48 



ECONOMIC GEOLOGY 



Placers represent the concentration of the heavier economic 

 minerals in the order of their decreasing density. In shallow 



FIG. 35. Theoretical section showing the origin of auriferous gravels. 

 The dark lines represent gold-bearing veins of which the coarser and heavier 

 materials accumulate in the valleys. 







Limestone 

 with Iron Ores 



FIG. 36. Map to illustrate the influence of ore-bearing rocks on the 

 alluvia of streams and rivers. (After Thomas and MacAlister's Geology of 

 Ore Deposits.) 



placers the metalliferous mineral is confined to one level. In the 

 deep leads there are two or more levels at which the values may be 



