The rocks composing the terrestrial crust have not always remained in
their original state. They have frequently undergone changes which have
altogether modified their properties, physical and chemical.
When they present these characteristics, we term them _Metamorphic
Rocks_. The phenomena which belong to this subject are at once important
and new, and have lately much attracted the attention of geologists. We
shall best enlighten our readers on the metamorphism of rocks, if we
treat of it under the heads of _special_ and _general_ metamorphism.
When a mass of eruptive rock penetrates the terrestrial crust it
subjects the rocks through which it passes to a special metamorphism--to
the effects of _heat_ produced by _contact_. Such effects may almost
always be observed near the margin of masses of eruptive rock, and they
are attributable either to the communicated heat of the eruptive rock
itself, or to the disengagement of gases, of steam, or of mineral and
thermal waters, which have accompanied its eruption. The effects vary
not only with the rock ejected, but even with the nature of the rock
surrounding it.
In the case of volcanic lava ejected in a molten state, for instance,
the modifications it effects on the surrounding rock are very
characteristic. Its structure becomes prismatic, full of cracks, often
cellular and scoriaceous. Wood and other combustibles touched by the
lava are consumed or partially carbonised. Limestone assumes a granular
and crystalline texture. Siliceous rocks are transformed, not only into
quartz like glass, but they also combine with various bases, and yield
vitreous and cellular silicates. It is nearly the same with argillaceous
rocks, which adhere together, and frequently take the colour of red
bricks.
The surrounding rock is frequently impregnated with specular iron-ore,
and penetrated with hydrochloric or sulphuric acid, and by divers salts
formed from these acids.
At a certain distance from the place of contact with the lava, the
action of water aided by heat produces silica, carbonate of lime,
aragonite, zeolite, and various other minerals.
From immediate contact with the lava, then, the metamorphic rocks denote
the action of a very strong heat. They bear evident traces of
calcination, of softening, and even of fusion. When they present
themselves as hydrosilicates and carbonates, the silica and associated
minerals are most frequently at some distance from the points of
contact; and the formation of these minerals is probably due to the
combination of water and heat, although this last ceases to be the
principal agent.
The hydrated volcanic rocks, such as the basalts and trappean rocks in
general, continue to produce effects of metamorphism, in which heat
operates, although its influence is inconsiderable, water being much the
more powerful agent. The metamorphosis which is observable in the
structure and mineralogical composition of neighbouring rocks is as
follows:--The structure of separation becomes fragmentary, columnar, or
many-sided, and even prismatic. It becomes especially prismatic in
combustibles, in sandstones, in argillaceous formations, in felspathic
rocks, and even in limestones. Prisms are formed perpendicular to the
surface of contact, their length sometimes exceeding six feet. Most
commonly they still contain water or volatile matter. These characters
may be observed at the junction of the basalts which has been ejected
upon the argillaceous strata near Clermont in Auvergne, at Polignac, and
in the neighbourhood of Le Puy-en-Velay.
If the vein of Basalt or Trap has traversed a bed of coal or of lignite,
we find the combustible strongly _metamorphosed_ at the point of
contact. Sometimes it becomes cellular and is changed into _coke_. This
is especially the case in the coal-basin of Brassac. But more frequently
the coal has lost all, or part of, its bituminous and volatile
matter--it has been metamorphosed into anthracite--as an example we may
quote the lignite of Mont Meisner.
Again, in some exceptional cases, the combustible may even be changed
into graphite near to its junction with Trap. This is observed at the
coal-mine of New Cumnock in Ayrshire.
When near its junction with a _trappean_ rock, a combustible has been
metamorphosed into _coke_ or anthracite, it is also frequently
impregnated by hydrated oxide of iron, by clay, foliated carbonate of
lime, iron pyrites, and by various mineral veins. It may happen that the
combustible has been reduced to a pulverulent state, in which case it is
unfit for use. Such is the case in a coal-mine at Newcastle, where the
coal lies within thirty yards of a dyke of Trap.
When Basalt and Trap have been ejected through limestone rock, the
latter becomes more or less altered. Near the points of contact, the
metamorphism which they have undergone is revealed by the change of
colour and aspect, which is exhibited all around the vein, often also by
the development of a crystalline structure. Limestone becomes granular
and saccharoid--it is changed into marble. The most remarkable instance
of this metamorphism is the Carrara marble, a non-fossiliferous
limestone of the Oolite series, which has been altered and the fossils
destroyed; so that the marble of these celebrated quarries, once
supposed to have been formed before the creation of organic beings, is
now shown to be an altered limestone of the Oolitic period, and the
underlying crystalline schists are sandstones and shales of secondary
age modified by plutonic action.
The action of basalt upon limestone is observable at Villeneuve de Berg,
in Auvergne; but still more in the neighbourhood of Belfast, where we
may see the Chalk changed into saccharoid limestone near to its contact
with the Trap. Sometimes the metamorphism extends many feet from the
point of contact; nay, more than that, some zeolites and other minerals
seem to be developed in the crystallised limestone.
When sandstone is found in contact with trappean rock, it presents
unequivocal traces of metamorphism; it loses its reddish colour and
becomes white, grey, green, or black; parallel veins may be detected
which give it a jaspideous structure; it separates into prisms
perpendicular to the walls of the injected veins, when it assumes a
brilliant and vitreous lustre. Sometimes it is even also found
penetrated by zeolites, a family of minerals which melt before the
blowpipe with considerable ebullition. The mottled sandstones of
Germany, which are traversed by veins of basalt, often exhibit
metamorphism, particularly at Wildenstern, in Würtemberg.
Argillaceous rocks, like all others, are subject to metamorphism when
they come in contact with eruptive trappean rocks. In these
circumstances they change colour and assume a varied or prismatic
structure; at the same time their hardness increases, and they become
lithoidal or stony in structure. They may also become cellular--form
zeolites in their cavities with foliated carbonate of lime, as well as
minerals which commonly occur in amygdaloid. Sometimes even the fissures
are coated by the metallic minerals, and the other minerals which
accompany them in their metalliferous beds. Generally they lose a part
of their water and of their carbonic acid. In other circumstances they
combine with oxide of iron and the alkalies. This has been asserted, for
example, at Essey, in the department of the Meurthe, where a very
argillaceous sandstone is found, charged with jasper porcellanite, near
to the junction of the rock with a vein of basalt.
Hitherto we have spoken only of the metamorphosis the result of volcanic
action. A few words will suffice to acquaint the reader with the
metamorphism exercised by the porphyries and granites. By contact with
granite, we find coal changed into anthracite or graphite. It is
important to note, however, that coal has seldom been metamorphosed into
coke. As to the limestone, it is sometimes, as we have seen, transformed
into marble; we even find in its interior divers minerals, notably
silicates with a calcareous base, such as garnets, pyroxene, hornblende,
&c. The sandstones and clay-slates have alike been altered.
The surrounding deposit and the eruptive rock are both frequently
impregnated with quartz, carbonate of lime, sulphate of baryta,
fluorides, and, in a word, with the whole tribe of metalliferous
minerals, which present themselves, besides, with the characteristics
which are common to them in the veins.