vii

a substance which, when pure, is of a white color and knows
the name of Kaolin; in its impure state, however, it is colored and
Accompanied by Lime, Sand, Oxyds of Iron, &c., forming the
various kinds of Clay.

A knowledge of the composition of Granite, therefore, explains
to us the origin and formation of two massy deposits: Sandstone
and Clay, which almost every where cover and often extend to
thousands of feet below the surface of the globe, and which also
form the fundamental constituents of the soil.                                 

The second class of crystalline rocks, headed: Silicates, con-
taining Lime and Protoxyd of Iron, consists either of rocks con-
taining Felspar, associated with Hornblende, or Felspar associated
with Augite; and their degradation under the influence of the at-
mosphere, is especially due to the influence of the atmospheric
Oxygen on Protoxyd of Iron. Hornblende, as well as Augite,
contains Protoxyd of Iron, a substance to which both minerals
owe their dark and somewhat green color, and which greedily
attracts oxygen from the air, in order to procure for itself a higher
degree of oxydation—that of the Peroxyd of Iron. The immediate
consequence of this change, is the formation of new combinations,
which produce an alteration in the position of the atoms, (the
smallest parts of the rocks;) so that the original solidity of the
rocks is diminished and a continued crumbling into small pieces,
and in the course of time a complete disunion of them takes place.
Common Copperas also contains the Protoxyd of Iron, and exposed
to the influence of the air, is modified in an analogous manner.

If Sulphurets (combinations of Metals with Sulphur) occur in
crystalline rocks, which is frequently the case, the degradation of
them takes place still more readily, inasmuch as these ingredients
also absorb Oxygen and become transformed into Sulphates, soluble
in water.

The capacity then of the crystalline rocks for decomposition, is
in proportion to the quantity of Silicate of Potash they contain, and
also of such other substances as absorb Oxygen, and in this way
have a change of their chemical composition. We always find
that the larger the quantity of these substances, the more quickly
and thoroughly is the exposed rock decomposed.                         

Formation and Constituents of the Soil.

We have thus represented to us in a sensible form, the influence
by which the gradual degradation of the primitive crystalline rocks
was effected; and also the consequences of this process, which
gave rise to the formation of new masses of an earthy and friable
character, permeable to the roots of plants, and thereby rendering
them their necessary mechanical support. We find these masses,
either in their original place, still covering the crystalline rooks by