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



dently the pyrite is not very readily decomposed, though the pyrrhotite 

 found in some parts of the valley is attacked rapidly. 



The solvents mentioned, carbonic acid and sulphuric acid, appear to 

 be the only ones which could be brought to bear under all the condi- 

 tions, unless the work was done at considerable depths, when water, 

 under pressure and more or less heated, would become an effective 

 solvent, which might attack the silica as well as the other components 

 of the rock. The last supposition, which seems very probable in early 

 times, when the breccias of granular silica were cemented with fresh 

 silica, can not hold for later times, when the valley was open to the air. 



Supposing the solvents at hand, why should certain parts of the iron 

 range be attacked and removed, while others, such as the top of Hema- 

 tite mountain, have shown a very high degree of resistance? Two rea- 

 sons suggest themselves to account for this : The shattering of the iron 

 range during the folding process was apparently quite local, and at the 

 shattered points waters charged with solvents could circulate, while they 

 were excluded from the unshattered portion of the range ; and again, the 

 materials of the iron range over the two basins may have been different 

 in composition from the rest and more easily attacked — for example, 

 there may have been unusual quantities of pyrrhotite at those points. 

 It is not impossible that both assumptions are true, for the pyrrhotite 

 which still occurs between the two lakes on the north side of the valley 

 is not a strong nor tough ingredient of the rock, and if present in large 

 quantities would probably render it more frangible than the adjoining 

 rock, as well as more soluble. 



Relation of the Iron Ores to the Basins 



A thorough understanding of the processes which made the great mass 

 of iron ore at the east end of Boyer lake would, no doubt, help greatly 

 in explaining the origin of the valley and its rock basins, and a brief 

 study of the known facts will be of service here. Unlike most of the ore 

 deposits of the iron ranges of Michigan and Minnesota, the ore of the 

 Helen mine is completely free from any capping of silicious iron-range 

 rock. Following the account of Professor Willmott, who has studied it 

 carefully, it stands as a separate hill, sloping in all directions from the 

 center and rising 100 feet above the original level of the lake, covered, 

 however, to some extent round the edges, toward the north, east, and 

 south, by boulder clay and debris rolled from the walls of the valley. 

 The ore, which consists of hematite mixed with limonite, is of a some- 

 what open, porous character, the walls of the cavities being lined with 

 more or less concretionary or stalactitic limonite, and from its arrange- 



