338 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
A fine-grained, close-textured, brownish-black rock, with semi-con- 
choidal fracture, from a large island near Cabmous-Neiding Point, thirteen 
miles southwest of Lamb Island Light, at the mouth of Nipigon Straits, 
proved to be a typical diabase or diabase-porphyrite, having as its con- 
stituents tabular plagioclase (anorthite), abundant augite in irregularly 
rounded granules, titaniferous magnetite, and much of an interstitial sub- 
stance now largely stained by red and brown iron oxide. 
A similar but more dense and less highly augitic rock forms the small 
islets at the west side of the mouth of the large bay of which the Roche de 
Bout is the eastern cape. S 
A coarse-grained black- and-white-mottled rock, which forms the north 
point of the large island lying directly in the mouth of Nipigon Straits, 
turned out to be a typical orthoclase-gabbro, with plagioclase, orthoclase, 
twinned diallage, large augite granules, titaniferous magnetite, and second- 
ary quartz as the constituents. A similar rock from the mainland immedi- 
ately to the northwest of the last rock yielded the same results in the thin 
section. 
Farther northward along the west side of Nipigon Straits the following 
rocks were encountered, among others: (1) A medium-grained to fine- 
grained, black rock, carrying numerous large brownish translucent por- 
phyritic plagioclases, which, in the thin section, shows a wholly crystalline 
groundmass, consisting of plagioclase, orthoclase, augite partly in grains, 
and partly filling the interstices between the feldspars, titaniferous magnetite, 
secondary quartz, and epidote. The porphyritic feldspars yield the polari- 
zation angles of labradorite. The rock lies between the ashbed-diabases 
and the orthoclase-gabbros. (2) A fine-grained, brownish-black rock con- 
sisting of fresh tabular labradorites, arranged so as to indicate flowage, 
abundant and partly fresh augites and magnetite. The rock is a diabase of 
the ordinary type. (3) A black, conchoidally fracturing, aphanitic ashbed- 
diabase consisting of predominating tabular labradorites, augite in grains, 
and a little magnetite. 
NIPIGON LAKE BASIN. 
The red sandstones, shales, and marls which in the region of Thunder, 
Black, and Nipigon bays lie at the base of the Keweenaw Series extend, as 
