258 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
7 W., between Frog and Cranberry rivers, there is a ridge having an altitude of over 
400 feet, and covered with many large angular bowlders of diabase and other Ke- 
weenawan eruptive rocks. It presents a striking resemblance to the range a short 
distance west of the Brulé river. On the NE. 4 of Sec. 20, T. 50, R. 6 W., a few 
rods north of the shore of Siskowit lake, there is a flat circular bowlder of brecciated 
conglomerate. 18 feet in diameter. A few of the angular and some of the round 
pebbles are diabase, but the most of them are felsitic porphyry. The matrix is red- 
dish sand. The bowlder is evidently stratified, and, as it lies, has an inclination of 
20° S. Owing to the frail nature of the rock, it probably never traveled any consid- 
erable distance. This is one of the numerous so-called outcrops of crystalline rocks 
to which I was directed by some of the citizens of Bayfield. Others were found to be 
large bowlders, trains of bowlders, or ledges of sandstone. 
The most important points established by Mr. Sweet’s investigations 
in this district are (1) the identity between the cupriferous rocks here ex- 
posed and those of Keweenaw Point; (2) the existence of a rather low 
southern dip, and of a northeasterly strike, ranging from east-northeast on 
the west, to north-northeast on the east; (*) the absence of diabase ledges 
on the highlands of Bayfield County; (4) the unconforamble contact be- 
tween the red sandstone which borders Lake Superior west of the Montreal, 
and the Keweenawan beds. 
The contacts between these formations described by Mr. Sweet as 
showing on Black River, Copper Creek, Aminicon River, and Middle River, 
are also of great interest on account of the peculiar disturbances they pre- 
sent. These disturbances find their explanation, in part, as it appears to 
me, in the irregularities of an unconformable contact, and in the pressure 
exerted by the deep-seated Keweenawan beds against the more shallow 
sandstone, but also in large measure in a faulting that has taken place 
along the contact line. There is much in common between this fault and 
that on the south side of Keweenaw Point. There are only two ways in 
which the conclusion might be avoided that we have here to deal with a 
true non-conformable contact. These are to suppose, either, as Whittlesey’ 
and Norwood® did, that the crystalline rocks here are true intrusive or dis- 
turbing masses, or dikes, and hence newer than the sandstone with which 
they are now in contact, or that the sandstones belong to the Upper Division 
of the Keweenaw Series, let down here by a great fault to a lower level 
than that occupied by the diabases which belong normally under them. 
1“ Physical Geology of Lake Superior.” Proce. Am. Assoc. Ady. Sci., Detroit Meeting, 1875, p. 67. 
2 Owen’s Geological Survey of Wisconsin, Iowa, and Minnesota, p. 305. 
