378 COPPER-BEARING ROCKS OF LAKE SUPERIOR. 
made any section of them. From Macfarlane’s analyses,’ however, it is- 
evident that the more arenaceous layers are made up chiefly of quartz- 
grains, along with some argillaceous and calcareous matter, while the shaly 
layers contain more of the argillaceous substance, which, from the presence- 
of alkalies, is plainly in the nature of decomposed and partly decomposed 
feldspars. Macfarlane also considers that the presence of some carbona- 
ceous matter is included in the loss on heating. The shaly layers never 
contain any lime or magnesia carbonates, but these carbonates are occa- 
sionally present in the more compact layers up to 20 per cent., or more. 
Rounding Thunder Cape, the slates are found continuing as far as. 
Silver Islet Landing, at which place they. are overlain by the white and 
red sandstones of the Keweenaw Series. Here, again, they are only a short 
distance above the water, an amount of sinking being thus indicated which 
the very flat southeastward dip will not account for, but which, as already 
shown, must be attributed to an intervening erosion. 
Between Thunder Cape and Silver Islet a large number of dikes are- 
seen cutting the slates. Only one of these dike rocks, which appear for 
the most part to be the same as those which form the numerous dikes of 
the southwest shore of Thunder Bay, already described, was examined. 
This is the rock which forms the dike at Silver Islet. It is a nearly black, 
rather fine-grained rock, distinctly composed of a greenish-black and a 
white mineral, the latter being, of course, the feldspar. According to- 
Macfarlane, its specific gravity is 2.7, and its silica content 53.34 per cent. 
It contains 5.02 per cent. of water, an amount indicating a considerable 
alteration; and this indication is fully borne out by a microscopic study 
of the thin section. This shows tabular plagioclases, with some ortho- 
-clases, as predominating ingredients. These feldspars are all much dulled 
by alteration, and are often penetrated by secondary quartz. In many 
places the larger feldspars have between them a mass of smaller, much 
crushed, and always highly altered feldspars. The augitic ingredient is only 
partly fresh, being commonly much altered to ocher and uralite, with which. 
alteration is connected the formation of some magnetite. Rather abundant 
titanic iron, for the most part altered to its characteristic gray decomposi- 
1Canadian Naturalist, New Series, Vol. IV, p. 37. 
