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.
Pages:
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428