1(53 



plays a prominent part in determining the physiography, caus- 

 ing migrating dunes and blowouts, where the sand shifts so 

 readily that it remains entirely without vegetation (PI. XII., 

 Fig. 1), or is colonized only by those few species able to adapt 

 themselves to the peculiar ecological conditions (PI. XIII.). 

 Some resistance to the wind action is usually offered by the 

 vegetation, so that the area occupied by the blow-sand is lim- 



TABLE III. PASTURED BUNCH-GRASS. 

 (In pasture up to time of count. ) 



ited. In the present case the covering of bunch-grasses, with 

 their fibrous roots, and of such other sand-binding plants as 

 the matlike Opuntia, or the dense clumps of Amorpha canescens 

 or Chrysopsis camporum, is very effectual. Blowing still takes 

 place, as is shown by the slight excavations between the bunches 

 on the hilltops (PI. XIV., Fig. 2), but it results merely in a gen- 

 eral redistribution of the sand, the quantity removed being vir- 



* See p. 159. 



