1903] FLORA OF NORTH CAROLINA 245 
believing that a heavy forest did not exist at that early date), was 
a mesophytic one with the addition of representative herbaceous 
plants that have not been preserved. 
With the uplift of this plain, followed by its planing down by 
streams, valleys and gorges were created, and rocky strata were 
exposed, which supported, as such physiographic formations 
support today, a xerophytic flora. The character of the rocky 
outcrop influences the particular kind of vegetation, so that we 
may have a different flora with the same exposure of light, heat, 
and moisture, if the rocky formations are different. This does 
not depend so much upon the chemical composition of two dif- 
ferent strata, as shown by Cowles,’ but it is because one forma- 
tion is further along in its life-history than is the other, so that 
the vegetation of the clay hill today may be seen on a sand hill 
in the future. With the widening out of the valley by erosion, 
and the slackening of the flow of any stream by the reduction of 
the elevations to base level, the xerophytes of the hillsides will 
be replaced by the mesophytes of the peneplain. The laws that 
control changes in the plant covering of a country are, there- 
fore, plainly physiographic and edaphic, if the meteorologic 
conditions remain the same. We may have broad flood plains, 
hills, cafions, lakes, and swamps, depending upon the history of 
the ever-changing topography. Wherever hills are being eroded, 
rivers widened, waterfalls eliminated, lakes filled, or coastal 
_ plains enlarged, there-:is found a constant change in the plant 
Societies, or a succession in definite order of plant groups. In 
any land which has undergone degradation from a mountainous 
topography to a peneplain, we ought to find a marked change in 
the organisms at the close of the cycle of denudation. In the 
first stages of change from original constructional topography, 
effects will be discernible. Sculptured slopes with ravines, 
sharp divides, and peaks cradle species and varieties by barriers 
which oppose ingress and egress. _ 
In the progress toward final base leveling, the repeated diver- 
sion of the streams, or the reversals of drainage, are a constant 
OWLEs, H. C., The influence of underlying rocks on the character of the vege- 
tation. Bulletin Raericen Bureau of Geography 2:— [pp. 26]. Je and D 1 
