"I cannot
tell."

If the further question be put, whether physical geology is in
possession of any method by which the actual synchrony (or the reverse)
of any two distant deposits can be ascertained, no such method can be
heard of; it being admitted by all the best authorities that neither
similarity of mineral composition, nor of physical character, nor even
direct continuity of stratum, are _absolute_ proofs of the synchronism
of even approximated sedimentary strata: while, for distant deposits,
there seems to be no kind of physical evidence attainable of a nature
competent to decide whether such deposits were formed simultaneously, or
whether they possess any given difference of antiquity. To return to an
example already given. All competent authorities will probably assent to
the proposition that physical geology does not enable us in any way to
reply to this question--Were the British Cretaceous rocks deposited at
the same time as those of India, or are they a million of years younger
or a million of years older?

Is palaeontology able to succeed where physical geology fails? Standard
writers on palaeontology, as has been seen, assume that she can. They
take it for granted, that deposits containing similar organic remains
are synchronous--at any rate in a broad sense; and yet, those who will
study the eleventh and twelfth chapters of Sir Henry De la Beche's
remarkable "Researches in Theoretical Geology," published now nearly
thirty years ago, and will carry out the arguments there most luminously
stated, to their logical consequences, may very easily convince
themselves that even absolute identity of organic contents is no proof
of the synchrony of deposits, while absolute diversity is no proof of
difference of date. Sir Henry De la Beche goes even further, and adduces
conclusive evidence to show that the different parts of one and the same
stratum, having a similar composition throughout, containing the same
organic remains, and having similar beds above and below it, may yet
differ to any conceivable extent in age.

Edward Forbes was in the habit of asserting that the similarity of the
organic contents of distant formations was _prima facie_ evidence, not
of their similarity, but of their difference of age; and holding as he
did the doctrine of single specific centres, the conclusion was as
legitimate as any other; for the two districts must have been occupied
by migration from one of the two, or from an int