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weak) acid, is capable of driving it from its combinations. Gran-
ite, a silicate, composed of quartz, feld-spar and mica, contains
both modifications of silica, as constituent parts of its composi-
tion. Its uncombined silica, (quartz,) is insoluble. Its combined
silica in the feld-spar and mica, is soluble. The feld-spar may be
considered, properly, a double silicate, composed of silicate of
alumina, and silicate of potash. The same is true of mica, or
isinglass, a mineral similar in composition to the feld-spar. We
thus can explain the rationale of the decomposition of granite, if
exposed to the influence of water and the atmosphere. The sili-
cate of potash is decomposed by the carbonic acid of the air, and
carbonate of potash and an acid silicate of potash formed, both
being soluble in water. If these substances are washed out, quartz,
pure sand, (the uncombined silicate,) and a white mass of kaolin,
silicate of alumina, (clay,) in a pulverulent condition, remains.

The action of atmospheric oxygen manifests itself in another
way in the decomposition of the silicates, many among which as
hyperate, syenite, basalt, and others, contain in chemical combina-
tion the protoxide of iron, which very greedily attracts oxygen from
the air, in order to procure for itself a higher degree of oxydation,
as pure iron does when exposed to air and moisture. The imme-
diate consequence of this change, is the formation of new combi-
nations, which produce an alteration in the position of the atoms,
the smallest parts of the minerals, so that their original solid-
ity is diminished, and a continued crumbling into small pieces
and in the course of time, complete disunion takes place. If
the silicates contain, as is frequently the case, combinations of metals
with sulphur, (sulphurets,) as iron pyrites, copper pyrites, &c,
diffused in the finest particles through this mass, their decomposi-
tion much more readily takes place, inasmuch as these components
have their ingredients transformed by the absorption of oxygen
into their corresponding salts; iron pyrites becomes sulphate of
iron, copper pyrites sulphate of copper, &c. The salts are very
soluble in water, and will be gradually washed out by its influ-
ence.

The capacity, then, of the silicates for decomposition, depends
on the quantity of the silicate of potash, and of such other sub-
stances as absorb oxygen, and have in this way 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.

The origin of the Neptunic rocks, already has shown us the in-
fluence which water and air have over their ingredients. They
are all soluble, to a greater or less degree, either in pure water,
or in water impregnated with carbonic acid.

The property inherent in water, to occupy a larger space when