at of the warmer parts of New Zealand at the
present day, and might have well supported the luxuriant vegetation of the
Miocene period, even without any help from similar changes in the western
hemisphere.[23]

_Condition of North America during the Tertiary Period._--But changes of a
somewhat similar character have also taken place in America and the
Pacific. An enormous area west of the Mississippi, extending over much of
the Rocky Mountains, consists of marine Cretaceous beds 10,000 feet thick,
indicating great and long-continued subsidence, and an insular condition of
Western America with a sea probably extending northwards to the Arctic
Ocean. As marine Tertiary deposits are found conformably overlying these
Cretaceous strata, Professor Dana is of opinion that the great elevation of
this part of America did not begin till early Tertiary times. Other
Tertiary beds in California, Alaska, Kamschatka, the Mackenzie River, the
Parry Islands, and Greenland, indicate partial submergence {84} of all
these lands with the possible influx of warm water from the Pacific; and
the considerable elevation of some of the Miocene beds in Greenland and
Spitzbergen renders it probable that these countries were then much less
elevated, in which case only their higher summits would be covered with
perpetual snow, and no glaciers would descend to the sea.

In the Pacific there was probably an elevation of land counterbalancing, to
some extent, the great depression of so much of the northern continents.
Our map in Chapter XV. shows the islands that would be produced by an
elevation of the great shoals under a thousand fathoms deep, and it is seen
that these all trend in a south-east and north-west direction, and would
thus facilitate the production of definite currents impelled by the
south-east trades towards the north-west Pacific, where they would gain
access to the polar seas through Behring's Straits, which were, perhaps,
sometimes both wider and deeper than at present.

_Effect of these Changes on the Climate of the Arctic Regions._--These
various changes of sea and land, all tending towards a transference of heat
from the equator to the north temperate zone, were not improbably still
further augmented by the existence of a great inland South American sea
occupying what are now the extensive valleys of the Amazon and Orinoco, and
forming an additional reservoir of super-heated water to add to the supply
poured into the North Atlant