GENERAL DISCUSSION. 



By D. T. MacDouoal. 



The Cahuilla Basin is a structural trough lying immediately to the eastward of an 

 abruptly rising mountain range which separates it from the Pacific. The depression is 

 shaped much like the bowl of a spoon, the tip of which comes to within a short distance of 

 the Gulf of California. An ancient beach-line, lying a few feet above present sea-level, 

 incloses an area of about 2,200 square miles; and this lowermost portion of the basin has 

 been designated the Salton Sink in the present paper. 



The original depression was of unknown depth; and it has been filled to within 284 

 feet below the mean tide-level by the alluvial outwash from the slopes of the mountains 

 which bound it on three sides. Borings to a depth of 1,700 feet show a series of interbedded 

 sands and clays, such as might be encountered in any one of the similar troughs in central 

 Arizona, which had been filled by material worn down by wind and water from the moun- 

 tain slopes. The bottom of the basin obviously lies far below the present level of the gulf; 

 yet it is clear that "Blake Sea," the ancient body of water which filled the basin to the 

 level of the highest beach, was composed of fresh water, at least during a part of its exist- 

 ence — as evidenced by the heavy layer of travertine formed on the rocks beneath its 

 surface at the highest level, by the presence of fresh-water shells, and by the composition 

 of the saline layer on the bottom of the Sink, evidently derived by condensation of fresh 

 water rather than from evaporation of sea water. (See discussion of this point by Free, 

 pages 22 to 34.) 



Essential differences of great importance to the naturalist are to be recognized between 

 the history of the undrained basins of California and Nevada and that of the Cahuilla. At 

 times the former contained bodies of water, the formation, fluctuation, and disappearances 

 of which were directly connected with climatic variations in the region in which they lie. 



The Cahuilla Basin, on the other hand, has probably been the scene of incursions from 

 the ocean in earlier times; and the formation and existence of Blake Sea may not be directly 

 connected with the climate of the immediate region. Likewise, the modern gathering of 

 the waters in the Sink is not to be attributed to climatic effects. The earliest existence 

 of the Salton Sea within historic times is that shown on Rocque's map (1762, see Plate 5). 

 Collated reports give the presence of flood- water in some volume in the Sink in 1828, 

 1840, 1849, 1852, 1859, 1862, 1867, and 1891. These occurrences do not seem to be correlated 

 with any climatic measurements, and are perhaps directly connected with the oscillations 

 of a silt-laden stream in alluvium. This view is strengthened by the fact that the fillings 

 of the Pattie Basin have not been synchronous with those of the Cahuilla. 



The study of the behavior of the soils of the emerged strands and uncovered beaches 

 has yielded some points of interest and value in the interpretation of lacustrine action, 

 especially with respect to the evaporation from saturated soils, and the surprisingly small 

 leaching effect resulting from prolonged submergence in water, the dissolved content of 

 which is lower than that of the soil solutions. 



It has been customary with geologists and geographers to assume that the waters of 

 ephemeral and fluctuating lakes are reduced and their salts dropped out as in a test tube. 

 The action of iron and sulphur bacteria has been well known for some time, but no good 

 opportunity had hitherto occurred for controlled observations of their possible influence 



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