68 POLAR PROBLEMS 



in the study of these coal-bearing beds, which are quite widely scat- 

 tered over the Arctic islands, it may, however, be unsafe to assume 

 that all of them are Mesozoic; and it is desirable, in order to determine 

 their age, that collections of fossils should be made from as many of 

 these localities as possible. 



Tertl\ry 



While formations containing records of events in our Arctic 

 sector during the Tertiary seem to be lacking, there is one bit of in- 

 direct evidence bearing on the subject. Toward the end of the Plio- 

 cene and the beginning of the Pleistocene the United States and 

 southern Canada were inhabited by many species of splendid mam- 

 mals, some of which probably developed in North or South America; 

 but others, including the mammoth and mastodon, a rhinoceros, 

 and some large carnivores, came from Asia or perhaps Africa. ^^ 



By what route did they reach North America? 

 . One naturally suggests a land connection between Kamchatka 

 and Alaska where the shallow Bering Strait now separates the two 

 continents, and, if there was one, there must also have been a Pliocene 

 climate permitting the growth of sufficient vegetation to support the 

 herbivores. It is to be hoped that remains of these Pliocene migrants 

 will be found somewhere along their line of march to give an idea of the 

 route they followed. 



Pleistocene 



With the on-coming of the Glacial Period much of the Arctic terri- 

 tory became covered with glaciers, but much remained free from ice, 

 as in the interior of Alaska and the Yukon Territory of Canada, which 

 were not glaciated. The botanist Fernald believes that large parts 

 of the far north were ice-free and that the ancestors of the present 

 Arctic plants survived the intense cold and were ready to colonize 

 the region to the south when the great Keewatin and Labrador ice 

 sheets, which covered most of Canada and part of the Northeastern 

 States, departed. ^2 



There is much need for a careful survey of the Arctic glacial de- 

 posits to show what parts remained unglaciated when more southern 

 regions of North America were buried under thousands of feet of ice ; 

 and one would be glad also to discover the causes which account for 

 such a reversal of present conditions. 



" O. p. Hay: The Pleistocene of North America and Its Vertebrated Animals from the States 

 East of the Mississippi River and from the Canadian Provinces East of Longitude 95°, Carnegie Instn. 

 Publ. No. 322, Washington, 1923. 



12 M. L. Fernald: Persistence of Plants in Unglaciated Areas of Boreal America, Memoirs Gray 

 Herbarium of Harvard Univ., No. 2, Cambridge, 1925 (also as Memoirs Amer. Acad, of Arts and Set., 

 Vol. IS, 1925, No. 3. pp. 237-342). 



