Davis and Wood.] 
392 
[Nov. 20, 
shales all dip at a moderate angle northwestward or thereabouts. 
It is, indeed, a more recent and more evident baselevel plain than 
that of the Highland plateaus. We have given it the name of the 
Central plain from its position in the state. The baselevel, with ref- 
erences to which it was denuded, may be called the Somerville 
baselevel, from the excellent development of the plain in the neigh- 
borhood of that town. 
The Central plain is developed with considerable distinctness 
over most of the Triassic formation, where the soft shales have been 
quickly degraded from the Scliooley baselevel. On crossing the 
Delaware to Pennsylvania at Yardley by the Bound Brook railroad, 
the broad fields that stretch far and wide at corresponding heights 
on either side of the Delaware are conspicuous illustrations of the 
surface of this extensive plain, showing that it is not limited to 
New Jersey. Thence northeastward, it forms the general surface 
of the lowlands between the trap ridges, until its smoothness is con- 
cealed by the addition of the morainic hills between Plainfield and 
Elizabeth. Along the Pennsylvania railroad it is almost unbroken 
from Princeton nearly to New Brunswick, being developed here on 
the lower strata of the Cretaceous formation. Farther southeast- 
ward, it is continuous for some distance, as along the railroad from 
Monmouth Junction, for example, almost to Jamesburg ; but there 
it is much broken, and further on it is difficult to trace ; south- 
westward, along the Delaware, it is trenched by streams, and its 
higher parts are much broken. 
The northern extension of the plain into the glaciated area has 
not been closely examined. 
The production of the Central plain on the Somerville baselevel 
illustrates to a nicety the different rates of development of a plain 
on rocks of different hardness. In the region of the crystalline 
rocks, the Somerville plain is not perceptible, unless in the lime- 
stone valleys ; and even there it is altogether subordinate to the 
great mass of the little broken plateaus on either side. In the Tri- 
assic region, the greater part of the rocks are soft enough to al- 
low the Central plain to take on good form ; but there are portions 
of the Triassic mass that are hard enough to retain in good preser- 
vation remnants of the Schooley peneplain, namely, the trap ridges 
and the Hunterdon plateau of hard sandstone ; the former are so 
hard as to form long, continuous ridges of even height, whose value 
in restoring the Schooley plain has been recognized. In the Cre- 
