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[Assembly 
stone; and the western in veins of granite and quartz; while the south- 
ern and eastern, as well as the middle portions, are more purely gneiss. 
The covering of the rock, in the southern part of the island, is in 
Some places more than 100 feet in depth, and consists generally of from 
10 to 80 feet of diluvium (loam, gravel, or boulders, one or all) on the 
surface, resting on the same or greater thickness of alluvial or tertiary 
sands, which last are highly stratified, and in many places even exhibit 
the appearance of ripples as from the retiring weaves of the ocean. As 
we proceed northward, the deposits grow thinner and thinner, until the 
rock makes its appearance above the surface. This occurs on the east 
side of the island, between 13th and I6th-streets, and on the west, from 
29th to 31st~street. The greatest thickness of diluvium is at the south- 
eastern extremity, where it was accumulated in conical hills, some of 
which were 70 or 80 feet above the grading of the streets. The gene- 
ral direction of the strata, corresponds very nearly with that of the ave- 
nues; and the dip, though generally to the west, averages within 10 de- 
grees of vertical. In speaking of different sections, the dip, on the 
west side, more especially from the city to Harlem valley, is with few 
exceptions, vertical; and the same remark applies to the middle por- 
tions, as far east as to the Eighth avenue; while on the east side, from 
Fourth avenue to the river, the dip is quite irregular; varying from 45"^ 
west to 45° east; although the composition is more uniformly gneiss^ 
than in other portions of the islands 
In a few cases, it has been found that carbonate of lime enters into 
the composition of the rock of the island. This is the case on the east 
side of the Fourth avenue, from the ll8th-street to the 120th-street. 
Here the rock retains its identity, as gneiss, to all appearance; although 
half of its mineral matter is carbonate of lime. Again, at 157th-street, 
and about 100 feet west of Tenth avenue, the rock is entirely changed, 
both in composition and structure. In composition, it is a mixture of 
limestone and serpentine; while in structure, it is destitute of strata^ 
and lies in beds. 
The following remarks will be arranged under seven heads, as fol- 
lows: •• 
1. Description of the region between the city of New-York and Har- 
lem, and Manhattan ville valley. 
2. The region between the above valley and Kingsbridge, or from Har- 
lem and Manhattan ville valley, to the northern extremity of the 
island. 
