396 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
of Thunder Bay are, in all probability, in large measure diabasic rather than 
dioritic rocks, if we may judge from the rarity of true dioritic rocks gene- 
rally in the Lake Superior region. I have already spoken of the uralitic 
nature of the hornblende of the Keweenawan and Huronian greenstones. 
Recent studies by myself and my assistant, Mr. C. R. Vanhise, of a number 
of hornblendic rocks from the valley of the Wisconsin River apparently 
plainly connected with the older gneisses, have resulted in showing that 
this uralitic change is found in many other kinds of rocks than the green- 
stones, true granites and even hornblende-schists frequently showing it." 
So common have we found this change that it has led to the suspicion that 
hornblende does not occur as a primary constituent in any of these ancient 
rocks. While such a generalization is of course unwarranted from the rel- 
atively small extent of our studies, our experience should be enough to 
render any one studying hornblendic rocks very watchful for indications of | 
the secondary origin of the hornblende. 
I proceed to give brief accounts of some of these schistose areas of 
doubtful relations. The positions and sizes of the areas mentioned are indi- 
cated on the accompanying general map of the Lake Superior region. 
In the Thunder Bay region, between the northern limit of the Animikie 
Group and the vicinity of Dog Lake, schists called by them Huronian have 
been studied by Murray, Logan and Bell. These schists appear nearly 
always, if not always, to be separated from the flat-lying Animikie rocks by 
a belt of gneiss and granite, which is, however, at times very narrow. Iso- 
lated areas of gneiss are found to occur within the schists, which are bounded 
on the north again by a large area of gneissand granite. Still farther west- 
northwest, as seen about the west end of Lake Shebandowan, are other 
schists, succeeded in turn by gneiss and granite. These so-called Huronian 
rocks consist, according to Bell, of ‘slates, some of them dark-green and com- 
posed of hornblende; some grayish-green and dioritic; others are light- 
colored, fine-grained, quartzose, somewhat nacreous micaceous schists; 
while dioritic slate-conglomerates, quartzites, fine-grained felsites, massive 
diorites, ribboned jasper, and iron ore also occur.”? 
These descriptions are of course not based on microscopic study, and 
1Geology of Wisconsin, Vol. IV, pp. 622-714. 
2 Report of the Geological Survey of Canada, 1867-69, p. 326. 
