GEOLOGIC RECORD OF CALIFORNIA 223 



The Santa Lucia limestone, in the Coast Ranges, of doubtful 

 Paleozoic age, also occur in large beds, amounting to several hundred 

 feet in thickness, now changed to marble. 



The Upper Triassic of Shasta and Plumas counties has lenses of 

 limestone in places four or five hundred feet thick, forming important 

 topographic features, and largely formed by the agency of corals. 



The Franciscan series of the Coast Ranges has similar limestone 

 masses of lenticular form, amounting in places to a few hundred feet 

 in thickness, and wholly destitute of fossils, except a few traces of 

 foraminifers. 



The Cretaceous lacks limestone beds, except a local accumulation 

 of shell limestone in the Knoxville formation of Colusa County, where 

 a thickness of only a few feet is developed. 



The Eocene of the Santa Cruz Mountains has some thin beds of 

 limestone, and the Miocene of Santa Barbara, San Luis Obispo, and 

 Orange counties has shell limestone amounting to as much as fifty 

 feet in thickness. With the exception of these local occurrences there 

 are no limestone masses in the marine beds of California from the 

 middle of the Jurassic to the Quaternary, the Jurassic and Knoxville 

 being characterized by thick beds of shale, and the other formations, 

 from the Horsetown up, by enormous beds of sandstone. 



Siliceous organic sediments.- — Among the most remarkable features 

 of the stratigraphy of California are the thick beds of siliceous organic 

 sediments. In the Monterey shale of the Middle Tertiary in the Coast 

 Ranges such sediments are extensively developed, and in places reach 

 a thickness of five thousand feet. These are not shales in the ordinary 

 sense, for they are chiefly organic in origin, the remains of microscopic 

 diatoms and radiolaria. Similar deposits are known also in the 

 Eocene of the middle Coast Ranges, but on a smaller scale. These 

 organic siliceous shales are of great economic importance, for they 

 have furnished nearly all of the petroleum of California. 



Similar masses of siliceous organic sediments are known in the 

 Coast Ranges in the Franciscan formation, of the earlier Mesozoic, 

 but they are no longer shales, rather hard, flinty rocks, with the 

 organic matter long since removed, and the fossil tests of radiolaria 

 almost entirely destroyed, so that the rocks now show little resem- 

 blance to organic sediments. 



