SURVEY OF THE ISLAND OF NEW-YORK. 
599 
dering on Harlem creek, and which is connected with Harlem valley, the rock appears in 
place only on the banks of the river, and varies a few degrees only in strike or dip, and 
affording an average -strike of'TSf 40° E. Dip vertical. 
North Part of the Iseand. 
4th avenue terminates on Harlem river, at 133d-street, from which point I followed the river 
banks, keeping on the high ground to Kingsbridge, and returned by the great road from 
Harlem to Kingsbridge, which passes a little east of the middle division of the island. 
As we follow up the southwestern shore of the river, the ground is low and marshy, and 
the Harlem flats extend to 142d-street, and west to 7th avenue. At this junction, about half 
a mile north-northwest of Harlem bridge, the gneiss first makes its appearance upon the 
surface. Strike N 25° E. Dip vertical, or 85° W in some places. 
The rock is generally well stratified, fissile and tender, containing thin veins of granite, 
and in some places highly charged with iron rust; in bthers, hornblende so predominates as 
to cause the rock to assume the columnar structure. Sometimes mica occurs in so large 
laminae, and black, as to give a showy exhibition. 
On a line with Tth avenue, from Harlem village northward, and in direction north-northeast 
and south-southwest, there is a range of elevated ridge, two hundred to four hundred feet 
wide, and fifty to eighty feet above the river on the east and the valley of 8th avenue on the 
west, and extending northward to M'Corab’s dam on the northern extremity of 8th avenue. 
The rock of this ridge is so impregnated with iron pyrites, as almost entirely to prevent its 
profitable use as a building material where it is to be exposed to atmospheric agents. 7th 
avenue is not opened any where beyond Harlem. 
The earthy covering of the ridge is fine drift loam, supporting good crops of grain and 
grassy and here and there boulders of greenstone, granite, and on its northern part abundance 
of white marble boulders. At this place, that is, near M'^Comb’s dam, the ridge is covered 
in many places from fifteen to twenty feet with drift loam, sand, gravel and pebbles, with large 
boulders in limestone or marble, which makes nearly one-third of the whole, the rest being 
greenstone like that of the palisades, and sandstone like that under them ; and the marble 
precisely like that of Kingsbridge. Boulders of gneiss and granite were also found in consi¬ 
derable quantities, but angular and rough, while the others were generally rounded and 
smooth. 
The gneiss here appears in place, cropping out in many places, full of granite veins, but 
well stratified in the main, and with here and there traces of carbonate of lime. Strike N 
35° E. Dip nearly vertical. 
The river and general course of the valley, for nearly two miles, and perhaps more, above 
this place, runs N 15 or 20° E, and from the materials, their position, and that of the valley 
together, the inference is strong that the materials here piled up have come from the Palisades, 
Kingsbridge, and the contiguous parts of the valley. 
