118 ALFRED R. C. SELWYN ON THE 
Nipigon were both younger than Keweenian (using the name Keweenian for Upper Cop- 
per-bearing) and may be Mesozoic, while Keweenian is Pre-Cambrian. The supposed 
Mesozoic or Permian age of this group of rocks is founded entirely on lithological and 
mineralogical considerations, and has, I believe, been advocated recently only by Hunt, 
Bell and Macfarlane. 
A full summary of the different opinions as regards the Upper Copper-bearing rocks 
is given, and the subject fully discussed by Professor Winchell, in the 10th annual report 
of the Wisconsin Survey, 1882, pp. 123-136. 
It may be well here to recapitulate the principal points I propose to refer to, and 
upon all of which there still appears to be a considerable divergence of opinion, arising, 
however, as I believe, chiefly from incomplete observation and study of the facts as pre- 
sented in all parts of the area, and from attaching too much importance to local uncon- 
formity and disturbances and to local lithological differences and resemblances, and far 
too little to the general physical characters ; also from not sufficiently recognizing the neces- 
sity of minutely accurate stratigraphical methods of observation and mapping. 
1. We have the question of conformity or otherwise between the Laurentian and the 
Huronian. 
2. The same question as regards the Huronian and the “ Keweenian.” 
3. What are the relative positions in the sequence of “ Animikie,” “ Nipigon ” group 
and “ Keweenian,” as defined by Dr. Hunt, pp. 241-42, Azoic Rocks E. 2nd Geological Sur- 
vey of Pennsylvania, and are any of these series either of Pre-Cambrian or of Mesozoic age ? 
4, Are the Chazy or St. Peters sandstones of Wisconsin, and the Calciferous or lower 
magnesian limestones, represented on or around the shores of Lake Superior, and if so, 
where and to what extent at or west of Sault St. Mary ? 
5. What relation, as regards age, do the great overflows of trap which form the 
summits of all the higher points, Thunder Cape, Pic Island, &c., at the west end of Lake 
Superior and in the Lake Nipigon basin, hold to the cupriferous rocks—“ Keweenian ”—of 
Isle Royale, St. Ignace and Michipicoten ? 
The foregoing are some of the principal, if not all, the points which have been dis- 
cussed, and which now require further investigation in connection with the geology of 
the region around Lake Superior. 
Last summer I visited and partly examined the north shore of the lake from Thunder 
Bay to Sault St. Mary, and thence eastward to Echo Lake, and I now propose to state 
briefly the conclusions which the evidence, so far as I have seen it, seems to me to establish 
as regards the points enumerated. 
1. I was unable to find any conclusive evidence whatever of the supposed unconfor- 
mity between the rocks which have been classed respectively as Huronian and Laurentian, 
and at present Iam unable to indicate any better reason than that of a considerable 
difference in the lithological characters of certain sets of beds for considering these so- 
called Laurentian and Huronian rocks to belong to distinct and .wholly unconformable 
systems. 
The Laurentians are essentially granitoid, gneissose and felspathic, while the Huro- 
nians are quartzose, hornblendic, schistose and slaty. As a whole, the latter haye a some- 
what less altered aspect, and they contain pebbles of rocks—granite, gneiss, quartzite, ete. 

