PLIOCENE AND PLEISTOCENE FOSSILS FROM ALASKA. 



25 



of Wales, the Diomede Islands of the Bering 

 Strait, and a considerable portion of the 

 Asiatic coast on the west side of the strait, 

 though probably not all of one age, are almost 

 certainly all older than the period during which 

 these beaches were in process of formation. 



The presence of widely distributed Eocene 

 or Oligocene leaf beds and lignitic coals, covered 

 in some localities by beds of sandstone con- 

 taining marine Miocene fossils, shows that a 

 milder climate in earlier Tertiary time, allowing 

 a profuse gi-owth of oaks, planes, figs, and other 

 trees, was succeeded during the Miocene by a 

 depression of much of the land below the sea 

 level with a much colder climate and consider- 

 able volcanic activity. This was followed by 

 a moderate amount of elevation, which has 

 been practically continuous to the present 

 time. During the Pliocene the chmate seems 

 to have moderated, judging by the character 

 of the marine fauna, to be later subjected to 

 the Arctic temperatures which came in with 

 the glacial epoch and still persist. 



ROUTES OF MIGRATION OF FAUNA. 



The bearing of these conditions upon the 

 theories relating to the immigration of Asiatic 

 land animals into America is important. A 

 superficial glance at an ordinary map is likely 

 to lead the observer who goes no deeper into 

 the subject to the conclusion that land bridges, 

 including the Bering Strait region and the 

 Aleutian Island chain, may reasonably be 

 assumed as the routes by which Asiatic immi- 

 grations took place. 



I have elsewhere^ pointed out the serious 

 objections to these assumptions that arise 

 from a more thorough knowledge of the 

 geology. So far as the Aleutian route is con- 

 cerned it must be positively rejected as imprac- 

 ticable. The Bering Strait region offers more 

 plausibility, yet the evidence so far gathered 

 from geologic exploration indicates not only 

 that no closer land connection than at present 

 has existed between the two continents at 

 Bering Strait since Miocene time but, on the 

 contrary, that the present separation is less 

 than at any period during that time. The 

 conclusion from our present knowledge is 

 inevitable either that the postulated land 

 bridges must have existed in some other 



1 Am. Anthropologist, vol. 14, pp. 12-18, 1912. 



locality or that the assumed migration must 

 have taken place over the ice of the strait 

 when frozen, possibly during the glacial epoch. 



INTERCOMMUNICATION OF ATLANTIC AND 

 PACIFIC FAUNAS IN PLIOCENE TIME. 



Still another important conclusion is indi- 

 cated by a study of the characteristics of the 

 Pliocene marine invertebrate fauna. I have 

 already stated that the fauna indicates a more 

 temperate sea than at present washes those 

 shores. Taken in connection with other data 

 derived from a study of the North Atlantic 

 Pliocene deposits in England, Iceland, and 

 on the New England coast, the present investi- 

 gation shows that a more free connection prob- 

 ably existed in Pliocene time between the 

 North Atlantic and the Bering Sea regions. 

 This is indicated by the presence in North 

 Atlantic Pliocene beds, as fossils, of species 

 still living in Bering Sea, and conversely by 

 the presence in the North Atlantic recent 

 fauna of species belonging to groups now extinct 

 on the Pacific side in the Pliocene at Nome. 

 F. W. Harmer,^ who is at present revising the 

 fauna of the Crag of England and Iceland, 

 has already found genera and species now 

 extinct in the North Atlantic region but living 

 in Bering Sea. It is probable that on the 

 completion of his monograph a still larger 

 number will be noted. 



In an investigation of the fossils of the marl 

 at Sankaty Head, Nantucket, the lower horizon 

 of which is probably upper Pliocene, J. Howard 

 Wilson,^ in 1904, found several Bering Sea 

 species now extinct in the Atlantic region. 

 The presence of Corbicula in a small Pliocene 

 lens discovered by Woodworth above the 

 Miocene beds of Gay Head, Marthas Vineyard, 

 is proof that on the Atlantic, as on the Pacific 

 coast, the Pliocene was an epoch in which the 

 temperature of the sea in that region was higher 

 than in the preceding and subsequent epochs. 

 Tlie presence of Rangia in the fossil fauna of 

 Cornfield Harbor, Chesapeake Bay, is confirm- 

 atory evidence to the same effect. 



There is also evidence of a southward exten- 

 sion of the cooler- water fauna of the Calif ornian 



2 Pliocene MoUusca of Great Britain, Paleontograpliical Society, 1913, 

 pi. 1, 1914; pt. 2, 1916. 



8 The Pleistocene formations of Sankaty Head, Nantucket: Jour, Geol- 

 ogy, vol. 13. pp. 713-734, 1905. 



