26 ROCK FLOOR OF GREATER NEW YORK. [bull, m 
The borings which have been utilized in making these sections com- 
prise some 90 well-distributed core borings, supplemented by 75 
or more intermediate wash borings. The rock was penetrated by 
the diamond drills in some cases as far as 100 feet, and revealed in all 
cases some phase of granite or gneiss, no limestone having been any- 
where found. 
A number of interesting results have followed from this work. It 
is first to be noted that the steep rock wall which forms the western 
margin of the island from 81st street northward to Manhattan ville is 
on the line of these sections continued beneath the pierhead line of 
the I liaison shore as an equally definite topographic feature. The 
eastern wall of the island is even steeper than the western. Its sur- 
face was followed down to depths of 100 feet beneath the bulkhead line, 
and was found to rise gradually from this depth in going eastward from 
the pierhead line. Between the bulkhead and pierhead lines the rock 
surface is of great depth and was determined with difficulty. Under 
the old A pier <>f the 34th street ferry, approximately on the extended 
center line of 34th street and immediately in the rear of the A pier as 
it now exists, thirty-five soundings with a water-pressure drill were 
first made before a passageway was found under the surface cribwork 
and riprap of the island. The casing was finally driven to a depth of 
208 feet. Below a depth of L29^ feet, mean low water, it is stated 
that the drill entered a rotten, decomposed gneiss. Though the 
diamond drill was put in it would not make a core, and the material 
crumbled under it." This apparently decomposed rock would appear 
to be in all respects like that which was found along the supposed 
fault crevice in the driving of the siphon for the New York aqueduct 
under Harlem River. As regards position it would correspond in 
this instance to the continuation of one of the possible faults of the 
Blackwells Island series. 
In the vicinity of the terminal site, near Eighth avenue, there is a 
deep depression in the rock surface, which has inward-sloping walla 
not only upon the east and west but to the north and south as well. 
To the east of Seventh avenue the surface is broken into a series ol 
remarkably sharp and regular troughs which run transverse to the 
section, or longitudinally along the island, the surface as a whole slop 
ing steeply to the south. 
LONGITUDINAL SECTIONS OF THE ISLAND. 
ALONG BROADWAY FROM THE BATTERY TO THIRTY-THIRD STREET. 
The extensive series ,,f borings made by the rapid transit connnis 
sioners in connection with the subway recently constructed/' ha 
"Information kindly furnished by A. Noble, chief engineer, East River division, and .1. Vipol 
Da vies, who was in charge of the drilling. 
bParsons, Win. H.. Borings in Broadway, New York: Trans. Am. Soc. civil Eng., vol. '28, 1893, pj 
13-18, pi. 6. Seealso Engineering News, vol.26, July 18, 1891, p. 62 (fig.)- 
