BLACK HILLS GEOLOGY. 309 



material of sandy limestones and lime shales by siliceous solu- 

 tions bearing the gold. The deposition has in all cases been a 

 metasomatic mterchange of silica and pyrite for carbonate of 

 lime, in which the latter has in all probability acted as the pre- 

 cipitating agent. Whether or not these solutions were in a 

 heated condition, it is not possible to say, but it is very probable 

 that such was the case. That the chemical activity of the solu- 

 tions was due to the eruptive activity seems probable because 

 at a distance from the eruptive centers, ore bodies are not found. 

 The gold remote from the eruptives is either in placers or in 

 finely disseminated colors in the Cambrian, and has been derived 

 in all probability by erosion from the Algonkian schists. The 

 ore shoots can invariably be traced to a so-called " vertical " or 

 crevice, now filled by silica of the same character as the ore 

 body itself 



At times these verticals occur at the sides of dikes of quartz 

 porphyry, but more frequently they are merely fractures in the 

 sedimentary rocks, probably caused by the same eruptions that 

 heated and rendered active the percolating waters to which the 

 ore bodies owe their origin. Prof Smith has said' "Wherever 

 rhineralization of the Potsdam beds has occurred, it can almost 

 always be traced to a quartz-porphyry or rhyolite dike, or 

 ' vertical ' which itself is usually mineralized, stained with oxide 

 of iron, and so much broken and decomposed, that its rock 

 character is distinguishable with difficulty." 



That all the verticals which occur in relation with these ore 

 bodies are shattered and subsequently mineralized dikes the 

 writer does not believe, because in many cases they show no 

 traces of the original rock, and the siliceous replacement of por- 

 phyry is something which is not frequently observed. That the 

 verticals do sometimes occur along the contacts of the dikes is 

 not to be denied, but this is to be attributed to their shattered 

 condition caused probably by the injection of later intrusions. 

 The verticls are to be considered simply as fractures, which have 

 afforded access to percolating waters. They have sometimes 

 occurred along the contacts of dikes, but are as frequently re- 

 moved from them. 



^ Trans. American Inst. Mining Eng., Vol. XXVII, pp. 416,417. July, 1897 



