go BOTANICAL GAZETTE [FEBRUARY 
exposed sandstone bluff. We may make the above statements 
in another form. Rock as such, or even the soil which comes 
from it, is of less importance in determining vegetation than are 
the aerial conditions, especially exposure. And it is the stage 
in the topography which determines the exposure. | 
All of the preceding statements as to topographic stages, | 


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FIG. 2.—Ravine at Glencoe with a mesophytic forest vegetation on the slopes 
(temporary climax). Presence of erosive forces indicated by leaning trees. Water 
in the stream bed only after rains. ( 
whether young or old, refer not to times but to constructional 
forms. Two ravines, equally youthful from the topographic 
standpoint, may differ widely as to actual age in years or centu- 
ries, Since erosion is more rapid in one rock than in another. 1 
In our region, however, elements of actual time are not very 
important, except as between rock and clay, since the limestone 
is less soluble and the sandstone is more easily eroded than is 
often the case. 
| B. The river bluff—As a valley deepens and widens, the 


