188 
ISLAND LIFE. 
[rART I. 
Mississippi, extending over much of the Rocky Mountains, con- 
sists 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 
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 coun- 
terbalancing, to some extent, the great depression of so much 
of the northern continents. Our map in Chapter XY. 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 im- 
pelled 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 Atlantic. 
It is not of course supposed that all the modifications here 
indicated co-existed at the same time. We have good reason to 
believe, from the known distribution of animals in the Tertiary 
