296 IRVING. 



any considerable importance, they will not be further discussed. 

 The important points are : 



I St. They are the oldest known gold deposits of the hills. 



2d. Their mineralogical character is largely free-milling in the 

 upper portions — and hence free-milling in those parts which, by 

 their disintegration, have furnished material for the formation of 

 later deposits. 



B. Ore Bodies in the Cambro-Silurian. 



From the disintegration of the Algonkian ore deposits we 

 have, as demonstrated by Devereux,^ ancient placers, in which 

 the gold exists in the free condition. These are in the ba.sal con- 

 glomerates of the Cambrian, and the gold has been worn from 

 the older deposits by the action of the waves upon the shores 

 of the Algonkian Island. They have been of especially large 

 development in the vicinity of Central City in Deadwood Gulch, 

 and in Blacktail Gulch. But the free gold of the Cambrian 

 formation is not confined to the basal conglomerate, which in- 

 deed is by no means auriferous throughout its entire extent. 

 The gold seems to be disseminated throughout the formation, 

 sometimes sinking to a very few cents per ton, but almost al- 

 ways giving colors when panned. Cambrian shales and sand- 

 stones, some two or three hundred feet above the basal quartzite 

 on the west banks of Spearfish Creek have yielded from 8o 

 cents to ^2.00 per ton, and are here entirely unaltered. They 

 contain the original high percentage of calcareous matrix be- 

 tween the quartz grains, and show no traces of induration. 

 Certain unaltered glauconite shales on Crown Hill have yielded, 

 according to Mr. Holmes of the Rua Mine, considerable colors 

 on panning. Many other instances of free gold in small quan- 

 tities in the unaltered Cambrian shales have been mentioned to 

 the writer. Time has not permitted an extended series of tests, 

 but if the numerous instances cited are correct, and there seems 

 to be no reason to doubt them, small quantities of free gold 

 would seem to be a common feature in many of the Cambrian 



1 Trans. Am. Institute Mining Eng., X, 465 and sqq. , Feb., 18S2. 



