32 NEW YORK STATE MUSEUM 



granitic intrusions, and were formed wholly under deep-seated con- 

 ditions and as a product of magmatic agencies. 



This hypothesis is essentially identical with that applied by the 

 writer 1 some years ago to the pyrite deposits of St Lawrence county, 

 although, in the latter case, the intervention of superficial agents was 

 recognized as a not improbable contributing factor. Further con- 

 sideration of these deposits has served to accentuate the potency 

 of magmatic agents in their formation and to eliminate superficial 

 conditions, thus bringing their origin into close agreement with that 

 of the zinc ores. The two groups of ores have much in common 

 and their history seems to be essentially the same. Certain striking 

 differences, however, exist in the character of the typical deposits, 

 as to composition and wall rock. 



The first of these differences appears in the fact that the zinc ores 

 thus far found always carry considerable pyrite, while the typical 

 pyrite ores, on the other hand, are nearly free from blende. The 

 writer found, at the Stella pyrite mines, two vugs, obviously of late 

 date as compared with pyrite ore, lined with crystals of blende and 

 quartz; while Mr Newland has seen one specimen of pyrite ore 

 which was a granular aggregate of pyrite and blende, but the rarity 

 of these exceptions serves to accentuate the rule. 



A second difference in composition between pyrite and zinc ores 

 lies in the fact that the former are notably rich in graphite, while 

 the zinc ores contain little or none, in spite of the fact that they 

 occur in crystalline limestone, a rock that, throughout the region, 

 often carries graphite. 



As to wall rock, the zinc ores are always in limestone, while the 

 typical pyrite ores are always in schist or gneiss, even when lime- 

 stone occurs within a few feet. This latter fact is paralleled by 

 many cases mentioned in the literature, but no entirely satisfactory 

 explanation of it has been given. 



As to the constant occurrence of the zinc deposits in limestone, it 

 would appear that, as zinc elsewhere shows a strong tendency to be 

 concentrated in limestone, this rock must, on account of its 

 chemical nature, exert a precipitating action on zinc compounds. 

 The possible nature of such action will be referred to later. 



This influence of the limestone suggests an explanation of the 

 absence of zinc in the typical pyrite ores : Zinc and pyrite solutions 

 carrying the constituents of both types might traverse purely 

 siliceous rocks, like the schists, and deposit pyrite, but not blende, 



1 Smyth, C. H. jr, On the Genesis of the Pyrite Deposits of St Lawrence 

 County, N. Y. ; N. Y. State Mus. Bui. 158, 1912, p. 143-82. 



