NEW YORK ACADEMY OF SCIENCES 643 



minerals. In the Black Hills of South Dakota there are sand- 

 stones with beds of calcareous mud rocks in them. Solutions 

 bringing gold have come up through insignificant-looking 

 crevices called "verticals" and have impregnated these mud 

 rocks with long shoots of valuable gold ores. In prospecting in 

 a promising locality the miner, knowing the systematic arrange- 

 ment 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 con- 

 tinuously 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 limestone 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 sym- 

 metrical arrangement. In the latter case the uprising waters 

 have first coated each wall with a layer. They have then 

 changed in composition and have deposited a later and different 

 one, and so on until the crack has become filled. Often cavities 

 are left at the centre or sides and are lined with beautiful and 

 shining crystals, which flash and sparkle in the rays of a lamp, 

 like so many gems. There are quartz veins in California which 

 are mined for gold and which seem to have filled clean-cut 

 crevices, wall to wall, for several feet across. More often there 

 is evidence of decided chemical action upon the walls, which 

 may be impregnated with the ore and gangue for some distance 

 away from the fissure. As the source of supply is left, however, 

 the impregnation becomes less and less rich, and finally fades 

 out into barren wall-rock. The enrichment of the walls varies 

 also from point to point, since where the rock is tight the solu- 



