THE CAHUILLA BASIN AND DESERT OF THE COLORADO. 9 



obtained here in beautifxil purple or violet-colored crystals. Garnets and beryls are also 

 obtained in these mountains. The beryls are sometimes colorless and almost as brilliant 

 as diamonds, and are often tinted a pale rose-pink, greatly enhancing their beauty. 



The mountains on the north of the great Cahuilla Valley are also granitic and form 

 a series of sharp ridges in constant sequence, separated by gravelly slopes and valleys for 

 a great distance up the valley of the Colorado. Some outcrops have been worked for gold 

 in the mountain ridges a few miles north of Indio, and also at Carga Muchacha, near Yuma. 



The geology south of the international boundary is but little known. At the Sierra 

 Giganta, between Muleje and Loreto, there are precipitous cUfTs of red sandstone. 



VOLCANISM. 



The evidences of volcanism are many and impressive in the lower Delta region of 

 the Colorado and on the Sonora side of the head of the Gulf, where numerous craters exist 

 and are dominated by the extinct volcano known as Pinacate. This mountain rises in a 

 volcanic center of great extent, only about 25 miles from the shores of the Gulf. It had 

 been recently visited by Dr. MacDougal, of the Carnegie Institution of Washington, in 

 company with Engineer Sykes; Messrs. Hornaday, of the Zoological Park, New York; 

 and Mr. Phillijjs of Pittsburgh, Pennsylvania. They found extensive pit-craters with 

 precipitous sides and a broad region covered with lava. 



Northward from this Pinacate region, volcanic outflows continue as far as Mohawk, 

 on the railroad in Arizona, on the north side of the international boundary-line. 



In close proximity to the deposits of the Colorado Delta we find the craters and out- 

 flows of the Cerro Prieto, or Black Butte, on the flanks of the Cocopah Mountains. The 

 chief crater is in the mountain, 750 feet high, and is flanked by smaller craters, all now 

 extinct but giving evidence of comparatively recent activity. Mud volcanoes, or salses, 

 occur there, from which hot water and steam are escaping. 



At the time of the great earthquake in San Francisco, April 18, 1906, there was an 

 earthquake in the Imperial region of the Delta, which suggests the probable continuation 

 of a fault-plane passing along the great interior valley of California and southward through 

 the valley of the Gulf. 



MUD VOLCANOES. 



In the year 1852, Major Heintzelman, U. S. A., then conunanding at Yuma, was 

 surprised to see clouds of steam arising from the southwest portion of the desert. Visiting 

 the place he found a great eruption of hot water and mud, with jets of steam, issuing from 

 conical hillocks of mud. Masses of dark-colored mud were thrown to a height of 40 feet. 

 These salses were later visited and described by Dr. John L. LeConte,' and by Dr. Veatch.^ 



An excellent graphic description was given by Dr. David P. Barrows in 1900.' He 

 correctly observes: 



"The volcanoes are doubtless immediately due to the infiltration of water from the Colorado 

 overflow down to the heated beds of rock not far beneath. Converted into steam, these waters 

 burst violently upward through the deposits of silt, and around their orifices throw up encircling 

 walls of mud." 



Similar outbursts have been seen not far from the line of the railroad opposite the 

 Salton Sea deposit, but are now covered by water. 



Dr. MacDougal described the volcanic phenomena as follows: 



"Hot springs and other manifestations of volcanic energy are to be found all along the geo- 

 logical axis on the eastern side of the Peninsula of Baja California, but the most pronounced feature 



■ American Journal of Science and Arts, (ii) XDC, May 1855. 



' Ibid., (ii) XXVI, 1858. 



» National Geographic Magazine, Sept. 1900, vol. xi, No. 9, p. 337. 



