Thus, we may suppose an
ocean to cover what is now the European area, and to be peopled
by certain species of animals. Beds of sediment--clay, sands,
and limestones--will be deposited over the sea-bottom, and will
entomb the remains of the animals as fossils. After this has
lasted for a certain length of time, the European area may undergo
elevation, or may become otherwise unsuitable for the perpetuation
of its fauna; the result of which would be that some or all of the
marine animals of the area would migrate to some more suitable
region. Sediments would then be accumulated in the new area to
which they had betaken themselves, and they would then appear,
for the second time, as fossils in a set of beds widely separated
from Europe. The second set of beds would, however, obviously
not be strictly or literally contemporaneous with the first, but
would be separated from them by the period of time required for
the migration of the animals from the one area into the other.
It is only in a wide and comprehensive sense that such strata
can be said to be contemporaneous.
It is impossible to enter further into this subject here; but it
may be taken as certain that beds in widely remote geographical
areas can only come to contain the same fossils by reason of a
migration having taken place of the animals of the one area to
the other.
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