102 



The subsequent successions of vegetation depend primarily upon 

 the physiographic changes. As the development of the blowout pro- 

 ceeds, the basin may begin to encroach on the windward slope. This 

 is caused either by an increase in depth or by a general movement 

 to the rear, or by both together. The sliding vegetation of the latter 

 association reaches the bottom of the slope, is undermined and blown 

 away, and its place is taken by plants of the basin. The windward 

 slope thus comes to occupy a place intermediate in time between the 

 basin and the Panicuni pscudopuhcsccns associations. If the basin 

 is moving backward without an attendant increase in size, the lee 

 slope will also extend backward over the extinct basin, constituting 

 another succession. If the general movement is forward, the condi- 

 tions are reversed and the basin association succeeds the blowsand 

 association. This forward movement, however, reduces the grade of 

 the windward slope, and eventually stops the settling of the sand. 

 With this change in its environmental condition, stabilization be- 

 gins, as will be described later, and the windward slope association is 

 succeeded by the bunch-grass. So, while the basin may succeed the 

 windward slope, the reverse does not take place. 



Succession may also take place in either direction between the 

 blowsand and the deposit associations. This depends in part on local 

 environmental changes, leading to the increase in size of one asso- 

 ciation and the corresponding restriction of the other, but principally 

 upon the direction of the general movement of the whole blowout. 

 If forward, the blowsand association succeeds the deposit association; 

 if backward, the reverse is true. 



As has been previously mentioned, the liasin of a blowout may 

 eventually become so wide or so deep that further erosion by the wind 

 is impossible. Erosion is the one factor of the environment which is 

 chiefly responsible for the development of the basin association, and 

 when that ceases the basin is at once replaced by a ditlerent type of 

 vegetation. Some of the basin plants may persist for a time as 

 relics. It seems probable that the first new vegetation is the blow- 

 sand association, mainly because of its proximity, its excessive seed 

 production, and its rapid de\'elopment. When the sand becomes sta- 

 tionary, it is no better suited to the blowsand plants than to a num- 

 ber of others, including bunch-grasses and perennials. These at 

 once begin to colonize in the blowout and the stabilization of the 

 basin is effected. 



With the extinction of the basin the source is destroyed from 

 which sand is added to the deposits, and they cease their growth. 

 The surface of the sand, so far as it is not protected by the dune- 



