of the atmosphere and of
water. Some of the minerals, such as certain silicates and
carbonates, dissolve relatively fast, others with extreme
slowness. In the process of solution chemical actions are
involved; oxidation in presence of the free oxygen of the
atmosphere; attack by the feeble acid arising from the solution
of carbon dioxide in water; or, again, by the activity of certain
acids--humous acids--which originate in the decomposition of
vegetable remains. These chemical agents may in some instances,
_e.g._ in the case of carbonates such as limestone or
dolomite--bring practically the whole rock into solution. In other
instances--_e.g._ granites, basalts, etc.--they may remove some of
the

34

constituent minerals completely or partially, such as felspar,
olivine, augite, and leave more resistant substances to be
ultimately washed down as fine sand or mud into the river.

It is often difficult or impossible to appraise the relative
efficiency of mechanical and chemical denudation in removing the
materials from a certain area. There can be, indeed, little doubt
that in mountainous regions the mechanical effects are largely
predominant. The silts of glacial rivers are little different
from freshly-powdered rock. The water which carries them but
little different from the pure rain or snow which falls from the
sky. There has not been time for the chemical or solvent actions
to take place. Now while gravitational forces favour sudden shock
and violent motions in the hills, the effect of these on solvent
and chemical denudation is but small. Nor is good drainage
favourable to chemical actions, for water is the primary factor
in every case. Water takes up and removes soluble combinations of
molecules, and penetrates beneath residual insoluble substances.
It carries the oxygen and acids downwards through the soils, and
finally conveys the results of its own work to the rivers and
streams. The lower mean temperature of the mountains as well as
the perfect drainage diminishes chemical activities.

Hence we conclude that the heights are not generally favourable
to the purely solvent and chemical actions. It is on the
lower-lying land that soils tend to accumulate,

35

and in these the chief solvent and the chief chemical denudation
of the Earth are effected.

The solvent and chemical effects which go on in the
finely-divided materials of the soils may be observed in the
laboratory. They proceed faster than wou