1887. | NEW YORK ACADEMY OF SCIENCES. 59 
past history of these beds that any particular or minute descrip- 
tion of their process of formation must necessarily be, as yet, 
largely conjectural. But they are undeniably of the same geo- 
logical age as the gneiss, a consideration of great importance in 
determining it, and which will be referred toagain. The extent 
of the limestone area is marked out on the map; for it I am al- 
most entirely indebted to the article on tke ‘‘ Limestones of 
Westchester Co.” by Prof. Dana, Amer. Jour. Sci., V.. XXI. 
(1881), p. 425, who has described the beds on the island with 
great minuteness. 
Structurally the island is a long ridge of gneiss, cut off from 
the mainland by the valley of Spuyten Duyvil Creek, and broken 
at Inwood and Manhattanville by two cross valleys, and in the 
upper portion somewhat divided longitudinally by small valleys. 
The gneiss is prolonged to the north in the main-land and 
runs into the Highlands. To the southward it appears in 
Bedloe’s Island! and Staten Island.2 To the west it disap- 
pears under the Hudson, reappearing only opposite the southern 
portion in a small outcrop in Jersey City,? now graded 
away. Opposite the northern portion are the Palisades with 
their trap and sandstone. The contact is not visible, being under 
the river. To the east the gneiss runs under the East River and 
outcrops in Astoria and Long Island City, and on Blackwell’s 
Island. But south of these it has not been found. It is proba- 
ble that it underlies the drift on which Brooklyn is built, but at 
too great a depth to be reached by any excavations yet made.* 
The island is thus a narrow gneiss ridge, with triassic trap and 
sandstone on the west, and very well connected north and south 
with the gneiss of the mainland. As to the age of the rocks 
there is wide diversity of opinion. The island is an outlying 
post ina geological battle ground which has been fought over 
with great vigor, as the last thirty years or more of Svl/iman’s 
Journal abundantly show. Nor is the discussion at rest yet. 
Briefly, they are pronounced by the one side to be Archean, and 
part of a connecting ridge between the Appalachians and the 
mountains of New England. Dr. T. Sterry Hunt, the most 
prominent advocate of this school, classes them with his Mon- 
1 Pep. Sci. Monthly, Vol. XIII., ‘‘Geol. N. Y. Island,” by Dr. J. S. 
erry. 
2 Ann. N. Y. Acad.Sci., Vol. II., p. 161. ‘* Geol. Richmond Co.,” 
Dr. N. L. Britton. 
3 Ann. N, Y. Acad. Sci., Vol. II., p. 27 seq. ‘‘Geol. HudsonCo.,” I. 
H. Russell. 
4 Russell (op. cit.) givessome results of test borings for the Brooklyn 
Approach of the East River Bridge. 
