400 The American Geologist. December, 1902. 
East river bridge No. 3. The limestone east of Blackvvell's island is 
enclosed between parallel fault walls, and appears to have been dropped 
down along therh. The numerous occurrences, however, of gneiss and 
gneiss only along, in and under the East river leave little doubt that 
the main portion of the bed is composed thereof. 
Regarding the bed rock beneath the North river, comparatively little 
is known, but the origin of its channel is sufificiently accounted for by 
its position along the contact of the Newark system with the crystal- 
lines. This contact seems surely to be a fault-border on account of 
its markedly rectilinear extension, the great scarp of basalt, the much 
inferior position of the newer terranes, and the borings along the 
route of the proposed tunnels of the Pennsylvania, New York and 
Long Island railroad compan}^ 
The author holds that the directions of the channels of Spuyten 
Duyvil creek and Harlem and East rivers have been determined largely 
by lines of jointing and displacement. Manhattan island borders di- 
rectly upon the Newark area, in which the existence of a network of 
faults has been established by the work of several observers, and the 
network probably extends beyond the. limits of the area. The strik- 
ing rectilinear outlines of the island, especially of the northern half of 
it, and its topographic development are favorable to the view that it 
represents an orographic -block left standing between dowmthrown 
strips of the crust. The rectilinear gorge of the upper Harlem between 
Washington Heights and Fordham Heights is continued, so far as its 
western wall is concerned some two and a half miles south of the river. 
It is parallel to the direction of the scarp of the Palisades, and of the 
Hudson. Besides the cross fractures indicated by the different parts 
of the Harlem river, which were pointed out by Stevens, several other 
cross fractures on and about Manhattan island were pointed out by 
the same author. Dana also considered that the Manhattanville cross 
valley was formed by a cross fracture. A considerable number of 
faults have been definitely established- Their directions correspond in 
general to the elements in the courses of the river channels. The ex- 
ceptions to this rule are the fissures in the East river east and west of 
Blackwell's island. 
The author cited a number of faults which have been disclosed by 
numerous borings and tunnels and in closing called attention to the 
fact that the buried rock surface in the lower part of the city (south 
of Twenty-third street), as well as that below the area of the Harlem 
flats (north of One Hundred Tenth street and east of Eighth avenue) 
is characterized by the most abrupt changes of level. In his opinion 
the area of these portions of the island represent orographic blocks 
depressed by faults, reefs of gneiss and limestone rising along the 
Harlem area, while reefs of gneiss alo^ne characterize the southern 
district. 
Profes.sor Hobbs' paper was discussed briefly by professors Kemp, 
Dodge and Stevenson, and it was evident that the author's theory 
would hot be accepted without considerable further investigation. 
