is being deposited over nearly the whole
bed of the sea, at a rate sufficiently quick to embed and preserve fossil
remains. Throughout an enormously large proportion of the ocean, the bright
blue tint of the water bespeaks its purity. The many cases on record of a
formation conformably covered, after an enormous interval of time, by
another and later formation, without the underlying bed having suffered in
the interval any wear and tear, seem explicable only on the view of the
bottom of the sea not rarely lying for ages in an unaltered condition. The
remains which do become embedded, if in sand or gravel, will when the beds
are upraised generally be dissolved {289} by the percolation of rain-water.
I suspect that but few of the very many animals which live on the beach
between high and low watermark are preserved. For instance, the several
species of the Chthamalinae (a subfamily of sessile cirripedes) coat the
rocks all over the world in infinite numbers: they are all strictly
littoral, with the exception of a single Mediterranean species, which
inhabits deep water and has been found fossil in Sicily, whereas not one
other species has hitherto been found in any tertiary formation: yet it is
now known that the genus Chthamalus existed during the chalk period. The
molluscan genus Chiton offers a partially analogous case.

With respect to the terrestrial productions which lived during the
Secondary and Palaeozoic periods, it is superfluous to state that our
evidence from fossil remains is fragmentary in an extreme degree. For
instance, not a land shell is known belonging to either of these vast
periods, with the exception of one species discovered by Sir C. Lyell and
Dr. Dawson in the carboniferous strata of North America, of which shell
several specimens have now been collected. In regard to mammiferous
remains, a single glance at the historical table published in the
Supplement to Lyell's Manual, will bring home the truth, how accidental and
rare is their preservation, far better than pages of detail. Nor is their
rarity surprising, when we remember how large a proportion of the bones of
tertiary mammals have been discovered either in caves or in lacustrine
deposits; and that not a cave or true lacustrine bed is known belonging to
the age of our secondary or palaeozoic formations.

But the imperfection in the geological record mainly results from another
and more important cause than any of the foregoing; namely, from t