How the Earth’s Crust Is Constructed
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.
GranitePorphyry
BasaltHornblende
COMPOSITION AND TEXTURE OF STONE AS REVEALED BY THE MICROSCOPE