,
which must have gradually filled up the great valley of that river since
the time when its waters, and the contiguous lands, were inhabited by
the existing species of freshwater and terrestrial mollusks. Showers of
ashes, thrown out by some of the last eruptions of the Eifel volcanoes,
fell during the deposition of this fluviatile silt, and were
interstratified with it. But these volcanoes became exhausted, the
valley was re-excavated through the silt, and again reduced to its
present form before the period of human history. The study, therefore,
of this shelly silt reveals to us the history of a long series of
events, which occurred after the testacea now living inhabited the land
and rivers of Europe, and the whole terminated without any signs of the
coming of man into that part of the globe.

To cite a still more remarkable example, we observe in Sicily a lofty
table-land and hills, sometimes rising to the height of 3000 feet,
capped with a limestone, in which from 70 to 85 per cent. of the fossil
testacea are specifically identical with those now inhabiting the
Mediterranean. These calcareous and other argillaceous strata of the
same age are intersected by deep valleys which have been gradually
formed by denudation, but have not varied materially in width or depth
since Sicily was first colonized by the Greeks. The limestone, moreover,
which is of so late a date in geological chronology, was quarried for
building those ancient temples of Girgenti and Syracuse, of which the
ruins carry us back to a remote era in human history. If we are lost in
conjectures when speculating on the ages required to lift up these
formations to the height of several thousand feet above the sea, how
much more remote must be the era when the same rocks were gradually
formed beneath the waters!

To conclude, it appears that, in going back from the recent to the
Eocene period, we are carried by many successive steps from the fauna
now contemporary with man to an assemblage of fossil species wholly
different from those now living. In this retrospect we have not yet
succeeded in tracing back a perfect transition from the recent to an
extinct fauna; but there are usually so many species in common to the
groups which stand next in succession as to show that there is no great
chasm, no signs of a crisis when one class of organic beings was
annihilated to give place suddenly to another. This analogy, therefore,
derived from a period of the ear