Minerals exceptionally consist of single elements, but ordinarily are combinations of two or more elements; for instance, quartz consists of a chemical combination of silicon and oxygen. The proportions of the common minerals in the outer ten miles of the lithosphere are in round numbers as follows: PERCENTAGE OF COMMON MINERALS IN LITHOSPHERE Feldspar 49 Quartz 21 Augite, hornblende, and olivine 15 Mica 8 Magnetite 3 Titanite and ilmenite 1 Kaolin, limonite, hematite, dolomite, calcite, chlorite, etc. 3 --- 100 In making up this table it is assumed that the rocks to a depth of ten miles are about 95 per cent of igneous type, that is, crystallized from molten magma, and about 5 per cent of sedimentary type, that is, formed from the weathering and erosion of igneous rocks or preexisting sediments, and deposited in beds or layers, either by water or by air (see pp. 16-17). More reliable figures for the relative abundance of the minerals are available for each of the two classes of rocks, igneous and sedimentary. The igneous rocks contain minerals in about the following proportions: PERCENTAGE OF COMMON MINERALS IN IGNEOUS ROCKS Feldspar 50 Quartz 21 Augite, hornblende, olivine, etc. 17 Mica 8 Magnetite 3 Titanite and ilmenite 1 --- 100 The sedimentary rocks contain minerals in about the following proportions: PERCENTAGE OF COMMON MINERALS IN SEDIMENTARY ROCKS Quartz 35 Feldspar 16 White mica 15 Kaolin (clay) 9 Dolomite 9 Chlorite 5 Calcite 4 Limonite 4 Gypsum, carbon, rutile, apatite, magnetite, etc. 3 --- 100