360 POPULAR SCIENCE MONTHLY. 



Among the facts brought out, a few may be briefly alluded to. 

 The early abandonment of Agassiz's original view of a vast exten- 

 sion of the polar snow caps, and the recognition of separate centers 

 of continental glaciation, now distinctly determined as three in num- 

 ber — a western, a central, and an eastern — the former being the 

 earliest, and the others following in succession; the recognition by 

 the Western geologists of the twofold character of the Glacial epoch, 

 as also determined in western Europe, but less markedly traceable in 

 our Eastern States, though now generally admitted; in close rela- 

 tion to this the determination of the line of the great terminal mo- 

 raine, traced by successive observers from the Atlantic seaboard to 

 Minnesota, and the subsequent recognition of an older, eroded, and 

 fragmentary morainal " fringe," marking the line of the earlier ice 

 sheet, somewhat beyond the later. With regard to the actual dis- 

 tance of the last glacial retreat, as expressed in years, Professor Eair- 

 child is both cautious and frank. He notes the general consensus of 

 recent observers toward a much shorter period than was formerly 

 supposed — from five to ten or perhaps fifteen thousand years. At 

 the same time, there are many elements of uncertainty involved, and 

 the problem is by no means settled. The Niagara gorge, so long 

 looked upon as a possible chronometer, grows more complicated as 

 it is further studied; the rate of erosion has evidently varied much 

 with the volume of water carried by the river; and this, in turn, has 

 varied with the changes of level, and consequently of drainage routes, 

 in the basin of the Great Lakes. There have been times when only 

 the Erie waters flowed through the Niagara outlet, the upper lake 

 drainage passing eastward independently, until a gradual northern 

 rise of the land, which is proved to be still going on, turned the entire 

 drainage into the present St. Clair route from Lake Huron into Lake 

 Erie, and so through Niagara. 



This point leads us to digress for a moment from the address under 

 consideration to allude to a very interesting department of study 

 that is now growing into prominence — to wit, the restoration of 

 pre-glacial geography and hydrography, and the genesis of our ex- 

 isting river and lake systems throughout the northern part of the 

 country. The discussions and results in regard to Niagara and the 

 Great Lakes are somewhat familiar, but the work on the rivers and 

 smaller lakes is not so widely known. Professor Fairchild himself 

 has done much in relation to the " central lakes " of New York State ; 

 and one very interesting paper of this kind on The Development of 

 the Ohio River was read before the section by Prof. William G. 

 Light, of Granville, Ohio, besides many papers by others on similar 

 topics. 



The work done within a few years upon the glaciers of Arctic 



