Physical geography. By Mary Somerville ...

CHAP. I. OUTLINE OF GEOLOGY. 29
which had nothing in common with the existing order of things, had
passed away, and given place to one more nearly approaching to that
which now prevails. Among the myriads of beings that inhabited
the earth and the ocean during the secondary fossiliferous epoch
scarcely one species is to be found in the tertiary. Two planets
could hardly differ more in their natural productions. This break
in the law of continuity is the more remarkable, as hitherto some
of the newly-created animals were always introduced before the older
were extinguished. The circumstances and climate suited to the one
became more and more unfit for the other, which consequently
perished gradually, while their successors increased. It is possible
that, as observations become more extended, this hiatus may be
filled up.
The series of rocks, from the granite to the end of the secondary
fossiliferous strata, taken as a whole, constitute the solid crust of the
globe, and in that sense are universally diffused over the earth's surface. The tertiary strata occupy the hollows formed in this crust,
whether by subterraneous movements, by lakes, or denudation by
water as in the estuaries of rivers, and consequently occur in irregular tracts, often, however, of prodigious thickness and extent. Indeed, they seem to have been as widely developed as any other formation, though time has been wanting to bring them into view.
The innumerable basins and hollows with which the continents
and larger islands had been indented for ages after the termination
of the secondary fossiliferous series had sometimes been fresh-water
lakes, and at other times inundated by the sea; consequently, the
deposits which took place during these changes alternately contain
the spoils of terrestrial and marine animals. The frequent intrusion
of volcanic strata among the tertiary formations show that, in Europe,
the earth had been in a very disturbed state, and that these repeated
vicissitudes had been occasioned by elevations and depressions of
the soil, as well as by the action of water.
There are three distinct groups in these strata: the lowest tertiary
or Eocene group, so called by Sir Charles Lyell, because, among
the myriads of fossil shell-fish which it contains, very few are identical with those now living; the Miocene, or middle group, has a
greater number of the exuvioa of existing species of shells; and
the Pliocene, or upper tertiary group, still more. Though frequently
heaved up to great elevations on the flanks of the mountain-chains,
as, for example, on the Alps and Apennines, by far the greater part
of the tertiary strata maintain their original horizontal position in
the very places where they are formed. Immense insulated deposits
of this kind are to be met with all over the world; Europe abounds
with them, London, Paris, and Vienna stand on such basins, and
they cover immense tracts both in North and South America.
The monstrous reptiles had mostly disappeared, and the main.
3,