HABITAT OF THE COAHUILLAS 29 



particular home at present of the Coahuilla Indians, has, however, 

 bizarre features peculiarly its own. 



13. This arm of the desert was in very recent geologic time an arm 

 of the Gulf of California. More than 1,600 square miles is still below 

 the level of the sea, the most depressed portion being 275 feet lower 

 than the ocean. 



The Colorado river in its course south to the ocean built up a flood plain 

 on a higher level that finally shut off the western part from a direct com- 

 munication with the sea, and evaporation, with a gradual uplifting of the 

 whole section, finally laid it bare, although leaving a great part of it below 

 the present sea level. 1 



The waters of the Colorado, 275 feet above sea level at Yuma, break 

 through their banks at the summer season of high water and flow south- 

 westward and then northward and inland forming a widely inundated 

 area with many sloughs, the best known being the New river. This 

 formerly took place in sufficient volume to form in the center of the 

 valley a huge inland lake, a vestige of which still remains in the Saline 

 lagoon at Salton. 



There are three principal soil levels noticed in crossing the desert : 

 an upper, made of great " alluvial fans," which skirt the western mountain 

 ranges and are formed of great masses of rock, gravel, and detritus, 

 washed by cloud-bursts from the hillsides and swept far out over the 

 sands ; mingled with this layer are bits of silicified wood and oyster 

 shells ; second, a sandy middle layer, representing the former bed of 

 the sea ; and, third, a lower layer of clay or fine silt, laid down in still 

 water and extending for many miles, representing the bed of the 

 former fresh-water lake and subsequent lagoons, that are even now 

 occasionally filled by water from the New river overflows. Over the 

 surface of this latter level are scattered great quantities of fresh-water 

 shells, mostly small univalves Amnicola protect and A. longinqua and the 

 Physa humor osa, and a single bivalve, a species of Anodon now found 

 in the Colorado river (A. Californiensis Lea). The physa is also some- 

 times found, still inhabiting springs on the New river. 2 



The great horror of the desert is, of course, its heat and the absence 

 of water. To venture upon it without a guide, to miss ones way, or to 

 find an expected water hole dry or obliterated by a shifting drift of 



iE. B. PRESTON, California State Mining Bureau, Fourth Report of the State Mineralogist, 1893. 



2 The desert was first carefully studied and its physiographic characters learned and beautifully 



described by Mr. WILLIAM P. BLAKE, of the Pacific Railroad Survey, 1853. His report on the Colorado 



still remains the only complete study of this most attractive region. Exploration and Surveys for 



Railroad Route from the Mississippi River to the Pacific Ocean, Vol. V. Washington, D. C., 1857. 



