PITCHING TROUGHS AND ARCHES. 1215 



In this connection it may be suggested that the positions of the ores 

 in reference to the limestone and porphyry in the Leadville district are 

 remarkably similar to those of the ores in the Mercur district in reference 

 to almost identical formations. The forms of the deposits, their irregular 

 under surfaces in the limestone, and the regular surfaces at the porphyry are 

 all identical. Both Emmons and Spurr agree that the ore in the Mercur 

 district was deposited as sulphides by ascending waters. If this be true, 

 the same explanation is probably applicable to the Leadville district. 



Another exceedingly interesting illustration of the deposition of ores 

 below an impervious stratum in pitching arches is that furnished by the 

 Enterprise inine of Rico, Colorado, described by Richard a and Ransome. 6 

 In this district above the ore bodies is an impervious shale, which is very 

 .rarely broken by the fissures. The ore occurs in two places: First, ore in 

 nearly vertical fissures extends indefinitely downward below the shale, but 

 not upward into it. The verticals are cut by cross fissures, and where the 

 intersections occur the fissures are likely to be unusually rich. (See p. 1084.) 

 Second, the larger masses of ore are in crushed or fractured limestone 

 below the black shale, but the rich blanket of ore replaces gypsum. 6 

 These ore bodies are narrow laterally, some being parallel to the strike of 

 the verticals and others parallel to the cross veins. Fig. 29 shows that 

 they occur below anticlinal flexures .of the shale made by the deformation 

 resulting in the fracturing and faulting in the more brittle rocks. Rickard 

 regards the deposits as the result of ascending waters, since the fissures 

 continue downward but do not extend upward into the shale. The anticlinal 

 arches have a pitch. Probably the waters issuing from the verticals and the 

 cross fissures followed these arches upward until the pitch somewhere 

 brought them to the surface, at which places the waters escaped as springs, 

 for the waters of the ascending circulation must have somewhere escaped, 

 and that they could not do through the impervious shale. 



Porous pitching troughs below, above, or between impervious strata 

 may have different origins from those mentioned. Very frequently such 

 troughs are produced in part or in whole by intrusive igneous rocks. For 

 instance, if an impervious sedimentary stratum has a monoclinal dip and a 



aRickard, T. A., The Enterprise mine, Rico, Colo.: Trans. Am. Inst. Min. Eng., vol. 26, 

 pp. 918-973. 



SRansome, F. L., The ore deposits of the Rico Mountains, Colorado: Twenty-second Ann.Ttept. 

 U. S. Geol. Survey, pt. 2, 1901, pp. 254-302. 



