made
up of their topmost films, would, in any case, be inevitable. But that
the deep deposits of sand and clay might represent shore sands and
clay beds formed in the weathering of the gneiss ridges of Manhattan
Island ages before the Ice Age or its succeeding quaternary epochs, is
a violent assumption. The results of the decomposition of the rocks of
Manhattan Island, formerly far more elevated than they are to-day,
were doubtless removed to the south, and may, indeed, have formed
cretaceous and tertiary beds beneath the surface of Long Island.
The
geological conditions are readily understood, as indicated by these
deposits. This lower section of the island is underlaid by the same
kind of rock as forms the hills, prominences and ridges northward, and
over this floor a burden of loose strata has been accumulating, rising
in a succession of beds, each individually homogeneous, or relatively
so, as clay, sand and graveL These beds originated by water action upon
drift material heaped up possibly north and south of them, at any rate
near their present position, an action which washed out the clay
particles and permitted their settling in clay beds. This action, also,
parted the sand and gravel, and under the impulse of torrents or slow
shore-washing completed the separation of each. During the long periods
required for this gradual re-assortment, the island underwent changes
of elevation, alternating, probably, over one hundred feet, since on
the one hand gravel beds formed in swift and shallow streams are found
fifty feet below water level, and must have been much nearer the
surface when made, and, on the other hand, sand hills, also indicating
water action, are recorded eighty feet above tide. In many places such
modified beds were capped by mingled aggregates brought hither by ice
which, in rolling hills of stones, pebbles, and boulders, formed the
original surface of the island over much of its extent, the higher
hills and more remarkable boulders seeming to have been concentrated
eastward about Corlears Hook. As present conditions supervened, river
mud and other debris accumulated and
