58 



University of California Publications. 



[Geology 



more primitive than Equus, but the small size of the fold indi- 

 cates a very advanced stage for a Miocene form. ' ' 



Lignitic Beds of Mark West Creek.— On Mark West Creek, 

 about half a mile above the point where it enters Santa Rosa 

 Valley, are lignitic beds which have been worked in a small way 

 for coal. The lignite occurs in a diatomaceous shale which here 

 underlies the Mark West andesite. The lignite deposit is small 

 and unimportant, but interesting geologically, since these beds 

 probably represent a fresh-water lake contemporaneous, if not 

 connected, with that at Lawlor's Ranch. 



Age. — Lignite is known to exist in these beds at two places, 

 and, at the lignite beds of Lawlor's Ranch, horse teeth of late 

 Miocene or early Pliocene age have been found, and imperfect 

 casts of fresh-water shells. Cyrena Calif ornica occurs abund- 

 antly on Petaluma Creek, and is known certainly from only one 

 other locality, viz: Kirker's Pass, in the very uppermost part of 

 the San Pablo section.* 



Andesite intervenes between these beds and the Sonoma Tuff, 

 and rests unconformably across the eroded edges of their strata. 

 Hence they are considerably older than the tuff, and, as will be 

 shown later, the latter is probably not later than early Merced. 

 These beds are, therefore, referred tentatively to the Orindan, 

 and will be discussed more fully later in the chapter on the cor- 

 relation of the Neocene. 



LATER PLIOCENE. 



The formations observed by the writer in the region under 

 discussion, and ascribed by him to the later Pliocene, make up 

 a series conformable in dip, but containing erosion intervals. 

 They consist of lava flows and sedimentaries ; the former of two 

 well marked characters, the one of intermediate acidity, the other 

 acid; the latter partly of marine, and partly of lacustrine, fluvia- 

 tile and aeolian deposition, but made up largely of volcanic detri- 

 tus of intermediate acidity. 



Starting at the base, the series consists of : 



The Mark West Andesite, -of varying thickness up to 1,500 

 feet. 



"Palaeontology of Cal. Gabb, Vol. 1 1, p. LIU, pi. 7, fig. 45. 



