re to waste away. Forests may be as dense and lofty as those of
Brazil, and may swarm with quadrupeds, birds, and insects, yet at the
end of ten thousand years one layer of black mould, a few inches thick,
may be the sole representative of those myriads of trees, leaves,
flowers, and fruits, those innumerable bones and skeletons of birds,
quadrupeds, and reptiles, which tenanted the fertile region. Should this
land be at length submerged, the waves of the sea may wash away in a few
hours the scanty covering of mould, and it may merely impart a darker
shade of color to the next stratum of marl, sand, or other matter newly
thrown down. So also at the bottom of the ocean where no sediment is
accumulating, sea-weed, zoophytes, fish, and even shells, may multiply
for ages and decompose, leaving no vestige of their form or substance
behind. Their decay, in water, although more slow, is as certain and
eventually as complete as in the open air. Nor can they be perpetuated
for indefinite periods in a fossil state, unless imbedded in some matrix
which is impervious to water, or which at least does not allow a free
percolation of that fluid, impregnated as it usually is, with a slight
quantity of carbonic or other acid. Such a free percolation may be
prevented either by the mineral nature of the matrix itself, or by the
superposition of an impermeable stratum: but if unimpeded, the fossil
shell or bone will be dissolved and removed, particle after particle,
and thus entirely effaced, unless petrifaction or the substitution of
mineral for organic matter happen to take place.

That there has been land as well as sea at all former geological
periods, we know from the fact, that fossil trees and terrestrial plants
are imbedded in rocks of every age. Occasionally lacustrine and
fluviatile shells, insects, or the bones of amphibious or land reptiles,
point to the same conclusion. The existence of dry land at all periods
of the past implies, as before mentioned, the partial deposition of
sediment, or its limitation to certain areas; and the next point to
which I shall call the reader's attention, is the shifting of these
areas from one region to another.

First, then, variations in the site of sedimentary deposition are
brought about independently of subterranean movements. There is always a
slight change from year to year, or from century to century. The
sediment of the Rhone, for example, thrown into the Lake of Geneva, is
now conveye