510 THE ICE AGE IN NORTH AMERICA. 



with corresponding elevation of other plateaus, which in turn 

 would become ice-covered, so that the epochs of glaciation of 

 the northern and southern hemispheres may have alternated 

 with each other;* and this may have been several times re- 

 peated, because of crustal oscillations due to ice-weight and its 

 removal, the effects of elevation and depression of the land 

 being re-enforced by climatic influences arising from the revolu- 

 tion each twenty-one thousand years in the place of the seasons. 

 When the building up of a great range of mountains ensued, 

 which may have been initiated and accelerated by the repeated 

 depressions under ice-weight and consequent transfers of the 

 earth's deformation from one region to another, the accumu- 

 lated strain in the earth's crust, with development of immense 

 lateral pressure, would be diminished below the limit of its 

 competency to cause glaciation. 



Such Quaternary mountain-building is known to have oc- 

 curred on a most massive scale in Asia, where the Himalayas, 

 stretching fifteen hundred miles from east to west, and tower- 

 ing twenty thousand to twenty-nine thousand feet above the 

 sea, are known to have been formed, at least in great part, and 

 perhaps almost wholly, during this latest geologic period,! 

 contemporaneously with the glaciation of North America, 

 Europe, and portions of the southern hemisphere. Within the 

 same time the great table-land of Thibet,]; and much of central 

 and northwestern Asia, have been uplifted ; the tract extend- 

 ing from the Black and Caspian Seas northeast to the Arctic 

 Ocean has risen to form a land-surface ; and the deep basin of 

 Lake Baikal has been probably formed in connection with 

 these crustal movements. Accompanying the formation of the 

 Himalayas, there has been doubtless much disturbance by 

 faults, local uplifts, and here and there plication of strata 



* Compare the opinions of Hutton, cited in A. Geikie's " Text-Book of Geolo- 

 gy," second edition, p. 912, that the former greater extension of glaciers in New 

 Zealand was caused by an increase in the elevation of the land, and that it be- 

 longed to a much earlier time than the Ice age in the northern hemisphere, 

 probably to the Pliocene period. 



f " Manual of the Geology of India," by H. B. Mcdlicott and W. T. Blanford, 

 Calcutta, 1879, Part I, pp. lvi, 372; Part II, pp. 569-571, 667-669, 672-681. 



% Ibid., Part II, pp. 585, 586, 669-672. 



