3O NEW YORK STATE MUSEUM 



origin. As to the second type, the evidence is so strong that the 

 zinc is deposited by vapors or hot waters of magmatic origin that 

 there is a very general acceptance of this theory of the genesis of 

 the deposits. 



In the case of the Edwards deposits, if the sulphides gave evi- 

 dence of having been deposited before metamorphism, it might be 

 argued that, since they occur in limestone, they belonged to the 

 first type, their association with igneous rock being a mere coin- 

 cidence. But, as the ores were introduced after the development of 

 the metamorphic silicates by the intrusion of granite, and during 

 a later stage of the metamorphic process, the conclusion is well- 

 nigh unavoidable than this intrusion of granite was the cause of 

 ore formation, while similar genetic relations in a large number of 

 cases, elsewhere, give added strength to the hypothesis. 



Blende is not, like some minerals, exclusively of high temperature 

 origin but, on the contrary, may form under a wide range of con- 

 ditions according to Lindgren, 1 from contact metamorphic con- 

 ditions nearly to surface conditions. It is a fact, however, as 

 previously stated, that the dark-colored, ferruginous blende is a 

 variety particularly characteristic of contact zones; but while this 

 may be taken as a suggestive indication, it can hardly be regarded 

 as conclusive. The mere presence of blende gives no positive evi- 

 dence as to the conditions under which it was formed. On the 

 contrary, these conditions must be determined by a study of the 

 geology of the ore deposit as a whole and the mineral association 

 of the blende. 



When, as often happens, blende occurs in nonmetamorphosed 

 limestones, associated with ordinary low-temperature vein minerals, 

 the natural inference is that the blende was gathered from a dis- 

 seminated condition in the sediment and deposited by meteoric 

 waters. When, however, as in the present instance, blende occurs 

 in contact metamorphic rocks, intimately associated with contact 

 minerals and their hydrothermal alteration products, a magmatic 

 source for the mineral is strongly indicated. 



In the Edwards deposits, as shown above, the blende is later than 

 the typical contact metamorphic minerals (diopside, tremolite etc.) 

 formed at high temperature, and precedes the alteration products of 

 these minerals (talc, serpentine etc.), which are formed at lower 

 temperature, in the later stages of contact metamorphism. 



If a long interval be assumed between the sulphides and the 



1 Lindgren, Waldemar, The Relation of Ore-Deposition to Physical Con- 

 ditions ; Econ. Geol. 1907, 2:122. 



