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CHAPTER V. 
Classification of tertiary formations in chronological order — Comparative value of 
different classes of organic remains — Fossil remains of testacea the most im- 
portant — Necessity of accurately determining species — Tables of shells by 
M. Deshayes — Four subdivisions of the Tertiary epoch — Recent formations — 
. Newer Pliocene period — Older Pliocene period — Miocene period — Eocene period 
— The distinct zoological characters of these periods may not imply sudden 
changes in the animate creation — The recent strata form a common point of 
departure in distant regions — Numerical proportion of recent species of shells 
in different tertiary periods — Mammiferous remains of the successive tertiary 
eras — Synoptical Table of Recent and Tertiary formations. 
CLASSIFICATION OF TERTIARY FORMATIONS .IN 
CHRONOLOGICAL ORDER. 
We explained in the last chapter the principles on which the 
relative ages of different formations may be ascertained, and we 
found the character to be chiefly derivable from superposition, 
mineral structure, and organic remains. It is by combining 
the evidence deducible from all these sources, that we deter- 
mine the chronological succession of distinct formations, and 
this principle is well illustrated by the investigation of those 
European tertiary strata to the discovery of which we have 
already alluded. 
It will be seen, that in proportion as we have extended our 
inquiries over a larger area, it has become necessary to interca- 
late new groups of an age intermediate between those first exa- 
mined, and we have every reason to expect that, as the science 
advances, new links in the chain will be supplied, and that the 
passage from one period to another will become less abrupt. 
We may even hope, without travelling to distant regions, — 
without even transgressing the limits of western Europe, to 
render the series far more complete. The fossil shells, for 
example, of many of the Subalpine formations, on the northern 
limits of the plain of the Po, have not yet been carefully col- 
