Granite is by far the most important of this class of rocks. Of its
thickness no estimate can be made, as no mining operations have ever
penetrated through it, and none of the most extensive displacements of
rocks by natural causes has brought to the surface any other rock on
which it rests. It may, therefore, be considered the foundation rock,
the skeleton of the earth, upon which all the other formations are
supported. The whole amount of granite in the earth's crust may be
greater than that of all other rocks, but it comes up through the other
formations so as to be exposed over only a comparatively small portion
of the surface, and this is generally the central portion of mountain
ranges, or the highest parts of broken, hill country. Still, it is not
unfrequently found in the more level regions, in the form of slightly
elevated ridges, with the stratified rocks reclining against it.
The structure of granite seems frequently to be a confused mixture of
the minerals which compose it, without any approach to order in their
arrangement; but in many cases it is found to split freely in certain
directions, and to work with difficulty in any other. This may result
from an arrangement of the integrant crystals, so that their cleavage
planes approach more or less nearly to parallelism. When this is the
case with the mica or felspar, it must diminish the cohesion in a
direction perpendicular to these planes, and thus facilitate the
cleavage of the mass.
[Illustration: Fig. 3.]
Granite is found to penetrate the stratified rocks in the form of veins.
The following section (Fig. 3) will show the relation of granite veins
to the granitic mass below. The granite which is quarried for
architectural purposes is often in comparatively small quantities,
disappearing at the distance of a few hundred yards beneath the
stratified rock; or else it exists in the form of isolated dome-shaped
masses. It is probable that, if they could be followed sufficiently far,
they would be found to be portions of dikes coming from the general mass
of granite below. Even the granite nuclei of the great mountain ranges
may be considered as injected dikes of enormous magnitude.
[Illustration: Fig. 4.]
Granite is itself intersected with granite veins more frequently,
perhaps, than any other rocks; but the vein is a coarser granite than
the rock which it divides. It is not uncommon to find one set of dikes
intercepted and cut off by a second set, and the second by a third. The
substance of the dikes was, of course, in a liquid state when it was
injected, and the first must have become solid before the second was
thrown in; hence the dikes are of different ages. The dikes _a b c_,
represented in Fig. 4, must have been injected in the order in which
they are lettered.
It is probable that, by the process of cooling, the liquid mass from
which these dikes have proceeded has been gradually solidifying from the
surface downwards. If so, it would follow that the granite nearest the
surface (1, Fig. 2) is the oldest, and the newest is that which is at
the greatest distance below (4). It is possible that at great depths
granite may be still forming, that is, taking the solid form, though of
this there can be no direct proof. There is, however, proof that it has
been liquid at periods of time very distant from each other; for the
dikes sometimes reach to the top of the coal formation (for example),
and then spread themselves out horizontally, as at _a_, showing that the
rock above the coal had not then been deposited. Another dike will
extend through the new red sandstone, as at _b_, and spread itself out
horizontally as before. These horizontal layers of granite, by their
position in strata whose ages are known, indicate the periods when
granite has existed in a liquid state. Granite veins have been
discovered in the Pyrenees as recent as the close of the cretaceous
period, and in the Andes they have been found among the tertiary rocks.
There are several other rocks, of minor importance, often found in
connection with granite. Hypersthene rock, in a few cases, forms the
principal part of mountain masses. Greenstone is more frequently
associated with the trappean rocks, but it sometimes passes
imperceptibly into syenite and common granite. Limestone is found in
considerable abundance, and serpentine in small quantities, as primary
rocks, and have evidently been formed like granite, by solidifying from
a state of fusion.