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



7!) 



of erosion which resulted in the formation of Napa Valley, and 

 the other large valleys, of a similar stage of topographical matur- 

 ity, Sonoma and Santa Rosa. Napa Valley seems to have had its 

 inception along the line of weakness due to the St. Helena fault. 

 At Calistoga numerous hot springs occur. The writer does not 

 know how far southward this fault extends. No evidence of it 

 was found in Section CD. 



SAN BRUNO FAULT. 



At Bodega and Point Reyes Peninsula the pre-Franciscan 

 granitics (diorite) have probably been brought to light by a 

 great fault along the line of Bodega and Tomales Bay, seem- 

 ingly the northwest extension of the San Bruno fault dis- 

 cussed by Lawson* in describing the geology of San Francisco 

 Peninsula. Anderson suggests that this granite block during 

 Tertiary times may have been independent in its oscillations 

 from the mainland. Certainly Point Reyes Peninsula was be- 

 neath the waves during Monterey times, having been previously 

 relieved by erosion of nearly all the pre-granitic sedimentaries 

 and Franciscan which may have covered it, since Monterey 

 shales are found resting on the worn surface of the granite. On 

 the mainland, however, no Monterey is found nearer than the 

 east shore of San Pablo Bay. The Monterey is greatly folded, 

 occupying a synclinal basin in the granite, and the latter is con- 

 sequently sheared and faulted. 



Evidence of Faulting at Bodega Bag. — The only evidence of 

 faulting the writer obtained at Bodega Bay was : 



1. The direction and shape of the bay, apparently merely a 

 northerly extension of the peculiarly long and narrow Tomales 

 Bay. 



2. The very general crushed and sheared appearance of the 

 Bodega diorite, and numerous minor faults shown in it by the 

 pegmatite dykes. 



3. The entire absence of Franciscan on Bodega Peninsula, 

 and of diorite on the east side of the bay. 



4. The general westward trend of the Franciscan strata along 

 the east shore, pointing across the bay more or less toward the 

 diorite, as if abutting upon it. 



*U. S. G. S, 15th Ann. Ept. 



