BLACK HILLS GEOLOGY. 305 



occurrence of Cambrian is not represented on the map as the hm- 

 ited time at the writer's disposal did not permit an accurate study 

 of the exposures. The district is still in the early stages of 

 development. 



TJie American Express Mine is situated in Sheeptail Gulch, a 

 short distance above its junction with Blacktail Gulch. It is 

 opened by a tunnel on the basal quartzite, which is here ex- 

 posed at some distance above the bottom of the gulch. The 

 quartzite dips slightly toward the northeast, and the ore shoots 

 run with the dip, being lateral enrichments of verticals. The 

 ores have come up from below, and spread out upon the quart- 

 zite floor, replacing the calcareous matter in the sandy limestone 

 and lime shales. The shoots vary from a few feet to thirty feet 

 in width, and in thickness they are about six feet. The roof of 

 the mine is a porphyry sheet. The average yield of the ore is 

 something between ^15.00 and ^25.00 per ton. It is of the 

 hard, bluish, unoxidized variety, and contains many vuggs filled 

 with quartz. Druses of pyrite can often be seen in the hand- 

 specimens. The verticals run northeast and are parallel with 

 the longer diameters of the shoots. Islands of unreplaced 

 "sand rock" (the "barren sand rock" previously mentioned) 

 or sandy limestone occur in the shoots. The accompanying 

 diagrams will illustrate the geological relations. (Figs. 17, 18.) 



Resume Regarding the Siliceous Ore Bodies. Form. — From 

 these data it will appear that the ore shoots are channel-like 

 masses of irregular shape, but generally longer than broad. 

 They sometimes attain a thickness of fifteen feet and again may 

 sink to a feather edge. The roof is either a porphyry sheet or 

 a bed of shale, and the floor is either the hard indurated basal 

 quartzite of the Cambrian or in the case of the upper con- 

 tact a bed of argillaceous and non-replaceable shales. Verti- 

 cal feeders run in a direction parallel to the longer diameters of 

 the shoots. The width of the ore bodies varies from 150 feet 

 to a mere vertical crack. The thickness seems to have been de- 

 termined partly by the thickness of the more easily replaceable 

 rock, /. e., that containing the most fissures, and the largest pro- 



