ANIMIKIE ROCKS OF THUNDER CAPE. 377 
tween the slates and the overlying white sandstones. The latter appears 
to me beyond doubt the true explanation, since all along the east coast of 
Thunder Bay, where these sandstones are found, the slates would naturally 
occur in large thickness, to judge from the exposures on the north and west 
sides of the bay. Such an unconformity is further strongly suggested by 
the fact that only a few miles to the east of Thunder Bay, on Black Bay, 
the sandstones are found lying directly against the older gneisses, without 
the intervening slate. This unconformity has been previously suggested 
by Hunt,! whom, however, I cannot follow in his supposition that both 
the slate and overlying sandstone are newer than the Keweenawan. 
The capping rock of Thunder Cape is, as already indicated, an olivin- 
itic gabbro, not in any respect differing from the rock described before as 
occurring in a number of places within the Keweenawan, and within the 
formation now under description, as dikes and overflows. It is medium- 
grained to coarse-grained, light-gray to dark-gray, and very highly erys- 
talline. The diallage is predominant in large areas, often including a num- 
ber of feldspars; the olivine is very plenty and coarse, and often fresh, but 
as often altered to an ocherous product; while the plagioclase is, as usual in 
these olivinitic rocks, anorthite, as shown by a large number of optical 
measurements. The rock is close to the coarse olivine-gabbros of Bad 
River, Wisconsin, standing between them and the coarser luster-mottled 
melaphyrs, which, as shown heretofore, are to be regarded as merely a 
finer phase of the olivinitic gabbros. This rock has been described by 
Macfarlane under the name of hyperite, with feldspar and hypersthene as 
the chief constituents, and hornblende and magnetite as accessories; a des- 
cription which illustrates well the unreliableness of rock determinations 
made without the use of the microscope, even when the observer is a skilled 
one like Macfarlane. 
The slates at Thunder Cape are quite arenaceous, in some layers so 
much so as to have received the name of sandstone from the Canadian 
geologists. They are from very fine-grained to aphanitic, in the more 
shaly layers, and vary from a dark-gray to black color. The layers are 
commonly very thin, and show a fine subordinate lamination. I have not 
1Second Geological Survey of Pennsylvania, Azoic Rocks, Part I, p. 289. 
