GEOLOGIC RECORD OF CALIFORNIA 221 



from the igneous rocks, so that they more often arkose and greywacke 

 than true sandstones. 



Thick beds of aluminous shales, now largely changed to slates, 

 are found in the Carboniferous and Jurassic rocks of the Sierra 

 Nevada, and to a less extent in the Franciscan formation of the Coast 

 Ranges. The Auriferous Slates also form the surface rocks of con- 

 siderable areas in the Klamath Mountains. 



Less altered shales are extensively developed in all the later for- 

 mations of the state, from the Lower Cretaceous upward, although 

 not on such a grand scale as in the older periods. 



The greatest individual mass of sediments in California is formed 

 by the Quaternary and Pliocene fiuviatile deposits of the Great Val- 

 ley. This mass is about four hundred miles long by fifty in width, 

 and is several thousand feet thick in the middle, thinning out toward 

 the edges, surpassing the enormous mass of Tertiary sediments. 

 These valley deposits have been bored to a depth of three thousand 

 feet, without reaching bed-rock, but there are too few deep borings 

 for an estimate of the average thickness to be possible. 



A second great mass of clastic sediments is seen in the Tertiary 

 sandstones of the Coast Ranges, which extend nearly the entire length 

 of the state, and have a total thickness of about fifteen thousand feet, 

 although not all of this at any one place. A remnant of this series 

 is seen along the western flank of the Sierra Nevada in the marine 

 and brackish-water lone formation, and the upland equivalent is 

 seen in the Auriferous Gravels. 



A third great mass of sandstones is found in the Cretaceous of the 

 Coast Ranges, where a thickness of about thirty thousand feet was 

 deposited. This thickness surpasses by far that of the Tertiary sand- 

 stones, but the areal extent is much less. These, too, overlapped 

 on the foot of the Sierra Nevada. 



Smaller masses of sandstone, now largely changed to quartzite, 

 are seen in the early Mesozoic and Paleozoic formations of the Sierra 

 Nevada and Coast Ranges, but nowhere forming extensive surface 

 areas. 



On the western flank of the Sierra Nevada, throughout the Gold 

 Belt, there are in the late Paleozoic and in the late Jurassic thick 

 beds of tuffs, or volcanic ash, now altered to greenstone schists. 



