222 JAMES PERRIN SMITH 



These in places have a thickness of several thousand feet, but do not 

 form considerable areas of the surface rocks. 



Organic sediments. — These do not make much of a figure on the 

 areal map of the state, but play a large part in its economic history. 

 They are limestones, siliceous shales, and plant accumulations in 

 the form of coal or lignite. 



The limestones are entirely of organic origin, with the exception 

 of some smaller occurrences of late spring deposits, or calcareous 

 tufa, which, however, are large enough to be used in the manufacture 

 of cement. 



The great masses of limestone are confined to the Paleozoic and 

 early Mesozoic, though as late as the middle of the Jurassic period 

 there are some large beds of limestone. They are formed of ground- 

 up shells, corals, and foraminifers that lived in quiet, clear waters, 

 but are now largely crystalline, most of the evidence of their organic 

 origin having been destroyed in the great mountain-making revolu- 

 tions that have passed over them. The formation of limestone on a 

 large scale in California was conlined to epochs that we know from 

 other evidence were warm, and also to epochs when sheltered, clear 

 seas covered portions of the state. In such seas corals and forami- 

 nifers abounded, and the evidence of their rock-forming activity is 

 still visible in the coral reefs of the Paleozoic and Triassic, and the 

 Fusulina limestone of the Carboniferous. 



From the middle of the Mesozoic up to the Eocene it was still 

 warm enough at times for reef-building corals, and foraminifers to 

 have flourished in the seas of California; but the warm epoch of the 

 Middle Jurassic was a time of igneous activity, and during the Cre- 

 taceous there was too much sand and mud poured into the water for 

 these organisms to find a favorable habitat. 



Limestones, at least in part formed by corals, have a thickness of 

 several thousand feet in the Cambrian of Inyo County, but the areal 

 extent is unknown. The Devonian of Shasta and Siskiyou counties 

 shows coral reef rock to the thickness of several hundreds of feet, of" 

 small area. These are all surpassed in the great masses of Carbon- 

 iferous limestone, of the White Mountains, the western flank of the 

 Sierra Nevada, and the Klamath Mountains, where the lenticular 

 beds sometimes attain a thickness of two thousand feet. 



