

\ 



V 





til 





1 H 



til 



ma. 







:k 



mi 



other 



riy 

 riods 



mm 



l1, and 



Teater 



mpos- 



and 

 The 



- 



. 



of tk 







20 



have 



• 



river 

 J into 



, tliat 



, been 



Hi 







Hk 













Ch. xviil] 



LAKE SUPEKIOR. 



421 



the cavity is an hypothesis which appears to me quite un- 

 tenable for reasons which I have explained elsewhere.* 



Lake Superior. — Lake Superior is the largest body of fresh 

 water in the world, being above 1,700 geographical miles in 

 circumference when we follow the sinuosities of its coasts, 

 and its length, on a curved line drawn through its centre, 

 being more than 400, and its extreme breadth above 150 

 Geographical miles. Its surface is nearly as large as the 

 whole of England. Its average depth varies from 80 to 150 

 fathoms ; but, according to Captain Bayfield, there is reason 

 to think that its greatest depth would not be overrated at 

 200 fathoms, so that its bottom is, in some parts, nearly 600 

 feet below the level of the Atlantic, its surface being about 

 as much above it. There are appearances in different parts 

 of this, as of the other Canadian lakes, leading us to infer 

 that its waters formerly occupied a higher level than they 

 reach at present; for at a considerable distance from the 

 present shores, parallel lines of rolled stones and sand are 

 seen rising one above the other, like the seats of an amphi- 

 theatre. These ancient lines of shingle are exactly similar to 

 the present beaches in most bays, and they often attain an 

 elevation of 40 or 50 feet, and sometimes several hundred 

 feet above the present level. The heaviest gales of wind do 

 not raise the waters more than three or four feet, and the 

 loose materials, says Agassiz, which lie within the action of 

 heavy storms are entirely deprived of vegetation, whereas 

 the set of beaches next above are covered by a few crypto- 

 gamous and herbaceous plants. At a still higher level, and 

 retreating more and more from the shores, are terraces on 

 which grow shrubs and small trees, and above these older 

 beaches are precipitous banks cut out of loose materials, 

 which must have been worn for a considerable time by the 

 action of the waves. Six, ten, and even fifteen such terraces 

 may sometimes be distinguished one above the other. All 

 these beaches and terraces are composed of remodelled glacial 

 drift, the stones having lost more or less of their scratches 

 and polished appearance, and having been rolled into ordinary 

 pebbles. M. Agassiz when discussing the Question how so 



* Elements of Geology, edition of 1865, p. 170. 



