1914] Grinnell: Mammals ami Birds of the Colorado Valley 79 



Favorable conditions for burrowing here attract heteromyid rodents, 

 notably the large Dipodomys deserti. A characteristic assemblage 

 results which might be appropriately called the aeolian sand associa- 

 tion. Its peculiarities are not, however, in the writer's mind, sufficient 

 to warrant giving it more than minor recognition. Future finer 

 analysis may possibly justify the separate recognition of the aeolian 

 sand association, especially when the reptiles of the desert at large are 

 taken into consideration. 



Elsewhere on the second bottom, depressions frequently occur where 

 rainwater from adjacent desert slopes leaves by evaporation more or 

 less alkali. In cases of excessive deposition, bare white stretches 

 result, without any vegetation at all. On somewhat less alkaline 

 ground there may be tracts of Spirostachys occidentalis and Suaeda 

 suffrutescens, both being shrubby plants popularly included under the 

 names pickle-weed and iceplaut. 



But the greater portion of the second bottom, as also a strip lead- 

 ing back along either side of the desert washes, is marked by the 

 saltbush. This Atriplex is quite uniform in appearance wherever it 

 grows, forming small but stout-branched bushes seldom more than 

 two feet tall. 



The creosote bush (Larrea divaricata) , although predominating in 

 the next-described association, occurs not infrequently as a minor 

 element in the saltbush association. Sometimes individual plants of 

 Larrea reach a very large size (pi. 7, fig. 8), much larger even than 

 on the desert mesa. Where small alluvial fans from the higher adjacent 

 mesa make down on to the second bottom, there is an influx of such 

 plants as the coyote melon (Cucurbita palmata), rattle-weed {Eriog- 

 onum inflatum), unicorn plant, locally called devil's-claw (Martynia 

 proboscidea), and sandbur (Franseria dumosa), all of which occur 

 on sandy parts of the upper mesa as well. Along shallow washes 

 through the second bottom there are often extensive thickets of Lycium 

 andersoni. 



In spite of the above indicated variations in floral constitution, 

 there is remarkable homogeneity in the animal life of the saltbush 

 association. In winter it is the preferred forage ground for a number 

 of xerophilous fringillids. as shown in the accompanying list of birds. 

 The generally loose, sandy soil seems to be an attractive feature for 

 burrowing rodents of limited fossorial powers such as Dipodomys mer- 

 riami merriami. These also find abundant food in the residual seeds 

 of numerous small annual plants which for brief periods thrive on 



