[graham] PHENACITE 193 



composed almost entirely of unaltered molybdenite, with only an 

 occasional layer replaced by the granular bismuthinite. 



It can hardly be doubted that the whole specimen consisted 

 originally of molybdenite alone, of which mineral two large crystals 

 had grown in parallel position, united by a pyramid face, as shown 

 diagrammatically in Plate I, figure 2, with several smaller crystals 

 grouped irregularly upon them. 



Although the association of bismuthinite with molybdenite is 

 by no means uncommon, pseudomorphs of the one after the other have 

 not hitherto been recorded. In the present occurrence, a study of 

 the veins has shown that the bismuthinite was deposited later than 

 the molybdenite, and it would seem that, where the exhalations or 

 solutions carrying the former have encountered crystals of the latter, 

 they have, under certain conditions, been able to effect a replacement 

 of the molybdenite by bismuthinite. 



Figure 2. 



In the particular case of the specimen described above, there 

 remains the difficulty of accounting for the fact that only one half 

 of it has been so affected, while the other half has undergone no 

 change. It might be suggested that the individual plates of molybd- 

 enite, whose aggregation has built up the larger crystals, were less 

 closely compacted in one case than in the other; or that the sur- 

 roundings of the specimen within the vein were of such a character 

 that, while one portion of it was freely exposed to the ascending bis- 

 muthinous fluids, the other was entirely . protected from their 

 influence. 



The only other minerals present on the specimen are quartz and 

 muscovite, both in small amount. 



Phenacite 



A specimen collected in 1910 by Dr. J. A. Bancroft from a peg- 

 matitic quartz vein, near the northern end of the Height of Land 

 Mining Company's property, proved on examination by the writer 

 to be composed partly of phenacite. The quartz of the vein is of the 



