508 THE ICE AGE IN NORTH AMERICA. 



area dotted with coral islands in the Pacific. The Quaternary 

 uplifts of the Andes and Kocky Mountains and of the West 

 Indies make it nearly certain that the Isthmus of Panama has 

 been similarly elevated during the recent epoch. On the line 

 of the Panama Railway the highest land rises only two hundred 

 and ninety-nine feet above the sea, and the highest on the 

 Nicaragua Canal is about one hundred and thirty-three feet, 

 while the isthmus nowhere attains the height of one thousand 

 feet.* It may be true, therefore, that the submergence of this 

 isthmus was one of the causes of the Glacial period, the con- 

 tinuation of the equatorial oceanic current westward into the 

 Pacific having greatly diminished or wholly diverted the Gulf 

 Stream, which carries warmth from the tropics to the northern 

 Atlantic and northwestern Europe. 



In view of the extensive recent oscillations of land and 

 sea both in glaciated and unglaciated regions, it seems a rea- 

 sonable conclusion that, while some of these movements have 

 resulted directly from the accumulation and dissolution of ice- 

 sheets, more generally, when the whole area of the earth is 

 considered, they have been independent of glaciation. May 

 not such movements of the earth's crust, then, have elevated 

 large portions of continents, as the northern half of North 

 America and the northwestern part of Europe, to heights like 

 those of the present snow-line on mountain -ranges, until these 

 plateaus became deeply channeled by fiords and afterward cov- 

 ered by ice-sheets ? For the recentness of the latest glaciation, 

 believed to have ended in the northern United States not more 

 than ten thousand to six thousand years ago,f forbids our re- 



* Charles Ricketts, " The Cause of the Glacial Period, with reference to the 

 British Isles," "Geological Magazine," II, vol. ii, 1875, pp. 573-580. A. R. 

 Wallace, "The Geographical Distribution of Animals," vol. i, p. 40. 



f N. H. WiuqflreU, "Geology of Minnesota," "Fifth Annual Report," for 

 1876, and "Final Report," vol. ii, pp. 313-341; "Quarterly Journal of the 

 Geological Society," vol. xxxiv, 1878, pp. 886-901. E. Andrews, "Transactions 

 of the Chicago Academy of Sciences," vol. ii. James C. Southall, " The Epoch 

 of ihe Mammoth and the Apparition of Man upon the Earth," 1878, chaps, xxii 

 and xxiii. G. F. Wright, " American Journal of Science," III, vol. xxi, pp. 120- 

 123, February, 1881 ; "The Ice Age in North America," chap. xx. G. K. Gil- 

 bert, "Proceedings of the American Association for the Advancement of Sci- 

 ence," vol. xxxv, IS 86. 



