136 REPOET UNITED STATES GEOLOGICAL SURVEY. 



rare occurrence is that of tlie pitchblende. It is to be regretted that 

 the mine is not worked at present, and has not been for several years. 

 Thus much that might be learned as to its distribution in the vein is 

 lost. 



Among those minerals classed as "ores," the argentiferous species are 

 prominent in Colorado. Compounds of sulphur, antimony, tellurium, 

 bismuth, arsenic, and other metals and metalloids with silver, are found in 

 varying ratio. Galenite may be regarded as invariably silver-bearing. I 

 have made more than a hundred assays of western galenites and have 

 never failed to find the precious metal. It remains to be said that the per- 

 centage is highly variable, and that, as a rule, it is small unless argentite 

 be present in the mineral. This is not unfrequently the case. It may 

 be noticed that many very coarse-grained galenites show, upon break- 

 ing, dark gray, or black, dull, cleavage-planes. This is, in many in- 

 stances, produced by a very thin coating of argentite. 



It is not to be supposed that the number of minerals occurring at any 

 particular locality could be fully ascertained by members of the survey 

 while examining the region. During the regular field-work only such 

 mineral localities will be obtained that happen to be found more or less 

 accidentally. It may be hoped, therefore, that in future years more 

 knowledge will be gained regarding the non-metalliferous minerals. Of 

 these the enumeration at the present time is rather meagre. 



Thus far but comparatively few ejugene minerals have been found in 

 Colorado, considering the large number of mines worked. With increas- 

 ing depth of the mines, and time, no doubt the mineralogist will event- 

 ually be rewarded, and will find many a secondary mineral-product that 

 now he looks for in vain. Upon undisturbed dumps a few such species 

 have been collected. 



Should further discoveries, or more comjilete examination of the min- 

 eral regions already known warrant it, I propose to prepare another 

 edition of the catalogue whenever such preparation may seem advisable. 



Agtinolite. — In radiated form, of light- green and bluish green color, on 

 Mount Ouray ; on Buffalo and Sopris Peaks ; crystallized at Bergen's 

 Eanch, Jefferson County ; on Boulder Peak. 



Agate. — See Quartz. 



Alabandite.— At Quartzville. 



Alabaster. — See Gypsum. 



Albite. — Quartz Hill, Central City ; Gold Hill, Boulder County. 



Almandite. — See Garnet. 



Allophantte. — Franklin Mine, in Gilson Gulch ; Fowler and Wells's 

 Branch, Sugar- Loaf district. 



Altaite. — Red Cloud and Cold Spring Mines, Gold Hill. It occurs in 

 various mines of the Sunshine district ; minute crystals were obtained 

 from the Eed Cloud. Analyses, published by Genth, * show the follow- 

 ing result. The specimen was from the Bed Cloud Mine : 



(1) (2) 



Per cent. Per cent. 



Quartz. 0.19 0.32 



Gold 0.19 0.16 



Silver 0.62 0.76 



Copper 0.06 0.06 



Lead 60.22 60.53 



Zinc 0.15 0.04 



Iron 0.48 0.33 



Tellurium 37.99 37.51 



99, 90 99. 74 

 "~ * Proc. Am. Phil. See, Philadelphia. Vol. XIV., p. 226, 1876. ' 



