376 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
nated with the usual slaty material of the series. Others show a peculiar 
concretionary arrangement. 
Still nearer the head of the bay come in the dolomitic sandstones of 
the overlying Keweenawan. Where first seen these sandstones are often 
conglomeratic, holding pebbles of the older slates and especially of the chert 
layers upon which they directly rest. Along the southeast shore of Thun- 
der Bay the slates are here and there seen at the water's edge, but above 
them the cliffs of newer sandstone, with one or more beds of chert-conglom- 
erate, rise to a height of 200 feet above the lake. These conditions continue 
for a number of miles. Before reaching the deep bay on the north side of 
Thunder Cape a number of narrow dikes were passed which intersect these 
slates. Fifteen of these dikes were counted in a distance of about ten miles. 
They are usually under 15 feet across, the widest not exceeding 20 feet, 
while some are only 4 feet in width. Most of them stand vertically, trend- 
ing from N. 75° E. to east and west. One was noticed dipping 60° south, 
with a strong columnar cross-jointing at right angles to this direction. It 
could not be made out that these dikes cut the overlying sandstones. ‘The 
one thin section of the dike-rock examined showed an olivine-free diabase 
or gabbro of moderately coarse grain, containing predominating augite, 
partly fresh and partly altered to greenish material, labradorite, a little 
orthoclase, and magnetite. The rock lies between the olivinitic gabbros 
and the orthoclase-gabbros proper. 
At the deep bay just above Thunder Cape the white sandstones, which 
have heretofore formed the cliffs of the entire southeast side of the bay, 
swing away from the coast to the southeastward, reappearing on the lake 
front near Silver Islet Landing, and leaving between them and the extremity 
of the cape a triangular area, which is occupied entirely by the slates I 
have been describing, capped by an immense overlying mass of olivinitic 
gabbro, upwards of 200 feet in thickness. The whole height of Thunder 
Cape is over 1,000 feet, from 700 to 800 feet of which must be occupied by 
the slates, which thus rise entirely across the horizon of the sandstones 
mentioned, and several hundred feet higher than they do. Such a relation 
can only be explained by supposing a great fault, as Logan did, by which 
these slates are brought up; or by supposing an erosion to intervene be- 
