ington.] OPEN-CUT MINING. 63 
in when occasion might require. The bucket elevator employed is 
t recommended, as the use of such contrivances for handling gravel, 
less they are specially and expensively constructed — as on large 
Bges — is condemned by experience. In place of this, the scraper 
3uld be dragged entirely to the point of final discharge to the sluice 
an inclined platform. The operation will necessitate the building 
a more elevated sluice or washing plant, surmounted by a hopper, 
i gravel being fed from this in order that the feed may be as nearly 
atinuous as possible. Assuming that the elevation of the head box 
the sluice shown in the illustration is 12 feet above the surface of 
i ground, the scraper could be dragged to double this elevation at a 
it not exceeding 10 per cent above that necessary to haul it up to 
1 12-foot elevation. One of the types of washing plants described 
pages 193 et seq. would be applicable with modifications to a steam 
•aping plant. 
[t may be well to consider here the reasons why, in wide, shallow 
>ek deposits, steam scraping appears to be preferable to other 
ichanical methods. In general, it may be said that this method is 
plicable to operations on schist bed rock, but is entirely impracti- 
)le on limestone or other hard bed rock. This is also true of the 
am-shovel and dredging methods. Grant that bed-rock conditions 
j suitable, and consider a solidly frozen deposit of characteristic 
all gravel with muck and moss overburden, 150 feet width of pay 
i 7 feet in total depth from grass roots to bottom of pay. After 
} moss is plowed up, the 3 feet depth of muck can be ground sluiced 
in the ordinary manner at an expense of, say, 15 cents per cubic 
rd. The ground sluicing should be done at the earliest possible 
>ment in the open season or, preferably, late in the fall. The 
erator now has to deal with a stripped block of frozen gravel and 
riferous bed rock 4 feet in depth, carrying pay, or the material 
rich it is desired to sluice. A vertical bank of this material can not 
attacked by any mechanical appliance yet devised, but if left 
covered for six weeks it will thaw to bed rock. The operator, 
wever, wishes to take advantage of the continual thawing of the 
rface by the sun, and to do this he must attack not a vertical but 
horizontal surface. The rate of natural thawing varies from 6 
;hes to 1 foot a week. The scraper armed with teeth rigged so as 
make a series of transverse cuts, say for 300 feet lengthwise of the 
mnel, will attack the ground, scraping off the thawed material 
:ter than an}' other mechanical appliance. 
fhe sluice boxes or washing plant built to a height of 25 feet above 
l> surface of the ground can now be approached by a broad incline 
tform, or, if desired, the scraper may dump to cars which convey 
| material by gravity to an isolated and conveniently situated wash- 
| plant. The system adopted will depend on the magnitude of the 
