OUTLINES OF LAKE SUPERIOR. 425 



therefore I can now only call the attention of geologists to these 

 facts, in the hope that they may, at some future time, be more fully 

 investigated. 



The whole range of rocks which constitutes the northern shore of 

 Lake Superior is so extensively metamorphic, and so thoroughly in- 

 jected in all directions by veins intersecting each other, that it is no 

 easy task to analyze their relations ; and for a full illustration of this 

 subject, minute maps of well-selected localities are required, such as 

 travelling geologists on an occasional visit can scarcely prepare. But 

 I should bo perfectly satisfied to see these hints more completely 

 wrought by others, satisfied, as I am, to have shown, at least, how a 

 minute investigation of the geological phenomena of a restricted 

 locality may lead to a better understanding of the origin of the geo- 

 graphical features of a country. 



But let me repeat that it were a great mistake to ascribe the 

 present form of Lake Superior to any single geological event. Its 

 position in the main is no doubt determined by a dislocation between 

 the primitive range north and the sedimentary deposit south. 



But the working out of the details of its present form is owing to 

 a series of injections of trap dykes of different characters, traversing 

 the older rocks, in various directions, which, from their mineralogical 

 differences, have no doubt been produced at different successive 

 periods. 



The diversity of rocks which occur on Lake Superior is very great, 

 and there are varieties observed there which seem to be peculiar to 

 that district, presenting innumerable transitions from one to another, 

 of which the Alps even do not present more extensive examples. 



Of these we have new red sandstone passing into porphyries, 

 into quartzites, granites, and gneiss, the metamorphisra being more or 

 less perfect, so that the stratification is sometimes still preserved, or 

 passes gradually into absolutely massive rocks. Again, the dykes 

 intersect other rocks almost without altering them, or the alterations 

 in the immediate contact are so intense as to leave no precise lines 

 of demarcation between the dyke and the injected rock. But here 

 again, the phenomena are so complicated, that unless the illustration 

 be accompanied by a very detailed map it were useless to enter into 

 more minute descinptions. 

 29 



