CLASSIFICATION OF TERTIARY STRATA. 
143 
cies coexisted at particular eras of the past: and having 
thus identified strata formed in seas with others which orig¬ 
inated contemporaneously in inland lakes, we are then ena¬ 
bled to advance a step farther, and show that certain quad¬ 
rupeds or aquatic plants, found fossil in lacustrine forma¬ 
tions, inhabited the globe at the same period when certain 
fish, reptiles, and zoophytes lived in the ocean. 
Among other characters of the molluscous animals, which 
render them extremely valuable in settling chronological 
questions in geology, may be mentioned, first, the wide geo¬ 
graphical range of many species; and, secondly, what is 
probably a consequence of the former, the great duration of 
species in this class, for they appear to have surpassed in 
longevity the greater number of the mammalia and fish. 
Had each species inhabited a very limited space, it could 
never, when imbedded in strata, have enabled the geologist 
to identify deposits at distant points ; or had they each last¬ 
ed but for a brief period, they could have thrown no light 
on the connection of rocks placed far from each other in the 
chronological, or, as it is often termed, vertical series. 
Classification of Tertiary Strata. —Many authors have di¬ 
vided the European Tertiary strata into three groups— 
lower, middle, and upper; the lower comprising the oldest 
formations of Paris and London before mentioned; the mid¬ 
dle those of Bordeaux and Touraine; and the ujDper all those 
newer than the middle group. 
In the first edition of the Principles of Geology, I divided 
the whole of the Tertiary formations into four groups, char¬ 
acterized by the percentage of recent shells which they con¬ 
tained. The lower tertiary strata of London and Paris were 
thought by M. Deshayes to contain only 3|- per cent, of re¬ 
cent species, and were termed Eocene. The middle tertiary 
of the Loire and Gironde had, according to the specific de¬ 
terminations of the same conchologist, 17 per cent., and 
formed the Miocene division. The Subapennine beds con¬ 
tained 35 to 50 per cent., and were termed Older Pliocene, 
while still more recent beds in Sicily, Avhich had from 90 to 
95 per cent, of species identical with those now living, were 
called Newer Pliocene. The first of the above terms. Eocene, 
is derived from t^wc, eos, davm^ and cainos, recent^ be¬ 
cause the fossil shells of this period contain an extremely 
small proportion of living species, which may be looked upon 
as indicating the dawn of the existing state of the testaceous 
fauna, no recent species having been detected in the older or 
secondary rocks. 
The term Miocene (from ^uftor, meion, Uss^ and Kaivoc^ cainos. 
