2 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
formations of the western end of the Lake Superior Basin, both on the Minne- 
sota and Wisconsin sides. The first of these obstacles was in part removed 
by the microscopic studies of Professor Pumpelly, whose conclusions as to 
the nature of the Keweenaw Point rocks were published in 1878.1 The 
second was also in part removed by the investigations of the Wisconsin 
Geological Survey, whose results as to that portion of Wisconsin which bor- 
ders on Lake Superior were first published in 1880.° There still remained 
unknown, however, a wide extent of the series in Minnesota, a most import- 
ant omission, since in this region only is it possible to connect the rocks of 
the South and North Shores. The rocks of the southern range of Keweenaw 
Point were also still very vaguely known, and the structure of the Porcu- 
pine Mountains had never yet been made out. é 
It thus became necessary for any one attempting to present anything like 
a general account of the series to cover these gaps, and extend the micro- 
scopic investigations begun by Professor Pumpelly, so far as possible, over 
the whole extent of the series. It was also necessary that he should famil- 
iarize himself as much as possible with those districts which had already 
been more or less thoroughly worked up, by studying the descriptions of 
others on the ground. This work, including also the preparation of this 
memoir, I have undertaken to accomplish during the months between July, 
1880, and March, 1882—short enough a time, especially when a large share 
of it has been given to other work. I was, however, already quite familiar 
with the series as developed in Wisconsin, with the microscopic characters 
of its rocks, so far as previously known, and with the literature of the sub- 
ject; so that, with the aid of several assistants, I have been enabled to 
accomplish the main objects aimed at. My chief regret in connection with 
the field work, which was of necessity confined to the season of 1880, has 
been my inability, on account of lack of time, to visit Isle Royale, the region 
about Lake Nipigon, and Michipicoten Island. The full descriptions of 
these places, given respectively by Foster and Whitney, Bell and Macfarlane, 
together with a type suite of specimens collected by the latter gentleman at 
1““Metasomatic Development of the Copper-Bearing Rocks of Lake Superior.” Proc. Am. Acad., 
1878, XIII, 253-309. 
2R. D, Irving, R. Pumpelly, E. T. Sweet, T. C. Chamberlin, and M. Strong, in the Geology of 
Wisconsin, Vol. III. 
