118 W. Upham — Epeirogenic Movements 



coast of South America, apparently however continuing for no 

 long time, lends much probability to the supposition that the 

 low Panama isthmus was somewhat deeply submerged for a 

 geologically short period contemporaneous with epeirogenic 

 uplifts of the circumpolar parts of this continent both at the 

 north and south, whereby the effects of high altitude in cover- 

 ing the northern and southern high areas with ice-sheets were 

 augmented by the passage of much of the Gulf Stream into 

 the. Pacific Ocean. 



The end of the Tertiary era and the subsequent Glacial 

 period have been exceptionally characterized by many great 

 oscillations of continental and insular land areas. Where 

 movements of land elevation have taken place in high lati- 

 tudes, either north or south, which received abundant precipi- 

 tation of moisture, ice-sheets were formed ; and the weight of 

 these ice-sheets, as was first pointed out by Jamieson, seems to 

 have been a chief cause, and often probably the only cause, of 

 the subsidence of these lands and the disappearance of their 

 ice. But the original sources of the energy displayed in the 

 earth-movements of uplift preceding glaciation, and why this 

 has been so extensively developed during the Quaternary era, 

 are very difficult questions which it is not the purpose of this 

 paper to consider, since I have attempted elsewhere to answer 

 them, in an appendix of Wright's " Ice Age in North America " 

 It may be properly noted, however, that the explanations 

 mentioned are entirely consistent with Dana's teaching that 

 the great continental and oceanic areas have been mainly 

 permanent from very early geologic times. 



Two formidable objections to this view that the accumula- 

 tion of the Pleistocene ice-sheets was preceded and caused by 

 great epeirogenic elevation deserve careful attention. The 

 first consists in an approximate identity of level with that of 

 to-day having been held by many drift-bearing areas at a time 

 very shortly preceding their glaciation. This is clearly known 

 to have been true of Great Britain and of New England. 

 Near Boston, for example, my observations of fragments of 

 marine shells in the till of drumlins in or adjoining the harbor 

 prove for that tract a preglacial height closely the same as 

 now at so late a time that the molluscan fauna, of which we 

 have a considerable representation, comprised only species 

 now living. In respect to this objection, it must be acknowl- 

 edged that the preglacial high elevation which I think these 

 areas experienced was geologically very short. With the steep 

 gradients of the Hudson, of the streams which formed the 

 now submerged channels on the Californian coast, and of the 

 Congo, these rivers, if allowed a long time for erosion must 

 have formed even longer and broader valleys than the still 



