texture, which have just been named. These crystalline rocks
have therefore furnished the material for all the immense masses
which, in parallel layers, now cover the surface of our globe, and
more particularly the material for that portion of the earth which
forms the soil—its outer crust. The study of the influences to
which, during thousands of years, these crystalline rocks were
exposed and in consequence of which they became loose and friable,
and were converted into new forms of various character, capable
of supporting vegetable life, must therefore form the natural basis
for understanding in what manner the wants of plants depend on
the soil, as also the manner in which those wants may be supplied.

The degradation of rocks,—a term applied to the natural changes
they undergo,—takes place under the double influence of mechani-
cal powers and chemical affinity.

The commencement of this process manifests itself in the forma-
tion of small clefts and fissures, which extend through the body of
the rocks, a phenomenon the consequence of changes in their tem-
perature. We know that all substances, without exception, are
expanded under the influence of heat and contracted again by cool-
ing; the degree of this expansion is however not the same in dif-
ferent substances submitted to the same temperature. The chrys-
talline rocks, of which alone we here speak, are always an aggre-
gation of different minerals. Changes in the temperature to which
they are periodically exposed, consequently effect an unequal
expansion or contraction of these different constituent minerals;
besides this, the surface of the rock is more directly warmed or
cooled, and therefore more expanded or contracted, than the interior.
The consequence of these influences, is the production of small
clefts or fissures, which originating on the surface, gradually extend
themselves towards the centre, and thus weaken the cohesion of
the whole mass.

To the effects of heat on solid masses, we must also add those
occasioned by water. The property inherent in water, to occupy a
larger space when frozen, than in the liquid state, aids materially
the preceding influences in destroying the cohesion of rocks. It
is a well known fact that this increase of the bulk of water when
frozen, is able to overcome any resistance however great, which
may oppose it. In Autumn, the season in which rains usually fall,
we find mountainous countries very moist; the caverns and fissures
of the rooks filled with water, which gradually leaks through the
smaller clefts, percolates through the pores and finally pervades
every part of the rock. The inclosed water converted by the first
frost into ice, expands and bursts the rocks, which oppose its ex-
pansion, and by these means aids materially in their comminution.

Besides, water when falling to the earth in the form of rain, and
running from mountainous regions to level countries, and also air,
by its weight when in motion, act mechanically on rocks and ma-
terially contribute to their degradation.

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