236 University of California Publications in Geology [Vol. 7 



during some stages than at others, and therefore spreading over 

 larger areas ; or both such agencies may have 'been active. 



As examples of such areas may be mentioned the country 

 to the north of Santa Clara Valley, the Los Angeles district, the 

 southern end of the San Joaquin valley, the Vallecitos, the Mount 

 Diablo region of Contra Costa County, and the Point Arena area, 

 already discussed in parts one and two of this paper. 



Chert. — Instead of diatomaceous earths or shales, the siliceous 

 members of the Monterey often appear as cherts — chiefly opaline 

 in character. They are generally yellowish, but sometimes brown 

 to nearly black. The dark color is usually due to bituminous 

 substances. Sometimes these cherts occur in thin beds one to 

 several inches thick, interstratified with earthy or siliceous earthy 

 shales, the alternation being repeated hundreds of times. 88 



Relation to the Occurrences of Petroleum. — The siliceous and 

 the earthy shales of the Monterey series are very commonly bitu- 

 minous, and are looked upon by most of the geologists working 

 in the California oil fields as the source of most of the commer- 

 cially utilizable petroleum of the state. Seepages and brea de- 

 posits are often associated with them. As far as the supplies 

 of oil for industries is concerned, it is commonly derived from 

 the sands of the series (the basal sands, or higher sands inter- 

 calated in the shale series), sometimes from zones of brecciated 

 chert, sometimes from the sands or other porous rocks of ad- 

 joining formation groups, stratigraphieally either higher or lower 

 — from Mesozoic to Pliocene. 



Volcanic Products in the Monterey Series 

 Tuffs. — Besides the terrigenous and biogenic deposits men- 

 tioned above, products of volcanic activity are frequently encoun- 

 tered within the Monterey Series. Over considerable areas ash 

 beds are more or less common and at different horizons. Along 

 the hills stretching from Lion Rock, near San Luis Obispo, to 

 north of Santa Maria, about 30 miles, a layer of volcanic ash 

 occurs, in part carrying coarse glass fragments, in part pumi- 

 eeous, in part coarsely agglomeratic, which according to Fair- 

 banks is rhyolitic and reaches a maximum thickness of 800 feet. 



ss Well shown in halftone, plate 11, opposite p. 365, Bull. Dept. Geol. 

 Univ. Calif., vol. 2. 



