An examination of the Earth’s crust shows us that it is constructed of
numerous strata of rocks, some of limestone, some of sandstone, and some
of clay; and some are very hard, others soft and crumbling, and readily
worn away by the action of running streams or the waves of the ocean. To
these several substances which form the materials of the earth’s crust
we give the name _rock_. Hence we see that while in ordinary language
the word rock denotes a great mass of hard stone, in geology a rock is
any mass of natural substance forming part of the earth’s crust. In this
sense, loose sand, gravel, and soft clay are as much rocks as hard
limestone and granite.
[Illustration: =Granite=
=Porphyry=
=Basalt=
=Hornblende=
=COMPOSITION AND TEXTURE OF STONE AS REVEALED BY THE MICROSCOPE=]