GROUND SQUIRRELS 113 



squirrel. Each species thus occupies a very 

 definite area; and neither trespasses on the 

 ground occupied by the other. 



The narrow, troughlike depression now 

 known as the Colorado Desert of California 

 was once a portion of the bed of the Gulf of 

 Lower California. Then as now the Colorado 

 River, brown with its heavy sediments of silt, 

 was emptying its waters into the Gulf and 

 pushing its delta across the narrow sea valley. 

 In time the sediments were deposited in such 

 quantities that the stream built up for itself a 

 channel higher than the waters of the Gulf it- 

 self. Not only this was done, but the delta was 

 built out sufficiently far to divide the narrow 

 arm of the ocean into two parts, one part still 

 opening into the ocean, and the other part 

 forming an inland sea. This latter body of 

 water is known to geologists as the Blake Sea, 

 it having been named in honor of Professor 

 William P. Blake who accompanied the expedi- 

 tion which first satisfactorily explored the re- 

 gion. The isolated waters of the inland sea soon 

 began to dry up under the intense heat of the 

 desert sun, and, as they receded from the moun- 



