Vol. XI] DICKERSON—PT. REYES AND SANTA ROSA QUADRANGLES 533 



males. Osmont 8 constructed a reconnaissance cross-section 

 across this general region. This paper was found very 

 suggestive, although the writer is in disagreement with sev- 

 eral portions of the text. The San Pablo (?) of Osmont 

 which was described as being in the vicinity of Freestone is 

 beyond doubt Merced, as fossils found at Freestone prove. 

 The area east of Petaluma which he regards as Orinda (?) 

 is the Petaluma formation of upper Miocene age described in 

 this paper. The teeth of the Pliocene horse, Neohipparion 

 gidleyi Merriam, were reported by Osmont as coming from 

 the San Pablo (?) formation beneath the lavas of Sonoma 

 Mountain, whereas they were found in strata which are in- 

 terbedded with these lavas of the Sonoma group. The syn- 

 clinal structure of Petaluma Valley as interpreted by Osmont 

 does not appear to be the explanation of this topographic 

 feature. The Sonoma tuff of Osmont does not prove to be 

 as accurate a horizon marker as he thought since there are 

 several strata of tuff in Sonoma Mountain which are sep- 

 arated from one another by lava flows. The writer is in- 

 debted to Osmont for his careful study of the igneous rocks 

 of the region. Professor J. C. Merriam 9 described the teeth 

 of Neohipparion gidleyi to which reference was made in a 

 brief paper concerning the occurrence of these interesting re- 

 mains. He states "that they probably came from a stratum 

 just below the coal seam at a mine on the Lawler Ranch, six 

 miles east of Petaluma, California. The formation has been 

 doubtfully referred to the San Pablo Miocene, but may rep- 

 resent a later period." Dr. G. K. Gilbert 10 in the California 

 Earthquake Commission's Report gives an excellent descrip- 

 tion of the fault features shown along the San Andreas Rift 

 in the vicinity of Olema and Tomales Bay. He also dis- 

 cusses the modifications in the drainage caused or controlled 

 by rifting. A trace of the fault of 1906 is given in the 

 atlas of this report. This same publication contains collateral 

 biologic articles by Dr. C. A. Kofoid 11 and Dr. Wm. E. 



8 Osmont, Vance, A Geological Section of the Coast Ranges North of the Bay of 

 San Francisco, Univ. Calif. Publ., Bull. Dept. Geol., vol. 4, No. 3, pp. 39-87 1904. 



* Merriam, J. C, New Species of the Hipparion Group from the Pacific Coast and 

 Great Basin Provinces of North America, Univ. Calif., Publ. Dept. Geol., vol. 9. No. 

 1. pp. 1-8. 1915. . . . 



10 Gilbert, G. K., Report of the State Earthquake Investigation Commission, vol. 1. 

 part 1, p. 66, Publ. Carnegie Institution of Washington, 1908. 



11 Kofoid, C. A., Idem, p. 89. 



