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oak forest below. This type of distribution has been observed only 

 on the narrower ridges, along- the crests of which the xerophytic 

 habitat is more emphasized and where the accumulation of 

 humus takes place more slowly. On the broader uplands Pteris 

 is usually common. Some of the latter show a transition to the 

 bur oak type in the presence of Pyrola elliptica, AmpJiicarpa 

 Pitchcri, J 'it is vulpina, and Agrimonia inollis, typical members of the 

 latter association. On the slopes from the black oak into the bur 

 oak association there is usually a well-defined tension zone (PI. 

 XV, Fig. 2) where the plants of both groups mingle. Among these 

 the most abundant are Pteris aquilina and Smilacina stellata of the 

 black oak association and Geranium maailatiim and Primus virginiana 

 of the bur oak. 



The sharpness of the tension line, coupled with the slow devel- 

 opment of the dominant species of the two associations, indicates a 

 condition approaching an equilibrium, between the two associations. 

 Their common boundary on the steeper slopes seems to depend upon 

 the water content of the sand as influenced by the height above the 

 water-table, while on the broader uplands the incipient succession 

 may depend not only upon the depth of the water-table, but to a 

 greater extent upon the increase in water capacity through the de- 

 velopment of humus. The present location of small ponds in the 

 sand deposits shows that the actual depth of the water-table is sev- 

 eral yards, and in all probability too great to explain the sharp tension 

 line already noted. Its origin must accordingly be referred back to 

 a past condition in which the general water-level was higher. Trees 

 of Qnercus velntina in the lower portions of the bur oak association 

 are very few in number and usually small in size, showing that they 

 are not relics, but recent sporadic invaders, and there is an unusually 

 small number of herbaceous relics. In the upland portions of the bur 

 oak association relic trees of black oak are numerous and frequently 

 of large size, while many relic herbaceous species also occur. 

 Throughout the black oak association pioneers of the bur oak group 

 are well represented, as is shown by the list given in the discussion 

 of the former association. All these peculiarities lead to a choice of 

 two conclusions : the depressions have never been occupied by black 

 oak, or the succession by the bur oak has been extraordinarily com- 

 plete. Further evidence leading to the acceptance of the first alter- 

 native is afforded by conditions in the Amboy area and to some extent 

 also in the Kankakee area. 



In the Amboy area the sand lies in similar ridges mostly parallel 

 to Green ri\-er and not over 60 feet (20 m.) above it. The inter- 



