171 
ance of original plant conditions by attempts at cultivation 
and the consequent introduction of adventive weeds, but so re- 
sistant have been the Barrens to wholesale modification by agri- 
culture and horticulture, that large blocks of them retain their 
original and peculiar species. 
Dr. Witmer Stone, in his monumental work on the Plants of 
Southern New Jersey, is unwilling to state any positive conclu- 
sions as to correlation of existing conditions of plant distribu- 
tion in the Barrens with geologic changes. He thinks that hy- 
potheses on this line are purely conjectural. He does point out 
these facts. The coastal plain, including what is now the Bar- 
rens, was submerged when the elevated Piedmont region to the 
west must have been covered with vegetation; and the region 
north of the terminal moraine, including most of the highlands 
of north Jersey, was almost wholly without plant life while it 
was covered by the ice sheets. The area between the coastal 
plain and the moraine, comprising some part of west Jersey and 
eastern Pennsylvania, must have been continuously covered 
with plants longer than the submerged plain and the glaciated 
region. When the coastal plain rose above sea it must have re- 
ceived its flora from the higher country to the west and south- 
west. The partial submergences and emergences of the coastal 
plain may have resulted in invasions of plants from outside and 
changes in the general character of the flora. 
Dr. Stone points out that the plant life of the eastern United 
States includes two elements, a boreal, more or less identical 
with the flora of northern Europe, and an austral, peculiarly 
American. If the austral or American flora covered the Pied- 
mont area at the time the coastal plain was elevated, it would 
have spread into the new territory, and the species best adapted 
to the sandy, dry Barrens, would have persisted there. If an in- 
vasion of the boreal species over the Piedmont area followed, 
from the southward migration forced by the advancing ice, “we 
should probably have,” he says, “exactly the conditions we find 
today, the survival of an earlier flora in the bogs and sandy areas 
and its disappearance where better soil has developed,” in the 
western part of the State, where later submergence in Pensau- 
ken time and formation of alluvial deposits by the Delaware, 
made a richer soil and perhaps led to the destruction of the Pine 
Barren elements and confined them to the arid interior of South 
