Geological Contemporaniety and Persistent Types of Life | Page 6

the unfossiliferous rocks in
question were not only 'contemporaneous' in the geological sense, but
'synchronous' in the chronological sense. To use the 'alibi' illustration
again. If a man wishes to prove he was in neither of two places, A and
B, on a given day, his witnesses for each place must be prepared to

answer for the whole day. If they can only prove that he was not at A in
the morning, and not at B in the afternoon, the evidence of his absence
from both is 'nil', because he might have been at B in the morning and
at A in the afternoon.
Thus everything depends upon the validity of the second assumption.
And we must proceed to inquire what is the real meaning of the word
"contemporaneous" as employed by geologists. To this end a concrete
example may be taken.
The Lias of England and the Lias of Germany, the Cretaceous rocks of
Britain and the Cretaceous rocks of Southern India, are termed by
geologists "contemporaneous" formations; but whenever any
thoughtful geologist is asked whether he means to say that they were
deposited synchronously, he says, "No,--only within the same great
epoch." And if, in pursuing the inquiry, he is asked what may be the
approximate value in time of a "great epoch"--whether it means a
hundred years, or a thousand, or a million, or ten million years--his
reply is, "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 paleontology able to succeed where physical geology fails? Standard
writers on paleontology, 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 intermediate spot, and the chances
against exact coincidence of migration and of imbedding are infinite.
In point of fact, however, whether the hypothesis of single or of
multiple specific centres be adopted, similarity of organic contents
cannot possibly afford any proof of the synchrony of the deposits
which contain them; on the contrary, it is demonstrably compatible
with the lapse of the most prodigious intervals of time, and with the
interposition of vast changes in the organic and inorganic worlds,
between the epochs in which such deposits were formed.
On what amount of similarity of their faunae is the doctrine of the
contemporaneity of the European and of the North American Silurians
based? In the last edition of Sir Charles Lyell's 'Elementary Geology' it
is stated, on the authority of a former President of this Society, the late
Daniel Sharpe, that between 30 and 40 per cent. of the species of
Silurian Mollusca are common to both sides of the Atlantic. By way of
due allowance for further