240 



with incrustations of calcium carbonate from tiio seepage water. It is 

 probable that the disintegration of the rock from weathering and stream 

 erosion combined is too rapid to permit the liverworts to maintain a foot- 

 hold. 



The succession from the xerophytic first stage to the ultimate meso- 

 phytic stage is very rapid, more so than that of the bluffs. The narrow- 

 ness of the ravines, the greater amount of shade, and the more constant 

 water supply account for this. The stages passed through are essentially 

 the same as in the case of the bluff. 



It is to be noted, however, that until the ultimate mesophytic stage is 

 reached the physiographic factors affect the succession, and that a purely 

 biogenic succession never occurs. In this point the ravine succession dif- 

 fers from that of the bluffs, as already discussed. 



That the ultimate stage of the ravine is mesophytic is indicated by the 

 following list of species found in a typical ravine of the region. 

 Acer saccharum Vitls vulpina 



Quercus prinus Rhus Toxicodendron 



Ostrya virginica Rubus .sp. 



Acer negundo Impatiens bifiora 



Fagus grandifolia Impatiens pallida 



Fraxinus americana Ipomoea pandorata 



Cercis canadensis Lobelia syphilitica 



Ulmus fulva Sedum ternatum 



Fraxinus quadrangulata Ambrosia trifida 



Gleditsia triacanthos Viola cucullata 



Celtis occideutalis Eupatorium urticfefolium 



Ulmus americana Sambucus canadensis 



Menispermum canadense Hyrangea arborescens 



The successions of the gorge floor are quite as interesting as those of 

 the bluffs and ravines. As already mentioned, the stream is ponded 

 through a large part of the region studied. The conditions which have 

 produced this result have been discussed. In the ponded portions, the 

 water varies in depth from two to five feet, and consequently the current 

 is very slow. As a result of this condition, a typical pond vegetation is 

 found in a number of points within the area. Sagittaria and Typha are 

 characteristic of this condition. Scirpus americanus occurs at a few 



