SKETCH OF THE GEOLOGY AND SOILS OF THE CAHUILLA BASIN. 33 



General observations at many other points about the beaches fully confirm the results 

 of the more intensive study of the Imperial Junction beach. The distribution of the beach 

 vegetation seems to be quite independent of the soil. The only important influence is an 

 indirect one, the influence of soil texture on water movement and water retention. These 

 influences may be summarized as follows: 1 



1. Capillary power is stronger in the soils of finer texture. Water will rise higher and spread 

 farther horizontally in such soils. 



2. Rain penetrates more rapidly into the coarser soils. From a silt or clay, most of the rain 

 will run off instead of being absorbed. 



3. Drainage is more rapid and complete from the coarser soils. A fine soil will hold more 

 interstitial (or "capillary") water than a coarse. 



4. When exposed to intense evaporation a coarse soil moistened with capillary water dries 

 out at and near the surface more rapidly than the feeble capillary powers of the soil can supply 

 more moisture from below. A dry surface layer is thus produced, in which layer the thin water 

 films responsible for capillary movement do not exist. This dry layer breaks the continuity of the 

 path by which capillary water is ascending, acts as a "dry mulch" or insulator to the water below, 

 and prevents the rise and loss of this water. On the other hand, the capillary powers of a fine soil 

 are so much better that water is usually supplied from below as fast as removed by evaporation, 

 with the result that all the water of the soil is lost down to the depth from which capillary rise can 

 act. Because of this different behavior toward intense evaporation, it frequently happens that 

 desert soils of sandy texture will retain moisture and nourish plants, when surrounding finer soils 

 are entirely desiccated and their vegetation killed. 



All four of these general principles find application to minor features of plant dis- 

 tribution both on the Salton beaches and on the higher slopes of the basin which the Salton 

 waters did not reach. Many specific examples are noted in Dr. MacDougal's contribution. 

 In all cases, however, these influences of soil texture are far subordinate to the influence 

 of major and minor topography as directly controlling water supply. The distribution 

 of the small dry washes which form channels for the occasional rains is far more important 

 than the variation in soil. 



It is true, of course, that the prevailing soil-alkali content of some regions, the pre- 

 vailing gravelly nature of others, and similar wider soil and topographic differences between 

 different parts of the Basin, have had considerable influence upon the content of the general 

 flora of these "provinces." Certain species have been barred out because of their unsuit- 

 ability to the general conditions of the province. In these larger ways, soil character has 

 perhaps had an important influence on the vegetation of the basin, fully discussed elsewhere 

 in this volume. 



1 For details the reader must consult the text-books of soil physics, e .</., Mitscherlich-Bodenkunde. 



