THE PROBLEM OF THE METALLIFEROUS VEINS. 195 



stratum, but in some such form it must exist. Naturallj^ porous 

 rock aftords the simjolest case and provides an easily understood 

 place of precipitation. For example, in the decade of the seventies 

 rather larg-e mines at Silver Reef in southern Utah were based upon 

 an open-textured sandstone into which and along certain lines silver- 

 bearing solutions had entered. Wherever they met a fossil leaf or 

 an old stick of wood which had been buried in the rock the dissolved 

 silver was precipitated as sulphide or chloride. Sometimes for no 

 apparent reason the solutions impregnated the rock with ore, but the 

 ore seems to follow along certain lines of fracturing. Again, at 

 Silver Cliff, near Rosita, in central Colorado, the silver solutions had 

 evidently at one time soaked through a bed of porous volcanic ash 

 and had impregnated it with ore, which, while it lasted, was quarried 

 out like so much rock. In the copper district of Keweenaw Point, on 

 Lake Superior, the copper-bearing solutions have penetrated in some 

 places an old gravel bed and impregnated it with copper; in other 

 places they have passed along certain courses in vesicular lava flows 

 and have yielded up to the cavities scales and shots of native copper. 



It has happened at times that the ore-bearing solutions, rising 

 through some crevice, have met a stratum charged with lime, and 

 having spread sidewaj^s have apparently been robbed of their metals 

 because the lime j^recipitated the valuable minerals. In the Black 

 Hills of South Dakota there are sandstones with beds of calcareous 

 mud rocks in them. Solutions bringing gold have come up through 

 insignificant-looking crevices called " verticals " and have impreg- 

 nated these mud rocks with long shoots of valuable gold ores. In 

 prospecting in a promising locality the miner, laiowing the syste- 

 matic arrangement of the verticals, and having found the lime shales, 

 drifts along in them, following a crevice in the hope of breaking into 

 ore. The very extended and productive shoots of lead-silver ores at 

 Leadville, Colo., which have been vigorously and continuously mined 

 since 1877, are found in limestone and usually just underneath sheets 

 of a relatively impervious eruptive rock. They run for long distances, 

 and suggest uprising solutions which followed along beneath the 

 eruptive, perhaps checked by it, so that they have replaced the lime- 

 stone with ore. The limestone must have been a vigorous precipitant 

 of the metallic minerals. 



The fracture itself up through which the waters rise may be of 

 considerable size and thus furnish a resting place for the ore and 

 gangue, as the associated barren mineral is called. A deposit then 

 results, which affords a typical fissure vein. The commonest filling 

 is quartz, but at times a large variety of minerals may be present 

 and sometimes in beautifully symmetrical arrangement. In the lat- 

 ter case the uprising waters have first coated each wall with a layer. 

 They have then changed in composition and have deposited a later 



