ON THE DIFFERENT AGES OF THE METAMORPHIC ROCKS.
Age of each set of metamorphic strata twofold--Test of age by fossils
and mineral character not available--Test by superposition
ambiguous--Conversion of dense masses of fossiliferous strata into
metamorphic rocks--Limestone and shale of Carrara--Metamorphic strata
of modern periods in the Alps of Switzerland and Savoy--Why the
visible crystalline strata are none of them very modern--Order of
succession in metamorphic rocks--Uniformity of mineral character--Why
the metamorphic strata are less calcareous than the fossiliferous.
According to the theory adopted in the last chapter, the age of each set of
metamorphic strata is twofold--they have been deposited at one period, they
have become crystalline at another. We can rarely hope to define with
exactness the date of both these periods, the fossils having been destroyed
by plutonic action, and the mineral characters being the same, whatever the
age. Superposition itself is an ambiguous test, especially when we desire
to determine the period of crystallization. Suppose, for example, we are
convinced that certain metamorphic strata in the Alps, which are covered by
cretaceous beds, are altered lias; this lias may have assumed its
crystalline texture in the cretaceous or in some tertiary period, the
Eocene for example. If in the latter, it should be called Eocene when
regarded as a metamorphic rock, although it be liassic when considered in
reference to the era of its deposition. According to this view, the
superposition of chalk does not prevent the subjacent _metamorphic_ rock
from being Eocene. If, however, in the progress of science, we should
succeed in ascertaining the twofold chronological relations of the
metamorphic formations, it might be useful to adopt a twofold terminology.
We might call the strata above alluded to Liassic-Eocene, or
Liassic-Cretaceous strata of the Hypogene class; the first term referring
to the era of deposition, the second to that of crystallization.
When discussing the ages of the plutonic rocks, we have seen that examples
occur of various primary, secondary, and tertiary deposits converted into
metamorphic strata, near their contact with granite. There can be no doubt
in these cases that strata, once composed of mud, sand, and gravel, or of
clay, marl, and shelly limestone, have for the distance of several yards,
and in some instances several hundred feet, been turned into gneiss,
mica-schist, hornblende-schist, chlorite-schist, quartz rock, statuary
marble, and the rest. (See the two preceding Chapters.)
But when the metamorphic action has operated on a grander scale, it tends
entirely to destroy all monuments of the date of its development. It may be
easy to prove the identity of two different parts of the same stratum; one,
where the rock has been in contact with a volcanic or plutonic mass, and
has been changed into marble or hornblende-schist, and another not far
distant, where the same bed remains unaltered and fossiliferous; but when
we have to compare two portions of a mountain chain--the one metamorphic,
and the other unaltered--all the labour and skill of the most practised
observers are required. I shall mention one or two examples of alteration
on a grand scale, in order to explain to the student the kind of reasoning
by which we are led to infer that dense masses of fossiliferous strata have
been converted into crystalline rocks.
_Northern Apennines--Carrara._--The celebrated marble of Carrara, used in
sculpture, was once regarded as a type of primitive limestone. It abounds
in the mountains of Massa Carrara, or the "Apuan Alps," as they have been
called, the highest peaks of which are nearly 6000 feet high. Its great
antiquity was inferred from its mineral texture, from the absence of
fossils, and its passage downwards into talc-schist and garnetiferous
mica-schist; these rocks again graduating downwards into gneiss, which is
penetrated, at Forno, by granite veins. Now the researches of MM. Savi,
Boue, Pareto, Guidoni, De la Beche, Hoffmann, and Pilla, have demonstrated
that this marble, once supposed to be formed before the existence of
organic beings, is, in fact, an altered limestone of the Oolitic period,
and the underlying crystalline schists are secondary sandstones and shales,
modified by plutonic action. In order to establish these conclusions it was
first pointed out, that the calcareous rocks bordering the Gulf of Spezia,
and abounding in Oolitic fossils, assume a texture like that of Carrara
marble, in proportion as they are more and more invaded by certain trappean
and plutonic rocks, such as diorite, euphotide, serpentine, and granite,
occurring in the same country.
It was then observed that, in places where the secondary formations are
unaltered, the uppermost consist of common Apennine limestone with
nodules of flint, below which are shales, and at the base of all,
argillaceous and siliceous sandstones. In the limestone, fossils are
frequent, but very rare in the underlying shale and sandstone. Then a
gradation was traced laterally from these rocks into another and
corresponding series, which is completely metamorphic; for at the top of
this we find a white granular marble, wholly devoid of fossils, and
almost without stratification, in which there are no nodules of flint,
but in its place siliceous matter disseminated through the mass in the
form of prisms of quartz. Below this, and in place of the shales, are
talc-schists, jasper, and hornstone; and at the bottom, instead of the
siliceous and argillaceous sandstones, are quartzite and gneiss.[483-A]
Had these secondary strata of the Apennines undergone universally as
great an amount of transmutation, it would have been impossible to form
a conjecture respecting their true age; and then, according to the
common method of geological classification, they would have ranked as
primary rocks. In that case the date of their origin would have been
thrown back to an era antecedent to the deposition of the Lower Silurian
or Cambrian strata, although in reality they were formed in the Oolitic
period, and altered at some subsequent and perhaps much later epoch.
_Alps of Switzerland._--In the Alps, analogous conclusions have been drawn
respecting the alteration of strata on a still more extended scale. In the
eastern part of that chain, some of the primary fossiliferous strata, as
well as the older secondary formations, together with the oolitic and
cretaceous rocks, are distinctly recognizable. Tertiary deposits also
appear in a less elevated position on the flanks of the Eastern Alps; but
in the Central or Swiss Alps, the primary fossiliferous and older secondary
formations disappear, and the Cretaceous, Oolitic, Liassic, and at some
points even the Eocene strata, graduate insensibly into metamorphic rocks,
consisting of granular limestone, talc-schist, talcose-gneiss, micaceous
schist, and other varieties. In regard to the age of this vast assemblage
of crystalline strata, we can merely affirm that some of the upper portions
are altered newer secondary, and some of them even Eocene deposits; but we
cannot avoid suspecting that the disappearance both of the older secondary
and primary fossiliferous rocks may be owing to their having been all
converted in this region into crystalline schist.
It is difficult to convey to those who have never visited the Alps a just
idea of the various proofs which concur to produce this conviction. In the
first place, there are certain regions where Oolitic, Cretaceous, and
Eocene strata have been turned into granular marble, gneiss, and other
metamorphic schists, near their contact with granite. This fact shows
undeniably that plutonic causes continued to be in operation in the Alps
down to a late period, even after the deposition of some of the nummulitic
or older Eocene formations. Having established this point, we are the more
willing to believe that many inferior fossiliferous rocks, probably exposed
for longer periods to a similar action, may have become metamorphic to a
still greater extent.
We also discover in parts of the Swiss Alps dense masses of secondary
and even tertiary strata, which have assumed that semi-crystalline
texture which Werner called transition, and which naturally led his
followers, who attached great importance to mineral characters taken
alone, to class them as transition formations, or as groups older than
the lowest secondary rocks. (See p. 92.) Now, it is probable that these
strata have been affected, although in a less intense degree, by that
same plutonic action which has entirely altered and rendered metamorphic
so many of the subjacent formations; for in the Alps, this action has by
no means been confined to the immediate vicinity of granite. Granite,
indeed, and other plutonic rocks, rarely make their appearance at the
surface, notwithstanding the deep ravines which lay open to view the
internal structure of these mountains. That they exist below at no great
depth we cannot doubt, and we have already seen (p. 445.) that at some
points, as in the Valorsine, near Mont Blanc, granite and granitic veins
are observable, piercing through talcose gneiss, which passes insensibly
upwards into secondary strata.
It is certainly in the Alps of Switzerland and Savoy, more than in any
other district in Europe, that the geologist is prepared to meet with the
signs of an intense development of plutonic action; for here we find the
most stupendous monuments of mechanical violence, by which strata thousands
of feet thick have been bent, folded, and overturned. (See p. 58.) It is
here that marine secondary formations of a comparatively modern date, such
as the Oolitic and Cretaceous, have been upheaved to the height of 12,000,
and some Eocene strata to elevations of 10,000 feet above the level of the
sea; and even deposits of the Miocene era have been raised 4000 or 5000
feet, so as to rival in height the loftiest mountains in Great Britain.
If the reader will consult the works of many eminent geologists who have
explored the Alps, especially those of MM. De Beaumont, Studer, Necker,
Boue, and Murchison, he will learn that they all share, more or less fully,
in the opinions above expressed. It has, indeed, been stated by MM. Studer
and Hugi, that there are complete alternations on a large scale of
secondary strata, containing fossils, with gneiss and other rocks, of a
perfectly metamorphic structure. I have visited some of the most remarkable
localities referred to by these authors; but although agreeing with them
that there are passages from the fossiliferous to the metamorphic series
far from the contact of granite or other plutonic rocks, I was unable to
convince myself that the distinct alternations of highly crystalline, with
unaltered strata above alluded to, might not admit of a different
explanation. In one of the sections described by M. Studer in the highest
of the Bernese Alps, namely in the Roththal, a valley bordering the line of
perpetual snow on the northern side of the Jungfrau, there occurs a mass
of gneiss 1000 feet thick, and 15,000 feet long, which I examined, not only
resting upon, but also again covered by strata containing oolitic fossils.
These anomalous appearances may partly be explained by supposing great
solid wedges of intrusive gneiss to have been forced in laterally between
strata to which I found them to be in many sections unconformable. The
superposition, also, of the gneiss to the oolite may, in some cases, be due
to a reversal of the original position of the beds in a region where the
convulsions have been on so stupendous a scale.
On the Sattel also, at the base of the Gestellihorn, above Enzen, in the
valley of Urbach, near Meyringen, some of the intercalations of gneiss
between fossiliferous strata may, I conceive, be ascribed to mechanical
derangement. Almost any hypothesis of repeated changes of position may be
resorted to in a region of such extraordinary confusion. The secondary
strata may first have been vertical, and then certain portions may have
become metamorphic (the plutonic influence ascending from below), while
intervening strata remained unchanged. The whole series of beds may then
again have been thrown into a nearly horizontal position, giving rise to
the superposition of crystalline upon fossiliferous formations.
It was remarked, in Chap. XXXIV., that as the hypogene rocks, both
stratified and unstratified, crystallize originally at a certain depth
beneath the surface, they must always, before they are upraised and exposed
at the surface, be of considerable antiquity, relatively to a large portion
of the fossiliferous and volcanic rocks. They may be forming at all
periods; but before any of them can become visible, they must be raised
above the level of the sea, and some of the rocks which previously
concealed them must have been removed by denudation.
_Order of succession in metamorphic rocks._--There is no universal and
invariable order of superposition in metamorphic rocks, although a
particular arrangement may prevail throughout countries of great extent,
for the same reason that it is traceable in those sedimentary formations
from which crystalline strata are derived. Thus, for example, we have seen
that in the Apennines, near Carrara, the descending series, where it is
metamorphic, consists of, 1st, saccharine marble; 2dly, talcose-schist; and
3dly, of quartz-rock and gneiss; where unaltered, of, 1st, fossiliferous
limestone; 2dly, shale; and 3dly, sandstone.
But if we investigate different mountain chains, we find gneiss,
mica-schist, hornblende-schist, chlorite-schist, hypogene, limestone,
and other rocks, succeeding each other, and alternating with each other,
in every possible order. It is, indeed, more common to meet with some
variety of clay-slate forming the uppermost member of a metamorphic
series than any other rock; but this fact by no means implies, as some
have imagined, that all clay-slates were formed at the close of an
imaginary period, when the deposition of the crystalline strata gave way
to that of ordinary sedimentary deposits. Such clay-slates, in fact, are
variable in composition, and sometimes alternate with fossiliferous
strata, so that they may be said to belong almost equally to the
sedimentary and metamorphic order of rocks. It is probable that had they
been subjected to more intense plutonic action, they would have been
transformed into hornblende-schist, foliated chlorite-schist, scaly
talcose-schist, mica-schist, or other more perfectly crystalline rocks,
such as are usually associated with gneiss.
_Uniformity of mineral character in Hypogene rocks._--Humboldt has
emphatically remarked, that when we pass to another hemisphere, we see
new forms of animals and plants, and even new constellations in the
heavens; but in the rocks we still recognize our old acquaintances,--the
same granite, the same gneiss, the same micaceous schist, quartz-rock,
and the rest. It is certainly true that there is a great and striking
general resemblance in the principal kinds of hypogene rocks, although
of very different ages and countries; but it has been shown that each of
these are, in fact, geological families of rocks, and not definite
mineral compounds. They are much more uniform in aspect than sedimentary
strata, because these last are often composed of fragments varying
greatly in form, size, and colour, and contain fossils of different
shapes and mineral composition, and acquire a variety of tints from the
mixture of various kinds of sediment. The materials of such strata, if
melted and made to crystallize, would be subject to chemical laws,
simple and uniform in their action, the same in every climate, and
wholly undisturbed by mechanical and organic causes.
Nevertheless, it would be a great error to assume that the hypogene rocks,
considered as aggregates of simple minerals, are really more homogeneous in
their composition than the several members of the sedimentary series. In
the first place, different assemblages of hypogene rocks occur in different
countries; and, secondly, in any one district, the rocks which pass under
the same name are often extremely variable in their component ingredients,
or at least in the proportions in which each of these are present. Thus,
for example, gneiss and mica-schist, so abundant in the Grampians, are
wanting in Cumberland, Wales, and Cornwall; in parts of the Swiss and
Italian Alps, the gneiss and granite are talcose, and not micaceous, as in
Scotland; hornblende prevails in the granite of Scotland--schorl in that of
Cornwall--albite in the plutonic rocks of the Andes--common felspar in
those of Europe. In one part of Scotland, the mica-schist is full of
garnets; in another it is wholly devoid of them: while in South America,
according to Mr. Darwin, it is the gneiss, and not the mica-schist, which
is most commonly garnetiferous. And not only do the proportional quantities
of felspar, quartz, mica, hornblende, and other minerals, vary in hypogene
rocks bearing the same name; but what is still more important, the
ingredients, as we have seen, of the same simple mineral are not always
constant (p. 369., and table, p. 377.).
_The Metamorphic strata, why less calcareous than the fossiliferous._--It
has been remarked, that the quantity of calcareous matter in metamorphic
strata, or, indeed, in the hypogene formations generally, is far less than
in fossiliferous deposits. Thus the crystalline schists of the Grampians in
Scotland, consisting of gneiss, mica-schist, hornblende-schist, and other
rocks, many thousands of yards in thickness, contain an exceedingly small
proportion of interstratified calcareous beds, although these have been the
objects of careful search for economical purposes. Yet limestone is not
wanting in the Grampians, and it is associated sometimes with gneiss,
sometimes with mica-schist, and in other places with other members of the
metamorphic series. But where limestone occurs abundantly, as at Carrara,
and in parts of the Alps, in connection with hypogene rocks, it usually
forms one of the superior members of the crystalline group.
The scarcity, then, of carbonate of lime in the plutonic and metamorphic
rocks generally, seems to be the result of some general cause. So long as
the hypogene rocks were believed to have originated antecedently to the
creation of organic beings, it was easy to impute the absence of lime to
the non-existence of those mollusca and zoophytes by which shells and
corals are secreted; but when we ascribe the crystalline formations to
plutonic action, it is natural to inquire whether this action itself may
not tend to expel carbonic acid and lime from the materials which it
reduces to fusion or semi-fusion. Although we cannot descend into the
subterranean regions where volcanic heat is developed, we can observe in
regions of spent volcanos, such as Auvergne and Tuscany, hundreds of
springs, both cold and thermal, flowing out from granite and other rocks,
and having their waters plentifully charged with carbonate of lime. The
quantity of calcareous matter which these springs transfer, in the course
of ages, from the lower parts of the earth's crust to the superior or newly
formed parts of the same, must be considerable.[487-A]
If the quantity of siliceous and aluminous ingredients brought up by
such springs were great, instead of being utterly insignificant, it
might be contended that the mineral matter thus expelled implies simply
the decomposition of ordinary subterranean rocks; but the prodigious
excess of carbonate of lime over every other element must, in the course
of time, cause the crust of the earth below to be almost entirely
deprived of its calcareous constituents, while we know that the same
action imparts to newer deposits, ever forming in seas and lakes, an
excess of carbonate of lime. Calcareous matter is poured into these
lakes, and the ocean, by a thousand springs and rivers; so that part of
almost every new calcareous rock chemically precipitated, and of many
reefs of shelly and coralline stone, must be derived from mineral matter
subtracted by plutonic agency, and driven up by gas and steam from fused
and heated rocks in the bowels of the earth.
Not only carbonate of lime, but also free carbonic acid gas is given off
plentifully from the soil and crevices of rocks in regions of active and
spent volcanos, as near Naples, and in Auvergne. By this process, fossil
shells or corals may often lose their carbonic acid, and the residual lime
may enter into the composition of augite, hornblende, garnet, and other
hypogene minerals. That the removal of the calcareous matter of fossil
shells is of frequent occurrence, is proved by the fact of such organic
remains being often replaced by silex or other minerals, and sometimes by
the space once occupied by the fossil being left empty, or only marked by a
faint impression. We ought not indeed to marvel at the general absence of
organic remains from the crystalline strata, when we bear in mind how often
fossils are obliterated, wholly or in part, even in tertiary
formations--how often vast masses of sandstone and shale, of different
ages, and thousands of feet thick, are devoid of fossils--how certain
strata may first have been deprived of a portion of their fossils when they
became semi-crystalline, or assumed the _transition_ state of Werner--and
how the remaining organic remains have been effaced when they were rendered
metamorphic. Some rocks of the last-mentioned class, moreover, must have
been exposed again and again to renewed plutonic action.
FOOTNOTES:
[483-A] See notices of Savi, Hoffmann, and others, referred to by Boue,
Bull. de la Soc. Geol. de France, tom. v. p. 317.; and tom. iii. p. xliv.;
also Pilla, cited by Murchison, Quart. Geol. Journ., vol. v. p. 266.
[487-A] See Principles, _Index_, "Calcareous Springs."