contribute potash and soda to soils and river waters. By the
removal of the soluble ingredients of feldspar there is left the
silicate of alumina, united with water or hydrated, in the
condition of a fine plastic clay which, when white and pure, is
known as KAOLIN and is used in the manufacture of porcelain.
Feldspathic rocks which contain no iron compounds thus weather to
whitish crusts, and even apparently sound crystals of feldspar,
when ground to thin slices and placed under the microscope, may be
seen to be milky in color throughout because an internal change to
kaolin has begun.

OXIDATION. Rocks containing compounds of iron weather to reddish
crusts, and the seams of these rocks are often lined with rusty
films. Oxygen and water have here united with the iron, forming
hydrated iron oxide. The effects of oxidation may be seen in the
alteration of many kinds of rocks and in red and yellow colors of
soils and subsoils.

Pyrite is a very hard mineral of a pale brass color, found in
scattered crystals in many rocks, and is composed of iron and
sulphur (iron sulphide). Under the attack of the weather it takes
up oxygen, forming iron sulphate (green vitriol), a soluble
compound, and insoluble hydrated iron oxide, which as a mineral is
known as limonite. Several large masses of iron sulphide were
placed some years ago on the lawn in front of the National Museum
at Washington. The mineral changed so rapidly to green vitriol
that enough of this poisonous compound was washed into the ground
to kill the roots of the surrounding grass.

AGENTS OF MECHANICAL DISINTEGRATION