trata 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 palaeontology, 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, or with completely separated deposits, the
mischief of confounding that "homotaxis" or "similarity of arrangement,"
which _can_ be demonstrated, with "synchrony" or "identity of date," for
which there is not a shadow of proof, under the one common term of
"contemporaneity" becomes incalculable, and proves the constant source of
gratuitous speculations