44 



arid depression, a quarter of which is below the level of the 

 sea, from the San Jacinto, Chuckawalla, and Chocolate Moun- 

 tains on the north, southward nearly two hundred miles to the 

 Gulf of Cahfornia. On the east it is bounded by the Sonoran 

 Mesa, and on the west by the main escarpment of the Rocky 

 Mountain coastal ridge which comes downward from the San 

 Gorgonio Pass as the backbone of the Lower Californian 

 peninsula. In the south-central part of this dry expanse is a 

 range of mountains, called the Cocopahs, running in a general 

 northerly and southerly direction, and dividing the desert 

 into two branches which merge in the south at the mouth of 

 the Colorado. The larger, northern branch, the chief part of 

 which comprises the Salton Basin, in Imperial County, Cali- 

 fornia — but with a Mexican appendage formed of the Colorado 

 Delta — is known as the Cahuilla Valley. The smaller, south- 

 westerly division of the desert, lying between the Cocopahs 

 and the Peninsula Range, has been distinguished by the name 

 "Pattie Basin." 



In the Salton Basin, and doubtless also in its counterpart west 

 of the Cocopahs, the average annual rainfall is less than three 

 inches, and even this is mainly in the form of cloudbursts. 

 During the last few centuries the lower areas of these basins, 

 one of which sinks to a depth of more than two hundred and 

 fifty feet below sea-level, have been alternately submerged and 

 desiccated many times. Further back in geologic history, 

 probably in the middle Tertiary, the whole of the Colorado 

 Desert appears to have been a great sea; above which the 

 peaks of the Cocopahs may have jutted as islands, and which 

 received the silt-laden waters of the Colorado River at some 

 point far above the site of Yuma, Arizona. Through the 

 pass between the San Jacinto and San Bernardino Mountains, 

 where reefs of fossil oysters are now to be found a thousand 



treeless plains, old lake-beds, and sand-hills — such conditions as are found 

 in the Sahara of Africa and in the delta regions of the Nile. The appella- 

 tion may properly be confined to the regions reached by the deposition of 

 the silt of the Colorado, whether in the form of deltas or at the bottom 

 of ancient lakes. I should also include the bordering detrital slopes 

 from the contiguous mountains. So restricted, the area is practically 

 coterminous with the ancient beach-lines and terraces of the lakes which 

 occupied the valley." 



