he sea extended
during some part of the Tertiary epoch northwards to the Arctic Ocean, and
there is nothing to show that this sea may not have been in existence
during the whole Tertiary period. Another channel probably existed over
Egypt[21] into the eastern {81} basin of the Mediterranean and the Black
Sea; while it is probable that there was a communication between the Baltic
and the White Sea, leaving Scandinavia as an extensive island. Turning to
India, we find that an arm of the sea of great width and depth extended
from the Bay of Bengal to the mouths of the Indus; while the enormous
depression indicated by the presence of marine fossils of Eocene age at a
height of 10,500 feet in Western Tibet, renders it not improbable that a
more direct channel across Afghanistan may have opened a communication
between the West Asiatic and Polar seas.

It may be said that the changes here indicated are not warranted by an
actual knowledge of continuous Tertiary deposits over the situations of the
alleged marine channels; but it is no less certain that the seas in which
any particular strata were deposited were _always_ more extensive than the
fragments of those strata now existing, and _often_ immensely more
extensive. The Eocene deposits of Europe, for example, have certainly
undergone enormous denudation both marine and subaerial, and may have once
covered areas where we now find older deposits (as the chalk once covered
the weald), while a portion of them may lie concealed under Miocene,
Pliocene, or recent beds. We find them widely scattered over Europe and
Asia, and often elevated into lofty mountain ranges; and we should
certainly err far more seriously in confining the Eocene seas to the exact
areas where we now find Eocene rocks, than in liberally extending them, so
as to connect the several detached portions of the formation whenever there
is no valid argument against our doing so. Considering then, that some one
or more of the sea-communications here indicated almost certainly existed
during Eocene and Miocene times, let us endeavour to estimate the probable
effect such communications would have upon the climate of the northern
hemisphere.

_The Indian Ocean as a Source of Heat in Tertiary Times._--If we compare
the Indian Ocean with the South Atlantic we shall see that the position and
outline of the former are very favourable for the accumulation of a large
body of warm water moving northwards. Its southern {82} op