But all of these mechanical influences which act on solid rocks,
powerful as they are and as great a share they take in their de-
struction, would not have been able to effect such changes by them-
selves as have taken place, and as were necessary to render the
surface of our globe fit for the support of vegetable life. Every
thing which happens in nature, is based upon endless and mutual
actions; no process whatever stands isolated, its whole economy
must be understood before even the most simple phenomena in all
their conditions and consequences can be explained—so in this
instance. The mechanical powers are not the only causes of the
degradation of rocks; these are accompanied and assisted by chemi-
cal affinity, the activity of which is due to the following facts:

A piece of granite protected against all mechanical influences
which could destroy it, would nevertheless lose its cohesion and
finally become changed into a white powder, commencing at the
surface, and gradually extending to the centre of the stone. This
change is due to the influence which the constituents of the atmos-
phere, most especially Oxygen, Carbonic Acid and Water, exercise
upon solid rocks. The different crystalline rocks are composed of
only a limited number of single minerals, which by the different
manner of their association, as well as by the different quantitative1
proportions of which they are united, produce the various forms
above named. These single minerals are: Quartz, Mica, Felspar,
Augite and Hornblende, of which the three first, in their various
combinations, form (he first class of crystalline rocks, headed :
Silicates, containing Alumina and Alkalies. The three latter, on the
other side, form the second class of crystalline rocks, headed:
Silicates, containing Lime and Protoxyd of Iron.

Granite is the representative of the first class, and an investiga-
tion of the causes which lead to its degradation, is sufficient to
explain those which effect that of all others of this class. Granite
consists of Quartz, Felspar and Mica, of which only the two latter
are subject to the influence of the atmosphere. Quartz is a form
of Silicic acid, a substance which presents itself in two different
modifications. In the one, its pure state, it is isolated, forming
Quartz, which is in no way attacked by the atmosphere, and only
liable to crumble to pieces under the influence of mechanical powers.
When thus broken, it gave rise originally and still adds to the im-
mense deposits of Sandstone, which we meet almost every where
on the surface of the earth. In its other modification, it forms with
Alumina and the Alkalies, Felspar and Mica, and in these it exists
in chemical combination, forming a Silicate of Alumina and Potash,
a substance which is subject to the influence of the atmosphere
In contact with the Carbonic Acid and the water of the air, it is
so decomposed that Carbonate of Potash and an Acid Silicate of
Potash are formed, both soluble in water. If these products are
washed out by water, nothing will remain, but Silicate of Alumina,

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