Rivers of Northern New Jersey. 107 
tilting of the plain seems to have reversed its direction of flow. 
It rises near the center of the State and flows northwestward till 
it joins the Raritan near Somerville, and on the way it crosses 
from the thrown or depressed to the heaved or elevated side of 
the “fall-line,”* and passes through a deep gap in the trap ridge 
of Rocky Hill back of Princeton. I believe there is no other 
Atlantic river which runs against the fall-line in this way ; and it 
is certainly at first sight remarkable that a stream of moderate 
size like the Millstone should have held its own against a displace- 
ment that sufficed to deflect great rivers like the Delaware and 
the Susquehanna from their courses. 
The Millstone appears to have been a stream of the normal 
kind in the previous cycle, before the tilting of the Central plain, 
when it probably ran southeastward with its fellows, and carried 
off its share of waste in the baselevelling process of that time. 
No other supposition than this seems consistent with the general 
history of the region. It was during that cycle that the deep 
gap was cut in the Rocky Hill trap ridge. Then came the 
deformation of the baselevelled plain, the relatively recent eleva- 
tion and gentle tilting that have permitted the streams to carve it 
into a pastplain; and with this, the dislocation along the fall- 
lme. The inclination of the interstream surfaces of the past- 
plain leaves no doubt that it was tilted to the northwest, and to 
this tilting we must ascribe the present direction of the Millstone 
flow: but why did not the accompanying dislocation on the fall- 
line throw this moderate sized stream off of its track and divert it 
southwestward to the Delaware at Trenton, or northeastward to 
the Raritan below New Brunswick. The effect of the dislocation 
appears with considerable distinctness along a line from Trenton 
towards Amboy, in the less altitude of the general surface of the 
pastplain to the southeast than to the northwest of the line, the 
difference of altitude of the two parts being about a hundred feet. 
The persistence of the Millstone against such a dislocation seems 
to require that we should postulate a slower and smaller move- 
‘ ment here than that which deflected the Delaware. 
The reversed course of the Millstone cannot be regarded as an 
example of inversion following a capture of its ancient northern 
headwaters by a branch of the Raritan ; for in such a case, surely 
the inversion could not have progressed farther south than the 
* For an account of the ‘‘fall-line” displacement, see McGee, Seventh 
Ann. Rep., U. 8. G. S., 1888, 616. 
