188 



ISLAND LIFE. 



[part I. 



Mississippi, extending over much of the Kocky 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 Eiver, 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. 



JEffed of these Changes on the Clmiate 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 



