SURFACE GEOLOGY. 73 



debris of the land was deposited in strata that subsequently rose to the 

 surface, and formed a broad, low margin to the central mountain belt ; 

 just as the Cretaceous and Tertiary strata flank the Alleghanies in the 

 Southern States. 



In the lapse of countless ages, all the mountain peaks and chains of 

 the Laurentian continent have been removed and carried into the sea, 

 and this has been done by rivers of water and rivers of ice. That these 

 mountains once existed there can be no reasonable doubt, for their trunc- 

 ated 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 highlands. 



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, and are 

 basins of excavation in the Palaeozoic 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 district, radial to the Laurentian 

 arch, given by Mr. Hind in his report on the Assinniboin country, will 

 be sure to find the comparison interesting and suggestive; suggestive 

 especially of a community of structure and history, and of an insepara- 

 ble connection between the lake phenomena and the topographical 

 features of the Laurentian highlands, flanked by the the Palaeozoic plain. 



In estimating the influences that might have affected the number and 

 magnitude of glaciers on the sides of the Laurentian mountains, it 

 should not be forgotten that the Cretaceous sea swept the western shore 

 of the Palaeozoic 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 to the for- 

 mation of great glacial masses of ice, i. e., a broad evaporating surface of 

 warm water swept by westerly winds, that carried all suspended moisture 

 immediately on to a mountain belt, which served as a sufficient con- 

 denser. 



This, at least, may be positively asserted in regard to the agency of 

 ice in the excavation 6f the lake basins, that their bottoms and sides, 

 wherever exposed to observation, if composed of resistant materials, bear 



