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CIHM/ICMH
Microfiche
Series.
CIHM/ICMH
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microfiches.
Canadian institute for Historical Microreproductions Institut Canadian de microreproductions historiques
1980
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1
2
3
THE
ORIGIN OF GNEISS
AND SOME *
OTHER PRIMITIVE ROCKS
By ROBERT BELL, B.A.Sc, M.D., LL.D.,
ASSISTANT DIKECTOU OF THE GEOLOGICAL SDKVKY OF CANADA, ROYAL
COMMISSIONER ON THE MINERAL RESOURCES OP ONTARIO, ETC.
[Being an Abstract of a Paper roacj before tbo American Association por the
Advancement of Science in Toronto, August, 1880.]
rniNTKD BY
THE SALEM PRESS rUHMSHINO AND PRINTING CO.,
SALEM, MASS.
im,
TlIK OUIOIN OK GNKISS AND SOMK OTIIEU riilMITIVK HOCKS. By 1)U. IlOBKKT
Hkij,, Assistant Director of the Geological Survey of Ciumda, Ot-
tawa, Cuniida.
[AnSTIlACT.]
Gni-.iss beiiiiL; proljuhly the most iil)mul!iiit rock In the crust of the cartli,
it is rather s!ny;ular that {jeolonists have not yet coine to a general aiiree-
nient as to its usual process of fornialion. I say usual, for gneissoid rocks
may be formed in more ways than one. For example, the jiranltic mate-
rials of some dykes are occasionally found to have assumed a i^ueissoid fo-
liation parallel to the walls of Hit; dykes, hut no one would pretend that
this was the normal mode of formation of the continuous gneiss of half a
continent.
It is conceivable that a gneissoid, or rather a streaked, api)earance may
have been developed in certain granitic or syenitlc rocks by a flowing move-
ment, while in a plastic state, l)nt such an appearance would, I think, l)e
only very liniited antl wouUl difl'er from ortliiiary gneissic bedeling. The
parallel mottling or clouding and isolated patches of a gneissoid character
sometimes met with in granite or syenite may even be accounted for lu
other ways than from How motion.
The bedding of gneiss dill'ers from the streaking of slags by flowing,
in I)eing in continuous sheets wldch generally ditter much in composition
as well as color. It is dilUcult to imagine how llow-structure could com-
nmnicate a distinctly and uniformly bedded arrangement throughout a
depth of about ten miles of strata which is the ascertained thickness of one
section of the Upper Laurentian alone. The presence of enormous beds of
limestone, traceable continuously for a hundred miles or more, in tlie midst
of these gneisses appears to me to be fatal to the tlow-structure theory.
Pressure will not account for the l)eddingof the Laurentian rocks, for no
amount of pressure would separate a homogeneous mixture into distinct
beds consisting of tlie ditferent minerals. Or if pressure could se|)arate
these constituents, why have certain areas remained as granite or syenite?
The composition of different superimposed beds of gneiss often differs
greatly, some being highly micaceous, while this mineral is scarce in
others lying above or below. Again, considerable thicknesses may con-
sist almost entirely cf felspar, (piartz or hornl)lende alone. I might men-
tion in passing that hornblende Is a very abundant constituent of our
(227)
228 SECTION E.
gneisses — li()riil)k'iide-{;nelHS Ix'liii,' sihoiit iis coninion as the so-called typ-
ical or inica-KiieisH. Now ii fused inixturc of mateiials would uot separate
iuto layers of lliedifVorcnt Inj^rcdieuts, and certaiidy uot into such innii-
uienil)ie line layers as in!il<e up tiie inimeuse tldclvuesses of tiicso fjueisses.
This is, I tliinlv. anotlier fatal ol))eetion to the al)ove tlieory. Tlien some
of the l)('(ldi'(i roclis of tlie Laurentiaii system l)ear sucliaclose resemblance
to or uiay ratlier l)e said to l)e identical witli rocl<s wliicii we can trace to
a clastic and even sedimentary oriftiu, that we are forced to conclude that
they too iiave iiad a similar orijjiu.
Tlie Upper l.aurentian rociis are dillcreutiated into l)elts or groups of
l)eds wiiidi can t»e traced over con8ideral)le distances on tlie ground and
can tlius l)e accurately mapped. After carefully woriiiiig out a large area
in this way Sir William Logan proved that the Upper Laurentian strata
obey the ordinary structural laws governing stratified rocks when thrown
into anticlinal and synclinal folds.
The lower liaurentinn gneisses cannot be so easily separated into dis-
tinct belts, and limestones appear to be ab.sentfrom this division, but many
of them resemble the gneisses of the upper series so closely tliat it is al-
together liliely they liad a sindlar origin. The Lower Laurentian strata
are more contorted and at tlie same time more solid or massive than the
Upper. Tliey also occupy a much larger geographical area. In Canada the
Upp<.-r Laurentian appears to l)e confined to a comparatively limited region
extending from Georgian Bay eastward through the Ottawa valley and
along the north side of the St. Lawrence. It also occurs in eastern Lab-
rador and perhaps in Hudson's Strait.
In the cotintry stretching from Lake Huron to LakeTemiscandng at the
great l)end of the Ottawa, ami thence northward and in other directions,
we liave the best known and one of tlie lai'gest areas of Iluroniau rocks in
Canada. Having worked on these rocks for a number of years, I have had
good opportunity for studying them and I beg to ofl'er a few suggestions
based on these oljsorvations, as to the origin of gneiss and some other prim-
itive rocks.
In the region referred to we have vast (piantithis of a rock of which very
little has hitherto l)eeii said Ity geologists. The late Mr. Alexander Mur-
ray nieiilioned certain varieties of it under tlio somewhat erroneous name
of slate conglomerates. It is a grey or asli colored rock somewhat resem-
bling sandstone, but usually massive or not separated into distinct parallel
beds. It bruises readily under tlie hammer and may be easily broken or
scratvlied, showing that it is largely composed of softer material than
(juartz. Tills rock as a matrix often contains large numbers of angularand
roumled fragments of all degrees of coarseness from the size of pins'
heads to that of pease, walnuts, a man's fist and even boulders. They
consist for the most part of reddish quartz-felspar rock, granulite or binary
granite, but fragments of other rocks occur in some varieties.
In tlie specimens which I have examined under the microscope tlie matrix
is seen to consist principally of angular grains of felspar and rounded
grains of quartz with a darker amorphous mineral tilling the interspaces.
OEOLOOY AND nEOOKAlMlY.
229
Much of tills rock mijilit he called a volciiiiic ash or brecclii. The por-
tions which lire most tree IVoiii pcljbics or (riiLtim-iits are like the golil-
heariiij; " whin rock " of the Nova Scotia miners. Some of It might be
called sandstone, and mncli of it passes t)y insensible dei;re('s into (piartz-
Ite. It is, however, desirable to have a jjeiieral name for tliis rock and I
know of no slna;ie word widcii will better descril)e it than y:re.vwack6.
Now this greywacke seems to me to be tlie raw material ont of which
some of the other Ilnroniaii rocks of tliis reu;ion have l)i'en fornn'd. And
from a stndy of the transfornnition of one rock into another 1 have come
to the conclusion tliat the oiigins of several kinds of crystalline rocks
hav(! points in common and are nnitually dependent on one another in a
larger measure than \f^' may have supposed. Thus if we can account for
the origin of gneiss we shall liave incidentally accounted for that of some
other crystalline rocks at the same time.
Another rock which is very common in some parts of tlie area referred
to is quartz-diorite. It presents consideralile variety of conditions with
the same constituents, passing from tlit; purely crystalline kind tlirough
several stages less or more moditieil liy water, as if it had tlowed out un-
der the sea and been variously att'ected by the water according to the vol-
ume of the rock-nnitter present; or in other cases it seems lo liave l)ei!n
entirely broken up and modilled by the sea, the constituents witliout the
addition of anything else resulting in an aipieons deposit. The (piartz-
diorlte is another parent from whii'h some of tlie other lluronian rocks are
derived.
Hut to return to the greywack6, there are plenty of examples in this
region which sliowthat it passes directly into ililfereiit varieties of (luarlz-
syenite and gneiss and also that these rocks, as well as quartzites, clay-
slates and dolomites may Ite derived from it.
In tiie direct formation of gneiss the materials of the more slaty varie-
ties of the greywacke in some cases appear to separate somewhat into
laminaj by a process of segregation. In other cases they gather them-
selves Into grains or spots by a species of concretion. Tlie complete con-
version of tliese into llnished gneisses is effected by a siiglitiy further altera-
tion through the agencies (whatever they may have been) which have
produced the metamorphism of the crystalline rocks in general.
On the other hand the more massive varieties of the greywacke may be
seen to change gradually into imperfect and then jierfect or thoroughly
cry.stalliiie quartz-syenite. The process seems to be largely due to con-
cretionary action, both on a small and large scale.
The breaking up and nioditlcalion of the materials of the greywack6 by
water appear to give rise to a great variety of (piarlzites on the one hand,
and to clny-slates on the other. These materials may have been thrown
Into the sea either as nind or dust and become separateil anil deposited at
once in different places as (luarlz-saiul anil clay, or the rock, after partial
or complete consolidation, may have become broken up and reduced by the
action of water so that the qiiartzose :. id the argillaceous portions could
become separated and each deposited by itself, to form the quartzites and
280 SECTION F.
claj'-slatfs, wlildi almost ahviiys accoinpiiny ciicli ollior. as well as the
yn'ywackd's of this region. Tin- very fact, of llic associatlun of tlicso
rocks sii;iy;est.s a connection of all llicst; or a ilcpcmlcncc of tln' (jnartzitos
and clay-slalcs upon tlic ;;ivy\vack6s. The clay-slalus frtM|iicntly liavu
layers of liornblcmlc ilcv<-lopt'(l In Miciii parallel to the heddiiiK.
The (luart/.llcs an? often largely mixed with llie softer material of the
jjrc'ywackC', hcin^ in these cases merely tlie more silicious variellfs of this
ro('k which |)ass hy ins<'n>i!)le decrees into onliiiary (piarl/ite. Tlie (piartz-
ile, iiowever, is never entirely free from dissendnated particles of felspar
ami this mliu'ral in a crystalline form may l)c present in all pro|)ortions
up to nearly lialf the whole? mass.
These crystalline j;rains of felspar are generally liijht red in color and
on very ohi surfaces they l)e(;ome dissolved out leaving little lioles or pits.
On slly:litly weatliered surfaces they assume an opacpie white, contrasting
with tin? vitreous lustre of llie (piartzite, .so that tliese surfaces have a
speckled appearance. Where the felspar is uniforudy and thickly scat-
tered throiijjh massive (|nartzite, liic rock is really a (piart/.-felspar or
l)imiry granite exce| ' tliat it lias not yet l)ecome entirely cryslalliiie l)Ut
this chanf;e ndjtht readily he accomplisheil in time.
Whert! tlie felspar grains are disposed more thickly in rows in cross sec-
tion, tlie rock looks very like f^neiss iven to tlit? nakeil eye, and sliil more;
so under the ndcroscope ; and it ndj^ht he converted into this rock hy
more complete alteration. On the west side of Lady Evelyn Lake between
Temayand Lake and the Montreal liiver there is a mountain ridiie 1,100
feet ]iijj;h,lar{^ely composed of this rock, standinj? nearly on edjie, which a
casual ol).>erver mi^lil easily taki; (or ;;neiss altliouiih on closer examina-
tion it nniy be seen to be of clastic nature. Tlie (piartziti! beds or quartz-
ose jjneisses of the Ui)per Laurentian in the county of Ottawa often con-
tain felspar jjrains or crystals which weather out just like those of the
lluronian iiuarlzites and the outward apjx'araiice of the two rocks is so
similar that hard specimens could not he distin}<[uished from each other.
The curious varielies of moditted ipiartz-diorite which have been men-
tioned are very abundant in some i)arts of the region under consideration
— notably in the township of Denison. Extensive sections may be seen
made up of beds of difl'erent thicknesses, l)ut mostly thin, in each of wliicli
the quartz grains and the other components have been roughly separated
from each other by the action of water. The bottom of each bed con-
sists almost entirely of rounded grains of quartz ; but, as we ascend, these
become more and more mixed with argillaceous material till at the top
there is pure cluy-slatc. The same arrangement is repeated in all the beds.
In the vicinity of tliese ijcds there are others entirely of quarlzite and of
clay-slate, which I l)elieve owe their origin to the disintegration of the
quartz-diorite (probal)ly wlien in a soft condition) and the separation
and transportation of the original ingredients by water. If the thin
alternating layers of quartz grains and slaty material were completely met-
amori)hosed they would result in gneiss. The rocks of this region there-
fore appear to show three ways by whicli gneiss may be formed, namely,
GEOLOGY AND CROORAPIIT. S81
l)y the (Urpct convorslon of tlie tliin hoddetl or sliity vnrictles of grey-
Wiuk^', the nllciiilioii of tlie niixi'il (luiiitz and felspar rock derived from
oilier varieties of it and the aileratioii of tlie iiiodilled (luarlz diorites. I
luive also found a case wlierc wliat appeared to liave l)een ii coniniinuled
niasH of f(!lspar and decomposed horiil)lende wliicli may have lieen derived
from diorile was separatin;; into interrupted layi-rs of pure felspar with
others of schist which by further alteration wouUl form a rock like f;iieiss.
The clay slates associated witli the greywack6s are, in some parts of the
rejjion, black or nearly so. On the Onapiiij; river these may be traced
into a l)lack breccia wliii h is clearly of volcanic oii;jin, largely developed
near ()Ma])in;; station on the Canadian Pucilh; Hallway, whicli would show
that the black color in Huronian slates is not necessarily organic and that
any theory baseil on the supposition that it is so may i)e erroneous.
Tlie dolomites of this rejiion are eviilently in most cases of a concre-
tionary or segrejiatcd mitnrc, and have probably been derived from the
decomposition of the honibleiide or tlie aup;ite of the rocks with which
lliey are associated. Small unstratiHed masses of this rock are not un-
coinmoii 111 tlie diorites, syenites and ;ireywackt's where they have been
formed in situ, while the layers and stratilled deposits may have been pre-
cipitated from waters which carrieil tiie carbonates of liiiie and ina};iiesiii
to short distances from the hornbleiidic or auj^itic-beariiig rocks undergo-
ing decomposition.
During the process of converting the grey wacke into .syenite, the difl'used
iron which it contained lias been ;;atliered either into great numi)ers of
strings or small veins of magnetite or into a lew larger ones of the same
ore which, however, I have never seen wide enough to be worked. Vein.s
of this kind are interesting us showing tlie probability of the formation
of more extensive larger ones of iron oxides witliout the existence of
organic life as a means for the concentration of tlie metal.
Tlie above notes touch on nearly all the stratified and some of the un-
stratiHed rocks of this Huronian region and it is hoped that they may iiave
suggested some jjoints of interest for discussion.
SALGH rUESS I'UBLISHINO AND rKINTINO CO.
