354 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



Chamberlin^^ and more recently advocated by Willis,^^ namely, that the 

 present deep-seated circulation of the ocean toward the equator is abnor- 

 mal, and that the more normal condition is a northward movement of 

 warm, highly saline waters in the depths instead of at the surface. The 

 special purpose of the hypothesis is to explain the extraordinarily mild 

 climates that prevailed in many geological ages in the Arctic regions. I 

 question, however, if it is competent to do so. I do not wish to say that 

 such a reversal of oceanic circulation did not occur, for indeed the argu- 

 ments on which the idea is based seem reasonable. I admit further that 

 it might be accepted as a satisfactory explanation of the presence of sub-« 

 tropical plants in Greenland in the Tertiary ; also that reversal may have 

 occurred toward the close of a highly emergent stage, or in the early part 

 of the succeeding submergent phase, of neither of which the marine 

 record is accessible ; but that it explains the occurrence of the same marine 

 faunas in the polar regions that are found in temperate and tropical zones 

 in most of the geological periods from the Canadian on to the Miocene I 

 can not admit. 



In the first place these faunas, with very few exceptions, are all lit- 

 toral or near-shore faunas, and their migrations, whether in the oceanic 

 or the continental basins, are almost confined to the shore and bottom of 

 the shallow seas in which they thrived. With organisms so sensitive as 

 these to changes in temperature and depth, extensive migration under any 

 but equable climatic and bathymetric conditions would have been highly 

 improbable if not impossible. As corals and other animals that are now 

 restricted to warm waters did so migrate, we must assume either that at 

 such times mild climates prevailed or that in ages preceding the present 

 these organisms were not affected by changes in temperature^a conclu-, 

 sion altogether repugnant to the zoologist. Assuming that they were then 

 as now sensitive to temperature changes, their fossil occurrence in all lati- 

 tudes must be accepted as proving at least occasional times of universally 

 mild climates — that is, such conditions prevailed in at least the relatively 

 brief geologic ages of which we have a marine sedimentary record in the 

 boreal continental basins. What occurred in the long intervening ages of 

 which no satisfactory life record is preserved in the far northern conti- 

 nental basins we may only surmise, or possibly infer from evidence found 

 elsewhere. 



Warm climates being thus established for the shallow northern conti- 

 nental basins during their occupancy by marine waters, similar condi- 



21 T. C. Chamberlin : Journal of Geology, vol. 14, 1906, p. 363. 



22 Bailey Willis : Science, N. S., vol. 31, No. 790, 1910, p. 245. 



