252 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
The lower falls of Black river are near the S. E. corner of Sec. 21, T. 47, R. 14 
W. In this vicinity, the exposures are the largest and most interesting of any observed 
in the district. Here the river has cut a gorge through the crystalline rocks of the 
Keweenawan and the breccia conglomerate and sandstone of the Lower Silurian, to a 
depth varying from 100 to 180 feet. and having a length of nearly one-half mile. Along 
the walls of the gorge, the measures are most beautifully exposed, but great difficulty 
has been experienced in satisfactorily making out their relations. * * * * At 
the head of the rapids, which extend about 100 feet above the falls, a dark-colored, 
fine-grained, hard diabase of the ashbed type occurs. Although quite indistinct, it 
appears to have a bedded structure. At the immediate head of the falls, this is suc- 
ceeded by a red felsitic porphyry, and this, again, by the common diabase. The third 
bed, over which is the main fall, is a dark-gray, fine-grained diabase, having also an 
indistinct bedding. About 75 feet above the foot of the great fall, in the left or west 
wall of the gorge, there is a vertical fissure 8 inches wide, formed by two smaller 
fissures dipping towards each other, and making an angle of about 40°. Owing to 
talus, the fissure can be seen only a distance of 10 or 15 feet. It is filled with a soft, 
clay-like sandstone. The walls on either side of the fissure are very dark-colored, soft 
and unctuous. The rock is a chloritic alteration of the diabase. 
Just above the head of a small fall, near the foot of the great fall, a fine-grained, 
reddish-brown diabase comes in, which is frequently amygdaloidal. The dip here is 46° 
in a direction about 8. 20° E. This extends along the wall of the gorge nearly 300 feet, 
and gradually grades on the west side into a diabase-breccia. The transition, in fact, 
is so imperceptible that it is impossible to locate exactly the point at which the dia- 
base ceases and the breccia commences. This is partially owing to the fact that both 
diabase and breccia walls are considerably decomposed, and partially to the further 
fact that the breccia contains immense inclosed masses of the diabase. In the east 
wall there is no breccia. Near the southern limit of the breccia the rock consists 
almost exclusively of angular grains and masses of diabase, while in the north, there 
is a notable proportion of reddish sand, which seems to be the matrix or cementing 
material. Imbedded in the face of the breccia are a number of highly indurated 
layers of reddish sandstone, from 4 to 50 feet in length, and inclined at different 
angles, some of them being vertical. About 50 feet above the stream, and near the 
foot of a cliff a hundred feet high, are two of these layers resting together. They are 
2 feet thick, 40 feet long, straight, and dip 26° to the N. 70° W. These layers are 
really quartzite. They have a dark-brown color, coarse granular strueture, and con- 
tain a few disseminated grains of delessite. 
One hundred and fifty feet from the breccia, in the left bank of the stream, there 
is a bed of conglomerate, arising directly from the stream, 30 feet in thickness. 
Interstratified with the conglomerate are a few layers of sandy shale. The dip of 
this bed of conglomerate is 25° in a direction 8. 20° W. The dip, however, is not 
uniform. For a few rods down the stream it is 20°, and 25 feet farther, only 15° in the 
same direction. The pebbles are from one-half an inch to three inches in diameter, 
and are principally white amorphous quartz. About a third of them are diabase, 
much more angular than the quartz pebbles, some are sandstone, and a few are them- 
selves conglomerate. The matrixisred sand. On the east or right side of the stream 
there is also a bed of conglomerate, which is underlaid by thin-bedded sandstone. This 
