Bulletin . trams (see Photo No. 5.—the dike crosses the hillfrom left to right, just above the dumps), though the outcrop is not abold one. The dip is N., about 45; and the width is at least 60 feetwhere most of the work has been done. The dike rock is a somewliatporous, finely granular, porphyritic rhyolite, showing quartz and ortho- 48 CALIFORNIA STATE MINING BUREAU. elase phenoerysts under the microscope (see Photo No. 6). Cinnabarcrj^stals are disseminated through the ground-mass, in some places tosuch an extent as to give the whole rock a pink color, of the rock w
Bulletin . trams (see Photo No. 5.—the dike crosses the hillfrom left to right, just above the dumps), though the outcrop is not abold one. The dip is N., about 45; and the width is at least 60 feetwhere most of the work has been done. The dike rock is a somewliatporous, finely granular, porphyritic rhyolite, showing quartz and ortho- 48 CALIFORNIA STATE MINING BUREAU. elase phenoerysts under the microscope (see Photo No. 6). Cinnabarcrj^stals are disseminated through the ground-mass, in some places tosuch an extent as to give the whole rock a pink color, of the rock which are white, and to the unaided eye appar-ently barren, show finely disseminated cinnabar under the this, the writer presumes that a considerable portion of the dikewould pay to mine and reduce with a large furnace equipment whichcould economically treat a large tonnage of low-grade ore. Of course,this can be definitely determined only by a careful sampling across thefull width of the Photo No. 6. Micro-photograph of porphyritic rhyolite carryingcinnabar, from Cuddeback Mine. The black specks arecinnabar. X 60 diam. magnification. Photo by S. A. Tibbetts. The richer accumulations of ore appear to be associated with cer-tain cross-fissures in the dike which are marked by the presence ofbrown clay seams. The mineralizing solutions apparently came upthrough these fissures, spread out into and impregnated the somewhatporous rhyolite, similarly to the impregnated sandstone ores as at OatHill, Napa County. So far as observed, the rhyolite is but littlealtered, and the adjacent granite is fairly fresh. At this point, themineralized portion of the dike is 0, mile long; and though the dikeitself is traceable beyond, it seems not to show enough cinnabar to beworkable beyond. The development worlv consists of several shortadits, crosscuts, drifts, small stopes, and open cuts; and when visitedin September, 1917, a depth of about 50 feet below the outcrop hadbeen atta
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