PRECIOUS METALS 



86 



the method adopted. Every yard of the gravel which is sluiced 

 must first be thawed either by artificial means or by exposing it 

 to the rays of the summer sun, after stripping off the muck that 

 overlies the auriferous gravels. It is impossible to work the frozen 



Fio. 64. Hydraulic mine at Cherokee; Butte County, California. (After 

 J. S. Diller, U. S. Geological Survey.) 



ground with pick and spade, or even with explosives to loosen the 

 gravel. 



Steam thawing is the most efficient method now in use. Iron 

 pipes, 4 to 6 ft. in length, tipped with steel nozzles, are inserted 

 into the gravel and then steam is forced through them at a pres- 

 sure of about 120 Ib. per square inch. These pipes are known as 



