ANCIENT GLACIERS. 109 



ging wells, the depth varying from twenty to fifty feet. 

 This county is near the highest divide in the State of Min- 

 nesota, and from it " flow the sources of the streams to the 

 north, south, and east." The wood encountered in this 

 stratum indicates the {prevalence f coniferous trees, and 

 the peat mosses indicate a cool and moist climate. 



Nor are intercalated vegetable deposits absent from the 

 vast region farther north over the area that drains into 

 Hudson Bay. At Barnesville, in Clay County, Minnesota, 

 which lies in the valley of the Bed Biver of the North, and 

 also in Wilkin County in the same valley, tamarack wood 

 and sandy black mud containing many snail-shells have 

 been found from eight to twelve feet below a surface of 

 till ; and Dr. Bobert Bell rejoorts the occurrence of limited 

 deposits of lignite between layers of till, far to the north- 

 west, in Canada, and even upon the southern part of Hud- 

 son Bay ; while Mr. J. B. Tyrrell reports * many indica- 

 tions of successive periods of glaciation near the northern 

 end of the Duck Mountain. The most characteristic in- 

 dications which he had witnessed consisted of stratified 

 beds of silt, containing fresh- water shells, with fragments 

 of plants and fish similar to those living in the lakes of 

 the region at the present time. 



Beviewing these facts with reference to their bearing 

 upon the point under consideration, we grant, at the out- 

 set, that they do indicate a successive retreat and re- 

 advance of the ice over extensive areas. This is specially 

 clear with respect to the vegetal deposits interstratified 

 with beds of glacial origin. But the question at issue 

 concerning the interpretation of these phenomena is, Do 

 they necessarily indicate absolutely distinct glacial epochs 

 separated by a period in which the ice had wholly disap- 

 peared from the glaciated area to the north ? That they 



* Bulletin of the Geological Society of America, vol. i, pp. 395- 

 410. 



