90 



margin. The fauna of the blowsand complex is thus on the whole 

 derived from the bunch-grass, and consists primarily of wandering 

 forms, both predaceous and phytophagous. 



THE DEPOSIT ASSOCIATION 



The manner of deposition of sand on the lee side of a blowout is 

 influenced greatly by the vegetation. If none is present the sand 

 spreads out over the general level in a broad, thin, fan-shaped layer. 

 Plants growing on the lee side of the blowout, particularly if they 

 be bunch-grasses, tend to check the velocity of the wind, which thus 

 becomes unable to carry its load. Sand is then deposited at the base 

 of the plants. If, now, the plants, by upward growth, can continue 

 to act as obstacles to the wind, the deposition of sand will continue, 

 and a dune will gradually be built up. The obstacle grows upward 

 with the dune. Many of the sand-prairie plants are efficient dune- 

 formers, and these species are c[uite common in the deposit associa- 

 tion. The principal species are Rhus canadensis var. illinocnsis, 

 Panicuin virgatujn, and Tcphrosia virginiana. A Rhus dune, the 

 side of which is being undermined by a large blowout, is shown in 

 Figure i, Plate IV. (See also PI. III. Fig. 2.) The process of dune- 

 fonnation at the lee side of blowouts is not well shown in the Havana 

 region; the deposit associations are not typical, and for this reason 

 very little study has been given them. A study of the animals of 

 the association was not even attempted, but the general character of 

 the assemblage is transitional between the animals of the blowsand 

 and those of the bunch-grass. Future work in the other sand areas 

 should include a study of this association. 



The deposit association is subject to two influences ; the stabiliz- 

 ing action of the vegetation and the destructive action of the wind. 

 The direction in which the succession proceeds, will depend upon 

 which set of conditions predominates. 



SuccEssiONAL Relationships in the Sand Prairie 



Most of the Illinois River sand area was covered originally by 

 the bunch-grass. The natural process of vegetative development 

 caused a gradual change in part of the area, until the black-soil transi- 

 tion stage was reached. In other places, where the exposure to the 

 wind is considerable, the sand between the tufts of grass is blown 

 away, and the Panicuni pscudopuhesccns association results. Con- 

 tinued wind action in parts of this association results in the formation 



