ANCIENT GLACIERS. 97 



a belt of a hundred miles or more, east of the Rocky Moun- 

 tains, which was never covered by true glacial ice. Mr. 

 Upham * is equally confident that the two ice-movements 

 became confluent, and united upon the western plateau of 

 Manitoba. The opportunity for such a difference of opin- 

 ion arises in the difficulty sometimes encountered of dis- 

 tinguishing between a direct glacial deposit and a deposit 

 taking place in water containing boulder-laden icebergs. 

 Where Mr. Upham supposes the ice-fields of the east and 

 of the west to have been confluent in western Manitoba, 

 Dr. Dawson supposes there was an extensive subsidence of 

 the land sufficient to admit the waters of the ocean. Leav- 

 ing this question for the present undetermined, we will 

 now rapidly summarise the glacial phenomena west of the 

 third meridian from Washington (which corresponds near- 

 ly with the western boundary of Pennsylvania), and east of 

 the Rocky Mountains. 



That the glacial movement extended to the southern 

 boundary just delineated is established by the presence 

 down to that line of all the signs of glacial action, and 

 their absence beyond. Glacial groovings are found irpon 

 the freshly uncovered rock surfaces at frequent intervals 

 in close proximity to the line all along its course, while 

 granitic boulders from the far north are scattered, with 

 more or less regularity, over the whole intervening space 

 between this line and the Canadian highlands. I have 

 already referred to a boulder of jasper conglomerate found 

 in Boone County, Kentucky, which must have come from 

 unique outcroppings of this rock north of Lake Huron. 

 Granitic boulders from the Lake Superior region are also 

 found in great abundance at the extreme margin men- 

 tioned in southern Illinois. West of the Missouri River 

 it is somewhat more difficult to delineate the boundary 



* American Geologist, vol. vi, September, 1890 ; Bulletin of the 

 Geological Society of America, vol. ii, pp. 243-276. 



