26 NEW YORK STATE MUSEUM 



Upon these grains in all stages of alteration, from perfectly fresh 

 to complete pseudomorphs of serpentine, the very frequent veining 

 of blende by serpentine, the absence of cracks in the serpentine, and 

 the character of the blende itself, all point to the deposition of the 

 major part of the blende before the period of hydrothermal altera- 

 tion, although, as elsewhere stated, some of the younger blende 

 seems clearly contemporaneous with the serpentine. 



In other words, the main deposition of blende, dark colored and 

 ferruginous, occurred at relatively high temperature as compared 

 with that of the hydrothermal alteration, but, during this latter stage, 

 limited quantities of blende continued to be formed, containing less 

 iron and having a correspondingly lighter color. 



To revert to the curve as representing time and temperature con- 

 ditions, diopside and related minerals being at its summit, indicating 

 the period of intense metamorphism, there can be no doubt that 

 the ores belong on the descending portion, which is also true of the 

 hydrous silicates. The former, according to the foregoing inter- 

 pretation, begin higher up and extend, though perhaps with a 

 break, beyond the point at which the latter begin. Indeed, it is 

 probable that the youngest, light-colored blende was deposited late 

 in the period of hydrothermal alteration. 



If the curve were continued down far enough to represent 

 weathering conditions and their products, the gap between these 

 and the hydrous silicate would, it is thought, be much greater than 

 between the latter and the diopside. 



SPHALERITE OF LATER DEPOSITION 

 While a very large proportion of the blende belongs to the one 

 continuous period of formation, there is, as stated above, a rela- 

 tively small amoimt of distinctly later deposition. In many cases, 

 grains of the ordinarv^ deep reddish, brown blende have been frac- 

 tured and subsequently recemented by a further deposition of the 

 same mineral. This later blende is distinguished from the earlier by 

 being pale gray or colorless, and it may occur as narrow borders 

 or projections on the margins of the cracks or may become quite 

 compact, entirely filling the cracks and forming a margin around 

 the whole cemented mass. When grains of the latter type are 

 viewed by reflected light, they present the appearance of a breccia 

 made up of dark-brown, angular fragments of older blende 

 cemented by the nearly white, younger blende. 



While a difference of age between the dark and light blende is 



