The Elements of Geology | Page 9

humus acids and carbon dioxide, both of which are
constantly being generated there by the decay of organic matter. So
both rain and ground water are charged with active chemical agents, by
the help of which they corrode and rust and decompose all rocks to a
greater or less degree. We notice now three of the chief chemical
processes concerned in weathering,-- solution, the formation of
carbonates, and oxidation.
SOLUTION. Limestone, although so little affected by pure water that
five thousand gallons would be needed to dissolve a single pound, is
easily dissolved in water charged with carbon dioxide. In limestone
regions well water is therefore "hard." On boiling the water for some
time the carbon dioxide gas is expelled, the whole of the lime carbonate
can no longer be held in solution, and much of it is thrown down to
form a crust or "scale" in the kettle or in the tubes of the steam boiler.
All waters which flow over limestone rocks or soak through them are
constantly engaged in dissolving them away, and in the course of time
destroy beds of vast extent and great thickness.
The upper surface of limestone rocks becomes deeply pitted, as we saw
in the limestone quarry, and where the mantle of waste has been
removed it may be found so intricately furrowed that it is difficult to
traverse.

Beds of rock salt buried among the strata are dissolved by seeping
water, which issues in salt springs. Gypsum, a mineral composed of
hydrated sulphate of lime, and so soft that it may be scratched with the
finger nail, is readily taken up by water, giving to the water of wells
and springs a peculiar hardness difficult to remove.
The dissolving action of moisture may be noted on marble tombstones
of some age, marble being a limestone altered by heat and pressure and
composed of crystalline grains. By assuming that the date on each
monument marks the year of its erection, one may estimate how many
years on the average it has taken for weathering to loosen fine grains on
the polished surface, so that they may be rubbed off with the finger, to
destroy the polish, to round the sharp edges of tool marks in the
lettering, and at last to open cracks and seams and break down the stone.
We may notice also whether the gravestones weather more rapidly on
the sunny or the shady side, and on the sides or on the top.
The weathered surface of granular limestone containing shells shows
them standing in relief. As the shells are made of crystalline carbonate
of lime, we may infer whether the carbonate of lime is less soluble in
its granular or in its crystalline condition.
THE FORMATION OF CARBONATES. In attacking minerals water
does more than merely take them into solution. It decomposes them,
forming new chemical compounds of which the carbonates are among
the most important. Thus feldspar consists of the insoluble silicate of
alumina, together with certain alkaline silicates which are broken up by
the action of water containing carbon dioxide, forming alkaline
carbonates. These carbonates are freely soluble and 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
HEAT AND COLD. Rocks exposed to the direct