594 
GEOLOGY OF THE FIRST DISTRICT. 
until we pass 129th-street, where it rises above the surface of grading, but has been removed 
by blasting. Enough, however, remains to estimate the strike, which is N 45° E. Dip 
80° W. 
134th-street: On the banks of Harlem river is the termination of the railroad. 
The 4th, or Railroad avenue, passing through the central part of the island, and including 
some of the deepest cuts hitherto made in solid rock, displays the most interesting part of the 
geology of its vicinity. I have therefore taken numerous observations on this line, and 
recorded a sufficient number for the purpose of drawing conclusions with regard to the strike, 
dip, etc. of any given neighborhood. I have repeatedly traversed the streets (not yet opened) 
from 4th avenue to 3d avenue, and from 4th avenue to 5th avenue, for differences in character; 
wherever they have been found, they have been recorded. 
Leaving the 4th avenue, pursued course on returning to the city by 3d avenue. 
Harlem. 
The gneiss approaches the surface in a number of places in the northern part of Harlem 
village, but southward it does not appear until we reach 104th or 105th-street, and no rock 
appears in place from lOSth^street to 128th'Street, and from Harlem river on the east to 
beyond 8th avenue on the west; the whole being an immense bed of loam, sand and gravel, 
containing few boulders, or even pebbles of considerable size. It is nearly or quite level, 
and grows continually narrower as we go westward until we reach the Hudson. 
Third Avenue. 
Returning by the 3d avenue, we first meet the rock in place between 104th and 105th~street, 
on the west side, just after we cross Harlem creek (which it will be remembered is not 
Harlem river; the former is only a small tide stream from Harlem river putting westward 
two hundred or three hundred rods, crossing 3d avenue near lOSth-street), where the strike is 
N 25° E, and dip 70° to 80° W. 
The same rock is in sight frequently from 105th to 94th'Street, especially on the east side. 
Dip nearly vertical. Strike as before, N 25° E. 
94th-street, and from that to 105th~street, is a salt marsh, on the east side of the avenue; 
but from 94th-street southward, the island widens very much. 
94th-street to 79th-street : The rock appears in many places on the east and on the west, 
but varying little in the strike from N 25° E, and the dip is on average vertical. It is hill 
and valley, with a difference of height of from ten to forty feet, and here and there boulders 
of greenstone and granite. 
At Yorkville, 86th-street, the gneiss is quite elevated, and forms very considerable heights 
between 3d and 4th avenues, but east of 3d avenue it descends towards the East river. Here 
