I 



166 LYELL'S ELEMENTS OF GEOLOGY. 



Classification of the Tertiary Formations. 



The number of tertiary fossil shells examined by M. Deshayes 

 was about 3000 ; and the recent species with which they had 

 been compared, about 5000. The result at which that naturalist 

 arrived was, that in the oldest tertiary deposits, such as those 

 found near London and Paris, there were about 3^ per cent, of 

 species of fossil shells identical with recent species ; in the next, 

 or middle tertiary period, to which certain strata on the Loire 

 and Gironde, in France, belonged, about 17 per cent. ; and in 

 the deposits of a third, or newer era, embracing those of the 

 Subapennine hills, from 35 to 50 per cent. In formations still 

 more modern, some of which I had particularly studied in Sicily, 

 where they attain a vast thickness and elevation above the sea, 

 the number of species identical with those now living was from 

 90 to 95 per cent. For the sake of clearness and brevity, I pro- 

 posed to give short technical names to these four groups, or the 

 periods to which they respectively belonged. I called the first or 

 oldest of them Eocene, the second Miocene, the third Older Plio- 

 cene, and the last or fourth. Newer Pliocene. The first of the 

 above terms. Eocene, is derived from j^wj, eos, dawn, and xawoj, 

 cainos, recent, because the fossil shells of this period contain an 

 extremely small proportion of living species, which may be look- 

 ed upon as indicating the dawn of the recent or existing state 

 of the testaceous fauna. 



The other terms, Miocene and Pliocene, are comparative ; the 

 first meaning less recent, (from (tt£tof , melon, less, and xmvo^, cainos, 

 recent,) and the other more recent, (from tc%uov, pleion, more, and 

 xaivos, cainos, recent,) they express the more or less near ap- 

 proach which the deposits of these eras, when contrasted with 

 each other, make to the existing creation, at least so far as the 

 mollusca are concerned. It may assist the memory of students 

 to remind them, that the iWiocene contain a minor proportion, 

 and PZiocene a comparative pZurality of recent species ; and 

 that the greater number of recent species always implies the 

 more modern origin of the strata. 



Two subjects of discussion have arisen respecting the tables 

 above alluded to ; first, whether the fossil shells were, upon the 

 whole, correctly identified with recent species by M. Deshayes ; 

 secondly, whether such a per-centage of recent species occurring 

 fossil in particular groups, affords the best criterion for estimat- 

 ing the relation of each fossil fauna to the living creation. 



Now in regard to the per-centage test, its application must 

 evidently depend on the extent to which conchologists are agreed 

 in their determination of species. In every branch of natural 

 history there is always some difference of opinion as to certain 

 species which are variable in their characters, and seem to pass 



