At
the same time, it is to be remembered that the evidence afforded
by the explorations carried out of late years as to the animal
life of the deep sea, renders it certain that the extinction
of marine forms of life at the close of the Cretaceous period
was far less extensive than had been previously assumed. It is
tolerably certain, in fact, that we may look upon some of the
inhabitants of the depths of our existing oceans as the direct,
if modified, descendants of animals which were in existence when
the Chalk was deposited.
It follows from the general want of conformity between the Cretaceous
and Tertiary rocks, and still more from the great difference in
life, that the Cretaceous and Tertiary periods are separated, in
the Old World at any rate, by an enormous lapse of unrepresented
time. How long this interval may have been, we have no means of
judging exactly, but it very possibly was as long as the whole
Kainozoic epoch itself. Some day we shall doubtless find, at some
part of the earth's surface, marine strata which were deposited
during this period, and which will contain fossils intermediate
in character between the organic remains which respectively
characterise the Secondary and Tertiary periods. At present, we
have only slight traces of such deposits--as, for instance, the
Maestricht beds, the Faxoee Limestone, and the Pisolitic Limestone
of France.
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