h
degrees of north latitude, that the bottom of the sea, at the depth of
from twenty to about fifty fathoms, consists of sand with a great
intermixture of shells, often entire, but sometimes finely comminuted.
Between the eleventh and ninth degrees of north latitude, on the same
coast, at soundings varying from twenty to about eighty fathoms, he
brought up abundance of corals and shells mixed with sand. These also
were in some parts entire, and in others worn and broken.

In all these cases, it is only necessary that there should be some
deposition of sedimentary matter, however minute, such as may be
supplied by rivers draining a continent, or currents preying on a line
of cliffs, in order that stratified formations, hundreds of feet in
thickness, and replete with organic remains, should result in the course
of ages.

But although some deposits may thus extend continuously for a thousand
miles or more near certain coasts, the greater part of the bed of the
ocean, remote from continents and islands, may very probably receive, at
the same time, no new accessions of drift matter, all sediment being
intercepted by intervening hollows, in which a marine current must clear
its waters as thoroughly as a turbid river in a lake. Erroneous theories
in geology may be formed not only from overlooking the great extent of
simultaneous deposits now in progress, but also from the assumption that
such formations may be universal or coextensive with the bed of the
ocean.

We frequently observe, on the sea beach, very perfect specimens of
fossil shells, quite detached from their matrix, which have been washed
out of older formations, constituting the sea-cliffs. They may be all of
extinct species, like the Eocene freshwater and marine shells strewed
over the shores of Hampshire, yet when they become mingled with the
shells of the present period, and buried in the same deposits of mud and
sand, they would appear, if upraised and examined by future geologists,
to have been all of the same age. That such intermixture and blending
of organic remains of different ages have actually taken place in former
times, is unquestionable, though the occurrence appears to be very local
and exceptional. It is, however, a class of accidents more likely than
almost any other to lead to serious anachronisms in geological
chronology.

CHAPTER L.

FORMATION OF CORAL REEFS.

Growth of coral chiefly confined to tropical regions--Principal