GEOLOGY OF LAKE SUPERIOR. 121 
immense 8. E. and N. W. dislocation, traversing the lake from west of the Slate Island to 
Sault St. Mary. If so, the movement has probably been on the Keweenaw Point side, and 
this would fully explain why we have the south-easterly dip of Michipicoten Island directly 
opposite, or on the strike of the north-westerly dip of Keweenaw Point; also the high 
angle of dip which the series presents on the Keweenaw peninsula, and it may likewise 
have some significance in connection with the sudden change—almost rectangular bend— 
in the shore of the lake. 
It may also be observed that this is also the direction of a very large number of the 
mineral veins and trap dykes which have cut all the strata beneath the Keweenian series, 
The relation to the amygdaloid series of the sandstones which appear at certain points 
on the coast south of Mamainse, and which have been noticed by Logan, Macfarlane, Bell, 
and others, requires further investigation, including an examination of Cariboo, Leach, 
Lizard and Montreal Islands, supposed to be occupied by these sandstones. 
The fifth point I have referred to seems to offer the most difficult problem, viz., the 
nature and origin of the so-called crowning overflows of trap. I have not had an oppor- 
tunity of examining these to the west of Thunder Bay; in McKay’s Mt. Pie Island and 
‘Thunder Cape, however, and in some of the smaller islands in Thunder Bay, this trap 
rests conformably and with a very slight easterly dip on the black shales, schists and 
dolomite of the Animikie series. 
It is usually a coarse, dark, somewhat rusty-looking diabase, and it generally presents 
an imperfect columnar structure, the columns being vertical. At the contact, the shales are 
altered and hardened, and there would appear to be almost a passage from the shales into 
the trap—the latter being marked by the dominant, divisional planes being vertical, while 
those of the shales are horizontal. From the statement, p. 57, 10th An. Rep., Minnesota 
Survey, Prof. Winchell considers similar traps in the vicinity of Pigeon River Falls to be 
dyke overflows—and this may likewise be true of those capping Thunder Cape and also 
the calcareous strata of Nipigon, both of which seem to shew almost a passage into the 
trap, a fact which was “also noticed by Logan, It is described on p, 28 of his Report for 1847, 
where he says: “The slope of the beds is about 5° and, as they approach the base of the trap, 
they appear to become obliterated, some of them proceeding further than others into the 
volcanic mass,” but in such a manner that it was difficult to say where any bed finally 
stopped. Whether there is more than one great “flow” represented is also uncertain. I am 
not aware of any single section in which two flows are shown ; but the great difference in 
the levels at which they are seen cannot be accounted for by the dip alone, but might be by 
faults. Big Island in Thunder Bay, for example, presents a section like that of Thunder 
Cape, some ten or twelve miles distant to the south. In the one the trap reaches the level 
of the water, in the other it is not less than 500 to 800 feet above it, while in the interven- 
ing distance along the shore of Black Bay, only narrow dykes are found cutting the black 
shales, these shales being immediately overlaid by conglomerate and red and white quart- 
zose sandstones, like the shales, in almost horizontal altitude, and which, passing up into 
the marls and dolomites, according to Dr. Bell, occupy nearly the whole of the peninsula 
between Thunder Bay and Black Bay, while the columnar trap is not again seen to the 
north-east for nearly fifty miles, or till we reach the Black Sturgeon and the Nipigon 
Rivers. Here heavy beds of coarse crystalline traps, showing vertical columnar structure, 
Sec. IV., 1883. 16 
