ECONOMIC GEOLOGY. 49 
ground. These methods have gradually developed in the Yukon 
Territory and in Alaska, and from year to year have become more 
efficient in solving the problems that are met. In the Fairbanks 
region in 1903 thawing was accomplished by the cruder methods 
mentioned, and equipments for thawing by steam, which had been 
found so effective in the Klondike region, were not plentiful. Since 
then extensive steam plants have been introduced, capable of thaw- 
ing and handling daily large quantities of gravel. 
The process in general includes the following operations: (1) 
The sinking of a shaft to bed rock, ranging in depth from 20 to 
300 or more feet; (2) the timbering of the shaft and the portion of 
the drifts near the shaft; (3) the opening up of the ground by drifts 
which are run either parallel to or across the pay streak and from 
which crosscuts are driven; (4) the extraction of the gravel from 
the crosscuts, beginning at the farther limits of the drifts and work- 
ing toward the shaft; (5) the hoisting of the pay gravel with as 
little waste as possible to the surface; and ((>) the recovery of the 
gold by ordinary sluicing. The main drift is usually carried to a 
maximum distance of about 200 feet in each direction from the shaft, 
and the ground is blocked off by crosscuts having a variable length up 
to about 100 feet. Fortunately but little timbering is generally 
required. Where the ground is weak, pillars are left at intervals 
of about 25 feet when working: back the faces toward the shaft. 
Ordinarily, as mining commences at the extreme limit of the area 
to be worked, the ground from which the pay dirt has been removed 
is allowed to settle if it will. Experience has shown that settling is 
generally so gradual that the work can be carried away from the 
settling ground with sufficient speed to avoid trouble. 
The steam-point method of thawing is the one most commonly in 
use. The steam point is a piece of one-half or three-eighths inch 
hydraulic pipe, 5 to 8 feet or more in length, with a blunt, hollow 
point of tool steel for piercing the ground and a solid head of tool 
steel or machine steel, sufficiently strong to withstand the impact 
of a maul or sledge. Steam is admitted through a pipe fitted laterally 
in a small aperture near the head. The points are placed about '2\ 
feet apart, and from a dozen to twenty or more are used in a plant 
of average size. The power needed is 1 to 2 horsepower per point 
and the duty of a point is 3 to 4 or more cubic yards per day of ten 
hours. In use the point is driven in gradually as the ground becomes 
thawed. It is customary in most places to use either hot water ;it 
a temperature of about 140° F. or a mixture of hot water and steam 
while driving the points, and then to complete the thawing by means 
of steam alone, since by employing hot water in a part of the opera- 
tion the atmosphere of the mine does not become so vitiated through 
24304— Bull. 337— OS 4 
