PITCHING TROUGHS AND ARCHES. 1213 



folded, openings are likely to be formed at the anticlines and synclines. 

 Rickard's explanation of the location of the ores is that the apices of the 

 anticlines furnish more open passages than the synclines. As already 

 explained, if it be assumed that there be less load above the anticlines 

 than above the synclines, this would be favorable to the production of larger 

 openings at the anticlines. This explanation may possibly be to some 

 extent applicable, but the pitching arches concentrating the ascending 

 solutions below impervious strata are believed to be an important factor in 

 the localization of the ores. Various other factors — for instance, intersecting 

 fractures — may be very important in the process. The Bendigo gold fields 

 are very interesting in a scientific way in that such a large number of 

 bonanzas have been found at a depth of 600 to 700 meters." At the present 

 time information is not available which adequately explains these bonanzas. 



A pervious layer or other opening furnishing a trunk channel for cir- 

 culating waters may be bounded on both sides by impervious strata. An 

 excellent illustration of ore deposits at the openings of anticlines between 

 relatively impervious strata, presumably formed by ascending waters, are 

 the gold-bearing quartz ores in the slates and quartzites of Nova Scotia, 

 described by Faribault. 6 Here are a great many parallel deposits 

 directly at the anticlines or on some parts of the anticlinal folds, the de- 

 posits being separated by layers of relatively impervious slate. Further- 

 more, the largest deposits are located on the great pitching anticlines rather 

 than on the subordinate ones. 



Another excellent illustration of ascending ore solutions concentrated 

 by an impervious roof is furnished by the Mercur district, Utah, described 

 by Spurr," where two ore-bearing beds, one called the silver ledge and the 

 other the gold ledge, about 30 meters apart, occur in a limestone below 

 seams or beds of very much altered porphyry resembling shale. The ores 

 are especially localized where fissures reach these beds, and thus displace 

 them, and in some cases form local arches. Moreover, the entire ore dis- 

 trict is located upon a general anticline, furnishing a gently pitching arch. 



«Rickard,.T. A., The Bendigo gold field: Trans. Am. Inst. Min. Eng., vol. 20, 1S92, pp. 538-539. 



6 Faribault, E. R., The gold measures s>i Nova Scotia and deep mining: Paper read before the 

 Canadian Mining Institute, March, 1899; published by the Mining Society of Nova Scotia, 1899. 



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

 U. S. Geol. Survey, pt. 2, 1895, pp. 365-367, 395, 399-401, 449, 454; see also Pis. XXV and XXXIV, 

 and figs. 44 and 45. 



