300 



Adventures in Scenery 



commonest of the sedimentary rocks. The average gold con- 

 tent of 4 shales from widely separated regions of the United 

 States represented a value of a little more than half a cent per 

 ton of rock. The average gold content of shales, so far as 

 known, is twice that of limestone. The average for sandstone 

 is one and one-half times that of shale, or three times that of 



Courtesy U. S. Bureau of Mines 

 FIG. 83. Typical ore from Mount Gaines Mine. (Actual size.) 



limestone. Coarser sedimentary rocks have a higher gold con- 

 tent than finer, conglomerates having the highest content, and 

 the coarser the conglomerate the greater the gold content. 

 This seems to indicate that, in the process of sedimentation, gold 

 tends to concentrate by lagging behind. 



Gold Content in Igneous Rocks 

 Variable 



There is very great variation in the gold content of igneous 



