irst place, it does not follow that because we have a tolerably
complete record of the succession of geological formations, we have
therefore any correspondingly complete record of their fossiliferous
contents. The work of determining the relative ages of the rocks does
not require that every cubic mile of the earth's surface should be
separately examined, in order to find all the different fossils which it
may contain. Were this the case, we should hitherto have made but very
small progress in our reading of the testimony of the rocks. The
relative ages of the rocks are determined by broad comparative surveys
over extensive areas; and although the identification of widely
separated deposits is often greatly assisted by a study of their
fossiliferous contents, the mere pricking of a continent here and there
is all that is required for this purpose. Hence, the accuracy of our
information touching the relative ages of geological strata does not
depend upon--and, therefore, does not betoken--any equivalent accuracy
of knowledge touching the fossiliferous material which these strata may
at the present time actually contain. And, as we well know, the
opportunities which the geologist has of discovering fossils are
extremely limited, if we consider these opportunities in relation to the
area of geological formations. The larger portion of the earth's surface
is buried beneath the sea; and much the larger portion of the
fossiliferous deposits on shore are no less hopelessly buried beneath
the land. Therefore it is only upon the fractional portion of the
earth's surface which at the present time happens to be actually exposed
to his view that the geologist is able to prosecute his search for
fossils. But even here how miserably inadequate this search has hitherto
been! With the exception of a scratch or two in the continents of Asia
and America, together with a somewhat larger number of similar scratches
over the continent of Europe, even that comparatively small portion of
the earth's surface which is available for the purpose has been hitherto
quite unexplored by the palaeontologist. How enormously rich a store of
material remains to be unearthed by the future scratchings of this
surface, we may dimly surmise from the astonishing world of bygone life
which is now being revealed in the newly discovered fossiliferous
deposits on the continent of America.

But, besides all this, we must remember, in the second place, that all
th