Sedimentary rocks sometimes exhibit all the symptoms of metamorphism
where there is no evidence of direct eruptive action, and that upon a
scale much grander than in the case of special metamorphism. It is
observable over whole regions, in which it has modified and altered
simultaneously all the surrounding rocks. This state of things is called
general, or normal, metamorphism. The fundamental gneiss, which covers
such a vast extent of country, is the most striking instance known of
general metamorphism. It was first described by Sir W. E. Logan,
Director of the Canadian Geological Survey, who estimates its thickness
at 30,000 feet. The Laurentian Gneiss is a term which is used by
geologists to designate those metamorphic rocks which are known to be
older than the Cambrian system. They are parts of the old pre-Cambrian
continents which lie at the base of the great American continent,
Scandinavia, the Hebrides, &c.; and which are largely developed on the
west coast of Scotland. In order to give the reader some idea of this
metamorphism, we shall endeavour to trace its effects in rocks of the
same nature, indicating the characters successively presented by the
rocks according to the intensity of the metamorphism to which they have
been subjected.
Combustibles, which have a special composition, totally different from
all other rocks, are obviously the first objects of examination. When we
descend in the series of sedimentary deposits, the combustibles are
observed completely to change their characters. From the _peat_ which is
the product of our own epoch, we pass to _lignite_, to _coal_, to
_anthracite_, and even to _graphite_; and find that their density
increases, varying up to at least double. Hydrogen, nitrogen, and,
above all, oxygen, diminish rapidly. Volatile and bituminous matters
decrease, while carbon undergoes a proportionate increase.
This metamorphism of the combustible minerals, which takes place in
deposits of different ages, may also be observed even in the same bed.
For instance, in the coal formations of America, which extend to the
west of the Alleghany mountains, the Coal-measures contain a certain
proportion of volatile matter, which goes on diminishing in proportion
as we approach the granite rocks; this proportion rises to fifty per
cent. upon the Ohio, but it falls to forty upon the Manon-Gahela, and
even to sixteen in the Alleghanies. Finally, in the regions where the
strata have been most disturbed, in Pennsylvania and Massachusetts, the
coal has been metamorphosed into anthracite and even into graphite or
plumbago.
Limestone is one of the rocks upon which we can most easily follow the
effects of general metamorphism. When it has not been modified, it is
usually found in sedimentary rocks in the state of compact limestone, of
coarse limestone, or of earthy limestone such as chalk. But let us
consider it in the mountains, especially in mountains which are at the
same time granitic, such as the Pyrenees, the Vosges, and the Alps. We
shall then see its characters completely modified. In the long and deep
valleys of the Alps, for example, we can follow the alterations of the
limestone for many leagues, the beds losing more and more their
regularity in proportion as we approach the central chain, until they
lose themselves in solitary pinnacles and projections enclosed in
crystalline schists or granitic rocks. Towards the upper regions of the
Alps the limestone divides itself into pseudo-regular fragments, it is
more strongly cemented, more compact, more sonorous; its colour becomes
paler, and it passes from black to grey by the gradual disappearance of
organic and bituminous matter with which it has been impregnated, at the
same time its crystalline structure increases in a manner scarcely
perceptible. It may even be observed to be metamorphosed into an
aggregate of microscopic crystals, and finally to pass into a white
saccharoid limestone.
This metamorphism is produced without any decomposition of the
limestone; it has rather been softened and half melted by the heat, that
is, rendered plastic, so to speak, for we find in it fossils still
recognisable, and among these, notably, some Ammonites and Belemnites,
the presence of which enables us to state that it is the greyish-black
Jurassic limestone, which has been transformed into white saccharoid or
granular limestone. If the limestone subjected to this transformation
were perfectly pure, it would simply take a crystalline structure; but
it is generally mixed with sand and various argillaceous matters, which
have been deposited along with it, matters which go to form new
minerals. These new minerals, however, are not disseminated by chance;
they develop themselves in the direction of the lamination, so to speak,
of the limestone, and in its fissures, in such a manner that they
present themselves in nodules, seams, and sometimes in veins.
Among the principal minerals of the saccharoid limestone we may mention
graphite, quartz, some very varied silicates, such as andalusite,
disthene, serpentine, talc, garnet, augite, hornblende, epidote,
chlorite, the micas, the felspars; finally, spinel, corundum, phosphate
of lime, oxide of iron and oligiste, iron pyrites, &c. Besides these,
various minerals in veins figure among those which exist more commonly
in the saccharoid limestone.
When metamorphic limestone is sufficiently pure, it is employed as
statuary marble. Such is the geological origin of Carrara marble, which
is quarried in the Apuan Alps on a great scale; such, also, was the
marble of Paros and Antiparos, still so celebrated for its purity. On
examination, however, with the lens the Carrara marble exhibits blackish
veins and spangles of graphite; the finest blocks, also, frequently
contain nodules of ironstone, which are lined with perfectly limpid
crystals of quartz. These accidental defects are very annoying to the
sculptor, for they are very minute, and nothing on the exterior of the
block betrays their existence. In the marble of Paros, even when it is
strongly translucent, specks of mica are often found. In the ancient
quarries the nodules are so numerous as to have hindered their being
worked, up even to the present time.
When the mica which occurs in granular limestone takes a green colour
and forms veins, it constitutes the Cipoline marble, which is found in
Corsica, and in the Val Godemar in the Alps. Some white marbles are
quarried in France, chiefly at Loubie, at Sost, at Saint-Béat in the
Pyrenees, and at Chippal in the Vosges. In our country, and especially
in Ireland, there are numerous quarries of marble, veined and coloured
of every hue, but none of a purity suitable for the finest statuary
purposes. All these marbles are only metamorphosed limestones.
The white marbles employed almost all over the world are those of
Carrara. They result from the metamorphism of limestone of the Lias.
They have not been penetrated by the eruptive rocks, but they have been
subjected upon a great scale to a general metamorphism, to which their
crystalline structure may be attributed.
It is easily understood that the calcareous strata have not undergone
such an energetic metamorphism without the beds of sandstone and clay,
associated with them, having also undergone some modification of the
same kind. The siliceous beds accompanying the saccharoid limestone
have, in short, a character of their own. They are formed of small
grains of transparent quartz more or less cemented one to the other in a
manner strongly resembling those of the saccharoid limestone. Between
these grains are usually developed some lamellæ of mica of brilliant and
silky lustre, of which the colour is white, red, or green; in a word, it
has produced a _quartzite_. Some veins of quartz frequently traverse
this quartzite in all directions. Independent of the mica, it may
contain, besides, the different minerals already mentioned as occurring
in the limestone, and particularly silicates--such as disthene,
andalusite, staurotide, garnet, and hornblende.
The argillaceous beds present a series of metamorphisms analogous to the
preceding. We can follow them readily through all their gradations when
we direct our attention towards such granitic masses as those which
constitute the Alps, Pyrenees, the Bretagne Mountains, or our own
Grampians. The schists may perhaps be considered the first step towards
the metamorphism of certain argillaceous rocks; in fact, the schists are
not susceptible of mixing with water like clay; they become stony, and
acquire a much greater density, but their chief characteristic is a
foliated structure.
Experiment proves that when we subject a substance to a great pressure a
foliated structure is produced in a direction perpendicular to that in
which the pressure is exercised. Everything leads us, therefore, to
believe that pressure is the principal cause of the schistous texture,
and of the foliation of clay-slates, the most characteristic variety of
which is the roofing-slate which is quarried so extensively in North
Wales, in Cumberland, and various parts of Scotland in the British
Islands; in the Ardennes; and in the neighbourhood of Angers, in France.
In some localities the slate becomes siliceous and is charged with
crystals of felspar. Nevertheless, it still presents itself in parallel
beds, and contains the same fossil remains still in a recognisable
state. For example, in the neighbourhood of Thann, in the Vosges,
certain vegetable imprints are perfectly preserved in the metamorphic
schist, and in their midst are developed some crystals of felspar.
Mica-schist, which is formed of layers of quartz and mica, is found
habitually associated with rocks which have taken a crystalline
structure, proceeding evidently from an energetic metamorphism of beds
originally argillaceous. Chiastolite, disthene, staurotide, hornblende,
and other minerals are found in it. Mica-schists occur extensively in
Brittany, in the Vosges, in the Pyrenees. In all cases, as we approach
the masses of granite, in these regions, the crystalline structure
becomes more and more marked.
In describing the various facts relating to the metamorphism of rocks,
we have said little of the causes which have produced it. The causes
are, indeed, in the region of hypothesis, and somewhat mysterious.
In what concerns special metamorphism, the cause is supposed to admit of
easy explanation--it is heat. When a rock is ejected from the interior
of the earth in a state of igneous fusion, we comprehend readily enough
that the strata, which it traverses, should sustain alterations due to
the influence of heat, and varying with its intensity. This is clear
enough in the case of _lava_. On the other hand, as water always exists
in the interior of the earth’s crust, and as this water must be at a
very high temperature in the neighbourhood of volcanic fires, it
contributes, no doubt, largely to the metamorphism. If the rocks have
not been ejected in a state of fusion, it is evidently water, with the
different mineral substances it holds in solution, which is the chief
actor in the special metamorphism which is produced.
In general metamorphism, water appears still to be the principal agent.
As it is infiltered through the various beds it will modify their
composition, either by dissolving certain substances, or by introducing
into the metalliferous deposits certain new substances, such as may be
seen forming, even under our eyes, in mineral springs. This has tended
to render the sedimentary deposits plastic, and has permitted the
development of that crystalline structure, which is one of the principal
characteristics of metamorphic rocks. This action has been seconded by
other causes, notably by heat and pressure, which would have an immense
increase of power and energy when metamorphism takes place at a great
depth beneath the surface. Dr. Holl, in an able paper descriptive of the
geology of the Malvern Hills, read before the Geological Society in
February, 1865, adopts this hypothesis as explanatory of the vast
phenomena which are there displayed. After describing the position of
this interesting and strangely-mingled range of rocks, he adds: “These
metamorphic rocks are for the most part highly inclined, and often in a
position nearly vertical. Their disturbance and metamorphism, their
being traversed by granitic veins, and still later their invasion by
trap-dykes and their subsequent elevation above the sea-level, were all
events which must have occupied no inconsiderable period, even of
geological time. I presume,” he adds, “that it will not be maintained
in the present day that the metamorphism of rocks over areas of any but
very moderate extent is due to the intrusion of veins and erupted
masses. The insufficiency of such agency becomes the more obvious when
we consider the slight effects produced by even tolerably extensive
outbursts, such as the Dartmoor granite; while in the case of the
Malverns there is an absence of any local cause whatever. The more
probable explanation in the case of these larger areas is, that they
were faulted down, or otherwise depressed, so as to be brought within
the influence of the earth’s internal heat, and this is the more likely
as they belong to an epoch when the crust is believed to have been
thinner.” When it is considered that, according to the doctrine of
modern geology, the Laurentian rocks, or their equivalents, lie at the
base of all the sedimentary deposits; that this, like other systems of
stratified rocks, was deposited in the form of sand, mud, and clay, to
the thickness of 30,000 feet; and that over an area embracing
Scandinavia, the Hebrides, great part of Scotland, and England as far
south as the Malverns, besides a large proportion of the American
continent, with certain forms of animal life, as recent investigations
demonstrate--can the mind of man realise any other cause by which this
vast extent of metamorphism could have been produced?
Electric and galvanic currents, circulating in the stratified crust, are
not to be overlooked. The experiments of Mr. R. W. Fox and Mr. Robert
Hunt suggest that, in passing long-continued galvanic currents through
masses of moistened clay, there is a tendency to produce cleavage and a
semi-crystalline arrangement of the particles of matter.[31]
[31] Report of the Royal Cornwall Polytechnic Society for 1837. Robert
Hunt, in “Memoirs of the Geological Survey of Great Britain,”
vol. i., p. 433.