purington.] DREDGING. 175 
still leaves much to be desired in connection with this phase of the 
investigation. In spite of these drawbacks the percussion drilling 
machine has no superior as a preliminary means of prospecting allu- 
vions thought to be suitable for working by the dredging method, and 
this is particularly true where large volumes of water may be encoun- 
tered near the surface. The sinking of shafts is to be preferred, how- 
ever, where not too costly. 
In several projects, where a large expenditure of money is in con- 
templation, so-called prospecting dredges have been used. These 
machines are small but complete dredges, and perform, on a small 
scale, the functions of the ordinary type. Their design is such that 
the} 7 may be readily moved, for they are equipped with lightly con- 
structed machinery and hulls and are of light draft. If, as is fre- 
quently the case, the area to be prospected is traversed by a river or 
sloughs, such a light dredge is easily moved from place to place. 
Where there are no waterways, a still smaller dredge, to be moved on 
rollers or skids, can be employed. 
Though the cost of installation and operation of such prospecting 
dredges is considerably more than that of drilling machines, the relia- 
bility of the results will often justify the additional expenditures, par- 
ticularly where large areas are being prospected. By their use the 
values per cubic yard, character of material, presence of large bowl- 
ders and clay, and depth and character of bed rock can be accurately 
ascertained. There is an additional advantage of great importance 
in the fact that the cost of operating a large dredge can at the same 
time be closely determined. 
The number of gold dredges in successful operation in the United 
States has greatly increased during the past decade, and especially in 
the last six years. The first successful one was operated at Bannock, 
in Montana, and was of the so-called "double-lift " type. The material 
was excavated by a chain of buckets and discharged from an eleva- 
tion of about 15 feet above the deck into a trommel, which had per- 
forations about 5 inches in diameter. Stones of larger diameter were 
discharged over the side of the dredge; the " lines" were elevated by 
a 12-inch centrifrugal dredging pump to a sluice and discharged about 
100 feet astern. This type of dredge is still advocated by some mana- 
gers, and unquestionably has a number of advantages over other 
types. It is especially good for handling material which has a clay 
content or which is so tenacious that gold is lost by incomplete disin- 
tegration and insufficient washing in the screens and sluices of the 
other types of gold dredges. The passage of the tines through the 
centrifugal pump of the double-lift dredge tends to disintegrate the 
material thoroughly and to completely liberate any particles of gold 
held in clay or other tenacious substances. 
