374 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
the slate for a short distance, as indicated in the preceding sketch, which: 
was taken on the north side of the south arm of Pigeon Bay. The slate- 
lamine run against the crystalline rock without alteration or disturbance, 
and the whole structure is an unmistakable one. The columnar rock is black, 
excessively fine-grained, and breaks with a conchoidal fracture. Under the 
microscope it shows a groundmass in which are recognizable tabular oligo- 
clases, rounded augite particles, magnetite, and some non-polarizing sub- 
stance. A few porphyritic oligoclases are seen, and the section is close to 
that of the so-called ashbed-diabases. It might very well be a very fine- 
grained phase of the coarser orthoclase-gabbro that forms the neighboring 
large dikes and overflows. 
The researches of Bell’ and Logan’ have shown that an area to the 
west of Thunder Bay, bounded on the north by a line from Gunflint Lake, 
on the national boundary line, to the Grand or Kakabika Falls, on the 
Kaministiquia, is very often underlain by the same series of slates that form 
the west side of Thunder Bay, intersected here by the same grand system of. 
dikes, which are often met with crossing the country in great walls or form- 
ing bold linear ridges. The same overflows and interbedded masses also 
occur, and often form the tops of table-like elevations. Among the boldest 
of these is the line of bluffs forming the south side of the lower stretch of 
the valley of the Kaministiquia, McKay’s Mountain, a portion of this 
elevation, rising to a height of 1,000 feet. Throughout all of this area the- 
flat southeasterly dip prevails, though at times there is a little variation from 
horizontality, while occasionally there is a slight slant northward for short 
distances. 
It is of the greatest interest in this connection that Bell reports the 
existence as part of the slate series in the valley of the Kaministiquia—as, 
for instance, at “a place called the Algoma mine”—of ‘“ thinly-bedded, 
flaggy, hard, dark-gray sandstone, largely composed of particles of mag- 
netic iron, and weathering to a rusty color.” There are exposed about 
twenty feet of the beds. An analysis showed 37.73 per cent. of metallic 
iron. ‘The same highly ferruginous sandstone, dipping very slightly east- 
1 Geology of Canada, 1863, pp. 67-70. 
2Geol. Survey of Canada, Report for 1866-’69, p. 321, et seq. 
