ORIGIN OF ORE DEPOSITS 



49 



obtained. Deep leads therefore mark a cessation in the deposi- 

 tion of sediments. It may be that the stream was turned into 

 some other channel by geological changes and subsequently re- 

 turned again to the same valley. (See Figs. 37 and 38.) 



Laterite is the name applied to deposits arising through the 



FIG. 37. Section through Tuolumue County, California, showing old 

 river auriferous gravels covered by a bed of lava, and the method of tunneling 

 to reach them, at the sides are shown river gravels of a later age. 



solution and oxidation of certain minerals. The metalliferous 

 minerals in such cases lie near the base of the hills, composed 

 largely of basic intrusives which have been the source of the ore. 

 Such deposits are represented in the Appalachian belt, India, 

 Madagascar and elsewhere. 



FIG. 38. Section through the Red Point and Damm Channels from 

 El Dorado Canyon (right) to Humbug Canyon, California, showing the 

 auriferous gravels covered by lava, and the method of reaching them by 

 tunneling. The dotted lines at the sides suggest the ancient outlines of 

 the hills. 



Gold. Placer deposits are one of the most important sources of 

 this precious metal. In the Black Hills of S. Dakota, and in 

 Alaska the gold has been concentrated by wave action. It is 

 derived from pre-Cambrian metamorphics traversed by quartz 

 veins. 



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