108 National Geographic Magazine. 
hard trap ridge of Rocky Hill, where a stable divide would have 
been formed: nor can the Millstone be regarded as an original 
stream, first developed and consequent upon the deformation of 
the Central plain, for in that case, it should consist of two sepa- 
rate parts ; One part running from the actual head of the river to 
the fall-line, where it would turn southwest and cross the faint 
flat divide that separates it from the Delaware ; the other part 
beginning by Princeton north of the fall-line, and running thence 
north to the Raritan. The continuity of these two parts in the 
actual Millstone seems to be explicable only by regarding the 
river as the upper portion of a single larger river that had 
reached an old age in the previous cycle ; it was then broken in 
two at the head of the present river where the greatest elevation 
of the Central plain occurred, and thus had its former head 
waters reversed from a southeast to a northwest direction of flow 
across and against the fall-line break by the tilting of the plain. 
Only in this way can the deep gap in Rocky Hill be explained. 
The river is thus consequent on the tilting of the plain, and yet 
antecedent to the accompanying faulting. It cannot be called an 
original stream, for it had an ancestor in its very channel. It is 
not a purely consequent stream, for it runs against the heaved 
side of a fault. It is not a strictly antecedent stream, for it flows 
in a direction determined by a disturbance that occurred late in 
its life. It is too exceptional a stream to have a generic name. 
We cannot expect to find many others like it. 
The result that has been of the greatest interest to me in these 
studies is the discovery of well-recorded and peculiar histories in 
the commonplace small-sized rivers of our Atlantic slope. We 
have looked for some years to the west as the region where river 
history should be illustrated, because it was there that the pioneers 
in this branch of study taught us the lessons on which our further 
work must depend. But home study as well as distant travel has 
its rewards, and with the progress of good topographic work on 
this side of the country we confidently await much instruction 
from a close acquaintance with the curious histories of many” 
of our rivers which we know now only by name. 
Harvard College, January, 1890. 
Supplementary Note.—Professor Albrecht Penck of Vienna 
has published a valuable essay on “ Die Bildung der Durchbruchs- 
thiler” (Verein zur Verbreitung naturwissenschaftlicher Kennt- 
