GOLD AND SILVER MINING 205 



tion was turned to (loop l^'ing auriferous pjravels which are 

 overlaid by a capping of volcanic rock or by a deep stratum 

 of barren gravel. These mines are worked by drifting. 



The drifting process consists in running long tunnels 

 under the lava capped divides and tapping the ancient river 

 channels, receiving only the lower and richer stratum of 

 gravel and then washing it, or, if cemented, crushing it under 

 light stamps. In some places shafts 70 or 80 feet deep are 

 sunk, and the lower gravel near bed rock is breasted out, 

 hoisted, and washed. The larger drift mines are usually 

 opened by tunnels. 



A quarter of a century ago the gravel was shoveled into 

 hand cars and wheeled out to the mouth of the tunnel. Since 

 that time the larger drift mines in following the gravel channel 

 have continued to extend their tunnels until they are miles 

 in length. 



To-day the gravel is removed by trains of cars operated 

 by compressed air or electricity, the power being generated 

 by the water flowing from the tunnels. The tunnels and 

 drifts are lighted by electricity. A material saving in cost 

 is made by the use of these compressed air or electric power 

 plants. The loaded cars run out on the grade, and the power 

 is used for hauling them back to the breasts at or near the 

 foot of the tunnel. The natural drainage of the mines fur- 

 nishes the power for haulage, lighting, and ventilating. 



The latest invention, one which promises to revolutionize 

 the conditions of placer mining, is the dredge. It originated 

 in New Zealand toward the end of the eighties, and after a 

 few years' successful operation in that country was intro- 

 duced in the United States and is fast gaining ground here. 

 The great advantage of the dredge is that it is not dependent 

 upon a large supply of water, but may be operated miles 

 away from any stream, in the midst of orchard lands. No 

 ditches or reservoirs are necessary, nor need there be a grade 

 or fall for dump. Ground w^hich has lacked suitable fall 

 or water supply may now be mined by means of dredges. 

 The working of these machines is thus described by Mr. Charles 

 G. Yale in a report to the director of the mint on gold mining 

 in California: 



