"1. Arenaceous or siliceous rocks. Beds of loose sand frequently met with, of which the grains consist entirely of silex, which term comprehends all purely siliceous minerals, as quartz and common flint. Quartz is silex in its purest form; flint usually contains some admixture of alumina and the oxide of iron. Silica is the mineral used in the manufacture of glass, mixed with a little potassium oxide and lime, or lead, etc. "2. Argillaceous rock. A mixture of silex or flint with a large proportion, usually about one-fourth, of alumina or argil; but in common language, any earth which possesses sufficient ductility, when kneaded up with water, to be fashioned like paste by the hand or by the potter's lathe, is called clay. Such clays vary greatly in their composition. They are, in general, nothing more than mud derived from the decomposition or wearing down of various rocks. The purest clay in nature is porcelain clay or kaolin, which results from the decomposition of a rock composed of feldspar and quartz, and it is almost always mixed with quartz. (The kaolin of China consists of 71.15 parts of silex, 15.86 of alumina, 1.92 of lime, and 6.73 of water.)... One general character of all argillaceous rocks is to give out a peculiar, earthy odor when breathed upon, which is a test of the presence of alumina. "3. Calcareous Rocks. These consist mainly of chalk--lime and carbonic acid. Shells and corals also are formed of the same elements, with the addition of animal matter. Any limestone which is sufficiently hard to take a fine polish is called marble. Many of these are fossiliferous; but statuary marble, which is also called saccharine limestone, as having a texture resembling that of loaf-sugar, is devoid of fossil. Siliceous limestone is an intimate mixture of carbonate of lime and flint, and is harder in proportion as the flinty matter predominates. Marl slate bears the same relation to marl which shale bears to clay, being calcareous shale. Magnesian limestone is composed of carbonate of lime and carbonate of magnesia; the proportion of the latter amounting in some cases to nearly one-half. It effervesces much more slowly and feebly with acid than common limestone. Gypsum is a rock composed of sulphuric acid, lime, and water. It is usually a soft whitish-yellow rock, with a texture resembling loaf-sugar, but sometimes it is entirely composed of lenticular crystals. Alabaster is a granular and compact variety of gypsum found in masses large enough to be used in sculpture and architecture. It is sometimes a pure snow-white substance. It is a softer stone than marble and more easily wrought." When geologists examine the earth's crust, they usually commence with the surface on which we live, and search downwards as far as possible. Lyell constructed a tabular view of the fossiliferous strata.