Plate XVII. 



Fig. 1. Natural openiug' in tlie blaciv oak association, Winnebag'o area, occupied by ttie 

 bunch-grass association. 



Fijjf. 2. Face of the river dune, Hanover area, showing- the upper and middle slopes, separa 

 ted by tde outcrop of an old soil la3-er. The thicket association caps the dune in the backaround 



