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

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
fo