240 
with incrustations of calcium carbonate froni tlie seépage 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 mesophytie 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 Vitis vulpina 
Quercus prinus Raus Toxicodendron 
Ostrya virginica Rubus sp. 
Acer negundo Impatiens biflora 
Fagus grandifolia Impatiens pallida 
Fraxinus americana Ipomca pandorata 
Cercis canadensis Lobelia syphilitica 
Ulmus fulva Sedum ternatum 
Fraxinus quadrangulata Ambrosia trifida 
Gleditsia triacanthos Viola cucullata 
Celtis occidentalis Hupatorium urticefolium 
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 
