ils remarkable for their fertility;
for these latter substances undergoing rapid decomposition furnish the
plants with abundant supplies of alkalies and lime, while the more
slowly decomposing hornblende affords the necessary quantity of
magnesia. In addition to these, the basaltic rocks are found to contain
appreciable quantities of phosphoric acid, so that they are in a
condition to yield to the plant almost all its necessary constituents.

The different rocks now mentioned, with a few others of less general
distribution, constitute the whole of our great mountain masses; and
while their general composition is such as has been stated, they
frequently contain disseminated through them quantities of other
minerals which, though in trifling quantity, nevertheless add their
quota of valuable constituents to the soils. Moreover, the exact
composition of the minerals of which the great masses of rocks are
composed is liable to some variety. Those which we have taken as
illustrations have been selected as typical of the minerals; but it is
not uncommon to find albite containing 2 or 3 per cent of potash,
labradorite with a considerable proportion of soda, and zeolitic
minerals containing several per cent of potash, the presence of which
must of course considerably modify the properties of the soils produced
from them. They are also greatly affected by the mechanical influences
to which the rocks are exposed; and being situated for the most part in
elevated positions, they are no sooner disintegrated than they are
washed down by the rains. A granite, for instance, as the result of
disintegration, has its felspar reduced to an impalpable powder, while
its quartz and mica remain, the former entirely, the latter in great
part, in the crystalline grains which existed originally in the granite.
If such a disintegrated granite remains on the spot, it is easy to see
what its composition must be; but if exposed to the action of running
water, by which it is washed away from its original site, a process of
separation takes place, the heavy grains of quartz are first deposited,
then the lighter mica, and lastly the felspar. Thus there may be
produced from the same granite, soils of very different nature and
composition, from a pure and barren sand to a rich clay formed entirely
of felspathic debris.

The sedimentary or stratified rocks are formed of particles carried down
by water and deposited at the bottom of the primeval seas from