206 KEMP'S ORE DEPOSITS. 



posures. Considerable masses of rocks formed of fracjmental 

 ejectamenta are also known. All these are crossed by immense 

 vertical veins, largely with quartz gangue, and containing argen- 

 tiferous minerals of the usual species, galena, tetrahedrite, pyrar- 

 gerite, and native silver, as well as bismuth compounds. Gold 

 has been quite subordinate, although late developments near Ouray 

 have shown some peculiar and interesting deposits. R. C. Hills, 

 in the Proc. Colo. Sci. Soc., 1883, traced three systems of veins. 

 (1) Silver-bearing, narrow (six inches to three feet), nearly verti- 

 cal veins, with base metal ores and no selvage. (2) Large, strong, 

 gold-bearing veins dipping 60 with selvages and intersecting (1). 

 (3) Like (l), but larger and more persistent, and carrying occa- 

 sionally bismuth and antimonial ores with gold and little or no 

 silver. T. B. Comstock (M. JK, XV. 218) has classified the veins 

 in three radiating systems. (1) The northwest, with tetrahedrite 

 (freibergite). (2) The east and west, with bismuth and less often 

 nickel and molybdenum. (3) The northeast, with tellurides and 

 antimony and sulphur compounds of the precious metals. Quite 

 recently a series of small caves near Ouray, in quartzite overlaid 

 by bituminous shales, have been found to contain native gold, and 

 have excited great interest. It is thought by Endlich that they 

 represent inclusions of shale, now dissolved away, and that, the 

 gold was precipitated on the walls. If this view is correct, they 

 mark one of the very few illustrations of chamber deposits which 

 are known. More extended mining work has proved them to be 

 in all cases connected with a supply fissure from which small lead- 

 ers guide the miners to the chambers. 



Placer gold mines (Example 44) are quite extensively worked 

 in San Miguel County. J. B. Farish has recently described the 

 veins at Newman Hill, near Rico, in a valuable paper cited below. 

 The lowest formation exposed is magnesian limestone, supposed 

 to be Carboniferous. It contains large ore bodies of low grade, 

 and is also, strangely enough, heavily charged with carbonic acid 

 gas. Above this for 500 feet are alternating sandstones and 

 shales, and then a narrow stratum of limestone 18 to 30 inches 

 thick. This is followed by about 500 feet additional of shales and 

 sandstone, regarded as Carboniferous. Fifty feet above the low- 

 est limestone a laccolite of porphyrite has been intruded. Two 

 sets of fissures are present one nearly vertical and striking north- 

 east, the second dipping 30 to 45 northeast and striking north- 

 west. The former are the richest, are banded (see Fig. 5) and per- 



