424 LAKE SUPERIOR. 



The large group of islands on the southern and eastern side of 

 Black Bay, and south-west of St. Ignace, consists of innumerable 

 islets, separated from each other by the close intersection of the three 

 systems of dykes, which appear more prominent and strongly marked 

 in their features further west, in Isle Royale and Victoria Islands, 

 and about Thunder Bay. 



But besides these six clearly defined systems, there seem to be 

 two more, or at least one other distinct system running due north-west 

 and south-east, cutting at right angles through Spar Island, and re- 

 appearing, as I understand from verbal communications of Mr. Foster, 

 further south upon Point Keewenaw. This system is perhaps the 

 cause of the bearing of the shores between Keewenaw Bay and Dead 

 River ; also of the outlet of Lake Superior between Point Iroquois 

 and Gros Cap along the river St. Mary, unless this eastern system 

 of intersection be distinct from the more western one. 



But however this may be, so much is plain ; — that at least six 

 distinct systems of dykes, with peculiar characteristic trap, forming 

 parallel ridges in the same system, but varying, for different angles, 

 between the different systems, intersect the northern shores of Lake 

 Superior, and have probably cut up the whole tract of rock, over the 

 space which is now filled by the lake, in such a way as to destroy its 

 continuity ; to produce depressions, and to have gradually created an 

 excavation Avhich now forms the lake, and thus to have given to it 

 its present outline. This process of intersection, these successive 

 injections of difierent materials, have evidently modified, at various 

 epochs, the relative level of the lake and land, and probably also 

 occasioned the modification which we notice in the deposition of the 

 shore drift, and the successive amphitheatric terraces which border, at 

 various heights, its shores. 



A more minute analysis of the mineralogical character of these 

 dykes would no doubt aflbrd satisfactory evidence of their original 

 independence, and perhaps lead, in connection. with a fuller investi- 

 gation of their intersections, to the means of ascertaining their rela- 

 tive age. But I became fully aware of the geological importance 

 and independence of these difierent systems of dykes only during 

 my return, after leaving the neighborhood of Thunder Cape, the 

 ground where this part of the subject might be best studied, and 



