ORIGIN AND DIFFERENTIATION OOO 



Continental elevation Jiypothesis. — It is probable that, all things con- 

 sidered, there is a wider acceptance among geologists and others of the 

 postulate that changes in temperature due to continental elevation are 

 sufficient to account for the inauguration and control of glaciation than 

 any other of the many hypotheses that are up for discussion. As Gregory 

 says, it is attractive from its simplicity. It is of course well established 

 that there was a measure of continental elevation and subsequent depres- 

 sion during, for instance, the Pleistocene invasion. This is proved by 

 the prolongation of fjords and land valleys on the adjoining ocean floor, 

 by the finding of littoral shells in submerged areas, and of marine organ- 

 isms on lands now elevated, though in some cases the last-mentioned oc- 

 currence may have resulted from an ice-mass that filled and plowed out 

 a shallow sea basin, thus pushing them onto and over the adjacent land. 

 But whether the hypothesis of continental elevation is fully competent 

 to account for the origin and control of glaciation at all times and in all 

 places, and under the usual postulate of a solar control similar to that 

 now obtaining, is much to be doubted. 



In discussing the several advances and retreats of the Pleistocene ice- 

 sheets, Ulrich^^ says : 



"Assuming that elevation is competent to bring al^out glacial conditions in 

 areas of abundant precipitation, it seems to me tliat the subsequent melting 

 and retreat of the ice-cap may be due cliiefly to the subsidence of tlie areas, 

 and that the subsidence resulted from overloading. In other words, that the 

 isostatic equilibrium had been disturbed by loading, and that subsidence set 

 in when the ice attained a certain limit of thickness. . . . Reaching the 

 level of melting, the ice-cap was gradually removed, only to be rebuilt when 

 the direction of movement was first stopped and then reversed." 



Coleman,^* in considering this problem of Pleistocene elevation, reaches 

 a quite different conclusion. He says : 



"Local elevations of thousands of feet can hardly be conceived as taking 

 place at the same time over most of North America, the whole length of the 

 Andes and Patagonia, all northern Europe, the Alps, the mountains of Tur- 

 kestan, the Himalayas and Altai Mountains, the Atlas region, Rewenzori, 

 Kenai, and Kilimanjaro, the New Zealand Alps, and Kosciusko in Australia, 

 not to mention other localities glaciated in Pleistocene times. The theory 

 breaks down of its own weight." 



Coleman also adds that in some cases elevation is actually hostile to the 

 formation of ice-sheets, as witness the observations of Scott, that on the 



37 E. o. Ulrich : Revision of the Paleozoic systems. Bull. Geol. Soc. Am., vol. 22. 

 1911. p. 353. 



38 A. P. Coleman: Glacial periods and thoii- bearing on geological theories. Bull. Geol 

 Soc. Am., vol. 19, 1908, p. 363. 



