94 F. R. VAN HORN PROUSTITE AND ARGKNTITE FROM COLORADO 



of the specimens under observation. In hand specimens the foliation of 

 the gneiss is not always distinct, and the rock is locally known as granite. 

 The vein varies in width, and frequently has a massive granular appear- 

 ance, but sometimes possesses a banded structure due to symmetrical 

 deposition of minerals ; at other times a more or less brecciated appear- 

 ance is noticeable, which was probably caused by movements of the vein 

 walls on each other. The surrounding region is more or less faulted, and 

 at one place in the mine, the main ore body has been dislocated and shows 

 slickenside surfaces. The vein is separated from the gneiss by a wide 

 clay selvage, or gouge, which causes more or less trouble by caving in. 



Principal Ore of Mine and Assays of Same 



The ore occurs in streaks in the vein, varying from 1 to 21 inches in 

 width, and consists principally of argentiferous galena, although in places 

 large amounts of sphalerite are mixed irregularly through it. As a rule, 

 both galena and sphalerite are coarsely granular, the crystals of each vary- 

 ing in size up to 3 inches in diameter. Some assays were made, which 

 show an interesting variation of silver and lead values in the different pay 

 streaks of the vein as follows : 



No. 1, 1 inch wide; silver, 86.5 ounces per ton; lead, 34 per cent. 

 No. 2, 2-3 inches wide ; silver, 11.0 ounces per ton ; lead, 33 per cent. 

 No. 3, 4 inches wide ; silver, 17.0 ounces per ton ; lead, 65 per cent. 

 No. 4, 4-5 inches wide ; silver, 34.0 ounces per ton ; lead, 64.5 per cent. 

 No. 5, 5 inches wide ; silver, 19.0 ounces per ton ; lead, 47 per cent. 

 No. 6, 14 inches wide ; silver, 25.8 ounces per ton ; lead, 80 per cent 



An inspection of the above assays shows that the streak which ran high- 

 est in silver values contained the lowest percentage of lead, and that the 

 one with highest lead contents gave results low in silver. This shows that 

 the amount of silver does not depend on the quantity of galena present. 

 It might be suggested that the variations in silver values could be ex- 

 plained by the presence of proustite and argentite mixed with the galena 

 and sphalerite. However, the above assays were made long before the 

 discovery of the rich proustite-argentite ore which will be described later 

 in this article, and, as far as the writer can learn, those minerals were 

 never found in the mine before the present discover}^ Therefore it would 

 seem that the variations in silver were probably caused by argentiferous 

 galena in which the silver contents varied considerably. Possibly there is 

 not sufficient evidence to warrant the speculation, but it occurs to the 

 author that the different streaks might possibly have been deposited from 



