y formations in England. Some even of these
newer groups have been raised to the height of three or four thousand
feet, and in proportion to their antiquity, they generally rise to
greater heights, the older of them forming interior zones nearest to the
central ridges of the Alps. We have already ascertained that the Alps
gained accessions to their height and width at several successive
periods, and that the last series of improvements occurred when the
seas were inhabited by many existing species of animals.

We may imagine some future series of convulsions once more to heave up
this stupendous chain, together with the adjoining bed of the sea, so
that the mountains of Europe may rival the Andes in elevation; in which
case the deltas of the Po, Adige, and Brenta, now encroaching upon the
Adriatic, might be uplifted so as to form another exterior belt of
considerable height around the southeastern flank of the Alps.

The Pyrenees, also, have acquired their present altitude, which in Mont
Perdu exceeds eleven thousand feet, since the deposition of the
nummulitic or Eocene division of the tertiary series. Some of the
tertiary strata at the base of the chain are raised to the height of
only a few hundred feet above the sea, and retain a horizontal position,
without partaking in general in the disturbance to which the older
series has been subjected; so that the great barrier between France and
Spain was almost entirely upheaved in the interval between the
deposition of certain groups of tertiary strata.

The remarkable break between the most modern of the known secondary
rocks and the oldest tertiary, may be apparent only, and ascribable to
the present deficiency of our information. Already the marles and green
sand of Heers near Tongres, in Belgium, observed by M. Dumont, and the
"pisolitic limestone" of the neighborhood of Paris, both intermediate in
age between the Maestricht chalk and the lower Eocene strata, begin to
afford us signs of a passage from one state of things to another.
Nevertheless, it is far from impossible that the interval between the
chalk and tertiary formations constituted an era in the earth's history,
when the transition from one class of organic beings to another was,
comparatively speaking, rapid. For if the doctrines above explained in
regard to vicissitudes of temperature are sound, it will follow that
changes of equal magnitude in the geographical features of the globe may
at different peri