34 
ECONOMIC GEOLOGY 
called the * Father of Dredging,” proposed the use of the 
harbour bucket dredge, and one was tried on the Otago River 
in 1886. It was not a financial success but showed that 
the process was practicable, and specially designed bucket 
dredges proved efficient and extraordinarily economical. It 
is claimed (E. B. Wilson, Hydraulic Mining, 1898, p. 100) 
that ground less than 60 feet below or 20 feet above water- 
level, which does not contain boulders more than a ton 
in weight, should be handled by dredges at 13d. to 23d. 
a cubic yard, though the cost is often 5d. a cubic yard. 
Wilson (ibid., p. 106) states that a dredge may pay on a re- 
covery of £ of a grain of gold to the ton of gravel, and some 
have paid dividends in Victoria with material of that grade. 
A dredge may haul from a river a cubic yard of earth, wash 
it, separate its gold, and yield a profit if it contains a penny- 
worth of gold. 
In some rocky river beds the gold lies in the depressions 
and a bucket dredge can only recover it by breaking off the 
projections unless the river bed has been blasted, so that 
the material can be scooped out. Such places can be worked 
by the suction dredge, which by a stream of water sucks up 
a pipe all loose material and gold dust on the river bed. 
Suction dredges are also used on river-side flats; the dredge 
is built in an excavation; it works forward, depositing the 
ground washed from the front of the pit behind it; it is 
floated forward to a new position by flooding the excavation, 
and thus gradually works its way through the whole alluvial 
plain. The coarse boulders should be deposited at the bottom 
and the fine material on the top, so that the ground may be 
left in better condition for agriculture than before the dredge 
began its work. 
Deep Leaps—The river placers first worked lay on the 
beds of rivers or the floors or sides of valleys and were known 
as ‘leads.” Some exceptionally rich deposits were due to 
a recent valley having been cut through the deposits of an 
older valley, with the reconcentration of its gold. Thus 
(at XX in Fig. 17) the gravel was especially rich because 
that of an ancient river had been rewashed and the gold 
further concentrated. The continuation of the old river 
was found under the hills of sand and clay which have filled 
its valley, and is a buried lead or * deep lead.”