508 THE ORfe-DEPOSITS OF BORAH CREEK, 



The second consists of masses of crystallised quartz, in whicli 

 many of tlie crystals have been attacked— some wholly, and 

 others partially replaced by zinc-blende. About one-half of the 

 original material has been attacked in this phase (Plate xv., 

 fig.2,B). 



The third consists of masses of zinc-blende, most, if not all, of 

 which has replaced the crystallised quartz. There still remain, 

 strangely enough, odd crystals of quartz which have quite pre- 

 served their integrity, though completely surrounded by zinc- 

 blende(Plate xv., fig.2,C). 



These facts have suggested to me that, in metasomatic changes, 

 the molecular structure of the crystals plays a very important 

 part. It would appear that a complete crystal may preserve its 

 integrity even where a wholesale metasomatic change is taking 

 place, and yet, as in the first phase, another crystal may entirely 

 break down in the midst of a resisting body of crystals. This 

 has suggested, further, that if, by some means, a portion of a 

 complete crystal be destroyed, the remaining part is in a state of 

 unstable equilibrium, and is thus much more readily attacked 

 than a complete crystal. This miglit be explained by supposing 

 that each molecule is less strongly attached to the crystal by tho 

 attractive forces of its fellow-molecules on a fractured surface, 

 than when in a complete crystal. This may also explain the 

 readiness with which metasomatic processes, in general, operate 

 .along cracks in various minerals, as sericite after quartz, etc. 



Galena was also found crystallised in the quartz in juxtaposi- 

 tion to the metasomatic zinc-blende, but no- definite evidence 

 could be obtained to indicate that it was a metasomatic product. 

 It is probable, however, that such is the case. 



At various places throughout the mine, numerous vughs occur. 

 These are lined with large quartz-crystals projecting inwards 

 towards tlie centre of the cavities. The vughs vary in size from 

 a few inches to several feet in diameter; and are generally bean- 

 shaped, and have their long axes vertical. It is worthy of note 

 that these cavities frequently contain hydrous iron-oxides. 



