208 ANNUAL EEPORT SMITHSONIAN INSTITUTION, 19 32 



portance as they now have, and effected the separation of the lighter 

 and heavier planetesimal material, the heavier being brought down 

 by descending currents over the oceans and the lighter being dis- 

 tributed over the present land surfaces. In this way he assigns a 

 cause for the generally accepted idea of the higher specific gravity of 

 the material of the ocean floors and the lighter material of the 

 continental areas. 



As a consequence, the suboceanic segments were habitually urged 

 to sink, while the continents were forced to rise to restore equilibrium. 

 This constitutes an enduring, though not an indefinitely enduring, 

 basis for isostatic action, because the actuating differentiation is 

 deeply inbred iji the formation of the earth. Lyell had already 

 pointed out that, with trifling exceptions, land is always antipodal 

 to water upon the earth, and, in the language of Chamberlin, the 

 heavy, relatively rigid, suboceanic cones stand opposite to the lighter, 

 weaker, yielding continents. Onl}^ one twenty-seventh of the land 

 of the globe has land antipodal to it. 



The theory of the tetrahedral plan of the land and water upon the 

 earth, as originally set forth by Lothian Green, also supports the idea 

 of continents and oceans having existed for long periods in much 

 the same positions as they occupy to-day. Other writers have sup- 

 ported the theory of the permanence of continents and ocean basins, 

 and Russel Wallace in his Island Life brings forward biological evi- 

 dence in support of this theory. He tabulated a number of points 

 which went far, as he thought, to prove it, but recent researches do 

 not support his original contentions. 



In order to explain the known distribution of animals and plants 

 in past and present times, land bridges of greater or lesser magni- 

 tude and permanency were erected by investigators, by means of 

 which the fauna and flora could have migrated from one land mass 

 to another. 



In the case of the present North Atlantic, which is my immediate 

 concern, an excellent paper was written by Doctor ScharfT, On the 

 Evidences of a Former Land Bridge between Northern Europe and 

 North America. In this he concludes, from the known distribu- 

 tion of plants in North America, Greenland, Faroes, Iceland, and 

 the British Isles, that plant migration has taken place in both direc- 

 tions along a land bridge connecting North America with Europe 

 by way of the islands named. According to Scharff this bridge 

 must have existed in later Pliocene times. 



Professor Gregory, in discussing this trans-Atlantic connection, 

 refers to the fact that remains of three mammals, the mammoth, 

 musk ox, and reindeer, occur in northeastern America and north- 

 western Europe, but their passage across this land bridge is not 



