general, they were found to be about as indicated in the preliminary
borings. The materials met in Tunnel _A_ may be taken as typical of all.

From the Manhattan shaft eastward, in succession, there were 123 ft. of
all-rock section, 87 ft. of part earth and part rock, 723 ft. of all
earth, 515 ft. of part rock and part earth, 291 ft. of all rock, and 56
ft. of part rock and part earth.

The rock on the Manhattan side was Hudson schist, while that in the reef
was Fordham gneiss. Here, as elsewhere, they resembled each other
closely; the gneiss was slightly the harder, but both were badly seamed
and fissured. Wherever it was encountered in this work, the rock surface
was covered by a deposit of boulders, gravel, and sand, varying in
thickness from 4 to 10 ft. and averaging about 6 ft.

The slope of the surface of the ledge on the Manhattan side averaged
about 1 vertical to 4 horizontal. The rock near the surface was full of
disintegrated seams, and was badly broken up. It was irregularly
stratified, and dipped toward the west at an angle of about 60 degrees.
Large pieces frequently broke from the face and slid into the shield,
often exposing the sand. The rock surface was very irregular, and was
covered with boulders and detached masses of rock embedded in coarse
sand and gravel. The sand and gravel allowed the air to escape freely.
By the time the shields had entirely cleared the rock, the material in
the face had changed to a fine sand, stratified every few inches by very
thin layers of chocolate-colored clayey material. This is the material
elsewhere referred to as quicksand. As the shield advanced eastward, the
number and thickness of the layers of clay increased until the clay
formed at least 20% of the entire mass, and many of the layers were 2
in. thick.