shown in the drawing, was erected in the bottom heading, and extended
through the break-up chamber. The plan was then to drill the entire face
above the bottom heading and blast it down upon the timber staging, thus
maintaining a passage below for the traffic from the heading and
break-ups farther down the line. Starting with the condition indicated
by Plate XIII, the face was drilled, the columns were then taken down
and the muck pile was shoveled through holes in the staging into muck
cars below. The face was then blasted down upon the staging, the drill
columns were set up on the muck pile, and the operation was repeated.
This method has the advantage that the bottom heading can be pushed
through rapidly, and from it the tunnel may be attacked at a number of
points at one time. It was found to be more expensive than the
top-heading-and-bench method, and as soon as the depression in the rock
at about Station 59 was passed, a top heading about 7 ft. high, and
roughly the segment of a 23-ft. circle, was driven to the next soft
ground in each of the four tunnels. The remainder of the section was
taken out in two benches, the first, about 4 ft. high, was kept about
15 ft. ahead of the lower bench, which was about the remaining 11 ft.
high.


EXCAVATION IN EARTH AND ROCK.

About 2,500 ft. of tunnel, the roof of which was in soft ground, was
excavated in normal air by the mining-and-timbering method. In the
greater part of this the rock surface was well above the middle of the
tunnel. The method of timbering and mining, while well enough known, has
not been generally used in the United States.

[Illustration: PLATE LXIII]