Vol. 4] Osmont. — Geological Section of Coast Ranges. 



59 



The Sonoma Tuff, andesitic in character, and interbedded 

 with thin flows of basalt, to the west with sandstones and vulcanic 

 conglomerates, and to the east with volcanic agglomerates and 

 breccias. The maximum thickness of the tuff and agglomerate 

 observed at any one place is 1,700 feet. 



The Marine Beds of Wilson's Ranch, made up of soft, friable 

 yellow sandstone and fine volcanic conglomerates of a thickness 

 probably exceeding 2,000 feet, and carrying typical Merced 

 fossils. 



The St. Helena Bhyolite, of varying thickness up to 2,000 feet. 



The maximum thickness observed for the whole series at any 

 one place was about 4,000 feet, which does not take into account 

 that portion of the rhyolite removed by erosion. 



This series mantles indifferently over the older formations 

 throughout the large portion of the area under discussion, and 

 engaged most of the writer's attention. Hence it will be described 

 in considerable detail. 



MARK WEST ANDESITE. 



Areal Distribution and Thickness. — The andesite flows in the 

 central and northern portion of the area under discussion aggre- 

 gate a great thickness, while to the south and west they thin out to 

 mere sheets. Where exposed by a fault on the southwest slope 

 of Mt. St. Helena they are over 1,500 feet thick, while the anti- 

 clinal fold just west of Mark West Springs shows a thickness of 

 about 700 feet. At Napa City the waterworks well shows them 

 to have a thickness of nearly 1,500 feet. Near Petaluma they 

 thin down to about 100 feet, and somewhere under Santa Rosa 

 Valley they run out to a thin edge, since they do not appear at 

 all on its west side. 



Structure. — They lie usually at small angles, mantling indif- 

 ferently over the various older formation beneath them. 



Age. — These andesites seem to be conformable in dip with the 

 Sonoma. Tuff above, but an erosion interval existed between the 

 two, as shown by the numerous andesite pebbles included in the 

 latter. The conformability of the dip with the Sonoma Tuff, the 

 age of which, as will be shown later, is pretty certainly late Plio- 

 cene, together with its unconformable relation to the blue San 



