54 GEOLOGY OF THE BLACK HILLS. 



the quartz. In many instances the quartz veins are undoubtedly aurifer- 

 ous and the larger portion of the gold found in the gravels of the Hills has 

 originated from them, but by reason of its sparse dissemination in the vein' 

 matter, specimens containing visible particles are not often found. Such 

 specimens, showing thin flakes of gold have, however, been obtained from 

 some of the ferruginous quartz veins of Custer Park. The very fine state 

 in which the gold is found in the gravels of the southern end of the Hills is 

 also an evidence of the fine state of division in which it occurs in the veins. 



The gold which since our exploration has been found in the ancient 

 gravels of the Potsdam formation must be referred, together with that our 

 party obtained from gravels of the most recent formation, to the quartzes 

 of the Archsean as its source. The fact that the concentration was begun 

 by the shore action of the Potsdam sea renders it probable that some at 

 least of the gold of the recent gravels is derived immediately from the Pots- 

 dam gravels, just as are the associated quartz pebbles. 



Hornblendic schist or hornblende rock was found in a few localities. 

 Near the stockade and on the headwaters of Amphibious Creek a very 

 silicious schist occurs containing a large proportion of hornblende, and on 

 French Creek near the point where it leaves the Hills hornblendic masses 

 were found at more than one place in the bluff of the canon. The latter is 

 a very compact and close-textured hornblende rock, containing consider- 

 able silicious matter. The general scarcity of hornblende among the schists 

 and its entire absence from the slate rocks, except as a microscopic element 

 in their structure, are facts deserving of remark. 



The minerals found in the schists and pertaining to them particularly 

 are few in number, and with the exception of garnets and mica are not 

 abundant. Staurotide, well crystallized, w r as found in one locality in the 

 schists east of Harney Peak, and a single specimen of crystallized epidote 

 was brought in for examination. Highly crystalline feldspar and mica are 

 found in thin veins in the gneissic rocks in some places. Scales of graphite 

 were found in small quantities in certain of the schists near Harney Peak. 



The granites are found only in the schists and gneiss of the western 

 series, and careful observations nowhere discovered them among the argil- 

 laceous slates of the eastern series. They occupy a large area entirely in 



