Purington.] DREDGING. 189 
The now general use of variable-speed motors on electric dredges 
has had good results. Constant-speed motors on headline dredges 
working in indurated material had brought the headline method of 
dredging into bad favor, whereas variable-speed motors permit a 
chain speed suitable for indurated material, with less surging and 
breakages. Under these conditions about the same amount of mate- 
rial is excavated, as a slow chain speed with low theoretical capacity 
has often in tenacious gravel a greater actual capacity than a high 
chain speed with high theoretical capacit}^. This is due to the incom- 
plete tilling of the buckets when running at too great a-chain speed. 
In Alaska, where much frozen ground is encountered, the conditions 
are not favorable for the successful operation of gold dredges, espe- 
cially as the long winter reduces the number of working days to about 
one hundred and thirty a year. Nearly all the deposits of aurifer- 
ous alluvions are frozen, though some areas, notably in the beds of 
rivers, are unfrozen. Such unfrozen zones are easily dredged and 
large volumes can be cheaply handled. The depths to bed rock are 
moderate, but the bed rock is generally unfavorable for easy excava- 
tion, and the ground is characterized by a great concentration of 
values near and in it. Lateral concentration also is great and the pay 
streaks are often narrow. High costs of installation and operation 
are other drawbacks in regions far from the coast, to which freight 
charges are high. Considerable areas of so-called i4 worked out" 
ground in this northern field, particularly in the Klondike district of 
the Yukon Territory, would furnish excellent dredging ground if the 
material were not frozen. Steam thawing now costs about 40 cents 
a cubic yard, but a cheaper method of thawing would make much 
ground available for highly profitable dredging. It is probable that 
if wide cuts were made the banks might thaw as rapidly under expo- 
sure to the sun as they could be removed by the dredge. Another 
factor which may be of importance is that ground which has once been 
worked does not freeze to any great depth. It is evident that the chief 
problem of gold dredging in this northern province is in the thawing of 
the material, for many of the other conditions are favorable for profit - 
able exploitation by this means. 
Had gold dredges been used in tne North when the first mining was 
done on the richer creeks much more gold would have been recovered 
and the cost would have been less, for the earlier methods were exceed 
ingly wasteful and costly. The overburden of muck could have been 
easily sluiced off and the thawing of the underlying gravels would 
have been easy when aided by exposure to sun and air. 
In this northern region, because of the small vertical section and 
absence of much line sand in the gravels, dredges with short digging 
ladders and stackers can be used. This equipment demands only small, 
light hulls and machinery. Large volumes of thawed materi-.il can be 
