Plate XX. 



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Fiy- 1- Uestruction of the stabilized river-dune and its mesophytic vegetation by river ero- 

 sion, and tiie reversion i)f the vegetation to the pioneer blowsand association. Oquawka area. 



Fig. 2- Destruction o1 tlie mesophytic vetreta'ion of the riverdune by i i vei eiosioii, showing 

 the coherent surface-layer of sand, and the sliding masses. 



