to the different circumstances and causes by which they have been produced.

The first two divisions, which will at once be understood as natural, are the
aqueous and volcanic, or the products of watery and those of igneous action at
or near the surface.

AQUEOUS ROCKS.

The aqueous rocks, sometimes called the sedimentary, or fossiliferous, cover a
larger part of the earth's surface than any others. They consist chiefly of
mechanical deposits (pebbles, sand, and mud), but are partly of chemical and
some of them of organic origin, especially the limestones. These rocks are
STRATIFIED, or divided into distinct layers, or strata. The term STRATUM means
simply a bed, or any thing spread out or STREWED over a given surface; and we
infer that these strata have been generally spread out by the action of water,
from what we daily see taking place near the mouths of rivers, or on the land
during temporary inundations. For, whenever a running stream charged with mud or
sand, has its velocity checked, as when it enters a lake or sea, or overflows a
plain, the sediment, previously held in suspension by the motion of the water,
sinks, by its own gravity to the bottom. In this manner layers of mud and sand
are thrown down one upon another.

If we drain a lake which has been fed by a small stream, we frequently find at
the bottom a series of deposits, disposed with considerable regularity, one
above the other; the uppermost, perhaps, may be a stratum of peat, next below a
more dense and solid variety of the same material; still lower a bed of shell-
marl, alternating with peat or sand, and then other beds of marl, divided by
layers of clay. Now, if a second pit be sunk through the same continuous
lacustrine FORMATION at some distance from the first, nearly the same series of
beds is commonly met with, yet with slight variations; some, for example, of the