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ON THE

7/1^

CHEMICAL AND MINERALOGICAL

IIELA.TION8

METAMORPHIC ROCKS,

T. sr^ HRY Hrar, m.a., f.r.s.

Reprinted from the Dublin Quarterly Journal of Science/or July, 186S.

Montreal :
PRINTED BY JOHN LOVBLL, ST. NICHOLAS STREET.

1863.

ON THE CHEMISTRY OF METAMORPHIC ROCKS,

T. STERRT HUNT, M.A., F.R.S.

At a time not very remote in the history of geology, when all
crystalline stratified rocks were included under the' common
designation of primitive, and were supposed to belong to a period
anterior to the fossiliferoi's formations, the lithologist confined his
studies to descriptions of the various species of rocks, without
reference to their stratigraphical or geological distribution. But
with the progress of geological science, a new problem is present-
ed to his investigation. While palaeontology has shown that the
fossils of each formation furnish a guide to its age and stratigraph-
ical position, it has been found that sedimentary strata of all ages
up to the tertiary inclusive, may undergo such changes as to
obliterate the direct evidences of organic life; and to give to the
sediments the mineralogical characters once assigned to primitive
rocks. The question here arises, whether in the absence of organic
remains, or of stratigraphical evidence, there exists any means of
determining, even approximately, the geological age of a given
series of crystalline stratified rocks;— in other words, whether the
chemical conditions which have presided over the formation of
sedimentary rocks, have so far varied in the course of ages as to
impress upon these rocks marked chemical and mineralogical
ditlerences. In the case of unaltered sediments it would be difficult
to arrive at any solution of this question withou t greatly multiplied
analvses; but in the same rocks, when altered, the crystalline
minerals which are formed, being definite in their composition
and varying with the chemical constitution of the sedi lents, may
perhaps to a certain extent, become to the geologist what organic
remains are in the unaltered rocks, a guide to the geological acre
and succession. '^

It was while engaged in the investigation of metamorphic rocks
of various ages in North America, that this problem suggested
itself; and I have endeavoured from chemical considerations, con-
joined with multiplied observations, to attempt its solution. In the
American Journal of Science for 1858, and in the Quarterly
Journal of the Geological Society of London tor 1869 (p. 488) will
be found the germs of the ideas on this subject, which I shall
e ndeavour to explain in the present p aper. It cannot be doubted

• Read before the Dublin Geological Society April 10, and reprintTd
from advance sheets of ths Dublin Qaarterly Jouraal for July, 1863.

4 ON THE CHEMISTRY OF METAMORPHIC ROCKS.

that in the earlier periods of the world's history, chemical forces
of certain kinds were much more fictive than at tlie present day.
Thus the decomposition of earthy and alkaline silicates under tho
combincid influences of water and carbonic acid, would be greater
when this acid was more abundant in the atmosphere, and when
the temperature was probably higher. The larger amounts cf
alkaline and earthy carbonates then carried to the sea from the
decomposition of these silicates, would furnish a greater amount
of calcareous matter to the sediments ; and the chemical eflfects of
vegetation, both on the soil and on the other atmosphere, must
have been greater during the Carboniferous period, for example,
than at present. In the spontaneous decomposition of feldspars,
which may be described as silicates of alumina combined with
silicates of potash, soda and lime, these latter bases are removed,
together with a portion of silica ; and there remains as the final
result of the process, a hydrous silicate of alumina, which consti-
tutes kaolin or clay. This change is favoured by mechanical
division ; and Daubree has shown that by the prolonged attrition
of fragments of granite under water, the softer and readily cleav-
able feldspar is in great part reduced to an impalpable powder,
while the uncleavable grains of quartz are only rounded, and form
a readily subsiding sand ; the water at the same tmie dissolving
from the feldspar a certain portion of silica, and of alkali. It has
been repeatedly observed, where • potash and soda-feldspars are
associated, that the latter is much the more readily decomposed,
becoming friable, and finally being reduced to clay, while the or-
thoclase is unaltered. The result of combined chemical and
mechanical agencies acting upon rocks which contain quartz,
with orthoclase, and a soda-feldspar such as albite or oligoclase,
would thus be a sand, made up chiefly of quartz and potash-feld-
spar, and a finely divided and suspended clay, consisting for tlie
most part of kaolin, and of partially decomposed soda-feldspar,
mingled with some of the smaller particles of orthoclase and of
quartz. With this sediment will also bo included the oxide of iron,
and Uie earthy carbonates set free by the sub-aerial decomposition
of silicates like pyroxene and the anorthic feldspars, or formed
by the action of the carbonate of soda derived from the latter upon
the lime salts and magnesia salts of sea-water. The debris of horn-
blende and pyroxene will also be found in this finer sediment.
This process is evidently the one which must go on in the wearing
away of rocks by aqueous agency, and explains the fact that

'xl^

ON THE CHEMISTRY OP METAMORPHIC ROCKS. 6

■while quartz, or an excess of combined silica, is for the most part
wanting in rocks which contain a large proportion of alumina, it
is generally abundant in those rocks in which potash-feldspar
predominates.

So long as this decomposition of alkali ferous silicates is sub-aerial,
the silica and alkali are both removed in a soluble form. The
process is often however submarine, or subterranean, taking place
in buried sediments, which are mingled with carbonates of lime
and magnesia. In such cases the silicate of 8(;da set free, re-acts
either with these earthy carbonates, or with the corresponding
chlorids of sea-water, and forms in either event a soluble soda-salt,
and insoluble silicates of lime and magnesia, which take the
place of the removed silicate of soda. The evidence of such a
continued reaction between alkaliferous silicates and earthy
carbonates is seen in the large amounts of carbonate of soda, with
but little silica, which infiltrating waters constantly remove from
argillaceous strata; thus giving rise to alkaline springs, and to
natron lakes. In these waters it will be found that soda greatly
predominates, sometimes almost to the exclusion of potash. This
is due not only to the fact, that soda-feldspars are more readily decom-
posed than orthoclase, but to the well-known power ot argillaceous
sediments to abstract from water the potash salts which it already
holds in solution. Thus when a solution of silicate, carbonate,
sulphate, or chlorid of potassium is filtered through common
earth, the potash is taken up, and replaced by lime, magnesia, or
soda, by a double decomposition between the soluble potash salt
and the insoluble silicates or carbonates of the latter bases. Soils
in like manner remove from infiltrating waters, ammonia, and phos-
phoric and silicic acids, the bases which were in combination with
these being converted into carbonates. The drainage-water of soils,
like thatof most mineral springs, contains only carbonates, chlorids,
and sulphates of lime, magnesia, and soda; the ammonia, potash,
phosphuric and silicic acids being retained by the soil.

The elements which the earth retains or extracts from waters
are precisely those which are removed from it by growing plants.
These, by their decomposition under ordinary conditions, yield
their mineral matters again to the soil ; but when decay takes
place in water, these elements become dissolved, and hence the
waters from peat bogs and marshes contain large amounts of
potash and silica in solution, which are carried to the sea, there
to be separated—the silica by protophy tes, and the potash by alore,

6 ON THE CUEMISTRY OP METAMORPHIC ROCKS.

which ktter, decayin}? on the shore, or in tho ooze at the bottom,
restore the alkali 'to the e^rth. The conaiticiis under which the
vegetation of the eoal formation grew, and was preserved, being
similar to those of peat, the soils became exhausted of potash, and
are seen in the fire-clays of that period.

Another eff>'('t of vegetation on swlimpnts is due to the reducing
or de-oxidizitig aicncy of the organic matters from its decay.
These, as is well known, reduce the peroxide of iron to a soluble
protoxide, and remove it from the soil, to be afterwards deposited
in the forms of iron ochre and iron ores, which by subsequent
alteration become hard, crystalline and insoluble. Thus, through
the agency of vegetation, is the iron oxide of the sediments with-
drawn ♦'rora the terrestrial circulation ; and it is evident that the
proportion of this element diffused in the more recent sediments
must be much lens than in those of ancient times. The reducing
power of organic matter is farther shown in the formation of
metallic sulphu rets; the reduction of sulphates having precipitated
in this insoluble form the heavy m-'tals, copper, lead, and zinc ;
which, with iron, appear to have been in solution in the waters of
early times, but are now by this means also abstracted from the
circulation, and accunuihited in beds and fahlbands, or by a sub-
sequent process have been redissolved and deposited in veins.
All analogies lead us to the conclusion that tho piimeval condition
of the metal?, and of sulphur, was, like that of carbon, one of
oxidation, and that vegetable life has been the sole medium of
their reduction.

The source of the carbonates of lime and magnesia in sediment-
ary strata is twofold :— first, the decomposition of silicates con-
taining these bases, such as anorihi.> feldspars and pyroxene ; and
second, the action of the alkaline carbonates formed by the decom-
position of feldspars, upon the chlorids of calcium and magnesium,
originally present iu sea-water; which have thus, in the course of
ages, been in great part replaced l>y chlond of sodium. The clay,
or aluminous silicate which has been deprived of its alkali, is thus
a measure of the carbonic acid removed from the air, of the
carbonates of lime and magnesia precipitated, and of the amount
of chlorid of sodium added to the waters of the primeval ocean.
The coarser sediments, in which quartz and orthoclase prevail,
are readily permeable to infiltrating waters, which gradually
remove from them the soda, lime, and magnesia, which they con-
tain ; and if organic matters intervene, the oxide of iron ; leaving

ON THB OHBMISTRY OF METAMORPHIO ROOKS.

jottom,
licli tho
1, being
^Hh, and

aducing
decay,
solnble
3positcd
isequent
through
ts with-
that the
idiraents
■educing
ation of
;i»)itated
nd zinc ;
vaters of
from the
ly a sub-
n veins.
;undition
), one of
edium of

ediraent-
ates con-
sne ; and
e decoin-
.gnesium,
course of
The clay,
li, is thus
ir, of the
e amount
ral ocean.
e prevail,
gradually
they con-
I ; leaving

at last little more than silica, alumina, and potash — the elemenU
of granite, trachyte, gneiss, and mica-schist. On the other hand,
the finer marls and clays, resisting the penetration of water, will
retain all their soda, lime, magnesia, and oxide of iron ; and con-
taining an excess of alumina, with a small amount of silica, will
by their metamorphism, give rise to basic limo and soda-feldspars,
and to pyroxene and hornblende — the elements of diorites and
doleritcH. In this way, the operation of the chemical and mechan-
ical causes which we have traced, naturally divides all the
crystalline silico-aluminous rocks of the earth's crust into two
types. These correspond to the two classes of igneous rocks, distin-
guished first by Professor Phillips, and subsequently by Durocher,
and by Buusen, as derived from two distinct niagmaj ; which these
geologists imagine to exist beneath the solid crust, and which the
latter denominates the trachytic and pyroxenic types. I have
however elsewhere endeavoured to show that all intrusive or exotic
rocks are probably nothing more than altered and displaced
sediments, and have thus thtir source within the lower portions of
the stratified crust, and not beneath it.

It may be well in this place to make a few observations on the
chemical conditions of rock-metamorphism. I accept in its
widest 8onse the view of Hutton and Boue, that all the crystalline
strntified rocks have been produced by the alteration of mechanical
and chemical sediments. The conversion of these into definit*" mineral
species lias been eSected in two ways : first by molecular changes ;
that is to say, by crystallization, and a re-arrangement of particles ;
and, secondly by chemical reactions between the elements of the
sediments. Pseudomorphism, which is the change of one mineral
species into another, by the introduction, or the elimination of
some element or elements, presupposes metamorphism ; since only
definite mineral species can be the subjects of this process. To
confound metamorphism with pseudomorphism, as Bischofi*, and
others after him, have done, is therefore an error. It may be
farther remarked, that although certain pseudomorphic changes
may take in some mineral species, in veins, and near to the surface,
the alteration of great masses of silioated rocks by such a process is
as yet an unproved hypothesis.

The cases of local metamorphism in proximity to intrusive rocks
go far to show, in opposition to the views of certain geologists,
(hat heat has been one of the necessary conditions of the change.
The source of this has been generally supposed to be from below ;

8 ON THE CHEMISTRY OP METAMORPHIO ROCKS.

but to the hypothesis of alteration by ancomling heat, Naumann
has obj(!cted that the inferior strata in some cases escape change,
and that in doBcending, a certain plane limits the niotanior()hi»m,
Bcparating the altered strata above, from the unaltered ones
beneath; there beinrr no apparent transition between tiio two.
This, taknn in connexion with the well-known fact that in many
cases the ititrnsion of ignectns rocks cames no apparent change in
the atljacent nnaltere(i sediments, shows that heat and moisture
are not the only conditions of metamorphism. In 1 867, I showed
bv experiments, that in addition to these conditions, certain chem-
ical reagents might be necessary ; and that water impregnated
with alkaline carbonates and silicates, would, at a temperature
not above that of 212° F., produce I'lemical reactions among the
elements of many sedimentary rocks, dissolving silica, and gene-
rating various silicates (1). Some months subsequently, Daubr^e
found that in the presence of solutions of alkaline solutions, at tem-
peratures above 100^ F., various silicious minerals, such as quartz,
feldspar, and pyroxene, could be made to assume a crystalline
form; and that alkaline silicates in solution at this temperature
weald combine with clay to form feldspar and mica (2). These
observations were the complement of my own, and both together
showel the agency of heated alkaline waters to be sufficient to
effect the metamorphism of sedimetita by the two modes already
mentioned, — namely, by molecular changes, and by chemical reac-
tions. Following upon this, Daubree observed that the thermal
alkaline spring of Plombieres, with a temperature of 10()« F., had
in the course of centuries, given rise to the formation of zeolites,
and other crystalline silicated minerals, among the bricks and
cement of the old Roman baths. From this he was led to sup-
pose that the metamorphism of great regions might have been
effected by hot springs; which, rising along certain lines of dislo-
cation, and thence spreading laterally, might produce alteration
in strata near to the surface, while those beneath would in some
cases escape change (3). This ingenious hypothesis may serve in

1. Proc. Royal See of London, May 7, 1857 ; and Philos. Mag. (4)
XV., 68 ; also Amer. Jour. Science (2), xxii., and xxv., 435.

2. Comptes Rendus de I'Acad., Nov. 16, 1857 ; also Bull. Soc. Qeol.
de France (2), xv., 103.

3. It should be remembered that normal or regional metamorphism is
in no way dependent upon the proximity of unstratitied or igneous rocks,
which are rarely present in metamorphic districts. The ophiolites,

ON TOE CHEMISTRT OF METAMORPHIO ROCKa.

lomc cjisea to meet the difficulty pointed out by Naumnnn ; but while
it is uiid<nibtedly true in certain iuRtanccH of local motainorphisra,
it seotiis to be utterly inadofjuate to explain the coinplolo and uni-
verHal alteration of areas of sedimentary rocks, embracing many
hundred thousands of Hquaro miloH. On the other hand, the study
of the oriojin and distribution of mineral springs, shows that alka-
line waters (whose action in metainorphism I first pointed ou
and whose efFicient agency Daubrde has since so well shown), aro
confined to certain sedimentary deposits, and to definite strati-
graphical horizons ; above and below which saline waters wholly
diff"erent in character are found impregnating the strata. This fact
seems to offer a simple solution of the difl^culty advanced by Naii-
mann, and a complete explanation of the theory of metamorphism
of deeply buried strata by the agency of ascending heat; which
is operative in producing chemical changes only in those strata in
which soluble alkaline salts are present. (4).

When the sedimentary strata have been rendered crystallino
by metamorphism, their permeability to wat.;, and their altera-
bility, become greatly diminished ; and it is only when again
broken down by mechanical agencies to the condition of soils and
sediments, that they once more become subject to the chemical
changes which have just been described. Hence, the mean com-
position of the argillaceous sediments of any geological epoch, or

amphibolites, euphotides, diorites, and granites of such regions, which
it has been customary to regard as exotic or intrusive rocitg, are in most
cases indigenous, and are altered sediments. I have elsewliere shown
that the -jreat outbursts o.' intrusive dolerites, diorites, and tracliytes in
Bouth-eastern Canada are found, not among the metamorphic roclta, but
among the unaltered strata along their margin, or at some distance
removed ; and I have endeavoured to expKin this by the consideration
that the groat volume of overlying sediments, which, by retaining the
central heat, aided in the alteration of the strata now exposed by denuda-
tion, jiroduced a depression of the earth's surface, and forced out the still
lower and softened strata along the lines of fracture which took place in
the regions beyond. See my paper " On some Points in American
Geology," Amer. Jour. Science (2), xxxi. 414., and Can. Nat. vi. 81,

4. See Report of the Geological Survey of Canada, 1853-6, pp. 479,
480 ; also Canadian Naturalist, vol. vii., p. 262. For a consideration
of tho relations of mineral waters to geologi'-al formations, see " Gene-
ral Report on the Geology of Canada," p. 561 ; also chap. xix. on " Sedi-
mentary and Metamorphic Rocks ;" where most of the points touched
in the present paper are discussed at greater length.

10 ON THE CHEMISTRY OF MSTAMORPHIC ROCKS.

in other words, the proportion between the alkalies and the alu-
mina, will depend not only upon the age of the formation, but
upon the number of times which its materials have been broken
up, and the periods during which they have remained unmetamor-
phosed, and exposed to the action of infiltrating waters. Thus
for example, that portion of the Lower Silurian rooks in Canada
which became metamorphosed before the close of the palaeozoic
period, will have lost less of its soluble bases than the portion of the
same age which still remains in the form of unaltered shales and
sandstones. Of these again, such parts as remain undisturbed by
folds and dislocations, will retain a larger portion of bases than
those strata in which such disturbances have favored the forma-
tion of mineral springs ; which even now are active in removing
soluble matters from these rocks. The crystalline Lower Silurian
rocks in Canada may be compared with those of the older Lau-
rentiah series on the one hand, and with the Upper Silurian or
Devonian on the other ; but when these are to be compared with
the crystalline strata of secondary or tertiary age in the Alps, it
cannot be determined whether the sediments of which these were
formed, (and which may be supposed, for illustration, to have been
directly derived from palieozoic strata), existed up to the time of
their translation, in a condition similar to that of the altered, or
of the unaltered Lower Silurian rocks of Canada. The proportion
between the alkalies and the alumina in the ari^'llaceous sedi-
ments of any given fcrma,.ion is not therefore in dire-t relation to
its age ; but indicates the extent to which these sediments have
been subjected to the influences of water, carbonic acid, and
vegetation. If however it may be assumed that this action, other
things being equal, has on the whole, be^n proportionate to the
newness of the formation, it is evident that the chemical and m- .-
eralogical composition of ditferent systems of rocks must vary w ' .
their antiquity; and it now remains to find in their comparative
study a guide to their respective ages.

Uwill be evident that silicioas deposits, and chemical precip-
itates, like the carbonates and silicates of lime and magnesia,
may exist with similar characters in the geological formations of
any age ; not only forming beds apart, but mingled with the im-
permeable silico-alurainous sediments of mechanical origin. Inas-
much as the chemical agencies giving rise to these (,-ompounds
were then most active, they may be expected in greatest abund-
auc3 in the rocks of the earlier periods. In the case of the per-

ON TSE CHEMlSTRTf OF METAMORPHIC ROCKS.

meable and more L'ghly silicious class of sediments already
noticed, whose chief elements are silica, alumina, and alkalies,
the deposits of diflferent ages will be marked chiefly by a pro-
gressive diminution in the amount of potash, and the disappearance
of the soda which they contain. In the oldest rocks the propor-
tion of alkali will be nearly or quite sufficient to form orthoclaso
and albite with the whole of the alumina present; but as the
alkali diminishes, a portion of the alumina will crystallize, on the
metamo'phism of the sediments, in the form of a potash-mica,
such as muscovite or margarodite. While the oxygen ratio be-
tween the alumina and the alkali in the feldspars just named is
3 : 1, it becomes 6 : 1 in margarodite, and 12 : 1 in muscovite. The
appearance of these niioas in a rock denotes then a diminution in
the amount of alkali, until in some strata the feldspar almost
entirely disappears, and the rock becomes a quartzose mica-schist
In sediments still farther deprived of alkali, n)etamorphism gives
rise to schisfs filled with crystals of kyanite, or of andalusite ;
which are simple silicates of alumina, into whose composition
alkalies do not enter ; or in case the sediment still retains oxide of
iron, sfaurotide and iron-alumina garnet take their place. The
matrix of all these minerals is generally a quartzose mica-schist.
The last term in this exhaustive process appears to be represented
by the disthene and pyrophyllite rocks, which occur in some
regions of crystalline schists.

In the second class of sediments we have alumina in excess,
with a small proportion of silica, and a deficiency of alkalies, be-
sides a variable proportion of silicates or carbonates of lime, magne-
sia, and oxide of iron. The result of the processes already de-
scribed will produce a gradual diminution in the amount of alkali,
which is chiefly soda. So long as this predominates, the meta-
morphism of these sediments will give rise to feldspars like oligo-
chse, labradorite, or scapolite (a dimetric feldspar) ; but in sedi-
ments where lime replaces a great proportion of the soda, there
appears a tendency to the production of denser silicates, like lime-
alumina garnet, and epidote, or zoisite, which replace the soda-
lime feldspars. Miiierals like the chlorites, and chloritoid, are
formed when magnesia and iron replace lime. In all these cases
the excess of the silicates of earthy protoxides over the silicate of
alumina is represented in the altered strata by hornblende, pyrox-
ene, 01. vine, and similar species; which give rise by their admix-
ture with the double aluminous silicates to diorite diabaso
euphotide, eklogite, and similar compound rocks.

'%«

ON THE CHEMISTRY OF METAMORPHIO ROCKS.

In eastern North America, the crystalline strata, so far as yet
studied, maybe conveniently classed in five groups, corresponding
to as many different geological series, four of which will be con-
sidered in the present paper.

I. The Laurentian system represents the oldest known rocks of
the globe, and is supposed to be the equivalent of the Primitive
Gneiss formation of Scandinavia, and that of the Western Islands
of Scotland, to which also the name of Laurentian is now applied. It
has been investigated in Canada along a continuous outcrop from
the coast of Labrador to Lake Superior, and also over a consider-
able area in northern New York.

IL Associated with this system is a series of strata characterized
by a great development of anortho8ites,of which the hypersthenite, or
opalescent feldspar-rock of Labrador, may be taken as a type.
These strata overlie the Laurentian gneiss, and are regarded as
constituting a second and more recent group of crystalline rocks,
to which the name of the Labrador series may be provisionally
given. From evidence recently obtained, Sir William Logan con-
ceives it probable that this series is unconformable with the older
Laurentian system, and is separated from it by a long interval
of time.

in. In the third place is a great series of crystalline schists,
which are in Canada referred to the Quebec group, an inferior
part of the Lower Silurian system. They appear to correspond
both lithologicaliy and stratigraphically with the Schistose group
of the Primitive Slate formation of Norway, as recognized by
Naumann and Keilhau, and to be there represented by the strata
in the vicinity of Drontheira, and those of the Dofrefeld. The
Huronian series of Canada in like manner appears to correspond
to the Quartzose group of the same Primitive Slate formation (5).
It consists of sandstones, imperfect varieties of gneiss, diorites,
silicious and feldspathic schists passing into argillites, with lime-
stones, and great beds of hematite. Though more recent tlian the
Laurentian and Labrador series, these stiata are older than the
Quebec group ; yet from their position to the westward of the
greatest accumulation of sediments, they have been subjected to a
less complete raetamorphism than the palaeozoic strata of the
East. The Huronian series is as yet but imperfectly studied, and
for the present will not be further considered.

(5) See Macfarlane— Primitive Formations of Norway and Canada
compared=-CanadiaQ Naturalist, vii., 113, iG2.

ON THE CHEMISTRY OF METAMORPHIC ROCKS.

IV. In the fourth place are to be noticed the metamorphosed
strata of Upper Sihirian and Devonian age, with which may also be
included those of the Carboniferous system in eastern New England.
This group has as yet been imperfectly studied, but presents inter-
esting peculiarities.

In the oldest of these, the Laurentian system, the first class of
aluminous rocks takes the form of granitoid gneiss, which is often
coarse grained and porphyritio. Its feldspar is frequently a nearly
pure potash orthoclase, but sometimes contains a considerable pro-
portion of soda. Mica is often almost entirely wanting, and is
never abundant in any large mass of this gneiss, although small
bands of mica-schist are occasionally met with. Argillites, which
from their general predominance of potash and of silica, are related
to the first class of sediments, are, so far as known, wanting through-
out the Laurentian series ; nor is any rock here met with, which
can be regarded as derived from the metamorphism of sediments
like the argillites of more modern series. Chloritic and chiastolite
schists, and kyanite are, if not altogether wanting, extremely rare
in the Laurentian system. The aluminous sediments of the second
class are however represented in this system by a diabase made
up of dark green pyroxene and bluish labradorite, often associated
with a red alumino-ferrous garnet. This latter mineral ?lso some-
times constitutes small beds, often with quartz, and occasionally
with a little pyroxene. These basic aluminous minerals form how-
ever but an insignificant part of the mass of strata. This system
is farther remarkable by the small amount of ferruginous matter
diffused through the strata; from which the greater part of the
iron seems to have been removed, and accumulated in the form of
immense beds of hematite and magnetic iron. Beds of pure
crystalline plumbago also characterize this series, and are generally
found with the limestones. These are here developed to an extent
unknown in more recent formations ; and are associated with beds
of crystalline apatite, which sometimes attain a thickness of several
feet. The serpentines of this series, so far as yet studied in Canada,
are generally pale colored, and contain an unusual amount of
water, a small proportion of oxide of iron, and neither chrome nor
nickel ; both of which are almost always present in the serpentines
of the third series.

The second or Labrador series is characterized, as already re-
marked, by the predominance of great b&ds of anorthosite, com-
uosed chiefly of triclinic feldspars, which vary in comoosition from
luiorthite to andesine. These feldspars sometimes form mountain

ON THE CHEMISTRY OF METAMORPHIC ROOKS.

masses, almost without any admixture, but at other times include
portions of pyroxene, which passes into hypersthene. Beds of
nearly pure pyroxenite are met with in this series, and others which
would be called hyperite and diabase. These anorthosite rocks
are frequently compact, but are more often granitoid in structure.
They are generally greyish, greenish, or bluish in colour, and be-
come white on the weathered surfaces. The opalescent labradorite-
rock of Labrador is a characteristic variety of these anorthosites;
which often contain small portions of red garnet and brown mica,
and more rarely, epidote, and a little quartz. They are sometimes
slightly calcareous. Magnetic iron and ilmenite are often dissem-
inated in these rocks, and occasiourtlly form masses or beds of
considerable size. These anorthosites constitute the predom-
inant part of the Labrador series, so far as yet examined.
They are however associated with beds of quartzose orthoclase
gneiss, which represent the first class of aluminous sediments, and
with crystalline limestones ; and they will probably be found, when
further studied, to offer a complete lithological series. These rocks
have been observed in several areas among the Laurentide Moun-
tains, from the coast of Labrador to Lake Huron, and are also met
with among the Laurentian rocksof the Adirondack Mountains ; of
which according to Emmons, they form the highest summits.

In the third series, which we have referred to the Lower Silurian
age, the gneiss is sometimes granitoid, but less markedly so than
in the first; and it is much more frequently micaceous, often pass-
ing into micaceous schist, a common variety of which contains
disseminated a large quantity of chloritoid. Argillites abound,
and under the influence of raetamorphism sometimes develop
crystalline orthoclase. At other times they are converted into a
soft micaceous mineral, and form a kind of mica-schist. Chias-
tolite and staurotide are never met with in the schists of this
series, at least in its northern portions, throughout Canada and
New England. The anorthosites of the Labrador series are repre-
sented by fine grained diorites, in which the feldspar varies from
albite to very basic varieties, which are sometimes associated with
an aluminous mineral allied to chlorite in composition. Chloritio
schists, freqently accompanied by epidote, abound in this series.
The great predominance of magnesia in the forms of dolomite,
magnesite, steatite and serpentine, is also charpcteristic of portions
of this series. The latter, which forms great beds (ophiolites), is
marked by the almost constant presence of small portions of the
oxides of chrome and nickel. These metals are also common in

ON THE CHEMISTRY OP METAMORPHIC ROCKS.

the other magnesian rocks of the series ; green chrome-garnets, and
chrome-mica occur; and beds of chrome iron ore are found in the
ophiolites of the series. It is also the gold-bearing formation o^
eastern North America, and contains large quantities of copper
ores in interstratified beds resembling those of the Permian schists of
Mansfeld and Hesse. In some parts of this seriee pure limestones
occur, which contain various crystalline minerals common also to
the Laurentian limestones, and to those of the fourth series. Th«
only graphite which has been found in the third series, is in the
form of impure plumbaginous shales.

The metamorphic rocks of the fourth series, as seen in south-
eastern Canada, are for the greater part quartzose and mica-
ceous schists, more or less feldspathic ; which in the neighboring
States become remarkable for a great development of crystals of
staurotide and of red garnet. A large amount of argillite occurs in
this series; and when altered, whether locally by the proximity of
intrusive rock, or by normal metamorphism, exhibits a micaceous
mineral, and crystals of andalusite ; so that it becomes known
as chiastolite slate in its southern extension. Granitoid gneiss is still
associated with these crystalline schists. Gold is not confined to
the third series, but is also met with in veins cutting the argilites
of Upper Silurian age. The crystalline limestones and ophiolites of
eastern Massachusetts, which are probably of this serieu, resemble
those of the Lanrentian system ; and the coal beds in that region
are in some parts, changed into graphite. It is to be remarked
that the metamorphic strata of the third and fourth serieb are
contiguous throughout their extent, so far as examined, but are
everywhere separated from the Laurentian and Labrador series by
a zone of unaltered palsezoic rocks.

Large masses of intrusive granite occur among the crystalline
strata of the fourth series, but are rare or unknown among the
older metamorphic rocks in Canada. The so-called granites of the
Laurentian and Lower Silurian appear to be in every case indigen-
ous rocks ; that is to say, strata altered in situ, and still retaining
evideJices of stratification. The same thing is true with regard to
the ophiolites and the anorthosites of both series ; in all of which the
gonera! absence of great masses of unstratified rock is especially
notiofuble. No evidences of the hypothetical granitic substratum
are lao with in the Laurentian system, although this is in one district
penetrated by great masses of syenite, orthophyre, and dolerite.
Granitic veins, with minerals containing the rarer elements, such as

16 ON THE CHEMISTRY OP METAMORPHIO ROCKS.

boron, fluorine, lithium, zirconium, and glucinum,are met with alike
in the oldest and the newest gneiss in North America. These
however, I regard as having been formed, like metalliferous veins,
by aqueous deposition in fissures in the strata.

The above observations upon the metamorphic strata of a wide
region seem to be in conformity with the chemical princiciples
already laid down in this paper; which it remains for geologists to
apply to the rocks of other regions, and thus determine whether
they are susceptible of a general application. I have found that
the blue crystalline labradorite of the Labrador series of Canada
is exactly represented by specimens from Scarvig, in Skye ; and
the ophiolites of lona resemble those of the Laurentian series in
Canada. Many of the rocks of Donegal appear to me lithologi-
cally identical with those of the Laurentian period ; while the ser-
pentines of Aghadoey, containing chrome and nickel, and the
andalusite and kyanite-schists of other parts of Donegal, cannot
be distinguished from those which characterize the altered palseo-
Boic strata of Canada. It is to be remarked that chrome and
nickel-bearing serpentines are met with in the same geological
horizon in Canada and Norway ; and that those of the Scottish
Highlands, which contain the same elements, belong t) tho
newer gneiss formation ; which, according to Sir Roderick
Murchison, would be of similar age. The serpentines of Cornwall,
the Vosges, Mount Rosa, and many other regions, agree in contain-
ing chrome and nickel ; which on the other hand, seem to be absent
from the serpentines of the Primitive Gneiss formation of Sca.'idina-
via. It remains to be determined how far chemical and mineral -
ogical differences, such as those which have been here indicated,
are geological constants. Meanwhile it is greatly to be desired that
future chemical and mineralogical investigations of crystalline rocks
should be made with this question in view ; and that the meta-
morphic strata of the British Isles, and the more modern ones
of Southern and Central Europe, be studied with reference to the
important problem which it has been my endeavour, in the present
paper, to lay before the Society.

Montreal, January 25, 1863.

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