1208 



A TREATISE ON METAMORPHISM. 



Sandy shale 



Blanket limestone 

 Black shale 

 Sandstone 

 Sandy shale 



circulations of their own, independent of the higher circulations, and this 

 circulation may produce ore bodies. This is beautifully illustrated by the 

 Enterprise mine, of Rico, Colorado (see fig - . 29), described by Richard" 

 and Ransome, 6 in which the ore is confined to fissured and broken lime- 

 stones, gypsum, and sandstone below a black shale, which, when bent, did 

 not fracture, and therefore afforded no channels for water circulation. 



In this connection it may be well to mention the Mercur district of 



Utah (see pp. 1213-1214), where a 

 silver ledge and a gold ledge, about 

 30 meters apart, occur in limestone, 

 each below a shale-like stratum of 

 altered porphyry. Spun- regards 

 the silver ledge as produced during 

 an earlier mineralizing period, and 

 the . gold ledge during a later one. c 

 It may be suggested that the true 

 explanation of the existence of two 

 mineral ledg-es so near together and 

 of such different mineral character 

 is that in this district there were two 

 independent cuculations separated 

 by impervious strata — the upper 

 one, producing the gold ledge, 

 lying between the two impervious 

 porphyry belts; while the lower 

 one, forming the silver deposit, was 

 below the lower impervious layer. 

 The lead and zinc ores of the Mississippi Valley d furnish another illus- 



oRickard, T. A., The Enterprise mine, Rico, Colo.: Trans. Am. Inst. Min. Eng., vol. 26, 1897, 

 pp. 918, 960-973, 976-977. 



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

 IT. S. Geol. Survey, pt. 2, 1901, pp. 299-397. 



c Spurr, J. E., Economic geology of the Mercur mining district, Utah: Sixteenth Ann. Rept. 

 U. S. Geol. Survey, pt. 2, 1895, pp. 367-369. 



<?Bain, H. F., with C. R. Van Hise and G. I. Adams, Preliminary report on the lead and zinc 

 deposits of the Ozark region: Twenty-second Ann. Rep't. IT. S. Geol. Survey, pt. 2, 1901, pp. 33-227. 

 Van Hise, C. R., Some principles controlling the deposition of ores: Trans. Am. Inst. Min. Eng., vol. 30, 

 1900, pp. 102-109. Van Hise, C. R., and Bain, H. F., The lead and zinc deposits of the Mississippi 

 Valley: Trans. Federated Inst. Min. Eng., England, 1902, pp. 1-60. 



Fig. 29.— Diagrammatic section of Enterprise mine, Colo- 

 rado, and its blanket pay shoot. After Ransome. 



