DISCUSSION. 



Dr. George M. Dawson : It is unfortunate that the time at Mr. Upham's 

 disposal has been so short that he has been able to present to the Society 

 only the outlines of his paper, but even these are sufficient to show that he 

 has been as diligent and untiring in this as in his former publications in the 

 collection and collation of facts. Since many of the observations quoted by 

 him are those made by myself, it is particularly interesting to me to note 

 how these may be rearranged under hypotheses which differ in part from 

 those which I have suggested ; and I may be permitted, even at this late 

 period of the meeting, to refer to one or two of the points upon which he has 

 touched. 



I infer from Mr. Upham's remarks that he now believes the southern and 

 western limit of the area of the Great Plains affected by the second maxi- 

 mum of glaciation to have been coextensive, or nearly so, with that of the 

 first maximum. This, I am pleased to find, nearly agrees with the view 

 which I have been led to favor and have lately advanced, viz., that the area 

 affected by the second glaciation was even greater thau that of the first in 

 the region in question. As Mr. Upham further adopts my definition of the 

 Laurentide and Cordillerau centers of glaciation, the most important out- 

 standing difference in our views, in so far as the western half of the continent 

 is concerned, is that connected with the mode of glaciation of the Canadian 

 Great Plains. If the bowlder-clay is to be explained as the bottom-moraine 

 of a great ice-sheet, he must be right in carrying this ice-sheet as far as the 

 bowlder-clay extends, or, in other words, practically to the western and 

 southern limits of the drift-covered region. My own view, based on ex- 

 tended opportunities of observation in the actual region, is that the bowlder- 

 clay, like the other drift deposits of at least the greater part of the plains, is 

 of glacio-natant origin. There is also, I think, much to show that even if the 

 Laurentide glacier did extend across the plains to the western limit of the 

 drift, it did not there become confluent with the Cordilleran glacier, but 

 reached that limit at a time when the latter was comparatively reduced, 

 and overlapped the deposits of an earlier and wider Cordilleran glaciation. 



Those valleys of southern Alberta which are adopted by Mr. Upham as 

 former channels of glacial lakes have been examined and in some cases 

 surveyed by me, and I have elsewhere discussed their origin, mentioning 

 the explanation advocated by the author as a possible one. It is doubtful 

 whether our present stock of information is sufficient to enable perfectly 

 definite or final conclusions to be reached respecting them, as they are sus- 

 ceptible of various explanations. Referring to Mr. Upham's allusion to the 



XLI— Bull. Geol. Soc. Am., Vol. 2, 1890. i'^''^) 



