286 HISTORICAL PALZONTOLOGY. 
shall doubtless find, at some part of the earth’s surface, marine 
strata which were deposited during this period, and which will 
contain fossils intermediate in character between the organic 
remains which respectively characterise the Secondary and 
Tertiary periods. At present, we have only slight traces of 
such deposits—as, for instance, the Maestricht beds, the Faxde 
Limestone, and the Pisolitic Limestone of France. 
CLASSIFICATION OF. THE TERTIARY Rocks.—The classifica- 
tion of the Tertiary rocks is a matter of unusual difficulty, in 
consequence of their occurring in disconnected basins, form- 
ing a series of detached areas, which hold no relations of 
superposition to one another. The order, therefore, of the 
Tertiaries in point of time, can only be determined by an ap- 
peal to fossils; and in such determination Sir Charles Lyell 
proposed to take as the basis of classification the proportion of 
living or existing species of Mollusca which occurs in each stratum 
or group of strata. Acting upon this principle, Sir Charles 
Lyell divides the Tertiary series into four groups :— 
I. The Zocene formation (Gr. eos, dawn ; kaznos, new), con- 
taining the smallest proportion of existing species, and being, 
therefore, the oldest division. In this classification, only the 
Mollusca are taken into account; and it was found that of 
these about three and a half per cent were identical with ex- 
isting species. 
Il. The ALzocene formation (Gr. meion, less ; kainos, new), 
with more recent species than the Eocene, but ess than the suc- 
ceeding formation, and less than one-half the total number in the 
formation. As before, only the J/o//usca are taken into account, 
and about 17 per cent of these agree with existing species. 
III. The P%ocene formation (Gr. pletion, more ; kainos, new), 
with generally sore than half the species of shells identical with 
existing species—the proportion of these varying from 35 to 
50 per cent in the lower beds of this division, up to go or 95 
per cent in its higher portion. 
IV. The Post-Tertiary Formations, in which all the shells 
belong to existing species. This, in turn, is divided into two 
minor groups—the Post-Pliocene and Recent Formations. In 
the Post-Pliocene formations, while all the J/o//usca belong to 
existing species, most of the JZammals belong to extinct 
species. In the Aecent period, the quadrupeds, as well as the 
shells, belong to living species. 
The above, with some modifications, was the original classi- 
fication proposed by Sir Charles Lyell for the Tertiary rocks, 
and now universally accepted. More recent researches, it is 
true, have somewhat altered the proportions of existing species 
