Northwest Seal Rock is nearly two miles outside of its nearest
neighbor. Southwest Seal Rock, with a clear and deep, but unused
passage between, and has close to it on all sides from one hundred-
and eight to one hundred and eighty feet of water, with no outlying
danger?. It is a mass of mctamorphic material, varying considera-
bly in character, extremely hard to drill, and brittle under the action
of explosives, but offering almost the resistance of glass to the action
of the sea.
The superficial area of the rock at the water-line is about forty-
six thousand square feet, and its general form is an oval with a high
central ridge running nearly east and west along its longer axis,
sloping gently on the north, but more steeply on the sides from its
crest to the sea level. To the westward is a prolongation, called
Little Black Rock. The greatest height of the ridge, fifty-four feet,
is at its eastern end.
The gentle lower slopes and smooth, water-worn surfaces, were
plain indications that the sea at times swept over its top. Yet, to
gain the requisite area for the foundation, it was necessary to exca-
vate at a point fully thirty feet below the crest. There was no space
available on the site where even temporary security of men or mate-
rial could be assured, and the frequency and quickness with which
all parts of the rock became untenable, greatly exceeded any previ-
ous anticipation. It is a peculiarity of this coast that a heavy sea.
