PLANT ECOLOGY AND PLOEISTICS OF SALTON SINK. 89 



SOILS OF THE SALTON SINK. 



The soils of the Sink may be classified under three general types. At the upper or 

 northern end the soil is a very fine sandy loam, deposited under water at the time that 

 the Sink was still an arm of the sea, as well as during the lacustrine period. It has a high 

 capillarity, so that the artesian water is able to seep up towards the surface, carrying 

 with it alkaline salts, which are concentrated on or near the surface. The water-table is 

 high, and the alkaline content is so great as to confine the vegetation largely to plants 

 of specialized type. 



Along the northeastern slope of the Sink, extending from Mecca to Frinks, the sur- 

 face is covered with wash from the adjacent mountains, consisting of sand, gravel, and 

 stones, assorted in increasing coarseness as the upper contour is approached, where it is 

 cut up by the broad shallow channels through which the flood waters are carried. This 

 soil is readily permeable by water, so that floods and rainfall sink deep, and are thus 

 protected from evaporation. The same porosity renders it easy for the roots of the plants 

 to penetrate and avail themselves of the stored moisture. It is in this wash-soil that 

 the greatest variety and abundance of xerophytic vegetation is found. 



The Imperial soils, which occupy the lower or southern part of the Sink, were deposited 

 by the Colorado River. They are loams of very fine compact grain, varying in the amount 

 of clay which they contain, but with very small percentages of sand. They are permeable 

 by water only to a slight degree. Consequently the water which they may receive readily 

 runs off, and only an inch or two of the surface is wetted, and this is rapidly dried again. 

 The mounds also are composed of similar loam, but of lighter particles which have been 

 selected by the winds and are consequently of a more permeable texture. All the Im- 

 perial loams are rich in materials for plant growth, needing only water to be abundantly 

 productive. 



DERIVATION OF THE SINK FLORA. 



A student of the flora of the Colorado Desert speedily discovers evidences of a migra- 

 tory movement from the south and east. Some plants from Arizona reach the Colorado 

 River, but fail to cross it, or, like the saguaro, barely gain a foothold on the California side. 

 Fouquieria splendens abounds on the desolate plains from Fort Yuma to Acolyta on the 

 Southern Pacific Railway, and then falls back to the mountains, along which it is able to 

 persist as far north as to Red Canon, near Mecca. The palo verde (Cercidium torreyanum) 

 pushes on farther and reaches Palm Springs. A few, like the desert willow (Chilopsis linearis) 

 and the mesquite (Prosopis glandulosa), even straggle over San Gorgonio Pass into the cis- 

 montane region of southern California. Instances of like character might easily be multi- 

 plied; indeed the one prominent fact in the distribution of the xerophytic flora of the desert 

 is the successive dropping out of species as one proceeds from the southeast to the north- 

 west. The Colorado Desert is the western fringe of the arid flora of Sonora, Arizona, New 

 Mexico, and regions still to the east and south, the so-called Lower Sonoran flora. 



Of this general flora of the Colorado Desert the xerophytic vegetation of the Sink is 

 a part, differentiated mainly by the great preponderance of Atriplices in its composition, 

 so that it may be fittingly denominated the Atriplex zone. As it is only in very recent 

 times, as geological periods are reckoned, that it has ceased to be submerged, it must 

 have directly received its population from the more elevated land bordering it. It has, 

 indeed, again and again been freed of vegetation over areas of greater or less extent, by 

 the repeated partial refilling of the depression, to be again repopulated as the water receded. 



The plants which border the diffluent rivers of Imperial Valley were brought in from 

 the delta of the Colorado. Most of them may be found in the river bottoms at Fort Yuma. 

 One most interesting member of this association has descended the Colorado from its 

 distant headwaters in Wyoming and Utah. 



