e that 60 per cent. of the species are common to the North
American and the British Silurians. Sixty per cent. of species in common
is, then, proof of contemporaneity.

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,
corresponding beds will occur in a similar order; but, however great may
be the probability, no man can say with absolute certainty that the beds
in the two sections were synchronously deposited. For areas of moderate
extent, it is doubtless true that no practical evil is likely to result
from assuming the corresponding beds to be synchronous or strictly
contemporaneous; and there are multitudes of accessory circumstances
which may fully justify the assumption of such synchrony. But the moment
the geologist has to deal with large areas, o