E. Suess — Paleogeoqraphy of North America. 107 



that life has proceeded from the ocean to fresh water and land. 

 In other cases of animal life no considerable change is visible ; 

 examples are the Medusa of Tanganyika, Victoria and the lower 

 Niger. In a like way the marine pelecypods Mysidae in the 

 upper Volga, now separated by twelve degrees of latitude from 

 the Caspian, are probably older than the separation of the 

 Caspian from the Mediterranean. Another example is the 

 siren ian Phoca oaikalensis. In Pyrgulifera a Cretaceous 

 freshwater gastropod has been preserved and I regard this as 

 a relict from the head of a Cretaceous river, because the marine 

 Cretaceous trangression and indeed every later marine inun- 

 dation seems in the center of Africa not to have extended far 

 beyond the southern limit of Sahara. In this way only can 

 we understand that Nile, Niger, Gambia, Senegal, Kongo, 

 Zambesi and lake Tchad (Boulangers Megapontic sub-region) 

 possess a very uniform fluviatile fauna. Further, the oldest 

 types of fluviatile fishes exist in the oldest continents, Amia 

 and Lepidosteus in Laurentia, Lepidosiren in Brazil, Polypterus 

 and Protopterus in Africa, Ceratodus in Australia. 



3. Difficulties in finding a satisfactory hypothesis as to the 

 causes of transgressions. 



When I wrote of eustatic movements in 1883 [" Face of the 

 Earth," vol. I] I confessed that I did not understand the trans- 

 gressions. I thought that variations in rotation might some- 

 how have influence. I also believed and still think that the 

 accumulation of sediment was a vera causa, but hardly suf- 

 ficient. Now, after twenty-seven years 1 cannot offer you 

 more than a loose heap of doubts regarding the explanation. 

 I have learnt more and know less about it. 



Regarding rotation, we must ask : Where was the pole ? and 

 has it always been fixed ? Many years ago Oswald Heer said 

 that its position was variable, as plants of a warmer climate are 

 known from the Devonian or lower Carboniferous through the 

 whole succeeding stratigraphic series and that signs of refrig- 

 eration begin to appear for the first time in the middle Tertiary. 

 Now, the Jurassic ferns from the Antarctic teach a similar 

 lesson and all these plants demand not only a warmer climate 

 but more light than the polar nights afford. Further, the 

 repeated glacial periods in different latitudes seem to hint of 

 great displacements of the poles ; several theories have been 

 proposed but none is adequate. 



It is quite true, as you remark, that the sinking of part of 

 an ocean's base or part of a continent must increase the rapidity 

 of rotation. The question remains, however, whether the 

 phenomenon is not accompanied by a displacement of the 

 planetary center of gravity. 



