Januabt 5, 1906.] 



SCIENCE. 



21 



into ore. The very extended and product- 

 ive 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 

 siiggest 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 af- 

 fords a typical fissure vein. The com- 

 monest filling is quartz, but at times a 

 large ■ variety of minerals may be present 

 and sometimes in beautifully symmetrical 

 arrangement. In the latter case the upris- 

 ing waters have first coated each wall with 

 a layer. They have then changed in com- 

 position and have deposited a later and 

 different one, and so on until the crack has 

 become filled. Often cavities are left at 

 the center 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 inined 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 iipon the walls, which may be im- 

 pregnated 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 solutions can not spread 

 laterally, but where it is open the impreg- 



nation may be extensive. The miner has, 

 therefore, to allow for swells and pinches 

 in his ore. 



Of even greater significance than the 

 lateral enrichment is the peculiar arrange- 

 ment of the valuable ore in a vein that may 

 itself be continiious for long distances al- 

 though in most places too barren for min- 

 ing. Cases are, indeed, known in which 

 profitable vein matter may be taken out 

 continuously for perhaps a mile along the 

 strike, but they are relatively rare. The 

 usual experience reveals the ore running 

 diagonally down in the vein filling, and 

 more often than not following the polished 

 grooves in the walls which are called slick- 

 ensides, and which indicate the direction 

 taken by one wall when it moved on the 

 other during the formation of the fracture. 

 The rich places may terminate in depth as 

 well, and again may be repeated, but they 

 must be anticipated, and for them allow- 

 ance must be made in any mining oper- 

 ation. 



Ores, therefore, gather along subter- 

 ranean waterways. They may fill clean- 

 cut fissures, wall to wall; they may im- 

 preg-nate porous wall rocks on either side, 

 they may even entirely replace soluble rocks 

 like limestones. 



We may now raise the question as to the 

 source of the water which accomplishes 

 these results and the further question as to 

 the cause of its circulations. 



The nature of ' the underground waters 

 which are instrumental in filling the veins, 

 presents one of the most interesting, if not 

 the most interesting, phase of the problem 

 and one upon which attention has been 

 especially concentrated in later years. 

 The crucial point of the discussion relates 

 to the relative importance of the two kinds 

 of groimd-waters, the magiuatic, or those 

 from the molten igneous rocks, and the 

 meteoric or those derived from the rains. 

 The magmatic waters are not phenomena 



