196 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
from belts of rock within the same formation, and have also shown the ex- 
istence of such a belt in that of which the Mount Houghton rock forms a 
part. 
I was also fortunate enough to find in position, south of Calumet, on the 
Torch Lake Railroad, a quartz-porphyry precisely like the pebbles of the 
conglomerate about Calumet. The rock is poorly exposed, but unquestion- 
ably in place and unquestionably original. It lies in the southern part of 
Sec. 36, T. 56, R. 33 W., near the southern border of the Trap Range, 
and is just about the horizon of the Mount Houghton felsite belt, of which, 
or of a closely parallel one, it may be reasonably regarded as the continua- 
tion. The rock exposed here shows a dark-red aphanitic matrix, through 
which are scattered very abundant and large porphyritic quartzes and feld- 
spars. The quartzes are black, and reach two-tenths inch in diameter; the 
feldspars red, partly quite fresh, partly much altered and whitened. They 
run from two-tenths to one-fourth inch in length. Some show striations 
plainly. Under the microscope the very large quartzes are seen to exhibit 
all the usual characters of the quartzes of such rocks. The grains are sin- 
gle crystals, or parts of single crystals, often showing plainly the rhombo- 
hedral planes, more rarely the prismatic. Commonly the grains are much 
rounded and penetrated by bays and lines of the matrix. The feldspars are 
chiefly oligoclase, but orthoclases are also frequent. Augite crystals, largely 
replaced by magnetite, appear porphyritically. The matrix presents, for 
the most part, a faint pinkish color and a cloudy appearance. Thickly 
scattered in it are minute brown particles and needles of ferrite, arranged 
in lines whose directions show most beautifully the so-called flowage struct- 
ure. This matrix affects the polarized light but very little, if at all. Other 
portions of the matrix, of a white color, appear to consist of individualized 
quartz and feldspar, the former perhaps secondary. Calcite is also seen in 
small particles in these places. This rock is pictured at Fig. 2, Plate XII, 
where is also figured a section of one of the common porphyry pebbles of 
the Calumet conglomerate. The identity of the two is readily apparent. 
The map and sections of Keweenaw Point accompanying this descrip- 
tion (Plates XVII and XVIID), along with the general map illustrating the 
structure of the Lake Superior basin, will help greatly to convey a correct 
