have
furnished refractory ware and brick. The trap rock has been used in
local construction, house and bridge building, and widely for pavements
and road metal. The serpentine in beds becomes fibrous, and has been,
mined for " asbestos," though such a use of it is very limited. The
very heavy and inexhaustible beach sands prevalent at Seguin's Point,
Ward's Point, and Princess Bay have been shipped to New York and
'Brooklyn. The black oxide of iron (magnetite) occurs in considerable
quantities through the beach sands on the southern shores, but it has
never been of any economic value.
The
rocks which will naturally control the attention of the teachers will
be the serpentine, whose configuration in a broad band of undulating
summits is so pleasingly seen from the lower bay. The serpentine is a
silicate of magnesia with water, and it is a mineral or, when occurring
in extended beds, a rock, over which a great deal of discussion and
speculation has arisen as to its origin. It is generally supposed not
to be an original deposit, but a result of changes in earlier rocks or
minerals by which a sort of residue, this hydrous silicate of magnesia
or serpentine, remains, the other chemical elements of the primary
mineral being removed by solution, or in some other form deposited
within or alongside of the serpentine itself. It will be recalled that
in the paragraph on the serpentine of Manhattan Island the original
mineral was found to be an amphibole (hornblende or actinolite) whose
change had produced the serpentine and secondary calcite.
The
great serpentine beds of Staten Island seem to have originated in a
similar way from identical conditions. If they have, this would also
justify an auxiliary inference that these basal rocks of the Borough of
Richmond are also the equivalents of and contemporaneous with those of
Manhattan. The theories regarding the origin of the serpentine may be
gathered under four heads. First, those that assign it to altered
eruptive and volcanic rocks of metamorphic schists; second, those that
trace it to replaced sedimentary beds of limestone or dolomite; third,
the abandoned hypothesis of Dr. Hunt that
