the atmosphere has been added to lime and magnesia of the
igneous rocks to make calcite and dolomite, water has been added to some
of the alumina and silica of the igneous rocks to make kaolin or clay,
and both oxygen and water have been added to the iron of the igneous
rocks to make limonite.

RELATIVE ABUNDANCE OF THE PRINCIPAL ROCKS OF THE LITHOSPHERE

Just as elements combine chemically to form minerals, so do minerals
combine mechanically, either loosely or compactly, to form rocks. For
instance, quartz is a mineral. An aggregation of quartz particles forms
sand or sandstone or quartzite. Most rocks contain more than one kind of
mineral.

Sedimentary rocks occupy considerable areas of the earth's surface, but
they are relatively superficial. It has been estimated that if spread
evenly and continuously over the earth, which they are not, they would
constitute a shell scarcely a half mile thick.[2] Igneous rocks are
relatively more abundant deep below the surface. If the sediments be
assumed to be limited to a volume equivalent to a half-mile shell, and
the remainder of the rocks be assumed to be igneous, it is evident that
to a depth of ten miles 95 per cent of the rocks are igneous. Our actual
observation is confined to a shallow superficial zone in which sediments
make up at least half of all the rocks.

Igneous rocks can be divided for convenience into two main types: (1)
granite and allied rocks, containing a good deal of silica and therefore
_acid_ in a chemical sense, and (2) basalt and allied types, containing
less silica and more lime, magnesia, iron, soda and potassa, and
therefore _basic_ in a chemical sense. The former are light-colored gray
and pink rocks while the latter are dark-colored green and gray rocks.
Granite and basalt as technically defined are very common igneous
rocks,--so common that the names are sometimes used to classify igneous
rocks in general into two great groups, the granitic and the basaltic.
It has been estimated that about 65 per cent of the igneous rocks are of
the granitic group and 35 per cent of the basaltic group.

Sedimentary rocks, as already indicated, consist principally of three
groups, which for convenience are named shale, sandstone, and limestone.
If we approximate the average composition of each group and the average
composition of the igneous rocks from which they are ultimately derived,
it can be calculated that sedimentary rocks must form in the proportions