METAMORPHIC ROCKS.
General character of metamorphic rocks--Gneiss--Hornblende-schist--
Mica-schist--Clay-slate--Quartzite--Chlorite-schist--Metamorphic
limestone--Alphabetical list and explanation of other rocks of this
family--Origin of the metamorphic strata--Their stratification is real
and distinct from cleavage--Joints and slaty cleavage--Supposed causes
of these structures--How far connected with crystalline action.
We have now considered three distinct classes of rocks: first, the aqueous,
or fossiliferous; secondly, the volcanic; and, thirdly, the plutonic, or
granitic; and we have now, lastly, to examine those crystalline (or
hypogene) strata to which the name of _metamorphic_ has been assigned. The
last-mentioned term expresses, as before explained, a theoretical opinion
that such strata, after having been deposited from water, acquired, by the
influence of heat and other causes, a highly crystalline texture. They who
still question this opinion may call the rocks under consideration the
stratified hypogene, or schistose hypogene formations.
These rocks, when in their most characteristic or normal state, are
wholly devoid of organic remains, and contain no distinct fragments of
other rocks, whether rounded or angular. They sometimes break out in the
central parts of narrow mountain chains, but in other cases extend over
areas of vast dimensions, occupying, for example, nearly the whole of
Norway and Sweden, where, as in Brazil, they appear alike in the lower
and higher grounds. In Great Britain, those members of the series which
approach most nearly to granite in their composition, as gneiss,
mica-schist, and hornblende-schist, are confined to the country north of
the rivers Forth and Clyde.
Many attempts have been made to trace a general order of succession or
superposition in the members of this family; gneiss, for example, having
been often supposed to hold invariably a lower geological position than
mica-schist. But although such an order may prevail throughout limited
districts, it is by no means universal, nor even general, throughout the
globe. To this subject, however, I shall again revert, in the last
chapter of this volume, when the chronological relations of the
metamorphic rocks are pointed out.
The following may be enumerated as the principal members of the metamorphic
class:--gneiss, mica-schist, hornblende-schist, clay-slate,
chlorite-schist, hypogene or metamorphic limestone, and certain kinds of
quartz-rock or quartzite.
[Illustration: Fig. 508. Fragment of gneiss, natural size; section at right
angles to planes of stratification.]
_Gneiss._--The first of these, gneiss, may be called stratified granite,
being formed of the same materials as granite, namely, felspar, quartz,
and mica. In the specimen here figured, the white layers consist almost
exclusively of granular felspar, with here and there a speck of mica and
grain of quartz. The dark layers are composed of grey quartz and black
mica, with occasionally a grain of felspar intermixed. The rock splits
most easily in the plane of these darker layers, and the surface thus
exposed is almost entirely covered with shining spangles of mica. The
accompanying quartz, however, greatly predominates in quantity, but the
most ready cleavage is determined by the abundance of mica in certain
parts of the dark layer.
Instead of these thin laminae, gneiss is sometimes simply divided into thick
beds, in which the mica has only a slight degree of parallelism to the
planes of stratification.
The term "gneiss," however, in geology is commonly used in a wider sense,
to designate a formation in which the above-mentioned rock prevails, but
with which any one of the other metamorphic rocks, and more especially
hornblende-schist, may alternate. These other members of the metamorphic
series are, in this case, considered as subordinate to the true gneiss.
The different varieties of rock allied to gneiss, into which felspar
enters as an essential ingredient, will be understood by referring to
what was said of granite. Thus, for example, hornblende may be
superadded to mica, quartz, and felspar, forming a syenitic gneiss; or
talc may be substituted for mica, constituting talcose gneiss, a rock
composed of felspar, quartz, and talc, in distinct crystals or grains
(stratified protogine of the French).
_Hornblende-schist_ is usually black, and composed principally of
hornblende, with a variable quantity of felspar, and sometimes grains of
quartz. When the hornblende and felspar are nearly in equal quantities,
and the rock is not slaty, it corresponds in character with the
greenstones of the trap family, and has been called "primitive
greenstone." It may be termed hornblende rock. Some of these hornblendic
masses may really have been volcanic rocks, which have since assumed a
more crystalline or metamorphic texture.
_Mica-schist_, or _Micaceous schist_, is, next to gneiss, one of the most
abundant rocks of the metamorphic series. It is slaty, essentially composed
of mica and quartz, the mica sometimes appearing to constitute the whole
mass. Beds of pure quartz also occur in this formation. In some districts,
garnets in regular twelve-sided crystals form an integrant part of
mica-schist. This rock passes by insensible gradations into clay-slate.
_Clay-slate_, or _Argillaceous schist_.--This rock resembles an
indurated clay or shale, is for the most part extremely fissile, often
affording good roofing slate. It may consist of the ingredients of
gneiss, or of an extremely fine mixture of mica and quartz, or talc and
quartz. Occasionally it derives a shining and silky lustre from the
minute particles of mica or talc which it contains. It varies from
greenish or bluish-grey to a lead colour. It may be said of this, more
than of any other schist, that it is common to the metamorphic and
fossiliferous series, for some clay-slates taken from each division
would not be distinguishable by mineralogical characters.
_Quartzite_, or _Quartz rock_, is an aggregate of grains of quartz,
which are either in minute crystals, or in many cases slightly rounded,
occurring in regular strata, associated with gneiss or other metamorphic
rocks. Compact quartz, like that so frequently found in veins, is also
found together with granular quartzite. Both of these alternate with
gneiss or mica-schist, or pass into those rocks by the addition of mica,
or of felspar and mica.
_Chlorite-schist_ is a green slaty rock, in which chlorite is abundant
in foliated plates, usually blended with minute grains of quartz, or
sometimes with felspar or mica. Often associated with, and graduating
into, gneiss and clay-slate.
_Hypogene_, or _Metamorphic limestone_.--This rock, commonly called
_primary limestone_, is sometimes a thick bedded white crystalline
granular marble used in sculpture; but more frequently it occurs in thin
beds, forming a foliated schist much resembling in colour and appearance
certain varieties of gneiss and mica-schist. It alternates with both
these rocks, and in like manner with argillaceous schist. It then
usually contains some crystals of mica, and occasionally quartz,
felspar, hornblende, and talc. This member of the metamorphic series
enters sparingly into the structure of the hypogene districts of Norway,
Sweden, and Scotland, but is largely developed in the Alps.
Before offering any farther observations on the probable origin of the
metamorphic rocks, I subjoin, in the form of a glossary, a brief
explanation of some of the principal varieties and their synonymies.
ACTINOLITE-SCHIST. A slaty foliated rock, composed chiefly of actinolite,
(an emerald-green mineral, allied to hornblende,) with some admixture of
felspar, or quartz, or mica.
AMPELITE. Aluminous slate (Brongniart); occurs both in the metamorphic
and fossiliferous series.
AMPHIBOLITE. Hornblende rock, which see.
ARGILLACEOUS-SCHIST, or CLAY-SLATE. _See_ p. 465.
ARKOSE. Term used by Brongniart for granular Quartzite, which see.
CHIASTOLITE-SLATE scarcely differs from clay-slate, but includes numerous
crystals of Chiastolite; in considerable thickness in Cumberland.
Chiastolite occurs in long slender rhomboidal crystals. For composition,
see Table, p. 377.
CHLORITE-SCHIST. A green slaty rock, in which chlorite, a green scaly
mineral, is abundant. _See_ p. 465.
CLAY-SLATE, or ARGILLACEOUS-SCHIST. _See_ p. 465.
EURITE and EURITIC PORPHYRY. A base of compact felspar, with grains of
laminar felspar, and often mica and other minerals disseminated
(Brongniart). M. D'Aubuisson regards eurite as an extremely fine-grained
granite, in which felspar predominates, the whole forming an apparently
homogeneous rock. Eurite has been already mentioned as a plutonic rock, but
occurs also in beds subordinate to gneiss or mica-slate.
GNEISS. A stratified or laminated rock, same composition as granite.
_See_ p. 464.
HORNBLENDE ROCK, or AMPHIBOLITE. Composed of hornblende and felspar.
The same composition as hornblende-schist, stratified, but not fissile.
_See_ p. 376.
HORNBLENDE-SCHIST, or SLATE. Composed chiefly of hornblende, with
occasionally some felspar. _See_ p. 464.
HORNBLENDIC or SYENITIC-GNEISS. Composed of felspar, quartz,
and hornblende.
HYPOGENE LIMESTONE. _See_ p. 465.
MARBLE. _See_ p. 465.
MICA-SCHIST, or MICACEOUS-SCHIST. A slaty rock, composed of mica and quartz
in variable proportions. _See_ p. 465.
MICA-SLATE. _See_ MICA-SCHIST, p. 465.
PHYLLADE. D'Aubuisson's term for clay-slate, from +phullas+,
a heap of leaves.
PRIMARY LIMESTONE. _See_ HYPOGENE LIMESTONE, p. 465.
PROTOGINE. _See_ TALCOSE-GNEISS, p. 464.; when unstratified
it is Talcose-granite.
QUARTZ ROCK, or QUARTZITE. A stratified rock; an aggregate of grains of
quartz. _See_ p. 465.
SERPENTINE occurs in both divisions of the hypogene series, as a stratified
or unstratified rock; contains much magnesia; is chiefly composed of the
mineral called serpentine, mixed with diallage, talc, and steatite. The
pure varieties of this rock, called noble serpentine, consist of a hydrated
silicate of magnesia, generally of a greenish colour: this base is commonly
mixed with oxide of iron.
TALCOSE-GNEISS. Same composition as talcose-granite or protogine, but
either stratified or laminated. _See_ p. 464.
TALCOSE-SCHIST consists chiefly of talc, or of talc and quartz, or of talc
and felspar, and has a texture something like that of clay-slate.
WHITESTONE. Same as Eurite.
_Origin of the Metamorphic Strata._
Having said thus much of the mineral composition of the metamorphic
rocks, I may combine what remains to be said of their structure and
history with an account of the opinions entertained of their probable
origin. At the same time, it may be well to forewarn the reader that we
are here entering upon ground of controversy, and soon reach the limits
where positive induction ends, and beyond which we can only indulge in
speculations. It was once a favourite doctrine, and is still maintained
by many, that these rocks owe their crystalline texture, their want of
all signs of a mechanical origin, or of fossil contents, to a peculiar
and nascent condition of the planet at the period of their formation.
The arguments in refutation of this hypothesis will be more fully
considered when I show, in the last chapter of this volume, to how many
different ages the metamorphic formations are referable, and how gneiss,
mica-schist, clay-slate, and hypogene limestone (that of Carrara for
example), have been formed, not only since the first introduction of
organic beings into this planet, but even long after many distinct races
of plants and animals had passed away in succession.
The doctrine respecting the crystalline strata, implied in the name
metamorphic, may properly be treated of in this place; and we must first
inquire whether these rocks are really entitled to be called stratified
in the strict sense of having been originally deposited as sediment from
water. The general adoption by geologists of the term stratified, as
applied to these rocks, sufficiently attests their division into beds
very analogous, at least in form, to ordinary fossiliferous strata. This
resemblance is by no means confined to the existence in both of an
occasional slaty structure, but extends to every kind of arrangement
which is compatible with the absence of fossils, and of sand, pebbles,
ripple-mark, and other characters which the metamorphic theory supposes
to have been obliterated by plutonic action. Thus, for example, we
behold alike in the crystalline and fossiliferous formations an
alternation of beds varying greatly in composition, colour, and
thickness. We observe, for instance, gneiss alternating with layers of
black hornblende-schist, or with granular quartz, or limestone; and the
interchange of these different strata may be repeated for an indefinite
number of times. In the like manner, mica-schist alternates with
chlorite-schist, and with granular limestone in thin layers.
As in fossiliferous formations strata of pure siliceous sand alternate with
micaceous sand and with layers of clay, so in the crystalline or
metamorphic rocks we have beds of pure quartzite alternating with
mica-schist and clay-slate. As in the secondary and tertiary series we meet
with limestone alternating again and again with micaceous or argillaceous
sand, so we find in the hypogene, gneiss and mica-schist alternating with
pure and impure granular limestones.
It has also been shown that the ripple-mark is very commonly repeated
throughout a considerable thickness of fossiliferous strata; so in
mica-schist and gneiss, there is sometimes an undulation of the laminae on a
minute scale, which may, perhaps, be a modification of similar inequalities
in the original deposit.
In the crystalline formations also, as in many of the sedimentary before
described, single strata are sometimes made up of laminae placed diagonally,
such laminae not being regularly parallel to the planes of cleavage.
[Illustration: Fig. 509. Lamination of clay-slate, Montagne de Seguinat,
near Gavarnie, in the Pyrenees.]
This disposition of the layers is illustrated in the accompanying diagram,
in which I have represented carefully the stratification of a coarse
argillaceous schist, which I examined in the Pyrenees, part of which
approaches in character to a green and blue roofing slate, while part is
extremely quartzose, the whole mass passing downwards into micaceous
schist. The vertical section here exhibited is about 3 feet in height, and
the layers are sometimes so thin that fifty may be counted in the thickness
of an inch. Some of them consist of pure quartz.
The inference drawn from the phenomena above described in favour of the
aqueous origin of clay-slate and other crystalline strata, is greatly
strengthened by the fact that many of these metamorphic rocks occasionally
alternate with, and sometimes pass by intermediate gradations into, rocks
of a decidedly mechanical origin, and exhibiting traces of organic remains.
The fossiliferous formations, moreover, into which this passage is
effected, are by no means invariably of the same age nor of the highest
antiquity, as will be afterwards explained.
_Stratification of the metamorphic rocks distinct from cleavage._--The beds
into which gneiss, mica-schist, and hypogene limestone divide, exhibit most
commonly, like ordinary strata, a want of perfect geometrical parallelism.
For this reason, therefore, in addition to the alternate recurrence of
layers of distinct materials, the stratified arrangement of the crystalline
rocks cannot be explained away by supposing it to be simply a divisional
structure like that to which we owe some of the slates used for writing and
roofing. _Slaty cleavage_, as it has been called, has in many cases been
produced by the regular deposition of thin plates of fine sediment one upon
another; but there are many instances where it is decidedly unconnected
with such a mode of origin, and where it is not even confined to the
aqueous formations. Some kinds of trap, for example, as clinkstone, split
into laminae, and are used for roofing.
There are, says Professor Sedgwick, three distinct forms of structure
exhibited in certain rocks throughout large districts: viz.--First,
stratification; secondly, joints; and thirdly, slaty cleavage; the two
last having no connection with true bedding, and having been superinduced
by causes absolutely independent of gravitation. All these different
structures must have different names, even though there be some cases where
it is impossible, after carefully studying the appearances, to decide upon
the class to which they belong.[469-A]
_Joints._--Now, in regard to the second of these forms of structure or
joints, they are natural fissures which often traverse rocks in straight
and well-determined lines. They afford to the quarryman, as Sir R.
Murchison observes, when speaking of the phenomena, as exhibited in
Shropshire and the neighbouring counties, the greatest aid in the
extraction of blocks of stone; and, if a sufficient number cross each
other, the whole mass of rock is split into symmetrical blocks.[469-B] The
faces of the joints are for the most part smoother and more regular than
the surfaces of true strata. The joints are straight-cut chinks, often
slightly open, often passing, not only through layers of successive
deposition, but also through balls of limestone or other matter which have
been formed by concretionary action, since the original accumulation of the
strata. Such joints, therefore, must often have resulted from one of the
last changes superinduced upon sedimentary deposits.[469-C]
In the annexed diagram the flat surfaces of rock A, B, C, represent exposed
faces of joints, to which the walls of other joints, J J, are parallel. S S
are the lines of stratification; D D are lines of slaty cleavage, which
intersect the rock at a considerable angle to the planes of stratification.
[Illustration: Fig. 510. Stratification, joints, and cleavage.]
Joints, according to Professor Sedgwick, are distinguishable from lines of
slaty cleavage in this, that the rock intervening between two joints has no
tendency to cleave in a direction parallel to the planes of the joints,
whereas a rock is capable of indefinite subdivision in the direction of its
slaty cleavage. In some cases where the strata are curved, the planes of
cleavage are still perfectly parallel. This has been observed in the slate
rocks of part of Wales (see fig. 511.), which consist of a hard greenish
slate. The true bedding is there indicated by a number of parallel stripes,
some of a lighter and some of a darker colour than the general mass. Such
stripes are found to be parallel to the true planes of stratification,
wherever these are manifested by ripple-mark, or by beds containing
peculiar organic remains. Some of the contorted strata are of a coarse
mechanical structure, alternating with fine-grained crystalline chloritic
slates, in which case the same slaty cleavage extends through the coarser
and finer beds, though it is brought out in greater perfection in
proportion as the materials of the rock are fine and homogeneous. It is
only when these are very coarse that the cleavage planes entirely vanish.
These planes are usually inclined at a very considerable angle to the
planes of the strata. In the Welsh chains, for example, the average angle
is as much as from 30 deg. to 40 deg.. Sometimes the cleavage planes dip towards
the same point of the compass as those of stratification, but more
frequently to opposite points. It may be stated as a general rule, that
when beds of coarser materials alternate with those composed of finer
particles, the slaty cleavage is either entirely confined to the
fine-grained rock, or is very imperfectly exhibited in that of coarser
texture. This rule holds, whether the cleavage is parallel to the planes of
stratification or not.
[Illustration: Fig. 511. Parallel planes of cleavage intersecting
curved strata. (Sedgwick.)]
In the Swiss and Savoy Alps, as Mr. Bakewell has remarked, enormous masses
of limestone are cut through so regularly by nearly vertical partings, and
these are often so much more conspicuous than the seams of stratification,
that an inexperienced observer will almost inevitably confound them, and
suppose the strata to be perpendicular in places where in fact they are
almost horizontal.[470-A]
Now these joints are supposed to be analogous to those partings which have
been already observed to separate volcanic and plutonic rocks into cuboidal
and prismatic masses. On a small scale we see clay and starch when dry
split into similar shapes, which is often caused by simple contraction,
whether the shrinking be due to the evaporation of water, or to a change of
temperature. It is well known that many sandstones and other rocks expand
by the application of moderate degrees of heat, and then contract again on
cooling; and there can be no doubt that large portions of the earth's crust
have, in the course of past ages, been subjected again and again to very
different degrees of heat and cold. These alternations of temperature have
probably contributed largely to the production of joints in rocks.
In some countries, as in Saxony, where masses of basalt rest on sandstone,
the aqueous rock has for the distance of several feet from the point of
junction assumed a columnar structure similar to that of the trap. In like
manner some hearthstones, after exposure to the heat of a furnace without
being melted, have become prismatic. Certain crystals also acquire by the
application of heat a new internal arrangement, so as to break in a new
direction, their external form remaining unaltered.
Sir R. Murchison observes, that in referring both joints and slaty cleavage
to crystalline action, we are borne out by a well-known analogy in which
crystallization has in like manner given rise to two distinct kinds of
structure in the same body. Thus, for example, in a six-sided prism of
quartz, the planes of cleavage are distinct from those of the prism. It is
impossible to cleave the crystals parallel to the plane of the prism, just
as slaty rocks cannot be cleaved parallel to the joints; but the quartz
crystal, like the older schists, may be cleaved _ad infinitum_ in the
direction of the cleavage planes.[471-A]
It seems, therefore, that the fissures called joints may have been the
result of different causes, as of some modification of crystalline action,
or simple contraction during consolidation, or during a change of
temperature. And there are cases where joints may have been due to
mechanical violence, and the strain exerted on strata during their
upheaval, or when they have sunk down below their former level. Professor
Phillips has suggested that the previous existence of divisional planes may
often have determined, and must greatly have modified, the lines and points
of fracture caused in rocks by those forces to which they owe their
elevation or dislocations. These lines and points being those of least
resistance, cannot fail to have influenced the direction in which the solid
mass would give way on the application of external force.
Professor Phillips has also remarked that in some slaty rocks the form of
the outline of fossil shells and trilobites has been much changed by
distortion, which has taken place in a longitudinal, transverse, or oblique
direction. This change, he adds, seems to be the result of a "creeping
movement" of the particles of the rock along the planes of cleavage, its
direction being always uniform over the same tract of country, and its
amount in space being sometimes measurable, and being as much as a quarter
or even half an inch. The hard shells are not affected, but only those
which are thin.[471-B] Mr. D. Sharpe, following up the same line of
inquiry, came to the conclusion, that the present distorted forms of the
shells in certain British slate rocks may be accounted for by supposing
that the rocks in which they are imbedded have undergone compression in a
direction perpendicular to the planes of cleavage, and a corresponding
expansion in the direction of the dip of the cleavage.[471-C]
Mr. Darwin infers from his observations, that in South America the
strike of the cleavage planes is very uniform over wide regions, and
that it corresponds with the strike of the planes of foliation in the
gneiss and mica-schists of the same parts of Chili, Tierra del Fuego,
&c. The explanation which he suggests, is based upon a combination of
mechanical and crystalline forces. The planes, he says, of cleavage, and
even the foliation of mica-schist and gneiss, may be intimately
connected with the planes of different tension to which the area was
long subjected, _after_ the main fissures or axis of upheavement had
been formed, but _before_ the final consolidation of the mass and the
total cessation of all molecular movement.[472-A]
I have already stated that some extremely fine slates are perfectly
parallel to the planes of stratification, as those of the Niesen, for
example, near the Lake of Thun, in Switzerland, which contain fucoids, and
are no doubt due to successive aqueous deposition. Even where the slates
are oblique to the general planes of the strata, it by no means follows as
a matter of course that they have been caused by crystalline action, for
they may be the result of that diagonal lamination which I have before
described (p. 17.). In this case, however, there is usually much
irregularity, whereas cleavage planes oblique to the true stratification,
which are referred to a crystalline action, are often perfectly
symmetrical, and observe a strict geometrical parallelism, even when the
strata are contorted, as already described (p. 470.).
Professor Sedgwick, speaking of the planes of slaty cleavage, where they
are decidedly distinct from those of sedimentary deposition, declares
his opinion that no retreat of parts, no contraction in the dimensions
of rocks in passing to a solid state, can account for the phenomenon. It
must be referred to crystalline or polar forces acting simultaneously,
and somewhat uniformly, in given directions, on large masses having
a homogeneous composition.
Sir John Herschel, in allusion to slaty cleavage, has suggested, "that
if rocks have been so heated as to allow a commencement of
crystallization; that is to say, if they have been heated to a point
at which the particles can begin to move amongst themselves, or at
least on their own axes, some general law must then determine the
position in which these particles will rest on cooling. Probably that
position will have some relation to the direction in which the heat
escapes. Now, when all, or a majority of particles of the same nature,
have a general tendency to one position, that must of course determine
a cleavage plane. Thus we see the infinitesimal crystals of fresh
precipitated sulphate of barytes, and some other such bodies, arrange
themselves alike in the fluid in which they float; so as, when stirred,
all to glance with one light, and give the appearance of silky
filaments. Some sorts of soap, in which insoluble margarates[472-B]
exist, exhibit the same phenomenon when mixed with water; and what
occurs in our experiments on a minute scale may occur in nature on
a great one."[472-C]
FOOTNOTES:
[469-A] Geol. Trans., 2d series, vol. iii. p. 480.
[469-B] The Silurian System of Rocks, as developed in Salop, Hereford,
&c., p. 245.
[469-C] Ibid., p. 246.
[470-A] Introduction to Geology, chap. iv.
[471-A] Silurian System of Rocks, &c., p. 246.
[471-B] Report, Brit. Ass., Cork, 1843, p. 60.
[471-C] Quart. Geol. Journ., vol. iii. p. 87. 1847.
[472-A] Geol. Obs. on S. America, 1846, p. 168.
[472-B] Margaric acid is an oleaginous acid, formed from different animal
and vegetable fatty substances. A margarate is a compound of this acid with
soda, potash, or some other base, and is so named from its pearly lustre.
[472-C] Letter to the author, dated Cape of Good Hope, Feb. 20. 1836.