393 
[Davis and Wood* 
taceous portion of the Central plain, even a greater share of the 
beds are soft enough to have been worn down to baselevel ; and the 
only parts that have withstood this reduction are certain loosely 
cemented conglomerates and sandstones, associated with the green 
sands, which still seem to stand above the general level of the plain 
that surrounds them. They appear in a range of disconnected hills 
from Navesink southwestward ; Pine Hill near Perrineville, at the 
head of the Millstone river, is of this class. It is in a country of 
remnants like these, especially when the region is complicated by 
the erosion of a later cycle still, that we find the greatest difficulty 
in tracing out ancient plains of denudation, and hence the restora- 
tion of the Somerville plain in the southeastern part of the state is 
at present uncertain. 
Inasmuch as the above mentioned Cretaceous hills remain on the 
seaward part of this plain, and as it extends with about as much 
distinctness back of Rock}'' Hill and Sourland mountain as in front 
of them, we have concluded that it is a product of subaerial and 
not of submarine erosion. 
23. Date of the Somerville cycle. The Central plain is manifestly 
of younger date than the Schooley plain, for the former is exca- 
vated below the surface of the latter. Baselevelling has been ac- 
complished at the Somerville level only on the softest rocks. The 
Highlands were subjected to erosion during the same period and yet 
were only channelled by deep valleys with strong sloping sides ; but 
the soft Cretaceous and Triassic beds were less successful in re- 
sisting erosion and hence were reduced so low as to form broad- 
open lowlands, broken chiefly by the trap ridges, whose crests re- 
tain something of the Highland altitude, as already described. The 
Somerville attitude was therefore maintained for a considerable 
period, but for by no means so long a time as the Schooley cycle 
endured. Finally, since the Central plain was lifted, the streams 
have accomplished only a small share of the new work offered 
them ; post-Somerville time has not been nearly so long as Somer- 
ville time. The moraine on the northeastern part of the plain has 
not been nearly as much eroded as the plain itself. These state- 
ments may be summarized as follows : the Somerville baselevel 
was maintained for a considerable part of post-Cretaceous, that is 
of Tertiary time ; the elevation that put an end to the develop- 
ment of the plain with reference to this baselevel took place in late 
