any rate, by an enormous lapse of unrepresented
time. How long this interval may have been, we have no means of
judging exactly, but it very possibly was as long as the whole
Kainozoic epoch itself. Some day we 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 Faxoee Limestone, and the Pisolitic Limestone
of France.

CLASSIFICATION OF THE TERTIARY ROCKS.--The classification of the
Tertiary rocks is a matter of unusual difficulty, in consequence
of their occurring in disconnected basins, forming 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 appeal 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 _Eocene_ formation (Gr. _eos_, dawn; _kainos_, new),
containing 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 existing
species.

II. The _Miocene_ formation (Gr. _meion_, less; _kainos_, new),
with more recent species than the Eocene, but _less_ than the
succeeding formation, and less than one-half the total number
in the formation. As before, only the _Mollusca_ are taken into
account, and about 17 per cent of these agree with existing species.

III. The _Pliocene_ formation (Gr. _pleion_, more; _kainos_, new),
with generally _more_ 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 90 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_