Vol. XI] DICKERSON—PT. REYES AND SANTA ROSA QUADRANGLES 551 



Sonoma Group 

 Basalts, andesites, rhyolites, tuff breccia, fine-grained tuff, 

 and other agglomerates, comprise the Sonoma group of vol- 

 canic rocks which are typically exposed on the western flanks 

 of Sonoma Mountain in the area. Osmont first used the 

 name, Sonoma, in connection with the tuff phase of this great 

 group, and he thought there was only one horizon of tuff 

 beds. His Mark West Andesite and St. Helena Rhyolite 

 were shown to be merely members of this great volcanic 

 mass by Dr. C. E. Weaver, 16 while working upon the Napa 

 Quadrangle. 



Dr. Weaver found that andesites or rather basalts, like the 

 ''Mark West Andesite" occur at various horizons. He also 

 found that rhyolites and tuffs were variously distributed. 

 Detail studies of the western slope of Sonoma Mountain show 

 the correctness of Dr. Weaver's views. 



The Sonoma group covers a large area of country east of 

 the region under discussion, extending over most of the Napa 

 Quadrangle and southward across Carquinez Straits where 

 it is known in the Pinole syncline as the Pinole tuff. The 

 areal mapping in the Petaluma and Santa Rosa quadrangles 

 shows that the lavas of this group probably did not extend 

 much further west than Spring Hill, two miles west of Peta- 

 luma, and that these lavas in part were actually interfingered 

 with Merced. The tuffaceous facies of the Sonoma group 

 are represented as interbedded tuff members of the marine 

 Merced, as was described in detail above. In the road cuts 

 a half-mile east of Freestone, a prominent tuff member oc- 

 curs, and, since numerous casts of marine fossils of Merced 

 age occur here, its origin as a sediment in the waters of the 

 Merced sea is clearly demonstrated. 



Osmont 17 made a careful microscopic study of the rocks 

 composing the Sonoma group. His descriptions are in part 

 as follows: 



A specimen of this rock from beneath the Sonoma tuff near the con- 

 tact on the west limb of the anticline near Mark West Springs showed 

 itself to be microscopically a dark, heavy rock, varying from dark green- 

 ish black to brown in color, according to degree of weathering, and 



ie Weaver C E., Unpublished manuscript of Napa Folio. U. S. Geological Survey. 



« Osmont' Vance, A Geological section of the Coast Ranges north of the Bay of 

 San FraSco, Univ. Calif. Publ., Bull. Dept. Geol., Vol. 4, No. 3, p. 60-61, p. 64. 

 p. 69-70. 



