420 LAKE SUPERIOR. 



for an investigation of their natural features ; some running east and 

 west, others straight north and south, and others forming a regular 

 crescent, with its convexity turned northwards. Their absolute po- 

 sition is at once characteristic. They are excavated chiefly between 

 the plutonic masses rising north, and the stratified deposits south of 

 the primitive range. 



Lake Superior, especially, fills a chasm between the northern 

 granitic and metamorphic range, and the oldest beds deposited 

 along their southern slopes in the primitive age of this continent. 

 Lake Ontario and Lake Erie, on the contrary, run between the suc- 

 cessive layers of different sets of beds of the same great geographical 

 period ; while Lakes Huron and Michigan fill up the cracks which 

 run at right angles with the main northern primitive range, and 

 which, no doubt, owe their origin to the elevation of the chains north 

 of Lake Huron and Lake Superior ; repeating, on a large scale, 

 what has been said above of the dependence of the Swiss lakes 

 upon their geological positions and relation to the mountain chains 

 which encircle them. 



Besides this general relation of the lakes in connection with their 

 shores, I have been able to trace a more intimate connection of the 

 outlines of their shores and their geological structure, especially in 

 Lake Superior. 



As a whole, that lake resembles a large crescent, with its convexity 

 turned northwards ; but it were a great mistake to imagine that this 

 form is actually the form of the shores, or that it is repeated upon 

 every point. On the contrary, the general outline of that lake is 

 the accidental result of the combination of many details, of many 

 geological events which have followed each other at different periods, 

 have modified the tract of land where the lake now exists, and have 

 cut up its foundation in such a manner as to break the continuity of 

 the solid rock, and allow it to be decomposed. Thus an extensive 

 crescent-shaped hole with innumerable islands has been formed, in 

 which the islands, in their various bearings, still indicate the direction 

 of the intersecting masses, and appear at present as the fragmentary 

 remains of a continuous tract of land, which is now replaced by a 

 deep lake. 



For many weeks I had been tracing the dykes which intersect the 



