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



lilt 



Solen sp. 

 Pleurotoma sp. 



Age and Correlation. — The similarity of the above List with 

 the species which occur in the Seven Mile Beach section and 

 those of Capitola, Half Moon Bay and the Scotia section of Hum- 

 boldt County is at once apparent and fixes the age of these beds 

 as Merced. The abundance of area trilincata points to their be- 

 ing of about the horizon represented at Capitola. 



The physical appearance of these sandstones also strongly 

 suggests the Mereed and Wild Cat series. They are soft and 

 friable and easily weather in the characteristic "Bad Land" 

 forms so commonly seen in the above mentioned formations. 



ST. HELENA EHYOLITE. 



Field Aspects, Areal Distribution mid Thickness. — Lying 

 conformably above the tuff are thick Mows of lava of a very 

 much more acid composition than that below. The section at 

 Mt. St. Helena shows a thickness of fully 2,000 feet, not taking 

 erosion into account, all of which is of the same general char- 

 acter, being a very light colored trachytic looking rock which 

 readily weathers and is conspicuous by reason of its lack of ferro- 

 magnesian phenocrysts. Lava of the same character forms the 

 crest of the high ridge east of Napa City, occurring in great 

 thickness and dipping toward the valley, so that a thin layer of it 

 still remains in the valley itself, just east of the town, capping 

 the tuff and andesite. It also forms the highest member in the 

 country between Napa and Sonoma, and between Sonoma and 

 Petaluma, the latter place being the farthest west it has been 

 found. 



Petrography. — A specimen from the top of Mt. St. Helena 

 showed the following characteristics. 



Macroscopically it is a very light colored, almost white, rock, 

 occasionally slightly reddish from iron stains, notably lacking in 

 ferromagnesian minerals. It has a rough, trachytic-like surface. 

 Numerous large, glassy feldspar phenocrysts can be seen, but no 

 quartz. The ground mass appears to be noncrystalline. 



Microscopically this rock is seen to consist of numerous rather 

 poorly formed phenocrysts of potash and soda-lime feldspars 



