BLACK HILLS GEOLOGY. 311 



The history of the formation of the sihceous ore bodies can 

 then be outhned as follows : 



First occurred the intrusion of the older quartz-porphyries, 

 which produced much shattering. Contemporaneous with 

 these, there may have been a certain amount of ore deposition, 

 but not that to which the main siliceous ore bodies owe their 

 origin. Later the eruption of the phonolites took place, cut- 

 ting and shattering the older eruptives, and adding to the num- 

 ber of fissures in the sedimentary rocks. Subsequent to all of 

 these intrusions, and probably separated from them by only a 

 brief interval of time, came a long period during which heated 

 solutions, containing fluorine and silica and other powerful min- 

 eralizers gradually replaced the carbonate of lime in the more 

 soluble strata of the Cambrian. The chemical activity of these 

 solutions was increased by the heat and mineralizers derived 

 from the newly injected phonolites. They passed up through 

 the Algonkian slates and schists, becoming much enriched by 

 the leaching out of the gold from these rocks. Finally they 

 reached the very calcareous and porous rocks of the Cambrian, 

 and by a metasomatic interchange, produced the horizontal ore 

 bodies that are found to-day. 



C. Ores in the Carboniferous Limestone. 



Of these ores there are two classes : Silver ores and gold 

 ores. The silver ores occur in the vicinity of Carbonate Camp 

 on the north side of Squaw Creek, and are mainly chlorides 

 and carbonates. They have not been studied with care. Of 

 the gold ores in the Carboniferous there is but one district. 



The Ragged Top District. — This includes two varieties of 

 deposits, one of which is represented by the Ulster Mine on the 

 divide to the northwest of Preston, the other by the verticals on 

 the Dacy Flat, and on the divide to the south of Ragged Top 

 Mountain. The Ragged Top verticals are seven in number, 

 the Dacy vertical being the largest producer. They are wedge- 

 shaped crevices in the limestone, of about ten feet in maximum 

 width at the top and narrowing down to mere crevices in 



