214 L. Y. Pirsson — Crystallization of Enargite. 



these localities as either massive, or in rude columnar structure 

 with cleavage surfaces or those of striated prisms, and this is 

 confirmed by an excellent suite of specimens from the above 

 occurrences in the Brush collection. 



More recently enargite in considerable quantities has been 

 discovered in various places in Colorado ; and that from the 

 mines at Red Mountain, especially, furnishes material suitable 

 for crystallographic investigation. A very fine suite of speci- 

 mens from the National Belle Mine was collected by Prof. 

 S. L. Penfield during the summer of 1891, while engaged in 

 work for the IT. S. Geological Survey. For the purpose of 

 carrying out the present investigation he has kindly placed 

 these specimens at the disposal of the author, who takes this 

 opportunity of expressing his thanks for the free use of the 

 material. 



The mineral, which is here mined for silver and copper, 

 occurs in vertical shafts or "chimneys" in a decomposed vol- 

 canic rock. It generally occurs accompanied by pyrite and is 

 in large cleavable masses, rarely in distinct crystals. When 

 crystallized it is usually in thick striated prisms up to two or 

 three mm. in length and of the habit shown in fig. 2, consisting 



Fig. 2. 



Fig. 3. 



Fig. 4. 



m 



m 



a v 



of the simple forms c, O (001) ; a, i-l (100) ; m, /(HO) and 

 A, i-2 (120). In some specimens the faces are more even and 

 the unit brachydome s. \-% (011) appears as shown in fig. 3. 

 On one specimen crystals were present which showed the unit 

 macrodome k, \-l (101) and in addition a prominent and well 

 defined brachydome was seen. This was invariably so rough, 

 from small pit-like markings and minute octahedra of pyrite 

 that only an approximate measurement of it could be made. 

 Measured on the base it gave an angle of 25° while 012^001 

 requires 22° 25'. It is therefore regarded as being most proba- 

 bly the \ brachydome. Besides the planes already mentioned 

 these crystals showed the macroprism v, i-2 (210) and their 

 habit is represented in fig. 4. 



Another very common habit, from this locality, is a flat tabu- 

 lar one, both bases being present and the crystal is attached by 

 one end of the macro-axis. This is shown in fig. 5. The 



