Vol, XI] DICKERSON—PT. REYES AND SANTA ROSA QUADRANGLES 557 



upon a basalt flow, but at the northwest end of the Burdell 

 Mountain mass (Petaluma Quadrangle), conglomerate con- 

 taining Merced fossils occurs, resting upon Franciscan with 

 several hundred feet of basalt and tuff-breccia above the 

 Merced strata. In the vicinity of Penn Grove the intimate 

 relations between Merced sandstones containing marine fos- 

 sils and tuff-breccia is quite clear. In this vicinity the nearly 

 horizontally bedded Merced rests unconformably upon the 

 Petaluma lake beds of Upper Miocene age, and a like relation 

 exists between the basalts and tuffs and the Petaluma for- 

 mation on the west side of Sonoma Mountain. The strati- 

 graphic relations are entirely clear and in this way the Mer- 

 ced group is thus shown to be the correlative of the Sonoma 

 group. As was shown above, the Neohipparion gidleyi beds be- 

 long with the Sonoma group and are not associated with the 

 Petaluma. 



Neohipparion and Hipparion, which are typical horses of 

 the Pliocene, occur also in beds of the Orinda formation of 

 the Berkeley Hills and in the Etchegoin and Jacalitos of the 

 western border of the San Joaquin Valley. Merriam" de- 

 scribes the Orinda and Siesta formations and their faunas as 

 follows : 



The Orindan and Siestan formations occurring in the hills immediately 

 to the east of Berkeley form the larger part of a thick accumulation of 

 freshwater and alluvial heds resting unconformably upon the marine Mio- 

 cene. The Orindan formation is the lower portion of these beds, and com- 

 prises a great thickness of clays, shales, sands, conglomerates, and tuffs, 

 with occasional beds of limestone. The Orindan is followed by a series 

 of igneous rocks consisting mainly of andesite and basalt. The Siestan 

 rests upon the lavas covering the Orindan, and is in turn covered by a 

 volcanic series made up largely of basalt. 



The section, from the base of the Orindan to the top of the lavas above 

 the Siestan, contains no marine fossils. It shows scattered through it a 

 few remains of freshwater Mollusca and Crustacea, land Mollusca, land 

 plants, and land or freshwater vertebrates. The accumulation as a whole 

 is evidently the result of deposition in a basin which was at some time oc- 

 cupied, at least in part, by freshwater, and at other times may have received 

 purely alluvial deposits. 



Remains of early horses have been found at two localities in the Orindan 

 beds. No specimens representing this group are certainly known from the 

 Siestan. It is stated that bones of a horse were found in a shaft sunk in 

 Siestan beds on Frowning Ridge near the upper end of Telegraph Canyon. 



" Merriam, J. C, Vertebrate Fauna of the Orindan and Siestan beds in Middle 

 California, Univ. Calif. Publ., Bull. Dept. Geol., Vol. 7, pp. 373-374; pp. 376-377, and 

 pp. 384-385. 



New Species of the Hipparion Group from the Pacific Coast and Great Basin, Univ. 

 Calif. Publ., Bull. Dept. Geol., Vol. 9, p. 3, 1915. 



Tertiary Vertebrate Faunas of the North Coalinga Region, Trans. Am. Philos. Soc, 

 Vol. 22, part 3, Philadelphia, 1915. 



