for its accumulation{318}.
Reflect on the years elapsed in many cases, since the latest beds
containing only living species have been formed;--see what Jordan Smith
says of the 20,000 years since the last bed, which is above the boulder
formation in Scotland, has been upraised; or of the far longer period
since the recent beds of Sweden have been upraised 400 feet, what an
enormous period the boulder formation must have required, and yet how
insignificant are the records (although there has been plenty of
elevation to bring up submarine deposits) of the shells, which we know
existed at that time. Think, then, over the entire length of the
Tertiary epoch, and think over the probable length of the intervals,
separating the Secondary deposits. Of these deposits, moreover, those
consisting of sand and pebbles have seldom been favourable, either to
the embedment or to the preservation of fossils{319}.

{318} _Origin_, Ed. i. p. 282, vi. p. 416.

{319} _Origin_, Ed. i. pp. 288, 300, vi. pp. 422, 438.

Nor can it be admitted as probable that any one Secondary formation
contains a fair record even of those organisms which are most easily
preserved, namely hard marine bodies. In how many cases have we not
certain evidence that between the deposition of apparently closely
consecutive beds, the lower one existed for an unknown time as land,
covered with trees. Some of the Secondary formations which contain most
marine remains appear to have been formed in a wide and not deep sea,
and therefore only those marine animals which live in such situations
would be preserved{320}. In all cases, on indented rocky coasts, or any
other coast, where sediment is not accumulating, although often highly
favourable to marine animals, none can be embedded: where pure sand and
pebbles are accumulating few or none will be preserved. I may here
instance the great western line of the S. American coast{321}, tenanted
by many peculiar animals, of which none probably will be preserved to a
distant epoch. From these causes, and especially from such deposits as
are formed along a line of coast, steep above and below water, being
necessarily of little width, and therefore more likely to be
subsequently denuded and worn away, we can see why it is improbable that
our Secondary deposits contain a fair record of the Marine Fauna of any
one period. The East Indian Archipelago offers an area, as large as most
of our Secondary deposits, in which there