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 preëxisting 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

Feldspar50
Quartz21
Augite, hornblende, olivine, etc.17
Mica8
Magnetite3
Titanite and ilmenite 1
100

The sedimentary rocks contain minerals in about the following proportions:

PERCENTAGE OF COMMON MINERALS IN SEDIMENTARY ROCKS

Quartz35
Feldspar16
White mica15
Kaolin (clay)9
Dolomite9
Chlorite5
Calcite4
Limonite4
Gypsum, carbon, rutile, apatite, magnetite, etc. 3
100

The sedimentary rocks comprise three main divisions: (1) The muds and clays, with their altered equivalents, shale, slate, etc.; (2) the sands, with their altered equivalents, sandstone, quartzite, quartz-schist, etc.; (3) the marls, limestones, and dolomites, with their altered equivalents, marble, talc-schist, etc. For brevity these groups are referred to respectively as shale, sandstone, and limestone. The proportions of minerals in each of these groups of rocks are as follows:

PERCENTAGE OF COMMON MINERALS IN SHALE, SANDSTONE, AND LIMESTONE

Average shaleAverage sandstoneAverage limestone
Quartz 31.91 69.76 3.71
Kaolin 10.00 7.98 1.03
White mica 18.40
Chlorite 6.40 1.15
Limonite 4.75 .80
Dolomite 7.90 3.44 36.251
Calcite 7.21 56.56
Gypsum 1.17 .12 .10
Feldspar 17.60 8.41 2.20
Magnetite .58
Rutile .66 .12 .06
Ilmenite .25
Apatite .40 .18 .09
Carbon .81
Total100.00100.00100.00
1Includes small amount of FeCO3.