334 THEORIES OF ORE DISPOSITION HISTORICALLY CONSIDERED. 



descending 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 were formed 

 above or below the ground- water level. 



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

 somewhat narroAver standj)oint — that of a practical field study, 

 extendinir over several 3^ears — explained the frequent occurrence of 

 bonanzas, or exceptionally rich portions 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 their rede])o- 

 sition as sulphides in contact with preexisting metallic sulphides 

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

 chemical reasoning and Avith a similar disregard of Posepny's 

 assumption that the ground-water level forms an effective barrier 

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

 the dee]) circulation, all three arrived at the same general conclusion 

 with regard to the continuance of rich ore in depth, a question which 

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

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

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

 is of comparatively low and uniform grade. Van Hise even went 

 so far as to say that in depth all deposits would become low-grade 

 pyritic ores, and that all veins would eventually Avedge out. 



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

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

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

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

 ing Avaters, but placed it aboA'e the ground-Avater or hydrostatic leA^el, 

 which in many veins had probably been displaced since their original 

 formation. 



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

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

 and microscopical study, and l)y a classification of veins according to 

 the predominant metasomatic mineral or process invoh'ed he made its 

 application nnich clearer to the student and observer. In his closing 

 remarks he suggested that of late sufficient attention had not been 

 given to the French theory of emanations from eruptive magmas, and 

 that in the case of metals Avith Ioav critical temperature they nuiy have 

 first been carried up under pneumatolytic conditions and with the aid 

 of mineralizers while still aboA'e the critical temperature, until they 

 reach(Ml tlie zone of circulating atmospheric Avaters. 



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

 gestion, served a useful genetic purpose by calling attention to and 

 clearly defining a group of deposits for Avhicli a piicuuiatolytic origin 

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



