202 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
From T. 48, R. 44 W., where it unites with the Main or Keweenaw 
Range, eastward for 50 miles, the South Range shows typical Keweenawan 
rocks. Further east and north, in T. 48, R. 36 W., between the east branch 
of the Ontonagon and Sturgeon River, I am informed? of the existence of 
exposures of diabase and porphyry-conglomerate, which from their posi- 
tion and character must belong to the same belt. Still further east and 
north, on the S. E. 4 of Sec. 1, T. 49, R. 36 W., a bold, rocky, isolated bluff, 
known as Silver Mountain, is composed of diabase, dipping northwestward 
30°, and apparently belonging to the same belt, which is thus carried nearly 
to Keweenaw Bay. It is not meant to assert that these rocks form a con- 
tinuous belt at surface, because the horizontal sandstone undoubtedly over- 
lies them very often, extending quite over them to the Huronian or even 
Laurentian on the south. Silver Mountain appears to be surrounded by 
sandstone, and west from here for many miles sandstone appears to cover 
the Keweenawan rocks. On the west branch of the Ontonagon, in See. 13, 
T. 46, R. 41 W., sandstone is to be seen cutting quite across the Kewee- 
nawan and lying against the older slates. 
Two or three of the points examined on this range deserve more 
detailed description. Beginning on the northeast, Silver Mountain first 
merits attention. This is a bold hill, three-fourths of a mile in length, 
trending northeast. It is on the S. E. 4 of Sec. 17, T. 49, R. 36. Its north- 
western face is a long gradual slope, its southeastern a bold precipice, 400 
to 500 feet high, this contour being evidently due to the northwestern dip. 
The rock is very fine-grained, varying from nearly black to light greenish- 
gray in much altered portions. Scattered through the fine-grained mass 
are numerous red and white patches, or pseud-amygdules, containing quartz 
and orthoclase. Some specimens show also porphyritie feldspar. Under 
the microscope the rock is seen to be very much altered, and to consist 
chiefly of tabular feldspars, and fibrous, faintly dichroic hornblende, often 
showing the characteristic prismatic cleavage. Augite also occurs, and in 
such a way as to suggest very strongly that all of the greenish hornblende 
constituent has come from it, 4. ¢., is uralite, as some undoubtedly is. 
Chlorite, titaniferous magnetite, and its usual grayish product of decompo- 
1By L. G. Emerson and B. N. White, both well acquainted with the formation. 
