Great Lakes, and t/ie Valley of the Mississippi. 231 



chains of the Laurentian continent have been removed and 

 carried into the sea, and this has been done by rivera of water 

 and rivera of ice. That these mountains once existed there can 

 be no reasonable doubt, for their truncated bases remain as 

 witnesses, and it is scarcely less certain that glaciers have 

 flowed down their slopes of sufficient magnitude and reach to 

 deeply score the plain which encircled them. 



It will-be noticed that all the great lakes of the continent 

 hold certain relations to the curving belt of Laurentian high- 

 lands. 



Some of them are embraced in the foldings of the Eozoic 

 rocks, and fill synclinal troughs; but most of the series, from 

 Great Bear Lake to Lake Ontario, exhibit the same geological 

 and physical structure, are basins of excavation in the paleo- 

 zoic plain that flanks in a parallel belt the Laurentian area. 

 Few of us have any conception of the enormous general and 

 local erosion which that plain has suffered. Those who will 

 take the trouble to examine the section across Lake Ontario, 

 from the Alleghanies to the Laurentian hills of Canada, and 

 compare it with the other sections in the Lake Winnepeg dis- 

 trict, radial to the Laurentian arch, given by Mr. Hind in his 

 report on the Assiniboin country, will be sure to find the com- 

 parison interesting and suggestive ; suggestive especially of a 

 community of structure and history, and of an inseparable con- 

 nection between the lake phenomena and the topographical 

 features of the Laurentian highlands, flanked by the paleozoic 

 plain. 



In estimating the influences that might have affected the 

 number and magnitude of glaciers on the Bides of the Lauren* 

 tian mountains, it should not be forgotten that the Cretaceous 

 sea swept the western shore of the Paleozoic and Laurentian 

 continent, from the Gulf of Mexico to the Arctic Ocean ; and 

 whether. we consider this sea as a broad expanse of water 

 simply dotted with islands, or a strait traversed by a tropical 

 current, we have in either case conditions peculiarly favorable 



