Transactions . e ore miusses usually surrounded by solution cavities which show all of theconcave surfaces and other criteria so characteristic of oixningsformed by solution. The main ore masses, however, seem tohave been formed by replacement. Impervious barriers are effective in stagnating solutions notonly when above or below the path of a fissure but also when theyintersect it in a horizontal direction and even if parallel to thezone of fracturing. Thus in the Mineral Farm mine, a portion ofwhich is shown in Fig. IS, a sheet of phonolite is intersected bya set of fractures w
Transactions . e ore miusses usually surrounded by solution cavities which show all of theconcave surfaces and other criteria so characteristic of oixningsformed by solution. The main ore masses, however, seem tohave been formed by replacement. Impervious barriers are effective in stagnating solutions notonly when above or below the path of a fissure but also when theyintersect it in a horizontal direction and even if parallel to thezone of fracturing. Thus in the Mineral Farm mine, a portion ofwhich is shown in Fig. IS, a sheet of phonolite is intersected bya set of fractures which piuss into it from the Hurn)un(ling shalylimestone. The ore has completely replacml the rock udjacont to 420 Replacement Ore-Bodies—Irving the phonolite; but has not connected the fissures at a distance of50 feet from it. In the phonolite itself only a slight alterationa fraction of an inch from the wall has been produced. / V Ouartr - rnoozonite-r^ ?^ ^ ^pcp^iyy \^ ^ I 500 BlacV carbonaceoos shale40±. Clay shale2 Fig. 16.— of the iVmcrican Nettie Mine, showing how pear-shaped shoots of ore have formed beneath imprevious barriers of shale.(After U. S. Geol. Sur. Bull. 260.) In the case of the Tornado Mogul shoot, the largest body ofrefractory siliceous ore in the Black Hills region, the ore bodyis bounded on one side by a dyke of phonolite which has servedto assist in the general stagnation of the solutions and has resultedin the production of a very largo body of ore. The same stagnationhas been produced in the Elkhorn mine in Montana, as describedby Weed, where solutions have risen and produced bodies of oreunder anticlinal domes of impervious rock.^ * Weed, U. S. Geol. Survey Twenty-second Ann. Report, pp. 22 to 92, Replacement Ore-Bodies—Irving 421
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