magnesian
limestone containing a little over twenty-one per cent, of magnesia,
and is not a true dolomite. Mr. E. C. Eckel recently discovered a point
of contact between the In-wood limestone and the Manhattan schists
which seemed clearly to show that the latter at this point overlay the
limestone beds.
The
two transverse depressions at Manhattanville and at Inwood, which are
features in this third topographical section of New York City, have
been regarded by Professor Dana as the results of an oblique wrenching
of the rocks, or a sort of lateral pull which has at these points
separated the ridge and permitted the agencies of weathering to effect
the widening and reduction of these initial crevices. It is evident
also that they have been former passages for the current of the Hudson
River to pour through eastward. To-day a lowering of the general level
of the land forty feet would bring these passages into tidal
communication with both the Hudson and Harlem. There are evidences in
both that they have been the channels of ice movements, and the
alluvial or detrital plains into which they enter, the Hudson flats on
the one hand and the upper basin of Harlem River from Marble Hill to
Morris Docks on the other, have originated in fluviatile motions
through these gateways, fluviatile motion which has modified drift
deposits previously accumulated in these hollows. The Spuyten Duyvil
Creek is considered a possible third break in the rocks, and is far
more irregular. It has been called by Stevens a " cross fracture," and
forms now a picturesque gorge.
The
limestone of Manhattan Island and its identical prolongations,
northward in Westchester County, illustrate very forcibly a stage in
metamorphism. Throughout the original calcareous and magnesian beds
were disseminated, in irregular sporadic and adventitious mixtures and
deposits, siliceous, ferruginous, aluminous, and alkali sediments which
quickly upon the supervention of chemical opportunities, combined with
the bases of the calcareous muds to form various sili-
