1902] AND ANCIENT GEOGRAPHY. 317 



masses of the Old and New Worlds is absolutely necessary, and the 

 similarity of the land faunas of both parts, which is not explained 

 by the present conditions, is so strong that these regions (northern 

 Eurasia and North America) have been united by certain authors 

 into one zoogeographical region, the Holarctic. As to the location 

 of this connection, two ways are possible : either from Siberia to 

 Alaska, or from Labrador over Greenland to Scandinavia. The 

 latter connection, which has been discussed, from à geological 

 standpoint, chiefly by Suess and Neumayr (for older times, Meso- 

 zoic and Tertiary), and, from a zoogeographical view by Scharff 

 (for the Pleistocene), may be disregarded for our present purpose ; 

 there is no indication for its existence among the crayfishes. But 

 the latter support strongly, as has been said, the other connection 

 over Bering Strait. 



Viewed from the tectonic side, this connection is quite possible. 

 The old rocks of northeast Asia are continued into northeastern 

 Siberia (east of the rivers Lena and Aldan) to the river Kolyma, x 

 and farther, toward the Arctic Ocean and Bering Sea, and similar 

 rocks are found in Alaska ; and further, the chain of the Aleutian 

 Islands is, according to Suess, another proof for the tectonic unity 

 of the lands east and west of Bering Sea. 



As regards the time of existence of this land bridge, we have to 

 assume it during almost the whole of the Tertiary period. Osborn 

 (1900) takes its existence for granted and demonstrates (p. 568) 

 that during Eocene, Miocene and upward to the Pliocene, a regu- 

 lar exchange of the faunas of Eurasia and North America took 

 place. In the older Pleistocene (p. 571) this connection still 

 existed, but was interrupted in the middle Pleistocene (p. 572). 



If we put the question, whether and how far this land bridge goes 

 back in Pretertiary times, we have to consult first Neumayr' s opin- 

 ion as to the distribution of the Jurassic oceans and continents 

 (1890, map, p. 336). It is true, in Siberia, deposits of Lower 

 Jurassic age are not known, and possibly Siberia was land during 

 this time 2 . There are found here, however, deposits belonging to 



1 See Tscherski, Sap. Akad. St. Petersb., Vol. 73, Append. 5, 1893 (Russian) ; 

 Review in N. Jahrb. Mineral., etc., 1896, Vol. 2, p. 318. 



2 Land and freshwater deposits of Jurassic age are largely distributed in Sibe- 

 ria as coal-bearing strata. Compare the geological investigations connected 

 with the great Siberian railroad, by Obrutchew, Gerassimow, Gedroiz, Jawor- 

 owksy. Reviews in N. Jahrb. Mineral., 1899, Vol., 2, p. 111-116. 



