FAULT FEATURES OF S ALTON BASIN, CALIFORNIA 219 



crystalline rocks, and forms the basement rock of all the region. 

 There are large bodies of schist and gneiss, and smaller amounts of 

 marble and quartzite. The age of these metamorphics is unknown. 

 Most of them are Paleozoic, but a part probably is pre-Cambrian. 

 Then there are still greater masses of granite, monzonite, and diorite 

 intruded into the metamorphic rocks. These intrusives are prob- 

 ably Mesozoic. The various crystalline rocks constitute the mass 

 of all the mountain ranges. 



The second division of rocks consists chiefly of sedimentary beds 

 of late Tertiary age. The series has a total thickness of several 

 thousand feet, and consists of sand, shale, and conglomerate, with 

 some salt and gypsum. Generally the beds are more or less folded 

 and broken, but they are everywhere soft and poorly consolidated. 

 On them is developed the badland topography mentioned above. 

 In the Carrizo Creek region these beds contain an abundant late 

 Tertiary marine fauna. Elsewhere they are apparently unfossilif- 

 erous, and probably terrestrial in origin. 



Interbedded with the Tertiary sediments at some places are 

 beds of basaltic lava and tuff sometimes 100 to 200 feet in thickness. 

 These volcanics constitute only a small portion of the series. 



Quaternary alluvium is the last division of the rock section. 

 It forms most of the basin floor, and fills many of the adjacent 

 valleys. Its areal extent is greatest of the three classes. 



FORMATION OF SALTON BASIN 



Salton Basin has evidently existed as a marked depression since 

 times previous to the latter part of the Tertiary period, for it has 

 received sediment during the formation of the last two rock series. 

 Neither the Quaternary nor the Tertiary sediments are displaced 

 by any such vertical movements as must have occurred during the 

 formation of the basin. The Tertiary beds are seldom found at 

 elevations above 1,000 feet, and the Quaternary shows but few 

 recognizable displacements and these are of small size. 



It has been tacitly assumed by geologists that the basin origi- 

 nated as a dropped fault block, or graben. This hypothesis, 

 suggested by the topographic features, is greatly strengthened by 

 the fact that the basin lies directly on the course of the San Andreas 



