To this there are two replies: the first, that the observational basis
of the assertion that the lowest rocks are nowhere fossiliferous is an
amazingly small one, seeing how very small an area, in comparison to
that of the whole world, has yet been fully searched; the second, that
the argument is good for nothing unless 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."