58. All rocks have a tendency to waste away under the influence of the
atmosphere. This is termed _weathering_. Under the influence of the
sun's heat, the external portions of a rock expand, and again contract
when they cool at night. The effect of this alternate expansion and
contraction is often strikingly manifest in tropical countries: some
rocks being gradually disintegrated, and crumbling into grit and sand;
others becoming cracked, and either exfoliating or breaking up all over
their surface into small angular fragments. Again, in countries subject
to alternations of extreme heat and cold, similar weathering action
takes place. The chemical action of the atmosphere is most observable in
the case of calcareous rocks. The carbonic acid almost invariably
present acts as a solvent, so that dew and rain, which otherwise would
in many cases have but feeble disintegrating power, are enabled to eat
into such rocks as chalk and limestone, calcareous sandstones, &c. The
oxygen of the atmosphere also unites with certain minerals, such as the
proto-salts of iron, and converts them into peroxides. It is this action
which produces the red and yellow ferruginous discolorations in
sandstone. Chemical changes also take place in the case of many igneous
rocks, the result being that a weathered 'crust' forms wherever such
rocks are exposed to the action of the atmosphere. Of course, the rate
at which a rock weathers depends upon its mineralogical and chemical
composition. Limestones weather much more rapidly than clay-rocks; and
augitic igneous rocks, as a rule, disintegrate more readily than the
more highly silicated species. The weathering action of the atmosphere
is also greatly aided by frost, as we shall see presently. The result of
all this weathering is the formation of _soil_--soil being only the
fine-grained debris of the weathered rocks. The angular debris found at
the base of all cliffs in temperate and arctic regions, and on every
hill and mountain which is subjected to alternations of extreme heat and
cold, is also the effect of weathering. But these and other effects of
frost will be treated of under the head of _Frozen Water_. The hillocks
and ridges of loose sand (_sand dunes_) found in many places along the
sea-margin, and even in the interior of some continents, as in Africa
and Asia, are due to the action of the wind, which drives the loose
grains before it, and piles them up. Sometimes also the wind carries in
suspension the finest dust, which may be transported for vast distances
before it falls to the ground. Thus, fine dust shot into the air by the
volcanoes of Iceland has been blown as far as the Shetland Islands; and
in tropical countries the dust of the dried-up and parched beds of lakes
and rivers is often swept away during hurricanes, and carried in thick
clouds for leagues. Rain falling through this dust soaks it up, and
comes down highly discoloured, brown and red. This is the so-called
_blood-rain_. Minute microscopic animal and vegetable organisms are
often commingled with this dust, and falling into streams, lakes, or the
sea, may thus become eventually buried in sediments very far removed
from the place that gave them birth.