422 



LAKE SUPEEIOE. 



[Ch. XVIII. 



many changes may have been produced in the level of the old 

 shore of the lake, inclines to the opinion that the land has 

 risen unequally, rather than that the waters have been 

 repeatedly lowered by the successive wearing down or 



of the barrier on the side where it is lowest at 



com 



removal of the barri 

 present.*" If we are 

 lities of movement have occurred in Post-glacial times, we 

 may well suppose that others of far greater extent contri- 

 buted, before and during the Glacial Period, to form the 

 basin of the great lake itself. 



The streams which 



discharge their 



waters into Lake 



Superior are several hundred in number, without reckoning 

 those of smaller size ; and the quantity of water supplied by 



than that discharged at the 



them is many times greater 

 Palls of St. Mary, the only outlet. The evaporation, there- 

 fore, is very great, and such as might be expected from so 

 vast an extent of surface. On the northern side, which is 

 encircled by mountains of old crystalline rock, the rivers 

 sweep in many large boulders with smaller gravel and sand, 

 chiefly composed of granitic and trap rocks. There are also 

 currents in the lake in various directions, caused by the con- 

 tinued prevalence of strong winds, and to their influence we 

 may attribute the diffusion of finer mud far and wide over 

 great areas ; for by numerous soundings made during Capt. 

 Bayfield's survey, it was ascertained that the bottom consists 

 generally of a very adhesive clay, containing shells of the 

 species at present existing in the lake. When exposed to 

 the air, this clay immediate 



become 



smart blow of the hammer 



It effer- 



vesces slightly with diluted nitric acid, and is of different 

 colours in different parts of the lake ; in one district blue, in 

 another red, and in a third white, hardening into a substance 

 resembling pipeclay. f Prom these statements, the geologist 

 will not fail to remark how closely these recent lacustrine 

 formations in America resemble the tertiary argillaceous and 



marls of lacustrine orig 



In 



most 



* Agassiz. Lake Superior, p. 416. 



t Trans, of Lit. and Hist. Soc. of Quebec, vol. i. p. 5. 1829. 



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