418 LAKE SUPERIOR. 



there is no cause left for its accumulation, and the changes in the 

 comparative level of the main land and the terraces remain equally 

 unaccounted for. 



Indeed, the terraces are so unequal in their absolute level when 

 compared to each other, that a gradual subsidence of the lake remov- 

 ing a barrier of loose materials at its outlet could never explain their 

 irregularity. But if we suppose that the innumerable dykes which 

 cross, in all directions, the rocks which form the shores of the lake, 

 have at various intervals lifted up these shores, we have at the same 

 time a cause for the change of the relative level between the terraces 

 and the lake, and also for the change of its absolute level, as it 

 removed larger and larger portions of materials accumulated at its 

 eastern extremity. 



That these dykes have produced such changes will not be doubted 

 by any one who may study the phenomena described in the follow- 

 ing chapter respecting the origin of the present outlines of the lakes, 

 as produced by the intersection of all the dykes traversing the 

 metamorphic and plutonic rocks of the northern shores. 



We should therefore conclude that, as there has been a general 

 gradual change between the relative level of the main land and sea, 

 so there has also been a gradual local change in the relative level of 

 the lake and its shores ; and hence the local phenomena would only 

 corroborate the induction derived from more general geological facts. 



