rtly mention the composition
of these rocks.

Granite is a mixture of quartz, felspar, and mica in variable
proportions, and the quality of the soil it yields depends on whether
the variety of felspar present be orthoclase or albite. When the former
is the constituent, granite yields soils of tolerable fertility,
provided their climatic conditions be favourable; but it frequently
occurs in high and exposed situations which are unfavourable to the
growth of plants. Gneiss is a similar mixture, but characterised by the
predominance of mica, and by its banded structure. Owing to the small
quantity of felspar which it contains, and the abundance of the
difficulty decomposable mica, the soils formed by its disintegration are
generally inferior. Mica slate is also a mixture of quartz, felspar, and
mica, but consisting almost entirely of the latter ingredient, and
consequently presenting an extreme infertility. The position of the
granite, gneiss, and mica slate soils in this country is such that very
few of them are of much value; but in warm climates they not
unfrequently produce abundant crops of grain. Syenite is a rock similar
in composition to granite, but having the mica replaced by hornblende,
which by its decomposition yields supplies of lime and magnesia more
readily than they can be obtained from the less easily disintegrated
mica. For this reason soils produced from the syenitic rocks are
frequently possessed of considerable fertility.

The series of rocks of which greenstone and trap are types, and which
are very widely distributed, differ greatly in composition from those
already mentioned. They are divisible into two great classes, which have
received the names of diorite and dolerite, the former a mixture of
albite and hornblende, the latter of augite and labradorite, sometimes
with considerable quantities of a sort of oligoclase containing both
soda and lime, and of different kinds of zeolitic minerals. Generally
speaking, the soils produced from diorite are superior to those from
dolerite. The albite which the former contains undergoes a rapid
decomposition, and yields abundance of soda along with some potash,
which is seldom altogether wanting, while the hornblende supplies both
lime and magnesia. Dolerite, when composed entirely of augite and
labradorite, produces rather inferior soils; but when it contains
oligoclase and zeolites, and comes under the head of basalt, its
disintegration is the source of so