184 
[Assembly 
ridge of limestone, and varied from north 25° east, to north 45° east, 
and the dip from 50° east to vertical. From Mr. Dykeman's house, 
which is near the southern line of the limestone, to the old tide-mill, 
(which is nearly opposite to Tubby-Hook,) at distance half a mile or a 
little more, is a line of abandoned quarries, which many years ago were 
extensively wrought for burning into lime; but as the lime was of infe- 
rior quality, other varieties at length superseded it. This limestone has 
been called dolomite, but whether from its granular structure, or from 
its composition, I am unable to determine. I have not met with any 
analysis of it, nor do I think there is any on record. 
The individual minerals, and the diluvial furrows, will be considered 
in another place. 
The extreme eastern part of this section lies east of the three divi- 
sions already considered, and is bounded east and north by the Harlem 
river, west by the valley of the 8th avenue, and on the south by Har- 
lem and Manhattanville valley. It terminates on the north at McComb's 
dam, about one mile north of the Harlem bridge, and consists of a low 
ridge of gneiss, following the line of the 7th avenue. It is from 200 
to 400 feet wide, and from 50 to 80 feet above the contiguous plains. 
The valley through which the 8th avenue passes, is throughout its 
course a perfect level, and but a few feet above the waters of the river. 
The strike of the strata of this ridge at 142nd-street is north 25° east, 
and the dip vertical. In the extreme north, at McComb's dam, the 
gneiss crops out. It has the strike north 35° east, and the dip vertical; 
and is covered in many places to the depth of 15 or 20 feet with dilu- 
vium, consisting of loam, sand, gravel and pebbles, with boulders of 
white limestone, like that of Kingsbridge, greenstone like that of the 
Palissades, sandstone and granite. The granite is generally rough and 
angular; while most of the others, especially greenstone, are rounded 
and smooth. In this vicinity the general course of the river, as well as 
the valley through which it runs, is SSE, so that any current from the 
northwest would be likely to accumulate any materials swept along in 
its course on such prominences as that at McComb's dam. 
III. 
General remarks on the transported materials of the island^ both alluvial 
and diluvial. 
The island contains in every part abundant evidence of a current 
having swept over it from northwest to southeast, both from the grooves 
and scratches still visible on the solid rocks of the island itself and its 
