Plan of investigation.

We find, according to this its internal structure, which, I believe,
with the assistance of Turner, can scarcely now be misunderstood, that
the earth may be considered as divided into three great classes of
formation, which geology has already named for us. Primary--the rocks,
which, though in position lower than all others, rise to form the
central peaks, or interior nuclei of all mountain ranges. Secondary--the
rocks which are laid in beds above these, and which form the greater
proportion of all hill scenery. Tertiary--the light beds of sand,
gravel, and clay, which are strewed upon the surface of all, forming
plains and habitable territory for man. We shall find it convenient, in
examining the truth of art, to adopt, with a little modification, the
geological arrangement, considering first, the formation and character
of the highest or central peaks; then the general structure of the lower
mountains, including in this division those composed of the various
slates which a geologist would call primary; and, lastly, the minutiae
and most delicate characters of the beds of these hills, when they are
so near as to become foreground objects, and the structure of the common
soil which usually forms the greater space of an artist's foreground.
Hence our task will arrange itself into three divisions--the
investigation of the central mountains, of the interior mountains, and
of the foreground.

CHAPTER II.

OF THE CENTRAL MOUNTAINS.

Sec. 1. Similar character of the central peaks in all parts of the world.

It does not always follow, because a mountain is the highest of its
group, that it is in reality one of the central range. The Jungfrau is
only surpassed in elevation, in the chain of which it is a member, by
the Schreckhorn and Finster-Aarhorn; but it is entirely a secondary
mountain. But the central peaks are usually the highest, and may be
considered as the chief components of all mountain scenery in the snowy
regions. Being composed of the same rocks in all countries, their
external character is the same everywhere. Its chief essential points
are the following.

Sec. 2. Their arrangements in pyramids or wedges, divided by vertical
fissures.

Their summits are almost invariably either pyramids or wedges. Domes may
be formed by superincumbent snow, or appear to be formed by the
continuous outline of a sharp