PRIMARY RX)CKS. 519 
western in veins of granite and quartz ; while the southern 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 
one hundred feet in depth, and consists generally of from ten to eightyfeet 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 waves 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 Thirteenth and Sixteenth-streets; and on 
the west, from Twentyninth to Thirtyfirst^street, The greatest -thickness of diluvium is at 
the southeastern extremity, where it was accumulated in conical hills, some of which were 
seventy or eighty feet above the grading of the streets.. The general direction of the strata 
corresponds very nearly with that of the avenues ; and the dip, though generally to the west, 
averages within ten degrees 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 portions, 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° W. to 45° E., although the composition is more uniformly gneiss, than in other 
portions of the island. 
“ 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 118th- 
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 
one hundred 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 section lying between the settled portion of the city and the level valley extending 
from Manhattanville on the Hudson to Harlem on the East river, is generally bounded on the 
south by the outcropping of the gneiss, which, as before stated, commences on the west at 
Thirtyfirst-street on the Tenth avenue, at Twentyninth-street on the Ninth avenue, at Twenty- 
eighth-street on the Eighth avenue, at Twentyfourth-street on the Seventh avenue, at Twen¬ 
tieth-street on the Sixth avenue, and at Sixteenth-street on the Fifth avenue, where it comes 
within four feet of grading; on Thirteenth-street, between University place and Broadway, 
it again approaches within three feet of the surface; and on Fourth, Third and Second 
avenues, at their several junctions with Sixteenth-street, it appears at or above the surface ; 
while on the First avenue, it crops out first at Twentyfirst-street, and continues as we pro¬ 
gress to the northward on the banks of the East river, until we reach Ninetyfourth-street, 
which is at the eastern termination of Harlem flats, where the rock dips below the surface, 
leaving a salt marsh which from this point continues northward. Again commencing on the 
west side of the island, we find the rock continues from Thirtyfirst-street northward to Man¬ 
hattanville, almost constantly above the surface at or near the water’s edge. 
