1881.] •-''^ 



I have never for an instant doubted that the erosive action of this river 

 was supplemented and modified by local glaciers. It is quite beyond the 

 reach of fluvial erosion, — of which in the canons of the Colorado, I have 

 studied the best examples extant, — to form basins like those of our great 

 lakes ; and while it gives me pleasure to find in Prof. Spencer's discovery 

 a confinuation of the prediction made years ago, and to give him credit 

 for the sagacity and industry which niai'ks his investigations, I cannot but 

 feel that before attempting to write a general history of the Lake basins, it 

 would have been well to have gone in person over all the ground under 

 discussion. 



Discvssion. 



Mr. Lesley remarked that in all controversies over the Glacial hypothesis, 

 as it used to be called, the Glacial theory as it has now well establishetl 

 itself to be, a vast number of observed facts are accepted on all hands as 

 part of the actual human knowledge. No one now thinks of disputing the 

 former extension of existing glaciers ; nor the former existence of sheets of 

 ice over large areas of the earth's surface, where nothing like a glacier is 

 now noticeable even at the close of the severest winters ; nor the meaning 

 of the scratches and grooves, clays and gravels, moraines and kames, pot 

 holes, ponds, terraces, sand dams, reversed drainage, and whatever else 

 are the characteristic marks and vestiges of the agency of the ice which 

 once covered such areas. All geologists who have studied existing glaciers 

 in the Alps, for instance, or who have acquainted themselves with their 

 character and action through good descriptions of them, take precisely the 

 same view of the circumstances. 



What geologists are not yet agreed upon is not whether moving ice once 

 covered now fertile districts, but the precise limits of these glaciated dis- 

 tricts ; not that all moving ice moves rocks, but precisely in what manner 

 the rocks move with, on, in or under the ice ; not that glaciers deposit 

 heterogenous materials, but precisely what part water, melted ice, plays in 

 the drama, and how one can best distinguish its work from that done by 

 the ice itself, unmelted, in and of itself; not whether there has been an 

 age of ice, but whether there were not two or more, and whether human 

 beings began to live in an earlier, in a medial, or in a later age ; and above 

 all, not whether the surface of glaciated regions was modified by the long 

 or short, single or repeated passage of ice over them, but precisely to what 

 extent this modification went. 



In a word, the Glacial Theory, perfectly well defined and accepted by 

 all in the clear light of long continued, thorough and consistent investiga- 

 tion, is still surrounded by a penumbra of Glacial Hypotheses, about 

 which very enthusiastic and dogmatic geologists are disposed to debate 

 with a great deal of personal warmth, as if their personal reputation for 

 genuine scientific ability was involved. The fact is, some of the questions 

 thus presented are so difficult of any precise definition that we must wait 

 Ions for their answers. 



