374 



PHYSICAL GEOLOGY 



the change from one to the other being sometimes so gradual that 

 it is difficult to say where one begins and the other ends. Veins 

 are known in which oxidized ores occur several hundred feet below 

 the water table. 



Magmatic Segregation. — Certain iron and* nickel deposits which 

 occur in igneous rocks were probably brought together while the 

 rocks were in a molten condition as the result of segregation. Few 

 workable deposits, however, have been formed in this way. 



Placer Gold Deposits. — In the early days of gold mining in many 

 countries the first gold was found in the gravels of stream beds. The 

 gold of the Klondike in northwestern Canada and the majority of 

 the early finds of Alaska were located in stream gravels, while that 



STREAM GRAVELS 



Fig. 359^- — Diagram showing the development of eluvial or residual placers, 

 which may be worked like ordinary stream placers, and stream gravels. In this case 

 the source of the gold is the quartz vein. (After Lindgren.) 



Fig. 359 B. — Diagram showing ancient aurif- 

 erous gravels (dotted) covered by a lava flow 

 (vertical lines). In mining the gravels a tunnel 

 is driven as indicated. 



of Nome was found in the 

 sands of the seashore. The 

 gold rush to California in 

 1849 was due to the find- 

 ing of stream or placer gold. 

 The source of the nuggets or 

 dust in stream gravels is evi- 



dently to be found in the rocks over which the streams or their 

 tributaries now flow or formerly flowed. 



I he way in which placer gold was transported and deposited 

 is simple:. The ^old occurred either in veins or scattered through 

 the country rock in small quantities. When these rocks were 

 disintegrated by weathering and the fragments carried away by 



