11. Having learned that all the rocks met with at the surface of the
earth's crust are capable of being arranged under a few classes, we have
now to investigate the matter more in detail. It will be observed that
the classification adopted above is based chiefly upon the external
characters of the constituent ingredients of the rocks, and the mode in
which these particles have been collected. In some rocks the component
materials are crystalline, in others they are rounded and worn; in one
case they have been brought together by precipitation from an aqueous
solution, or they have crystallised out from a mass of once molten
matter; in another case their collection and intimate association is due
to the mechanical action of the atmosphere or of water, or to the agency
of the organic forces. We have next to inquire what is the nature of
those crystals and particles which are the ingredients of the rocks?
The answer to this question properly belongs to the science of
mineralogy, with which, however, the geologist must necessarily make
some acquaintance.
12. _Granite--its composition._--It will tend to simplify matters if we
begin our inquiry by selecting for examination some familiar rock, such
as _granite_. This rock, as one sees at a glance, is crystalline, nor is
it difficult to perceive that three separate kinds of ingredients go to
compose it. One of these we shall observe is a gray, or it may be, clear
glassy-looking substance, which is hard, and will not scratch with a
knife; another is of a pink, red, gray, or sometimes even pale green
colour, and scratches with difficulty; while the third shews a
glistering metallic lustre, and is generally of a brownish or black
colour. It scratches easily with the knife, and can be split up into
flakes of extreme thinness. If the granite be one of the coarse-grained
varieties, we shall notice that these three ingredients have each more
or less definite crystalline forms; so that they are not distinguished
by colour and hardness alone. The metallic-looking substance is _mica_;
the hard gray, or glassy and unscratchable ingredient is _quartz_; and
the remaining material is _felspar_. The mineralogist's analysis of
granite ends here. But there is still much to be learned about quartz,
felspar, and mica; for, as the chemist will tell us, these are not
'elementary substances.' Quartz is a compound, consisting of two
elements, one of which is a non-metallic body (silicon), and the other
an invisible gas (oxygen). Felspar[A] is a still more complex compound,
being made up of two metals (potassium, aluminium) and one non-metallic
body (silicon), each of which is united to an invisible gas (oxygen).
Mica, again, contains no fewer than four metals (potassium, magnesium,
iron, calcium) and one non-metallic body (silicon), each of which is in
like manner chemically united to its share of oxygen. Thus the
rock-forming substances, quartz, felspar, and mica, have each a definite
chemical composition.
13. _Minerals._--Now, any inorganic substance which has a definite
chemical composition, and crystallises in a definite crystalline or
geometric form, is termed a _mineral_. Having once discovered that
quartz is composed of silicon and oxygen--that is, silica--and that the
faces of its crystals are arranged in a certain definite order, we may
be quite sure that any mineral which has not this composition and form
cannot be quartz. And so on with mica and felspar, and every other
mineral. The study of the geometric forms assumed by minerals
(crystallography) forms a department of the science of mineralogy. But,
in the great majority of cases, the mineral ingredients of the rocks are
either so small individually, or so broken, and rounded, and altered,
that crystallography gives comparatively little aid to the practical
geologist in the field. He has, therefore, recourse to other tests for
the determination of the mineral constituents of rocks. Many of these
tests, however, can only be applied by those who have had long
experience. The simplest and easiest way for the student to begin is to
examine the forms and appearance of the more common minerals in some
collection, and thereafter to accustom his eye to the aspect presented
by the same minerals when they are associated together in rocks, of
which illustrative specimens are now to be met with in most museums. The
microscope is largely employed by geologists for determining the
mineralogical composition of certain rocks; and, indeed, many rocks can
hardly be said to be thoroughly known until they have been sliced and
examined under the microscope, and analysed by the chemist. But with a
vast number such minute examination is not required, the eye after some
practice being able to detect all that is needful to be known.
[A] There are various kinds of felspar; the one referred to above is
_orthoclase_, or potash-felspar.