165 



by whatever means, blowing proceeds rapidly, resulting in the 

 saucer- or bowl-shaped excavations known as blowouts (PI. VIII., 

 IX., XV.). Their depth may finally be as great as the depth of 

 the sand itself, and their area is sometimes several acres in ex- 

 tent. Their sides usually have a gentle slope, but are some- 

 times quite steep where the bunch-grass along the crest has 

 restricted the blowing (PI. XIV., Fig. 2). The sand removed 

 from the blowouts is piled up on the leeward side in a more or 

 less fan-shaped heap, and this in turn is blown on by the wind 

 as a traveling dune (PI. XII., Fig. 2). Blowouts may be par- 

 tially or wholly refilled by sand, being thus transformed into 

 level tracts (PI. XIII. ), generally called blow-sand, and when 

 these become large the individual blowouts lose their identity 

 and the whole tract becomes a vast undulating surface of shift- 

 ing sand. The limit of size is reached when the blowout be- 

 comes so large that it no longer offers much resistance to the 

 wind, or so deep that the wind does not have sufficient force to 

 carry the sand from the bottom up over the sides, or when 

 moister layers of sand are reached which can not be blown. 



In young blowouts, when the excavation is being carried 

 on most rapidly, vegetation is very sparse, and the few species 

 able to grow in such conditions constitute the typical blow- 

 sand association. They are mostly plants with a short period 

 of development, which may mature before the shiftiug of the 

 sand has undermined their root-system, and they frequently 

 possess methods of seed distribution by which they are enabled 

 to colonize rapidly on barren areas of sand. The most charac- 

 teristic species are Ambrosia psilostackya, Cassia chamcecrista, 

 Cenchrus tribuloides, Cycloloma atriplicifolium,t 'ristatellaJamesii, 

 and Aristida tuberculosa. Of these, Cassia and Cenchrus are the 

 most abundant. Six to ten thousand plants of Cenchrus, none 

 of them more than six inches high, sometimes grow on a single 

 quadrat ten feet square, and a third of them produce seeds. 

 Cassia prefers sand that is loose from blowing or that has been 

 otherwise disturbed. Wagon tracks across the sand are quickly 

 occupied by it, and are marked by long parallel lines of the 

 plants, which are very conspicuous in the blooming season. 



