82 



University of California Publications. 



[Geology 



the ones described. But the crest-line of the coastal ridge for a 

 considerable distance north and south is very nearly horizontal, 

 and at an elevation of about 750 to 800 feet above sea-level. 

 From a point on the upper road from Bodega Bay to the town of 

 Bodega, where it passes over the summit, the writer found nu- 

 merous pholas-borings in large Franciscan boulders which may 

 be residual stacks. At another place, about midway on the road 

 between Freestone and Occidental, and at nearly the same eleva- 

 tion, as determined roughly by aneroid, pholas-borings were 

 also observed. 



Fault Origin of Ht. St. Helena. — Mt. St. Helena owes its 

 height mainly to a great fault, previously described, which has 

 elevated it at least 2,500 feet higher than the corresponding flows 

 of lava on the west side of Napa Valley. There is no evidence 

 of its ever having been a volcano. No crater, or residual neck, or 

 heterogeneity of materials is present. On the contrary, it is made 

 up of even, well defined flows of lava, which frequently show 

 evidence in columnar structure of having cooled as surface flows. 

 Mt. Cobb appears to be simply another point on this high ridge, 

 though the writer has no personal knowledge of it. 



A nearly straight line through Cobb Mountain and St. Helena 

 southward would follow the high ridge east of Napa Valley, 

 crossing the bay near the Straits of Carquines, passing south 

 through Mt. Diablo, and, if continued farther south, would pass 

 through Mt. Hamilton. This line is evidently the axis of the 

 Range. Napa Valley has been determined partly by the St. 

 Helena Fault, and partly by its synclinal nature toward the 

 south. 



Becent Submergence. — The recent subsidence which has 

 affected this region is well illustrated at the mouths of all of the 

 streams flowing into the ocean and the bay. Russian River, 

 Salmon Creek, the Esteros Americano and San Antonio, and 

 Drake's Bay at their mouths are wide, fiord-like bodies of water 

 with very precipitous shores, while the streams such as Petaluma 

 Creek and Napa Creek are mere sloughs in their lower limits, 

 meandering through broad, flat tule land bordered by steep hill- 

 sides. That they represent a submerged area is apparent to the 

 casual glance. Certain evidence exists to show that the sub- 



