298 A. P. COLEMAN ROCK BASINS OF HELEN MINE 



The crushing took place after the silicious part of the iron range had 

 consolidated in the shape of polygonal crystalline grains, but before the 

 deposit of granular silica was complete, for the angular fragments of 

 white "grained" silica are cemented by less pure brownish silica hav- 

 ing the same structure. It is probable that the crushing of the iron- 

 range rocks was one of the most important factors in the hollowing of 

 the valley and its rock basins, since it gave opportunities for the circu- 

 lation of fluids. 



Probable Causes of the Rock Basins 



We are now prepared to consider the forces which excavated the basin. 

 It is evident that ice has done a great amount of work in the region, 

 since blocks of ore weighing several tons are found on top the ridge 

 toward the south, no doubt transported from the mass of ore beneath, 

 now forming the mine at the east end of the valley, and it is also clear 

 that the ice had at least some effect at the very bottom of the valley, for 

 a shelving point of rock on the south side of Boyer lake, exposed by the 

 partial drainage referred to before, is smoothed and striated, the motion 

 of the ice being in a direction 10 degrees east of south across the strike 

 of the schists and the direction of the ridges on the north and south. 



However, the action of ice can have had little effect in scouring 

 out a trough which lies transverse to its direction of motion, and 

 there is good evidence to prove that the original hollowing of the valley 

 took place, at least in part, long before the Glacial period, for the great 

 ore body which occupies its eastern end is pre- Glacial in age, as shown 

 by the erratic blocks of ore to the south and by the finding of glacial 

 striae on top of the hill of ore by Doctor Bell* The shape of the valley, 

 with its steep, sometimes almost vertical, northern wall, is such as could 

 not be accounted for by the action of ice coming from the north, though 

 a flow of ice toward the west, of which there is no evidence, might have 

 done work of the kind. 



Nor can we consider running water capable of cutting the valley 

 with its rock basins unless under conditions very different from the 

 present ; for the whole surrounding region is distinctly lower than the 

 rock walls of the valley, which slope away in all directions, except along 

 the narrow ridge of Hematite mountain, and afford no opportunity for 

 a stream to enter the valley. At present there is no stream flowing into 

 Boyer lake, and the small stream flowing out of it represents only the 

 rainfall of the valley itself. It has done practically no work since the 

 Ice age, though running over a somewhat decomposable rock, silicious 



*(ieol. Survey of Canada, Summary Rept., 1900, p. 116. 



