Now suppose that, a million or two of years hence, when Britain has made
another dip beneath the sea and has come up again, some geologist
applies this doctrine, in comparing the strata laid bare by the
upheaval of the bottom, say, of St. George's Channel with what may then
remain of the Suffolk Crag. Reasoning in the same way, he will at once
decide the Suffolk Crag and the St. George's Channel beds to be
contemporaneous; although we happen to know that a vast period (even in
the geological sense) of time, and physical changes of almost
unprecedented extent, separate the two.

But if it be a demonstrable fact that strata containing more than 60 or
70 per cent. of species of Mollusca in common, and comparatively close
together, may yet be separated by an amount of geological time
sufficient to allow of some of the greatest physical changes the world
has seen, what becomes of that sort of contemporaneity the sole
evidence of which is a similarity of facies, or the identity of half a
dozen species, or of a good many genera?

And yet there is no better evidence for the contemporaneity assumed by
all who adopt the hypothesis of universal faunae and florae, of a
universally uniform climate, and of a sensible cooling of the globe
during geological time.

There seems, then, no escape from the admission that neither physical
geology, nor paleontology, possesses any method by which the absolute
synchronism of two strata can be demonstrated. All that geology can
prove is local order of succession. It is mathematically certain that,
in any given vertical linear section of an undisturbed series of
sedimentary deposits, the bed which lies lowest is the oldest. In many
other vertical linear sections of the same series, of course,