266 ASSINNIBOINE AND SASKATCHEWAN EXPEDITION. 



LIMITS OF DENUDATION. 



A curious and very interesting example of parallelism 

 in bold limits of denudation, can be traced in the basins 

 of Lake Winnipeg and the St. Lawrence. The abrupt 

 escarpment of the Grand Coteau de Missouri preserves a 

 direction throughout the greater portion of its denuded 

 face, parallel to the escarpment of the Niagara limestone 

 which enters Canada from the State of New York at 

 Lewiston, on the Niagara river, and sweeping round the 

 head of Lake Ontario passes up the Indian Peninsula and 

 thence to the Grand Manitoulin Island in Lake Huron. 

 The denuded flanks of the Eiding and Duck Mountains 

 with their northern and southern prolongations also pre- 

 serve the same general direction. Lines drawn on a map 

 of North America to show these well-marked limits of 

 denudation at different periods, will at once suggest the 

 existence of a uniformly acting force operating under 

 similar physical conditions. Like beaches or ridges formed 

 at different sea-levels they have probably a common origin, 

 which the supposition of their being ancient coast-lines is 

 not sufficient to explain, neither do their geographical 

 relations appear to be altogether dependent upon their 

 geological structure. They seem to point to the action of 

 currents, of which the Gulf stream and the compensating 

 Arctic currents are modern illustrations. 



DISLOCATIONS. 



The basin of Lake Winnipeg from the Laurentides to 

 the Eocky Mountains, has been subjected to a series of 

 dislocations, which have acted very uniformly over areas 

 far apart. The courses of rivers show the general direc- 

 tion of these gradual disturbances, which may have given 

 rise to the numerous " Elbows " which form such a 

 marked feature in the water channels, and consequently 



