MOVEMENTS OP VEGETATION IN THE SALTON SINK. 125 



moisture was sufficient to produce germination, with the result that narrow and well- 

 defined ranks of plants were formed in these angles in the succeeding years of the observa- 

 tions. The ancient beach-lines higher up the slopes were taken to have been produced by 

 prolonged maintenance of the water at about the same general level, as the amount of 

 sorting which had been done would not occur within a single season. (Plates 29 a, 30 b.) 



The recession of the water during the summer months was checked in some seasons 

 by rains, and the consequent maintenance of level, together with storm action, produced 

 another series of beach ridges and cut banks which varied greatly not only in their height 

 and general character, but also in the composition of the rank of vegetation, as the seeds 

 which were being disseminated at this season were of different species in part from those 

 being carried about by winds and waves during the cooler season. 



It is to be noted that the terraces lay across the crest of a detrital ridge or bajada and 

 that it was thus barred from the liability of invasions by run-off streams from the slopes 

 above, the only introductions from landward possible being those which might fall by 

 gravity from the extreme edge of the crumbling bank. It was therefore determined to 

 make an examination of the slope to the southward, which comes down from a greater 

 distance and at a much gentler gradient from the same range of mountains. Here the 

 recession of 1907 had laid bare a gravelly shore nearly 900 feet in width, with a fall of about 

 1 in 250. 



The first glance at this area showed numerous invaders that had come down the run- 

 off streams, which had been strong during the midsummer of 1907. Plantlets of Franseria 

 dumosa, Encelia eriocephala, and an unrecognizable species were found, as well as mature 

 individuals of Wislizenia refracta and Oenothera scapoidea aurantiaca, while Cryptanthe 

 barbigera and Heliotropium curassavicum were abundant and in bloom. Young shoots of 

 Pluchea sericea were also found and this plant and Heliotropium were the only ones which 

 had not arrived by flotation on the run-off streams of the slopes. The results from this 

 place and the terraces previously examined gave in marked relief the relative importance 

 of flotation from the body of the lake and down the run-off streams on the slopes. Nowhere 

 else were the conditions so favorable to invasion by the latter method. 



Obsidian Island, which lay westward from the Imperial Junction beach about 5 miles, 

 had been selected as offering shore phenomena in good condition for analysis, and this 

 place was reached on the leg of the voyage from the western shore in February 1908. The 

 hill-tops making this island consist of two crests joined by a low neck which was covered, 

 with the exception of a portion about 100 yards in width, by the high water of 1907. The act- 

 ual beach line measured 2 miles as paced with a pedometer in February 1908. The shore was 

 rocky and precipitous in places, but about half of the total was sandy, with the admixture of 

 alluvium in East Bay and of coarse gravel at the southern end and in West Bay. (Plate 21.) 



A large number of species was found on the beaches at this time and the collections 

 were as follows: Cryptanthe barbigera on upper portions of emersed beach and also above 

 flood-level. Eriogonum thomassii, distributed above and below flood-level, was possibly 

 brought in by the wind or by birds, the soil offering suitable conditions near the water. 

 Psathyrotes ramosissima was found at extreme high-water level, as if the seeds had come 

 in on the waters of the lake. Oenothera scapoidea aurantiaca occurred on the upper level 

 of emersed band; Rumex midway of the beach. Sonchus asper, represented by three indi- 

 viduals in separate places, may have come from seeds floated ashore or may have been 

 carried there directly by the wind. Encelia eriocephala was represented by a single speci- 

 men, which probably reached the beach by the flotation of a seed; it was abundant at 

 Imperial Junction, 8 miles distant and 3 miles back from the shore to the northeastward, 

 but may have come from some other source. Atriplex canescens was found in a tide pool; 

 A. polycarpa was represented by a few plantlets on the beaches. A. hymenelytra and A. 

 fasciculata, with the two preceding species, are indigenous to the island and had come down 



