Denudation has, however, other methods of work than purely
mechanical; methods more noiseless and gentle, but not less
effective, as the victories of peace ate no less than those of
war.
Over the immense tracts of the continents chemical work proceeds
relentlessly. The rock in general, more especially the primary
igneous rock, is not stable in presence 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
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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 would be anticipated. The
observation is made by passing a measured quantity of water
backwards and forwards for some months through a tube containing
a few grammes of powdered rock. Finally the water is analysed,
and in this manner the amount of dissolved matter it has taken up
is estimated. The rock powder is examined under the microscope in
order to determine the size of the grains, and so to calculate
the total surface exposed to the action of the water. We must be
careful in such experiments to permit free oxidation by the
atmosphere. Results obtained in this way of course take no
account of the chemical effects of organic acids such as exist in
the soils. The quantities obtained in the laboratory will,
therefore, be deficient as compared with the natural results.
In this manner it has been found that fresh basalt exposed to
continually moving water will lose about 0.20 gramme per square
metre of surface per year. The mineral orthoclase, which enters
largely into the constitution of many granites, was found to lose
under the same conditions 0.025 gramme. A glassy lava (obsidian)
rich in silica and in the chemical constituents of an average
granite, was more resistant still; losing but 0.013 gramme per
square metre per year. Hornblende, a mineral