980 HISTORICAL GEOLOGY. 



B. The amelioration of the cold, and the retreat of the ice-sheet. — Setting 

 aside CroU's theory, for the reasons ah-eady stated, the disappearance of the 

 cold and wet climate that was the occasion of the ice period is naturally 

 attributed to a reversal of the conditions that produced it — that is, to a 

 subsidence of the land over the higher latitudes, and a deepening again of 

 the sea over the submarine plateau between Scandinavia and Greenland, 

 thereby restoring the Gulf Stream to its circuit in the Arctic regions. 



But there appears to be good evidence that the melting had made 

 great progress before there had been much subsidence of glacial regions. 

 The facts stated on page 969 bear strongly in this direction ; for they show 

 that, however great the loss from melting and subsidence may have been, 

 the southward slope of the ice-surface continued, and the Mississippi drained 

 a large part of British America, even when the ice was making its last 

 moraine on the northern borders of Minnesota. 



These facts from the Continental Interior are sustained by others from 

 the eastern border. It has been shown by K. L. Britton (1872), that the 

 Pine Barren flora of the ISTew Jersey coast region formerly occupied Staten 

 Island and Long Island ; and others have added to the range of its distribu- 

 tion the southern shores of Rhode Island and Massachusetts, with the 

 adjoining islands. A. Hollick has reviewed the facts (1893) and referred the 

 northward distribution of this southern flora to the period of Glacial 

 emergence, which made New Jersey, Staten Island, Long Island, with the 

 islands east of it and southern New England, continuous dry land. 



The migration northward of the Pine Barren flora must have been during 

 the later part of the time of high latitude elevation. The flora was first 

 driven south by the ice, and long kept there. But finally, after the ice had 

 retreated from New Jersey it was again restored ; and when the retreat 

 had made so great progress that the climate of southern New England was 

 right for the flora, it completed its northward migration. It is tlius proved 

 that southern New England had a climate warmer than now, long before 

 the alleged subsidence had completed its work in southern New England. 



These facts do not prove, however, that no subsidence had taken place 

 in higher latitudes. That of the submarine plateau between Europe and 

 Greenland may have been so far completed as to have caused a great modi- 

 fication in climate. Each stage in the retreat was a contraction of the area 

 of perpetual frost, and a widening of the range of tropical winds, ensuring 

 further encroachment. In view of all the facts, it is probable that before 

 the subsidence had made large progress, the ice-sheet had retreated to Cana- 

 dian territory, excepting the portions left about the higher mountains of 

 eastern and western America. 



It seems also to be true that the conclusion of Becker, deduced from his 

 discussion of the question of Glacial climate, on page 979, suggests the 

 right explanation of the initiation of the warmer climate and retreat. 



