460 THE CANADIAN NATURALIST. [Dcc. 



sandstones, which have been described as succeeding, in ascend- 

 ing order, the same grey ai'gillaceous sandstones on the eastern 

 part of the location, are found to crop out beneath the crystalline 

 covering, and to be unconformably overlaid by the latter. An 

 attempt has been made to represent the relations of these various 

 rocks in the section which accompanies the map. From this it 

 will be apparent that the summit rock of Thunder Cape is not a 

 bed iuterstratified between the grey argillaceous and the white 

 and red dolomite sandstones, but an overflow which has spread 

 over both these groups after their deposition and partial dis- 

 turbance. The cliffs at the east end of Thunder Cape would 

 form the eastern extremity of this overflow, unless it should be 

 found that the rock of the Paps between Black Bay and Lake 

 Superior, and those of other overflows in the Nipigon dis- 

 trict, resemble that of Thunder Cape. Southwest from Thunder 

 Cape large areas are occupied by the same rock, which contri- 

 butes in a marked degree to form the picturesque coast lying 

 between Fort William and Pigeon River. 



The rock whose geological relations have just been described, 

 and which, with its roughly columnar structure, adds so much to 

 the imposing appearance of Thunder Cape, is very hard and 

 crystalline, and exhibits no appearance of stratification or paral- 

 lelism among its constituent minerals. It is very little acted on 

 by the atmosphere, but separates into large rectangular Mocks, 

 which, becoming detached from the rock above, form an enormous 

 talus at the bottom of the cliffs, sometimes completely obscuring 

 the debris from the underlying strata. The rock is readily recog- 

 nizable as a compound one, its constituents being of distinctly 

 different colours. These vary in size from one-twentieth of an 

 inch in diameter to very minute grains, and in general the fine- 

 grained varieties are found immediately over the underlying 

 strata, while the coarse-grained varieties are more frequent on 

 the summit of the cliffs. The principal constituents are greyish 

 white feldspar and black hyperstheue, the former mineral being 

 generally the most abundant. The hypersthene is but slightly 

 influenced by the atmosphere, and on exposed surfaces retains 

 most of its lustre and hardness, while the accompanying feldspar 

 is bleached and decomposed. A softer dark greenish coloured 

 mineral, probably hornblende, is also present, as well as magnetite 

 m small quantity. It would therefore appear that the rock in 

 (juestion is hypersthenite, or, as the name has been more recently 



