Rocks are formed of various materials called minerals. If we take a
piece of sandstone rock, or a piece of granite, we shall probably be
able to notice that the rock is made up of different substances.
On looking at a piece of _sandstone_, for example, especially if we use
a magnifying glass, we see that it is composed of little rounded grains
of a glassy-looking substance cemented together. In some specimens these
grains are larger than in others. This cementing material is not the
same in all sandstones, but in our specimen it is formed of _calcium
carbonate_, for when we drop a little diluted hydrochloric acid on the
rock there is an effervescence. The cementing material is dissolved, but
the little rounded grains, which consist of _quartz_, are not affected
by the acid. The sandstone, then, consists of quartz grains cemented
together by calcium carbonate. It is called a calcareous sandstone.
Now take a piece of granite, and break it with a hammer to get a
clean-cut face. On looking at this face we see that the rock is made up
of _three_ different substances.
One of these has a glassy appearance like the grains in the sandstone,
and is so hard that we cannot scratch it with a knife. This is _quartz_.
Another of the substances is of a dull white or pinkish color. It lies
in long, smooth-faced crystalline patches, which easily break along a
number of smooth parallel surfaces having a pearly lustre. It can be
scratched with difficulty by the point of a knife. This substance is
called _felspar_. The third substance consists of bright glistening
plates, sometimes of a dark color, which can be easily scratched, and
which readily split into transparent leaves. This is _mica_. Notice that
these substances do not occur in any definite order, but are scattered
about through the stone irregularly, the felspar occurring in some
specimens in larger crystals than in others.