ay and gravel were irregular, but they rose
gradually. After rock was encountered, the formations of gravel and clay
were roughly parallel to the rock surface.

As the surface of the rock rose they disappeared in order and were again
encountered when the shields broke out of rock on the east side of the
Blackwell's Island Reef. East of the reef a large quantity of coarse
open sand was present in the gravel formations before the clay appeared
below the top of the cutting edge. In Tunnels _C_ and _D_ this was
especially difficult to handle. It appears to be a reasonable assumption
that the layer of clay was continuous across the reef. Wherever the clay
extended above the top of the shield it reduced the escape of air
materially. It is doubtless largely due to this circumstance that the
part-rock sections in the reef were not the most difficult portions of
the work.

While sinking the lower portions of the shafts the tunnels were
excavated eastward in the solid rock for a distance of about 60 ft.,
where the rock at the top was found to be somewhat disintegrated. This
was as far as it was considered prudent to go with the full-sized
section without air pressure. At about the same time top headings were
excavated westward from the shafts for a distance of 100 ft., and the
headings were enlarged to full size for 50 ft. The object was to avoid
damage to the shaft and interference with the river tunnel when work was
started by the contractor for the cross-town tunnel.

[Illustration: PLATE LXVI, FIG. 1.--REAR OF SHIELD SHOWING COMPLETE
FITTINGS.]

[Illustration: PLATE LXVI, FIG. 2.--SHIELD WITH LOWER PORTION OF
BULKHEAD REMOVED.]

The shields were erected on timber cradles in the shaft, and were shoved
forward to the face of the excavation. Concrete bulkheads, with the
necessary air-locks, were then built across the tunnels behind the
shields. The shields were erected before the dividing walls between the
two contracts were placed. Rings of iron tunnel lining, backed by
timbers spanning the openings on the west side, were erected temporarily
across the shafts in order to afford a bearing for the shield jacks
while shoving into the portals. The movement of the shield eastward was
continued in each tunnel for a distance of about 60 ft., and the
permanent cast-iron tunnel lining was erected as the shield advanced.
Before breaking out of rock, it was necessary to have air pressure in
the tunnels. This required the building of