302 A. P. COLEMAN — ROCK BASINS OF HELEN MINE 



terranean leaching had progressed far, crumbling the brecciated rocks 

 which filled it, and that when this process was complete or nearly so 

 the basins were excavated in parts of the bed more easily soluble than 

 others, the carbonates of iron, lime, and magnesia, which make up nine- 

 tenths of the rock, being removed* and the silica left behind in the 

 basins as part of the green mud covering the bottom to depths of from 

 20 to 60 feet. An analysis of this mud, made in the laboratory of the 

 Lake Superior Power Company, at Sault Sainte Marie, Ontario, showed 

 that more than 47 percent of it is silica, the next largest solid ingredient 

 being iron, which forms 11 per cent. 



This account of the hollowing of the lake basin seems to conflict with 

 Professor Willmott's theory of the formation of the ore body, which he 

 supposes to have been deposited in a cavity or cavities before the over- 

 lying iron-range rocks had been removed, for the ore apparently occupies 

 the east end of the Boyer Lake basin, and, if so, must have been deposited 

 after that was formed. It may be, however, that the ore, which has 

 been found by boring to go 188 feet below the original lake level, repre- 

 sents largely or entirely the oxidation of siderite in place, and that the 

 lake basin is later in age and formed partly by solution of the ore itself. 



It is of interest to note in this connection that iron is still going into 

 solution and still being deposited in the basin, for at the east end of the 

 lake, north of the ore body, a considerable thickness of bright yellow 

 ocher containing 49 <} per cent of iron, was discovered when the water 

 was lowered by cutting down the outlet, evidently a distinct and much 

 later deposit than the ore of the mine, since it rests in part on the lower 

 slope of the mass of ore. In addition all the rock surfaces which had 

 been below water are covered with a film of bog or lake ore, which in 

 some places forms flat concretionary rims at certain levels or round 

 cakes a few inches across and half an inch thick in the upper part of the 

 green mud. These deposits represent work done since the Ice age, for 

 they are found on glaciated surfaces and on drift boulders ; and if we 

 multiply the thousands of years since the ice departed to equal the mil- 

 lions of years of pre-glacial action, perhaps the present rate might ac- 

 count for the great mass of ore at the mine, if all was accumulated at 

 one place instead of being spread over the basin. Whether more ma- 



*See Bureau of Mines of Ontario, 1901, p. 193, for analysis of the siderite : 



1. 2. 



Insoluble 4.38 per cent. 14.76 per cent. 



Carbonate of iron 78.57 " " 74.07 " " 



Carbonate of magnesia 12.84 " " 8.75 " " 



Carbonate of lime 4.09 " " 0.79 " " 



Total 99.88 " " 98.37 " 



