The Colorado Desert 117 



crossing the delta plain. The extreme northern end of the great 

 valley or trough, San Gorgonio Pass, lies between two moun- 

 tain ranges, San Jacinto and San Bernardino, 200 miles north of 

 the present Gulf of California. The great trough is due to 

 crustal movement, the subsidence being along fault lines. The 

 character of the topography of the west side of the desert indi- 

 cates fault lines. The east edge of the Peninsular Range is 

 marked by a fault scarp. The character of the slopes on the 

 east side of mountain salients of the Peninsular Range, Santa 

 Rosa, Carrizo, San Felipe, and Black mountains, indicates a 

 break or fault fracture in the crust of the earth. The San 

 Andreas fault runs through San Gorgonio Pass into the Colorado 

 Desert. At the time of the great earthquake in San Francisco 

 in 1906 there was an earthquake in the Imperial Valley, which 

 suggests the probable continuation of the fault southward 

 through the valley toward the Gulf of California. 



Closely associated with the faults in the desert region are 

 evidences of volcanic activity. In the Salton Sea about 7 miles 

 southwest of Imperial Junction is a row of knobs of obsidian, 

 pumice, scoriaceous lava, and tuff. Near these an interesting 

 group of mud volcanoes existed before the area was flooded by 

 the rising of the lake. Some of these were nearly perfect cones 

 with little craters at their tops. Accompanying these were hot 

 pools, gaseous emanations, sulphur and salt deposits, acidulated 

 waters, and boiling mud pools. A few miles northwest of Im- 

 perial, near the bank of New River, is a group of mud volcanoes 

 and boiling pools. A more extensive field of mud craters is 

 found about 40 miles south of the international boundary along 

 the shore of Volcano Lake. These are similar to those just men- 

 tioned, but on a larger scale. These mud volcanoes are regarded 

 as representing the last phase of the volcanic activity of which 

 there are many evidences in this desert region. 



