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base of the cliff. After this erosion ceases, tlie phint succession is deter- 

 mined for a time by the slope of the cliff. At first, only shelf plants can 

 gain a foothold. As the slope is reduced, the number of species is increased. 

 After a time, though, when a soil of considerable depth has formed, the 

 succession becomes biogenic. The plants^ hold the soil, and the reduction 

 of slope proceeds very slowly, if at all, particularly after grasses become 

 prominent. The slope of the gorge wall where it is covered with a meso- 

 [)hytic forest is but little gentler than that of the wall where only bushes 

 occur. From the herb stage to. the ultimate mesophytic forest, each plant 

 stage prepares the way for the next in the succession by holding the soil, 

 accumulating humus and furnishing shade. 



The herb stage is succeeded by a bush stage. The most prominent 

 species is Rhus canadensis, which often forms large colonies. Cornus 

 paniculata and Salix longifolia are commonly associated with it. Rubus, 

 Ribes, Rhus toxicodendron, Vitis vulpina, Cratregus, Psedera, Ptelea trifo- 

 liata and others occur, together with a number of species characteristic 

 of the preceding stage, such as Dipsacus sylvestris, Heracleum lanatum, 

 etc. 



This shrub stage is probably very brief and pioneer trees soon appear. 

 Two parallel tree stages appear. Considerable areas are found occupied 

 by Ulmus americana, Celtis occidentalis and Cratjegus spp. In other sit- 

 uations simihir in all respects, Cersis canadensis, Robinia pseudo-acacia 

 and Rrunus americana dominate the vegetation. In both cases, the trees 

 are accompanied by a large number of undergrowth herbs and shrubs, 

 among them the following : 



Gleditsia triacanthos Heracleum lanatum 



.Juglans nigra Daucus cnrota 



Cornus paniculata Taraxacum officinale 



Sambucus canadensis Aster spp. 



Itibes cynosbati Verbascum thapi-r.s 



Vitis vulpina Nepeta cataria 



Psedera quinquefolia Poa compressa 



Menispermum canadeuse Solanum nigrum 



Dipsacus sylvestris 



Following these two ])arallel stages appears the ultimate stage of the 

 region, the mesophytic forest. This stage occurs only on the east bluff of 

 the gorge, inimediately above Test's Mills. That this forest is really 



