As water falls on the earth in rain it has already absorbed from
the air carbon dioxide (carbonic acid gas) and oxygen. As it sinks
into the ground and becomes what is termed ground water, it takes
into solution from the soil 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