ORES OF GOLD 57 
height at different lines of bores indicates the river gradient, 
from which can be inferred the amount of meander between 
the two lines, and where the current would have been suffi- 
ciently powerful to concentrate the gold. - The samples from 
the drill holes indicate the relative amount of gold, but not 
the actual yield, for they are a concentrate, much of the lighter 
material having been washed away during the drilling. } 
A deep lead cannot be worked until it has been drained. 
A shaft is sunk to a suitable depth below that of the lead. 
A drive known as the ** reef drive,” as it is in the bedrock, 
1s made under the lead, which is drained by bores put into 
it from this drive. Some mines had to pump several million 
gallons of water a day for years before it was possible to enter 
the lead. Lead mining under favourable conditions has 
been very profitable; the Madame Berry Mine, e.g. paid 
£1,300,000 in dividends on a capital of £15,000. When part 
of the lead is drained it is entered by the upper or wash 
drive,” from which the gravel or * wash ” is dropped through 
shoots to the reef drive, whence it is raised to the surface. 
The gold is washed out of the sand; none is found in the 
pebbles, 
The costs of working are estimated per square fathom, 
as most of the gold is in the lower part of the gravel, and the 
yield depends more on area than on thickness. 
The probable value of a lead depends on constant re- 
enrichment, as the gold usually travels but a short distance. 
A lead which crosses rocks intruded by granodiorite and 
likely to contain numerous gold-quartz veins, may be ex- 
pected to be richer than areas without igneous rocks. 
The distribution of the leads depended on the nature of 
the plateay basalts, and the mining has shown that they 
were formed by the confluence of lavas from many vents 
and not by eruption from fissures. 
_The Kanowa Lead in Western Australia, as its gold has been 
dissolved and redeposited, raised the question whether the 
mines had to comply with the regulations for alluvial or 
lode mining. Though the evidence proves that the gold 
as it now occurs, was deposited from solution, it has been 
wisely treated as alluvial. The problem bears on the nature 
of the gold of the Rand (cf. p. 61). 
RAND BaANkET Marine placer deposits occur on the coasts