ORES OF GOLD 53 
foot of the volcano. This ore contains about 6 dwt. per 
ton of gold and 2% per cent. of copper; the mine which in 
ts early days contained the purest of recorded native gold 
ended as a low-grade copper mine yielding gold as a bye- 
product. 
Section C. ALLUVIAL GOLDFIELDS 
Pracers—Alluvial deposits or placers (cf. p. 32) and their 
altered representatives are especially important in gold 
mining, though the same methods are applied to piers 
Containing tin and platinum. The first economic process 0 
working low-grade placers was by hydraulic sluicing which 
Was developed in California. Water is impounded in a high 
level reservoir and brought to the alluvial deposit by a lead 
or trench; it falls through an iron pipe to a nozzle, which 
directs it against the gold-bearing material. A ‘* Giant 
nozzle weighing 2000 1b. can control a stream with a head 
of 500 feet; the water leaps from it with a velocity of some- 
times 2 miles a minute, and in a jet so strong that a stick 
may be broken across it as over a metal bar. The jet digs 
into the gravel, and washes the material along a chain of 
sluice boxes, on the floor of which the gold is caught in between 
ridges or “rifles,” or on canvas, or by mercury. The chain 
of sluice-boxes may be from 100 feet to miles in length. 
This process is most effective in working gravels about 
80 feet thick ; if the beds are thinner, time is lost in frequent 
movements of the nozzle ; if the beds are thicker the material 
falls in unmanageable masses and for safety the nozzle has 
to be too far from the cliff for the jet to have its full excavating 
power. Thicker beds have therefore to be worked in two 
layers. The cost of hydraulic sluicing is low, and in Cali- 
fornia is from 13d. to 6d. per cubic yard. Hydraulic sluicing 
has been successfully used in alluvial fields. in most parts 
of the world. 
Dredging is usually the cheapest method of working a 
low-grade placer in river valleys. The first device was a 
floating timber platform, on to which gravel from a river 
bed was shovelled by a man standing in the water, or by 
2 Spoon dredge worked from a boat. Another hand system 
Was to drop a bucket on to the river bed and haul it ashore. 
In 1882 McQueen, a New Zealand miner who has been