METAMORPHIC ROCKS--_continued_.
Strata near some intrusive masses of granite converted into rocks
identical with different members of the metamorphic series--Arguments
hence derived as to the nature of plutonic action--Time may enable
this action to pervade denser masses--From what kinds of sedimentary
rock each variety of the metamorphic class may be derived--Certain
objections to the metamorphic theory considered--Lamination of
trachyte and obsidian due to motion--Whether some kinds of gneiss have
become schistose by a similar action.
It has been seen that geologists have been very generally led to infer,
from the phenomena of joints and slaty cleavage, that mountain masses, of
which the sedimentary origin is unquestionable, have been acted upon
simultaneously by vast crystalline forces. That the structure of
fossiliferous strata has often been modified by some general cause since
their original deposition, and even subsequently to their consolidation and
dislocation, is undeniable. These facts prepare us to believe that still
greater changes may have been worked out by a greater intensity, or more
prolonged development of the same agency, combined, perhaps, with other
causes. Now we have seen that, near the immediate contact of granitic veins
and volcanic dikes, very extraordinary alterations in rocks have taken
place, more especially in the neighbourhood of granite. It will be useful
here to add other illustrations, showing that a texture undistinguishable
from that which characterizes the more crystalline metamorphic formations,
has actually been superinduced in strata once fossiliferous.
In the southern extremity of Norway there is a large district, on the west
side of the fiord of Christiania, in which granite or syenite protrudes in
mountain masses through fossiliferous strata, and usually sends veins into
them at the point of contact. The stratified rocks, replete with shells and
zoophytes, consist chiefly of shale, limestone, and some sandstone, and all
these are invariably altered near the granite for a distance of from 50 to
400 yards. The aluminous shales are hardened and have become flinty.
Sometimes they resemble jasper. Ribboned jasper is produced by the
hardening of alternate layers of green and chocolate-coloured schist, each
stripe faithfully representing the original lines of stratification. Nearer
the granite the schist often contains crystals of hornblende, which are
even met with in some places for a distance of several hundred yards from
the junction; and this black hornblende is so abundant that eminent
geologists, when passing through the country, have confounded it with the
ancient hornblende-schist, subordinate to the great gneiss formation of
Norway. Frequently, between the granite and the hornblende slate, above
mentioned, grains of mica and crystalline felspar appear in the schist, so
that rocks resembling gneiss and mica-schist are produced. Fossils can
rarely be detected in these schists, and they are more completely effaced
in proportion to the more crystalline texture of the beds, and their
vicinity to the granite. In some places the siliceous matter of the schist
becomes a granular quartz; and when hornblende and mica are added, the
altered rock loses its stratification, and passes into a kind of granite.
The limestone, which at points remote from the granite is of an earthy
texture, blue colour, and often abounds in corals, becomes a white granular
marble near the granite, sometimes siliceous, the granular structure
extending occasionally upwards of 400 yards from the junction; and the
corals being for the most part obliterated, though sometimes preserved,
even in the white marble. Both the altered limestone and hardened slate
contain garnets in many places, also ores of iron, lead, and copper, with
some silver. These alterations occur equally, whether the granite invades
the strata in a line parallel to the general strike of the fossiliferous
beds, or in a line at right angles to their strike, as will be seen by the
accompanying ground plan.[474-A]
[Illustration: Fig. 512. Altered zone of fossiliferous slate and limestone
near granite. Christiania.
_The arrows indicate the dip, and the straight lines the strike,
of the beds._]
The indurated and ribboned schists above mentioned bear a strong
resemblance to certain shales of the coal found at Russell's Hall, near
Dudley, where coal-mines have been on fire for ages. Beds of shale of
considerable thickness, lying over the burning coal, have been baked and
hardened so as to acquire a flinty fracture, the layers being alternately
green and brick-coloured.
The granite of Cornwall, in like manner, sends forth veins into a coarse
argillaceous-schist, provincially termed killas. This killas is converted
into hornblende-schist near the contact with the veins. These appearances
are well seen at the junction of the granite and killas, in St. Michael's
Mount, a small island nearly 300 feet high, situated in the bay, at a
distance of about three miles from Penzance.
The granite of Dartmoor, in Devonshire, says Sir H. De la Beche, has
intruded itself into the slate and slaty sandstone called greywacke,
twisting and contorting the strata, and sending veins into them. Hence some
of the slate rocks have become "micaceous; others more indurated, and with
the characters of mica-slate and gneiss; while others again appear
converted into a hard-zoned rock strongly impregnated with felspar."[475-A]
We learn from the investigations of M. Dufrenoy, that in the eastern
Pyrenees there are mountain masses of granite posterior in date to the
formations called lias and chalk of that district, and that these
fossiliferous rocks are greatly altered in texture, and often charged with
iron-ore, in the neighbourhood of the granite. Thus in the environs of St.
Martin, near St. Paul de Fenouillet, the chalky limestone becomes more
crystalline and saccharoid as it approaches the granite, and loses all
trace of the fossils which it previously contained in abundance. At some
points, also, it becomes dolomitic, and filled with small veins of
carbonate of iron, and spots of red iron-ore. At Rancie the lias nearest
the granite is not only filled with iron-ore, but charged with pyrites,
tremolite, garnet, and a new mineral somewhat allied to felspar, called,
from the place in the Pyrenees where it occurs, "couzeranite."
Now the alterations above described as superinduced in rocks by volcanic
dikes and granite veins, prove incontestably that powers exist in nature
capable of transforming fossiliferous into crystalline strata--powers
capable of generating in them a new mineral character, similar, nay, often
absolutely identical, with that of gneiss, mica-schist, and other
stratified members of the hypogene series. The precise nature of these
altering causes, which may provisionally be termed plutonic, is in a great
degree obscure and doubtful; but their reality is no less clear, and we
must suppose the influence of heat to be in some way connected with the
transmutation, if, for reasons before explained, we concede the igneous
origin of granite.
The experiments of Gregory Watt, in fusing rocks in the laboratory, and
allowing them to consolidate by slow cooling, prove distinctly that a rock
need not be perfectly melted in order that a re-arrangement of its
component particles should take place, and a partial crystallization
ensue.[475-B] We may easily suppose, therefore, that all traces of shells
and other organic remains may be destroyed; and that new chemical
combinations may arise, without the mass being so fused as that the lines
of stratification should be wholly obliterated.
We must not, however, imagine that heat alone, such as may be applied to
a stone in the open air, can constitute all that is comprised in
plutonic action. We know that volcanos in eruption not only emit fluid
lava, but give off steam and other heated gases, which rush out in
enormous volume, for days, weeks, or years continuously, and are even
disengaged from lava during its consolidation. When the materials of
granite, therefore, came in contact with the fossiliferous stratum in
the bowels of the earth under great pressure, the contained gases might
be unable to escape; yet when brought into contact with rocks, might
pass through their pores with greater facility than water is known to do
(p. 35.). These aeriform fluids, such as sulphuretted hydrogen, muriatic
acid, and carbonic acid, issue in many places from rents in rocks, which
they have discoloured and corroded, softening some and hardening others.
If the rocks are charged with water, they would pass through more
readily; for, according to the experiments of Henry, water, under an
hydrostatic pressure of 96 feet, will absorb three times as much
carbonic acid gas as it can under the ordinary pressure of the
atmosphere. Although this increased power of absorption would be
diminished, in consequence of the higher temperature found to exist as
we descend in the earth, yet Professor Bischoff has shown that the heat
by no means augments in such a proportion as to counteract the effect of
augmented pressure.[476-A] There are other gases, as well as the
carbonic acid, which water absorbs, and more rapidly in proportion to
the amount of pressure. Now even the most compact rocks may be regarded,
before they have been exposed to the air and dried, in the light of
sponges filled with water; and it is conceivable that heated gases
brought into contact with them, at great depths, may be absorbed
readily, and transfused through their pores. Although the gaseous matter
first observed would soon be condensed, and part with its heat, yet the
continual arrival of fresh supplies from below might, in the course of
ages, cause the temperature of the water, and with it that of the
containing rock, to be materially raised.
M. Fournet, in his description of the metalliferous gneiss near Clermont,
in Auvergne, states that all the minute fissures of the rock are quite
saturated with free carbonic acid gas, which rises plentifully from the
soil there and in many parts of the surrounding country. The various
elements of the gneiss, with the exception of the quartz, are all softened;
and new combinations of the acid, with lime, iron, and manganese, are
continually in progress.[476-B]
Another illustration of the power of subterranean gases is afforded by the
stufas of St. Calogero, situated in the largest of the Lipari Islands.
Here, according to the description published by Hoffmann, horizontal strata
of tuff, extending for 4 miles along the coast, and forming cliffs more
than 200 feet high, have been discoloured in various places, and strangely
altered by the "all-penetrating vapours." Dark clays have become yellow, or
often snow-white; or have assumed a chequered or brecciated appearance,
being crossed with ferruginous red stripes. In some places the fumaroles
have been found by analysis to consist partly of sublimations of oxide of
iron; but it also appears that veins of chalcedony and opal, and others of
fibrous gypsum, have resulted from these volcanic exhalations.[476-C]
The reader may also refer to M. Virlet's account of the corrosion of hard,
flinty, and jaspideous rocks near Corinth, by the prolonged agency of
subterranean gases[477-A]; and to Dr. Daubeny's description of the
decomposition of trachytic rocks in the Solfatara, near Naples, by
sulphuretted hydrogen and muriatic acid gases.[477-B]
Although in all these instances we can only study the phenomena as
exhibited at the surface, it is clear that the gaseous fluids must have
made their way through the whole thickness of porous or fissured rocks,
which intervene between the subterranean reservoirs of gas and the external
air. The extent, therefore, of the earth's crust, which the vapours have
permeated and are now permeating, may be thousands of fathoms in thickness,
and their heating and modifying influence may be spread throughout the
whole of this solid mass.
We learn from Professor Bischoff that the steam of a hot spring at
Aix-la-Chapelle, although its temperature is only from 133 deg. to 167 deg. F., has
converted the surface of some blocks of black marble into a doughy mass. He
conceives, therefore, that steam in the bowels of the earth having a
temperature equal or even greater than the melting point of lava, and
having an elasticity of which even Papin's digester can give but a faint
idea, may convert rocks into liquid matter.[477-C]
The above observations are calculated to meet some of the objections which
have been urged against the metamorphic theory on the ground of the small
power of rocks to conduct heat; for it is well known that rocks, when dry
and in the air, differ remarkably from metals in this respect. It has been
asked how the changes which extend merely for a few feet from the contact
of a dike could have penetrated through mountain masses of crystalline
strata several miles in thickness. Now it has been stated that the plutonic
influence of the syenite of Norway has sometimes altered fossiliferous
strata for a distance of a quarter of a mile, both in the direction of
their dip and of their strike. (See fig. 512. p. 474.) This is undoubtedly
an extreme case; but is it not far more philosophical to suppose that this
influence may, under favourable circumstances, affect denser masses, than
to invent an entirely new cause to account for effects merely differing in
quantity, and not in kind? The metamorphic theory does not require us to
affirm that some contiguous mass of granite has been the altering power;
but merely that an action, existing in the interior of the earth at an
unknown depth, whether thermal, electrical, or other, analogous to that
exerted near intruding masses of granite, has, in the course of vast and
indefinite periods, and when rising perhaps from a large heated surface,
reduced strata thousands of yards thick to a state of semi-fusion, so that
on cooling they have become crystalline, like gneiss. Granite may have been
another result of the same action in a higher state of intensity, by which
a thorough fusion has been produced; and in this manner the passage from
granite into gneiss may be explained.
Some geologists are of opinion, that the alternate layers of mica and
quartz, or mica and felspar, or lime and felspar, are so much more
distinct, in certain metamorphic rocks, than the ingredients composing
alternate layers in many sedimentary deposits, that the similar particles
must be supposed to have exerted a molecular attraction for each other, and
to have thus congregated together in layers more distinct in mineral
composition than before they were crystallized.
In considering, then, the various data already enumerated, the forms of
stratification in metamorphic rocks, their passage on the one hand into the
fossiliferous, and on the other into the plutonic formations, and the
conversions which can be ascertained to have occurred in the vicinity of
granite, we may conclude that gneiss and mica-schist may be nothing more
than altered micaceous and argillaceous sandstones that granular quartz may
have been derived from siliceous sandstone, and compact quartz from the
same materials. Clay-slate may be altered shale, and granular marble may
have originated in the form of ordinary limestone, replete with shells and
corals, which have since been obliterated; and, lastly, calcareous sands
and marls may have been changed into impure crystalline limestones.
"Hornblende-schist," says Dr. MacCulloch, "may at first have been mere
clay; for clay or shale is found altered by trap into Lydian stone, a
substance differing from hornblende-schist almost solely in compactness and
uniformity of texture."[478-A] "In Shetland," remarks the same author,
"argillaceous-schist (or clay-slate), when in contact with granite, is
sometimes converted into hornblende-schist, the schist becoming first
siliceous, and ultimately, at the contact, hornblende-schist."[478-B]
The anthracite and plumbago associated with hypogene rocks may have been
coal; for not only is coal converted into anthracite in the vicinity of
some trap dikes, but we have seen that a like change has taken place
generally even far from the contact of igneous rocks, in the disturbed
region of the Appalachians.[478-C] At Worcester, in the state of
Massachusetts, 45 miles due west of Boston, a bed of plumbago and impure
anthracite occurs, interstratified with mica-schist. It is about 2 feet in
thickness, and has been made use of both as fuel, and in the manufacture of
lead pencils. At the distance of 30 miles from the plumbago, there occurs,
on the borders of Rhode Island, an impure anthracite in slates, containing
impressions of coal-plants of the genera _Pecopteris_, _Neuropteris_,
_Calamites_, &c. This anthracite is intermediate in character between that
of Pennsylvania and the plumbago of Worcester, in which last the gaseous or
volatile matter (hydrogen, oxygen, and nitrogen) is to the carbon only in
the proportion of 3 per cent. After traversing the country in various
directions, I came to the conclusion that the carboniferous shales or
slates with anthracite and plants, which in Rhode Island often pass into
mica-schist, have at Worcester assumed a perfectly crystalline and
metamorphic texture; the anthracite having been nearly transmuted into that
state of pure carbon which is called plumbago or graphite.[479-A]
The total absence of any trace of fossils has inclined many geologists to
attribute the origin of crystalline strata to a period antecedent to the
existence of organic beings. Admitting, they say, the obliteration, in some
cases, of fossils by plutonic action, we might still expect that traces of
them would oftener occur in certain ancient systems of slate, in which, as
in Cumberland, some conglomerates occur. But in urging this argument, it
seems to have been forgotten that there are stratified formations of
enormous thickness, and of various ages, and some of them very modern, all
formed after the earth had become the abode of living creatures, which are,
nevertheless, in certain districts, entirely destitute of all vestiges of
organic bodies. In some, the traces of fossils may have been effaced by
water and acids, at many successive periods; and it is clear, that, the
older the stratum, the greater is the chance of its being
non-fossiliferous, even if it has escaped all metamorphic action.
It has been also objected to the metamorphic theory, that the chemical
composition of the secondary strata differs essentially from that of the
crystalline schists, into which they are supposed to be convertible.[479-B]
The "primary" schists, it is said, usually contain a considerable
proportion of potash or of soda, which the secondary clays, shales, and
slates do not, these last being the result of the decomposition of
felspathic rocks, from which the alkaline matter has been abstracted during
the process of decomposition. But this reasoning proceeds on insufficient
and apparently mistaken data; for a large portion of what is usually called
clay, marl, shale, and slate does actually contain a certain, and often a
considerable, proportion of alkali; so that it is difficult, in many
countries, to obtain clay or shale sufficiently free from alkaline
ingredients to allow of their being burnt into bricks or used for pottery.
Thus the argillaceous shales and slates of the Old Red sandstone, in
Forfarshire and other parts of Scotland, are so much charged with
alkali, derived from triturated felspar, that, instead of hardening when
exposed to fire, they sometimes melt into a glass. They contain no lime,
but appear to consist of extremely minute grains of the various
ingredients of granite, which are distinctly visible in the
coarser-grained varieties, and in almost all the interposed sandstones.
These laminated clays and shales might certainly, if crystallized,
resemble in composition many of the primary strata.
There is also potash in fossil vegetable remains, and soda in the salts by
which strata are sometimes so largely impregnated, as in Patagonia.
Another objection has been derived from the alternation of highly
crystalline strata with others having a less crystalline texture. The heat,
it is said, in its ascent from below, must have traversed the less altered
schists before it reached a higher and more crystalline bed. In answer to
this, it may be observed, that if a number of strata differing greatly in
composition from each other be subjected to equal quantities of heat, there
is every probability that some will be more fusible than others. Some, for
example, will contain soda, potash, lime, or some other ingredient capable
of acting as a flux; while others may be destitute of the same elements,
and so refractory as to be very slightly affected by a degree of heat
capable of reducing others to semi-fusion. Nor should it be forgotten that,
as a general rule, the less crystalline rocks do really occur in the upper,
and the more crystalline in the lower part of each metamorphic series.
There are geologists, however, of high authority, who admit the
metamorphic origin of gneiss and mica-schist even on a grand scale in
some mountain-chains, and who nevertheless believe that gneiss has in
some instances been an eruptive rock, deriving its lamination from
motion when in a fluid or viscous state. Mr. Scrope, in his description
of the Ponza Islands, ascribes "the zoned structure of the Hungarian
perlite (a semi-vitreous trachyte) to its having subsided, in obedience
to the impulse of its own gravity, down a slightly inclined plane, while
possessed of an imperfect fluidity. In the islands of Ponza and
Palmarola, the direction of the zones is more frequently vertical than
horizontal, because the mass was impelled from below upwards."[480-A] In
like manner, Mr. Darwin attributes the lamination and fissile structure
of volcanic rocks of the trachytic series, including some obsidians in
Ascension, Mexico, and elsewhere, to their having moved when liquid in
the direction of the laminae. The zones consist sometimes of layers of
air-cells drawn out and lengthened in the supposed direction of the
moving mass. He compares this division into parallel zones, thus caused
by the stretching of a pasty mass as it flowed slowly onwards, to the
zoned or ribboned structure of ice, which Professor James Forbes has
so ably explained, showing that it is due to the fissuring of a
viscous body in motion.[480-B] Mr. Darwin also imagines the lamination
or _foliation_, as he terms it, of gneiss and mica-schist in South
America to be the extreme result of that process of which cleavage
is the first effect.[480-C]
M. Elie de Beaumont, while he regards the greater part of the gneiss and
mica-schist of the Alps as sedimentary strata altered by plutonic action,
still conceives that some of the Alpine gneiss may have been erupted, or,
in other words, may be granite drawn out into parallel laminae in the manner
of trachyte as above alluded to.[480-D]
Opinions such as these, and others which might be cited, prove the
difficulty of arriving at clear theoretical views on this subject. I may
also add another difficulty. In many extensive regions experienced
geologists have been at a loss to decide which of two sets of divisional
planes were referable to cleavage and which to stratification; and that,
too, where the rocks are of undisputed aqueous origin. After much doubt,
they have sometimes discovered that they had at first mistaken the lines of
cleavage for those of deposition, because the former were by far the most
marked of the two. Now if such slaty masses should become highly
crystalline, and be converted into gneiss, hornblende-schist, or any other
member of the hypogene class, the cleavage planes would be more likely to
remain visible than those of stratification.
But although the cause last-mentioned may, in some instances, be a "vera
causa," as applied to gneiss and mica-schist, I believe it to be an
exception to the general rule. Nor would it, I conceive, produce that kind
of irregular parallelism in the laminae which belongs to so many of the
hypogene rocks of the Grampians, Pyrenees, and the White mountains of North
America, where I have chiefly studied them.
But it will be impossible for the reader duly to appreciate the propriety
of the term metamorphic, as applied to the strata formerly called
primitive, until I have shown, in the next chapter, at how many distinct
periods these crystalline strata have been formed.
FOOTNOTES:
[474-A] Keilhau, Gaea Norvegica, pp. 61-63.
[475-A] Geol. Manual, p. 479.
[475-B] Phil. Trans., 1804.
[476-A] Poggendorf's Annalen, No. xvi., 2d series, vol. iii.
[476-B] See Principles, _Index_, "Carbonated Springs," &c.
[476-C] Hoffmann's Liparischen Inseln, p. 38. Leipzig, 1832.
[477-A] See Princ. of Geol.; and Bulletin de la Soc. Geol. de France,
tom. ii. p. 230.
[477-B] See Princ. of Geol.; and Daubeny's Volcanos, p. 167.
[477-C] Jam. Ed. New Phil. Journ., No. 51. p. 43.
[478-A] Syst. of Geol., vol. i. p. 210.
[478-B] Ibid., p. 211.
[478-C] See above, pp. 327, 333.
[479-A] See Lyell, Quart. Geol. Journ., vol. i. p. 199.
[479-B] Dr. Boase, Primary Geology, p. 319.
[480-A] Geol. Trans., 2d series, vol. ii. p. 227.
[480-B] Darwin, Volcanic Islands, pp. 69, 70.
[480-C] Geol. Obs. in S. America, p. 167. See also above, p. 471.
[480-D] Bulletin, vol. iv. p. 1301.