the neighbouring depths and there deposited, became the components of
the successive series of stratified rocks, extending from the basal
envelope of granite to the earth's surface, and which it will be proper
briefly to describe.


DEPOSITS OR ROCK FORMATIONS.

The first of the series is the _Gneis and Mica Slate System_, of which
examples are exposed to view in the Highlands of Scotland and the west
of England. These earliest stratified rocks contain no matters which are
not to be found in the primitive granite. They are the same in
material--silica, mica, quartz, or hornblende--but changed into new
forms and combinations, and hence called by Mr. LYELL metamorphic rocks.
Some of them are composed exclusively of one of the materials of
granite; the _mica schist_, for example, of mica; the _quartz rocks_,
of quartz. In the metamorphic rocks no organic remains have been found,
and they are geologically below all the rocks that do contain traces of
animal life.

From the primary rocks we pass into the next ascending series, called
the _Clay Slate and Grauwacke Slate System_, which in some places is
found resting immediately on the granite, the antecedent bed being there
wanting. This deposit has been well examined, because some of its slate
beds have been extensively quarried for domestic purposes. By some
geologists it is called the _Silurian System_, it being largely
developed at the surface of a district of western England formerly
occupied by the Silures. It is found also in North Wales and in the
north of England, in beds of great thickness, and in Scotland, but there
the Silurian rocks are more feebly represented.