£98 | 
In this study, the various plant associations have been considered as 
members of a succession, and therefore, in general, transitional. The ulti- 
mate stage in this region, i .e., the permanent association, is held to be a 
very mesophytic forest, dominated by Fagus and Acer saccharum. Ali 
other plant associations are held to be transitional stages between a plant- 
less condition and this ultimate forest condition. The position of any 
given plant association in this succession may be determined accurately 
only by observation over a long period of time. The successive stages 
have been worked out carefully in many cases, however, and the usual 
succession for this region is well known. Two kinds of successions are 
recognized, namely, biogenic and physiogenic. A biogenic succession may 
be defined as one influenced only by plant and animal life, and therefore 
such a succession will occur only where the physiography is static. In 
physiogenic successions, physiographic changes are the controlling factors. 
In general we have endeavored to determine two points with regard to 
each plant association, namely, its place in the succession and whether 
the controlling factors of that succession at that stage are biogenic or phys- 
iogenic. Lists of species given are usually incomplete but as representative 
as possible. 
The walls of the gorge and the ravine branching from it are quite 
favorable for the study of plant successions in such situations. Within 
the region studied, almost all stages from the bare plantless cliff to the 
ultimate mesophytic forest can be found. The stage of development of the 
vegetation on the walls of the gorge seems to be dependent largely upon 
the length of time that has elapsed since active erosion by the river ceased. 
The succession is very rapid for a rock cliff. This is explained by the very 
unstable nature of the rock, the abundance of shale and the favorable con- 
ditions of rainfall and climate. Very often stages that are usually suc- 
cessive occur combined, or telescoped, here. Lichens, which usually form 
the first vegetation on rock cliffs, are absent. No liverworts or ferns occur. 
The oaks, which commonly form a stage immediately preceding the ulti- 
mate forest, seem to be replaced by elms and black locust. Juniperus is 
the only conifer found. 
The earliest stage of the succession occurs at one point where a cliff 
occupies the outside of 2 curve of the stream, and active erosion of its 
foot is going on. As a result of this condition, the cliff is very steep, even 
overhanging to a slight extent. The wall is bare of plants, except for algae 
