250 D. T. MacDougal—The Salton Sea. 



The arrangement of the pioneers on any beach would be 

 characterized as open, and on the gentler alkaline slopes the 

 tendency in general was toward a decrease both in the number 

 of species and individuals, with few or no secondary introduc- 

 tions, thus making a direct change toward the true open or 

 desert formations. The gently sloping beach at Mecca, how- 

 ever, receiving some seepage water, did not exhibit such simple 

 results. Steeply sloping beaches, as represented by the Traver- 

 tine Terraces and the shores of Obsidian Island, showed two 

 phases of succession, differing - chiefly in degree. The open for- 

 mations on the shores of Obsidian Island showed some tendency 

 to becoming closer or denser in irregular areas, which soon 

 began to thin in the final change toward the open desert forma- 

 tion (fig.6). The benches of Travertine Terrace were character- 

 ized by the development of dense ranks comprising a half 

 dozen species or less at the upper margins of the annually bared 

 strands, which were soon thinned in accordance with the 

 general tendency toward desert formations, but at the same 

 time the greater part of the surface of the terrace was knit to- 

 gether in a close formation by a mat of Distichlis. This 

 closed formation, however, soon began to show the effects of 

 desiccation and the progression toward the desert formation 

 with the introduction of xerophytes would be seen within 

 three or four years after the zone had been laid bare by the 

 lake. The transition is so rapid and abrupt that species 

 appearing on strands two years old are also included in ancient 

 beach ranks marking the positions of strands 300 to 400 years old. 

 The revegetation of the area submerged by the waters of the 

 lake in Salton Sink is therefore seen to be chiefly influenced 

 by water or other edaphic conditions during the first year or 

 two after emersion, after which the formations become increas- 

 ingly open in the progression toward the extreme desert type. 



The evaporating power of the air becomes an agency of in- 

 creasing importance, as the age of the plant-formation or the 

 period since emersion lengthens. 



Submergence and consequent extermination of the flora of 

 portions of the Salton Sink has occurred many times in the last 

 few centuries and the reoccupation of the bared strands has 

 taken place with the complex interplay of biological and me- 

 chanical agencies partly suggested and partly described in the 

 preceding pages. 



