334 THEORIES OF ORE DISPOSITION HISTORICALLY CONSIDERED. 



(lescoiuiiiiiLj waters. In prevailing composition the first class are 

 sulphides, tellnrides, etc.; the second oxides or oxide salts, while the 

 third are chiefly the one or the other, according as they Avere formed 

 above or beloAv the ground-water level. 



Emmons and Weed, coming to the subject from a different but 

 somewhat narrower standpoint — that of a practical field study, 

 extending over seA^eral yeiirn — explained the frequent occurrence of 

 bonanzas, or exceptionally rich j^ortions of deposits just below the 

 oxidized zone or ground-water level, as the result of leaching by sur- 

 face waters of the upper portions of these deposits and tlieir rede])o- 

 sition as sulphides in contact with preexisting metallic sulphides 

 (especially pyrite) in the zone below. Through similar processes of 

 chemical reasoning and with a similar disregard of Posepny's 

 assumption that the ground-Avater level forms an effective barrier 

 separating the action of the surface or vadose Avaters from that of 

 the deep circulation, all three arrived at the same general conclusion 

 Avith regard to the continuance of rich ore in depth, a cpiestion which 

 has occupied the attention of geologists and miners since the days of 

 AVerner. This conclusion Avas that in most ore deposits a deeper 

 region exists beyond the influence of surface Avaters in which the ore 

 is of comparatiA'ely Ioav and uniform grade. Van Hise even Avent 

 so far as to say that in depth all deposits avouIcI become loAV-grade 

 pyritic ores, and that all veins Avould eA'entually wedge out. 



De Launay, in his generalizations on Mexican deposits, had already 

 recognized three zones: (1) an upper oxidized zone, (2) a middle 

 zone of rich sulphides, and (3) a loAver zone of loAv-gracle sulphides. 

 He assumed the enrichment of the middle zone had been by descend- 

 ing Avaters, Init 2:)laced it aboA'e the ground-water or hydrostatic le\"el, 

 Avhich in many A"eins had probably been displaced since their original 

 formation. 



In his article " On metasonuitic processes in fissure veins," Lindgren 

 placed this theory for the first time on a scientific basis of chemical 

 a))d microscopical study, and by a classification of veins according to 

 the predcmiinant metasomatic mineral or process involved he made its 

 applicaticm much clearer to the student and obserA'er. In his closing 

 remarks he suggested that of late sufficient attention had not been 

 given to the French theory of emanations from eriiptiA^' magmas, and 

 that in the case of metals with Ioav critical temperature they nuiy ha\"e 

 first been carried up under pneumatolytic conditions and Avith the aid 

 of mineralizers Avhile still aboA^e the critical temperature, until they 

 reached the zone of circulating atmospheric Avaters. 



His paper '' On contact deposits,'' 1001, folloAving out this sug- 

 gestion, serA^ed a useful genetic purpose by calling attenti(,n to and 

 clearly defining a group of deposits for Avhich a pneumatolytic origin 

 would readily be admitted, but of wdiich no important examples had 



