280 GEOLOGICAL SURVEY 
ed through a closely woven cloth, and well squeezed; this leaves the gold combined with some mer- 
cury, which is driven off by heat. About 20 bushels of ore may be washed, with such an imple- 
ment as this, in a day. 
Fie. 42. b The Burke rocker, 
(Fig. 42,) is an im- 
provement on this. 
a is an iron plate, on 
which the ore to be 
washed is placed ; 6 
is a spout of water, 
pouring on, with 
considerable veloci- 
ty; ¢ is a perforated 
plate of sheet iron, 
the holes about half 
an inch in diameter, 
through which the 
A 
BENS 
finer parts fall into 
| | the riffle-box, below. 
B is the riffle-box, with shallow eayneges five inches apart. In the rocker its position is seen 
at e. 
Mercury is placed in the compartments, and as the particles of gold pass over it, they are taken 
up. This machine, placed in an inclined position, is rocked by means of a lever, attached to the 
side, and is manipulated by a boy or girl; a second person being employed to throw on the ore, and 
remove the coarser gravel, &c. More than double the work of the preceding, can be accomplished 
by this machine. 
There are some modifications of these, that scarcely deserve notice: such as a long simi-cylindric 
trough, with transverse grooves, in which the mercury is placed, and a stationary rocker, where the 
ore is washed by the force of a stream of water, and pushed along by the shovel. 'These are all 
the means used at the deposit mines. 
The practice of working deposits over, after the lapse of some time, is common; and it does 
not follow that the previous working was imperfect, because gold is found. 'The iron pyrites 
which the beds contain, by exposure becomes oxydized, and gold is liberated. 
