comprising the oldest formations of the environs of Paris and
London, with others of like age; the Middle, those of Bordeaux and
Touraine; and the Upper, all that lay above or were newer than the
last-mentioned group.

When engaged in 1828 in preparing for the press the treatise on
geology above alluded to, I conceived the idea of classing the
whole of this series of strata according to the different degrees
of affinity which their fossil testacea bore to the living fauna.
Having obtained information on this subject during my travels on
the Continent, I learnt that M. Deshayes of Paris, already
celebrated as a conchologist, had been led independently by the
study of a large collection of Recent and fossil shells to very
similar views respecting the possibility of arranging the Tertiary
formations in chronological order, according to the proportional
number of species of shells identical with living ones, which
characterised each of the successive groups above mentioned. After
comparing 3000 fossil species with 5000 living ones, the result
arrived at was, that in the lower Tertiary strata there were about
3 1/2 per cent identical with Recent; in the middle Tertiary (the
faluns of the Loire and Gironde), about 17 per cent; and in the
upper tertiary, from 35 to 50, and sometimes in the most modern
beds as much as 90 to 95 per cent.

For the sake of clearness and brevity, I proposed to give short
technical names to these sets of strata, or the periods to which
they respectively belonged. I called the first or oldest of them
Eocene, the second Miocene, and the third Pliocene. The first of
the above terms, Eocene, is derived from Greek eos, dawn, and Greek
kainos, recent; because an extremely small proportion of the fossil