the hydrostatic preBsure of tho lava- 
GolamD in the funnel, whioh may be taken to bo about 120 lb. per 
sqnare inch, or nearly 8 tons on the square foot, for each 100 feet of 
depth. Both of these canses may act simnltaneously. 

Into the rents thus formed, the molteu lava naturally finds its way 




Fig. M.— VIqw of Liv»-dyk«i, Vail'? ilcl Dove, ElnB (AWth). 

or is forced, and it solidifies there like iron in a mould. Tho clifis of 
many an old crater show how marvellously they have been injected 
hy snoh veiiu or dykes of lava. Those of Somma, and the Vallo del Bovo 
wi Etna (Fig. 35), which have long been known, project now from tho 
•nfter tufls like walls of masonrj'.' Tho crater cliffs of Sautorin also 
present an abundant series of dykes. Tho permanent separation of 
the walls of fissures by tho consolidation of the lava that rises in 
them as dykes must widen the dimensions of a cone, for the fissures are 
not due to shrinkage, although doubtless the loosely pilod fragmentary 



'. Kor a notice of flsanros oi 



■a Bilvcatri, Bdl. Jt. Ued. Com. Ital. IW4. 



DrifAMICAL GEOLOGY. [Book Ul. Part I 



196 

iiiut«rialii, ill the uouise of their conuolidation, develop Uqcb of joint. 
Sometimes the lava has evidently liBeii in a state of extreme fluidity, 
and has at once filled the reut« prepared fur it, cooling npidly 
on tho outside as a true volcauiu glass, but aBsumiag a distinctly 
ciystalliuo structure inside (ante, p. 153). Dykce of this kind, with 
a vitreous crust on their sides, may be seen on the crater-vrall of 
Somma, and uot uncommonly among basalt dykes in Iceland and Soot- 
laud. In other cases, the lava had probably already acquired a more 
viucouu or ovun lithoid character ere it rose in tho fissure, and in this 
eonditiou was able to push aside and even contort tho strata uf tuff 
thruugh which it made its way (Fig. 3t}). There can bo little doubt 




tig 39— Drii c 



1) gbedi fc ff 



rVcnvlaXAbUh) 







that in the architeoturo of a volcano d}kes must act tho jiart of huge 
beams and girders (Fig 37) binding the loose tnfb and intercalated 
lavas together and strengthening the cone 
against the effects of subsequent oonvul- 
Bions. 

From this pomt of view an explaua- 
tiOQ suggests itself of the observed alter- 
nations in the character of a volcano s erup- 
tions These alternations may depend in 
great measure upon the relation between the 
height of the cone on the one hand and 
the strength of its sides, on the other. When the sides have been vrell 
braced together by interlaoing dykes, and further thickened by tho 
spruad of volcanic materials all over their slopes, they may insist tho 
effects of explosion and of the pressure of tho ascending lava-column. 
Ill this case, the volcano may find relief only from its summit, and 
if the lava flows forth, it will do so from the top of the cone. As the 
Cone increases in elevation, however, tho pressure from within upon 
its sides augments. Eventually cgrcBS is once more established on 
the flanks by means of fissures, and a new series of lava-streams 
is poured out ovet tho lower slopes. 

Though lava very commonly issues from the lateml fisaures on u 



Smct. L § 2.] rOLCANia EXPLOSIONS. 197 

volcanic cone, it may sometimeB approach the sarface in them witli- 
out actually flowing out. Tho great fissure on Etna in 1669, for 
example, was Tisible even from a distance, by the long line of vi-vid 
light which rose &om the incandosccnt lava within. Again, it 
freqaently happens that minor volcanic cones arc thrown up on the 
line of a fiBBure, either from the congelation of the lava round the 
point of emission, or from the accumulation of ejected scoria round 
the fiasnre-vent, 

XzploBionB. — Apart from tho appearance of viHililo fiBsures, volcanic 
energy may be, aa it were, concentrated on a given point, which will 
UBuaUy be the weakest in the structure of that part of the terrestrial 
crnst, and from which tho solid rock, shattered into pieces, is hnrlctl 
into the air, followed hy the ascent of volcanic materials. This operation 
has often been observed in volcanoes already formed, and has even been 
witnessed on ground previonsly unoccupied by a volcanic vent. The 




history of tho cone of Vesuvius brings before ns a long series of such 
eiplodions, l>eginningwiththatof a.t>. 7i>, and coming down to the present 
day. Even now, in spit* of all tho lava and ashos ponred nnt dnring the 
last eighteen centuries, it is easy to see liow stupendous must have been 
that earliest explosion, by which the southern half of tho ancient crater 
VBB blown out. At every successive important eruption, a similar but 
minor operation takes place within the present cone. Tho hardened 
fake of lava forming the floor is burst open, and with it there usually 
4inappeara much of the upper part of tho cone, and sometimes, as in 1872, 
■* lai^e segment of the crater-wall. Tho Vallo del Hovo on the eastern 
Bank of Etna is a chasm probably duo mainly to some gigantic pre- 
historic explosion.' The islands of Santorin (Figs, ilfi and 6ft) bring 
Iwfiire tis evidence of a pre-historic catafitrni>ho of a similar nature, by 

' ■DprAotna.'p. 100. 



198 DYNAMICAL GEOLOGY. [Book III. Pabt I. 



which a largo volcanic cono was blown up. The existing outer islands 
are a chain of fragments of the periphery of the cone, the centre of which 
is now occupied hy the sea. In the year 1538 a new volcano, Monte 
Nuovo, was formed in 24 hours on the margin of the Bay of Naples. 
An opening was drilled by successive explosions, and such quantities of 
stones, scorisd, and ashes were thrown out from it as to form a hill that 
rose 440 English feet above the sea-level, and was more than a mile and 
a half in circumference. Most of the fragments now to be seen on the 
slopes of this cone and inside its IxjautifuUy perfect crater are of 
various volcanic rocks, many of thom being black scorisd ; but pieces of 
Roman pottery, together with fragments of the older underlying tuff, 
and some marine shells, have been obtained — doubtless part of the soil 
and subsoil dislocated and ejected during the explosions. 

One of the most stupendous volcanic explosions on record was that 
of Krakatau in the Sunda Strait on the 26th and 27th of August, 1883.* 
After a series of convulsions, the greater portion of the island was blown 
out with a terrific detonation which was felt over an area fully 3000 
miles in diameter. On the site of the volcano there now lies an abyss 
of ocean water, fathomless with a sounding line of 1000 feet. A mass 
of matter, probably some cubic miles in bulk, was hurled into the air 
ill the form of lapilli, ashes, and the finest volcanic dust. The effects 
of this volcanic outburst were marked both upon the atmosphere and 
the ocean. A series of barometrical disturbances passed round the globe 
in opposite directions from the volcano at the rate of about 700 miles 
an hour. The air-wave, travelling from east to west, is supposed to have 
passed three and a quarter times round the earth (or 82,200 miles) 
l)efore it ceased to be perceptible.^ The sea in the neighbourhood was 
thrown into waves, one of which was computed to have risen 100 feet 
above tide-level, and to have destroyed towns, villages and vast numbers 
of people. Oscillations of the water wore perceptible even at Aden, 
1000 miles distant, at Port Elizabeth in South Africa, 6450 miles, 
and among the islands of the Pacific Ocean, and they are computed to 
have travelled with a maximum velocity of 407 statute miles in the 
hour.'* 

It is not necessary, and it does not always happen, that any actual 
solid or liquid volcanic rock is erupted by explosions that shatter the 
rocks through which the funnel passes. Thus, among the cones of the 
extinct volcanic tract of the Eifel, some occur consisting entirely, or 
nearly so, of comminuted debris of the surrounding Devonian grey wacke 
and slate through which the various volcanic vents have been opened 
(see pp. 187, 227, 236). Evidently, in such cases, only elastic vapours 
forced their way to the surface ; and we see what probably often takes place 
in the early stages of a volcano's history, though the fragments of the 

* H. O. Forbes, Proc. ItouaX Geog. Soc, March, 1884. 

' Scott and Strachey, Proc. Hoy. Soc. xxxvi. (1883). Rykatohew, Melanges^ Bull 
Arnd. 8t. Petershourg, xii. (1884), p. 167. 

' Major A. W. Baird, op. cit. p. 250. A Committee of tlie Boynl Society Iiab been 
appointed to investigate the phenomena of this great explosion. 



Sect. L f 2.] VOLCANIC DUST AND STONES, 199 



nnderlying disrupted rocks are in most instances huviod and lost under 
the far more abundant subsequent volcanic materials. Sections of small 
ancient Tolcanic "necks" or pipes sometimes afford an excellent 
opportunity of observing that these orifices were originally opened by 
the blowing out of the solid crust and not by the formation of fissures. 
Examples will be cited in later pages from Scottish volcanic areas of 
Old Bed Sandstone, Carboniferous, and Permian age. The orifices are 
there filled with fragmentary materials, wherein portions of tho 
BorroTinding and underlying rocks form a noticeable proportion.^ 

Bhowen of Dust and Stones. — ^A communication having been 
opened, either by fissuring or explosion, between the heated interior and 
the surface, fragmentary materials are commonly ejected from it, 
consisting at first mainly of the rocks through which the orifice has 
been opened, afterwards of volcanic substances. In a great eruption, 
vast nnmbers of red-hot stones are shot up into the air, and fall back 
partly into the crater and partly on the outer slopes of the cone. 
According to Sir W. Hamilton, cinders were thrown by Vesuvius, 
dnring the eruption of 1779, to a height of 10,000 feet. Instances are 
known where large stones, ejected obliquely, have described huge 
parabolic curves in the air, and fallen at a great distance. Stones 

8 lb. in weight occur among the ashes which buried Pompeii. The 
Tolcano of Antuco in Chili is said to send stones flying to a distance 
of 36 (?) miles, and Cotopaxi is reported to have hurled a 200-ton block 

9 miles.* 

But in many groat eruptions, besides a constant shower of stones 
and scoriaB, a vast column of exceedingly fine dust rises out of the 
crater, sometimes to a height of more than a mile, and then spreads 
outwards like a sheet of cloud. So dense is this dust-cloud as to 
obscure the sun, and for days together the darkness of night may reign 
for miles around the volcano. In 1822, at Vesuvius, the ashes not 
Only fell thickly on the viDages round the base of the mountain, but 
travelled as far as Asooli, which is 56 Italian miles distant from 
the volcano on one side, and as Casano, 105 miles on the other. Tho 
oruption of Cotopaxi, on Juno 26th, 1877, began by an explosion 
that sent up a column of fine ashes to a prodigious height into 
the air, where it rapidly spread out and formed so dense a canopy 
CIS to throw the region below it into total darkness. So quickly did it 
diffuse itself, that in an hour and a half, a previously bright morning 
l)eeame at Quito, 33 miles distant, a dim twilight, which in the after- 
noon passed into such darkness that the hand placed before tho eye 
could not be seen. At Guayaquil, on the coast, 150 miles distant, tho 
shower of ashes continued till the first of July. Dr. Wolf collected the 
ashes daily, and estimated that at that place there fell 315 kilogrammes 
on every square kilometre during the first thirty hours, and on tho 
30th of June, 209 kilogrammes in twelve hours.'^ The explosion of 

* Tram. Boy, Soc, Edin. xxix. p. 458. ' D. Forbes, Gnol. Mag, vii. p. 320. 

» Neu€8 Jahrb, 1878, p. 141. 



200 DYNAMICAL GEOLOGY. [Book HL Past I. 



Krakatan in August 1883 was accompanied by the discharge of enormous 
quantities of volcanic dust, some of which was carried to vast distances. 
The diflfusion and continued suspension of the finer dust in the upper 
air has l)een regarded as the probable cause of the remarkably brilliant 
sunsets of the following winter and spring over a large part of the earth's 
surface. One of the most stupendous outpourings of volcanic ashes on 
record took place, after a quiescence of 26 years, from the volcano Cose- 
guina, in Nicaragua, during the early part of the year 1835. On that 
occasion, utter darkness prevailed over a circle of 35 miles radius, 
the ashes falling so thickly that, even 8 leagues from the mountain^ 
thoy covered the ground to a depth of about 10 feet. It was estimated 
that the rain of dust and sand fell over an area at least 270 geographical 
miles in diameter. Some of the finer materials, thrown so high as to 
come within the influence of an upper air-current, were borne away 
eastward, and fell, four days afterwards, at Kingston, in Jamaica — a 
distance of 700 miles. During the great eruption of Sumbawa, in 
1815, the dust and stones fell over an area of nearly one million of 
square miles, and were estimated by 2iOllinger to amount to fully fifty 
cubic miles of material, and by Junghuhn to be equal to one hundred 
and eighty-five mountains like Vesuvius. 

An inquiry into the origin of these showers of fragmentary 
materials brings vividly before us some of the essential features of 
volcanic action. We find that bombs, slags, and lapilli may be thrown 
up in comparatively tranquil states of a volcano, but that the showers 
of fine dust are discharged with violence, and only appear when the 
volcano becomes more energetic. Thus, at the constantly, but quietly, 
active volcano of Stromboli, the column of lava in the pipe may be 
watched rising and falling with a slow rhythmical movement. At 
every rise, the surface of the lava swells up into blisters several feet in- 
diameter, which by-and-by burst with a sharp explosion that makes the 
walls of the crater vibrate. A cloud of steam rushes out, carrying with 
it hundreds of fragments of the glowing lava, sometimes to a height of 
1200 feet. It is by the ascent of steam through its mass, that a 
c^:^lumn of lava is kept boiling at the bottom of the crater, and by the 
explosion of successive larger bubbles of steam, that the various bombs, 
slags, and fragments of lava are torn ofiF and tossed into the air. It has 
often been noticed at Vesuvius that eiwjh great concussion is accompanied 
by a huge ball-like cloud of steam which rushes up from the crater. 
Doubtless it is the sudden escape of that steam which causes the 
explosion. 

The varying degree of liquidity or viscosity of the lava probably 
modifies the force of explosions, owing to the different amounts of 
resistance ofiFered to the upward passage of the absorbed gases and 
vapours. Thus explosions and accompanying scoria; are abundant at 
Vesuvius, where the lavas are comparatively viscid; they are almost 
unknown at Kilauea, where the lava is remarkably liquid. 

In tranquil conditions of a volcano, the steam, whether oollecting 



Sbct, i. S 2.] VOLCANIC DUST. 201 

into larger or smaller vesicles, works its way upward througli the 
Bubotaaoe of the molten lava, and as the elasticity of this compressed 
vapour overcomes the pressure of the overlying lava, it escapes at the 
Borfaoe, and there the lava is thus kept in ebullition. But this com- 
X>aratively qniet operation, which may be watched within the craters of 
many active volcanoes, does not produce clouds of fine dust. The 
collision or friction of millions of stones ascending and descending in the 
dark oolmnn above the crater, though it must doubtless cause much 
dust and sand, can give rise to but an insignificant proportion of what 
is aotnally reduced to the condition of extreme subdivision necessary to 
prodnoe widespread darkness and a thick far-reaching deposit of ashes. 
The explanation now accepted calls in the explosive action of steam as 
the immediate cause of the trituration. The aqueous vapour, by which 
many lavas are so largely impregnated, must exist interstitially far 
down in the lava-column, under an enormous pressure, at a temperature 
fSstr above its critical point, even at a white heat, and therefore probably 
in a state of dissociation. The sudden ascent of lava so constituted will 
relieve the pressure rapidly without sensibly affecting the temperature 
of the mass. Consequently, the white-hot gases or vapours will at 
length explode, and reduce the molten mass to the finest powder, like 
water shot out of a gun. 

Evidently no part of the operations of a volcano has greater 
geological significance than the ejection of such enormous quantities 
of fragmentary matter. In the first place, the fall of these loose 
materials round the orifice of discharge is one main cause of the growth 
of the volcanic cone. The heavier fragmionts gather around the vent, 
and there too the thickest accumulation of finer dust takes place. 
Hence, though successive explosions may blow out the upper part of 
the crater-walls and prevent the mountain from growing so rapidly in 
height, every eruption must increase the diameter of the cone. In tho 
second place, as every shower of dust and sand adds to the height of tho 
ground on which it falls, thick volcanic accumulations may bo formed 
far beyond the base of the mountain. The volcano of Sangay, in Ecuador, 
for instance, has buried tho country around it to a depth of 4000 feot under 
its ashes.^ In such loose deposits are entombed trees and other kinds of 
vegetation, together with the bodies of animals, as well as the works of 
man. In some cases, where the layer of volcanic dust is thin, it may 
merely add to the height of the soil, without sensibly interfering with 
the vegetation. But it has been observed at Santorin that though this is 
true in dry weather, the fall of rain with the dust at once acts detri- 
mentally. On the 3rd of June, 1866, the vinos were there withered 
up, as if they had been burnt, along the track of tho smoke cloud.^ By 
the gradual accumulation of volcanic ashes, new geological formations 
arise which, in their component materials, not only bear witness to the 
volcanic eruptions which produced them, but preserve a record of tho 

» D. Forbes, Geol Mag, vii. 820. - Fonqiie, * Santorin,* p. 81. 



202 DYNAMICAL GEOLOGY, [Book HI. Part I. 



land-surfaces over which they spread. In the third place, besides the 
distance to which the fragments may be hurled by volcanic explosions, 
or to which they may be dififused by the ordinary aerial movements, we 
have to take into account the vast spaces across which the finer dust 
is sometimes borne by upper air-currents. In the instance already cited, 
ashes from Coseguina fell 700 miles away, having been carried all that 
long distance by a high counter-current of air, moving apparently at 
the rate of about 7 miles an hour in an opposite direction to that of 
the wind which blow at the surface. By the Sumbawa eruption, al«:» 
referred to above, the sea west of Sumatra was covered with a layer of 
ashes two feet thick. On several occasions ashes ra>m the Icelandic 
volcanoes have fallen so thickly between the Orkney and Shetland 
Islands, that vessels passing there have had the unwonted deposit 
shovelled off their decks in the morning. In the year 1783, during the 
memorable eruption of Skaptar- Jokull, so vast an amount of fine dust 
was ejected that the atmosphere over Iceland continued loaded with it 
for months afterwards. It fell in such quantity over parts of CaitlinesB 
— a distance of 600 miles— as to destroy the crops ; that year is still 
spoken of by the inhabitants as the year of '* the ashie." Traces of the 
same deposit have been observed in Norway, and even as far as HoUaiid.^ 
Hence it is evident that volcanic accumulations may take place in 
regions many hundreds of miles distant from any active voloano. A 
single thin layer of volcanic detritus in a group of sedimentaiy atrata 
would thus not of itself prove the existence of contemporftneouB voloanic 
action in its neighbourhood. Unsupported by other proof of adjaoent 
volcanic activity, it might be held to have been wind-borne from a 
volcano in a distant region. 

Lava-streams. — At its exit from the side of a volcano, lava 
glows with a white heat, and flows with a motion which has been 
compared to that of honey or of melted iron. It soon becomes red, and 
like a coal fallen from a hot fireplace, rapidly grows dull as it moves 
along, until it assumes a black, cindery aspect. At the same time the 
surface congeals, and soon becomes solid enough to support a hca^y 
block of stone. The aspect of the stream varies with the composition 
and fluidity of the lava, form of the ground, angle of slope, and rapidity 
of flow. Viscous lavas, like those of Vesuvius, break up along the surface 
into rough brown or black cinder-like slags, and irregular ragged cakes, 
which, with the onward motion, grind and grate against each other 
with a harsli metallic sound, sometimes rising into rugged mounds or 
becoming seamed with rents and gashes, at the bottom of which the 
red-hot glowing lava may be seen (Fig. 39). In lavas possessing 
somewhat greater fluidity, the surface presents froth-like, curving lines, 
as in the scum of a slowly flowing river, or is arranged in curious 
ropy folds, as the layers have successively flowed over each other 
and c(nigealed. These, and many other fantastic coiled shapes were 

' N«n(leu8ki«"iUl, Geol Mag. 2d dec. iii. p. 292. G. vom Rath, Munatslmr, K. ri-eu^. 
J An//. HVkx. 1870, p. 282. AVm^« Jahrb. 1876, p. 52. 



S»jr. LS2.3 LAFA-STSEAM8. 203 

exhibited by the Vesuvian hiva of 1858.' Boaalts posBesaing extreme 
liquidity have flowed for great distances with singnlarly smooth Burfaces, 
A hkTge area whioh has been flooded with lava is perhaps the moat hideous 
and appalling soene of desolation anywhere to be found on tho snrface 
of the globe. 

A lava-stream nsually spreads ont as it doBconds from its point 
of eeoape, and moves more slowly. Its sides look like huge embankments, 
or like some of the long mounds of " clinkeTs " in a great mannfacturing 
district. The advancing end is often much steeper, creeping ouwanl 
like a great wall q^ rampatt, down the &oe of whioh the rough blocks 
of hardened lava ate ever rattling (Fig. 40). 

Outflow of Lava. — This appears to be immediately duo to tho 




wpannion of the absorbed vapours and gaaea in the molten rock. 
Thongh these vapours may reach the surface, and oven produce 
ttemenilous eiploeioua, without an actual outcomo of lava, yet so 
inlimattly are vapours and lava commingled in the subterranean 
twenroirs, that thoy commonly rise together, and tho explosions of 
tb* one lead to the outflow of the other. The first point at which 
thu lava makes its appearance at tho surface will largely depend 
ipon tho structure of the ground. Two cauBeu have been assigned 
''u a foregoing page (p. 195) for the fisBuring of a volcanic cone. As 
'lie molten mass rises within the chimney of the volcano, continued 

' Pot deteriptioD* of VosnTiaii lava-Btteams, see Bchmidt'a ' EmpUon ile» Vcmiv, 
"I, Mai IW5,' nnd Mtratlli's ' Vulcani, Ac." p, 51. For tiioso of Ehm, Sartoriiw voii 
WilirmLaiuon and A. tou Ias^uIx, ' Dor Aetiiu,' ii. p. !190. 



SM 



r-TTjJlICAl. •JOjLOQT. 



[Book HL Put I. 



'rzplfMfU'iRH 'li -wnz -;>ic6 puce liinm iis ^ipp^ mr&ce. The violence 
''f thiMe mav r.*r im>rr^ ^m toe v^a i:]ij-aAa of nwm, aahefl and stones 
iiaried Eo ifi 'ifiM ri ii^iicht into uie iir, md fimi the coDooiBionB of tite 
^rrrmnd which may be feit ac 'linuiGes ■•f mure tbui 100 milefl from the 
vnXtxais. It n'^I d'ji be % maner r't' aozpnae. thereforei, that the side§ of 
n i^Tcatvenr. ■:ipo«e*i toabockaot'snch insensitT, should at last gireiray, 
iind that larp: 'Uv^nreDi diBares soonld be opened down the oooe. 
Again, the iiv'Irbsta tic [>r>:aBiiiv of thecolanm of laTaniiwt,at a depth of 
I'XMt ffKt below thr; top <>f ihe column, exert a pFeesiire of between 70 
and !^0 tons on >.-ticit i<<inaie toot <jf the smroonding walLs. We may 
well Ijelieve that inch a fon.'e. acting npon the walls of a fannel alieadf 
nhutteretl \tj a suc(.-eHirion of terrific expIosionB, will be apt to prove too 




»1S.U»Vlrw.itl 



indrduhtputlrilnHlMwll7rb>Ijvi.Trr<iiTtii^ 1^ 



jrwat f.-r their resistanix'. ^^^)en this happenis the lava ponrs forth 
fr«i ih* ontside of the wne. On a mnch fistmred cone, lava may ifisno 
fnrly fr\-.m niauv iviuts. <<*> that a voli-ano w affected haB been 
£rkF<hicalIy described as '■ sweatini: firv." 

In a l.'fty Tvktino. lava nocasionidlr rise* to the lip of the crater 
«nd d<™:s •■at thtre; but more freyjaently it eMviies from some fiasore 
'T 'irifiv-e in a weak ran of (be >vi-.<'. hi minor volcanoes, on the otlier 
•■lAud, wE^rv tiie tiplvieior.s ar*' K-ss violent, aud where the tliickness of the 
wut iti pp .(»irtio[i to the di^ttifter of the fmiuol is often greater, the 
l.*vi» vvrv wnim^'nly n«>tf into the or*ttr. :>h«uld the crater-wallti 1* 
<*>*/ wvikk tu Twist the prvwiire I'f the moltieu uuu«. they give way, and 
'.>iv hiv.» i-Wtiwv ottt irvm th> brwch. 'Vliii* is seen to have happened 
!. ■M.-kvrW ;f »hw j.'itv* of AnvtT^tw. SI' well fi<tire<l and deficribed by 



Smtt. i. S 2-] LAVA-STREAMS. 205 

Scrope (Fig- 41).* Bnt if the crater bo masBive enough to withstand tho 
pnosure, the lava, if still impelled upward by the struggling vapour, 
will at last flow out from the lowest part of the rim. 

In a tall column of molten lava, there may be a variation in the 
density of its different parts, the heaviest naturally gravitating to the 
bottom. It has been observed by Ch, V^laiu thut at the Isle of Bourbon 
(B^nnion), the lavaseBcapingfrom tbobaseof the volcanie cone are denser 
and more basic than those which flow out from the Up of the crater.^ 

Ab soon as tho molten rook roachee tho surface, the suporhoatcd 
water or steam, imprisoned within its mass, escapes copiously, and bange 
u B dense white cloud over the moving current. The lava stroania 
of VeouviuB HOuietimeB appear with as dense a steam-cloud at their 
lower cuds as that which escapes at the same time from the main 
oater. Even after the molten mass has flowed several miles, steam 
omtinnee to riso abundantly both from its end and from i 




.Vkit of OIK «f tbe Tuff- 



I'liuU along its surface, and continues to do so for laany weeks, muuths, 
w it may be for several years. 

Should the point of escape of a lava-stream lie well down on tho 
«mi', far below the summit of the lava-column iu the funnel, the molten 
Kick, on its first escape, driven l>y hydrostatic pressure, will sometimes 
"l-oiit up high into the air— a fountain of molten rouk. This was 
oWn-od in 1704 on Vesuvius, and in 1832 on Etmi. In the eruption of 
^^'i2 at Maiina Loa, an unbroken fountain of lava, from 20l> to 700 feet 
in lieij^ht and 1000 feet broiid, burst out at the base of the cone, 
'■iimilar " geysers " of molten rock have subsequently been noticed in 
iWiMuie region. Thus in March and April 1868, four fiery fountains, 
ilirowiug lava to heights vnrj-ing from r)00 to 1000 feet, continued to 
iJay for several weeks. According to Mr, C'oan, such outbursts take 

' Fur (ie»criiJtioiLS of tliin n'gioii. bli; Sutojit'i) '(itijlncy iind Kititict Vnlcaiioca 
« (a-oIihI FmiHt,' 2iid. edit. 1HS8. H. LLiaxfs ' EptMjuea gtolut-iquos <tc I'Auvurgtuj,' 

' 'tts Volcanit,' p. 30. For lefureuoeu mlatiug tu thU blaod, see p. 22G. 



206 DYNAMICAL GEOLOGY. [Book m. Pabt L 



place from tho bottom of a column of lava 3000 feet high. The ▼oleano 
of Mauna Loa strikingly illustrates another feature of volcanic dynamics 
in t]ie position and outflow of lava. It bears upon its flanks at a 
distance of 20 miles, but 10,000 feet lower, the huge crater Eilauea. 
As Dana has pointed out, these orifices form part of one mountain, yet 
the column of lava stands 10,000 feet higher in one conduit than in tho 
other. On a far smaller scale the same independence occurs among the 
several pipes of some of the geysers in the Yellowstone region of North 
America. 

From the wide extent of basalt-dykes, such as those of Tertiary age in 
Britain, which rise to the surface at a distance of 200 miles from the main 
remnants of tho volcanic outbursts of their time, and are found over an 
area of perhaps 100,000 square miles, it is evident that molten lava may 
sometimes occupy a far greater space within the crust than might be 
inferred from the dimensions and outpourings even of the largest volcanic 
cone. There can be no doubt that vast reservoirs of melted rock, impreg- 
nated with superheated vapours, must formerly have existed, if they do 
not exist still, beneath extensive tracts of country (p. 239). Yet even in 
these more stupendous manifestations of volcanism, the lava should be 
regarded rather as the sign than as the cause of volcanic action. It is 
the pressure of the imprisoned vapour, and its struggles to get free, 
which produce the subterranean earthquakes, explosions, and out- 
pouring of lava. As soon as the vapour finds relief, the terrestrial 
commotion calms down again, until another accumulation of vapour 
demands a repetition of the same phenomena. 

Rate of flow of Lav a. — The rate of movement is regulated by 
the fluidity of the lava, by its volume, and by the form and inclination 
of the ground. Hence, as a rule, a lava-stream moves faster at first 
than afterwards, because it has not had time to stiffen, and its slope of 
descent is usually steeper than further down the mountain. One of the 
most fluid and swiftly flowing lava-streams ever observed on Yesayius 
was that erupted on 12th August, 1805. It is said to have rushed 
down a space of 3 Italian (3^ English) miles in the first four minutes, 
but to have widened out and moved more slowly as it descended, yet 
finally to have reached Torre del Greco in throe hours. A lava erupted 
by Mauna Loa in 1852 went as fast as an ordinary stage-coach, or fij^een 
miles in two houra. Long after a current has been deeply crusted over 
with slags and rough slabs of lava, it continues to creep slowly forward 
for weeks or even months. 

It happens sometimes that, as the lava moves along, the still molten 
mass inside bursts through the outer hardened and deeply seamed crust, 
and rushes out with, at first, a motion much more rapid than that of the 
main stream. Any sudden change in the form or slope of the ground 
affects the flow of the lava. Thus, reaching the edge of a steep defile 
or difif, the molten rock pours over in a cataract of glowing, molten rock, 
with clouds of steam, showers of fragments, and a noise utterly 
indescribable. Or on the other hand, encountering a ridge or hill across 



Sect. L § 2.] LAVA-STREAMS. 207 

its path, it accumulates until it either finds egress round the side or 
actually overrides and entombs the obstacle. The hardened cmst or shell, 
within which the still fluid lava moves, serves to keep the mass from 
spreading. Here and there, inside this crust, the lava subsides, leaving 
cavernous spaces and tunnels into which, when the whole is cold, one 
may creep, and which are sometimes festooned with stalactites of lava. 

Size of Lava-stream s. — In some cases, lava escaping from 
craters or fissures comes to rest before reaching the base of the slopes, 
like the obsidian current which has congealed on the side of the little 
volcanic island of Volcano,^ In other instances, the molten rock not 
only reaches the plains but flows for many miles away from the point 
of eruption. Sartorius von Waltershausen computed the lava emitted by 
Etna in 1865 at 92 millions of cubic metres, that of 1852 at 420 millions, 
that of 1669 at 980 millions, and that of a pre-historic lava-stream 
near Bandazzo at more than 1000 millions.^ The most stupendous 
outpouring of lava on record was that which took place from Skaptar- 
JclkuU in Iceland in the year 1783. Successive streams issued from 
the volcano, flooding the country far and wide, filling up river-gorges 
which were sometimes 600 feet deep and 200 feet broad, and advancing 
into the alluvial plains in lakes of molten rock 12 to 15 miles wide and 
100 feet deep. Two currents of lava which flowed in nearly opposite 
directions extended for 45 and 50 miles respectively, their usual tliick- 
nefls being 100 feet. Bischof estimated that the total amount of lava 
poured forth during this single eruption " surpassed in magnitude the 
bulk of Mont Blanc." 3 

Varying liquidity of Lava. — All lava, at the time of its 
expulsion, is in a molten condition, that is, consists of a glassy magma in 
which, by reason of the high temperature, most or all of the mineral 
constituents exist dissolved. Considerable differences, however, have 
been observed in the degree of liquidity. Humboldt and Scropo long 
ago called attention to the thick, short lumpy forms presented by 
masses of solidified trachytic rocks, which are lighter and more siliceous, 
and to the thin, widely extended sheets assumed by basalts, which are 
heavy and contain much iron and basic silicates.* It may be inferred 
that, as a rule, the basalts or basic lavas have been more liquid than 
the trachytes or siliceous lavas. The cause of this difference has 
been variously explained. It may depend partly upon chemical com- 
pucition, the siliceous being naturally less fusible than the basic rocks. 
But as great differences of fluidity are observable even among lavas 
having nearly the same composition, there would seem to be some 
farther cause for the diversity. Reyer has ingeniously maintained that 
we must look to original differences in the extent to which the sub- 
terranean igneous magma that supplied the lava has been saturated 

^ Bfeoent eraptionB in this island have conBisied entirely of ashes. A. Baltzer, Zeitsch . 
0nte&. GeoL Ge$. xxvi. (1875) p. 36. 

« 'Da Aetna,' ii. 393. » Lyell, • Principles,' ii. p. 49. 

* Bciope, 'Considerations on Volcanoes' (1825), p. 93. 



208 DYNAMICAL GEOLOGT. tfitxx m. Vixr t 

witli vapuurs and gaaea. Molten rock higUy impregnated gives rise, 
ho holilit, to fragmentary diucLargeti, wliile when feebly impregnated it 
flows out trantiuilly.' On the other hand, Captain G. E. Dutton, who 
hae etndied thu volcanic phenomena of Western America and Hawaii, 
suggests that the different degrees of liquidity may depend not only on 
chemical differenceB, hut on variationH of temperature. He snppoeea 
that the basaltic lavas which have spread so far in thin sheets, and 
which muflt have had a comparatively great liquidity, flowed at tem- 
peratures far above that of their melting point, and were, to nse his 
phnise, " superfuseil." ' 

Tho varying degrees of liquidity arc manifested in a characteristio 
way on tho surface of lava. Thus, in the great lava-pwds of Hawaii, 
the rock exhibits a remarkable liqoidity. 
Daring its ebullition in the crater-pools, 
jets and driblets, a quarter of an inch in 
diameter, are tossed up, and falling back on 
one another, make " a colmnn of hardened 
tears of lava," one of which (Fig. 43) was 
found to have attained a height of 40 feet, 
while in other places, tho jets thrown up 
and blown aside by the wind give rise to 
long threads of glass which lie tliickly to- 
gether like mown grass, and are known by 
the natives under tiie name of Fele's Hair, 
after one of their divinities.^ 

On the other hand, the lavas of Vesuvius 
and of most modem volcanoes, which issue so saturated with vapour as 
to bo nearly concealed from view in a cloud of steam, are accompanied 
by abundant explosions of fragmentary materials. Slags and clinkers, 
tern by explosions of steam from the molten rock, are strewn abundantly 
over Ute cone, while the surface of the lava is likewise ni{!^;ed wiUi 
similar clinkers, which may now and then be observed piled up round 
some more energetic steam-spiracle (Fig. 43). So vast an amount of 
steam rushes out from one of these orifices, and with such boiling and 
explosion, tliat the cone of bombs, slags, and irregular lumps of lava 
forms a miniature or parasitic volcano, which will remain as a nuirked 
cone on its parent mountain long after tho eruption which gave it birth 
has ceased. The lava of the eruption at Suntoriu in 186()-07 at first 
welled out tranquilly, but after a few days its outflow was accom- 
panied with explosions and discharges of incandescent fragments, which 
increased until they had covered the lava domo with ejected scorife, and 
had opunud a number of crateriform mouths on its summit.* 

There can Ije no doubt, as above remarked, that the condition of 
' * Beitratc mir Pliyaik dci Eiujiliuueii,' p. 77. 

' "HighPlatuuiix of Utali," (Iwj. and Gml Hareei/ uf TerritvHtn. Wasliiiigtou, 
1880, chiip, V. 

' Duiiu, (ito/. V.ti. Kxpliir, Jixjied; ''Geology,' p. 17'J. 
' Fouque, ' anntoriii,' p. iv. 




SEtrr. LIS.] 



LAVA.STJtSAMS. 



liquidity of the lava has in Bome measure detormined the form of the 
cmptioiiB. In one case, there are qniot oiit-wollings of the more liquid 
laTaa,u at Hawaii; in another, there arooxplasiTedischargoaandciader- 
cones aooompanying the more viscid lavas, au at moet modern Tolcanoc^H. 
The former has been the condition favourable to the moat colossal 
OU^tonringB of molten rook, as we see in the bnsalt-platoaux of Britain, 
Faroe, Greenland, Idaho, and Oregon, tho GhnutH, Abyssinia, &c. This 
anbject in again referred to at p.^2;)@. 




Fig. 43.— laTa-CBliimn {elgbl fwt hlgU), Vwiivlos (AUr! 



Cryatallizfttion of Lava. — Pouring forth irith n liquidity 
like that of molten iron, lavn speedily aSBumea a more viscous condition 
and ft slower motion. Obsidian and other vitreous rocks have conw.li- 
dated as glass: yet that they are not always extremely fliii.l in 
indicated by tho nrreirt, of the obsidian Btroam half way down the stoop 
northern slope of Volcano. Even in such perfect natural glass as 
obdidian, micrOBCopic crystallites and crystals are usually present, and 
in pTodigions numlicrs (pp. Ill, 146). In most lavas, devitrification has 
proceeded so far before the dual atiffening, that the original glassy magma 
has passed into a more or lees completely Htlioid or crystalline mass. 



210 DYNAMICAL GEOLOGY. [Book IH. Pabt I. 



That lava may posHess an appreciably cryBtalline Btractxire while 
Ktill in motion, has often been proved at Vesuvinfl, where weil-defined 
crystals of the infusible Icucito may be observed in a molten magma of 
the other minerals, portions of the white-hot rock in this oondition 
being ladled out, impressed with a stamp and suddenly congealed. 
The ilnxion-stnicture above (p. Ill) described, furnishes intereeting 
evidence of this fact in many ancient as well as modem lavas. 

The crystalline structure of lava has been supposed to be in some 
measure determined by the presence of the volcanic vapours and gases 
with which tlic molten rock is impregnated, the rapid escape of these 
vapours preventing the formation of the crystalline structure, and 
leaving the lava in the condition of a more or less perfect glass. But 
the experiments of MM. Fouquo and Jj^vy, to be afterwards re- 
ferred to, liave shown that rocks, having in every essential particular 
the characters of volcanic lavas, may be artificially produced under 
ordinary atmosphoric pressure by simple dry fusion. What the influenoe 
may be of the dissolved vapours upon the ultimate consolidation of 
molten lava has therefore yet to be ascertained. Difference in the rate 
of cooling has doubtless been an important, if not the main, factor in 
determining the various conditions of texture of lavas. The crystal- 
line structure may be expected to be most perfect where, as within thick 
maRses of rock, the cooling has been prolonged, and where, consequently, 
tlie crystals have had ample time and opportunity for their formation. 
Oil the other hand, the glassy structure vnW naturally be most perfectly 
shown where tlie cooling has been most rapid, as in the vitreous crust on 
the walls of dykes already (pp. 112, 196) referred to. Bocks crystallizing 
in the doe^x^r parts of a volcano appear usually to possess a more coarsely 
cryHtalline structure than those which crystallize near the surface. 

Temperature of Lava. — It would be of the highest interest 
and importance to know accurately the temperature at which a lava- 
stream first issues. Measurements not altogether satisfiftctory have 
been taken at various distances below the point of emission, whero the 
moving lava could be safely approached. Experiments made at 
Vesuvius by Scacchi and Sainte-Claire Deville in 1865, by thrusting 
thin wires of silver, iron, and copper into the lava, indicated a 
temperature of scarcely 700° C. (1228° Fahr.). Observations of a similar 
kind, made in 1819, when a silver wire J^th inch in diameter at onco 
melted in the Vesuvian lava of that year, gave a greatly higher 
temperature, the melting point of silver being about 1800° F^r. But 
copper wire has also been melted, the point of fusion of this metal being 
about 2204° Fahr. Evidence of the high temperature of lava has 
likewise been adduced from the alteration it has effected upon 
refra(^.tory substances in its progress, as where, at Torre del Greco, it 
overflowed the houses, and was aftei-wards found to have fused the fine 
edges of flints, to have decomposed brass into it« component metals, the 
copper actually crystallizing, and to have melted silver, and even 
sublimed it into Rmall octahedral crystals (p. 214). The lava of Santorin 



Sbot. L } 2.] LAVA'STREAMS, 211 



has canght np pieces of limestone, and has formed out of them nodules 
containing crystallized anorthite, augite, sphene, black garnet, and 
particularly wollastonite.^ The initial temperature of lava, as it first 
issnes from the Yesuvian funnel, is probably considerably more than 
2000° Fahr. Obviously the absorbed water (or water-substance, for, as 
already remarked, the temperature is far above the critical point of 
water, and its component gases may exist dissociated) must possess as 
high a temperature as that of the white-hot lava in which it is contained. 
The existence of the elements of water at a white heat, even in rocks 
which have reached the surface, is a fact of no little significance in 
the theoretical consideration of hypogene action. 

Inclination and thickness of lava-flows. — It was at one 
time sappoeed that lava could not consolidate in beds on such steep slopes 
as those of most volcanoes. Hence arose the " elevation-crater theory " 
(described at p. 224), in which the inclined position of lavas round a 
volcanic vent was explained by upheaval after their emission. Observa- 
tions all over the world, however, have now demonstrated that lava, with 
all its characteristic features, can consolidate on slopes of even 35° and 
40°.* The lava in the Hawaii Islands has cooled rapidly on slopes of 25°, 
that from Vesuvius, in 1855, is here and there as steep as 30°, while the 
older lavas in Monte Somma are sometimes inclined at 45°. On the 
east side of £tna, a cascade of lava, which in 1689 poured into the vast 
hollow of the Cava Grande, has an inclination varying from 18° to 48°, 
with an average thickness of 16 feet. On Mauna Loa some lava-flows 
are said to have congealed on slopes of 49°, 60°, and oven 80°, though in 
these cases, it could only be a layer of rock, stiffening and adhering to 
the surface of the declivity. Even when it consolidates on a steep 
slope, a stream of lava forms a sheet with parallel upper and under 
surfaces, a general uniformity of thickness, and often greater evenness 
of surface than where the angle of descent is low. The thickness varies 
indefinitely ; many basalts which have been poured out in a remarkably 
liquid condition have solidified in bods not more than 10 or 12 feet 
thick. On the other hand more pasty lavas, and lavas which have 
flowed into narrow valleys, may be piled up in solid masses to a thick- 
ness of several hundred feet (p. 207). 

Structure of a lava-stream. — Lava-streams are sometimes 
nearly homogeneous throughout. In general, however, they each show 
three component layers. At the bottom lies a rough, slaggy mass, 
produced by the rapid cooling of the lava, and the breaking up and 
continued onward motion of the Bcoriform layer. The central and 
main portion of the stream consists of solid lava, often, however, with a 
more or less carious and vesicular texture. The upper part, as wo have 
seen, may be a mass of rough broken-up slabs, scoriae, or clinkers. The 
proportions borne by these respective layers to each other vary con- 
tinually. Some of the more fluid ropy lavas of Vesuvius have an 

> Fouaue, ' Santorin/ p. 206. 

• Lyoll on tho oonsolidation of lava on steep slopes, Fhih Traw. 1858. 

P 2 



212 DYNAMICAL GEOLOGY. [Book m. Paw L 



inconstant and thin slaggy onist ; others may be said to oonsiBt of little 
else than scorisB from top to bottom. Thronghont the whole man of a 
lava-onrrent, but more especially along its upper surfaoe^ the absorbed 
vapours expand as the pressure diminishes, and pushing the molten rock 
aside, segregate into small bubbles or irregular cavities. Henoe, when 
the lava solidifies, these steam-holes are seen to be sometimes so abundant 
that a detached portion of the rock containing them will float in water 
(pumice). They are often elongated in the direction of the motion of 
the lava-stream (Fig. 44). Sometimes, indeed, where the oelU are 
numerous, their elongation in one direction gives a fissile straotnre to 

the rock. 

In passing from a fluid to a solid condition, and thus oontraotingi 
lava acquires different structures. Lines of divisional planes or 
joints traverse it, especially perpendicular to the upper and under 
surfaces of the sheet. These sometimes assume prismatio forms, 
dividing the rock into colunms, as is so frequently to be observed in 
basalt. They are described in Book IV. Part II., together with other 
forms of joints. 




h 

Fig. 44.— Elongation of vesicles In direction of flow of lava. 

Vapours and sublimations of a lava-stream. — ^Besides 
steam, many other vapours, absorbed in the original subterranean 
molten magma, escape from fissures of a lava-stream. The points at 
which such vapours are copiously disengaged are termed /wnarokt. 
Among the exhalations, chlorides abound, particularly chloride of 
sodium, which appears, not only in fissures, but even over the cooled 
crust of the lava, in small crystals, in tufts, or as a granular and 
even glassy incrustation. Chloride of iron is deposited as a yellow 
coating at fumaroles, where also bright emerald-green films and 
scales of chloride of copper may bo more rarely observed. Many 
chemical changes take place in the escape of these vapours. Thus 
specular-iron, cither the result of the mutual decomposition of steam 
and iron-chloride, or of the oxidation of magnetite, forms abundant 
scales, plates, and small crystals in the fumaroles and vesicles of some 
lavas. Sal-ammoniac also appears in large quantity on many lavas^ 
not merely in the fissures, but also on the upper surface. In these cases, 
it is not directly a volcanic product, but results from some decom- 
position, possibly from the gases evolved by the sudden destruction of 
vegetation. It has, however, been observed also in the crater of Etna, 
whore the co-operation of organic substance is hardly conceivable, and 
where perhaps it may arise from the decomposition of aqueous vapour. 



SiCT. L S 2.] LAVA'STRtlAMS. 213 



whereby a oomfaination is formed with atmospheric nitrogen. Sulphur, 
bieidakite, Beaboite, tenorite, alum, sulphates of iron, soda and potash, 
and other minerals are also found. 

Slow cooling of lava. — The hardened crust of a lava-stream 
is a bad condactor of heat. Consequently, the surface of the stream 
may have beoome cool enough to be walked upon, though the red- 
hot masB may be observed through the rents to lie only a few inches 
below. Many years, therefore, may elapse before the temperature of 
the whole mass has fallen to that of the surrounding soil. Eleven 
months after an eruption of Etna, SpaDanzani could see that the 
lava was still red-hot at the bottom of the fissures, and a stick 
throBt into one of them instantly took fire. The Yesuvian lava of 
1785 was found by Breislak, seven years afterwards, to be still hot and 
steaming internally, though lichens had already taken root on its 
KorfiEMie. The ropy lava erupted by Vesuvius in 1868 was observed 
by the author in 1870 to be still so hot, even near its termination, 
ihat steam issued abundantly from its rents, many of which were too 
warm to allow the hand to be held in them, and three years later it 
was still steaming abundantly. Hoffmann records that from the lava 
which flowed from Etna in 1787, steam was still issuing in 1830. 
Tet more remarkable is the case of JoruUo, in Mexico, which sent out 
lava in 1759. Twenty-one years later a cigar could be lighted at its 
fissures; after 44 years it was still visibly steaming; and even in 
1846, that is, after 87 years of cooling, two vapour-columns were still 
rising from it.^ 

This extremely slow rate of cooling has justly been regarded as a 
point of high geological significance, in regard to the secular cooling 
and probable internal temperature of our globe. Some geologists 
have argued, indeed, that if so comjmratively small a portion of 
molten matter as a lava-stream can maintain a high temperature 
under a thin, cold crust for so many years, we may, from analogy, 
feel little hesitation in believing that the enormously vaster mass of 
the globe may, beneath a relatively thin crust, still continue in a 
molten condition within. More legitimate deductions, however, 
might be drawn from more accurate and precise measurements of 
the rate of loss of heat, and of its variations in different lava-streams. 
Sir William Thomson, for instance, has suggested that, by measuring 
the temperature of intrusive masses of igneous rock in coal-workings 
and elsewhere, and comparing it with that of other non-volcanic 
rocks in the same regions, we might obtain data for calculating the 
time which has elapsed since these igneous sheets were erupted 
(anfe, p. 48). 

Effects of lava-streams on superficial waters and 
topography. — ^In its descent, a stream of lava may reach a water- 
course, and, by throwing itself as an embankment across the stream, 
may pond back the water and form a lake. Such is the oiigiii 

* E. SchloideD, quoted by Naumaun, ' Geognosio,* i. p. 160. 



214 DYNAMICAL GEOLOGY. [Book HI. Past L 



of the picturesque Lake Aidat in Anvergne. Or the molten current 
may usurp the channel of the stream, and completely bury the 
whole valley, as has happened again and again among the vast 
lava-fields of Iceland. Few changes in physiography are so rapid 
and so enduring as this. The channel which has required, doubtless, 
many thousands of years for the water laboriously to excavate, is 
sealed up in a few hours under 100 feet or more of stone, and 
another vastly protracted interval may elapse before this newer pile 
is similarly eroded.^ 

By suddenly overflowing a brook or pool of water, molten lava 
sometimes has its outer crust shattered to fragments by a sharp 
explosion of the generated steam, while the fluid mass within rushes 
out on all sides.* The lavas of Etna and Vesuvius have protruded 
into the sea. Thus a current from the latter mountain entered the 
Mediterranean at Torre del Greco in 1794, and pushed its way for 
360 feet outwards, with a breadth of 1100 and a height of 16 feet. So 
quietly did it advance, that Breislak could sail round it in a boat 
and observe its progress. 

By the outpouring of lava, two important kinds of geological 
change are produced. (1) Stream-courses, lakes, ravines, valleys, in 
short, all the minor features of a landscape, may be completely over- 
whelmed under a thick sheet of lava. The drainage of the 
district being thus effectaally altered, the numerous changes which 
flow from the operations of running water over the land are arrested 
and made to begin again in new channels. (2) Considerable altera- 
tions may likewise be caused by the effects of the heat and vapours of 
the lava upon the subjacent or contiguous ground. Instances have been 
observed in which the lava has actually melted down opposing rocks, 
or masses of slags on its own surface. Interesting observations, already 
referred to (p. 210), have been made at Torre del Greco under the lava- 
stream which ovei-flowed part of that town in 1794. It was found that 
the window -i)anes of the houses had been de vitrified into a white, 
translucent, stony substance ; that pieces of limestone had acquired an 
open, sandy, granular texture, without loss of carbon-dioxide, and tliat 
iron, brass, lead, copper, and silver objects had been greatly altered, 
some of the metals being actually sublimed. We can understand, there- 
fore, that, retaining its heat for so long a time, a mass of lava may 
induce many crystalline stnictures, rearrangements, or decomposi- 
tions in the rocks over which it comes to rest, and proceeds slowly to 
cool. This is a question of considerable importance in relation to the 
})chaviour of ancient lavas which ha\e been intruded among rocks 
iKaieath the surface, and have subsequently been exposed (Book IV. 
Part YII.). 

* For an example of the conversion of a lava-buried river-bed into a hill-top by long- 
continued denudation, see Quart. Journ. Ckol, Soc. 1871, p. 303. 

^ Explosions of this nature have been observed on Etna, where the lava haa enddenly 
come in contact with water or snow, considerable Iobs of life being sometimes the 
result gartorius von Waltershausen and A. von Lasaulx, ' Der Aetna,' L pp. 295, 800. 



Sect. L f 2.] LAVA-STBEAMS. 215 



But on the other hand, the exceedingly trifling change produced, 
even by a massiye sheet of lava, has often been remarked with astonish- 
ment On the flank of Vesnvius, vines and trees may be seen still 
flotuishing on little islets of the older land-surface, completely sur- 
rounded by a flood of lava. Dana has given an instructive account of 
the descent of a lava-stream from Kilauea in June, 1840. Islet-like 
Bi>aceB of forest were left in the midst of the lava, many of the trees being 
still alive. Where the lava flowed round the trees, the stumps were 
nsnaUy consumed, and cylindrical holes or casts remained in the lava, 
either empty or filled with charcoal. In many cases, the fallen crown 
of the tree lay near, and so little damaged that the epiphytic plants on 
it began to grow again. Yet so fluid was the lava that it hung in pen- 
dent stalactites from the branches, which nevertheless, though clasped 
round by the molten rock, had barely their bark scorched. Again, for 
nearly 100 years there has lain on the flank of Etna a large sheet of ice, 
which, originally in the form of a thick mass of snow, was overflowed 
by lava and has thereby been protected from the evaporation and thaw 
which woidd certainly have dissipated it long ago, had it been exposed 
to the air. The heat of the lava has not sufficed to melt it. Extensive 
tracts of snow were likewise overspread by lava from the same 
mountain in 1879. In other cases, snow and ice have been melted 
in large quantities by overflowing lava. The great floods of water 
which rushed down the flank of Etna, after an eruption of the mountain 
in the spring of 1755, and similar deluges at Cotopaxi, are thus 
explained. 

One further aspect of a lava-stream may be noticed here — the effect 

of time upon its surface. While all kinds of lava must, in the end, 

crumble down under the influence of atmospheric waste and, where 

other conditions permit, become coated with soil, and support some kind 

of yegetation, yet extraordinary differences may be observed in the 

facility witli which different lava-streams yield to this change, even on 

the flank of the same mountain; Every one who ascends the slopes of 

Vesuvius remarks this fact. After a little practice, it is not difficult 

there to trace the limits of certain lavas even from a distance, in some 

cascH by their verdure, in others by their barrenness. Five hundred 

yoare have not sufficed to clothe with green the still naked surface of the 

I'atanian lava of 1381 ; while some of the lavas of the present century 

bave long given footing to bushes of furze.^ Some of the younger 

lnvaa of Auvergne, which certainly flowed in times anterior to those of 

history, are still singularly bare and rugged. Yet, on the whole, where 

lava is directly exposed to the atmosphere, without receiving protection 

frjin cx«asional showers of volcanic ash, or where liable to be washed 

We by heavy torrents of rain, its surface decays in a few years 

snfficiently to afford soil for stray plants in the crevices. When these 

liave taken root they help to increase the disintegration ; at last, as the 

» On the weathering of the Etna lavaa, see ♦ Der Aetna," ii. p. 897. 



216 DYNAMICAL GEOLOGY. [Book IIL Pabt I. 



rock \a ovcrHpread, the traces of its yolcanic origin fade away from its 
surface. Some of the Vesuvian lavas of the present oentury already 
support vineyards. 

Elevation and Subsidence. Proofis of elevation are frequent 
among volcanic vents which, lying near the sea and containing marine 
sediments among their older erupted materials, supply, in the enclosed 
marine organisms, evidence of the movement. In this way, it is known 
that Etna, Vesuvius and other Mediterranean volcanoes, began their 
history as submarine vents, and that they owe their present dimensions 
not only to the accumulation of ejected materials, but also, to some 
extent, to an elevation of the sea-bottom. Proof of subsidence is 
less easily traced, but indications have been observed of a sinking 
of the ground beneath a volcanic vent, as if the crust had settled 
down upon the cavity made by the discharge of so much volcanic mate- 
rial. During the eruption of Santorin in 1866-67, very decided but 
extremely local subsidence took place near the vent in the centre of the 
old crater. 

Torrents of Water and Mud. — ^We have seen that large 
ijuantities of water accompany many volcanic eruption& In some 
cases, where ancient crater-lakes or internal reservoirs, shakeu by 
repeated detonations, have been finally disrupted, the mud which has 
thereby been liberated issues at once from the mountain. Such " mud- 
lava " (lava d'acqua)^ on account of its liquidity and swiftness of motion, 
is more dreaded for destructiveness than even the true melted lavas. 
On the other hand, rain or melted snow or ice, rushing down the cone 
and taking up loose volcanic dust, is converted into a kind of mud that 
grows more and more pasty as it descends. The mere sudden rush of 
such largo bodies of water down the steep declivity of a volcanic cone 
cannot fail to effect much geological change. Deep trenches are cut out 
of the loose volcanic sIojkjs, and sometimes large areas of woodland arc 
swept away, the debris being strewn over the plains below. 

One of these mud-lavas invaded Herculaneum during the great 
eruption of 79, and by quickly enveloping the houses and their 
contents, has preser\'ed for us so many precious and perishable monu- 
ments of antiquity. In the same district, during the eruption of 1622, 
a torrent of this kind poured down upon the villages of Ottajano and 
Massa, overthrowing walls, filling up streets, and oven burying houses 
with their inliabitants. During the great eruption of Cotopaxi, in June 
1877, enonnous torrents of water and mud, produced by the melting of 
the snow and ice of the cone, poured doAvn from the mountain. Among 
the debris hurried along were vast numbers of large blocks of ice. 
The villages all round the mountain to a distance of sometimes more 
than ton gooj^raphical miles were left deeply buried under a deposit of 
mud mixed with blocks of lava, ashes, pieces of wood, &c.^ Many of the 
volcanoes of Central and South America discharge large quantities of 
mud directly from their craters. Thus, in the year 1G91, Imbabuni, one 

> Wolf, Nem6 Jahth, 1878, p. 133. 



Sect. i. S 2.] BOLFATARA STAGE OF VOLCANIC ACTION. 217 



of the Andes of QnitO) emitted floods of mud so largely charged with dead 
fish that pestilential fevers arose from the subsequent effluvia. Seven 
years later (1698), during an explosion of another of the same range of 
lofty mountains, Carguairazo (14,706 feet), the summifc of the cone is 
said to have fallen in, while torrents of mud containing immense 
numbers of the fish Pymehdus Gyclopum, poured forth and covered the 
ground over a space of four square leagues. The carbonaceous mud 
(locally called moya) emitted by the Quito volcanoes sometimes escapes 
from lateral fissures, sometimes from the craters. Its organic contents, and 
notably its siluroid fish, which are the same as those found living in the 
streams above ground, prove that the water is derived from the surface, 
and accumulates in craters or underground cavities until discharged by 
volcanic action. Similar but even more stupendous and destructive 
oatpoorings have taken place from the volcanoes of Java, where 
wide tracts of luxuriant vegetation have at different times been buried 
under masses of dark grey mud, sometimes 100 feet thick, with a rough 
hiUocky surface from which the top of a submerged palm-tree occasion- 
ally protruded. 

Between the destructive effects of mere water-torrents and that of 
those mud-floods there is, of course, the notable difference that, whereas 
in the former case, a portion of the surface is swept away, in the latter, 
while sometimes considerable demolition of the surface takes place at 
first, the main result is the burying of the ground under a new 
tumultuous deposit by which the typography is greatly changed, not 
only as regards its temporary aspect, but in its more permanent features, 
such as the position and fonn of its watercoiirsoH. 

Exhalations of Vapours and Gases.— A volcano, as ite activity 

wanes, may pass into the Solfatara stage, when only volatile emanationH 

are discharged. The well-known Solfatara near Naples, since its 

last eruption in 1198, has constantly discharged steam and suli)hurous 

vapours. The island of Volcano has now passed also into this phase. 

Numerous other examples occur among the old volcanic tracts of Italy, 

where they have been tenned aoffionu Steam, escaping in conspicuous 

jets, sulphuretted hydrogen, hydrochloric acid, and carbonic acid are 

particularly noticeable at these orifices. The vapours in rising condense. 

The sulphuretted hydrogen, partially oxidises into sulphuric acid, which 

powerfully corrodes the surrounding rocks. The lava or tuff through 

which the hot vapours rise is bleached into a white or yellowish crumbling 

clay, in which, however, the less easily coiToded crystals may still bo 

recognised in situ. At the same time, sublimates of sulphur or of chlorides 

may l)e formed, or the sulphuric acid attacking the lime of the silicates 

^ves rise to gypsum, which spreads in a network of threads and veins 

through the hot, steaming, and decomposed mass. In this way, at the 

i»laud of Volcano, obsidian is converted into a snow-white, dull, clay- 

»t4jne-like substance, with crystals of sulphur and gypsum in its crevices. 

Silica is likewise deposited from solution at many orifices, and coats 

the altered rock with a crust of chalcedony, hyalite, or some form of 



218 DYNAMICAL GEOLOGY, [Book IIL Pakt L 



Biliceous sinter. As the result of this action, masses of rock are decom- 
posod below the surface, and new deposits of alum, sulphur, sulphides of 
iron and copper, <S:c., are formed above them. Examples have been de- 
scribed from Iceland, Lipari, Hungary, Terceira, Teneriffe, St. Helena, 
and many other localities.^ The lagoons of Tuscany are basins into 
which the waters from suffioni are discharged, and where a precipita- 
tion of their dissolved salts takes place. Among the substances thus 
depouited are gypsum, sulphur, silica, and various alkaline salts; but 
the most important is boracic acid, the extraction of which constitutes a 
thriving industry'. 

Another class of gaseous emanations betokens a condition of volcanic 
activity further advanced towards final extinction. In these, the gas 
is carbon-dioxide, either issuing directly from the rock or bubbling up 
with water which is often quite cold. The old volcanic districts of 
Europe furnish many examples. Thus on the shores of the Laacher 
See — an ancient crater-lake of the Eifel — the gas issues from numerous 
openings ccdled moffettCj round which dead insects, and occasionally 
iiiico and birds, may be found. In the same region, occur hundreds of 
Bprings more or less charged with this gas. The famous Valley of 
Death in Java contains one of the most remarkable gas springs in 
the world. It is a deep, bosky hollow, from one small space on the 
bottom of which carbon-dioxide issues so copiously as to form the 
lower stratum of the atmosphere. Tigers, deer, and wild-boar, enticed 
by the shelter of the spot, descend and are speedily suflfocated. 
Many of their skeletons, together with those of man himself, have been 
observed. 

As a distinct class of gas-springs, we may group and describe here 
the emanations of volatile hydrocarbons, which, when they take fire, 
are known as Fire-wells. These are not of volcanic origin, but arise 
from changes witliin the solid rocks underneath. They occur in many 
of the districts where mud-volcanoes appear, as in northern Italy, on 
tlie Caspian, in Mesopotamia, in southern Kurdistan, and in many parts 
of the United States. It has been observed that they frequently rise 
in regions where beds of rock-salt lie underneath, and a& that rock 
has been ascertained often to contain compressed gaseous hydrocarbons^ 
tho solution of the rock by subterranean water, and the consequent 
liberation of the gas, has been offered as an explanation of these 
lire -wells. 

In the oil regions of Pennsylvania, certain sandy strata occur at 
various geological horizons whence large quantities of petroleum and 
gas are obtained (p. 174). In making the borings for oil-wells, reser- 
voirs of gas as well as subterranean courses or springs of water are met 
with. When the supply of oil is limited but that of gas is large, a 
contest for possession of the bore-hole sometimes takes place between 
the gas and water. When the machineiy is removed and the boring 

* Von Bach, *Oauar. InaelD/ p. 282. Hofibian, Pogg. Ann. 1832, pp. 3d, 40, 60. 
BuDBen, Ann, Chem, Plutrm, 1847 (Ixii.), p. 10. Darwin, * Volcanic iBlandf*,' p. 29. 



Sbct. i § 2j GET8JSB8. 219 



is abandoned, the contest is allowed to proceed unimpeded and results 
in the intermittent discharge of columns of water and gas to heights 
of 130 feet or more. At night, when the gas has been liglitod, the 
spectacle of one of these " fire-geysers " is inconceivably grand.^ 

Oeysers. — ^Eruptive fountains of hot water and steam, to which the 
general name of Geysers (i.e. gushers) is given, from the examples in 
Iceland, which were the first to be seen and described, mark a declining 
phase of vdcanio activity. The Great and Little Geysers, the Strokkr, 
and other minor springs of hot water in Iceland, have long been cele- 
brated examples. More recently another series has been discovered in 
New Zealand. But probably the most remarkable and numerous 
assemblage is that which has been brought to light in the north-west 
part of the territory of Wyoming, and which has been included 
within the "Yellowstone National Park" — a region set apart by the 
Congress of the United States to be for ever exempt from settle- 
ment, and to be retcdned for the instruction of the people. In this 
singtdar region, the ground in certain tracts is honeycombed with 
passages which communicate with the surface by hundreds of openings, 
whenoe boiling water and steam are emitted. la most cases, the water 
remains dear, tranquil, and of a deep green-blue tint, though many of 
the otherwise quiet pools are marked by patches of rapid ebullition. 
These pools lie on mounds or sheets of sinter, and are usually edged 
round with a raised rim of the same substance, often beautifully fretted 
and streaked with brilliant colours. The eruptive openings usually 
appear on small, low, conical elevations of sinter, from each of which 
one or more tubular projections rise. It is from these irregular tube- 
like excrescences that the eruptions take place. 

The term geyser is restricted to active openings whence columns of 
hot water and steam are from time to time ejected ; the non-eruptive pools 
are only hot springs. A true geyser should thus possess an underground 
pipe or passage, terminating at the surface in an opening built round 
with deposits of sinter. At more or less regular intervals, rumblings and 
sharp detonations in the pipe are followed by an agitation of the water in 
the basin, and then by the violent expulsion of a column of water and steam 
to a considerable height in the air. In the Upper Fire Hole basin of the 
Yellowstone Park, one of the geysers, named " Old Faithful " (Fig. 45), 
lias ever since the discovery of the region, sent out a column of mingled 
water and steam every sixty-three minutes or thereabouts. The 
column rushes up with a loud roar to a height of more than 100 feet, 
the whole eruption not occupying more than about five or six minutes. 
The other geysers of the same district are more capricious in their move- 
ments, and some of them more stupendous in the volume of their 

* Ashbumer, Froc. Atner. PhiL 8oc. xvii. (1877), p. 127. StoweWs Petroleum 
Reporter, 15th Sept 1879. Second OeoL Survey of Pennsylvania, containing Reporta by 
J. OeutII, 1877, 1880. On the naphtha districts of the Caspian 8ea, Abich, JahrK Geol. 
Bek^. ixix. (1879), p. 165 ; see also for phenomena in Gallicia the same work, xv. pp. 
199, 851 ; zriL p. 291 ; xyiii p. 811 ; xxxi. (1881) p. 131. Proc. ImU Civ, Engineers, xlii. 
(1875) p. 343. 



DTNAMJCAL OEOLOGT. 



y>%. m. Paw I. 



disohiirge. The eruptions of the Castle, Giant, and Beehive vents are 
marvellouBly impressive.' 

In uxamining the Yellowstone Qeyser region in 1879, the author was 
sjKcially struck by the evident independence of the vents. This was 
shown l>y their very different levels, as well aa by their capricionB and 
unsympathetic eruptions. On the same hill-slope, dozens of qoiet pools, 
as well as some true geysers, were noticed at different levels, from the 
edge of the Fire Hole Eivor up to a height of at least 80 feet above it 
Yet the lowor pools, from which, of course, had there been underground 
connection between the different vents, the drainage should have princi' 
pally discharged itself, woi-e often found to.be quiet steaming pools 
without outlet, while thoso at higher jmints were oocasionaUy in aotivo 
eruption. It seemed also to make no difference in the height or tiau- 




(|uillity of one of the quietly boUiug cauldrons, when an active projec- 
tion of steam and water was going on from a neighbouring vent on the 
samo gcutlo slope. 

Buiisen and Dcscluitieaux sj>ont some days experimenting at the 
Icelandic geysers, and ascertained that in the Great Geyser, while the 
Kurfacc temiieraturo is about 212° Fahr,, that of lower portions of the 
tul>c is much higher — a thermometer giving as high a reading as 266° 
Fahr. The water at a littlo depth must couBcqxiontly be 54° above the 
normal boiling-point, l)ut it is kept in the fluid state by the pressure of 
tlic overlying cidiimn. At the baein, however, tho water cools quickly. 
After an cKplosion it accumulates there, and eventually begins to boil. 

> Bee Hajdeo's licporta for ISTII and fur 1878, in tLu latter U wliicli wlU be fonnJa 
voluminouij muuo^ph on tho Hot Springs by A.C.Foalo; Cowitook'aBeportillJanw'* 
Beoonnaiwance of N. W. Wyoming, ice, 1871. 



Sect, i § 2.] MUD-VOLCANOES. 221 



The preesare on the column below being thus relieved, a portion of the 
superheated water flashes into steam, and as the change passes down 
the pipe, the whole column of water and steam rushes out with great 
violence. The water thereafter gradually collects again in the pipe, and 
after an interval of some hours the operation is renewed. The experi- 
ments made by Bunsen proved the source of the eruptive action to lie in 
the hot part of the pipe. He hung stones by strings to different depths 
in the funnel of the geyser, and found that only those in the higher 
part were cast out by the rush of water, sometimes to a height of 
100 feet, while, at the same time, the water at the bottom was hardly 
disturbed at alL These observations give much interest and importance 
to the phenomena of geysers in relation to volcanic action. They show 
that the eruptive force is steam ; that the water column, even at a com- 
paratively small depth, may have a temperature considerably above 212*^ ; 
that this high temperature is local ; and that the eruptions of steam and 
water take place periodically, and with such vigour as to eject largo 
stones to a height of 100 feet.^ 

The hot water comes up with a considerable percentage of mineral 
matter in solution. According to the analysis of Sandberger, water 
from the Gh'eat Geyser of Iceland contains in 10,000 parts the following 
proportiofns of ingredients : silica 5*097, sodium-carbonate 1*939, ammo- 
ninm-carhonate 0*083, sodium-sulphate 1*07, potassium-sulphate 0*475, 
m^nennm-solphate 0*042, sodium-chloride 2*521, sodium-sulphide 0*088, 
carbonio acid 0*557,= 11 *872.> 

As soon as the water reaches the surface, and begins both to cool and 

to evaporate, it deposits the silica as a sinter on the surfaces over which 

it flows or on which it rests. The deposit naturally takes place fastest 

along the margins of the pools. Hence the curiously fretted rims by 

which these sheets of water are surrounded, and the tubular or cylindrical 

protuberances which rise from the growing domes. Whore numerous 

liot springs have issued along a slope, a succession of basins gives a 

curiously picturesque terraced aspect to the ground, as at the Mammoth 

Springs of the Yellowstone Park and at Kotamaliana in New Zealand. 

In course of time, the network of underground passages undergoes 
alteration. Orifices that were once active cease to enipt, and even tlie 
water fails to overflow them. Sinter is no longer formed round them, 
an<l their surfaces, exjwsed to the weather, crack into fine slialy rubbish 
like comminuted oyster-shells. Or the cylinder of sinter grows upward 
until, })y the continued deposit of sinter and the failing force of the 
geyser, the tube is finally filled up, and then a dry and crumbling white 
pillar is left to mark the site of the extinct geyser. 

Mad-Volcanoes. — Tliese are of two kinds: Ist, where tlie chief 

' Ompie$ Bendm, xxiii. (1«46), p. 034; Fogy. Annal. Ixxii. (1847), p. 150; Ixxxiii. 
nSol , p. 197. Ann. C%irnt«, xxxviii. (1853), pp. 215, 385. Tlio explanation propOHcd 
for the phenomena observed at the Great Geyser is probably not applicable in 
tlKMe caaefl where the mere local aocnmulation of steam in suitable reservoirs may l>e 
ntBcient 

' Annal, Chem, und Pliarm. 1847, p. 49. 



1 



"3ooK in. Pabt L 



.."-""Ltz^ ■ - — -jiZ' - . ,s=-^3 St*" trze* : 2nd, where the 

... ■ ._ . ." '" .-ill- z. 'Jir "TT"^^ =*=*3e z I'zs Tezin« oertam 

:=:.: - -"tz r. ._l~ -r L.zsi'^i jjere. to which the 

■ - •. ^•. --■-.. :fiti*-;*. .iiL •i«iorxiWMM have been 

"-■^r:::!, r^m-^n >. These are 



■i=. n. : tn^ xnd -isnallj valine 

r - :iaL7 r inremiittently 

:i. ■;!:; -riiire. TlieT <x?CTir in 
. :^rd -in height. 



.-...« -T-^iir_zd _*r^ r u-ire- Like tme vol- 

^ .— . .- ■ — ~- ..— -.-: .j -"3^. ^^.r-z. -iiier no diiicharge 

J.:- .i.v -. - ._ : - :.-« zz T-in.., nil'- rr.m the crater, and 

^tlt - • • — - ■- "i z. *r:r- ~ ' — >--' : -z^is* joid sometimes 

LiiLL-- ■ ■:_r. ■•-£_ ii -r^ niadtr-i.-r -..Ltnce uid explosion, 

:.■.■-_-• i: : -i - :-i« .•.■:«j.: : -^rVierai Iinndred feet. 

Z':-.- ^ r^.-f . --_ _-- -**!:*: tr*, z.-rr-[-.rv. .n '2.e4e phenomena that 

-ca;- r« ... .-.'-r : •7':»r -viiAii'.ir*. TLt" vHisisi "'l maTsh-gafl and 

.-•rr _ ir- -.ij-." tis. . ir- ' ri-il-iii-. :-iiiiii:vcr^iLviir.^^n. ;ind nitrogen, 

^.::i --.r- -vi-ii ■.;.■. lt^. ""..- .:: -. > -.sJUal" .viL In rhe water oocnr 

i*:- .- T-k.^.- .: rv-i--:- -. -ji«.i::i •*■»::.. :i :.n::i;--ii -fait j:eneTaIlv appears ; 

■ L'.T •• i^r. ?•«•«*. y iLMi:iiLi:i JisewisH :re»iiientiy present. Large 

.•.^•.-!' : -:.:-T. .-r»*r.:i^ r-yji "ii.ae u '::-_• Mirhi '••urhooi.L ha ve been 

-ftti'v.-,. i.iii-L-;, -i-r -c^.TiLcaw -:i'^«.-:ie:vw ...iiPtiese -i' I s^.«mewhat deeper 

-oup.t -.-.lixii .'A . '.iniiir- .aiie». liv-ivv .mns? :nav -.viiaii dt»wu the minor 

■ii'id-5.uiit»aijL-.L syiviui ^i: -iie ziaiumii vor *•:« :;r»>nnd : but ga»-bnbbles 

liivkiii L^i««rar "iir.*:.:- 'iie ^ii»^t t .u'i»i* in-i ■ v leicrees a new series of 

I'htre :in •r .iiui*.- :•. ^i- t 'iiLic "iiLs "vrt.* if mud- volcano is to I h^ 

Tnw.tnL 'J . iitiiii'.tii iuiiii:»?fe '^ ^^r.'icrvss mdemeath. Dr. Danbeny 

» xj'laiiicd :i:tiu ".n Triciiv " v -he sicw .jombmstion of Ijeds of snlphnr. 

Ihv Tt^ucuc . o.!irrviiL-e f :iaphiha ind •*( indammable g;i8 points, m 

■LiK r ■.■!4B*«s tl- :he iiiie!iir^ui»^m*r!it -i iiv-lrnur'^'us fr»jm subterranean 

-' I'hv >eo.'iitL liUsis 'I !iiu«i-^uic:ino pr«.'!?ents itself in true vulcanic 
rejourns, ind is due ru rue '.'S^iire ylL livi water iind steam through lieiis 
m{' lutf or si.^me othtT fnablf kinti ^f r?ck. The mud is kept in 
LdmUitiuu ''v rue riise of steam tkn^vrh it. As it becomes more pastr 
ill id ilio steam meers wirh iTeat^r resistance, large bubbles are f»>rmed 
wlikli : Mirer, and the more liquM mud from Mow oozes out fn~'m the 
vont. l:i '•iiis way. small cones are built up, many of which have 
l-rrfui-t iTirot^ iti'p. In tho < Peyser tractn uf the Yellowstone region, 
L 111' IV arv instructive oxamplos of such active and extinct mud- vents. 
Si»iiio of the extinct cones tliere are not more than a foi>t high, auil 
might Ih) carefully removed an niusoum Hpecimens. 

' Thu "burning hills ** of TiirltcBtnii nn^ n^fornMl to tho Bubtorranean oombnstion oi 
ihU of Jnmsaic Ck»l. J. MuHclikoU»ff. i\V««N« J,ihrh. 1876, p. 516. 



Sect. L § 3.] 8TRU0TURE OF VOLCANOES. 223 

Mud-Yoloanoes oocnr in Iceland, Sicily (Maccalnba), in many dis- 
trictB of northern Italy, at Tamar and Kertoh, at Baku on the Caspian, 
near the month of the Indus, and in other parts of the globe. ^ 

§3. Structure of Volcanoes. 

We haye now to consider the manner in which the various solid 
materials ejected by volcanic action are built up at the sur&ce. This 
inquiry will be restricted here to the phenomena of modem volcanoes, 
including the active and dormant, or recently extinct, phases. Obviously, 
however, in a modem volcano we can study only the upper and external 
portions, the deeper and fundamental parts being still concealed from 
Tiew. But the interior structure has been, in many cases, laid open among 
the volcanic products of ancient vents. As these belong to the architec- 
ture of the terrestrial crust, they are described in Book lY. The student 
18 therefore requested to take the descriptions there given, in connection 
with the foregoing and present sections, as related chapters of the study 
of volcanism. 

Confining attention at present to modem volcanic action, we find that 
the solid materials emitted from the earth's interior are arranged in two 
ibtmct types of structure, according as the eruptions proceed from largo 
central cones or from inconspicuous vents connected with an extensive 
qnrtem of fissures. In the former case, volcanic cones are produced ; in 
the latter, volcanio plateaux or plains. The type of the volcanic cone, or 
ordinary volcano, is now the most abundant and best known. 

i. Volcanic Cones, 

From some weaker point of a fissure, or from a vent opened directly 
^y explosion, volcanic discharges of gases and vapours with their 
liquid and solid accompaniments make thoir way to the surface and 
gradually build up a volcanic hill or mountain. Occasionally, eruptions 
Uve proceeded no further than the first stage of gaseous explosion. A 
cauldron-like cavity has been torn open in the ground, and ejected 
fragments of the solid rocks, through which the explosion has emerged, 
hiTe fallen back into and round the vent. Subsequently, after possible 
mbridence of the fragmentary materials in the vent, and even of the 
Aiee of the orifice, water supplied by rain and filtering from the 
neighbouring ground may partially, or wholly, fill up the cavity, so as 
to produce a lake either with or without a superficial outlet. Under 
fcvourable circumstances, vegetation creeping over bare earth and stone, 
iDay 80 conceal all evidence of the original volcanic action as to make 
the quiet sheet of water look as if it had always been an essential part 

' On mnd-volcanoes, see Bansen, Liebig*$ Annual^ Ixiii. (1847\ p. 1 ; Abich, Mem. 
^«d. 8L Petertburg, 7* ser. t. vi. No. 5, ix. No. 4 ; Daubeny's Volcanoes, pp. 264, 539 
Bairt, Trant. Bombay (hograph, Soo, x. p. 154 ; Roberts, /oum. Roy. Asiatic 8oc. 1850 
^ Vemeuil. Mem. i^tc. Oloh France, iii. (1838). p. 4 ; Stiffe. Q. /. Oeol 8oc xxx. p. 50 
V«m Ldiaunlx, Z, DeuUeh. Geol Qes. xxxi. p. 457 ; Gum1)el, Sitzb. Akatl. Munch. 1879 
^ K. Kallet, Bee. Geol, Surv. India, xi. p. 188. 



224 DTXAMICAL GEOLOGY. [Bo« m. Pabt L 



of the landscape. Exploeion-lakes (Crater-lakes) of this kind oocnr in 
districts of extinct volcanoes, as in the Eifel (maare)^ oential Italy, and 
Auvergne. A remarkable example is supplied by the Lonar Lake in the 
Indian peninsula, half-way between Bombay and Xagi)ur. It lies in the 
midst of the volcanic plateau of the Deccan traps, which extend around 
it for hundreds of miles in nearly fiat beds that slightly dip away from 
the lake. An almost circular depression, rather more than a mile in 
diameter, and from 300 to 400 feet deep, contains at the bottom a 
shallow lake of bitter saline water, depositing crystals of trona (native 
carbonate of soda, the nUrum of the ancients). Except to the north and 
north-east, it is encircled with a raised rim of irregularly piled blocks 
of basalt, identical with that of the beds through which the cavity has 
been opened. The rim never exceeds 100 feet, and is often not more than 
40 or 50 feet in height, and cannot contain a thousandth part of the 
material which once filled the crater. No other evidence of volcanic 
discharge from this vent is to be seen. Some of the contents of the 
cavity may have been ejected in fine particles, which have subsequently 
been removed by denudation; but it seems more probable that the 
existence of the cavity is mainly due to subsidence after the original 
explosion.^ 

In most cases, explosions are accompanied by the expulsion of so 
much solid material that a cone gathers round the point of emission. 
As the cone increases in height, by successive additions of ashes or lava to 
its 8urfiBU)e, these volcanic sheets are laid down upon progressively steeper 
slopes. The inclination of beds of lava, which must have originaUy 
issued in a more or less liquid condition, offered formerly a difficulty 
to observers, and suggested the famous theory'of Elevation- craters (Er- 
hebungtkraiere) of L. von Buch,^ jl^ie de Beaumont,^ and other geologists. 
According to this theory, the conical shape of a volcanic cone arises 
mainly from an upheaval or swelling of the ground, round the vent from 
which the materials are finally expelled. A portion of the earth's crust 
(represented in Fig. 46 as composed of stratified deposits, ah g h) was 
believed to have been pushed up like a huge blister, by forces acting 
from below (at c) until the summit of the dome gave way and volcanic 
materials were emitted. At first these might only partially fill the 
cavity (as at /), but subsequent eniptiouK, if sufficiently copious, would 
cover over the truncated edges of the pre- volcanic rocks (as at g h)^ and 
would 1)0 liable to further upheaval by a renewal of the original upward 
swelling of the site. 

It was a matter of prime importance in the interpretation of volcanic 
action to have this question settled. To Poulett Scrope, Constant 

* ThiB cavity may poeisibly mark one of the vents from whioli the basalt floods istncd. 
On explosion-eratera and lakes, see Scrope's * Volcanoes.* Leooq, *■ Epoqnes geologiques 
de rAureigne,' tome it. ; compare also Vogelsang, * Vnlcane der Eifel/ and in J\i>h<» JoM*, 
1870^ Jip. 190. 326^ ifiO. On Lonar Lake, see ^[ulc^ilmsou, Tranf. fiffJ, Snc, 2nd ter„ v. 
p. M^^wPhdUwtt and Blanford's ' Geology of India,* p. 87!). 

^ ' kiz^z^zxTii., p. 1(>9. 



MK'JiBAIwtt 



Nt ftSi QM, Aanetf it. p. 357. Ann, de$ Miius^ ix. and x. 



axrr. L I &] 



ElEVATJOK-CSATEB THEORY. 



PrfvoBt, and Lyell, belongfl the merit of diBproving the Elevation-crater 
theoiy. Sotop© ahowed ooncluaively that the steep alope of the lava-bede 
of a volcanic cone was original.* Conatant Prfvoet pointed out that there 
w« no more reason why lava should not consolidate on steep slopes 
than that tears or drops of wax should not do so.» Lyell, in Bnoooesive 
editions of his works, and snbseqnontly by an examination of the Canary 
Wands with Hartung, brought forward cc^nt arguments against the 




M.— 5«t)an mntntlT* oT Ihi 



Hmtion-CTster theory.' A oompariaon of Pig. 46 with Fig. 47 will 
•hftwat a glance the diffei-enoe between this theory and the views of 




'olcania structure now universally accepted. The steep declivities on 
»bioh lava can actually consolidate have been referred to on p. 211. 

The cone grows by additions made to its surface during Buccessive 
^nptiooB. Its angle of Riope depends mainly upon the angle of repone 
D Volcanix'fl,' 1625. Qmii. JburTi. Geol. 8oe. xiL p. 326. 



' Pha. IVoiw. 1858, p. 703, 6ce the remarks of Fonqn^ ' Santorin,' pp. 40(M2Z 



226 D7NAMICAL OEOLOOY. [B«w m. Pibt I, 



of the erupted matoriaU, but is apt to be modified by the effect of nin 
and torrents, in sweeping down the loose detritus and excavating ravine* 
on the sides of the cone.^ 

The orator donbtlese owee ite generally circular form to the equal 
ezpansioiL in all directions of the explosive vapunra from below. Di 
some of the mnd-oones already noticed, the crater is not more than a fisw 
inches in diameter and depth. From this minimam, every gradation of 
size may be met with, np to huge precipitous depressions, a mile or 
more in diameter, and several thousand feet in depth. In the crater of 
an active volcano, emitting lava and scorite, like VesnviuB, the walls are 
steep, rugged cliffs of searched and blasted rock— red, yellow, and Uaak. 
Where the material erupted is only loose dust and lapilli, the sidee of 
the crater are slopes, like those of the outside of the cone. 

The crater bottom of an active volcano of the first claas, when 
quiescent, forms a rough plain dotted over with hillocks or oones, from 
many of which steam and hot vapours are ever rising. At night, the 
glowing lava may be seen lying in these vents, or in fissures, at a depth 
of only a few feet Axim the surface. Occasional intermittent eruptions 
take place and miniature cones of slag and scoriae are thrown up. In 
some infltances, as in the vast crater of Guiung Tengger, in Java, the 
crater bottom stretches out into a wide level wasle of volcanic sand, 
driven by the wind into dunes like those of the African deserts. 

A volcano commonly possesses one chief crater, often also many minor 
ones, of varying or of nearly equal size. The volcano of the Isle of 
Bourbon (or Reunion) has three craters.^ Not infrequently craters 
appear suceessively, owing to the blocking up 
of the pipe below. Thus in the accompanying 
plan of the volcanic cone of the island of Vol- 
canello (Fig, 48), one of the Lipari group, the 
volcanic funnel has shifted its position twice, 
so that three craters have successively appeared 
upon the cone, and partially overlap each other. 
It may be from this cause that some volcanic 
mountains are now destitute of craten, or in 
other cases, because the lava hss welled out in 
>bo»LnaihrMiu«MsfveCrBi«n dome form without the production of scorin. 
Mount Ararat, for example, is said to have no 
crater ; but so late as the year 1840 a fissure opened on its side whence 
a considerable eruption took place. 

Though the interior of modern volcanic cones can be at the best 
but very partially examined, the study of the sites of long-extinct cones, 

r'„P",I''*^*'"P?* °^ volcftuio cones, see J. Milne, Geol Mag. 1878, p, 339: 1879, 
p. .m. Vn Iheir donudation, H. J. John gton-La vie. Q. J. Geoi. Sue-, xl. p. lOS. 
17 i j^ reP"iit iDformtttion rFgardiug tliie volcnnip ialnad, see R, von Dnucho, in 
^"Sf^,A.-£f"^'^"''- '875, p. 266, uud in TsclierraakB Jfin. M.itteiV. 1873 (8), 
■"; ,. , ."■'•v?- ^' ""'' '■'8 fo* '!>'« 'nsel He'imjon (Bourbon),' «o, Vieons, 1878. 
»■ T' iS^'P*.'^. K<^'*«'<l"e de la Prwqu-Ile d'A.leii. de VHe de U Beunion. fa.' 
I'liriB, nn, 1878 : niid b\s work, ' Lp» Volcaua,' 18P4. 




Sect. L § 3.] VOLCANIC CONES. 227 



laid bare after denudation, shows that subsidence of the ground has 
commonly taken place at and round a vent. Evidence of sul^idence has 
also been observed at some modem volcanoes (ante, p. 216). Theoreti- 
cally two causes may be assigned for this structure. In the first place, 
the mere piling up of a huge mass of material round a given centre 
tends to press down the rock underneath, as some railway embankments 
may be observed to have done. This pressure must often amount to 
Beveral hundred tons on the square foot. In the second place, the 
expulsion of volcanic material to the surface must leave cavities under- 
neath, into which the overlying crust will naturally gravitate. These 
two causes combined, as suggested by Mr. Mallet, afford a probable 
explanation of the saucer-shaped depressions in which many ancient and 
some modem vents appear to lie.^ 

The following are the more important types of volcanic cones : * — 

1. Cones of Non-voloanio Materials.— These are due to the disoharge of steam 
or other aeriform product through the solid cmst without the emission of any true ashes 
or lava. The materials ejected from the cavity are wholly, or almost wholly, parts of 
the fonoimding rocks through which the volcanic pipe has been drilled. Some of the 
eoDea surrounding the crater-lakes (maare) of the Eifel consist chiefly of fragments of 
the underlying Devonian slates (pp. 187, 198, 236). 

2. Taff-Ck>ne8, Cinder-Cones.— Sucessive eruptions of floe dust and stones, 
often rendered pasty by mixture with the water so copiously condensed during an 
eroption, form a cone in which the materials are solidified by pressure into tuff. Cones 
Dide up only of loose cinders, like Monte Nuovo in the Bay of Baise, often arise on the 
flinks or round the roots of a great volcano, as happens to a small extent on Vesuvius, 
ind on a larger scale upon Etna. They likewise occur by themselves apart from any 
Uva-producing volcano, though usually they afford indications that columns of lava have 
r»en in their funnels, and even now and then that this lava has reached the surface. 

The cones of the Eifel district have long been celebrated for their wonderful perfec- 
tion. Though small in size, they exhibit with singular clearness many of the leading 
features of volcanic structure. Those of Auvergne are likewise exceedingly instructive.* 
The high plateaux of Utah are dotted with hundreds of small volcanic cinder-cones, 
tbc' lingular positions of which, close to the edge of profound river-gorges and on the 
Qpthrow side of faults, have already (p. 191) been noticed. Among the Oarboniferous 
Toloanic rocks of central Scotland the stumps of ancient tuff-conc8, frequently with a 
<*ntial core of basalt, or with dykes and veins of that rock, are of common occurrence.* 

The materials of a tuff-cone are arranged in more or less regularly stratified beds. 



* ^lallet, Q. J. Geol. Soc. xxxiii. p. 740. See also tho account of " Volcanic Necks,** 
in Book IV. Part VII. 

* Von Seebach {Z, Deutsch. Geol. Ges. xviii. 644) distinguished two volcanic types, 
ht, Bedded VoUanoeB (Strato-Vulkane), compose<l of successive sheets of lava and tuffs, 
WHi embracing the great majority of volcanoes. 2nd, Dome VolcatweSy forming hills 
^^-inpcwed of homogeneous protrusions of lava, with little or no accompanying fragmen- 
tary discharges, without craters or chimneys, or at least with only minor examples of 
th4; volcanic features. He believed that the same volcano might at different periods 
'!« its history belong to one or other of these types — the determining cause being tho 
latore of the erupte<l lava, which, in the case of the dome volcanoes, is less fusible and 
Jwwe viscid than in that of tho bedded volcanoes. (See above, under " Lava-cones.*') 

* For Auvergne, see works cited on p. 205. For the Eifel, consult Hibbert, * History 
f'f th*i Extinct Volcanoes of the Basin of Neuwied on the Lower Rhine, Edin. 1832. 
Von Dechen, * Geognostischer Fiihrer zu dcm Laacher See,* Bonn, 1864. * Geognos- 
tucber Fiilirer in das Siebengebirge nm Rhein,* Bonn, 1861. 

* Tratu. Boy. Soc. Edin. xxix.^ p. 455. See poHea, Book IV. Part VII. 

Q 2 



228 DTSAillCAL GEOLOOT. [Bomt m. Paw I. 

On the uuler Biilf, ihfj dip doim the slopea ot Ota Moe at the Bvenge angle oF T^poae, 
whkh may range bttveen SO' and 40*. Fmn the mmmit of the cmler lip th^ 
likewiae dip inwanl lomiTd the cnter-bottotn at nmilar anglee of Inclination (Fig. 50). 
S. Knd-eonaB Teaemble tiiff-«one* in Tona, bnt are nmallj imaller in iiize and lea 
■l«ep. The; are pfodoced b; the baidening of aoeeemite outponringi of mud bom the 
orifice* already dneribed (p. SSI). Li the Kgion of the Lower Indus, where they are 




Fig. M.— TkworibcTnS-coBCa^af AiiTtT|lM,UkxnfrtiiiiUietop(ittluceaeiiidcnlciatPii; Fuhni. 

abnndentlj distribnted ovet an area of 1000 eqaare miles, Bome of them attain a, height 
of 100 feet, with cntera 30 yarda across.' 

4. ItaTa-coneB. — Volcanic cones composed entirely of lava are compatatiTely taw, 
bnt occur in Kme younger tertiaij and modern volcanoos. Foiiqnc draoribet the la™ 






of ISCG cit SBiit)riti as having formed n dome-sliaped elovation, flowing out quietly and 
ratiiilly witlionl pxploeions. AfttT Bereral days, Itowevor, its emission was nccompsnieJ 
with po|iioiw clisrliftrpeB of fragmonlory materials and tlio formation of sercral crateri- 
fiirramnnlbs on (lie t<ip of tlie dome. Where lava posseases extreme Itqaidity. anJ 

' Lyell, ' Prinoipleg,' ii. p. 77, 



Sicr. L f 3.] 



VOLCANIC CONES. 






to littlo or no frftgrneubu? niattar, it muy build np a loigr couo 
jnplea dcaciibed b/ Danu from tlio lluwuii luliiniLi.' Un the » 
SlaoDB Loa (Fig. 51). a flat laTOHMne 13,7liO Ibct above tlic ka, liiia h ciHtLT, 
it* deepeit port is about 8000 feot bruul, witli vvrticul wulla of slrutUiiiU 
l*Ta riling on me aide to a height of 7(t4 feet above tho block lavu-plftin 
of tht) orater-bottom. From the edge* of tbis elevated cauldron, the 
luaantain ilope* ontmiTd at an angle of not more than 6°, until at a level 
of about 10.000 foet lowor, ita suifaoo is indented by tlie vast pit-cmtcr, 
Kilanea, about two milca long, and noiLily a mile broiul. Ho low lire tho 
■nrroauding ilopea that these vast craters hnvu been compuxed to opou 
qimnlM on a hill or moor. The bottom of Kilauoa in a lava-plain, dotted 
vttb Uket of extremely flniil Uva in conataut ubullitiou. The level of 




aro nwk y o. ge 
■«. e h gh o a 
he a uompanyiDg ti 



Hj. t: 



« In Kiluca (Dun). 



ri-iiig aWi: Ihu lower pit (j) p') were found to be 342 feet high, thoeo 
kimdiDg the htsjlicr tcmice (o » n' o') were 650 fett higli. all being iwm- 
I"«ed of innumerable buds of lava, as iu Ldifb of htiatifled rock*. MupIi 
"I rLe hnttom of the lower lava-plain lioa been ctualwl ovt-r by the soUdi- 
Oft.ij.n of the moltin rock. But large area^ wliieh shift thoir position 
fn*D tiMU.- to timt-, rtmaiu in perpetual rapid ebullition. Tho glowing 
M, UB it boils up with a Suidity more like that of water than wlLat ii 
"Winnjiily uliowii by molten rock, surgtB against tho lorroundiug twraco 
■Jit. Larjfo Btgmoula of tho cliffu undermined by the fusion of tlicir 
la«!, lall at iiiti.'r%-ahi into tho flery waves and are eoou melted. Bccvut 



' In WUk«'s Btport of V. S. Exploriny t'^rpcdHiaa, 1838-*i See the wotka cited 



230 



DYNAMICAL GEOLOOY. [BoMt IIL Pakt L 



obeerratioiK by Captua Dnttou poiut to a dimintition of the oetiTitj of thia h 

In Iceland, uid in the Weatem Tenitorica of North Ameriou, low dtmcfl of 1>t> *W>ta 

to mark the veats fioiii which cxteuaivo baeelt'floods htiTe issued. 

Wheie the lava asaumea a more viscid character, dome-shaped eminencea nwy bs 
ptotnided. As the mass iDcreaaes in aize bj the advent of iVeah material iiijeoted ban 
below, the outer layer will be puabed outward, and Bucoeaaive ahelb will in like mumar 
be enlarged a* the eniptiou adranoea. On the oeaiation of diachargea, we maj conoeim 
that a volcanic hill formed in thia way will preaent an onion-like anangement of its 
component aheeta of rock. Hote or leea perfect examplea of thIa atraetaie havo bean 
observed in Bohemia, Aavergne, and the Eifel.' 

5. ConraofTuffaiidliaTft.— Thia ia by far the moat abundant type of voloanie 
atruotuio, and inclndeB the great voloanoea of the globe Beginnug, perfaapa, aa men 
tuff-eonee, these emmenoea have giadoally been bnilt np by aucoeaaiTe on^nringa of 
lava &om diiferent aides, and b/ ahowera of dnat and accoin. At flnt, tha Uva, If tlte 
aidei of the cone are stropg enough to reaiit its preaniie may rue nntil it tnerflowi 





-^.- 




■■ ^ 




1^ * 




^^ 


r/ ^'f^jT^W^^ 


KV^ 



t1g. H.— Flu ot the summit of tha Piak of TenmUft, showing tlie Urge enter tad mlnot ana. 

from ttie crater. Snbeeqnently, as the funnel becomes choked up, and the cona ia 
shattered by repeated eiploaioQS, the lava finds egreea from different flaaorca and 
npeninga on the cone. Aa the mountain increaaes in height, the unmber of lava- 
curronla from its summit will usually decrease. Indeed, the taller a volcanio oooe 
growB, the leas freqaently as a rule does it erupt. The lofty volcanoes of the Andea 
liaTB each seldom been mora than once in eraption daring a century. The peak of 
Teneriffe (Fig. 54) waa three times active during 370 years prior to 1798. The earlier 
efforts of a volcano tend to increase iU height, as well as its breadth ; the later 
eruptions chiefly augment the breadth, and are often apt to diminish the height by 
blowing away the upper pact of the oone. The formation of fissures and the oonseqnent 

' E. Reycr {Jakrh. Geo!. Seiefu. 1879, p. 463J lias oipcrimentnUy imitated the 
prooeas ot eitrusion bv forcing up piaster of Paris tlirough a hole in a board. Vat 
drawings of the Puy de Barcouv and other dome-shaped hills which presumably btm 
had this mode of origin, see Scrope'a 'Geology and Extinct Volcunoea of (jential 
Fmnce.' Refer also to the remarks already made on the liquidity of lava lanU, 
pp. 207-209), and the account of " Vulkanische Kuppen," poitea, p. 238. 



imn. i. f S.] 



VOLCANIC CONHS. 



231 



k network of Uvs-djke*, tend to bind the fitunework of tbe volcano and 
tnngOMn ft againit labaequent eiplosioQs. lu thia yraj, a kind of oaoillatioii ia 
litabliiLed in the fonn of the cone, pcrioda of crater«rnptioiu being anucceded by 
ttben wheD the emiadou lake plaoe only lattjmlly (ante, p. I9G). 

One oonaeqnencc of lateial eniption ia the fonuation of minor parasitic cones on the 
Hanks of the parent volcano (p- 180). Those oo Etna, more than 200 iu number, tun 
rMUjf ndnlatnte valoaoow, Mme of them reaching a height of 700 feet- Aa tho lateral 
«aili cBOoewTely become extinct, tho cones are buried under aheeta of lava and ahowcra 
ei (Ubria thrown ont troiu younger oponings or from the parent cone. 




I KSJ C~3 CZZ] 1133 



».AjiclmtUv«uruukiwwiidMe; 7, CuDM.udCi.Mn, a. Non-yolunlc Itotlu. 

%«il that the original fnnnci U dianavd, and that the eruptions of the i-olcano tako 
!**» faua a newer main vent. Vesavius, for eiample (as shown in Fig. 56), atunda on 
^ "le of a portion of the rira of the wore uuoicnt und much larger vent of Monte 
*»»«. The presort crater ot Etna lies to tho nortli-wi-st of tho furmer vaster crater. 
"" pretty litUe eiample of thU shilling furnishod by Volcanello haa been already 
'Wi<:«d(p,22e> .^ . 

Wlile,lliefefore.a volcano, aud wore particularly one of great aize throwing ou 
•** bmt and fraginentary materials, is liable to contiuoal modiflmticm of its external 



232 



DTNAMICAL GEOIOOT. [B«« in.-PABT I. 



tbnn, M tlie Ttealt <a «iicceiid*<i eraptionB, ita coDtonr ii likewiM OMotllj ezpoaad to 
exl«iiBiTi) allemtion by the effects of oidinvj atmcBpherio erodca, u wdl M from 
the condcDBation of the Tolcuiia Tuponn. Heaxy and mddeD 
floodi, produced by the mpid nmbll eottBeqveiit upon m 
copions digcharge of ateun, mah down the ilopea with andi 
Tcdmne and force u to rut deep galliea in the looas or onlj 
partiallf coiualklated tuffg nnd Mwrin. Ordinuy rain oon- 
tinnea the erosion nntS the onter alopeo, nnleaa occanctuUf 
Teuewed by tteth ahowen of detritiu, aaiQine k ctukKulf fnt- 
rowed aapect, like a half-opened umbrella, the ridgn htiag 
aepusled bj fanoiTi that narrow upwards towaida the nimmit 
□f the cone. The outer declivitiea of Monte Bonuna allbid an 
excellent iUnitratian of this form of rarfiuw, the nnmenHU 
rsTinea on that aide of the mountain presenting inatmcKvo 
(WctionB of the pre-hiatoric Uvaa and tnfb of the earlier aod 
mora important period ia the hiitory of thia Toloano.' Similar 
trenehea have been eroded on Uie aoathani or VeaoTian cida 
of Uie ori^nal cone, bat these bave in great meaanre been 
filled up by the lavaa of the yonnger mountain. The raTinea, 
in &et, fiiim natnral channela for the lara, aa may nnforto- 
nately be aeen round the YeauTiau obaerratory. Thia bidlding 
ia placed on one of tbe ridget between two deep larlnet ; bat 
the laTaratreams of recent yeaie have poored into time raTioee 
on either ride, and are mpUly filling them np. 

Submarine Voloanoea. — It ia not only on the 
sorfaoe of the land that voloanio action BhowB 
itaelf. It takes place likewiae nnder the aea, and 
as the geological records of the earth's past history 
are chiefly marina formationa, the oharaoteristics of 
submarine volcanio action have no small interest fbr 
the geologist. In a few instances, the actual out- 
break of a submarine eruption has been witneaeed. 
Thus, in the early summer of 1783, a volcanic emp- 
-3% " tion took place about thirty miles from Gape Bey- 
||e kjanaea on the west coast of Iceland. An island 
"I ° was built up, ftx)m which fire and smoke oontloned 
||.s to issue, but in leas than a year the waves had 
1^1 washed the looae pnmice away, leaving a submerged 
^|| reef &om five to thirty fathoms below sea-level. 
fe|= About a month after this eruption, the Irightfal 
i^.l outbreak of Skaptar Jiikull, already (p. 207) re- 
^■■^ ferred to, began, the distance of this mountain 
||iE from the submarine vent being nearly 200 miles.* 
|lg A century afterwards, riz., in July 188i, another 
J g^ volcanic island is said to have been thrown up near 
|Sl the Bamo spot, having at first the form of a llat- 
ot| tened cone, but soon yielding to the power of the 
||2 breakers. Again, in the year 1831, a new volcanic 



I 
I-: 
it 
II 

fe 
lit 

m 

I ■■b 

is* 

111 
II' 



Sect, i I 3.] 8UBMABINE VOLCANOES. 233 

inland (Qraham'u Liland, lie Julia) was tlirown up, with abundant diu- 
cbargo of steam and showers of Hcoriro, between Sicily and the coaut 
of Africa. It reached an eitremo height of 200 feet or more above the 
sea-level (800 feet above Bea-bottom) wtli a circumference of if uiilea, 
but on the cessation of the eruptions, vma attacked by the waves and 
soon demolished, leaving only a ehoal to mark itx Bite' In tlio year 
1811, another island wan formed by eubmarine eruption off the coaut 
uf St. Michael's in the Azores (Fig. 57). ConsiHting, like thu Meditei- 
mnean example, of loose cinderB, it rose to a height of about iJOQ feet, 
with a circumference of about a mile, but bubse^uontly disappeared.'-' 
In the year 1796 the island of Johanna Bogoslawa, in Alaska, appeared 




i\ ST.— Sketcb of nbcurir 



*boTe the water, and iu four years had grown iuto a lurge vok-Hnic 
ti«e, the summit of which was 3000 feet above sea-level.^ 

Unfortunately, the phcnoniena of recent volcanic ei-uptions under 
"" eeji are for the most part inaccessible. Hcru and there, as in thu 
"J of Naples, at Etna, among the inlands of the Greek Archipelago, 
*iiJ at Tahiti, elevation of tlie oea-Lcd has taken place, and brought lg 
tie mrface beds of tuff or of lava which have consolidated nnder water. 
T'-'fi VesDvins and Etna began their career aw Kulnnarini,' volcanoes.' 

■ JU Trav. IWti. Couxluiit FnivuBt, A.-h. <le» »:/. -Vcif. ixiv. Mc,». S.^. (Jo,l. 
/'""*'■ U. p. Bi. Mcn.a!li'B 'Vulcani, &v.' i>. 117, wlwHjotbermibniarino truirtiwiB in 
±» o M^tattama uru notlcal. 

, * 1« Bectiu, ■ Geological OlmiTvur," ii. 7U. 



' J- fVbtt, deoJ. 3Iay. vii. p. 323. 

' ***»iMgard«Etn«,'DerAetii»,'ii. p. 827. 



PYKAMICAL QEOLOOY. [Book III. Pabt L 




It will be Koen from tho aooom- 
IHiuying chart (Fig. 58), tlint 




Fig. BK.— Uip of [■nlBllr-folmiMgcd VDktaoor 
r. Ulkn 

tbc dotted Unr nwrtLj 

IbelMbilbointlLui'. 

Ihc islands of Santorin and Tho- 
inwia form Ihe imflubmerged por- 
tioiiB of a ^ruat crater-rim rising 
round a cintcr which dcsccndM 
1271? fi'Ct below Bea-level. The 
lUHteriald of these islands oousist 
of a nucIeuH of marbles and 
schists, nearly buried nnder a 
pile of tiiffs (trass), scorite and 
Eheeta of lava, the bedded cha- 
racter of which IB well shown 
in the accompanying sketch by 
Admiral Spratt (Fig. 59), who, 
with tho late ProfeHaor Edward 
Forbes, ozamined the geology of 
lliiu interesting district in 1841. 
They found sonic of the tafis to 
contain marine Hhells and thiu 
to bear witDeaa to ao elevation 
of the aea-floor since Toloanio 
aotim began. More reoentlr 
the ialMidB have been cftrefolly 
ttodifld by vaiioiu ofaaerron 



Sect. L f 3.] SUBMARINE VOLCANOES. 235 

E. von Fritsch has found recent marine Bliellti in many places np to 
heights of nearly 600 feet above the sea. The etraia containing these 
remains he estimates to be at leaet 100 to 120 metres thick, and he 
remarks that in every case he found them to consist essentially of vol- 
canto debris and to rest upon volcanio rocks. It is evident, therefore, 
that these ahell-bearinf; tafb were originally deposited on the sea-flooi- 
after volcanic action had begun here, and that during later times they 
were upraised, together with the submarino lavas aesociatod with them.' 
Fouque concludes that the volcano formed at one time a large island 
with wooded slopes, and a somewhat civilised human population, culti- 
vating a fertile valley in the Bouth-westem district, and that in pre- 
historic times the tremendous explosion occurred whereby the centre of 
the island was blown out. 

The similarity of the structure of Santorin to that of Sonima and Etna 
is obvious. Volcanic action still continues there, though on a diminished 
ncale. In 1866-67 an eruption took place on Neo Eaimeni, one of the 
later-formed islets in the centre of the old crater, and greatly added to 
its area and height. The recent eruptions of tjantorin, which have been 
■tudied in great detail, are specially interesting from the additional 
information they have supplied as to the nature of volcanio vapours and 
gues. Among these, as already stated (p. 183), free hydrogen plays 
u important part, constitnting, at the foons of discharge, thirty per 
cent, of the whole. By their eruption under water, the mingling of 
tlKw gases with atmospheric air and the combustion of the inflammable 
cuDpounds is there prevented, so that the gasLous dischai'gcs can bo 
collected and analysed. Probably weio 
upwations of this kind more practicable 
■t terrestrial volcanoes, free hydrogen 
»iid its comi)ounds would be more 
abundantly detected than has hitherto 
been possible. 

The numerous volcanoes which dot 
the Pacific Ocean, probably in moEt Cdaeii 
legui their career as submarine \euts 
their eventual appearance as subaerial 
oaaet being mainly due to the accumu 
btion of empted material, but also par- 
tislly, as in the case of Sautoriu, to 
ictnal uphtu-ial of the sea-bottom. Tho i-'k m.— ^■"iejiuii|er.iiT..t>i. i-^u] m-i»i. 
Itmily island of St. Paul (Figs. 60 and 

lllii. iniiy in the Indian Ocean more than 20iJ0 niiku from th..- i.c-aicsl 
Uh'I, is a notable example of the summit of a volcani': inuuniiiiu 

' See Fritwb, Z DtmUek. <hoL G,:'. niii. (1871) pp. li">-21». TIi. <d.«.i ■^.l..|.^;^. 
unl -lnbunW wnitk ii Foaque'* moDograph (alri'tuly cittnl). 'Siint>-riii i-l au Kriiplniir.' 
I'lTji, t1<>. I88l>, when m^oiu Boalywd of rocke, miiiL-tuIii, uurl fWHUu, I'luuiuirjiir. 

I'iiiiii'cniibT of the loealitT *iU be Ibimd. Compare C. Docltvr ou tlit KiLki IttituLt. 
l)'rJ..rh. .ttirf. irbMiMdL Vienna, xxitL p. HI. 




236 DYNAMICAL GEOLOGY. tBooK HI. Pabt I. 



rising to the sea-level in mid-ocean. Its cironlar crater, broken 
down on the north-east side, is filled with water, having a depth of 
30 fathoms.^ 

Eecent observations by K. von Drasche have shown that at Bourbon 
(Reunion), during the early submarine eruptions of that volcano, coarBely 
crystalline rocks (gabbro) were emitted, that these were succeeded by 
andesitio and trachytic lavas : but that when the vent rose above the 
sea, basalts were poured out.^ It is interesting to find that the order of 
appearance of the lavas in a submarine volcano so closely reeemblee that 
generally noticed in terrestrial volcanic districts. Fouqu6 observes that 
at Santorin some of the early submarine lavas are identioal with those 
of later subaerial origin, but that the greater part of them belong to an 
entirely different series, being acid rocks, belonging to the group of 
hornblende-andesites, while the subaerial rocks are augite-andesites. 
The acidity of these lavas has been largely increased by the infoBion 
into them of much silica, chiefiy in the form of opal. They differ much 
in aspect, being sometimes compact, scoriaoeous, hard, like millstcme, 
with perlitic and spherulitic structures, while they frequently present 
the characters of trass impregnated with opal and zeolites. Among the 
fragmontal ejections there occur blocks of schist and granitoid rooks, 
probably representing the materials below the sea-floor through whioh 
the first explosion took place (pp. 187, 198, 227). During the eruption of 
1866, some islets of lava rose above the sea in the middle of the bay, near 
the active vent. The rock in these cases was compact, vitreous, and 
much cracked.^ 

Among submarine volcanic formations, the tuffs differ from those laid 
down ou land chiefly in their organic contents ; but partly also in their 
more distinct and originally less inclined bedding, and in their tendency 
to the admixture of non-volcanic or ordinary mechanical sediment widi 
the volcanic dust and stones. No appreciable difference either in ex- 
ternal aspect or in internal structure seems yet to have been established 
between subaerial and submarine lavas. Some undoubtedly submarine 
lavas are highly scoriaoeous. There is no rettson, indeed, why slaggy 
lava and loose, non-buoj^ant scoriae should not accumulate under the 
pressure of a deep column of the ocean. At the Hawaii Islands, on 25th 
February, 1877, masses of pumice, during a submarine volcanic explosion, 
were ejected to the surface, one of which struck the bottom of a boat 

^ For a general account of the volcanic islands of the ocean, see Darwin's ^ Volcanic 
Islands/ 2nd edit. 1876. For the Philippine volcanoes, see R. von Drasche, TtchermaJ^s 
Mineraloguiche Mittheil. 1876; Souiper's * Die Philippinen und ihre Bewohner,' Wiirz- 
burg, 18(>9. For the Kurile Islands, J. Milne, Geol. Mag, 1879, 1880, 1881 ; Yolcanoett 
of Bay of Bengal (Barren Island, &c.), V. Ball, Geol. Mag. 1879, p. 16; St. Piwil 
(Indian Ocean), C. Velain, Assoc. Fran. 1875, p. 581 ; also his * Description g^logique 
de la Presquilo d'Aden,* &c., 4to, Paris, 1878: and *Les Volcans,* 1884. For Isle of 
Bourbon, see authorities cited on p. 226, and for the Sandwich Islands, the references 
ou p. 192. 

^ Tschermak's MinercUogische Mitthtil. 1876, pp. 42, 157. A similar straotoie oocun 
at Palma. Ck>hen, Neues Jdhrh 1879, p. 482. 

' Fouque, * Santorin/ 



^CT.t fa] VOLCANIC ISLANDS. 237 

■with oonndenble violence and then floated. When we reflect, indeed, 
to what R oondderaHe extent the bottom of the great ocean-baeinB ia 
dotted over with volcanic cones, rising often solitary from profound 
depths, we can believe that a Urge proportion of the actual eruptions in 
ooeanio areas may take place under the sea. The immense abnndance 
and wide diffusion of vokanic detritua (including blocks of pumice) over 




flf. *!.— Vlewof the P«k a( Teiurlffo ind 



the bottom of the Pacific and Atlantic ooeans, even at distances remote 
from land, as made known by the voyage of the Challenger, doubtless 
indicate the prevalence and persifitence of submarine volcanic action, 
even though, at the same time, an extensive diffasion of volcanic debris 
from the islands is admitted to be effected by -winds and ocean-en rrents. 
Tolcanic islands, nnless continually augment«d by renewed eruptions, 
are attacked by the waves and cut down. Graham's Island and the 



Fig. •».— View of «. Pinl Ilium, lodlu Ooein, from lbs e»rt (Cipt. I 
■, Nlne^nln Ruck. ■ itack of hinter rock left bf the ki; fr, cn1nn« lo < 

Clin ninpoHd Df btddcd Tslunlc nulFrilli dtjiplni Uiwird-i thr Mum. uia maca •'rmmi HI luE iiiKuer 
•od (e) by w»w« «ml siihteilil wMt ;/, iBiiihpm point of the Mnnd. 11kf«l"ocii[aitoY loioa rliff. 

other examples above cited show how rapid this disappearance may 
be. The island of Volcano has tho base of its slopes truncated by a line 
of cliff due to marine erosion. The island of Teneriffe ehowa, in the 
same way, that the sea is cutting back the land towards the great cone 
fPig. 61). The island of St. Paul (Figs. 60, 62) brings before us in a 



238 DYNAMICAL GEOLOGY. [Book EL Paot L 



more impressive way the tendency of volcanic islands to be destroyed 
unless replenished by continual additions to their surface. At St. 
Helena lofty cliffs of volcanic rocks 1000 to 2000 feet high bear witness 
to the enormous denudation whereby masses of basalt two or three miles 
long, one or two miles broad, and 1000 to 2000 feet thick, have been 
entirely removed.^ 

ii. Fissure {McLSsive) Eruptions, 

Under the head of massive or homogeneous volcanoes some geologists 
have included a great number of bosses or dome-like projections of onoe- 
melted rock which, in regions of extinct volcanoes, rise oonspicnously 
above the surface without any visible trace of oones or craters of 
fragmentary material. They are usually regarded as protrusions of 
lava, which, like the Puy de Dome in Auvergne, assumed a dome-form 
at the surface without spreading out in sheets over the surrounding 
country, and with no accompanying fragmentary discharges. But the 
mere absence of ashes and scoriae is no proof that these did not once 
exist, or that the present knob or boss of lava may not originally have 
solidified within a cone of tuff which has been subsequently removed in 
denudation. The extent to which the surface of the ground has been 
changed by ordinary atmospheric waste, and the comparative ease with 
which loose volcanic dust and cinders might have been entirely removed, 
require to be considered. Hence, though the ordinary explanation is no 
doubt in some cases correct, it may be doubted whether a large propor- 
tion of the examples cited from the Bhine, Bohemia, Hungary, and 
other regions, ought not rather to be regarded as the remaining roots 
of true volcanic cones, like the " necks " so abundant in the anoient 
volcanic districts of Britain (Book IV. Part VII.). If the tuff of a 
cone, up the funnel of which lava rose and solidified, were swept away, 
we should find a central lava plug or core resembling the volcanic 
" heads " (yulkanische Kuppen) of Germany. Unquestionably, lava has 
in innumerable instances risen in this way within cones of tuff or 
cinders, partially filling them without flowing out into the surrounding 
country.^ 

But while, on either explanation of their origin, these volcanic 
"heads" find their analogues in the emissions of lava in modem 
volcanoes, there are numerous cases in old volcanic areas whero the 
eruptions, so far as can now be judged, were not attended with the 
production of any central dome, cone or crater. In former geological 
ages, and perhaps even in the existing period,^ extensive eruptions of 
lava, without the accompaniment of scoriae, with hardly any fragmentary 

' Darwin, * Volcanic lalanda,* p. 104. For a more detailed account of this island, 
see J. C. Melliss* * St. Helena,' London, 1875. 

« Von Seobach, Z. Deutseli. Geol Qtf. xviii. p. 643. F. von Hochstetter, Neue$ Jahrb, 
1871, p. 469. Reyer, Jahrb. K. K. Geol, Beichmrat^iU, 1878, p. 81 ; 1879, p. 468. 

' oome of the modem lava-floods of Iceland may possibly be examples of the 
structure above described. See W. L. Watts* " Across the Vatna Jokull," Proo, Bov. 
Geog.Soc. 1876. H. Thoroddsen, Oeol Mag. 1880, p. 458; Nature, Oct 1884. QeoL 
Fdren. Stochholm F&rhand. vii. (1884) p. 148. W. G. Lock, Oeol. Mag. 1881, p. 212. 



S«cr. ii. I 3.] FISBUBE-ERUPTIONS. 239 

materlkU, and Trith, at the most, onl; low, dome-Bhaped cones at the 
pointa of emieaion, have taken place over wide areas from scattered 
Tents, along lines or systems of fiseurea. Vast sheets of lava havo 
in this manner been poured out to a depth of many hundred feet, 
completely burying the preTions surface of the land and forming wide 
plains or plateaux. These truly "massive eruptions" have been held 
by Bichthofen * and others to represent the grand fundamental cha- 
racter of vnlcsnism, ordinary volcanic cones being regarded merely as 
parasitic excrescences on the subterranean lava-reservgiix, verj' much in 
the relation of minor cinder cones to their parent volcano.^ 

Though a description of these old fissure or massive -eruptions 
cmght properly to be included in Book IV„ the subject is so closely 
connected with the dynamics of existing active volcanoes that an 




FI«. U.— view tbegn 



Mcouut of the inhject may be given hero. Perhaps the rooDt MtiijH.-udiii 
aumple of this fype of volcanic structure occurs in woHti;ru Nurl 
Amerioa. The extent of country which has been flooded witli litmu 
inOragon, Washington, California, Idaho and Montana lias wA \i-i l"->- 
Mtnntsly anrveyed, but has been estimated to cover alur^iK' 11I>:^^ liia 
^nsoe and Great Britain combined, with a thiclcneai averugiji;' 'i'l'i'i m 
fwliins; in «ome places to 3700 feet.^ The Snake Hivti ]>iuM. ... I...,. 
lH;;.ti3) fonuspartof this lava-flood. Surroundc<l(>ii tin- imiiii .m - ...i 
livl-ftymonntoiiiB, it stretches westward as an app-m.-iifly l«l•lrl.ll.;.-^..^„., 

■ -uid and bare sheets of black basalt. A few BlMrnuii- u."- > 

tli^ lilsin gtHD the hills, are soon swallowed nj' mi-''- •«- 



' IVmu. Aeai. Bet. California, l>-(iS. 

' JVoa Ay. Ptft. 8m. Edin. v. 730. .Vu(». 

■ J. LeOonte. Amtr. Jbnm. Sei. 3rd i«r t 



240 DYNAMICAL GEOLOGY. [Book HI. Paot I 



Eiver, however, flows across it, and has cut out of its lava-beda a Beiiei 
of picturesque gorges and rapids. Looked at from any point on. iti 
surface, it appears as a vast leyel plain like that of a lake-bottom 
though more detailed examination may detect a slope in one or man 
directions, and may thereby obtain evidence as to the sites of the ohie 
openings from which the basalt was poured forth. The unifonnit] 
of surface has been produced either by the lava rolling over a plaii 
or lake-bottom, or by the complete effacement of an original and nndn 
lating contour of the ground under hundreds of feet of volcanic rook ii 
successive sheets. The lava rolling up to the base of the mountains hai 
followed the sinuosities of their margin, as the waters of a lake fbllon 
its promontories and bays. The author crossed the Snake Biver plaii 
in 1879, and likewise rode for many miles along its northern edge. Hi 
found the surface to be everywhere marked with low hummocks oi 
ridges of bare black basalt, the surfaces of which exhibited a reticulatec 
pavement of the ends of columns. In some places, there was a per 
ceptible tendency in these ridges to range themselves in one genera 
north-easterly direction, when they might be likened to a series of long 
low waves, or ground-swells. In many instances the crest of eaci 
ridge hAd cracked open into a long fissure which presented along iti 
walls a series of tolerably symmetrical columns (Fig, 63). That these 
ridges were original undulations of the lava, and had not been prodnoec 
by erosion, was indicated by the fact that the columns were perpendicnla] 
to them, and changed in direction according to the form of the groimc 
which was the original cooling surface of the lava. Though the basali 
was sometimes vesicular, no layers of slag or scoriee were anywhen 
observed, nor did the surfaces of the ridges exhibit any specially scori- 
form character. 

There are no great cones whence this enormous flood of basalt could 
have flowed. It probably escaped from many fissures still concealed 
under the sheets which issued from them, the points of escape being 
marked only by such low domes as could readily bo buried under the 
succeeding eruptions from other vents. That it was not the resnll 
of one sudden outpouring of rock is shown by the distinct bedding oi 
the basalt, which is well marked along the river ravines. It arose from 
what may have been, on the whole, a continuous though locally inter 
mittent welling-out of lava, probably from many fissures extending 
over a wide tract of Western America during a late Tertiary period, if 
indeed, the eruptions did not partly come within the time of the 
human occupation of the continent. The discharge of lava continued 
until the previous topography was buried under some 2000 feet of lava 
only the higher summits still projecting above the volcanic flood.^ At 8 
few points on the plain and on its northern margin, the author observed 
some small cinder cones (Fig. 63). These were evidently formed 
during the closing stages of volcanic action, and may be compared to th< 

' Professor J. LeConte believes that the chief fissures opened in the Cascade anc 
Blue Mountain ranges. Amer. Journ. Set. 3rd series, vii. (1874), p. 168. 



Sect. L f 4.] DISTRIBUTION OF VOLCANOES. 241 



minor oones on a modem voloano, or better, to those on the surface of a 
reoent lava-stream. 

In Europe, during older Tertiary time, similar enormous outpourings 
of basalt covered many hundreds of square miles. The most important of 
these 18 that which occupies a large part of the north-east of Ireland, 
and in diaoonnected areas, extends through the Inner Hebrides and the 
Far5e Islands into Iceland. Throughout that region, the paucity of 
evidence of volcanic vents is truly remarkable. So extensive has been the 
denudation, that the inner structure of the volcanic plateaux has been 
admirably revealed. The ground beneath and around the basalt-sheets 
haa l)een rent into innumerable fissures which have been filled by the 
rise of basalt into them. A vast number of basalt-dykes ranges from the 
Tokanio area eastwards across Scotland and the north of England. To- 
wards the west the molten rock reached the surface and was poured out 
there, while to the eastward it does not appear to have overflowed, or, at 
least, all evidence of the out-flow has been removed in denudation. When 
▼e reflect that this system of dykes can be traced from the Orkney Islands 
soothwaTds into Yorkshire and across Britain from sea to sea, over a 
total area of probably not less than 100,000 square miles, we can in some 
nwasure appreciate the volume of molten basalt which in older Tertiary 
times underlay large tracts of the site of the British Islands, rose up 
in so many thousands of fissures, and poured forth at the surface over so 
wide an area in the north west. 

In Africa, basaltic plateaux cover large tracts of Abyssinia, where by 
the denuding effect of heavy rains they have been carved into picturesque 
iiUs, valleys, and ravines.^ In India, an area of at least 200,000 square 
miles is covered by the singularly horizontal volcanic plateaux of the 
** Becxjan Traps " (lavas and tuffs), which belong to the Cretaceous period 
and attain a thickness of 6000 feet or niore.^ The underlying platform 
of older rock, where it emerges from beneath the edges of the basalt 
table-land, is found to be in many places traversed by dykes ; but no 
cones and craters are anywhere visible. In these, and probably in many 
other examples still undescribed, the formation of great plains or 
plateaux of level sheets of lava is to be explained by " fissure-eruptiouH " 
wther than by the operations of volcanoes of the familiar "cone and 
^ter" type. 

5 i Geographical and geological distribution of 

volcanoes. 

Adequately to trace the distribution of volcanic action over the 
globe, account ought to be taken of donnant and extinct volcanoes, 
^wiHe of the proofs of volcanic outbreaks during earlier geological 
Friodg, When this is done, we learn, on the one hand, that innumerable 
^strictH have been the scene of prolonged volcanic activity, where there 

> Blaiiford'a » AbvBfiinia,' 1870, p. 181. 

' Medlicott and filanfonl, * Geology of ludin, p. 299. 

n 



242 DYNAMICAL GEOLOGY. [Book IH. Past L 



is now no token of undorground commotion, and on the other, that 
volcanic outburats have been apt to take place again and again after 
wide intervals on the same ground, some modem active volcanoes being 
thus the descendants and representatives of older onoa Some of the 
facts regarding former volcanic action have been already stated. Othera 
will be given in Book IV. Part VII. 

Confining attention to vents now active, of which the total number 
may be about 330, the chief facts regarding their distribution over the 
glolx) may Ix) thus summarised. (1.) Volcanoes occur along the margins 
of the ocoan-basins, particularly along linos of dominant mountain- 
ranges, which either form part of the mainland of the continents or 
extend as adjacent lines of islands. The vast hollow of the Pacific ia 
girdled with a wide ring of volcanic foci. (2.) Volcanoes rise, as a 
striking feature, from the submarine ridges that traverse the ocean 
basins. All the oceanic islands are either volcanic or fonned of coral, 
and the scattered coral-islands have in all likelihood been built upon the 
tops of submarine volcanic cones. (3.) Volcanoes are situated close to 
the sea. The only exceptions to this rule are certain vents in Mant- 
cliouria and in the tract lying between Thibet and Siberia ; but of the 
actual nature of these vents very little is yet known. (4.) The dominant 
arrangement of volcanoes is in series along subterranean lines of weak- 
ness, as in the chain of the Andes, the Aleutian Islands and the Malay 
Archipelago. A remarkable zone of volcanic vents girdles the globe 
from Central America eastward by the Azores and Canary Islands to 
the Mediterranean, thence to the Bed Sea, and through tlio chains of 
islands from the south of Asia to New Zealand and the heart of the 
Pacific. (5.) On a smaller scale the linear arrangement gives 2)lace to 
one in groups, as in Italy, Iceland, and the volcanic islands of the great 
oceans. 

In the European area there are six active volcanoes — Vesuvius, Etna, 
Stromboli, Volcano, Santorin, and Nisyros. Asia contains twenty-four, 
Africa ten, North America twenty. Central America twenty-five, and 
South America thirty-seven. By much the largest number, however, occur 
on islands in the ocean. In the Arctic Ocean rises the solitary Jan Mayen. 
On the ridge separating the Arctic and Atlantic basins, the group of 
Ic(>landic volcanoes is found. Along the great central ridge of the 
Atlantic l)ottom, numerous volcanic vents have risen aliove the sur- 
face of the sea — the Azores, (Canary Islands, and the extinct degraded 
volcanoes of St. Helena, Ascension and Tristan d'Acunha. On the 
eastern Tx»rder, lie the volcanic vents of the islands off the African coast, 
and to the west, those of the West India Islands. Still more remarkable 
is tlie ilevelopment of volcanic energy in the Pacific area. From the 
Aleutian Islands southwards, a long line of volcanoes, numbering 
upwards of fifty active vents, extends through Kamtschatka and the 
Kurile Islands to Japan, where another series carries the volcanic l)and 
far south towards the Malay Arehii)elaf^o, which must Ik) regarded as 
the chief centre of the ]>resent volcjinic activity of our planet. In 



Sect. L S 4.] DISTRIBUTION OF VOLCANOES. 243 



Sumatra, JaTa, and adjoining islands, no fewer than fifty active vents 
occur. The chain is continued through New Guinea and the groups of 
idands to New Zealand. Even in the Antarctic regions, Mounts Erehus 
and Terror are cited as active vents ; while in the centre of the Pacific 
Ck»an rise the great lava cones of the Sandwich Islands. In the Indian 
Ocean, the Bed Sea, and off the east coast of Africa a few scattered vents 
appear. 

Beddes the existence of extinct volcanoes which have obviously 
been active in comparatively recent times, the geologist can adduce 
proofs of the former presence of active volcanoes in many countries 
where cones, craters, and all the ordinary aspects of volcanic mountains, 
have long disappeared ; but whore slieots of lava, beds of tuff, dykes, and 
necks representing the sites of volcanic vents havo been recognised 
abundantly (Book IV., Part VII.). These manifestations of volcanic 
action, moreover, have as wide a range in geological time as they havo in 
geographical area. Every great geological period, back at least as far as 
the Lower Cambrian, has had its volcanoes. In Britain, for instance, 
there were active volcanic vents in the Lower Cambrian period when the 
tnffs and diabases of Pembrokeshire were erupted, and still more vigorously 
in the Lower Silurian period, when the lavas and tufk of Snowdon, Aran 
Mowddwy, and Cader Idris were ejected. The Lower Old Bed Sandstone 
ex)oeh was one of prolonged activity in central Scotland. The earlier 
half of the Carboniferous period likewise witnessed the outburst of 
innumerable small volcanoes over the same region. During Permian 
time, a few scattered vents existed in the south-west of Scotland, and in 
the epoch of the New Bed Sandstone some similar points of eruption 
appeared in the south-west of England. The older Tertiary ages were 
distinguished by the outpouring of the enormous basaltic plateaux of 
Antrim and the Inner Hebrides. 

In France and Germany, likewise, Palaeozoic time was marked by the 
emption of many diabase and porphyrite lavas, followed in the Permian 
epoch by a great outburst of porphyries, while on the other hand the 
late Tertiai*y volcanoes of Auvergne, the Eifel, Italy,^ Bohemia, and 
Hungary belong almost to the existing period. Bcccnt research has 
brought to light evidence of a long succession of Tertiary and post- 
tertiary volcanic outbursts in Western America (Nevada, Oregon, Idaho, 
Utah, &o.). Contemporaneous volcanic rocks are associated with 
Palseosoic, Secondary and Tertiary formations in New Zealand, and 
volcanic action there is not yet extinct. 

Thus it can be shown that, within the same comi aratively limited 
ge<^raphical space, volcanic action has been rife at intervals during a 
long succession of geological ages. Even round the sites of still active 
vents, traces of far older eruptions may be detected, as in the case of the 
existing active volcanoes of Iceland, which rise from amid Tertiary lavas 

* For early and classical aoconnts of tho Italian volcanic districts, sco Spallnnzani's 
• Voyoget dans les deux Siciles,* and Breislak's * Voyages riiysiquos ct Lytholopiqucs 
dans la Campanic* Consult also Morcalli's * Yulcani,' &c. 

K 2 



244 DYNAMICAL QEOLOOr. [Book in. Pabt I. 



and tuffe. Volcanic action, whicli now manifests itfielf 80 conBpicnuuHly 
along certain linoR, scorns to liavo continued in tliat linear development 
for protracted pcrioda of time. The actual vcnta have changed, dying 
in one place and Iji'eaking out in anotlier, yet keeping on tlie 'whole 
along tlio Kame tracts. Taking all tho manifestations of volcanic action 
together, both modom and ancient, wo see that the Bubterranean forces 
have oi)erated along great lines in tho earth's crust, and that tho existing 
volcanoes form but a small proportion of the total number of once aotivc 
vents. 

§ 5, OauBOR of Volcanic Action. 

The modus operandi whereby the internal heat of the globe manifests 
itttelf in volcanic action Ih a problem to wliich an yet no satisfactory 
solution h.iB been found. '^Voro tliis action merely an expression of tho 
intensity of tho heat, wo miglit expect it to have manifeHtcd itself in a 
far more powerful manner in former periods, and to exhibit a regularity 
and continuity commensurate with the exceedingly slow diminution of 
tho eartli's temperature. But there is no geological evidence in favonr 
of greater volcanic intensity in ancient than in moro rocout jieriods ; on 
the wmtrary, it may bo doubted whether any of the Paleeozoic volcanoes 
ciiuallcd in magnitude those of Tertiaiy and perhaps even post-Tertiary 
times. On the other hand, no feature of volcanic action Is more 
conspicuous than its Hpaamodio fitfulncss. 

As physical considerations negative the idea of a comparatively thin 
crust, surmounting a molten interior whence volcanic energy might Iw 
derived (anfc, p. 52), geologists have found thcmselveB involvctl in 
great perplexity to explain voloanio phenomena, for tho production of 
which a source of no great depth would seem to bo necessary. Some 
have supiKiBcd the existenco of pools or hikes of liquid lava lying 
beneath the crust, and at an inconsiderablo depth from tbc surface. 
Utliei-s liavc appealed to the influence of the contraction of tlio earth's 
niaMS, assuming tlie contraction to bo now greater in the outer tlian in 
the inner portions, and that the effect of this external contraction must 
1k! to siiueczo out some of tho internal molt«u matter through weak 
parts of the crust.' 

That volcanic action is one of the results of terrestrial contraetum 
can hardly Ije doubted, though wo are atill without satisfactory data 
as til the connection between the cause and tho effect. It will be 
obscrvoil that volcanoes occur chiefly in lines along the cresta of 
teiTcstrial ridges. There is probably, therefore, a connection liotween 
tlic elevation of these ridges and the extravasation of molten rock at the 
Kiii'ra<'c. The fomiation of I'uutiuenta and mountain chains hoa already 
been referred to as probably consequent on the suhflideuoe and readjust- 

' Conlitr, f.r r\ii<::y ■ , ' i i',„tiiif.|inn of only b slogle milUmcln 

alK>ut,|jth..f !■'■;■■ "tit tJ. I.!)P gurfmca Itraenonrti fcrBPO 

craiitionfl, iiitriut lUOO million enbk jBida) ftr cwk 

oriiptioa. iii mnlniotiaiiot the cmt m tbe initial nwr 



Sect. i. S 5.] CAUSES OF VOLCANIC ACTION. 245 



iiient of tho cool outer shell of the planet upon the hotter and more 
rapidly contracting nucleus. Every such movement, by relieving 
pressure on regions below tho axis of elevation, will tend to bring up 
inolt<;n rock nearer the surface, and thus to promote the formation and 
continued activity of volcanoes. 

The fissure-oruptions, wherein lava has risen through innumerable 

rents in tho ground across the whole breadth of a country, and has 

boon ix)ured out at tho surface over areas of many thousand square 

miles, flooding them 'sometimes to a depth of several thousand fe(jt, 

undoubtedly prove that molten rock existed at some depth over a largo 

extent of territory, and that by some means still unknown, it was 

forced out to the surface (<m/^, p. 20G). In investigating this subject, it 

wouLl bo important to discover whether any evidence of great terrestrial 

eniinpling or other movement of the crust can be ascertained to havo 

taken place about the same geological period as a stupendtms out- 

|iouring of lava — whether, for example, the great lava fields of Idaho 

may have had any connection ivith contemporaneous flexure of the 

N'urtli American mountain-system, or whether the basalt-plateaux of 

Antrim, Scotland, Fariie and Iceland may ])ossibly have Ixjen in their 

origin sympathetic with the post-Eocene upheaval of tho Alps and 

uthfr Tertiarj" movements in Europe. 

In the onlinary phase of volcanic action, marked by the copious evo- 
lution of steam and the abundant production of dust, slags and cinders, 
from one or moro local vents, the main proximate cause of volcanic 
excitement is obviously the expansive force exerted by vapours present 
iu the molten magma from whicli lavas proceed. Whether and to wliat 
ext^'nt these vapours are parts of the aboriginal constitution of tho 
lartL's interior, or are derived by descent from the surface, is still an 
niisolve<l pn)blem. The abundant occluHion of hydrogen in meteorites, 
And ihe capacity of many terrestrial sul)st^nccs, notably melted metals, 
to alworb large quantities of gases and vapours without chemie^il com- 
liinatiun, and to emit them on cooling with eruptive phenomena, not 
nulike those of volcanoes, have led some observers to conclude that tho 
Jptttous ejections at volcanic vents arc portions of tlio original ron- 
«litutiou of tho magma of tho glol>e, and that to their CKcjipe the aciivKy 
ofTolcanic vents is due. Prof. Tsc 'her mak in ])arlicular has a<lvo<atiH| 
this uiiinion.^ 

Ou the other hand, since so large a proportion of the vapour «»!' a*! iv « 
volcanoes cousists of steam, it is diilimlt to resist the convii-tion that 1ili^ 
Itti in great measure been supplied by the descent of wati.T tVoin alM.M 
Sraand. The floor of the sea and tlie beds of rivers ami laki> ai< iH 
Inky. TJ '*'" ^iTlking beneath the surface of the laud, |»en.oIai«'.- fl..». i 
'■ . iiitH. and infiltrates through the very porrs of tin i 

' ii^f^CBtcd that if 190 cubic kilometres, of th<3 <;iMibliiHt.i<»ii (•* < a.-* it 
- viiliiiify Rnnually, and to j^ivo off 50 timcH Hh v(,iiiiiii o* ^-.k.^ ^ ir.i'' 

-•' imiintAia 20,000 active volcanoes. Sitz. Ahwl. H7>^#/' h/*/ '•^'. 

! It ; IT (* Ueitrag xor Physik dor Eniptionen,* V miuxw , 1 m 7 / ,■ . j i < • v . - : -> t uj • 



246 DYNAMICAL GEOLOGY. [Book IlL Pabt L 



The presence of nitrogen among the gaseous discharges of voloanoes 
indicates, no doubt, the decomposition of water containing atmospherio 
gases. The abundant volcanic sublimations of chlorides are saoh as 
might probably result from the decomposition of sea-water. 

Whatever may be its source, we cannot doubt that to the enormona 
expansive force of su][)erheated water (or of its component gases, disso- 
ciated by the high temperature), in the molten magma at the roots 
of volcanoes, the explosions of a crater and the subsequent riso of a 
lava column are mainly due. It has been supposed that, some- 
what like the reservoirs in which hot water and steam accumulate 
under geysers, reservoirs of molten rock receive a constant influx of 
water from the surface, which cannot escape by other channels, but is 
absorbed by the internal magma at an enormously high temperature 
and under vast pressure. In the course of time, the materials filling up 
the chimney are unable to withstand the upward expansion of this 
imprisoned vapour or water-substance, so that, after some premonitory 
rumblings, the whole opposing mass is blown out, and the vapour escapes 
in the well-known masses of cloud. Meanwhile, the removal of the over- 
lying column relieves the pressure on the lava underneath, satniated 
with vapours or superheated water. This lava therefore begins to rise in 
the funnel until it forces its way through some weak part of the oone, 
or pours over the top of the crater. After a time, the vapour being 
expended, the energy of the volcano ceases, and there comes a variable 
period of repose, until a renewal of the same phenomena brings on 
another eruption. By such successive paroxysms, the forms of the 
internal reservoirs and tunnels may be changed ; new spaces for the 
accumulation of superheated water being opened, whenoe in time fi^sfa 
volcanic vents issue, while the old ones gradually die out. 

An obvious objection to this explanation is the difficulty of con- 
ceiving that water should descend at all against the expansive force 
witbin. But Daubree's experiments have shown that, owing to 
capillarity, water may permeate rocks against a high counter-preasure 
of steam on the further side, and that so long as the water is supplied, 
whether by minute fissures or through pores of the rocks, it may, undei 
pressure of its own sux)erincumbent column, make its way into highl]| 
heated regions.^ Exj>erience in deep mines, however, rather goea tc 
show that the permeation of wator through the pores of rocks getf 
feebler as we descend. 

Reference may be made here to a theory of volcanic action in whicl 
the influence of terrestrial contraction as the grand source of volcanic 
energy was insisted upon by the late IMr. Mallet.^ He maintained that al 
the pre8(5iit manifestations of hypogono action are due directly to thi 
more rapid contraction of the hotter iuterniil mass of the earth and thi 

' Daubrco, *Geologio Expcrimentale,' p. 274. Soo also Tsclicrmak, as cited oi 
previous pa^. Royer, * Boitnig zur Physik der Eniptioncu,* § i. 

' Phil. Tram.y 1873. Seo ulso Daubree's experiincntul determiuaUon of tb 
(luuutity of boat evolved by the iuioruol crushing of rocks. * Geologic Ezpdrinientale, 
p. 148. 



Sect. L { 6.J CAUSES OF VOLCANIC ACTION. 247 



ooMeqnent crushing in of the outer cooler shell. He pointed to the 
admitted di£Bcalties in the way of connecting volcanic phenomena with 
the existence of internal lakes of liquid matter, or of a central ocean of 
molten rock. Observations made by him, on the effects of the earth- 
quake Bhocks accompanying the volcanic eruptions of Vesuvius and of 
Etna, showed that the focus of disturbance could not be more than a 
few miles deep ; that, in relation to the general mass of the globe, it 
was quite superficial, and could not possibly have lain under a crust of 
800 miles or upwards in thickness. The occurrence of volcanoes in 
lines, and especially along some of the great mountain-chains of the 
planet, was likewise dwelt upon by him as a fact not satisfactorily oxpli- 
caWe on any previous hypothesis of volcanic energy. But ho contended 
that all these difficulties disappear when once the simple idea of cooling 
and eontracticm is adequately realised. " The secular cooling of the 
globe,** ho remarks, "is always going on, though in a very slowly 
descending ratio. Contraction is therefore constantly providing a store 
of energy to bo expended in crushing parts of the crust, and through 
that providing for the volcanic heat. But the crushing itself does not 
take place with uniformity ; it necessarily acts per saltum after accumu- 
lated pressure has reached the necessary amount at a given point, where 
some of the pressed mass, unequally pressed as we must assume it, gives 
way, and is succeeded perhaps by a time of repose, or by the transfer of 
the crashing action elsewhere to some weaker point. Hence, though 
the magazine of volcanic energy is being constantly and steadily 
replenished by secular cooling, the effects are intermittent." He offered 
an experimental proof of the sufficiency of the st4»re of heat prmhu*cd ]>y 
this internal crushing to cause all the phenomena of existing volcanoes. ^ 
The slight comparative depth of the volcanic foci, their linear urrAuge- 
'"Wnt, and their occurrence along lines of dominant elevation become, ho 
contended, intelligible under this hypothesis. For since the crushing in 
®^ the crust may occur at any depth, the volcanic sources may vary in 
*J^th indefinitely; and as the crushing will take place chiefly along 
^es of weakness in the crust, it is precisely in such lines that crumpled 
'^^^Imtain-ridges and volcanic funnels should appear Moreover, by this 
^^{)lanation its author sought to harmonise the discordant observations 
'^.^arding variations in the rate of increase of temperature downward 
'^thin the earth, which have ali*eady been cited and referred to unequal 
^^ductivity in the crust (p. 49). He pointed out that in some parts of 
H« crust the crushing must be much greater than in other parts ; and 
^i^ice the heat ** is directly proportionate to the local tiingential pressure 
"^kich produces the crushing and the resistance thereto," it may vary 
'^definit^^lv up to actual fusion. So long as tlu; crnshod rock remains 

* Tlio elaborate and careful oxix>riineiital reswirches of tliia observer will reward 
HUcBtivc pcrnsal. Mallet estimates from experiment the amount of lieat j^iven out by 
the cruahiuf? of (lifferent rocks (Byeuite, granite, sandstone, slate, limestone), and con- 
cludes tbat a cubic mile of the crust taken at the mean density would, if crushed into 

powder, give out heat enough to melt nearly IIJ cubic miles of similar rock, assuminfr 

tho mcltSig point to be 2000° Fabr. 



248 DYNAMICAL GEOLOGY. [Book HI. Pabt I. 



out of reach of a sufficiont acccsH of subtciTanean water, there would, of 
course, be no diBturbanco. But if, through the weaker part«, water 
enougli Hhouhl descend and be al)8orl)ed by the intensely hot crushed 
mass, it would be raised to a very high temi)erature, and, on sufficieut 
diminution of pressure, would iiash into steam and produce the com- 
motion of a volcanic eruption. 

This ingenious tlieory requires the operation of sudden and violent 
movements, or at least that the heat generated by the crushing should 
bo more than can be immediately ctjnducted away through the cruai 
"Were tlie crushing slow and oqualde, the lieat developed by it might be 
so tranquilly dissipated that the temperature of the crust would not be 
sensibly affected in the process, or not to such an extent as to cause any 
appreciable molecular rearrangement of the particles of the rocks. But 
an amount of internal crushing insufficient to generate volcanic action 
may have been accompanied by such an elevation of temperature as to 
induce important changes in the structure of rocks, such as are embraced 
under the term " metamorphio." 

There is, indeed, strong evidence that, among the consequences 
iirising fi*om the secular contraction of the globe, masses of sedimentary 
stratii, many thousands of feet in thickness, have been crumpled and 
crushed, and that the crumpling has often been accompanied by such an 
amount of heat and evolution of chemical activity as to produce an 
interchange and i-earrangement of the elements of the rocks, — this 
change sometimes advancing to the point of actual fusion. (See posUa^ 
p. 274, and IV^ok IV. Part VIII.) There is reason to believe that some 
at least of these periods of intense terrestrial disturbance have been 
followed by periods of prolonged volcanic action in the disturbed areas. 
jVlr. ]\lallet*s theory is thus, to some extent, supported by independent 
geological testimony. The existence, however, of large reservoirs of 
fused rock, at a comparatively small depth beneath the surface, may be 
conceived as probable, ajiart from the effects of crushing. The connec- 
tion of volcanoes with lines of elevation, and consequent weakness in 
the earth's crust, is what might have been anticipated on the view that 
the nucleus, though practically solid, is at such a temperature and 
pressure that any diminution of the pressure, by corrugation of the 
crust or otherwise, will cause the subjacent portion of the nucleus to 
melt. Ahmg lines of elevation the pressure is relieved, and consequent 
molting may take place. On these lines of weakness and fracture, 
therefore, the conditions for volcanic excitement may l^e conceived to be 
Ijest developed. Water, able soonest to roach there the intensely 
heated mat<>rials underneath the crust, may give rise to volcanic 
oxplosioiis. 'i'he periodicity of eruptions may thus depend upon the 
longth of time required for the storing up of sufficient steam, and on 
the amount of resistance in the crust to be overcome. In some 
volcanoes, the inten-als of activity, like those of many geysers, return 
with considerable regularity. In other cases, the shattering of the 
crust, or the upwelling of vast masses of lava, or tho closing of 



fiiBCT. L J 6.] CAUSES OF VOLCANIC ACTION, 249 



tfabterranean passagos for the descending water, or other causes may 
vary the oonditions so much, from time to time, that the eruptions 
follow each other at Tery unequal periods, and with very discrepant 
energy. Each great outburst exhausts for a while the vigour of the 
volcano, and an interval is needed for the renewed accumulation of 
vapour. 

But beside the mechanism by which volcanic eruptions are j)roduccd, 

a Turther problem is presented by the varieties of materials ejected, and 

tho differences which these exhibit at neighbouring vents, and even 

BometimeB at successive eruptions from the same vent. It is common 

to find that tho earlier lavas of a volcano have been acid (trachytes, 

lipaiites, obsidians, drc), while the later are basic (andesites, basalts, 

•fec.^. Bichthofen has deduced from observations in Europe and North 

America a general order of volcanic succession which has been well 

sustained by subsequent investigation. He states that volcanic rocks 

»Mty be arranged in five great groups, and that all over the world 

^Cico groups have appeared in the following sequence. 1. Propylite ; 

2- i^ndesite; 3. Trachyte; 4. Rhyolite; 5. Basalt.^ The sequence is 

*^ldom or never complete in any one locality; sometimes only one 

**^oinber of the series may be found, but when two or more occur they 

*^k:e, it is affirmed, this order, basalt being everywhere the latest of 

*^o series. Instances have been noticed, of apparent or real excep- 

*^Oii8 to Bichthofen's law. But continued study of the great volcanic 

P*^teaux of Western America has supplied many new examples of its 

*^ Implication.*'* 

Reference has already (p. 69) been made to the speculation of 

r-^trocher as to the existence within the crust of an upper siliceous 

^^er with a mean of 71 per cent, of silica, and a lower basic layer with 

^ ■ -N)ut 51 per cent, of silica. Bunsen also came to the conclusion that 

^^^^Icanic rocks are mixtures of two original normal magmjis — tho 

^^^rmal trachytic (with a mean of 76 • 67 silica), and the normal pyroxenic 

V Xx'ith a mean of 47 • 48 silica). The varying proportions in which these 

"^Vo original magmas have been combined are, in Bunsen*s view, the 

^^use of the differences of volcanic rocks. We may conceive these 

^"Vro layers to be superposed upon each other, according to relative 

^^fnsities, and the composition of the last erupted at the surface to 

^^pend upon the depth from which it lias been derived.^ The earlier 

^ explosions of a volcano may be supposed to take place usually from the 

^ pper lighter and more siliceous layer, and the lavas ejected should be 

^vnsequently acid, as in fact they usually are, while the later eruptions, 

"^^aching down to deej^er and heavier zones of the magma, would bring 

' ** The Natural System of Volcanic Rocks," F. Richthofen, Calijorn. Acad. Sci. 1868. 

* See in particular Captain Duttou*8 valuable Report on the Geology of the High 
M*lateau of Utah, Washington, 1880, p. 64. 

» See B. Bunsen, Pong. Ann. Ixiii. (1851) p. 20i; Sartorius von Waltershausen, 
• BicilicQ nnd Island,' p. 410 ; Royer, * Reitrng zur Thysik dcr Emptioncn,' iii. Scropo 
\iim1 Jong before suggcetcd u cla»Jsilicationof volcanic rockd into Trachyte, Groystone, and 
Bjuult, Jouru, Science, xxi. 



250 DYNAMICAL QEOLOQT. [Book UI. 



up such basic lavas as basalt. Certainly the general similarity of the 
volcanic rocks all over the globe would appear to prove that there must 
be considerable uniformity of composition in the zones of intensely hot 
material from which volcanic rocks are derived, and the general order 
of succession in the ap|)earance of lavas, shows that some arrangement 
in relation to density probably exists within the crust.* 

Many difficulties, however, remain yet to be explained before our 
knowledge of volcanic action can be regarded as more than mdimentaiy. 
For example, why should two adjoining vents, like Manna Loa and 
Kilauoa, have their lava column at such widely different levels as to 
show that there can be no real connection between them ? Why should 
two neighbouring vents sometimes eject, the one acid, the other basic 
lavas ? Why should even the same vent occasionally exhibit an alterna- 
tion of acid and basic eruptions? To these and other questions in the 
mechanism of volcanoes no satisfactory answers have yet been given. 
In Book IV., Part VII., a description is given of the part volcanic rocks 
have played in building up what we see of the earth's crust, and the 
student will there find other illustrations of facts and deductions which 
have been given in the previous pages. 

Seotionii. Earthquakes.^ 

By the more delicate methods of observation which have been invented 
in recent years, it has been ascertained that the ground beneath our feet 
is apparently everywhere subject to continual slight tremors and to 
minute pulsations of longer duration. The old expression " terra fimia " 
is not only not strictly true, but in the light of modem research seems 
singularly inappropriate. Rapid changes of temi>erature and atmos- 
l)heric pressure, the fall of a shower of rain, the patter of birds' feet, 

^ In the memoir by Captain Dotton, cited in a previous note, the hypothesis is main- 
tained that the order of appearance of the lavas is determined by their relative density 
and fusibility, the most basic and heaviest, though most easily fused, requiring the 
higliest temperature to diminish their density to such an extent as to permit them to be 
erupted. 

^ On the phenomena of earthquakes consult Mallet, Brit. A»8oe. 1847, part iL p. 30 ; 
1850, p. 1 ; 1851, p. 272 ; 1852, p. 1 ; 1858, p. 1 : 1861, p. 201. • The Great Neapolitan 
Earthquake of 1857,' 2 vols., 1862. D. Milne, EcUn, Neto rhil Joum. xxxi.-xxxvi. A 
Perrey, Mem. Couronn. Bruxelles, xviii. (1844) Compt^ rendus, lii. p. 146. Otto Volger, 
* I'litersuchungen uber die Phiinomene aer Erdbeben in der Schwoiz,' Gotha, 1857-8; 
Z Deutsch. Oeol. Ges. xiii. p. 667. R. Falb, * Grundziige einer Theorie der Erdbeben 
UD(1 Vulkanonsausbriiche,' Uraz, 1871 ; * Gedankcn und Studien iiber den Yidkanismas, 
&(\,' 1874. Pfaff, * Alljiijcincino Geologic als exacto Wissenschaft,* Txjipzijj, 1873, p. 224. 
Kcoonls of f>l)served oarthqiuikcs will bo found in the memoirs of Mallet and Perrey; 
also in jjapera by Fuchs in NeiKjs Jahrb. 1865-1871, and in Ttiohormak's Minerahtij, 
Mitthcihnufen. \>^^'^ and subsequent years. Schmidt, * IStudien iiber Erdbeben,' 2ud edit. 
1S7(» ; • Stiulicii iiber Vulkano und Erdbeben,' 1881 ; Dieffenbach, NeuiS Jahrb. 1872. 
p. 1.j5. (i. Merealli, in his * Vulcani e Fenomeni Vulcanici in Italia' (1883), g^vosan 
aecoiint of the Italian earthquakes from 1450 b.o. to 1881 a.d. ; he separately describes 
the great Itjcliian earlht|uako of 1883 : * L* Isola d' Ischia,* Milan, 1884. Much interesting 
information will be found in the Transactions of the Seismological Society of Japan — 
a society instituted in the year 1880 for the investigation of earthquake pheunmena 
especially in Jajum, where they are of frequent occurrence. Other pejiers aro quoted 
in the fulluwini^ pages. 



Part I. Sect. iL] EARTHQUAKES, 251 



and Btill more the tread of larger animals, produce tremors of the 
ground which, thongh exceedingly minute, are capable of being made 
dearly audible by means of the microphone and visible by means of the 
galTanometer. Some tremors of varying intensity and apparently of 
irregular occnrrenoe, may be due to minute movements or displacements 
in the crust of the earth. Less easily traceable are the slow pulsations 
of the crust which in many cases are periodic, and may depend on such 
eausos as the diurnal oscillation of the thermal or barometric conditions 
of the atmosphere, the rise and fall of the tides, &c.^ So numerous and 
well marked are these tremors and pulsations, that the delicate observa- 
tions which were set on foot to determine the lunar disturbance of 
gravity had to be abandoned, for it was found that the minute movements 
Bought for were wholly eclipsed by these earth tremors.^ 

The term Earthquake denotes any natural subterranean concussicjn, 

varying from such slight tremors as to be hardly perceptible up to 

severe shocks, by which houses are levelled, rocks dislocated, landslips 

precipitated, and many human lives destroyed. The phenomena are 

analogous to the shock communicated to the ground by explosions of 

mines or powder-works. They may be most intelligibly considered as 

wave-like undulations propagated through the solid crust of the earth. 

In Ur. Mallet's language, an earthquake may be defined as " the transit 

of a wave of elastic compression, or of a succession of these, in parallel 

or intersecting lines through the solid substance and surface of the 

distorbed country." The passage of this wave of shock constitutes the 

real earthquake. 

Besides the wave of shock transmitted through the solid crust, waves 
are also propagated through the air, and, where the site of the impulse 
is not too remote, through the ocean. Earthquakes originating under 
the Boa are specially destructive in their effects. They illustrate well 
the three kinds of waves associated with the progress of an earthquake. 
These are, 1st, The true earth-wave through the earth's crust ; 2nd, a 
wave propagated through the air to which the characteristic sounds 
of rolling waggons, distant thunder, belloAving oxen, &c., are duo ; 
Srd, Two sea- waves, one of which travels on the back of the earth- wave 
wd reaches the land with it, producing no sensible effect on shore ; the 
other an enormous low swell, caused by the first sudden blow of the 
«wth-wave, but travelling at a much slower rate, and reaching land often 
several hours after the earthquake has arrived. 

V e 1 o c i t y. — Experiments have been made to determine the velocity 
of tlie earth-wave, and itfl variation with the nature of the material 

' Plantainour, Archives S^'Jenfvs Phys. Nat. (iciicva, ii. p. G41 ; v. p. 97 ; viL p. 601 ; 
^ii.,l». .5r»l ; X. p. 610; xii. (1884) p. 388. G. II. Darwin, Brit Aasitc. 1882, p. D5. In 
^^^ imper Prof. Darwin «li»oii8scs the amount ol' disiurbanco of the vertical near tho 
**»tg of contineDts, cunacd by the rise and fall of tho tide. J. Milne, Trans. Seismo* 
^<oI Sijc. Japan, vi. (1883) p. 1. 

* For a Dotico of proviouB rcHcarches in this subject eee Prof. Darwin's Rojiort, 
^^ Atioe, 1882, p. 90; Prof. J. Milne. Geol. Mag. 1882, p. 482 ; Nature, xxvi. p. 125. 
^ennnieroas observations ina<lo by Rossi in Italy aro summariso-i by G. Mcrcalli in hid 
*wk cited above. 



. 1 r 'Book IIL 

. _.-- :-_-!-: •■;•.• shock 
1 -1 •.-.—"-„-:*: the rate 

:_. ■ __li - ■ .trTLiinako 

-r . ^ -ir^.A "_ivv siiowa 

. :-. ~ '_? z. ri- '.'.tiabrisin 

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Past L Sect, ii.] 



EARTHQUAKES. 



253 



did not diaappear dnring the earthquake of 1662 were built npoD solid 
white limostone, while the parts built on sand were shaken to pieces.^ 

It has been observed that an earthquake shock will pass under a 
limited area without disturbing it, while tho rt'gion all around has boon 
ftfiected, AB if there were some suporGoial stratum protected from the 
earth-wave. Humboldt oitod a case where miners were driven up from 
below-ground by earthquake shocks not perceptible at tlio surface, and 
on the other hand, an instance where they experienced no sensation of 
an earthquake which shook the surface with considerable violence.'' 
Such &ct8 liring impressively before the mind tho extent to which tho 
oonne of the earth-wave must bo modified by geological Btnicturc. In 
aome instances, tho shock extends outwards from a common centre, 
HO that a sories of conecntrio ciroles may be drawn round the focus, caoh 
of which will denote a certain approximately uniform intensity of shock 
("ooseianuc lines" of Mallet), this intensity, of coutro, diminishing 
with distance from the focus. Tho Calabriun earthquake of 1857 and 




Hg. ai^-nu at Port Royol, Junalu, slw» iiic tliF Fffwti uf ihc t:«nbquake o[ imi ( B.)' 
F(;h(liaaa<iribetv<rn liulU un llinolonf and IpH ttaudlriK itt^ the ranhqaakr?; on, L.tlif bnnntiij 
>< IW (own prioi lu tbe nrtbqiuhe : N N. Ilrniiid |til>i''d by tho iLrdttlng o[ sanr] up to the f tid of In-'t 
■stiBji ILH, AcIdWopi from Ihc sum'' moseilurlnp; the flrnlqmrtwijtthe prennilrcnliirr- 

ft«t of Central Europe in 1872, may lie taken in illustration of this 
wntral typo. In other cases, however, tho carthiiHako travels obiufly 
•kiDg a certain band or Kono (particularly along tho flanks of a 
"Wimtain-ehain) without advancing far from it laterally. This type of 
liwar earthquake is exemplifioil by the frequent shocks which traverse 
Chili, Peru and Ecuador, lx.'tween tho line of the Andes aiid tlio Pacific 

Extent of CO un try a f f ected.^The area shaken by an earth- 
•pake varies with tho intensity of the shock, from a more local tract 
there ft slight tremor lias lieeu experienced, up to such catastrnphes as 
thituf Lisbon in 1765, which, l)eBides convulsing tho Portuguese cuasts, 

' Tbe oppoeitc tOect Iihs been observed on the isknd of lacliia, tlio hoQECS built on 
W nUoil genemUr baying lufforcd muoli lesa tban Ibe otbcra. 

' 'CiwmiM,' Art EaTittquakts. 

' FocalUtorPeruTiancarthqmutesfmm a.d. 1570 to 1875, boo OeograiJi. Miig. It. 
{\tn) p. 806. The ewthnoako of 9 May, 1877, at InuLiine, iiml its oci'un-ivitve nrw 
teenlitd by B. Geinitz, ffora Ael. Ai-- Cai. T^opold. Car., si. (IS78) vi>- 383^44. 



254 



DYNAMICAL GEOLOGY. 



[BoME m. 



extended into the north of Africa on the one liand and to Scandinavu 
on the other, and was even felt as far as the oast of North America. 
Hnmholdt computed that the aroa shaken by this great earthquake was 
four times greater than that of the whole of Europe. The South 
AnAiioan earthquakes are remarkable for the great distanceB to whiph 
their ofTocts extend in a linear direction. Thus the strip of oonatry in 




P-, 1 


s£% - X 


^ 


E-l^ 


m 


mm 


^m 


^^t^k'T"^ 








hlchUu" 


' iMtk of Mn»D«~ h» bMn In tb* 


rr™. (Altar M.nrt.) 



n ■hown b; the I 



Peru and Ecuador eoveroly shaken by the earthquake of 1808, had a 
length of 2000 miles. 

Depth of so 11 re e. — According to ]t[allet's obterrations, over tbe 
centre of origin the shock vi felt as a vortical up-and-down movement 
{Seismic vertical) i while, receding from this centre in any direction, 
it is felt as an undnlatory movement, and comes up more and mon 
obliquely. The angle of emergence, as he termed it, was obtained 1^ 




him by taking the mean of observations of the rents and displaoementi 
of walls and buildings. In Fig. 65, for example, the wall there repre- 
sented has been rent by an earthquake which emerged to the soifaou 
iu the patli marked by the arrow. 

By observations of tliis nature. Mallet estimated the approximate 
depth of origin of an earthquake. Let Fig. 06, for example, repneent 
a portion of the earth's crimt in which at n nn earthquake arises. The 



Paot L Sect, iij EARTHQ UAKE8. 255 



wave of shook will travel outwards in successive spherical shells. At 
tlie point e it will bo felt as a vertical movement, and loose objects, 
lach as paving-stonos, may be jerked up into the air, and descend 
bottom uppermost on their previous sites. At c2, however, the wave 
will emerge at a lower angle, and will give rise to an undulation of 
the ground, and tho oscillation of objects projecting above the surface. 
In Tent buildingSy the fissures will be on the whole perpendicular to 
the path of emergence. By a series of obHorvations made at dififerent 
points, as at ^ and /, a number of angles are obtained, and the point 
where the various lines cut the vei*tical (a) will mark the area of 
origin of the shock. By this means, Mallet computed that the depth 
at which the impulse of the Calabrian earthquake of 1857 was given 
was about five miles. As the general result of his inquiries, he concluded 
that, on the whole, the origin of earthquakes must be sought in com- 
paratively superficial parts of the crust, probably never exceeding a depth 
of 30 geographical miles. Following the same method. Von Seel)ach 
calculated that the earthquake which aflfectod Central Europe in 1872 
originated at a depth of 9*6 geographical miles ; that of Belliuio in 
the same year was estimated by Hofer to have had its source rather 
more than 4 miles deep ; while that of Herzogenrath in 1 873 was placed 
bj Von Lasaulx at a depth of about 14^ miles, and that of 1877 in the 
same region at about 14 miles.^ 

Geological Effeets. — These are dependent not only on the 
strength of tho concufision but on the structure of tho ground, and on 
the site of the disturbance, whether underneath land or sea. They 
include changes 8uperinduce<l on the surface of the land, on terrestrial 
^^ oceanic waters, and on the relative levels of land and sea. 

1. Effects upon the soil and general surface of a 
country. — The earth-wave or wave of shock underneath a countrj' 
^ traverse a wide region and aifect it violently at the time, without 
leaving permanent traces of its passage. Blocks of rock, however, 
already disengaged from their parent masses, may be rolled down 
into the valleyB below. Landslips are produced, which may give rise 
^ oonsiderable subsequent changes of drainage. In some instancufl, 
*he Burfeoes of solid rocks are shattered as if by gunpowder, as 
^^ p»irticularly noticed to have taken place among the Primary rocks 
"» tho Concepcion earthquake of 1835.2 It has often been observed 
*^ that the soil is rent by fissures which vary in size from mere 
^^'^cks, like those due to desiccation, up to deep an<l wide chasms. Per- 
'^^^'lent modifications of the landscape may thus be produced. Trees 
^^ thrown down, and buried, wholly or in part, in the rents. These 
"*P«rficial eflfects may, indeed, be soon eifaccd by the levelling i)ower of 



^ ' Ree papera by Hofer and A. von Lasaulx, citc<l on p. 2,52. The method adopted by 
•■•l^^tdoea not opivar to be ai)plicablo in Japan, whore e«rth(iuakes are abundant, and 
where accorate moaflurementa are beinj^ made of the phenomena. We may hope that 
"y a poutiiiuation of th(t obsiTvalions now ko carefully eonducte<l there, a thoroupldy 
■•*»»factor\- Rolution of the problems of earthquake-physioH will l)e obtaincMl. 
' Darwin, *.Tonni!il of Hi'siarehifl/ 1815, p. 30:i. 



256 DYNAMICAL GEOLOGY. [Book HI. 



the atmosphere. Where, however, the ohasms are wide and deep 
enough to intercept rivulets, or to servo as channels for heavy rain- 
torrents, they are sometimes further excavated, so as to become 
gradually enlarged into ravines and valleys, as has happened in the case 
of rents caused by the earthquakes of 1811-12, in the Mississippi 
viillcy. As a rule, each rent is only a few yards long. Sometimes it 
mjiy extend for half a mile or even more. In the earthquake which 
shook the South Island of New Zealand in 1848, a fissure was formed, 
averaging 18 inches in width and traceable for a distance of 60 miles 
parallel to tlie axis of the adjacent mountain-cliain. The subsequent 
earthquake of 1855, in the same region, gave rise to a fracture which 
could be traced along the base of a line of cliflf for a distance of about 
90 miles. Dr. Oldham has described a remarkable series of fissurings 
which ran parallel with the river of Calhar, Eastern British India, 
varying with it to every point of the comjmss and traceable for 100 
miles. ^ 

Kemarkable circular cavities have been noticed in Calabria and 
elsewhere, formed in the ground during the passage of the earth-wave. 
In many cases, these holes serve as funnels of escape for an abundant 
discharge of water, so that when the disturbance ceases they appear as 
pools. Tliey are believed to be caused by the sudden collapse of 
subtenaiiean water-channels and the consequent forcible ejection of the 
water to the surface. 

2. Effects upon terrestrial waters.^ — Springs are tem- 
porarily aflfected by earthquake movements, becoming greater or smaller 
in volume, sometimes muddy or discoloured, and sometimes increasing 
in temperature. Brooks and rivers have been observed to flow with an 
interrupted course, increasing or diminisliing in size, stopping in their 
flow so as to leave their channels dry, and then rolling forward with 
increased rapidity. Lakes are still more sensitive. Their waters 
occasionally rise and fall for several hours, even at a distance of many 
hundred miles from the centre of disturbance. Thus, on the day of the 
groat LislKjn earthquake, many of the lakes of central and north-western 
Europe were so afifected as to maintain a succession of waves rising to a 
height of 2 or 3 feet above their usual level. Cases, however, have been 
observed where, owing to excessive subterranean movement, lakes liavo 
been emptied of their contents and their beds have been left per- 
manently dry. On the other hand, areas of dry ground have been 
depressed, and have become the sites of new lakes. 

Some of the most important changes in the fresli water of a region, 

lio\v«.-vi*r, are produced by the fall of masses of rock and earth, which, 

liy •laiiijiiiug up a stream, may so arrest its water as to form a lake. If 

! rrier bo of sufficient strength, the lake will Ix) permanent ; though, 

the uiraally loose, incoherent character of its materials, tiie 

^'JS^ xxviii p. 257. Far a catalogue of Indian Earthquakes down to the 
JJftT •-.«• tow. flW. Amt. JiMWa, xix. part 2. 
Ht^ •ffM Bl. «iL 777. 



Part I. Sect, ii.] EARTHQUAKES, 257 



dam thrown across the pathway of a stream runs a groat risk of Ix-'iiig 
undermiued by the percolating water. A sudden giving way of the 
barrier allows the confined water to rusli with great violence down 
the valley, and to produce perhaps tenfold more havoc there than may 
have been caused by the original earthquake. When a landslip is of 
sufficient dimensions to divert a stream from its previous course, the 
new channel thus taken may become permanent, and a valley may be 
cut out or widened. 

3. Effects upon the sea. — The great sea- wave propagated 
outward from the centre of a sub-occaiiio earthquake and reaching 
the land after the eiirth-wave has arrived there, gives rise to much 
destruction along the maritime parts of the distnrlKjd region. As it 
approaches the shore, the littoral waters retreat seawards, sucked up, as 
it were, by the advancing wall of water, which, reaching a height of 
Bumetimes 60 feet or more, rushes over the bare beach and sweeps inland, 
carrying with it everything which it can dislodge and bear away. 
LooBo blocks of rock are thus lifted to a considerable distance from theit 
former position, and left at a higher level. Deposits of sand, gravel, 
and other superficial accumulations are torn up and swept away, while 
the surface of the country, as far as the limit reached by the wave, is 
strewn with dubris. If the district has been already shattered by the 
passage of the earth-wave, the advent of the great sea-wave augments 
and completes the devastation. The havoc caused by the Lislx)n earth- 
qnake of 1755, and by that of Peru and Ecuador in 18G8, was much 
aggrai'ated by the co-operation of the oceanic wave. 

4. Permanent changes of level. — It has been observed, 
after the passage of an earthquake, that the level of tlie disturbed 
country has sometimes been changed. Thus after the terrible earth- 
quake of 19th November, 1822, tlie coast of Chili, for a long distance, 
*a8 found to have risen from 3 to 4 feet, so that along shore, littoral 
shells were exixised still adhering to the rocks, amid multitudes of dead 
fish. The same cuast-line has been further ui)raised by subseciuent 
earthquake shocks. On the other hand, many instances have been 
ohservcd where the effect of the earthcpiakc has been to depress 
permanently the disturbed ground. For example, by the Bengal earth* 
quake of 17G2,an area of 00 square miles on the coast, near Chittagong, 
suddenly went down beneath the sea, leaving only the tops of the 
higher eminences alx>ve water. The succession of earthquakes which in 
the years 1811 and 1812 devastated the basin of the Mississippi, gave 
rise to widespread depressions of tlie ground, over some of which, a}x)ve 
alluded to, the river spread so as to form new lakes, with the tops of the 
trees still standing al)ove the surface of the water. 

DiBtribntioxi of Earthquakes. — While no large space of the earth's 
nrface seems to 1x3 free from at least some degree of earthquake-move-' 
ment, there are regions more especially liable to the visittition. As a 
rule, earthquakes are most frecxuent in volcanic districts, the explosions 
of a volcano being generally i)rcceded or accoiiii>anied by tremors of 

s 



258 DYNAMICAL GEOLOGY. [Book IH. 



greater or loss intonsity. lu the Old World, a great belt of earthquake 
disturbanco st retches in an east and Avest direction, along that tract of 
remarkable depressions and elevations lying between the Alps and the 
mountains of northern Africa, and S2)reading eastward so as to oncloso 
the basins of the Mediterranean, Black Sea, Caspian, and Sea of Aral, and 
to rise into the great mountain-ridges of Central Asia. In this zone lie 
numerous volcanic vents, both active and extinct or dormant, from the 
Azores on the west to the basaltic plateaux of India on the east. The 
Pacific Ocean is surrounded with a vast ring of volcanic vents, and its 
borders are likewise subject to frc(iuent earthquake shocks. Some of 
the most terrible earthquakes within human experience have been those 
which have affected the western seaboard of South America. 

Origin of Earthquakes. — Though the phenomena of an earthquake 
become intelligible as the results of the transmission of waves of shock 
arising from a centre where some sudden and violent impulse has been 
given within the terrestrial crust, the origin of this sudden blow can 
only be conjectured. Various conceivable causes may, at dififerent times 
and under difTerent conditions, communicate a shock to the subterranean 
regions. Such are the sudden flashing into steam of water in the 
s2}heroidal state, the sudden condensation of steam, the explosions of a 
volcanic orifice, the falling in of the roof of a subterranean cavity, or the 
sudden snap of deep-seated rocks subjected to prolonged and intense strain. 

In volcanic regions, the frequent earthquakes which precede or 
accompany erui)tions are doubtless traceable to explosions of olastio 
vapours, and notably of steam. As earthquakes originate also in districts 
remote from any active volcano, and, so far as observation shows, at 
comparatively sliallow depths, these cannot bo oonnected with ordinary 
volcanic action, though it is possible that by movements of molten or 
highly-heated matter within the crust and its invasion of the upper 
layer, to which meteoric water in considerable quantities descends, 
sudden and extensive generation of steam may occasionally take place.^ 
In minor cases, whore the tremor is comparatively slight and looal, -we 
may conceive that the collapse of the roof or sides of some of tlie 
numerous tunnels and caverns dissolved out of underground rocks by 
permeating water may suffice to produce the observed shocks.^ There 
appears reason to believe that the most convulsive earthquakes originate 
under the sea, as in the cases of the great Lisbon earthquake and those of 
Peru, Chili, and Japan. For these it is as yet difficult to imagine an 
adequate cause. One of the most obvious sources of disturbanco is the 
rupture of rocks wthin the crust under the intense strain produced 
by subsidence upon the more rapidly contracting inner hot nucleus. 

* Pfnff, * AUgemoine Geologic als czacto Wissenschaft,' p. 280. 

^ lu tho Yisp Thai, Canton Wallis, for example, -where tliero are aomo twenty 
springs currying up gypsum in solution (one of them to the extent of 200 cubic metvu 
annually), continued rumblings and sharp shocks are from time to time oxperionoed. In 
July and August, 1855, these movements Listed upwards of a month, and gave rise to 
the fissuring of buildings and the precipitation of landslips. In tho honeycombed lime- 
stone tract of tho Karst, also, earthquakes of varied intensity arc of constant oconrrenoe. 



Part I. Sect, iiij UPHEAVAL AND DEPBESSIOy, 259 



This may oonceivably afifect mountainous torrostrial areas ; but wo do 
not know how it would affect the sea-floor. In mountainous districts, 
many different degrees of shook, from more tremors up to imjiortaut 
earthquakes, have been observed, and these are not improbably due to 
sudden more or less extensive fractures of rocks still under great 
strain.^ Hoemes, from a study of European earthquake phenomena, 
concludes that though some minor earth-tremors may be due to the 
collapse of underground caverns, and others of local character to 
volcanic action, the greatest and most important earthquakes are the 
immediate consequences of the formation of mountains, and he connects 
the lines followed by earthquakes with the structural lines of moun- 
tain-axes.^ 

From what was stated at the beginning of the present section, it is 
evident that some connection may be expected to be traceable between 
the frequency of earthquakes and the earth's ix)sition with regard to the 
moon and sun, on the one hand, and changes of atmospheric conditions, 
on the other. Accordingly, a comparison of the dates of recorded earth- 
quakes seems to bear out the following conclusions : 1st. An earthquake 
maximum occurs about the time of new moon ; 2nd. Another maximum 
appears two days after the flrst quarter ; 3rd. A diminution of activity 
oocnrs about the time of full moon ; 4th. The lowest earthquake 
minimum is on the day of the last quarter.^ There is likewise 
observable a seasonal maximum and minimum, earthquakes occurring 
most frequently in January, when the earth is nearest the sim, and least 
frequently in July, when it is farthest from him.* Out of 656 earth- 
quakes chronicled in France up to the year 1845, threo-fiftlis took place 
in the winter, and two-fiftlis in the summer months. In Switzerland 
thev have been o])Scrved to be about throe times more numerous in 
winter than in summer. Tlie same fact is remarked in tlio history even 
of the slight earthquakes in Britain. A daily maximum ap^iears to 
occur about 2.30 a.m., and a minimum a})out thrce-quarterH of an hour 
after noon. No connection has yet ]x)en natisfactorily estiiblished 
between the occurrence of earthquakes and sun-spots. The j^roater 
frequency of earthquakes in winter miglit Ik) expecteil txj indicate a 
relation Ijetween their occurrence and atmospheric pressure. And it 
appears probable that, as a rule, earthquakes are more frequent with a 
low than with a high barometer.^ 

Section ill. Secular Upheaval and Depression. 

Ik^ides scarcely i>erceptible tremors and more or k'ss violent move- 
ments due to earthquake-shocks, the crust of the earth undergoes in 
many places oscillationrt of an cxtn^mely (juii^t and uniform character, 

• Bee potiraj p. 287. Suewj, * Kntslehunp tl<;r Al[M'n,* Vicinm, 1875. 
« ••Eidbobcn Stiulien," JaJirh. Oeof. liekhs. xxviii. (1878; p. 118. 

» J. F. J. Schmidt, "Studim ubor Erdbebcn," 2ii(l (il. (1871) J p. 18. 

• Ibid, p. 20. Sco tho works of Perrcy cited on p. 250. 

• 6ehmidt» op. cU. p. 23. F. Griigcr, Neuet Jahrft, 1878, p. 1)28. 

s 2 



260 DYNAMICAL GEOLOGY. [Book HI. Part L 



Koiiit' til IK'S in an ui)ward, soniotimes in a downward direction. So trau- 
cpiil may these changes bo, as to produce from day to day no appreciablo 
alteration in the aspect of the ground aflfected, bo that only after the 
lapse (.)f several generations, and by means of careful measurements, 
can they re^illy be proved. Indeed, in the interior of a country nothing 
but a series of accurate levellings from some unmoved datuni-line 
might detect the change of level, unless the effects of the terres- 
trial disturbance showed themselves in altering the drainage. It is 
only along the sea-coast that a ready measure is afforded of any such 
movement. 

It is customary in popular language to speak of the sea rising or 
falling relatively to the land. AVe cannot conceive of any })os8iblo 
augmentation of the oceanic waten*, nor of any diminution, save what 
may Ikj due to the extremely slow processes of abstraction by tho 
liydration of minerals and absoi-^^tion into the earth's interior. Any 
changes, therefore, in tho relative levels of sea and land must be duo 
to some readjustment in the form either of tho solid glolx) or of its 
watery envelope or of both. Playfair ix)inted out at the beginning of 
this century that no subsidence of the sea-level could be local, but must 
extend over the globe. ^ 

Various suggestions have been made regarding possible causes of 
alteration of the sea-level. Thus a shifting of the present distribution 
of density within the nucleus of the planet would affect the position and 
level of the oceans {ante^ p. 45). A change in the earth's centre of gravity, 
such as might result from the accumulation of large masses of snow and 
ice as an ice-cap at one of the poles, has been already (p. 18) referred 
U) as tending to raise the level of the ocean in the hemisphere so 
affected, and to diminish it in a corresi)onding measure elsewhere. The 
return of the ice into the state of water would produce an ojjjKisite 
(jffect. A still further conceivable source of geogi'aphical disturlmnce is 
to Ix) found in the fact that, as a consequence of the diminution of 
centrifugal forw owing to tho retardation of the earth's rotiition causetl 
by the tidal wave, the sea-level must have a tendency to subside at the 
equator and rise at the i)oles.^ A larger amount of land, however, need 
not ultimately be laid bare at the e(j[uator, for the change of level 
ivsulthig from this cause would bo so slow that, as Dr. CroU has pointed 
out, the general degradation of the surface of the land might keep jmoe 
with it and diminish the teiTestrial area as much as the retreat of the 
ocean tended to increase it. Tho same writer has further suggested 
!]• if iIk' waste of the equatorial land, and the deposition of the detritus 
;.. lii-lu'V latitudes, maystill further counteract the effects of retardation 
\\\A the consequent change of ocean -level .^ 

« • lUoBtra^nui of the Huttnnian Theory,' 1 802. The same concluBion fas aunoonceJ 
iv 1^ ^,^W. • BdM duich Norwegcn und Lapland/ 1810. 

J^ CwU, fML Mag. 1868, p. 382. Sir W. Thomson. Tran$, Geol 8oc. Glatgow. iii. 

I V^«A*^ ULife* ^ ^^^^ ®^ ^^^^ ^^y ^*"*i deiicnd Upon corUiu lar^v 

kiiimnwir *« «» Bbitioal figwro of tho txjvanic envelope ; tlmt not only »w 



Sectt. iii. S Ij EVIDENCE OF UPnEAVAL. 261 

The balance of evidence at present available seems decidedly adverse 

to any theory which would account for ancient and modem changes in 

the relative level of sea and land by variations in the figure of the 

oceanic envelope, but to be in favour of regarding such changes as due 

to movements of the solid crust. The proofs of upheaval and sub- 

udence, though sometimes obtainable from wide areas, are marked by a 

want of uniformity and a local and variable character, indicative of an 

action local and variable in its operations, such as the folding of the 

terrestrial crust, and not unifonn and wide-spread, such as might be 

piedioated of any alteration of sea-level. While admitting therefore 

that, to a certain extent, oscillations of the relative level of sea and land 

may have arisen from some of the causes above enumerated, we must 

hold that, on the whole, it is the land which rises and yinks rather than 

the sea.^ 

{ 1. Upheaval. — Various maritime tracts of land have been ascer- 
tained to have undergone in recent times, or to be still undergoing, 
a gradual elevation above the sea. Thus, the coast of Siberia, for 
60t) miles to the east of the river Lena, the islands of Spitzbergen and 
Kovaja Zemlja, the Scandinavian peninsula with the excepticm of a 
small area at its southern apex, and a maritime strip of western 
South America, have been proved to have been recently upheaved. In 
learching for proofs of such movements the student must be on his 
guard against being deceived by any apparent retreat of the sea, which 
may be due merely to the deposit of gravel, sand, or mud along the 
Aore, and the consequent gain of land. Local accumnlations of gravel, 
or "storm beaches," are often throvn up by storms, even ahovo the 
level of ordinary high-tide mark. In estuaries, also, considerable tracts 
of low ground arc gradually raised above the tide level l)y the slow 
deposit of mud. The following proofs of actual rise of the land are 
chiefly to be relied on.^ 

Evidence from dead orpjanisme. — Rooks covered with barnacles or other 
littoral adherent animals, or pierced by litliodomous shells, afford presmnptivo proof of 
the prej«nco of the sea. A single stone with these creatures on its surface would not 
1» tttidfactory evidence, for it might be 04ist up by a storm ; but a lino of largo 
bnolders, whicli had evidently not been moved since the cirripedcs and molhmks lived 
apon thorn, and still more a solid clilT witli these marks of littoral or sub-littoral life 
apen itit bo«e, now raised above high-water mark, wouM bo suffioiout to demonstrate a 
rwp of hind. The amount of the upheaval might l>o pretty accurately detcnnined by 



'*nutt«l beaches" to bo thus explained, but that there are abiiolutoly no vertical movo- 
■(-nt« of the crust save such as may fonn part of the plication arising from uocular 
eootnrtion ; and Uiat the doctrine of secular fluctuations in tlie levol of the contincMits 
ii merely a laanant of the old ** Krhebungstheorie/* destined to speedy extinction. 
(S« Vtrhand. Gtol, Iteichs, 1880, No. 11.) * Antlitz der Erde/ Leipzig, 1883. Pfuff 
d^end!! the general opinion against these views in ZviUch. DnUsch. (hoi. Crtx. 1884. 

' For the arguments against the view above adopted and in favour of tlio doctrine 
thit the increase of the land above sea-level is duo to Ww retirement of tlu^ s<;i, acv 
H. Tnntachold, Bulletin Societe Imp. des Naturalhtts de Moscou, xlii. (18(11)) jmrt i. 
f- 1; 1888, Ka 2, p. 341; Bull. Soc. Otol France (3) viii. (187i0 p. 134; Suess, 
IB the ptpen above cited. 

' See*'£arlhi|imkes and Volcanoes" (A. O.), Olmmberr^'fj Mhrnhnuj of Trar(», 



262 DYNAMICAL GEOLOGY. [Book IH. Pabt L 



measuring the vei-tical distance between the upper edge of the barnacle lone upon the 
upraised rock, and the limit of the same zone on the present shore. By tluB kind of 
evidence, the recent uprise of the coast of Scandinavia has been proved. The shell- 
borings on the pillars of the temple of Jupiter Serapis in the Bay of Naples proye firsl 
a depression and then an elevation of the ground to the extent of more than twenty 
fcet.^ Raised coral-reefs, formed by living species of corals, are a conspionous feeioxe 
of the geology of the West Indian region. The terraces of Barbodoes are partioolarly 
striking. In Cuba, a raised coral-reef occurs at a height of 1000 or 1100 feet above the 
sea.^ In Peru, modern coral-limestone has been found 2900 or 3000 feet above sea- 
level.' Again, in the Solomon Islands, evidence of recent uprise is furnished by ooial 
reefs lying at a height of 1100 feet.* 

The elevation of the sea bottom can in like manner be proved by dead cnrgaiiifms 
lixcd in their position of growth beneath high-water mark. Thus dead specimens of 
Mya truncaia occur on some parts of the coast of the Firth of Forth in considerable 
numbers, still placed with their siphuncular end uppermost in the stiff clay in which 
they harrowed. The position of tliese shells is about high-water mark, but as their 
existing descendants do not live above low-water mark, we may infer tliat the coast has 
been raised by at least the difference between high and low-water mark, or eighteen feet* 
Dead shells of the large Photos dactylus occur in a similar position near high-water 
mark on the Ayrshire coast. Even below low^-water, examples have been noted, as in 
the interesting case obser\'ed by Bars on the Diul)aksbank in the Chtistiania Fjord, 
where dead stems of Oculina prolifei-a (L.) occur at depths of only ten or fifteen fiEithonia. 
This coral is really a deep-sea form, living on the western and northern coasts of 
Norway, at depths of one hundred and fifty to three hundred fathoms in cold water. It 
mmst have been killed as the elevation of the area brought it up into upper and wanner 
layers of water.' It lias even been said that the pines on the edges of the Norwegian 
snow-iields are dying in consequence of the secular elevation of the land bringing them 
up into colder zones of the atmosphere. 

Any stratum of rock containing marine organisms which have manifestly lived and 
died where their remains now lie, must be held to prove upheaval of the land. In this 
way, it can be shown that most of the solid land now visible to us has once been under 
the sea. High on the flanks of mountain-chains (as in the Alps and Himalayas), 
undoubted marine shells occur in the solid rocks. 

Sea- worn Cave s. — A line of sea-worn caves, now standing at a distance above 
high- water mark l)eyond the reach of the sea, affords evidence of recent uprise. In the 
accomi)anying diagram (Fig. 67) examples of such caves are seen at the base of the 
cliff, once the sea margin, now separated from the tide by a platform of meadow-land. 

Uaised Beaches furnish one of the most striking proofs of upheaval. A beach 
or space between tide-marks, where the sea is constantly grinding down sand and 
<;ravel, mingling with tliem the remains of shells and other organisms, sometimes piling 
the deiK)sits up, sometimes sweeping them away out into opener water, forms a iamiliar 
terrace or platform on coast lines skirting tidal seas. When land is upraised, and this 
margin f»f littoral deposits is carried above the reach of the waves, the flat terrace thus 
elevated is known as a "raised Ixjacii " (Figs. C7, (58). The former high-water mark 
tiioii lies inland, and while its sea-woni caves are in time hung with ferns and mosses, 
th(i beach across which the tides once flowed fumirthes a platform on which meailows, 

' Babbttge. Edin. Phil. Journ, xi. (1824), 91. J. I). Forbes, JEatii. Joum. 8ci, i. 
(18210, p. 2G0. Lyell, * Principles,' ii. p. 1(54. 
' A. Agassiz, Amer, Acad. xi. (1882) p. 119. 
' A. Agassiz, Bull. Mu». Comp. Zool. vol. iii. 

* H. B. Guppy, Nature, 8rd Junuaiy, 1884. 

* Hugh Miller's * Edinburgh and its Neighbourhood,* p. 110. 

« Quoted by Vom Rath in a paper entitled " Aus Norwegen," Newn Jahrb. 18G9, 
p. 422. For another example, see (5wyn Jeffnjys, PHt. Asmc. 18(57, p. 431. 



Sht. iii. i t] 



EVIDENCE OF UI'JIKAVAL. 



fields, gaideiu, tcaAt, hoiuea, Tillagea, and towiM tpring np, while n new bench U miido 
btlow the mugiu of the nplifbxl one. A BnceesMOO of raised be&chea may occur at 
mioiu heights above the sea. E»ch terrace mnrks a former lower level of the lond 
vith ngud to tbo lea, and probably a leng^thened itay of the land at that level, while 
Iho tnteiTalB between them represent the vertical amiiunt of each mocearivo uplift, 




Htfl.— View DlaUutof 

>^ thaw that the Luul, in ila upward xaoremout, did not 
Bfiliite pointa for the fonnatiou of turracoa. A 
>^ the present sca-lcrel, nifty therefore bo token 
M pniuting to a former intcrmtltcnt uphouval of 
Qkt (onutry, interrupted hj long pauses, during 
■Uch the general level did not materially chnn{;e. 
Baited bcoehce abound In the kigiicr lutifudcs 
I'tllie northern and southern licmisplicrca. Tlicy 
*R fonod, for example, round many parts of the 
^wl-)inc of Britain. De la Bcchc gives the suh- 
jtued view (Fig. 69) of a Cornish locality nhcie 
tlw uiating beach ia flanked by a cliff of slale, 
\ NntiDiially ent away by the Ka so lliat the 
ortrlfing raised beach, a, e, will ere hmg dia- 
•Wear. The ooast-lliie on both sides of Hcotknd '•"'S^ 
'■Utwise fringed with miseil l>etH'hc-0. fouic-times 
liKBcir Sto ticcurting iibuve each oilier at hcij^htsof 
^ W. SO, GO, Tii anil 100 feet abo\-e tliu prewiit 
Ulli-WBtct mark.' Olliers are found oit botli xiclort 
•dlw Knglish Choniirl.' Tlic Bi'Ica of tlio 



KO-wom caves anil rroutcil 1^ n 




-jmc British niisfd hwiclirn, foc De la Bctlie, ' Runirt on Hoology 

"I imon and Oonwall,' chap. xUi. ; C. Maclnrcn, ' GcoUigy of Fife and tho liotliiuiis,* 
li* ; B. Chamber!, ' Ancient Sea BlarcinB ; ' rrcslwirb. Q. J. Grol. Soe. xiviii. p. 38 ; 
>txL n E3; R. BmMcll and T. V. Holmes, Bril. Anoe. ISTG, tkcts. p. t)S; Lasher, 

(M Jrs>i. 1979, p. lec. 

' On the railed beach of Sanj^tte, near Calais. Rec Prostwieh, BiiU. Snr. Giol. 
'Wi (3) TuL (ISSO) jj. 547. On those of Finistorrc, C. »irroi», Anu. Snr. tliii. 
*'wl.tt.<l8M). 



DYNAMICAL OEOLOOY. [Book IIL Pabt L 



264 

to moro thnii GOO fcot aboro iM-level, are nmrkcd with conipicDoiu linea of tenaoM 
(Fig. 70). Thc»o tfrrncefl are portly onlinury bench depoBita, portly notches ent tmt 
of rook, probably with tho aid of drifting coast-ice.' Proofi of recent elerotion of 
the Bkorca of the MeJitcrraautu ate fiunislied by laiwid benchei at varioo* helgbti 




>1g. «*.— View o[ Qilwd Bench, Kellr'a C«vc,Ooin»«ll iB.). 

abOTo tlio present vatar-Iorel. In Corsioa auch terraces occur at boiglits of boa 
ir> to 20 metres.* 

On the vrest ooaat of South Ameri(«, linea of raised tcnnco containing recent shells 
linvo bocii traced by Darwin, which prove a great nphenTal of that part of the gfabe 




in modem geologionl time. Tiio terraces arc 
tlin Boutii. On the frontier of lioliviii, they i 
ej[inting wa-leTcl, hut ncnrcr Uio liiglior tnusi 



not quite horizontal bnt rise townidt 
cenr at from 05 tn 80 fort above the 
of the Chllinn Andes they are fomtd 



' Sec It. Chambers, 'Tracinga of the Xnrlh of Enropo' (1850), p. 172, rf wo. 
ItrsTais, ' Voyages de la Comnussion Sciontiflqne du Nord,' &c., translated In Q. J. OeoL 
fhe. i.p. 534. Xjernlf, Z. DnUieh. Gfol Get. xxii. p. I. 'Die Geologle dea sQd. uod 
inittl. Norwtwjn,' 1880. p. 7. fifof. Mag. viii, p. 74. S. A, Si'xo, " On Rise of Land in 
Scandinavia, Jitdfz SchdIaTttm of VnirertUy, Cliriaiianin, 1872. H. Motin. NyU Mag. 
Kai. xxii. p. 1. Dakjnii, Geol. mig. 1B77, p. 72. K. Pettcrsen, ^reS. Mafk. Nat. 
Chritllaaia, 1878, p. IB2, Geol. Mag. 1870, p. 298. TromrO Muimmi Aar^fta; lU. 
18S0. Lebmann, ' Uebcr^?licmnlige 8trandlinier,' &c., Halle, ISTO; Zeit»fk. gn. 
NalHririM. 1880, p. 28". = Jlall. Sob. Gt'J. Fnin'v (3), iv. p. 8a 



Skct. iii. §. 2.] EVIDENCE OF f^UBf^JDENCE. 205 



at 1000, ami near Yalpaniirio at 1300 feet. That H^itno of Ihoso nnoii-iit H(*a-nmr<<;iiiu 

lit-long tu the human |K'rkx1, was Hhowu hy Mr. Diirwiu'n tliHcovcry of hhcllrt wiili bones 

of binU, cars of maize, plaited nMxU and cuttnn throad in one of tlio terraced op|K)sito 

Calhiu at a lieight of 85 fuC't' Kairiiul Ijoaolies occur in X(;w Zealand, and indi(Miie a 

'pOX'atc'r elevation of tlie southern tlian tho northern part of tlio country.' It shouKl 

be obserrod that this increased rise of the terraces polewards occurs both in tho 

Bortbem and southern hemisphere, and is odb of the facts insisted niK>n by those who 

would explain tlic terraces by displacements of the sea rather than of the land. 

Uuman Records and Traditions. — In countries which have been long 

H'ttlal by a human population, it is sometimes possible to prove, or at h^nst to 

Muler probable, the fact of recent upriso of tho land by reference to tra<lition, t) local 

naiiu-s, and to works of human construction. Piers and har1)onrs, if now foun<l to 

Rtaud a1>ov6 tlio upper limit of high-water, furnish indd^l indisputable evi(h-nce ot 

a rise of hind sinc^j their erection. Xumerous proofs of a recent upheaval of the coast 

liDe uf the Arctic Ocean from Spitzbergen eastward hav<( Ixittn observed. U'ho Finninh 

f'tti't is re|>orted to have rist^n 6 feet 4 inches in 127 years.' At Spitzbergcn itself, 

U-viiU-B its raised beaches, bearing witness to previous elevations, small islands which 

eiiitcil two hundred years ago are now joined to larger ])ortions of land. At Xov^ja 

Ztmlja, where six raised beaches were found by Nordimskjuld, the highest being GOO feet 

abjTv scu-lcrel,^ there seems to have been a rising of the sea-bottom to the extent of 

l'"Hi-et or more since tho Dutch expedition of 1594. ()\\ the north coast of SilxTia 

tlie iiland of Diomida, observed in 17G0 by Chalaourof to the east of Cape Sviatoj, was 

f'»anil by Wrangel sixty years afterwards to have been united to tho nuiiuland.* From 

Qia^ Diadc on the coast in the middle of last century it apj>cars that the north of 

^veicD has risen about 7 feet in the last 154 years, but that tho movement has lessened 

Katliwards until in ):>cania it has been replaced by one in a downward direction (see p. 2(>8). 

§ 2. Subsidence. — It iH more difficult to trace a downiward luove- 

meiit of land, for tho evidence of each succx^ssive Bea-niargiii ik carried 

4iiwu and washed away or covered up. The etudont will tnkf^ can* to 

jniurd hiniHolf against lK*ing misled l:»y mere proofs of the advance of tlie 

*>«i«mthe laud. In tlie great majority of caseH, wlirn; siuli an advance 

i"5 taking places it is due not to suhsidence of the land, but to erosiun of 

till: shores. It is, indeed, tlie converse of tlie deposition kIkivo mentioned 

d'. 'JiHj as lialde to Ik; mistaken for pr(»of of upheaval. Tlu? resultH of 

'nere erosion by the sea, however, and those of a<^tual do^jrcKsion of tluj 

level of the land, cannot always be dLstinguished without some care. 

TIm' encroachment of the wa upon the land may involve the dis- 

Bppoarance of successive fields, roads, housr-s, villages, and even whole 

iNirishcB, without any actual change of level uf the land. I'he following 

l^iuds of evidence may Ik? held to prove the fn(rt of suhsidenee. 

Submerged Forests. — ^As the land is bnmirht ilown within reaoh of the wavrs, 
•*! it* oharactcriBtic snrface-ffatnres arc eflaocd. the subnn'rjfc»l area may ntain liith" 
•riifi ividence of it« liaving Ix-en a land-surfaoo. It will \y^' covenil, us a nile, with seu- 
'- "i Kiad or rilt. Hence, no dnnbt, the reaw>n why, ann»ng the marine strata whi«"lj 

' M ^illogical Olwervatious,' chap. ix. See (i*oL Muf. Jv^TT, j). 2x. 

' HaaAt'a * Geology of Cautcrbury/ 1871», p. HGG. 

' Aafwrr, xxtL p. 231. » Urol. xv. p. 12:5. 

• «inid. Bu/L the. Uwl. France, Snl ffcr. ii. p. .'^4S. 'J'race.-i <»f < Hiiiati'His of hv. I 

^iiltiii historic limes have been ^bserve<l in tlie N\tlKrliindH, FlundiTH, und rpj-i-r 

^ litdL Soe. 6W. France, 2nd ser. xix. p. ."ijH: 3r«l ser. ii. pp. W; 222; Anu. So*-. 

'■ ' \''*rd, V. p. 218. For nllc^^ed chaii;^s of li?vi-l in tin- cMnary <*i' the <;arinii". n «■ 

■ • 'ii'*,iW. Aie, iiim. D«>nU'anx. xxxi. 'I«7»J; p. 2s7, and tin- n ply *'\' !»• Ifi-riri.-. 



DYNAMICAL OEOLOOY. [Book III. Pact L 



Form BD muoh of the stratified portion of the earth's cnut, and ooutaiD m qmdj yndU 
of UtpreswoB, ACtaal truces of land-anrfaccB are comparatiTcly rare. It is only nnder 
very favournblo circumitanMB, as, for inetancc, vhero tho area is sheltered bom 
prevalent winds nod wavos, and wLero, tliereforo, tho sorftice of the land can ainlt 
tmnquilly under tho sra, that fiagmoQta of that sarfaoe may be presoTred tudBr orei- 
lyin(; marino accnmulatioDe. It is in snoh places that "submerged foiests" oooor. 
TLp£o are stumps of trees still in their positions of growth in their native soil, oRm 
ossncinted with beds of peat, full of troe-roots, hazcl-uuts, branches, leaves^ and other 
inilications of a terrestrial surface. 

De la Bccho has described, round tho shores of Devon, Cornwall, and wcrtein 
Somerset, a Tcgctablo aecnmulatinn, consisting tii plants of the some »pecie« aa thrao 
which now grow freely on the adjoining land, and iicciirring as a bed at the moutha of 
vnllcys. at tlio bottoms of sheltered bays, and in front of and under low tracts of land, 
of wliicli tho seaward side dips beneath tho present level of the sea.' Over tUi 
Kiil>uii'rgod land-surface, sand and silt containing estnarine shells have generally beoD 
dcjiositcd, whence we may infer that, in the submergenn-, the valleys first boeama 
cstuarieH, and then sen-lmys. If now, in the winrso of bkw, a series of mich snlimergcd 



Jj^i 



r-^ £/!^r— 3^-^ i-„^-«*J-*.« 




KUn. ill (ouiw of llmc, mOer Kme of Uie Irem liiul Ulen (fi). und ■ qnnntftir oT TFcetablc Mil Imi 
ncrnioulunl. mclMliiB here unillhcrf Ui* biio« or ilMr uid men (t e), Uie ■«* MBt, uid the »• OTW- 
floMng It threw dovn upoti lu Hurfkce undy or muddy di^puslti (//). 

forests shotild be formed one over tho otlier, and if, finally, they should, by nphcaval of 
the sea-bottom, be once mora laid dry, so as to be capable of examioation by boring, 
well-sinking, or otherwise, they would prove a former long-contiaued depression, wiUi 
ititerrals of rest. These interrala would be marked by the buried forests, and the 
progress of depression by the strata of sand and mud lying botween them. In shcol, 
tho evidence would bo strictly on a posallel with that furnished by a snceesBion of 
raised bcoclies as to a former protrooted intermittent elevation. 

Coral-islands. — Kvidenco of wide-spread depression, over tho am of tho 
I'ocitlc and Indian Oceans, has been adduced from the stniclurc and growth of eoral-redii 
nnd islands. Mr. Darwin, many yvan ago, stated bis belief that, as tho revf-buildinf: 
eniuls do not livu at depths of more tlian 20 to 30 fathonia, and yet their reefs rise out 
iif di-cp water, the sites on which they lutvo formed those structures must bare subsided, 
the rate of subsidence being so slow, tliat tlie upward growth of the reefs has on tho 
wikolo kept pace wllli it.* More recent reeeorchcs, however, show that the phenomena 

' "Geology of Devon and Cornwall," Jlfem. (7«il. Surrey. For further aocounto ct 
British submergoil foresls, see Q. J. Geol. Soe. xiii. p. 1 ; miv. p. 447. GeoL Mag. t^. 
p. 76; vii. p. 04 ; iii. 2nd scr. p. 491 ; vi. pp. 80. 251. 

' See Durwin's ' Coral Islands,' Dana's ' (Vimls and Coral Islands,' and the worici 
cited poafea, Book III. Part II. Section iii. § 3, under " Coral-reefs." 



Sect. iii. § 2.] EVIDENCE OF SUBSIDENCE. 267 



of coral-ieefd are capable of satisfactory explaiiation without the uocessity of siibsidonco, 
and hence that they can no longer bo adduced as evidence of the subnideuce of lar^o 
areas of the ocean.* The formation of coral-reefs is described in Book III. Part II. 
Section iii., and Mr. Darwin's theory is there more fully explaine<l. 

Distribution of plants and a n i m a 1 s.—^ince the appearance of Edward 

Foibea'a essay upon the connection between the distribution of the existing fauna and 

loia of tlie British Isles, and the geological changes wliich have afTt-ct^i tliat area,' 

much attention has been given to the evidence furnished by the googrophical 

diiitribntion of plants and animals as to geological revolutions. In some cas^H, tlie 

former existence of land now submerged has been iufcrred with considerable confidetice 

from the distribution of living organisms, although, as Mr. Wallace has shown in tlio 

(itt of the supposed ** Lemuria," some of the inferences have been unfounde<l and 

nmccesaary.* The ])resent distribution of plants and animals is only intelligible in the 

light of former geological cliangcs. As a single illustration of the kind of reasoning 

6om present zoological groupings as to former geological subsidence, ruffrencc may 1)0 

B»le to the fact, that while the fishes and mollusks living in the seas on the two sides 

of the Isthmus of Panama are on the whole very distinct, a few shells and a large 

Dombcr of fishes are identical ; whence the inference has been drawn that though a 

Iwul Wiit;.'r-channel originally separated North and South America in Miocene times, 

a 8crie8 of elevations and subsidences has since occurred, the most recent submersion 

htriog lasted but a short time, allowing the passage of locomotive fishes, yet not 

Kbitting of much change in the comparatively stationary mollusks.^ 

Fjords. — ^An interesting proof of an extensive depression of the north-west of 
Europe is furnished by the fjords or sea-lochs by which tliat region is indented. A 
Q<wd ia a long, narrow, and often singularly deep inlet of the sea, which terminat(« 
i^Qtl at the mouth of a glen or valley. l*he word is Norwegian, and in Norway fjords 
tie characteristically developed. The English wonl " firth,** however, is tlie same, and 
the ire«ti*m coasts of the British Isles furnish many excellent examples of Qords, such 
•* the Hcottish Loch Ileum, Loch Nevis, Loch Fyiie, Garelocli ; and the Irish Lough 
^^'Vtc, Ix>ugh Swilly, Bautry Bay, Dunmanus Bay. Similar indentations ul>ound on 
thpwttit coast of British North America. Some of the Alpine lakes (Lucerne, Ganhi, 
^Ui^inrt* and others), as well as many in Britain, arc inland examples of fjordn. 

There can bo little doubt that, though now fille<l with salt water, fjonls have Ix-en 
^ginally Lind-valleys. Tlie long inlet was first excavated as a vall*:y or glen. Thu 
^jtoent valley exactly corresponds in form and chamcter with the hollow of the fjonl, 
^l muiit be regarded as merely its inland prolongation. That the glens have been 
*i«ivatefl by sub-aerial agents is a conclusion l>onie out by a great weight of evidence, 
vhieh will be detailed in later parts of this volume. If, therefore, we admit the sub- 
*criil origUi of the glen, we must also grant a similar origin to its seaward prolongation. 
CvRy Qord will thus mark the site of a submerged valley. This inference is confirnunl 
^7 the diet that fjords do not, as a rule, occur singly, but. like glens on land, lie in groups ; 
*^thtt, when found intersecting a long line of coast, such as that of the west of Norway 
**t]ie west of Scotland, they serve to show tliut thu land has there sunk down so an to 
l^ouit the sea to ran far up and fill submerged glens. 

Human eonstruotions and historical records. — Should the sea Ih^ 

'"'^-rrcd to rise to the level of roads and buildings which it n^-vt-r U8e«l t<) touoli, nliould 

' "nncr half-tide xocks cease to be visible even nt low water, and should nx'kn, pnrvionsly 

**ti.vo the reach of the highest tide, Ikj turm-d first inlo shore-reefs, then int/> skerrii-.s 

'^^^\ Ubfts.wc infer tiiat the coast-line is sinking. Suoh kind of evidence is found in 

• Soe Prot J2oy. Phy», Soc, Etlinhurgh, viii. p. 1. 
^ Vrm. Gtai, Surrey, vol. i. IHiO, p. ii^G, 

* 'U\bm\ Life,' 1880, p. 394. In this work tho <jue^tion of distribution in its 
-•' '^l^i^uail relations is treated with arlmimble lucidity and fulncFH. 

^ ^'allaec, * rieogruphical Diatribution of Animals,' i. ]•]>. 40, 7<>. 



268 DYNAMICAL GEOLOGY. [Book HI. Part I. 



Scania, the moat southerly part of Bweden. Streets, built of course ahoYe higfa-waler 
mark, now lie below it, with older streets lying beneath them, so that the sabsidence ii 
of some antiquity. A stone, the position of which had been exactly determined by 
Linnseus in 1749, was found after 87 years to be 100 feet nearer the watet^s cdge.> The 
west coast of Greenland, for a space of more than 600 miles, is perceptibly sinkiDg. It 
has there been noticed that, over ancient buildings on low shores, as well as orer entlvB 
islets, the sea has risen. The Moravian settlers have been more than once driveo to 
shift their boat-poles inland, some of the old poles remaining visible imder irftter.' 
Historical evidence likewise exists of the subsidence of ground in Holland and 
Belgium.' On the coast of Dalmatia, Boman roads and villaB are said to be Tisibls 
below the sea.^ ' 

§ 3. Causes of Upheaval and Depression of Land. — ^These 
movements must again bo traced back mainly to consequences of the 
internal heat of the earth. There arc various ways in which the heat 
may have acted. As rocks expand when heated, and contract on 
cooling, we may suppose that, if the crust underneath a tract of land has 
its temperature slowly raised, as no doubt takes place round areas oi 
nascent volcanoes, a gradual uprise of the ground above will be the 
result. The gradual transference of the heat to another quarter may 
produce a steady siibsidence. Basing on the calculations of Colonel 
Totten, cited on p. 275, Lyell estimated that a mass of red sandstone one 
mile thick, having its temperature augmented 200° Fahr., would raise 
the overlying rocks 10 feet, and that a portion of the earth's crust oi 
similar character 50 miles thick, with an increase of 600° or 800°, might 
produce an elevation of 1000 or 1500 feet.* Again, rocks expand by 
fusion and contract on solidification. Hence, by the alternate melting 
and solidifying of subterranean masses, upheaval and depression of the 
surface may possibly bo produced (see pp. 275, 280). 

But processes of this nature can evidently effect changes of level 
only limited in amount and local in area. When we consider the wide 
tracts over which terrestrial movements are now taking place, or have 
occurred in past time, the explanation of them must manifestly be 
sought in some far more wide-spread and generally effective force in 
geological dynamics. It must be confessed, however, that no altogethei 
satisfactory solution of the problem has yet been given, and that the 
subject still remains beset with many difficulties. 

* According to Erdmann, the subsidence has now ceased, or has even been exchanged 
for an upward movement {Qeol. F6r. Stockholm F6rharull. i. p. 93). Nathorst also (hinki 
that Scania is now sharing in the general elevation of Scandinavia {ibid, p. 281). II 
api)cars that the zero of movement now passes through Bomholm and Lnaland. 

' These observations, which have been accepted for at least a generation naet (JProe. 
Geol, Soe. ii. 183.5, p. 208), havo recently beou called in question, but tne alleged 
disproof is not convincing, and they are hero retained as worthy of credence^ Bee 
Suess, Verhand, Geol Reichmnstalt, 1880, No. II. 

' Lavnicyc, * Afiaissemcnt du sol et cnvasemcnt dcs fleuvcs, survcnus dans lea temps 
historiques,' Brussels, 1859. Grad, Bull Soc. Geol France, ii. 3rd ser. p. 4C. Aremu, 
'Physische Geschichte der Nordseekiiste,* 1833. Compare also R. A. Peacock on 
* Pliysioal and Uistorical Evidences of vast Sinkings of Land on ilio North and West 
Coasts of France,' &o., London, 18G8. For submerged peat-beds on French coast, ace 
A. Gaspard, Ann. Soc. Gtol Nord. 1870-74, p. 40. • On oscillations of French coast, 
T. Girard, Bull Son. Geograph. Paris, ser. G, vol. x, p. 225; E. Delfortrio, Act. Soe. 
Linn. Bordeaux^ b6t. 4. vol. i. p. 79. 

^ flnll a>m. Gvol Jhliauoy 1874, p. 57. » * Principles,' ii. p. 235, 



Sect. iii. S 3.] UAUSSS OP UPHEAVAL AND SUBSIDENCE. 269 

Professor Darwin, in one of his recent nieiuoirb already cited (antCy 
\K 19), has snggcstod a possible determining cause of the larger 
features of tho earth's surface. Assuming for his theory a certain 
degree of viscosity in tho earth, he jwints out that, under the combined 
influenco of rotation and the moon's attraction, the polar regions tend 
to ontstrii) tho equator, and to acquire a consequent slow motion from 
west to east relatively to tho equator. The amount of distortion pro- 
duced hy this screwing motion ho finds to have been so slow, that 
45,iX)0,000 years ago, a i^oint in lat. 30^ would have Ixjcn 4J', and a 
point in lat. 60^ 14^' further west, with reference to the cfpiator, tlian 
they are at present. This slight transference shows us, ho remarks, 
that the amount of distortion of the surface strata from this cause must 
fe excoe^lingly minute. But it is conceivable that, in earlier conditions 
uf the planet, this screwing action of tho earth may have had some 
influence in determining tho surface features of the planet. In a body 
not i^erfectly homogeneous it might originate wrinkles at the surface 
ninning jKjrpendicular to tho direction of greatest pressure. ** In the 
c;«c of tho earth, tho wrinkles would run north and south at the equator, 
*iid would bear away to the eastward in noi'therly and southerly 
latitudes, so that at the north pole the trend would be north-east, and 
*t the south pole north-west. Also tho intensity of the wrinkling force 
"V'aries as the square of the cosine of the latitude, and is thus greatest at 
tie e(|nator and zero at the poles. Any wrinkle, when once formed, 
"^unld have a tendency to turn slightly, so as to become more nearly 
c-4urt and west than it was when fii*st made." 

Accor<ling to the theory, the highest elevations of the earth's surface 

Hhoiild 1)0 equat<.>rial, and should have a general north and south trend, 

vhile in the northern hemisphere the main direction of the masses of 

laud should l>end round towards north-east, and iu tlie opposito hcmi- 

**l'li«'re towards south-east. Prof. Darwin thinks that the general facts of 

tvni'strial geography tend to coiToborate his theoretical views, though 

lie admits that some are very unfavourable to them. In tho discussion 

*jf Huch a theorj", however, we must rememl)er that the present mountain- 

^-'liainu on the earth's surface are not aboriginal, but arose at many 

wucccHsive and widely-separated epochs. Now it is quite certain that the 

Jimnger mountain-chains (and these include the loftiest on the surface 

^>f the glolxs) arose, or at least received their chief upheaval, during tho 

Ti'rtijiry periods — a comparatively late date in geological history. Un- 

lew we arc to enlarge enormously the limits of time which physicists 

»Te willing to concede for the evolution of the whole of that historj% we 

canhanlly suppfiso that the elevation of the great mountain-chains took 

pUce at an eixx^h at all approaching an antiquity of 45,000,000 years. 

^'et. according to Prof. Darr^'in's showing, the sui)erficial effects of 

^ttnial distortion must have been exceedingly minute during the past 

'*'VH)0,000 years. Wo must either therefore multiply enonnously the 

V^Aoi% Tequired for geologiad changes, or find some cruise which could 

W elevated great mountain-chains at more recent intervals. 



270 DYNAMIOAL QEOLOOY. [Book IH. Part L 



But it is well worth consideration wlictlier the oauso BUggested by 
Prof. Darwin may not liave given their initial trend to the masses of 
land, BO that any subsequent wrinkling of the terrestrial snrfaco, due 
to any other cause, would be apt to take jdace along the original lines. 
To be able to answer this question, it is necessary to ascertain the 
dominant line of strike of the older geological formations. But in- 
formation on this subject is still scanty. In Western Europe, the 
prevalent line along whicli terrestrial plications took place during 
ralasozoic time was certainly from S.W. or S.S.W. to N.E. or N.N.E., 
and the same direction is recoguisiible in the eastern States of North 
America. But the trend of later formations is more varied. The 
striking contradictions between the actual direction of so many 
mountain-cliains and masses of land, and what ought to be their line 
according to the theory, seem to indicate that while the effects of 
internal distortion may have given the first outlines to the land-areas 
of the globe, some other cause must have been at work in later times, 
acting sometimes along the original lines, sometimes transverse to 
them. 

The main cause to which geologists are now disposed to refer the 
corrugations of the carth^s surface is secular cooling and consequent 
contraction. If our planet has l>eon steadily losing heat by radiation 
into space, it must have progressively diminished in volume. The 
cooling implies contraction. According to Mallet, the diameter of the 
earth is less by at least 189 miles since the time when the planet 
was a mass of liquid.^ But the contraction has not manifested itself 
imifonnly over the whole surface of the planet. The crust varies much 
in structure, in thermal resistance, and in the position of its isogeo- 
tliermal lines. As the hotter nucleus contracts more rapidly by cooling 
than the cooled and hardened crust, the latter must sink down by its 
own weight, and in so doing requires to accommodate itself to a con- 
tinually diminishing diameter. The descent of the crust gives rise to 
enormous tangential pressures. The rocks arc crushed, crumpled and 
broken in many places. Subsidence must have been the general rule, 
but every subsidence would doubtless be accompanied with upheavals of 
a more limited kind. The direction of these upheaved tracts, whether 
determined, as Prof. Darwin suggests, by the effects of internal distortion, 
or by some original features in the structure of the crust, would be apt 
to be linear. The lines, once taken as linos of weakness or relief frcnn 
the intense strain, would probably l)e made use of again and again at 
successive paroxysms or more tranquil periods of contraction. Mallet 
ingeniously connected these movements with the linear direction of 
mountain chains, volcanic vents and earthquake shocks. If the initial 
trend tcj the land-masses wore given as hypothetically stated by Prof. 
Darwin, wo may conceive that after the outer parts of the globe had 
attained a considerable rigidity and could then be only slightly in- 
fluenced by internal distortion, the effects of continued secular oontrao- 

• Fhil. Tram. 1873, p. 205. 



'. iiL § 3.] CAUSES OF UPHEAVAL AND SUBSIDENCE. 271 



tlon would be seen in the intermittent Bubsidence of the ocoanio basins 
siJx^^y existing, and in the successive crumpling and clovation of the 
jxi.'tervening stiffened terrestrial ridges. 

This view, varionslj modified, has been widely accepted by goolo- 

^ifits as famishing an explanation of the origin of the u])hcaval8 and 

subsidences of which the earth's crust contains such a long record. 

^iit it is not unattended with objections. The difficulty of conceiving 

tli.Bt a globe possessing on the whole a rigidity equal to that of glass or 

st^eel could be corrugated as the crust of the earth has Ix^en, has led 

90ine writers to adopt the hypothesis already described {ante, p. 54), of 

axi. intermediate viscous layer between the solid crust and the solid 

n.'UcleuBy while others have suggested that the observed subsidence may 

bAve been caused, or at least aggravated, by the escape of vapours from 

volcanic orifices. But with modifications, the main cause of terrestrial 

movements is still sought in secular contraction. 

Some observers, following an original suggestion of Babbage,^ have 

supposed that upheaval and subsidence, together with the solidification, 

cryBiallization, and metamorphism of the layers of the earth's crust, 

may have been in large measure due to the deposition and removal of 

niinoral matter on the surface. There can be no dou])t that the lines 

of equal internal temperature (isogeothermal lines) for a considerable 

depth downward, follow approximately the contours of the surface, 

^^lu^'ing up and down as the surface rises into mountains or sinks into 

plains. The deposition of a thousand feet of rock will, of course, cause 

* corrosponiling rise in the isogeo therms, and if we assume the average 

n«o of temperature to be 1'^ Fahr. for every 50 feet, then the teiupera- 

^o of the crust immediately below this deposited mass of rock Avill 

^ i^uised 20^. But masses of sediment of mu(;h greater thickness have 

'*eix laid down, and wo may admit that a much greater increase of 

^^^i:)erature than 20^ has been effected by this means. On the other 

l^^d, the denudation of the land must lead to a depression of the 

^g^otherms, and a consequent cooling of the upper layers of the 

c^UBt. 

It may be conceded that in so far as the internal structure of rocks 
'''^y bo modified by such progressive increase of temperature iis would 
*^^^^© from superficial deposit, this cause of change must have a place in 
8*^logical dynamics. But it has been urged that Ijcsidcs this effect, 
thfi removal of rock by denudation from one area and its accumulation 
'^Pon another affects the equilibrium of the cnist; that the portions 
^'Ixfire denudation is active, being relieved of weight, rise, while those 
^l^cre deposition is prolonged, being on the contrary loaded, sink.^ 
Txiig hypothesis has recently been strongly advocated by some of the 
S'^logists who have been exploring the Western Territories of America, 

» J(mm. Geol. Soc, iii. (1831) p. 206. 

^ Similarly it has beon coutondcd that tho nccuniulatioii of a massive ice-sheet on 
'•^0 land would cause a depression of tho terrestrial surface. N. S. Shaler, Froc, Boston 
^at. im. 8oc, xvii. p. 288. T. F. Jamicson, Qtuirt. Journ, GeoL Soc, 1882, and Geo. 
*Iag. 1882, p. 400, 526. Fisher, * Physics of Earth's Cruat/ p. 223. 



272 DYNAMICAL GEOLOGY. [Book IIL Pabt I. 



and who iK)iut in proof of it« truth to evidence of continuous Bubsideuoe 
in tracts whore there was prolonged deposition, and of the uprise and 
cui-vature of uiiginally horizontal strata over mountain ranges like the 
Uinta Mountains in Wyoming and Utah, which have been for a long 
time out of water. To suppose, however, that the removal and deposit 
of a few thousand feet of rock should so seriously affect the equilibrium 
of the crust as to cause it to sink and rise in proportion, would evince 
such a mobility in the earth as could not fail to manifest itself in a fiir 
more powerful way under the influence of lunar and solar attraotion. 
That there has always been the closest relation between upheaval and 
denudati<m on the one hand, and subsidence and deposition on the 
other, is undoubtedly true. But denudation has been one of the con- 
sequences of upheaval, and deposition has been kept up only by continual 
subsidence. 

We are concerned in the present part of this volume only with the 
surface foiitures of the land in so far as they l)ear on questions of 
geological dynamics. The history of these features will bo more 
conveniently treated in Book VII. after the structure and history of the 
crust have been described. Before quitting the subject, however, we 
may observe that the larger terrestrial features, such as the great ocean 
basins, the lines of submarine ridge surmounted here and there by 
islands chiefly of volcanic materials, the continental masses of land, 
tind at least the cores of most great mountain chains, are in the main 
of high antiquity, stampeil as it were from the earliest geological ages 
on the physiognomy of the glol)e, and that their present aspect has 
been the result not merely of original hypogene operations, but of 
long-continued superficial action by the epigene forces described in 
Book III. Tart II. 



Section iv. H3rpogene Causes of Changes in the Texturoi 
Structure, and Composition of Rocks. 

Tlie phenomena of hyi)ogene action considered in the foregoing 
jiages relate almost wholly to the effects produced at the surfiuse. It is 
evident, however, that these phenomena must be accomi)anied by veiy 
considerable internal changes in the rocks which form the earth's outer 
crust. These rocks, subjected to enormous pressure, have boon contorted, 
crumpled, and folded back upon themselves, as if thousands of feet of 
solid limestones, sandstones, and shales had been merely a few layers of 
carpet; they have been shattered and fractured; they have in some 
places been pushed far above their original jiosition, in others depressed 
far beneatli it : so great has been the compression which they have 
undergone that their component particles have in many places been re- 
armngcd, and even crystallized. They have here and there actually 
been reduced to fusion, and have been abundantly invaded by masses of 
nudten rock from below. 

In the present section, the student is asked to consider chiefly the 



Sect. iv. S 1.] EFFECTS OF HEAT ON HOOKS. 273 

nature of the agencies by which such changes can be cflfected; the 

remits achieved, in so far as they constitute part of the architecture 

or structure of the earth's crust, will bo discussed in Book IV. At 

the outset, it is evident that we can hardly hope to detect many of these 

processes of subterranean change actually in progress and watch their 

effiscts. The very vastness of some of them places thom beyond our 

direct reach, and we can only reason regarding them from the ohanges 

which we see them to have produced. But a good number arc of a 

Und which can in some measure be imitated in laboratories and furnaces. 

It is not requisite, therefore, to speculate wholly in the dark on this 

wbject. Since the early and classic researches of Sir James Hall, great 

progress has been made in the investigation of hypogene processes by 

experiment. The conditions of nature have l)ecn imitated as closely as 

poMible, and varied in different ways, with the result of giving us 

ui increasingly clear insight into the physics and chemistry of sub- 

temuiean geological changes. The following pages are chiefly devoted 

to an illustration of the nature of hypogene action, in so far as that can 

1)6 inferred from the results of actual experiment. The subject may be 

conveniently treated imder three heads — 1. The effect* of mere heat ; 

2. the influence of the co-operation of heated water ; 3. the effects of 

oompression, tension and fracture. 

§ 1. Effects of Heat. 

The importance of heat among the transformations of rocks has 
leen fully admitted by geologists, since it used to l>e the watchword of 
the Huttonian or Vulcanist school at the end of last century. Three 
wnrces of subterranean heat may have at different times and hi different 
'legrees co-operated in the production of hypogene changes — the original 
internal heat of the glolw, the heat arising from chemical changes within 
4e crust or beneath it, and the heat due to tlio transformation of 
i&eohanical energy in the crumpling, fracturing, and crushing of the 
J^TkB of the crust. 

Rise of temperature by depression. — As stated above (p. 271), 

^he mere recession of rocks from the surface owing to superposition of 

Viewer deposits upon them will cauwi the isogeotherms, or lines of equal 

•tiliterranean temperature, to rise — in other words, will raise the 

temperature of the masses so withdrawn. This can take place, however, 

to but a limited extent, unless combined with such depression of ihu 

cmst as to admit of thick sedimentary formations. From the rate of 

increment of temperature downwards it is obvious that, at no great 

depth, the rocks must be at the temperature of boiling water, and 

that ferther do^*Ti, but still at a distance which, relatively to the earth's 

ndiuB, is small, they may reach and exceed the temperatures at which 

tbey would fuse at the surface. Mere descent to a great depth, 

Iwwever, will not necessarily result in any marked lithological change, 

tt has been shown in the cases of the Nova Scotian and South Welsh 

ood-fields, where sandstones, shales, clays, and coal-seams can l)e proved 



»r 



274 DTXAMICAL GEOLOGY. [Book HT. Part L 



to have been once depressed 14,000 to 17,000 feet below the sea-level, 
nnder an overlying mass of rock, and yet to have sustained no more 
serious alteration than the partial conversion of the coal into anthracite. 
They must have been kept for a long period exposed to a temperature of 
at least 212^ Fahr. Such a temperature would have been sufficient to 
set some degree of internal change in progress, had any appreciable 
quantity of water been present, whence the absence of alteration may 
perhaps be explicable on the supposition that these rocks were ocmi- 
paratively dry (p. 281). 

Rise of temperature by chemical transformatilon. — To what 
extent this cause of internal heat may be operative, forms part of an 
obscure problem. But that the access of water from the surface, and the 
consequent hydration of previously anhydrous minerals must prodace 
local augmentation of temperature, cannot be doubted. The conversion 
of anhydrite into gypsum, which takes place rapidly in some mineH) 
gives rise to an increase of volume of the substance. Besides the 
remarkable manner in which the rock is torn asunder by minute clefii, 
the crystals of bitter-spar and quartz are reduced to fragments.^ The 
amount of heat evolved during this process is capable of measnre- 
nient. The conversion of limestone into dolomite, on the other hand, 
which involves a diminution of volume, might likewise be made the 
subject of similar experimental enquiry. Experiments with varioiM 
kinds of rocks, such as clay-slate, clay and ooal, show that when 
those substances are reduced to powder and mixed with water, thejr 
evolve heat.* 

Rise of temperature by rock-crushing. — A further store of heat 
is provided by the internal crushing of rocks during the collajise and 
re-adjustment of the crust. The amoimt of heat so produced has heffl 
made the subject of direct experiment. Daubree has shown that, by 
the mutual friction of its parts, firm brick-clay can be heated in three- 
quarters of an hour from a temperature of 18° to one of 40' C. (do' to 
104° Fahr.).3 The most elaborate and carefully conducted series of 
experiments yet made in this subject are those conducted by MaDet 
He subjected 16 varieties of stone (limestone, marble, porphyry, granite. 
and slate) in cubes averaging rather less than ll^ inches in height to 
pressures sufficient to crush them to fragments, and estimated the 
amount of pressure required, and of heat produced. The following 
examples may be selected from his table : * — 

' The microflcopio gtructure of tliegtages in the conversion of anhTdrite into gypiO" 
is described by F. Hammerechmidt, Tfchermak^s Mineral Mittheil v.* (1883) p. 272. 

' W. Skey, Chem, NetcB, xxx. p. 290. 

* * Geo!. Expe'rimentale,' pp. 448 et acq. This distingnished chemist and godiof^^ 
during the last forty years devoted much time to researches designed to fllostiatetf; 
pcrimentally the processes of geology. His numerous important memoirs are sostterw 
thnmgh the AnnaU» de$ Mines, Chmptes JRendtu de rAeatlemie, BnUetin de la Sociflf 
QMMqiie de France^ and other publications. But he has reoentiv collected aod tt- 
naUidied tfaem as ' Etndes Synthetiqnes de Geologic Exi)crimentalo,* 8vo, 1879-« s^' 
oooae of infoniiatioii. The admirable memoirs of Dolciwo in the same journals «hnnn 
alwbertvttid. 
. . ' nAnWf^Vm, p. 187. 




Sect. iv. ^ 1.] 



SOURCES OF HEAT. 



275 



Bock. 



Tempentare 

(FdirO In 

1 cubic foot of 

rock dae to work 

of crushing. 



Number of cubic 
ftet of water-at 
32 deg. evapo- 
rated into Rteam 
at 312 deg. 



Volume of ice at 

32 deg. melted to 

water at 32 deg. 

by one volume of 

rock. 



Gben Stone, Oolite . . . . 
Sandstone, Ayre Hill, Yorkshire 

Slate, Conway 

Granite, Ab(ndeen .... 
Seotch fhrnace-clay porphyry 
Bowlej Rag (basalt) .... 



8° -004 
470.79 

132*' -85 

198'='-97 
213°-23 












004G 

0234 

07 

072 

083 

109 



0- 04008 

0-2026 

0-59fi 

0-017 

0-724 

0-925 



Within the crufit of the earth, there are abundant proofs of onormoiiB 
stresses nnder which the rocks have been crushed. The weight of rock 
involved in these movements has often been that of masses several miles 
thick. We can conceive that the heat thus generated may have been 
mfficient to promote many chemical and mineralogical re-arrangements 
thnnigh the operation of water (jpostea, p. 283), and may even have been 
here and there enough for the actual fusion of the rocks by the cnishing 
nf which it was produced. 

Rise of temperature by intrusion of erupted rock. — The great 
heat of lava, even when it has flowed out over the surface of the earth, 
iias been already referred to, and some examples have been given of its 
dfect8(pp. 210, 214). Where it does not reach the surface, but is injected 
into subterranean rents and passages, it must effect considerable changes 
upon the rocks with which it comes in contact. That such intruded 
igneous rocks have sometimes melted down portions of the crust in their 
pMBige, can hardly be doubted. But probably still more extensive 
dumges may take place from the exceedingly slow rate of cooling of 
erupted masses, and the consequently vast period during which their 
fcett is being conveyed through the adjacent rooks. Allusion will bo 
tttde in later pages to the observed amount of such " contact meta- 
morphism " (p. 559). 

Expansion. — Rocks are dilated by heat. The extent to which this 
tdrea place has been measured with some precision for various kinds of 
TOck, as shown in the subjoined table : — 



Rock. 



Bkrk marble, Oalway, Ireland 

Gvey granite, Aberdeen . . . 
we, Penfkyn, Wales . . . 
White marble, Sicily . . . 
Bed imdifaine, Portland, Conncc 
tlwt 



f 



Expansion for 
every 1° Fahr. 



00000247 

•00000438 
"0000057G 
"00000613 



Anthority. 



/Adie, Trartf. Boy, Soc. Edin. xiii. 
,\ p. 366. 

lUd. 
: Ibid. 



00000963 Totten, iiincr. Joum. Sci. xxii. 136. 



Aooording to these data, the expansion of ordinary rocks ranges f lom 



.m. O 



276 DYNAMICAL GEOLOGY. [Book IIL Paw I 



about 2*47 to 9'63 millionths for 1° Fabr. Even ordinary daily anc 
Hoasonal cbanges of temperature suffice to produce considerable snperficia 
changes in rocks (see p. 304). The much higher temperatures to whicl 
rocks are exposed by subsidence within the earth^s crust must hain 
far greater effects. Some experiments by Pfaff in heating from ai 
ordinary temperature up to a red heat, or about 1180^ C, small oolnmiu 
of granite from the Fichtolgebirge, red porphyry from the Tyrol, an< 
basalt from Auvergno, gave the expansion of the granite as 0*016808, o 
the porphyry 0*012718, of the basalt O'OllOO.^ The expansion and con 
traction of rocks by heating and cooling have been already referred to ai 
possible sources of upheaval and depression (p. 268). 

Crystallization. — In the experiments of Sir James Hall, poandec 
chalk, hermetically enclosed in gun-barrels and exposed to the tern 
l)eraturo of molting silver, was melted and partially crystallized, bu 
still retained its carbonic acid. Chalk, similarly exposed, with thi 
addition of a little wat^r was transformed to the state of marble. 
These experiments have been repeated by G. Bose, who produoec 
by dry heat from lithographic limestone and ohalk, fine-grained marU 
without melting. The distinction of marble is the independen 
crystalline condition of its component granules of calcite. Thi 
structure, therefore, can be superinduced by heat under pressare. L 
nature, portions of limestone which have been invaded by intnuiT 
masses of igneous rock, have been converted into marble, the gradatiooi 
from the unaltered into the altered rock being distinctly traceable, ft 
-will be shown in subsequent pages (p. 661). 

Production of prismatio structure.— The long-continned higl 
temperature of iron-furnaces has been observed to have superinduoed \ 
prismatic or columnar structure upon the hearth-stones, and on th* 
sand in which these are bedded.^ This fact is of interest in geologj 
seeing that sandstones and other rocks in contact with eruptive maase 
of igneous matter have at various depths below the surface assnmed i 
similar internal arrangement (p. 558). 

Dry ftision.— In an interesting series of experiments, the illiiBtiioii 
De Saussure (1779) fused some of the rocks of Switzerland and Franoc 
and inferred from them, contrary to the opinion previously expreflsei 
by Desmarest,^ that basalt and lava have not been produced froi 
granite, but from hornstone (pierre de come), varieties of " schorly 
calcareous clays, marls, and micaeeous earths, and the cellular varietie 
from different kinds of slate.* He observed, however, that the artifida 
products obtained by fusion were glassy and enamel-like, and did no 
always recall volcanic rocks, though some exactly resembled poroa 
lavas. Dolomieu (1788) also contended that as an artifioiaUy-fufle* 

* Z. Deuinch. QtoL Ges. xxiy. p. 403. 

« Trans, Roy. Soe. Edin. vi. (1805) pp. 101, 121. 

* C. Cochrane, Proc. Dudley Geol &oc. iii. p. 54. 

* Mem, Acad, Seien. 1771, p. 273. 

* De SauMure, * Voyages dans les Alpes,* edit. 1803, tome i. p. 178. 



Swt iv. § ij SXl^ERtMENTS IN FUSION. 277 

kt» becomes a glass, and not a crystalline mass with crystals of easily 
fiisiUe minerals, there must be some flux present in the original lava, 
and ho supposed that this might be sulphur.^ 

Sir James Hall, about the year 1790, began an important investiga- 
tion, in which he succeeded in reducing various ancient and modern 
Tdouiio rocks to the condition of glass, and in restoring them, by slow 
oooling, to a stony condition in which distinct crystals (probably 
pyroxMie, oliyine, and perhaps enstatite) wore rccognisaljle.^ Gregory 
Watt afterwards obtained similar results by fusing much larger quanti- 
tiei of the rocks. In more recent years, this method of research has 
leen resumed and pursued with the much more effective appliances 
of modem science, notably by Mitscherlich, 6. Eose, C. Sainte-Claire 
Seville, Delesse, Daubr^e, Fouqu6, I>3vy, Friedel and Sarasin. It has 
been experimentally proved that all rocks undergo molecular changes 
wlien exposed to high temperature, that when tlie heat is sufficiently 
niaed, they become fluid, that if the glass thus obtained is rapidly 
ouded it remains vitreous, and that, if allowed to cool slowly, a more 
Of teas distinct crystallization sets in, the glass is devitrificd, and a 
Uthoid product is the result. 

A glass is an amorphous substance resulting from fusion, 
perfectly isotropic in its action on transmitted polarized light (ante^ 
ppi 106, 111). Its specific gravity is rather lower than that of the same 
rabatanoe in the crystallized condition. By being allowed to cool 
slowly, or being kept for some hours at a heat which softens it, glass 
MBumes a dull x>orcelain-like aspect. This devitrification possesses 
much interest to the geologist, seeing that most volcanic rocks, as has 
been already (p. 110) described, present the characters of devitrificd 
glaaaea. It consists in the appearance of minute crystallites, and 
^r imperfect or rudimentary" crystalline forms, accompanied with an 
increaae of density and diminution of volume. It must be regarded as an 
intermediate stage between the perfectly glassy and the crystalline con- 
ditiona. Eocks exposed to temperatures as high as their melting-points 
^ into glass which, in the great majority of cases, is of a bottle-green 
w black colour, the depth of the tint depending mainly on the proportion 
^^ iron. In this respect they resemble the natural glasses — pitchstones 
^ obaidians. Microscopic investigation of artificially-fused rocks shows 
***t, even in what seems to be a tolerably homogeneous glass, there are 
•bnndant minute hair-like, feathered, needle-shaped, or irregularly- 
fgpegated bodies diffused through the glassy paste. These crystallites, 
*^ some cases colourless, in others opaque, metallic oxides, particularly 
oxides of iron, resemble the cr^^'stallites observed in many volcanic rocks 
(p. 106). They may be obtained even from the fusion of a granitic or 
P^Jiitoid rock, as in the well-known case of the Mount Sorrel syenite 

' * llctj PoDOos,' pp. 8 ti 9eq. 

' Ttwm. Boy, 86c. Edin. v. p. 43. The uctuul products obtaiuod by Hall have 
2*** nibjecied to tuicruscopic exaininatiou by Foiwiuc aud Levy. Compttf Remh 
^Ji I88L 



278 DYNAMICAL GEOLOGY. [Book IH. Paw I. 



near Leicester, whioh, being fused and glowly cooled, yielded to Mr. 
Sorby abundant crystallites, including exquisitely-grouped octohedra of 
magnetite.^ 

According to the observations of Delesse, volcanic rocks, when 
reduced to a molten condition, attack briskly the sides of the Hessian 
crucibles in which they are contained, and even eat them through. 
This is an interesting fact, for it helps to explain how some intmsive 
igneous rocks have come to occupy positions previously filled by 
sedimentary strata, and why, under such circumstances, the oompositioii 
of the same mass of rock should be found to vary considerably from 
place to place.* 

The most elaborate and successfal experiments yet made regard- 
ing the fusion of igneous rocks, are those of MM. Fouqu6 and 
Levy. These observers, by mixing the chemical elements and, in other 
cases, the minoralogical constituents, of certain minerals and rocks, and 
fusing these in platinum crucibles in a gas-furnace, have been able to 
produce both rock-forming minerals, such as several felspars, angite, 
leucite, nopheline and garnet, and also rocks possessing the composition 
and microscopic structure of augite-andesites, leucite-tephrites, and true 
basalts. By rapid cooling, they obtained an isotropic glass, often 
full of bubbles, and varying in colour with the nature of the mixture 
from which it was formed. Where the mixture contains the elements 
of pyroxene, enstatite or melilite, it must be cooled very rapidly to 
prevent these minerals from* partially crystallizing out of the glaas. 
Nopheline also crystallizes, easily. The felspars, on the other hand, 
])ass much more slowly from the viscous to the crystalline condition. 
In these experiments, use was made of the law that the fusion- 
tomperature of a crystallized silicate is usually higher than that of the 
same substance in the glassy state. Hence if such a glass be kept 
sufficiently long at a temperature slightly higher than that at which it 
softens, the most favourable conditions are obtained for the production 
of molecular arrangements and the formation of those crystalline bodies 
which can solidify in the midst of a viscous magma. The limits of 
temperature for the production of a given mineral must thus be 
comprised within the narrow range between the fusion-point of the 
mineral and that of its glass. By varying the temperature in the 
experiments, distinct minerals can be obtained from the same magma. 
Thus an artificial basalt, like a natural one, always shows that its 

1 Zirkel, iVt/e. Bucli. p. 92; Sorby, Address Geol. l6eci. BrU Assoc. 1880. On the 
microscopic structure of slags, &o.. see Vogelsang's * Krystalliten/ 

« Bull. Soc. Geol France, 2nd ser. iv. 1382 ; see also Trans. Edin. Roy. 8oe, xxix. 
p. 492. In the more recent experiments by Doelter and Hussak uo change mmt 
observed in the porcelain crucibles in which basalt, andcsito and phonolite were melted. 
Neues Jahrb. 1884, p. 19. Bischof has described i\ scries of experiments on the (tia&m of 
lavas with different proportions of clay-slate, lie found that the lava of Niedermendig, 
kept an hour in a Dellows-fumace, was reduced to a black glassy subfltance without 
iK)res, and that a similar product was obtained even after 30 per cent of clay-slate had 
been added and the whole had been kept for two hours in the famace. * Chem. nnd 
Thys. Geol/ supp. (1871) p. 98. 



Bcr. iv. S IJ EXPEBIMENTS IN FUSION, 279 



liYine has dystallized first. Minerals Buch as oliyine, leucite and folspar, 
rhioh solidify at higher temperatures than the others, appear first, and 
he later forms are moulded round them. By providing fiujilities for 
he crystallization of the minerals in the inverse order of their fusi- 
nlities, the eharacters of naturally formed crystalline rocks can thus be 
Officially produced by simple igneous fusion. 

Certain well-known facts which appear to militate against the 
irinciple of these experiments have been successfully explained by 
IQL Fouqu6 and Levy. Some minerals, very difficult to fuse, con- 
ktin crystals of others which are easily fusible, as if the latter had 
(systallized first, as in the case of pyroxene enclosed within leucite. 
But in reality the pyroxene has slowly crystallized out of inclusions oi 
the surrounding glass which were caught up in the leucite. WTiere the 
■me silicates are found to have crystallized first in large and sub- 
leqiiently in smaller forms, they may reveal stages in the gradual cooling 
and consolidation of the mass, one set of crystals, for example, being 
farmed in a lava while still within the vent of a volcano, and another 
doling the more rapid cooling after expulsion from the vent. 

The rocks obtained artificially by the se observers are thus classed by 
them: — 1. Andesites and andesitic poi|)liy rites — from the fusion of a 
luxtnre of four parts of oligoclase and one of augite. 2. Labradorites 
md labzadoric porphyrites — from the fusion of three parts of labrador 
•nd one of augite. 3. A microlithic rock formed of pyroxene and 
morthite. 4. Basalts and labradoric melaphyres — from the fusion of 
I mixture of six parts of olivine, two of augite and six of labrador. 
5. Nephelinites — from the fusion of a mixture of three parts of iicpheline 
Uid 1"3 of augite. 6. Leucitites — from the fusion of nine parts of leucite 
ind one of augite. 7. Leucite-tephrite — froui the fusion of a mixture 
of nlica, alumina, potash, soda, magnesia, lime and oxide of iron, 
tvpresenting one part of augite, four of labrador and eight of leucite. 
). Lherzolite. 9. Meteorites mthout felspar. 10. Meteorites with 
Edspar. 11. Diabases and dolerites with ophitic structure. In these 
irtificially produced compounds, the most complete resemblance to 
mtmal rocks was observed, down even to the minutiae of microscopic 
itmctiire. The crystals and microliths ranged themselves exactly as 
in natural rocks, with the same distribution of vitreous base and 
vitnoua inclusions. It is thus demonstrated that a rock like basalt 
■ny be produced in nature in the dry way, by a process entirely 
igneous.^ 

Hore recently, another series of experiments has been carried on by 
Measn. Doelter and Hussak of Gratz, to determine the effect of 
inuaersing various minerals in molten basalt, andesite or phonolite. 
i^aoog the results obtained by them are the ])roduction of a granular 
■tractme in pyroxene and hornblende, es2>ecially along the bordei's, as 
■■y he observed in the hornblende of recent eruptive rocks; the 

' Bm the woik of Meaurs. Fouque and Levy, * Hynth^so des Miueraux vi detj Koched,' 
UQi bom whidi the above digest of their researches is taken. 



280 DYNAMICAL GEOLOGY. [Book HI. Paw I. 



couvorBion of a hornblende crystal, whicli still retains its form, into an 
aggregate of angite prisms and magnetite, as observed also in some 
basalts; the conversion of garnet into variotis other minerals, saoh 
as meionite, melilite, anorthite, lime-olivine, lime-nepheline, spemdar 
iron, and spinel, the garnet itself never reappearing in the molten 
magma. ^ 

While experiment has thus shown that certain eruptive rocks of the 
basic order, such as basalts and augite-andesites, may be produced by 
mere dr}^ fusion, the acid rocks present difficulties which have as 
yet proved insuperable in the laboratory. MM. Fouqu6 and Ij6vy 
liave vainly endeavoured to reproduce by igneous fusion rocks vrith 
quartz, orthoclase, white mica, black mica and amphibole. We may 
therefore infer that these rocks have been produced in some other way 
than by dry igneous fusion. The acid rocks, terminating in granite, form 
a remarkable series, regarding the origin of which we are still completely 
ignorant. Some data relating to their production, will be given in § 2, 
in connection with the co-operation of underground water. 

Contraction of rocks in passing firom a glassy to a stony 
state. — Hoferenco has been made (pp. 275 et seq.^ to the expansion of 
rocks by heat and their contraction on cooling; likewise to the diffe- 
rence between their volume in the molten and in the solid state. It 
woiild ap][)ear that the diminution in density, as rocks pass from a 
crystaUino into a vitreous condition, is, on the whole, greater the more 
silica and alkali are present, and is loss as the proportion of iron, lime, 
and alumina increases. According to Delesse, granites, quartziferons 
porphyries, and such highly silicated rocks lose from 8 to 11 per cent. 
of their density when they are reduced to the condition of glass, basalts 
lose from 3 to 5 per cent., and lavas, including the vitreous varieties, 
from to 4 per cent.* More recently, Mallet observed that plate- 
glass (taken as representative of acid or siliceous rocks) in passing 
from the liquid condition into solid glass contracts 1*59 per cent., 100 
parts of the molten liquid measuring 98*41 when solidified; while 
iron-slag (having a composition not unlike that of many basic igneons 
rocks) contracts 6*7 per cent., 100 parts of the molten mass measuring 
03*3 when cold.^ By the contraction due to such changes in the 
internal condition of subterranean masses of rock, minor oscillations of 
level of the surfistce may be accounted for, as already stated (p. 268). 
Thus, the vitreous solidification of a molten mass of siliceous rook 1000 
feet thick might cause a subsidence of about IG feet, while, if the rook 
were basic, the amount of subsidence might be 67 feet. 

» Neues Jahrb. 1884, pp. 18, 158. 

' BuU. 8oc. Ged. France, 1847, p. 1390. Bisohof had dctcnulned the ooniiaotioa 
i»f granite to be as much as 25 per cent (Leonhard unci Bronn, Jahrb. 1841). The 
correctness of tliis determination was dijspnted by D. Forbes (Gfeol. Mag. 1870« 
p. 1), who found from his own experiments that the amount of contraction must 
ix) much less. The values given by him were still so mucli in excess of those recently 
obtained with much care by Mallet, that some defect in their determination may be 
suspected. 

» PhU. Tratis. dxiii. pp. 201, 204 ; dxv. ; Proc. Roy. ISoc. xxii. p. 328. 



Sktt. iv. I 2.] INFLXmtrCE Of HEAtED WAtEtt. 281 



SabUmation^— It has long been known that many mineral 
ffulsianoeB can be obtained in a crystallino fonu from the condensation 
of TaponrB (pp. 62, 184). This process, called Sublimation, may bo tho 
result of the mere cooling and reappearance of bodies which have 
been ▼aporised by heat and solidify on cooling, or of the solution 
of these bodies in other vapours or gases, or of the reaction of 
diflerent yaponis upon each other. These operations, of such common 
ooonirence at volcanic vents, and in the crevices of recently erupted 
and still hot lava-streams, have been successfully imitated by 
experiment. In the early researches of Sir James Hall on tho effects of 
heat modified by compression, he obtained by sublimation '< transparent 
and well-defined oystals," lining the unoccupied portion of a her- 
meticallyHBealed iron tube, in which he had placed and exposed to a high 
temperature some fragments of limestone.^ Numerous experiments 
have been made by Delesse, Daubree, and others, in the production of 
minerals by sublimation. Thus, many of the metallic sulptiides found 
in mineral veins have been produced by exposing to a comparatively 
low temperature (between that of boiling water and a dull-red heat) 
tubes containing metallic chlorides and salphide of hydrogen. By 
varying the materials employed, corundum, quartz, apatite, and other 
minerals have been obtained. It is not difficult, therefore, to understand 
how, in the crevices of lava-streams and volcanic cones, as well as in 
mineral veins, sulphides and oxides of iron and other minerals may 
have been formed by the ascent of heated vapours. Superheated steam 
is endowed with a remarkable power of dissolving that intractable 
substance, silica ; artificially heated to the temperature of the melting- 
point of cast-iron, it rapidily attacks silica, and deposits the mineral 
in snow-white crystals as it cools. Sublimation, however, can hardly bo 
conceived as having operated in tho formation of rocks, save here and 
there in the infilling of open fissures. 

S 2. Influence of Heated Water. 

In the geological contest fought at the beginning of tho century 
between the Neptunists and the Plutonists, the two great battle-cries 
were, on the one side, Water, on tho other. Fire. The progress of 
science since that time has shown that each of the parties had some 
troth on its side, and had seized one aspect of the problems touching 
the origin of rocks. If subterranean heat has played a large part in the 
construction of the materials of the earth's crust, water, on the other 
hand, has performed a hardly less imix)rtant share of the task. They 
have often co-operated together, and in such a way that the result 
must 1)0 regarded as their joint achievement, wherein the re8j)ective 
share of each can hardly Ijc exactly ai)portioned. In Part II. of this 
Book, the chemical o|)eration of infiltratinjj; water, at ordinary' tem- 
peratures at the surface, and among rocks at limited depths, is described. 

* TroM. Hoy, Soc* Ediiu vi. p. 110. 



282 DYNAMICAL GEOLOGY. [Book IIL Pabt L 



We are here concerned mainly with the work done by water when 
within the influence of subterranean heat. 

Presence of water in all rocks. — Besides its combinations in 
hydrous minerals, water may exist in rocks either (1) absorbed among 
minute crevices and pores, or (2) imprisoned within the microscopic 
cells of crystals. 

1. By numerous observations it has been proved that all rooks within 
the accessible portion of the earth*s crust contain interstitial water, or, 
us it is sometimes called, quarry-water (eau de carriere). This is not 
chemically combined with their mineral constituents, but is merely re- 
tained in their pores. Most of it evaporates when the stone is taken 
out of the parent rock, and freely exposed to the atmosphere. The absor- 
bent powers of rocks vary greatly, and chiefly in proportion to iheix 
degree of porosity. Gypsum absorbs from about 0*50 to 1*50 per cent 
of water by weight ; granite, about * 37 per cent. ; quartz from a vein 
iu granite, 0*08; chalk, about 20*0; plastic clay, from 19*5 to 24*6* 
These amounts may be increased by exhausting tiie air from the speci- 
mens and then immersing them in water.^ No mineral substance ifi 
strictly im^)ervious to the passage of water. The well-known artificial 
colouring of agates proves that even mineral substances, apparently 
the most homogeneous and impervious, can be traversed by liqtdds. In 
the series of experiments above (p. 246) referred to, Daubree has illus- 
trated the power possessed by water of penetrating rocks, in virine 
of their porosity and capillarity, even against a considerable counter' 
pressure of vapour ; and, without denying the presence of original water, 
he concludes that the interstitial water of igneous rocks may all have 
been derived by descent from the sui-face. The masterly researches of 
roiscuiUe have shown that the rate of flow of liquids thi-ough capillariefl 
is augmented by heat. He proved that water at a temperature of 45^ & 
iu such situations moves nearly three times faster than at a temperature 
of 0^ C.^ At the high temperatures under which the water must exist 
at some depth within the crust, its power of penetrating the capillary 
interstices of rocks must be increased to such a degree as to enable it 
to become a powerful geological agent. 

(2) Bcfcrence has already (p. 101) been made to the presence of 
minute cavities, containing water and various solutions, in the crystals 
of many rocks. The water thus imprisoned was obviously enclosed 
with its gases and saline solutions, at the time when these minerals 
crystallized out of their parent magma. The quartz of granite is 
usually full of such water-vesicles. " A thousand millions," says Mr. J, 
Clifton Ward, ** might easily be contained within a cubic inch of quartz, 
and sometimes the contained water must make up at least 5 per cent, of 
the whole volume of the containing quartz." 

* Bee an interestlug paper by Delesse, BuU, Soc. (Jtol, France j 2me ser. xiz. 
(18G1-2) p. 65. 

' Comptes Bendus (1840), xi. p. 1048. Pfaff (* Allgcmeine Geolo^e,' p. 141) ooooliidev 
from calotUationa as to the relations between presaure and tension that watar may 
dcioend to any depth hi fissurcB and remain in a liuid state even at high tempexBtorea 



'. vr. I 2.] EFFECTS OF WATER AND PRESSURE. 283 




Solvent power of water among rocks. — Tho presence of 
tentitial water Tnnst affect the chemical constitution of rocks. It is 
well understood that there is probably no terrestrial substance 
hioh, under proper conditions, is not to some extent soluble in water. 
7 an interesting series of experiments, made many years ago by W. B. 
. jnd H. D. Bogers, it was ascertained that the ordinary mineral con- 
'^tuents of rocks could be dissolved to an appreciable extent even by 
water, and that the change was accelerated and augmented by 
be presence of carbonic acid. ^ Water, as pure as it ever occurs in a 
^taral state, can hold in solution appreciable proportions of silica, 
3kaliferous silicates, and iron oxide, even at ordinary temperatures, 
he mere presence, therefore, of water within the pores of subterranean 
cannot but give rise to changes in the composition of these rocks, 
of the soluble materials must be dissolved, and, as the water 
t^vaporates, must be redeposited in a new form.* 

This power increased by heat. — The chemical action of water is 
■wressed by heat, which may be either the earth's original heat or that 
hich arises from internal crushing of the crust. Mere descent from 
"^be sorSeKse into successive isogeothcrms raises the temperature of per- 
meating water until it may greatly exceed the boiling point. But a high 
^^fliperature is not necessary for many important mineral rearrange- 
ments. Daubr6e has proved that very moderate heat, not more than 50° C. 
(122® Fahr.) has sufficed for the production of zeolites in Roman 
•cricks by the mineral waters of Plombi^res.^ He has experimentally 
demonstrated the vast increase of chemical activity of water with 
^i^^mentation of its temperature, by exposing a glass tube containing 
about half its weight of water to a temperature of about 400*^ C. At the 
®ttd. of a week he found the tube so entirely changed into a white, opaque, 
pOMrdery mass, as to present not the least resemblance to glass. The 
"^iiiaining water wcw highly charged with an alkaline silicate containing 
,. P^r cent, of soda and 37 per cent, of silica, with traces of potash and 
*^^. The white solid substance was ascertained to bo composed 
**^i^OBt entirely of crystalline materials, partly in the form of minute 
P^^fectly limpid bipyramidal crystals of quartz, but chiefly of very 
""^^^U acicular prisms of woUastonite. It was found, moreover, that tho 
P^^^^on of the tube which had not been directly in contact with the 
^ler was as much altered as the rest, whence it was inferred that, 
^ these high temperatures and pressures, the va2)our of water acts 
^^mically like the water itself. 

Co-operation of pressure. — The effect of pressure must be 

^^*^iognised as most important in enabling water, especially when heated, 

S^ dissolve and retain in solution a larger quantity of mineral matter 

*^Hu it could otherwise do,* and also in preventing chemical changes 

' American Journ, Science (2), v. p. 401. 

' Bee farther on this subject. Part II. pp. 317, 338. 

' ^G^logie Exp^rimentalo,' p. 462. 
. * Sgrt>]r has shown that the solubility of all salts which exhibit ooutraotion in solution 
U ittnaikably increased by pressure. Proc. Bay, Soc. (1862-3), p. 340. 



284 DYNAMICAL OEOLOGT. [Book IlL Pabt L 



which take place at once when the preBsure is removed.^ In Danbrfe's 
oxporiincnts above cited, the tubes were hermetically sealed and aecored 
against fracture, so that the pressure of the greatly superheated vapour 
had full effect. By this means, witli alkaline water, he not only 
produced the two minerals above mentioned, but also felspar and 
diopside. The enormous pressures under which many crystalline rooks 
have solidified is indicated by the liquid carbon-dioxide in the vesidleB of 
their crystals. Besides the pressure due to their varying depth bom 
the surface, they must have been subject to the enormous expansion of 
the superheated water or vapour which filled all their cavities, and some- 
times, also, to the compression resulting from the secular contraction of 
the globe and consequent corrugation of the crust. Mr. Sorby inferred 
that in many cases the pressure under which granite consolidated must 
have been equal to that of an overlying mass of rock 60,000 feet* or 
more th^n 9 miles, in thickness, while De la Yallee Poussin and Benard 
from other data deduced a pressure equal to 87 atmospheres (p. 103). 

Aquo-igneous fosion. — As far back as the year 1846, Scheerer 
observed that there exist in granite various minerals which could not 
have consolidated save at a comparatively low temperature.' He 
instanced especially gadoliuites, orthites, and aUanites, which cannot 
endure a higher temperature than a dull-red heat without altering their 
physical characters ; and he concluded that granite, though it may have 
possessed a high temperature, cannot have solidified from simple igneous 
fusion, but must have been a kind of pasty mass containing a con- 
siderable proportion of water. It is common now to speak of the 
" aquo- igneous " origin of some eruptive rocks, and to treat their pro- 
duction as a part of what are termed the " hydro-thermal " operations 
of geology. 

Scheerer, Elie de Beaumont, and Daubree have shown how the 
presence of a comparatively small quantity of water in eruptive igneous 
rocks may have contributed to suspend their solidification, and to 
X)romote the crystallization of their silicates at temperatures considerably 
below the point of fusion and in a succession different from their 
relative order of fusibility. In this way, the solidification of quartz in 
granite after the crystallization of the silicates, which would bo un- 
intelligible on the supposition of mere dry fusion, becomes explicable. 
The water may be regarded as a kind of mother-liquor out of which the 
silicates crystallize without reference to relative fusibility. 

Artificial production of minerals. — As the result of experi- 
ments, both in the dry and moist way, various minerals have been 
produced in the crystalline form. Among the minerals successfully 
reproduced are quartz, tridymite, olivine, pyroxene, enstatite, wollas- 
tonito, zircon, emerald, melanite, melilite, several felspars, leucite, 

» See Cailletot, Natur/orsclier, v.; Pfiiflf, Neaes Jahrh. 1871 ; W. Hpriug, BulL Acad. 
Hoy. Belgique, 2iul uer. xlix. (1880) p. 369. Pfttff found tliat plaater does not absorb 
Water under a pressure of 40 atmospheres. 

2 BuU. Soe, UeoL France^ iv. p. 468. 



iBCT. Iv. I 2.] EXPEBIMENT8 IN METAM0BPHI8M. 285 



lepheline, meionite, petalite, soTeral zeolites, dioptase, rutile, brookite, 
fifttaae, perowskite, sphene, oalcite, aragonite, dolomite, witHerite, 
idexite, oenuite, malachite, corundum, diasporo, spinel, haematite, 
iTianite, apatite, anhydrite, with many metallic ores.^ 

Kxperlxnents in metamorphism. — Besides showing the solvent 
ower of superheated water and vapour upon glass in illustration of 
rhat happens within the crust of the earth, Daubr^o's experiments 
o oaoBB a high interest and suggestiveness in regard to the internal re- 
rrangementB and new structures which water may superinduce upon 
xska* Hermetically sealed glass tubes containing scarcely one-third of 
tieir weight of water, and exposed for several days to a temperature 
alow an incipient red heat, showed not only a thorough transformation 
r stmctare into a white, porous, kaoHn-Uke substance, encrusted with 
miimerable bipyramidal crystals of quartz like those of the drusy 
avities of rocks, but had acquired a very distinct fibrous and even an 
minently schistose structure. The glass was found to split readily 
ato eonoentric laminas arranged in a general way parallel to the original 
orfaces of the tube, and so thin that ten of them could be counted in a 
veadth of a single millimetre. Even where the glass, though attacked, 
etained its vitreous character, these fine zones appeared like the lines 
f an agate. The whole structure recalled that of some schistose and 
systalline rooks. Treated with acid the altered glass crumbled and 
lennittod the isolation of certain nearly opaque globules and of some 
ninnte transparent infusible acicular cr^^stals or microliths, sometimes 
pnouped in bundles and reacting on polarized light. Eeduced to thin 
ilices and examined under the microscope with a magnifying power 
)f 300 diameters, the altered glass presented : 1st, Spherulites, ^jj of a 
nillimetre in radius, nearly opaque, yellowish, bristling with points 
irhich i)erhaps belong to a kind of crystallization, and with an internal 
radiating fibrous structure, (these resist the action of concentrated 
bydrochloric acid, whence they cannot be a zeolite, but may be a 
mbstance like chalcedony) ; 2nd, innumerable colourless acicular micro- 
liths, with a frequently stellate, more rarely solitary distribution, 
resisting the action of acid like quartz or an anhydrous silicate ; 3rd, 
lark green crystals of pyroxene (diopside). Daubree satisfied himself 
that these enclosures did not pre-exist in the glass, but were developed 
in it during the process of alteration.^ 

But beside the effects from increase of temperature and pressure, we 
bave to take into account the fact that water in a natural state is never 
ahemically pure. Rain, falling through the air, absorbs in particular 

' Fouque and Levy, * Synlhow^ des Minemiix.* 

* * Gcol. Experim.' p. 158 et seq. The production of oirstals and niicrolitha in the 
deritrification of glass at comparatively low temperatures by the action of water is of 
mat interest. The first observer wlio described the phenomenon appears to have 
been Brewster, who, in the second decade of tliis century, studied the effect upon 
poburixed light of glass decompose<l by ordinary meteoric action. {Phil. Tran9. 
1814, TVofu. Boy, 8oc. Edin. xxii. (18G0) p. G07. See on the weathering of rocks, 
p. 319.) 



286 DYNAMICAL GEOLOGT. [Book IIL Paw 



oxygen and carbon-dioxide, and filtering through the soil, abstract 
more of this oxide as well as other results of decomposing orga&j 
matter. It is thus enabled to effect numerous decompositions of sal 
terranean rocks, even at ordinary temperatures and pressures. Bat as i 
continues its underground journey, and obtains increased solvent powe 
the very solutions it takes up augment its capacity for effecting minen 
transformations. The influence of dissolved alkaline carbonates in pn 
moting the decomposition of many minerals was long ago pointed oi 
by Bischof. In 1857 Sterry Hunt showed by experiments that wati 
impregnated with these carbonates would, at a temperature of not moi 
than 212° Fahr., produce chemical reactions among the elements i 
many sedimentary rocks, dissolving silica and generating yarioi 
silicates.^ Daubree likewise proved that in presence of dissolve 
alkaline silicates, at temperatures above 700"^ Fahr. various siliceoi 
minerals, as quartz, felspar and pyroxene, could be crystallized, an 
that at this temperature the silicates would combine with kaolin to fori 
felspar.^ 

The presence of fluorine has been proved experimentally to have 
remarkable action in facilitating some precipitates, especially tin oxide 
as well as in other parts of the mechanism of mineral veins.^ Farth< 
illustrations of the important part probably played by this element i 
the crystallization of some minerals and rocks have been published b 
Ste. Claire Deville and Hautefeuille, who by the use of compoands i 
fluorine have obtained such minerals as rutile, brookite, anatase as 
corundum in crystalline form.^ Elie de Beaumont inferred that tl 
mineralizing influence of fluorine had been effective even in tl 
crystallization of granite. He believed that *'the volatile oompoan 
enclosed in granite, before its consolidation contained not only wate 
chlorine, and sulphur, like the substance disengaged from cooling lava 
but also fluorine, phosphorus and boron, whence it acquired mac 
greater activity and a capacity for acting on many bodies on which tl 
volatile matter contained in the lavas of Etna has but a comparatiTel 
insignificant action." * 

§ 8. Effects of compression, tension and fraotare. 

Among the geological revolutions to which the crust of the eart 
has been subjected, its rocks have been in some places powerfolly con 
pressed ; elsewhere they have undergone enormous tension, and almo 
everywhere they have been more or less ruptured. Heuoe intern; 
structures have been developed which were not originally present i 

» Pha. Mag. XV. p. 68. 

* BvU, 8oe, Oeog. France, xv. p. 103. 

' First Buggeetod by Daubree, Ann. de$ Mines (1841), .3me s^r. xx. p. 65. 

* Compte$ Sendut, xlvi. p. 764 (1858) ; xlvii. p. 89 ; Ivii. p. 648 (1865). Fouqu^ ai 
L^yy, *SyDtbte des Min^raux et des Boches.' 

* "Sor les Emanations VolcaniqneB et M^tallif^res," Butt, 8oo, QM. l^VafiM^ i 
(1846): p. 1249. This admirable and exhaustive memoir, one of the greatest nummnflo 
of £lie de Beaumont's genius, should be consulted by the student. 



Sbct. It. I 8.] . 00MPBE88I0N, TENSION, FR AC TUBE. 287 



the rocks. These strnctures will he more properly considered in Book IV. 

We are here concerned mainlj with the natni'e and operation of the 

agencies hy which they have heen produced. 
The most ohvious resnlt of pressure upon rocks is consolidation, aR 

where a mass of loose sand is gradually compacted into a more or less 

ooherent stone, or where, with accompanying chemical changes, a layer 
of Tegetation is compressed into peat, lignite, or coal. The cohesion of 
h sedimentary rock may he due merely to the pressure of the superin- 
camhent strata, hut some cementing material has usually contrihuted 
to hind the component particles together. Of these natural cements 
the most frequent are peroxide of iron, silica, and carhonate of lime. 
Koderate pressure equally distrihuted over a rock presenting every- 
where nearly the same amount of resistance will promote consolidation, 
hot may produce no further internal change. Whore the component 
particles are chiefly crystalline, pressure may induce a crystalline 
itracture upon the whole mass, as recent experiments have shown.* 
It however, the pressure hecomes extremely unequal, or if the rock 
•objected to it can find escape from the strain in one or more directions, 
It may undergo shear along certain planes, or may be crumpled, or the 
lii&it of its rigidity may he passed, and rupture take plaoe. Some 
consequences of ihese movements may he briefly alluded to here in 
iUnfltration of hypogene action in dynamical geology. 

(1.) Minor Bnptores and Noises. — ^Among mountain-valleys, in 
iiQway-tunnels through hilly regions, or elsewhere among rocks 
nbjected to much lateral pressure, or where owing to the removal of 
ttlterial by running water, and the consequent formation of cavities, sub- 
sidence is in progress, sounds as of explosions are occasionally heard. In 
miny instances, these noises are the result of relief from great lateral 
compression, the rocks having for ages been in a state of strain, from 
which as denudation advances, or as artificial excavations are made, 
they are relieved. This relief takes place, not always uniformly, but 
iometimes cumulatively by successive shocks or snaps. Mr. W. FT. 
Niles of Boston has described a number of interesting cases where the 
effects of such expansion could be seen in quarries ; large blocks of rock 
b«ng rent and crushed into fragments, and smaller pieces being even 
^charged with explosion into the air.^ If this is the condition of rocks 
eren at the surface, we can realise that at great depths, where escape 
from strain is for long periods impossible, and the compression of the 
naases must he enormous, any sudden relief from this strain may well 
pre rise to an earthquake-shock (p. 258). A continued condition of 
strain must also influence the solvent power of water permeating the 
rocks (p. 283). 

(2.) Oonsolidation and Welding. — That pressure consolidates 
TOchi is familiar knowledge. Loose sedimentary materials may by mere 
pRarare he converted into more or less firm and hard masses. Experiments 

« W. Spring, Bun, Acad, Roy. Belg. 1880, p. 375. 
« Pr/v?. Bo»Um Soc, Nat. UM. xviii. p. 272 (187(J). 



DYNAMICAL OEOLOOr. [Book HL Pin 1 



by Mr. W. Spring upon many BubstanceB in tho state of powder h*« 
Rhown that under higli pressure they become welded into aolid mb 
stances. Under a prossuro of 6000 atmospheres, coal-duet beoomeB i 
brilliant solid block, taking tho mould of tbo cavity in which it is placed 
and thereby giving evidence of plasticity. Feat, in like maucer, beoome 
a brilliant black substance in which all trace of the original stnictore i 

(3.) Cleavage. — Over extensive tracts of country a peonliar strao 
tnre has been superinduced, especially ujion fine-grained aigillaceons rooki 
then termed slates. They split along a set of planes which, •■ a mlc 
are highly inclined or vertical, and independent of the original bedding 
Examined more minutely, it is found that their component partiolef 
which in meet cases have a longer and shorter axis, have grouped them 
selves with their long axes generally in one common direction, oni 
parallel with the planes of fissility. An ordinary shale may prosen 




It.— fVxtloii at compnaHd >rTll]u«nu FIr. 13.— StctionufBili 

.^k In •rhkh clnvige-nnictaTe hu bnn und«iKDne till* modllkaUon. 

ilerelopi^. X*gnlD«l. |Cuni[iuF Fig. 14t.) 

under the microscope such a structure as is shown in Fig. 73. Ba 
where it has undergone the change here referred to, it has acquired tfa 
structure represented in Fig. 72, Bocks wliich, having been thus acte> 
on, have acquired this auporinduced fissility, aro said to bo cleaved 
and the fissile structure is tonned cleavage. In Fig. 74, for examplt 
wheio the strata, at first in even parallel l>Qds, liave been Bubjecto 
to great comprosaion from the directions (a) and (b), the origins 
planes of stratification are roprosented by wavy lines, and the iiei 
system of cleavage-planes by fine upright lines. The fineness of th 
(Jeavuge depends in largo measure uiwii tho texture of the origins 
rock. Snndstones, consisting as they do of rounded obdurate quarti 
grains, take either a very rude cleavage (or jointing) or none at ol) 
Fine-grained argillaceous rocks, consisting of minute partioles o 
flakes, that can adjust their long axes in a new direction, are those ii 

' Bali. Aead. Son. Belg. 1680, p. 325, and ante, p. 171. 



Sect. iv. I 3.] COMPRESSION, TENSION, FBACTUBB. 289 

vMch the structure is best developed. In a series of cleaved rocks, 
tWefore, cleavage may be perfect in argillaceous bods (6 6, Figs. 75 
aT>il 76), and imperfect or abeent in interstratilied beds of Bandstoiio 
(<i a. Fig. 75) or of limestone (as at Clonea OasUe, Waterford, a a 

Tip 76). 

That cleavage may be produced in a mechanical way by lateral 

pTMnire has been proved experimentally by Sorby, who effected perfect 




Ftg. t4,-~Cnrv«d qnuti-TOCk tnrencd b; vortlul and bigtalj'EncUned Cleavage. 
SoMh Btack LighUwow, Anglwa (A,). 

cleavage in pipe-clay through which scales of oxide of iron bad 
Iirerionsly been mixed.' Tyndall superinduced cleavage on bees-wax 
•ad other Rn1«tanccs by subjecting them to severe pressure. More 
ncently, Fisher has proposed the view that in nature it is not to the 
(icwnre which plioated the rocks that cleavage is to be attributed, but 
to the shearing movements geneiated in large masses of rock left in a 
jnrition too lofty for eriuilibrinm.' If such, however, had been the origin 




DFfenlnKc of Clearagt apon Ibc grain of the 



of the itructure it is difGcult to understand wby there should be such a 
pvralent relation between the strike and the cleavage, for if descent by 
(notation were the main cause wc sbould expect to find the rocks 
■heartd far more irregiilarly than even tlif most irregular disposition of 
•'lavage. That in cleavago there lias heeu a true shearing of the rocks 

' EUa. Stw FUI. Junra. Iv. {l&W, p. 137. \V. King, Jioy. Iriih Amd. xxv. (187il) 
^ (W. Tha itwient vitl find ttoent inttfH'Hlinc nilclitions ti> nur knowlctlj-e of tb<> 
Biicz'Mn^ ttnietnre RDd thehinlory of rliuved nnrka in Mr. Sorb; 'b oddreBs. <ii. /, Geol. 

A>c. ii»i.p.72. Bee also E. Jnontttaz, Bull. Sf-: Giol. Fraiiet, ix. (1881) p. 19t; ; 

atL (l!M)p.Sll. 



290 DYNAMICAL GEOLOGY. [Book IIL Pam I. 



is indubitable ; and the amount of shear may be ascertained by the 
extent of the distortion of fossils in the planes of cleavage (Figs. 
77-80). Microscopic study of cleaved rocks shows that their fissility 
is not always due merely to a rearrangement of original clastic par- 
ticles, but to the development of new minerals, particularly varieties 
of mica, along the planes of cleavage. This relation is well seen in the 
folded and cleaved Devonian and Carboniferous rocks of S.W. Ireland 
and Cornwall, in the Carboniferous shales of Laval, Mayenne, and in 
the Jurassic and Eocene shales of the Alps.* Just as shales graduate 
into true cleaved slates, so slates by augmentation of their saperinduced 
mica pass into phyllites, and these into mica-schists. The structure of 
districts with cleaved rocks is described in Book IV. Part V. . 

(4.) Deformation. — Further evidence of the internal movements of 
rocks is furnished by the way in which contiguous pebbles in a con- 
glomerate have been squeezed into each other, and even sometimes 
have been elongated in a certain general direction. The coarseness of 
the grain of such rocks permits the effects of compression or tension to 
be readily seen. Similar effects may take place in fine-grained rocks and 
escape observation. Daubr6o has imitated experimentally indentations 
produced by the contiguous portions of conglomerate pebbles.^ 

In discussing the cause of these indentations it must be remembered 
that imprints of pebbles upon each other, particularly when the 
material is limestone or other tolerably soluble rock, may have been 
to some extent produced by solution taking place most actively 
where pressure was greatest (p. 283). But there are indubitable 
evidences of crushing and deformation, even in what would be 
termed solid and brittle rocks. Of these evidences, perhaps the most 
instructive and valuable are furnished by the remains of plants and 
animals occurring as fossils, and of which the unaltered shapes are well 
known. "Where fossiliferous rocks have undergone a shear, the exteni 
of this movement, as above remarked, can be measured in the resultaal 
distortion of the fossils. In Figs. 77 and 79 drawings are given o: 
two Lower Silurian fossils in their natural forms. In Fig. 78 f 
specimen of the same species of trilobite as in Fig. 77 is represented 
where it has been distorted during the shearing of the enclosing 
rock. In Fig. 80 four examples of the same shell as in Fig. 79 an 
shown greatly distorted by a strain which has elongated the rock in tb< 
direction a h? Amorphous crystalline rocks (pegmatite, granite, diorite' 
have been so crushed as to acquire a schistose structure (pp. 575, 578). 

Another illustration of the effects of pressure in producing deforma 

* Jannettaz, Renevier and Lory. BuU. Soe. Geol. France^ ix. p. 649. 

' Comptes Eendtis, xliv. p. 823 ; nlso his * Gdologie Expdri mental c/ part i. sect, li 
chap, iii., where a seriefi of important experiments on deformation is given. For yarioiu 
examples and opinions, soe Rothpletz, Z. DfiiUsch. Ged. Ges. xxxi. p. 355. Heim 
• Mecnanismns der Ocbirgsbildung,' 1878, vol. ii. p. 31. Hitchcock, * Geology of Vermont 
i. p. 28. Proe. Bost, Soo. Nat. llht. vii. pp. 209, 353 ; xviii. p. 97 ; xv. p. 1 ; xx. p. 313 
Amer. Assoc. 1866, p. 83. Amer. Jour. Set. (2) xxxi. p. 372. Sorby, i?g>. OardUt Nai 
Soc. 1873, p. 21. Bt. H. Rensch, * Fossilien-fuhrender Kryst. Schiefer,' p, 25. 

■ See D. Sharpe, Q. J. Geol Soc, iii. (1846) p. 75 ; O. Fisher, Geol Mag. 1884. p. 399. 



l.fT. ja.] C0MPBE88I0K, TENSION, FS ACTUS E. 



291 



tun in rocln, is Hnpplied by the so-called " lif^iliteg," " epsomiteB," or 
"itfloUtes." Them are cylindrical or columnar bodies varying in 
tengtli up to more than four inohcB, and in diameter up to two or more 
indies. The sides are longitudinally etriated or grooved. Each column, 
nmlly with a conical or rounded oap of day, beneath which a shell or 




■'tber organism may frequently be detected, is placed at right angles to 
'^ licdding of the limestones, or calcareous shales through which 
■' panes, and consiBts of the same material. This Htmoturo has 
"^i referred by Professor Marsh to the difTerence between the 
"'"wtsiice offered by the column under the sbcll, and by the surrounding 




^^trii to SHiwrincumbent pressure. Tlio Htriated surface in this view 
'* a cose of " Hliokeiisides." The same observer has suggested that the 
''^oie complex structure known as " cone-in-cono " may bo duo to thf 
^'^on of pressure upon concretions in tho course of formation.' 

I8C7. GUmbol, feftx*. Dfriheh. Otot. Ott. sixiv. 

V 2 







\Bo^ 




fto^ 




\lft^e 



o« «^^T tocV- 






i*V 



ftTV 



t\vft*' , ^ \eftA' 



ot 



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i»U^S 












fttv' 






to ft"^" ' o€ ^"^ 






Cft^'^^ 



^».vi'ssi-> -r^ 



v-;-, .«,»v--rj.»»»r;»*»',r:;'ct «<•* "";i»«**»w 



^" I*A «!"'':Sj =»»'."■ cowV-n, B.'* 



^ '^^^ 



Vf^^C*^ 



t\vefy 









r^^^\ w^p. r::^\^'s>^ ?!?r>» *'!?.**^«« ^^ 



•^"^^ fS^-^'^^za^ ^. 



,\>\e g*^:::i.t eWaAtv, 



AoTig 






acq; 



aoii 



e 



,«eBe«® 



j,\lW« 



'" ic to -- 



<Son>^"" 



t^eeti 



ti^ 






tai 






the 
o 









ateft 



.aVnS.-:,,, tF-r.hong »g« Sa te^: 



Bl 






>ovfW 



.\e 



tot^* 



A < 



ftte<^«- ,,«xo«8 «^*rVccotAf 6 ala«^f ^' ong^*' , . 






a;ii^ 









ea^ 



I 



cots 



^s^^--r^^>^"' 



^^"^ .eB t^« "^ «aV, *^* ruBt *»* ToOxet tT^^J^.^ 
^^^' W tV^« *°^^ 14 «^^*'- At o« «^^^te ot t^\ 






Bt 






tb® 



«»?JSr«-. 



0O«- 






Sect. if. I 8.] C0MFBE88I0N, TENSION, FR' AC TUBE. 



293 



intricate pnckenngB are visible (Fig. 10). So iutouse has been the 
pienaxe, that even the tiny flakes of mica and other minerals have been 
forced to arrange themselves in complex, friJled, crimped and goffered 
ibklings- On an inferior scale, local compression and contortion may bo 
Gtued by the protrusion of eruptive rocks. The characters of plicated 
rocb as part of the framework of the terrestrial crust are given in 
Book IV. Part IV. 

As may be supposed, it is difficult to illustrate experimentally the 
prooeans by which vast masses of rock have been plicated and crumpled. 
The early devices of Sir James Hall, however, may be cited from their 
interest as the first attempts to demonstrate the origin of the contortion 
of rocks. He placed layers of cloth under a weight, and by com- 
preenng them f^m two sides produced corrugations closely resembling 
iboee of the Silurian strata of the Berwickshire coast (Fig. 81). 
Professor Favre of Geneva has devised an experiment which more 
doBely imitates the conditions in nature. Upon a tightly stretched 
band of india-rubber ho places various layers of clay, making them 
adheie to it as firmly as possible. By then allowing the band to 
contract he produces in the overlying 
stnta of day a series of contortions, 
inTenions, and dislocations which at 
onoe recall those of a great mountain 
duun.^ 

(6.) Jointing and Dislocation. 
—Almost all rocks are traversed by 
vertical or highly inclined divisional 
planes termed joints (Book IV. Part 
n.). These have been regarded as 
^ne in some way to contraction dur- 

"*g consolidation (fissures of retreat); and this is no doubt their 
^^'igin in innumerable cases. But, on the other hand, their frequent 
'^g^ilarity and persistence across materials of very varying texture 
^'"K^t rather the effects of internal pressure and movement within 
the crust In an ingenious series of experiments, Daubree has imitated 
jointa and fractures by subjecting different substances to undulatory 
ittovement by torsion and by simple pressure, and he infers that they 
"*ve l)een produced by analogous movements in the terrestrial crust.^ 

Bnt in many cases, the rupture of continuity has been attended with 
'I'lative displacement of the sides, producing what is termed a fault, 
^ubrte also shows experimentally how faults may arise from the same 
movements as have caused joints and from bending of the rocks. As 
^^^ solid cnist settles down, the subsidence, where unequal in rate, 
'^'*y cause a rupture between the less stable and more stable areas, 
"k'u a tract of ground has been elevated, the rocks underlying it 

\ iVaturc, xiv. (1878), p. 103. 
(\sr ' ^^^ Experim.' Part I. sect. ii. chap. ii. See W. King, Boy. Irith Acad. xxv. 
v»o75), p. 605, and the theories of jointing given postea, p. 490. 




Fig. 81.— Hall's Experiment illiutratiug 
contortiou. 



294 DYNAMICAL GEOLOGY. [Book IIL PABfr I. 

get more room by being pmilied up, and are placed in a positicm. of 
more or less instability. As tbey cannot occupy the additional space 
by any elastic ex2)ansion of their mass, they accommodate them- 
Helves to the new position by a series of dislocations.^ Those segmentii 
having a broad base rise more than those with narrow bottoms, or the 
latter sink relatively to the former. Each broad-bottomed segment 
is thus bounded by two sides sloping towards the upper ]»rt oi 
the block. The plane of dislocation is nearly always inclined from the 
vertical, and the side to which the inclination rises, and from which it 
'* hades," is the upthrow side. Faults of this kind are termed Manual. 
and are by far the most common in nature. In mountainous regions, 
however, instances frequently occui* where one side has been pushed 
over the other, so that lower are placed above higher beds. Such a faull 
is said to be reversed. It indicates an upward thrust within the crast 
and is often to be found associated with lines of plication. Where f 
Hharp fold, of which one limb is pushed forward over the other, gives wa^ 
along a line of rupture, the result is a reversed fault. The details o! 
these features of geological structure are reserved for Book IV. Part VI 

§ 4. The Metamorphism of Rocks. 

Mctamorphism is a crystalline (usually also a chemical) rearrange- 
ment of the constituent materials of a rock. In its production the fol- 
lowing conditions have been mainly operative. (1) Temperature, from 
the lowest at which any change is possible up to that of complete fusum; 
(2) nature of the materials operated upon, some being much more 
susceptible of change than others ; (3) mechanical movements, whiob 
so often have induced molecular rearrangements in rocks ; (4) pressoret, 
the potency of the action of heat being, ^vithin certain limits, increased 
with increase of pressure ; (5) presence of water, usually containing 
various mineral solutions, whereby chemical changes might be effidcted 
which would not bo possible in dry heat. 

Since experiment has proved that in presence of water under 
pressure, even at comparatively low temperatures, mineral substanoofi 
are vigorously attacked, we may expect to find that as these conditiona 
abundantly exist within the earth's crust, the rocks exposed to them 
have been more or less altered. A large proi)oi'tion of the accessiUe 
ci-ust consists of sedimentary materials which were laid down on the 
ocean bottom, and which were sfcill abimdautly soaked with sea-watei 
even after they had been covered over with more recent formations. 
The gradual growth of submarine accumulations would of course 
deprive the lower strata of most of their original water, but some 
proportion of it would probably remain. If, according to Dana, the 
average amount of interstitial water in stratified rocks, such as lime- 
stones> bandstones and shales, l>e assumed to be 2*67 per cent., which is 
pi-obably less than the truth, *' the amount will correspond to two quarts 

;[ Sec J. M. Wilson, Geol Mag, v. p. 20G ; O. Fieher, ojt. cit. 1884. 



BexTLTf.i 4.] METAM0RPHI8M. 295 



of witer for eveiy cubio foot of rock." ^ There is certainly a consider- 
able store of wat^ ready for chemical action when the required conditions 
of heat and pressure are obtained. We must also remember that, as the 
water in which the sedimentary formations of the crust were formed 
wiB mostly that of the ocean, it already possessed chlorides, sulphates, and 
other ndts with which to begin its reactions. The inference may therefore 
be drawn^ that rocks possessing not more than 3 per cent, of interstitial 
water cannot be depressed to depths of several thousand feet beneath the 
level of the earth's surfistce, and undergo great pressure and crushing, 
without suffering more or less marked internal change or metamorphism. 
A metamorphosed rock is one which has suffered such a minera- 
logical rearrangement of its substance. It may or may not have been a 
dystalline rock originally. Any rock capa]>lc of alteration (and all 
rucb must bo so in some degree) will, when subjected to the required 
wnditions, be metamorphosed. The resulting structure, however, will, 
MTe in eztrome cases, bear witness to the original character of the mass. 
In some instances, the change has consisted merely in the rearrangement 
ur crystallization of one mineral originally present, as in limestone 
inverted into marble ; in others, the constituents have been forced by 
mechanical movements to range themselves in parallel laminee, as where a 
<liorite or pyroxenic rock becomes a hornblende-schist ; in others, the intro- 
dnctiou of mineral solutions has involved the partial or complete trans- 
formation of the original constituents, whether crystalline or clastic, 
iiito new crystalline minerals. Quartzite is evidently a compacted 
sandstone, either hardened by mere prossui-e, or most freciuently by the 
<iepoBit of silica between its granules, or a slight solution of these 
g'^^ules by permeating water so that they have become mutually 
''^^herent A clay-slate is a hardened, cleaved, and partially metamor- 
phosed form of muddy sediment, which on the one hand may be found 
^"U of organic remains, like any common shale, while on the other, by the 
[Appearance and gradual increase of some form of mica and other minerals, 
'^ Hiajbe traced becoming more and more crystalline, until it passes into 
P^yUite, chiastolite-slatc, or some other schistose rock. Yet remains 
^^ fossils may be obtained even in the same hand-specimens with 
^^stals of andalusite, ganiet, or other minerals. The calcareous matter 
^^ Oorals is sometimes replaced by hornblende, garnet, and axinite, with- 
^^t deformation of the fossils.^ A few illustrative examples of meta- 
''^^^hism may be given here; the structure of metumorphic rocks, 
''^th the phenomena of " regional " and " contact " metamorphism, will 
*^ discussed in Book IV. Part VIII. 

traduction of marble from limestone, — One of the most obvious 
e« of alteration — the conversion of ordinary limestone into crystal- 
*^^*i saccharoid marble — has been already (p. 276) referred to.^ The 

' * Hauuiil,' Snl od. (1880), p. 758. 
. • Ann. dea Mine^f, .5me Bcr. xii. ji. ^18. II. II. KoiiBch, 'Din Fcwsilieii filhreiideii 
•^"■yBtalliniachcn Schicfcr vou IJeixeu * (tmnaltttcil by K. IJulduiif), lA'ipzijr, 1883. 

' See alBO •• Marmiiro»i« " in Book IV. Part VIII. 



296 DYNAMICAL GEOLOGY. [Book UI. Part 



calcito liaviug imdergouc completo transformation, its original stru* 
tiire, wlietlier organic or not, has been effaced, and a new Btmctnre hs 
Ijoou dovelopud, consisting of an aggregate of minute rounded grain 
each with an independent cr^'stalliue arrangement. The productic 
of a crystalline structure in amorphous calcite, may be effected I 
the action of mere meteoric water at or near the surface {ante^ p. 11 
and 2H)8t€a, p. 340). But the generation of the peculiar granuL 
structure of marble always demands heat and pressure, and probabl 
usually the presence of water ; the details of the process are, howevc 
still involved in obscurity. We know that where a dyke of basalt • 
other intrusive rock has involved limestone, it has sometimea been ab 
t<; convert it for a short distance into marble. The heat (and perha] 
the moisture) of the invading lava have sufficed to produce a granuL 
structure, which even under the microscope is identical with that 
marble. The conversion of wide areas of limestone into marble is 
regional metamorphism, associated usually with the alteration of oth 
sedimentitry masses into schists, &c. 

Dolomiiization, — Another alteration which, from the labours of V< 
Buch, received in the early decades of this centurj* much attentv 
Irum geologists, is the conversion of ordinary' limestone into dolomil 
»Somo dolomite appears to be an original chemical precipitate fro 
the saline water of inland lakes and seas (p. 384). But calcareoi 
formations due to organic secretions are often weaklj' dolomitic at t' 
time of their formation, and may have their proportion of magncsir 
carbonate increased by the action of i>ermeating water, an is proved 
the convfrsion into dolomite of shells and other organisms, cMUsist 
originally of calcito or aragonite and forming portions of what wm 
doubt originally a limestone, though now a continuous mass of dolim 
This change may have Hometimes consisted in the mere abstTBCtiQ 
carlnmatu of lime fi\)m a limestone already containing carlioiu^ 
magnesia, so as to leave the rock in the form of dolomite; orpi^- 
re usually in the action of the magnesium salts of m^ 

ecially the chloride, upon organically-fonned limestone; ^^ 
times lo(jally in the action of a solution of carbonate of 



csiMJCiuii^y tiiu uiiiorioe, upon orgaiucaiiy-iormeu iimosujne; ^^ 
times lo(jally in the action of a solution of carbonate of in%I*^ 
«|jirbonated water upon limestone, either magucsian or noiKt^%F 



VlVw dc Beaunuiut (calculated that on the assumption tli^^\j 
every twt> equivalents of carbonate i>f lime was replaoQ^ x ^ 
of magnesia, the conversion of limestone into dolomite Wi ,^^> v^V 
witli a reductitin of the volumo of the mass to the .. '^ 

<eiil. It is certainly remarkal)le in this connection \\. 
dolomite, wliicli may bo conceived to have once bcHsi\ y 
cavernous, iissured structure which, on this theory ^,c 
liave Ikm'U l(»oked for. 

Dolomite has been produced Inith on u «»■ 
In the north of England and elsewhen* ' 
hiis been altered for a few feet ■•• 
into a dull yellow dolomite. 1- 



Sm. hr. f 4] METAMOBPHISM. 297 



zones of dolomite occur also in the Carboniferous Limestone of Ireland. 
HarknesB pointed out that the dolomite appears in vertical ribs where the 
rookB are much jointed, and in beds where they have few or no joints.^ 
Ko doubt percolating water has been the agent of change in the vertical 
ames. The beds, however, which in Ireland and elsewhere constitute 
important masses in the Carboniferous Limestone, were more probably 
fonned contemporaneously with the rocks among which they lie. They 
may ha^e been deposited as limestone in shallow lagoons where the 
magnesian salts of concentrated sea-water would act upon them. Dole- 
mite sometimes forms great ranges of mountains, as in the Eastern Alps, 
where it has by some writers been regarded as altered ordinary limestone. 
In all probability, however, these masses, also, became dolomite at the 
beginning, by liie action of the magnesian salts of the concentrated 
waters of inland seas upon organic or inorganic calcareous deposits 
aooomnlatod previous to the concentration, their metamorphism having 
oomdsted mainly in the subsequent generation of a crystalline structure 
analogous to thai of the conversion of limestone into marble.^ 

Comoenum of vegetable substance into coal. — Exposed to the atmo- 
spbere, dead vegetation is decomposed into humus, which goes to 
increase the soil. But sheltered from the atmosphere, exposed to the 
action of water, especially with an increase of temperature, and under 
some pressure, it is converted into lignite and coal. An example of 
thia alteration was observed a few years ago in the Dorothea mine, 
ClansthaL Some of the timber in a long-disused level, filled with slate 
nibbish, and saturated with the mine-water from decomposing pyrites, 
was found to have a leathery consistenco when wet, but, on exposure to 
the air, hardened to a firm and ordinary brown-coal, with the typical 
brown colour and external fibrous structure, and having the internal 
fracture of a black glossy pitch-coal.^ This change must have been 
produced within less than four centuries — the time since the levels wore 
^pened. According to Bischofs determinations the conversion of wood 
into coal may take place, 1st, by the separation of carbonic acid and 
^^buretted hydrogen ; 2nd, by the separation of carbonic acid, and the 
formation of water either from oxidation of hydrogen by meteoric 
®^ygen, or from the hydrogen and oxygen of the wood ; 3rd, by the 
*P*ration of carbonic acid, carburetted hydrogen and water.* The 
^circumstances under which the vegetable matter now forming coal has 
^^ accumulated were favourable for this slow transmutation. The 
^bon-dioxide (choke-damp) of old coal-mines and the carburetted 

[ Q. jr. Gtol. Soe. XV. p. 100. 
^ On dolomitization, sec L. von Buch, in Lconhurd^H Mineralog. Tafcfiehbuchj 1824 ; 
*J*jo»ann'8 • Geog^osie,' i. p. 763 ; Bischofs * Chemical Geology,' iii. ; Elie de Beaumont, 
i!!t '^' ^'^^' France, viii. (1836) p. 1 74 ; Porby, BHi. Aswc. Rep, 1856, part ii. p. 77, and 
•*«a«^, q, J. Gtol, Soc, 1879. A full statement of the litemture of tliia subject wiU bu 
mn^ in a suggestive memoir by 0. Doelter and R. Hoenie:*, Jahrb. Geol. Reichmuslalt ^ 
^* See also potteaj p. 562. The dolomite mountains of the Eastern Alps have been 
^1 described by Mojsisovics. See account of Triossic system, potleaf Book VI. 
Hinchwald, Z, Deuitch, Geol, Ges, xxv. p. 364. 

* BiKhof, ' Chem. Geol.* i. p. 274. 



298 DYNAMICAL GEOLOGY. [Book HL Paw 

hydrogen (fire-damp CH4) given ofif in such large quantitieB 1^ c 
ueams, are productH of the alteration which would appear to 
accelerated by terrestrial movementti, Bucli as those that oompresii 1 
2)licato rocks. During the proceus, these gases escape, and the pio^ 
tion of carbon progressively increases in the residue, till it reachee 
most highly mineralized anthracite (p. 173), or may even pass into nea 
pure carbon or graphite. In the coal-basins of Mons and Yalencienj 
the same seams which are in the state of bituminous coal {graa) at 
surface, gradually lose their volatile constituents as they are tra 
downward till they pass into anthracite. In the Pennsylvanian a 
field, the coals become more anthracitic as they are followed into 
eastern region, where the rocks have undergone great plicatioiiy I 
where, possibly during the subterranean movements, they were expo 
to an elevation of temperature.^ Daubree has produced from wo 
exposed to the action of superheated water, drop-like globules of antknu 
which had evidently been melted in the transformation, and wb 
presented a close resemblance to the anthracite of some mineral veijui 

Production of the schistose structure, — All rocks are not equally j 
meable by water, nor is the same rock equally permeable in all di] 
tions. Among the stratified rocks esix^cially, which form so larg 
proportion of the visible ten-ostrial crust, there are great differez 
in the facility with which water can travel, the planes of sedimentat 
(or those of cleavage or sliearing where these have been develop 
being naturally those along which water passes most easily. It 
in these planes that dififerenccs of mineral structure and oompoeii 
are ranged. Alternate layers of siliceous, argillaceous, and caloaxe 
material vary in porosity and ca])ability of being changed by ] 
meating water. We may, therefore, expect that unless the ozigi 
stratified structure has been efifaced or rendered inoperative by any ot 
superinduced structure, it will guide the metamorphic action of \uii 
groimd water, and will remain more or less distinctly traceable e^ 
after very considerable mineralogical transformations have taken pli 
Even without this guiding influence, superheated water can, to a oerl 
extent, produce a schistose structure, parallel to its bounding surfa 
as Daubree*B experiments \i\ion glass, above cited, have proved. ' 

The stratified formations consist largely of silica, silicates of alumi 
lime, magnesia, soda, and potash, and iron oxides. These mineral b 
stances exist there as original ingredients, partly in recognisable w 
crystals, partly in a granular or amor^)hous condition, ready to be ac 
on by permeating water under the requisite conditions of temperat 
and pressure. We can understand that any re-combination and 
crystallization of the silicates will prol)ably follow the lamince of dep 
or of cleavage, and that in this way a ciystalline foliated structure ma^ 
developed. Kound masses of granite erupted among Palaeozoic ro< 

* Daubree, * Geologic Expuriuicntale/ p. IGJ. Part of the framework below a ate 
hiiiiiiucr had been fuuud after twenty years to be coQvertecl into li^iite. F. Seek 
Verh. Geol. Beichs. 1883, p. 192. ' Op, cit, p. 177. 



Ski. if. | 4] METAMOBPHISM. 299 

iDstractiTe sectioiiB may be obeerved whore a transition can be traced from 
ordinary mialtered sedimentary strata, bucL as sandstones, greywackcs 
and Bhales containing foesilB, into foliated crystalline rocks, to which 
the nunes of micaHMduBt and even gneiss may be applied. (Book IV. 
Ftft Tin.) Not only can the gradual change into a crystalline 
fdiated straoture be readily followed with the naked eye, but Avith the 
aid of the mioroaoope the finer details of the alteration can be traced. 
IGnute plates of some micaceous mineral and small concretions of anda- 
luite, garnet, quartz, d:c., may be observed to have crystallized out of the 
soiroaiiding amorphous sediment. These, especially the mica, can be 
leen gradually to increase in size and number towards the granite, until 
the rock assumes a thoroughly foliated structure and passes into a true 
achiit Tet even in such a schist, traces of the original and durable 
water-worn quartz-granules may be detected.^ Foliation is thus a 
ciyataUine segpregation of the mineral matter of a rock in certain 
dominant planes which may be those of original stratification, of joints, 
of deavage, of shearing or of fracture.^ Mr. Sorby has recognised 
foliation in three sets of planes even among the same rocks.^ 

Sorope many years ago called attention to the analogy between the 
foliation of soldsts and the ribbanded or streaked structure of trachyte, 
obsidian and other lavas.^ This analogy has even been regarded as an 
identity of structure, and the idea has found supporters that the 
Bchiitoee rocks have been in a condition similar to or identical with that 
of nuiny volcanic masses and have acquired their peculiar fissility by 
differential movements within the viscous or pasty magma, the solidified 
niinerals being drawn out into layers in the direction of shearing. 
Daubree, availing himself of the researches of Tresca on the flow of 
*>lidfl (p. 292), has endeavoured to imitate artificially some of the 
pbenomena of foliation by exposing clay and other substances to great 
but Qnequal pressure.* 

That the production of the schistose structure has been largely 

influenced by mechanical movements cannot be doubted. A relation 

^ be commonly traced between the completeness of this structure and 

^® extent of the corrugation of the rocks, the most highly puckered 

'flaases being also as a rule the most coarsely schistose. We may 

^ceive the shearing action that produced cleavage to be still further 

"^^^Icpeil. It might be accompanied with sufficient augmentation of 

^I>«rature to permit of extensive mineralogical transformation along 

^® oleavage-planes or sheariug-planes. But probably a rise of tempera- 

^/^ was not essential. The conversion of pyroxene into hornblende, 

^i^^oli has been observed in regions of crystalline schists, points indeed 

*^ lower temperature than that required for the crystallization of 

^^ original mineral.® A schistose structure of almost any degree of 

^ ^knrby, Q. J. GeoL 8oo. xxxvi. p. 82. 
^ Darwin, ' Geological Obttcn'ations,' p. 1G2. llauiday, ** Goologv of North Walen," iii 
**>V>iw of Gtol. Survey, vol. iii. p. 182. '» Op. cil. p. 84. 

* •Volcanoes,' pp. 14U, 30U. * •(ieoh.gie Expe'rimentale/ p. 410. 

Bee for example Q. U. Williamti, Anter. Juum, i:y*:i, 3nl aer. xxviii. (1884) p. 25U. 



300 DYNAMICAL GEOLOGY. [Book H 



coarseness might oonoeivably be produced. A mixed rock like gxaail 
might be converted into a foliated gneiss. Diorite, diabase or gabbi 
might likewise by mere mechanical movement be made to aflBume 
schistose structure and pass into amphibolite-schist. That snoh hi 
been really the origin of at least some gneisses and sohistB is in tl 
highest degree probable. 

Again, in some places the schistose structure may be found ' 
disappear and to be replaced by one of a thoroughly amorphouB kin 
indistinguishable from that of ordinary eruptive rocks. Where Teii 
or injected portions of such an amorphous rock penetrate the adjoiniii 
highly crystalline foliated masses, they may be i)ortiouB of the foliat< 
rocks reduced to the ultimate stage of crystalline rearrangement, eveo 
trace of foliation or original structure having been effaced, and ti 
rocks having been brought into a plastic condition, in which, dnrix 
the crumpling of the crust, they were actually forced into crackB of ti 
less highly altered members of their own series. Gneiss in a plast 
state and squeezed into fissures, or between beds of firmer oond 
tence, might conceivably consolidate as granite. The solution of sn< 
problems will be best obtained by an exhaustive study of the snooe 
sive phases of metamorphism, beginning with the initial stages whe 
the original characters of the altered rocks are still indisputable.^ 

The study of metamorphism and metamorphic rocks leads us &o 
unaltered mechanical sediments at the one end, into thoroughly orysti 
line masses at the other. We are presented with a cycle of ohan; 
wherein the same particles of mineral matter pass from crystalline roo 
into sedimentary deposits, then by increasing stages of alteration ba 
into crystalline masses, whence, after being reduced to detritus and i 
deposited in sedimentary formations, they may be once more launch 
on a similar series of transformations. The phenomena of metani< 
phism appear to be linked together with those of igneous action 
connected manifestations of hypogene change. The author has fhrtl 
suggested a relation between periods of extensive metamorjthism a 
periods of volcanic eruption. He has pointed out that in the geologic 
history of Britain there are indications of such a relation, the voloa] 
eruptions of the Old Bed Sandstone period, for example, succeeding i 
time when the Silurian rocks of the Scottish Highlands were crumpled a 
metamorphosed.^ 

Part II. Epicene or Surface Action : 

An Inquiry into the Oeoloyical Changes in progrcBS upon the EarlKs Sur/oi 

On tLe surface of the globe and by the operation of agents worki 
there, the chief amount of visible geological change is now effect 
This branch of incjuiry is not involved in the preliminary difficul 
i-cgarding the very nature of the agents, which attends the investigati 
of plutonic action. On the contrary, the surface agents are carry! 

' See tbe account of the GryBtnlline Scliista, p. 554, whiih is necessarily in iMurt t 
tioipated above. * Tram. Gad, 8oe, Edin. il p. 287. 



Pin IL Bmr. i] EPIGENE ACTION'-Ain. 301 



on their work tinder our eyes. Wo can watch it in all its stages, 
measure its progrefls, and mark in many ways how well it represents 
similftr changee which for long ages previously mnst have been effected 
by similar means. But in the systematic treatment of this subject, a 
difficulty of another kind presents itself. While the operations to be 
discoflnd are numerous and often complex, they are so interwoven into 
one great network that any separation of them under different sub- 
diyisions is sure to be more or less artificial, and is apt to convey an 
enoneous impression. While, therefore, under the unavoidable ne- 
oeenty of making use of such a classification of subjects, we must bear 
slwajB in mind that it is employed merely for convenience, and that in 
natnre, superficial geological action must be viewed as a whole, since 
the work of each agent has close relations with that of the others, and 
is not properly intelligible unless this connection be kept in view. 

The movements of the air ; the evaporation from land and sea ; the 

Wl of rain, hail, and snow ; the flow of rivers and glaciers ; the tides, 

cttirents, and waves of the ocean ; the growtli and decay of organised 

^ristenoe, alike on land and in the depths of the sea : — in short, the 

^hole circle of movement, which is continually in progress upon the 

^'Uface of our planet, are the subjects now to be examined. It would be 

desirable to adopt some general term to embrace the whole of this range 

^^ inquiry. For this end the word e p i g e n e may be suggested as a con- 

^^ient term, and antithetical tohypogene, or subterranean action. 

The simplest arrangement of this part of Geological Dynamics will 
''G into three sections : — 

I. Air. — The influence of the atmosphere in destroying and form- 
'^K rocks. 

II. Water. — The geological functions of the circulation of water 
**^irough the air and between sea and land, and the action of the sea. 

III. Life. — The part taken by plants and animals in preserving, 
^^^troying, or originating geological formations. 

The words destructive, reproductive, and conservative, employed in 

^^^cribing the operations of the epigene agents, do not necessarily 

^J*^ply that anything useful to man is destroyed, reproduced, or pre- 

*^x-Ted. On the contrary, the destructive action of the atmosphere may 

co^rer bare rock with rich soil, while its reproductive effects may bury 

'"^"rtile soil under sterile desert. Again, the conservative influence of 

vegetation has sometimes for centuries retained as barren morass what 

T'^'^ight otherwise have become rich meadow or luxuriant woodland. 

Trie terms, therefore, are used in a strictly geological sense, to denote 

"le removal and re-deposition of material, and its agency in preserving 

^tat lies beneath it. 

Section i. Air. 

The geological action of the atmosphere arises partly from its 

chemical composition and partly from its movements. The composition 

^* the atmo8])heric envelo])e has been already discussed fp. 30), and 

"Either information will Ix? found under the liead of Eain. The move- 



302 DYNAMICAL GEOLOGY. [Book m. Pakt n. 



ments of the atmosphere are due to variations in the distribtition of 
pressure or density, the law being that air always moves spirally from 
where the pressure is high to where it is low. Atmospherio preasiire 
is understood to be determined by two causes, temperature and aqueous 
vapour. Since warm air, being less dense than cold air, asoends, wUUe 
the latter flows in to t-ake its place, the unequal heating of the earth's 
surface, by causing upward currents from the warmed portions, picdnoee 
liorizontal currents from the surrounding cooler regions inwards to the 
central ascending mass of heated air. The familiar land and sea breeces 
offer a good example of this action. Again, the density of the air lessens 
with increase of water-vapour. Hence moist air tends to rise as wanned 
air does, with a corresponding inflow of the drier and consequently 
lieavier air from the surrounding tracts. Moist air, ascending and 
diminisliing atmospheric pressure, as indicated by the fall of the 
Imrometer, rises into higher regions of the atmosphere, where it expands, 
cools, condenses into visible cloud and into showers that descend again 
to the earth. 

Unequal and rapid heating of the air, or accumulation of aqneoaf 
vapour in the air, and possibly some other influences not yet properly 
understood, give rise to extreme disturbances of pressure, and con- 
sequently to storms and hurricanes. For instance, the barometer some- 
times indicates in tropical storms a fall of an inch and a half in an hour, 
showing that somewhere about a twentieth part of the whole mass of 
atmosphere has, in that short space of time, been displaced over a certain 
area of the earth's surface. No such sudden change can occur without 
the most destructive tempest or tornado. In Britain the tenth of an 
inch of barometric fall in an hour is regarded as a large amount, suoh as 
only accompanies great storms.^ The rate of movement of the air 
depends on the difference of barometric pressure between the regi<ma 
from and to which the wind blows. Since much of the potency of the 
air as a geological agent depends on its rate of motion, it is of interest 
to note the ascertained velocity and pressure of wind as expressed 
in the subjoined table : — 

Velocity in Mile* PreteuK In Poundii 

per hour. per square font. 

Calm 

Light breeze .... 14 1 

Strong breeze .... 42 9 

Strong gale 70 25 

Hurricane 84 36 

While the paramount importance of the atmosphere as the vehicle 
for the circulation of moisture over the globe, and consequently as 
powerfully influencing the distribution of climate and the growtii of 
plants and animals, must l)e fully recognised by the geologist, be is 
specially called upon to consider tlie influence of the air in directly pro- 
ducing geological changes upon the surface of the land and in augment- 
ing the geological work done by water. 

1 Biichsn'8'll«teoroLogy,'p. £66. 



Sect. i. | 1.] GEOLOGICAL WORK OF THE AIR, 303 



5 1. Geological work of the atmosphere on land. 

Viewed in a broad way, the air is engaged in the twofold task of 
promoting the disintegration of superficial rocks and in removing and 
redistributing the finer detritus. These two operations however are so 
intimately bound up with each other that they cannot be adequately 
understood unless considered in their mutual relations. 

L Destractive action. — Still dry air, not subject to much range of 
temperature, has probably little or no effect on minerals and rocks. The 
diemical action of the atmosphere takes place almost entirely through 
diflBolved moisture. This subject is discussed in the section devoted to 
Bam. But sunlight produc