D. T. MacDougal — The Salton Sea. 



249 



the lake and deposited on various beaches owes this dissemina- 

 tion to structures which could hardly be attributed to any excita- 

 tion action on the part of water or to any previous selecting 

 action. The same mechanical qualities of flotation and dissem- 

 ination are displayed by the fragments of pumice which were 

 carried about at the same time. 



The communal life and successions on the beaches showed 

 some interesting diversities. The manner of the occupation 

 of the zones laid bare by the receding waters was chiefly deter- 



Fig. 6. 



Fig. 6. Ranks of vegetation marking ancient strands near the high level 

 of Blake sea. 



mined by the water, and hence the first communities of plants 

 were of the nature of strand-steppes. The history of such for- 

 mations showed two distinct phases, both also determined chiefly 

 by edaphic conditions. The strands of the more gently sloping 

 alkaline beaches were at first occupied by a greater number of 

 species than the bared strips on the steeper gravelly shores, and 

 the pioneers gained a foothold earlier. This difference may be 

 attributed in greater part to the fact that the narrower, more 

 steeply sloping beaches were subject to the action of storm waves 

 a longer period than the broader zones on the gentler slopes, 

 and also to the fact that the latter actually presented a greater 

 area of soil for the reception of seeds (fig 5). 



