56 



University of California Publications.. 



[Geology 



Age. — The question of the age of these strata cannot at pres- 

 ent be conclusively settled. Gabb* referred the beds near Free- 

 stone doubtfully to the Miocene. Pecien canrinus is not sup- 

 posed to extend back of the Pliocene, while dementia subdia- 

 phana is not known back of the Pliocene. These beds lie con- 

 formably beneath the Sonoma Tuff, which will be shown later is 

 probably of late Pliocene age. But on the eastern side of Santa 

 Rosa Valley a thick flow of andesite lies conformably beneath 

 the tuff, with no intervening sedimentaries, while beneath the an- 

 desite is a marked unconformity separating the latter from fresh- 

 water beds of probable Orindan age. Hence, since the marine 

 sedimentaries near Freestone beneath the tuff are non-volcanic 

 in their nature, they are tentatively referred by the writer to the 

 San Pablo. 



The sedimentaries beneath the Sonoma Tuff in Pleasant and 

 Capay Valleys, while conformable in dip with the latter, are 

 also non-volcanic in nature. That they are marine in deposition 

 is shown by a bed of marine shells found in Pleasant Valley 

 carrying numerous species of poorly preserved shells of the San 

 Pablo appearance. These beds are also referred tentatively to 

 the San Pablo. ORINDAN ( ?). 



Red Gravels of Santa Rosa Valley. — Certain strata of un- 

 known age are here inserted, since they are certainly older than 

 the Merced, and probably younger than the Cretaceous. They 

 occur on the west side of Santa Rosa Valley, between Trenton 

 and Healdsburg, where they form a low ridge of hills which, on 

 account of their very red color, form a striking feature of the 

 landscape. Where exposed in favorable places, as at landslides, 

 it is found that this peculiar brick-red soil is derived from a 

 formation which is characterized by being made up almost en- 

 tirely of chert and sandstone of unmistakable Franciscan appear- 

 ance. Sandstone predominates, about 60% being sandstone, 

 10% radiolarian chert, 5%. quartzite, and 25% red clay, which 

 holds it loosely together and gives it its striking color. There is 

 a notable absence of volcanics, only a few pebbles of quartz- 

 porphyry being seen. The gravel is of the average size of a 

 pigeon's egg, with a small percentage of large pebbles up to 

 *Geol. Calif., pp. 83, 84. 



