VALUE OF THE ORE. . 275 



quartz, are found, similar to the " Mammoth ledge" on Spring Creek. 

 One of these altered strata contained large masses of limonite iron ore 

 intermixed with quartz, colored and stained by copper, and resembling 

 closely some specimens of Colorado gold ores. It was a portion of a 

 quartzite belt dipping 60° to the south, and could be traced east and west 

 for a mile across the low hills until the outcrop was concealed beneath the 

 Potsdam sandstone. Several parallel strata of quartz and quartzite were 

 found within a few hundred yards. That forming the "lode" was from 20 

 to 40 feet wide, a mixture of massive milk-white quartz and limonite iron 

 ore, with considerable " gossan" ore, resulting from the decomposition of 

 copper and iron pyrites. Samples were carefully taken from the best- 

 appearing ore in the outcrop of this ledge and submitted to Mr. P. De P. 

 Ricketts for assay, who reports that the quartz contains small quantities of 

 gold, but, unless richer than the samples assayed, the deposit is of little 

 value Where the outcrop of this ledge was covered by the Potsdam 

 sandstone the conglomerate forming the lowest layer of that formation was 

 full of large rounded bowlders partly derived from it. Some of these 

 bowlders were 3 to 4 feet in diameter, and contained all the varieties of 

 rock found in the vicinity, including clay-slate, quartzite, ferruginous and 

 ribbon quartz, and gossan ore, exactly as they occurred in the adjacent 

 ledges, proving that very little change had taken place in the slates and the 

 inclosed quartz veins since the commencement of the Potsdam period. 

 Previous to the deposition of this conglomerate the slate and quartzite 

 rocks were subjected to a very great erosion, which must have removed a 

 great thickness of the strata, but since the elevation of the Black Hills the 

 Archaean rocks in this district have been but little denuded, the remnants 

 of the Potsdam sandstone capping hills and ridges from 50 to 150 feet 

 above the present surface ; and although the formations which covered" this 

 area at the close of the Cretaceous period have been swept away over 

 extensive tracts, the erosion has extended but a short distance into the 

 slates. In the Spring and Rapid Creek districts, however, the slates and 

 quartzites have been very greatly denuded and furnished a large amount of 

 material to form the placer gravels. 



On Box Elder the gravel deposits seem to be largely composed of 



