SKETCH OP THE GEOLOGY AND SOILS OF THE CAHmLLA BASIN. 29 



peared, and contain terraces, strand-lines, travertine deposits, and other records similar 

 to those of Blake Sea. However, the records of these northern lakes are much more com- 

 plex, and indicate a long and complicated history of rises and falls, with long periods of 

 nearly stationary level and probably long periods of complete desiccation. The.se lakes 

 were fluctuating lakes. Blake Sea was simply a falling lake. There are no records in the 

 Cahuilla Basin of anything comparable to the fluctuations which are so marked a feature 

 of the histories of Lake Lahontan, Lake Bonneville, and their smaller analogues. The 

 reason is apparent. The fluctuations of the northern lakes were almost certainly climatic 

 in origin, and due to changes of rainfall, or evaporation, or both. These changes were 

 recorded in the fluctuations of the lakes because the basins were inclosed and varying 

 balance of rainfall and evaporation found expression in expansions or contractions of the 

 final evaporating surface, the lake. In the Cahuilla Basin these conditions did not exist. 

 Whether we believe the trough to have been occupied by the sea, or whether we believe 

 it a high and essentially a drained valley subsequently depressed, it is apparent that 

 the rainfall would have free or nearly free egress to the sea and that variations of climate 

 would not find so ready a record as in the permanently inclosed and high-walled basins 

 to the north. Even if there were a period of true basin conditions following a separation 

 from the ocean and preceding the origin of Blake Sea, it is scarcely likely that climatic 

 fluctuations would leave a record now readable. The rainfall area tributary to the Salton 

 Sink is not very large, and a very considerable increase of rainfall would be necessary to 

 maintain a permanent lake in it. Records of shallow transient lakes, formed by rainfall 

 only, may exist in the clay and salt beds of the sink bottom, but these can not now be 

 examined, and it is doubtful if any such lake would rise high enough to leave records 

 comparable with those of Blake Sea. 



For these reasons the history of the basin ofifers little either for or against the theories 

 of fluctuating climate recently advanced by Huntington and others. The evidence of 

 climatic variations furnished so richly by the ancient lake basins to the north is altogether 

 lacking. It is possible that the gravel-sand-clay alternations exhibited by Tertiary and 

 Quaternary alluvium aUke may be connected with climatic change, but it is just as possible 

 that they are due to shifting of stream channels, gradual changes of grade, and the like. 

 The decipherable events in the history of the basin seem to be connected with orographic 

 or other changes with which climatic change had little or nothing to do. It is possible, 

 of course, that the changes in the course of the Colorado, noted as accidental, are really 

 related to climatic changes in the drainage basin of that river with their resulting changes 

 in degree of overloading and in seasonal regimen, but these remain undeciphered. 



THE SOILS.' 



In origin and general character there are two types of soil represented in the basin: 

 the desert soils of the bajada, and the river alluvium soils of the Colorado Delta. The 

 former are mainly sandy and gravelly, though occasionally stream-action or local depres- 

 sions have produced soils of silty or clayey type. The minerals composing the soil are 

 mostly angular fragments, fresh and unweathered, and indicating in their character the 

 shortness of the journey which they have undergone. Being derived originally from the 

 granitic and similar rocks of the mountains, the soils are mineralogically very heterogeneous 

 and are amply supplied with the useful soil-forming minerals. That they are very fertile when 

 suppUed with water is indicated by their very successful agricultural use in the Coachella 

 Valley. (See Plate 15.) The alluvial soils of the Delta, being derived from the whole drain- 

 age area of the Colorado, are similarly diverse and fertile, but differ in that their mineral 



' Considerable use has been made in this section of the reports of soil surveys in the area made by T. H. MeaM, 

 J. Gamett Holmes, and others for the Bureau of Soils, U. S. Department of Agriculture, and published m the Field 

 Operations" of that Bureau for 1901 and 1903. 



