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 | Average | Average | shale | sandstone | 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 | 36.25[1] 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 | | -----------+----------+-----------+----------- Total | 100.00 | 100.00 | 100.00 -----------+----------+-----------+----------- 1: Includes small amount of FeCO_{3}.