OUTLINES OF LAKE SUPERIOR. 421 







shores of Lake Superior in almost all directions, when I was one day 

 most forcibly struck with the fact, that these dykes agree, in their 

 bearings, with the bearings of the shores ; and that even the greatest 

 complications in the outlines of the shores could be accounted for, by 

 the combinations of dykes intersecting each other in different direc- 

 tions. And indeed, now that I have the key for such an analysis, 

 I find no difficulty in referring, even short lines of the coast, to 

 the different systems of dykes which I know to exist there, and 

 wherever my memoranda are sufficiently full, I find indications of 

 dykes running in the direction of the coast. As soon as my attention 

 had been called to these phenomena, I lost no opportunity of invest- 

 igating the nature of the rock of these different systems of dykes, 

 and I ascertained, to my great astonishment, that there are consid- 

 erable differences in their mineralogical characters ; some being am- 

 phibolic trap ; others being injected with epidote ; others having 

 more the appearance of pitchstone ; and, what is particularly inter- 

 esting, the dykes which run in the same direction preserve the same 

 mineralogical character, as well as the same bearing. 



The systems of dykes which run directly north and south, and 

 which form the inlets between Neepigou Bay and the main lake, and 

 intersect the large island of St. Ignace, and separate St. Ignace 

 stielf from the main land, all run north and south, and consist of 

 very hard, tough, unaltez-able hornblende trap, of a crystalline 

 aspect, and a grayish color ; while the dykes, which run east and 

 west, and mark out the northern and southern shores of those same 

 islands, consist mostly of a greenish trap extensively injected with 

 epidote, and breaking with the greatest ease into angular, irregular 

 fragments. The northern shore east of the Pic has the same general 

 bearing, due east and west ; and here, also, we find the dykes more 

 or less epidotic, and the metamorphic rocks talcose. 



Again, the long shore running clue east and west from Michipico- 

 tin Avestwards, is, also, along its whole extent, intersected by epidotic 

 dykes running east and west. 



The dykes of the north-eastern coast of the lake between the Pic 

 and Michipicotin Island, which run north north-east to south south- 

 west, consist of a pitchstone trap, like black glass, which, notwith- 

 standing its external hardness, readily decomposes, and forms almost 



