THE MIOCENE PERIOD. 263 



of organic origin, were deposited. Such thicknesses of such shale, if 

 their interpretation is correct, imply prodigious lapses of time. The 

 whole system here has a thickness of 5000 to 7000 feet. 



In the vicinity of San Francisco, the Monterey series has a thick- 

 ness of more than 5000 feet, and is composed chiefly of sandstone, but 

 subordinately of bituminous shale. 1 In the interpretation of the great 

 thickness, the considerations previously mentioned should be borne 

 in mind. The sections at other points would show notable variations 

 from those here given. One of the singular features of the Miocene 

 tuffs of the Santa Cruz mountains, near San Francisco, is the occurrence 

 of limestone dikes in them. These dikes are clastic, and the cal- 

 careous material of which they are composed is thought to have been 

 forced up into the tuff as ooze from below. 2 



The Miocene is one of the oil-producing horizons of California, 

 and the most important source of bitumen in that State. 3 



The Miocene of western California does not possess the simple struc- 

 ture which characterizes the corresponding beds along the Atlantic and 

 Gulf coasts. Instead of dipping gently to seaward, the strata have been 

 deformed in many places so as to stand at high angles (Figs. 443 and 

 444). Locally (Mount Diablo range), the beds have been folded, and 

 the folds overturned so that the Chico (p. 160) and Tejon (p. 201) series 

 overlie the Miocene. 4 In the Santa Cruz mountains, the early Miocene 

 beds constitute a part of the metamorphic Pascadero series on which the 

 Later Miocene 5 rests unconformable'. The Miocene beds are found in 

 some parts of the Coast Range 6 up to elevations of 2500 feet, and their 

 altitude, position, and stratigraphic relations give some indication of 

 the extent of the deformative movements which have affected this 

 region since the Miocene. 



Farther north, considerable parts of western Oregon, including some 

 of the coastal ranges, were under water during the period, and Miocene 

 (Empire) beds a few hundred feet thick, and containing volcanic ash, 



1 Lawson, Science, N. S., Vol. 15, p. 416, 1902. 



2 Haehl and Arnold, Proc. Am. Phil. Soc, Vol. XLIII, p. 10. 

 8 Eldridge, Bull. 213, U. S. Geol. Surv., p. 306. 



4 Turner, The Geology of Mount Diablo, Bull. Geol. Soc. Am., Vol. 2, 1891. 



6 Ashley, Jour. Geol, Vol. Ill, p. 434. 



6 Lawson, Bull. Dept, Geol., Univ. of Cal., Xo. 1, 1893, and No. 4, 1894; Lawson 

 and Palache, idem, Vol. II, p. 364; Ashley, Jour. Geol., Vol. Ill, p. 434; and Fair- 

 banks, Jour, of Geol., Vol. VI, p. 561. 



