Page:Handbook for Boys.djvu/134

Rh made of material deposited from solution in water. Many valuable minerals and ores occur in such viens, and fine specimens can sometimes be obtained from them.

Sedimentary rock are formed of material usually derived from the breaking up and wearing away of older rocks. When first deposited, the materials are loose, but later, when covered by other beds, they become hardened into solid rock. If the

layers were of sand, the rock is sandstone; if of clay, it is shale. Rocks made of layers of pebbles are called conglomerate or pudding-stone; those of limy material, derived perhaps from shells, are limestone. Many sedimentary rocks contain fossils, which are shells or bones of animals or the stems and leaves of plants living in former times, and buried by successive beds of sand or mud spread over them.. Much of the land is covered by a thin surface deposit of clay, sand, or gravel, which is yet loose material and which shows the mode of formation of sedimentary rocks.

Some rocks have undergone, since their formation, great pressure or heat and have been much changed. They are called metamorphic rocks. Some are now made of crystals though at first they were not; in others the minerals have become arranged