206 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
Section IV.—THE REGION BETWEEN THE ONTONAGON RIVER AND 
NUMAKAGON LAKE OF WISCONSIN; INCLUDING THE PORCUPINE 
MOUNTAINS. 
This region is one of especial interest. Within it, as indicated on the 
map and sections of Plates XXII and XXIII, is included the entire thick- 
ness of the Keweenaw Series, both of the Upper and Lower Divisions. 
Within it the Eastern Sandstone reaches its western termination, and the 
ranges of Keweenawan rocks south and north of the valley occupied by 
the sandstone unite. Both the underlying formations—the iron-bearing 
Huronian and the gneissic Laurentian—are here largely developed, and 
in close proximity to the Keweenawan. Midway the district, in the vicinity 
of the Montreal River, a line drawn southeasterly crosses, in a distance of 
some twelve miles, the entire thickness of the Huronian and Keweenawan. 
Within this region we have also the eastern termination of the horizontal 
Western Sandstone. Here we have again far more extensive developments 
of quartziferous and felsitic porphyries than on Keweenaw Point. Augite- 
syenites also occur. Here, too, we have, in the Bad River country, a great 
showing of the coarse gabbro, which is developed in only one other district 
about Lake Superior, viz: about Duluth, in Minnesota. 
In this area, moreover, occurs the peculiar fold producing the Porcupine 
Mountains, and here we have the axis of the great trough in which most 
of Lake Superior lies running on to the land. Here are found all degrees 
of inclination, from near horizontality to complete verticality. With all of 
this there is also a perfect development of the usual bedded amygdaloids, 
melaphyrs, conglomerates, etc. Thus, save that the structural relations of 
the acid porphyries are not to be so distinctly made out as on the Min- 
nesota coast, had this restricted region been worked in such detail as those 
portions of Keweenaw Point examined by Pumpelly and Marvine, the 
Keweenaw Series would be nearly as well understood as from a study of 
the entire basin of Lake Superior, and what is to be seen elsewhere about 
the lake would be of little more than confirmatory value. 
Unfortunately the whole district is wilderness, and with the exception 
