CAUSES OF THE BASINS 299 



siderite, heavily charged with p} T rites and sometimes also pyrrhotite, 

 and the same is true of the slightly larger stream draining Sayers lake 

 into Talbot lake, which begins its course by tumbling over rock very 

 similar to that at the outlet of Boyer lake. In any case, one could not 

 imagine any stream short of a powerful waterfall excavating such deep 

 basins, 133 feet below the level of the outlet, even if it had previously 

 dug the valley down to its present slope. 



There is no evidence of late faulting to produce the basins, and warp- 

 ing of the crust is out of the question for cavities so small; so that we 

 are reduced to solution as their cause. 



The materials inclosing the basins are largely of a soluble kind — sid- 

 erite, pyrite, pyrrhotite, and iron oxides — but there must have been a 

 very large amount of silica removed also and of a very insoluble variety, 

 for the granular silica of the iron range is not opaline nor even chalce- 

 donic, but distinctly crystalline, as seen in thin sections, and not only 

 the banded rock, but also the massive siderite, contains a considerable 

 percentage of these quartz grains, which have to be disposed of in some 

 way. In connection with this it may be mentioned that a pale-gray, 

 flour-like silt found in large quantities a few miles southwest of the mine 

 consists of silica, no doubt derived from the iron range, though not nec- 

 essarily from this part of it. 



The Action of Solvents 



The solvent that one thinks of first is carbonic acid, obtained by sur- 

 face waters in the soil of the hills around, and this is no doubt capable 

 of attacking the impure carbonate of iron which formed a large part of 

 the rock to be removed. It is a curious fact, however, that the summit 

 of Hematite mountain, the highest point for many miles, consists of sid- 

 erite which has been weathered to the depth of a half inch, or at most a 

 few inches, since the Glacial period, but has not been removed bodily 

 since there still remains a crust of limonite well shown in certain costean 

 pits across the top of the ridge. 



The only other solvent likely to have been present is sulphuric acid, 

 resulting from the action of water and oxygen on the pyrites distributed 

 in large amounts through all the country rocks of the valley and form- 

 ing more than half the volume of the rock at some points, as on the 

 south shore of Sayers lake, where a prospector has driven a tunnel into 

 the pyritous mass. However, caution is necessary in assuming that 

 products of the decomposition of pyrites were the only important solv- 

 ents, for it is often found that inside a mass of siderite completely 

 changed to limonite the crystals of pyrites are almost untouched. Evi- 



