the muck was handled by steam shovels.

In the double-heading method, shown on Plate LVII, the top headings for
each tunnel of the pair were driven separately, leaving a short rock
core-wall between them. The headings were drilled from columns in the
manner described for the single tunnels. The temporary rock dividing
wall between the headings was drilled by a tripod drill on the bench of
one of the headings, and was fired with the bench.

In the center-heading method, also shown on Plate LVII, only one heading
was driven. It was rectangular in shape, about 8 ft. high and 14 ft.
wide. It was located on the center line between the tunnels. In general,
the face was from 6 to 12 ft., or the length of one or two rounds, in
advance of the remainder of the face at the top. The center heading was
drilled by four drills mounted on two columns. By turning these drills
to the side, they were used for holes at right angles to the line of the
tunnels, by which the remainder of the face of the heading was blasted.
By turning the drills downward, the bench holes under the center heading
were also drilled. The center heading explored the rock in advance of
the full-width heading, and gave a good idea as to the care needed in
firing.

For the full-width-heading method, Fig. 2, ten drills were mounted on
five columns set abreast across the face. Holes were drilled to form a
cut near the center line between the tunnels. The remainder of the holes
were located so that they would draw into the cut. The bench was
frequently drilled from the same set-up of columns by turning the
drills downward. In sound rock this method proved to be the most rapid
of any.

Practically all trimming was left until immediately before the
concreting. It was then taken up as a separate operation, but proved to
be costly and tedious, and a hindrance to the placing of the lining.

_Materials Encountered._--All the rock encountered was the familiar
Hudson schist, but it varied widely in its mineral constituents and in
its physical characteristics. In many places where the rock surface was
penetrated, a fine sand was found that was probably quicksand. The
material above the rock in the open-cut sections was mostly sand.

[Illustration: FIG. 2. METHOD OF EXCAVATING WITH FULL-WIDTH HEADING
CROSS-TOWN TUNNELS, MANHATTAN
SIDE ELEVATION
FRONT ELEVATION
PLAN SHOWING POSITION OF COLUMNS FOR DRILLING FACE]

The concurrence of the watercourse, shown on