D. T. MacDougal—The Salton Sea. 233 



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, encloses an area 

 of about 2,200 square miles ; and this lowermost portion of the 

 basis has been designated the Salton Sink in the present 

 paper. (See the map accompanying the original memoir.) 



The original depression was of unknown depth ; it has been 

 filled to within 28-4 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 en- 

 countered in any one of the similar troughs in central Arizona, 

 which had been filled by material worn down by wind and 

 water from the mountain 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 existence; 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. 



The Cahuilla Basin is subject to a mixed type of climate. 

 It lies far enough inland so that overheating should result in a 

 continental type of climate, particularly with respect to the 

 rainfall. Its great bowl, however, lies immediately in the lee 

 of a great mountain range which rises abruptly from its south- 

 western side, with the result that fringes of mountain storms 

 reach out over part of its area at times, while the topographical 

 conditions favor the development of the intense and localized 

 precipitation known as cloudbursts. 



The annual average rainfall from data covering 36 years is 

 2'74 inches and the character of the precipitation phenomena 

 suggests a high degree of aridity. The maximum amount 

 received in one year was 7*10 inches (1906) and the lowest, a 

 "trace" (less than '01 inch) in 1904, giving a variation as 1 to 

 1000, a proportion occurring in deserts of a pronounced degree 

 of aridity only. In addition to this expression, the ratio of 

 possible evaporation from a free-water surface to the annual 

 amount of precipitation has been useful in characterizing 

 deserts. About 116 inches of water would evaporate from the 

 surface of a small vessel on the ground in the open in the 

 Cahuilla during a year ; this is 15 times the amount which has 

 fallen in any one year, 43 times the average, and many thou- 

 sands of times the minimum. The possible evaporation in 

 northern Africa at Algiers is about 60 inches annually, but in 

 the interior of the Sahara it is much £;reater. 



