PRIMARY ROCKS. 
523 
or bearing of the strata, it widens until it becomes from four hundred to six hundred yards in 
width, and from ten to thirty feet in height, forming a low ridge immediately west of the road> 
but east of the main ridge of gneiss which flanks the eastern shore of the Hudson, and which 
terminates at Tubby hook, about half a mile north of the commencement of the limestone. 
As the gneiss runs out at the mouth of the Spuytenduyvel creek, the whole ridge northward 
is limestone, the form and extent of which may be learned by inspecting the map of the island. 
The strike was examined in a number of places on the ridge of limestone, and varied from 
N, 25° E. to N. 45° E., and the dip from 50° E. to vertical. From Mr. Dykeman’s house 
(which is near the southern line of the limestone), to the old tide-mill (which is nearly oppo¬ 
site to Tubby hook), at a 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of inferior quality, other varieties at length superseded it.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 extreme eastern part of this section lies east of the three divisions already consi¬ 
dered, and is bounded east and north by the Harlem river, west by the valley of the Eighth 
avenue, and on the south by Harlem and Manhattanville valley. It terminates on the north at 
M'Comb’s dam, about one mile north of the Harlem bridge, and consists of a low ridge of 
gneiss, following the line of the Seventh avenue. It is from two hundred to four hundred 
feet wide, and from fifty to eighty feet above the contiguous plains. The valley through 
which the Eighth 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 142d-street is N. 
25° E., and the dip vertical. In the extreme north, at M‘Comb’s dam, the gneiss crops out. 
It has the strike N. 35° E., and the dip vertical; and is covered in many places to the depth 
of fifteen or twenty feet with the diluvium, consisting of loam, sand, gravel and pebbles, 
with boulders of white limestone like that of Kingsbridge, greenstone like that of the Palli- 
sades, 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 south-southeast; 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 M‘Comb’s dam,”* 
* Prof. L. D. Gale’s Report to W. W. Mather, on the Geology of New-York Island, in the Third Annual Geological Report 
of New-York, 1839, p. 177, 184. 
66* 
