Igneous rocks, while still molten, have been forced into other rocks from below, or poured out on the surface from volcanoes. They are chiefly made of crystals of various minerals, such as quartz, felspar, mica, and pyrite. Granite often contains large crystals of felspar or mica. Some igneous rocks, especially lavas, are glassy; others are so fine grained that the crystals cannot be seen.
In places one may find veins filling cracks in the rocks, and {113} made of material deposited from solution in water. Many valuable minerals and ores occur in such veins, and fine specimens can sometimes be obtained from them.
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Fold in stratified rock
Wearing the soft and hard beds by rain and wind
Quartz vein in rock
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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 the 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 {114} in layers closely resembling the beds of sedimentary rocks; still others, like slate, tend to split into thin plates.