160 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
lower of the island, and the two together seem to indicate a total thickness 
of as much as 25,000 feet 
About Black and Nipigon bays the Keweenawan flows dip at a low 
angle to the southeast and south, and reach a total thickness which does 
not exceed 7,500 to 8,000 feet, measuring from the older rocks upward. 
The greater part of the Lower Division must here be concealed beneath the 
lake. ‘ 
Michipicoten Island, according to Macfarlane’s measurements, displays 
a total thickness of 18,500 feet, all of which belongs to the Lower Division, 
neither limit of which is in sight. From the position of the island it appears 
that the thickness here must be 25,000 feet or over. At the promontory of 
Mamainse, again, according to the same authority, there are 16,000 feet of 
Keweenawan strata displayed, all belonging to the Lower Division. This 
thickness is measured from the base of the series, and does not reach as 
high even as that of Michipicoten. Judging from the lithological characters 
of the Michipicoten and Mamainse successions Macfarlane considers the 
rocks of the former to rise to a higher horizon by 4,000 feet, and so esti- 
mates the thickness of the Mamainse series at 20,000 feet, without any in- 
dication that the upper limit is reached. 
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