hobbs.] LONGITUDINAL SECTIONS OF THE ISLAND. 27 
furnished a detailed profile of the rock surface between these lim 
South of Union square this profile is characterized by the most abrupt 
changes in level, the greatest depression being- found at Duane street, 
though less marked interruptions of its course are encountered at 
Walker, Canal, and Houston streets and at Clinton place. Just 
south of 14th street (Union square) also, a marked though some- 
what less profound trough is disclosed. North of Union square and 
south of Chambers street the rock surface shows a comparatively 
regular slope (PI. V). At Duane street, where the maximum depth 
of 163.25 feet was found, a second or check boring was put down on 
the opposite side of Broadway, where a depth of 149 feet (13.75 feet 
less) was found. Records obtained by the writer from other sources 
show that the deepest part of the depression is not upon the line of 
Broadway but to the southwest (PL I). 
ALONG LENOX AND WESTCHESTER AVENUES FROM NINETY-SIXTH 
STREET TO MELROSE AVENUE. 
This section (fig. 5) has been reproduced from the map and profile 
prepared b}^ the rapid transit commission. It shows that the high 
bluff of gneiss, which forms the northern boundary of the Manhattan 
uplands near the northern line of Central Park, descends beneath the 
| level of the grade in the Harlem flats. Rock was nowhere encountered 
I in tunneling between this point and 145th street, where the tunnels 
descend in order to go beneath Harlem River. At that point, how- 
ever, limestone was met with and was penetrated by the tunnels beneath 
the river. Records from borings derived from other sources show 
that the rock surface beneath the Harlem flats is at distances vary- 
ing from 40 to 125 feet below its comparatively level grade. ^ Out of 
these depths rise, like islets, the reefs of rock which are now so fast 
being leveled to grade. 
It has been rather generally assumed that the area of the Harlem 
flats has been eroded in limestone. In the opinion of the writer it 
appears more probable that it represents a depressed composite oro- 
graphic block of gneiss and limestone, outlined on the west and south 
by faults along Eighth avenue and the northern wall of the Manhattan 
uplands. From this depressed composite block reefs of gneiss and 
limestone alike rise along precipitous slopes to and above the present 
urface. A limestone reef of this kind has been described by Gale." 
Stevens, 5 Ries. c and Dana/ and the latter was evidently at some pains 
:o explain his theory of the New York river channels because of this 
a Mather, Wm. W., Geol. New York, pt. 1, 1843, pp. 581-604. 
| & Stevens, R. P., Hist. Geol. New York Island: Annals Lyceum Nat. Hist. New York, vol. 8, 1865, pp. 
16-117. 
I cRies, Heinrich, Trans. New York Acad. Sci., vol. 10, 1891, pp. 113-114. 
dPana, J. D., Geological relations of the limestone belts of Westchester County and northern New 
fork Island: Am. Jour. Sci., vol. 21, 1881, p. 440. 
