SILVER MINING IN COLOKADO. 593 



The ore consists chiefly of galena and zincblende with some iron and 

 copper pyrites ; with these are associated some rich silver minerals, compris- 

 ing stephanite, silver glance, fahlerz, some native silver, and, occasionally, ruby 

 silver. The galena is thought to be quite rich in silver, which may also be 

 true of the zincblende, but the great value of the ore is probably due to the 

 abundant occurrence of true silver minerals. The gangue mineral most fre- 

 quently found with those just named is crystallized quartz, with which are 

 associated fluor spar, heavy spar, and others in small proportions. 



The ore-seam, where seen by the writer, was generally compact, occur- 

 ring sometimes on the south wall and sometimes on the north wall of the 

 vein. Occasionally, however, it is split up, forming a number of parallel 

 seams. A narrow selvage of clay divides it from the wall of the vein, and 

 sometimes from the accompanying belt of gangue rock. The minerals com- 

 posing the seam are often crystallized, and vugs, or cavities with crystalline 

 linings, are frequently met with. 



Fig. 2, on Plate XXXV, represents a transverse section of this vein at 

 a point observed and sketched by Professor Schirmer, an educated mining 

 engineer, formerly residing at Georgetown and connected with the Terrible 

 mine. The alternating seams of ore and accompanying gangue are clearly 

 shown. According to the view of Mr. Schirmer, who has had occasion to 

 study closely the formation of this vein, the middle mass of gangue rock, e, is 

 of later origin than the other material on either side, having intruded itself 

 into the vein, widening the fissure, and producing the slickensides, or polished 

 surfaces, that may be observed on the planes of contact between it and the 

 older rock, a. This view is also supported by the occurrence, in the supposed 

 newer rock, of imbedded fragments of quartz several inches thick. 



Fig. 3, on same Plate, is a somewhat similar section, taken in the 

 Brown lode, a vein occurring near the Terrible; but in this case the difier- 

 ences between the masses of gangue rock are less clearly distinguished. 



The value of the ore may be best shown by the following statement. 

 The developments made in the mine comprise the sinking of the main shaft, 

 185 feet deep; the driving of the three levels shown in the section, the upper 

 one being 310 feet in length, and the stoping, which is also indicated. The 



whole extent of ground stoped, at the time referred to, was 60 or 70 fathoms. 



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