126 HARSHBERGER : PHYTO-GEOGRAPHIC SKETCH 
Delaware river, striking across several geologic formations. The 
Laurentian syenites are cut into by the headwater tributaries of 
these streams, which make gorges in the newer gneissic rocks of 
the region, finally flowing across the alluvial plain found along the 
Delaware river. 
The shorter streams with less volume of water have not cut 
across the Laurentian divide, but they are and have been con- 
stantly at work widening and deepening their gorges, so that in 
the last few miles of their courses they often run practically at 
base-level, as indicated by the sluggishness of their flow. During 
their existence, these creeks, as well as the master-streams men- 
tioned above, have been subjected to various vicissitudes through 
oscillations of the earth’s surface. Several times their lower 
stretches have been subjected to depression and elevation. Dur- 
ing periods of depression, their mouths have been drowned by the 
encroachment of the sea upon the preceding land-surface, the 
Delaware river becoming an estuary of the Atlantic ocean, with 
the deposit of sediments in the form of mud, sand and gravels. 
Nevertheless the streams of the region have been constantly at 
work reducing the country to a peneplain, eating away the sand 
and gravel deposited during submergence and leaving undisturbed 
patches to tell the tale of their former existence. During perio 
of subsidence, erosion has been less active, but during long periods 
of elevation the streams have been reawakened and started afresh 
into active earth-leveling. 
The Piedmont plateau and the alluvial coastal strip in south- 
east Pennsylvania were, with the similar region in New Jersey; 
subjected to movements of the earth’s surface. The following 
may be recognized as of importance in the consideration of the 
aes from a phyto-geographic aspect : 
. The post-Triassic uplift and the subsequent dicen of 
the mu peneplain. 
2. The Cretaceous subsidence and deposition. 
3. The post-Cretaceous uplift. 
4. The Miocene submergence and the deposition of the Mio- 
cene beds. 
5. The post-Beacon-Hill submergence and the development of 
the pre-Pensauken peneplain. 
