Rock

in geology any solid portion of the earth's crust. This definition includes sand, clay, and the other kinds of earth, as well as the various sorts of stone. From the viewpoint of the chemist, oxygen forms a half of all the rocks, and silicon, an element of sand, forms a fourth. The remaining fourth is composed chiefly of aluminum, iron, calcium (lime), sodium (salt), potassium, and magnesium. The remaining sixty elements, all taken together, contribute less than one per cent of the rock material of the globe. See Granite; Quartz; Feldspar; Mica; Kaolin; Salt; Gypsum; Clay; Limestone; Sandstone; also articles on the several metals and precious stones.