Vol. XI] DICKERSON—RT. REYES AND SANTA ROSA QUADRANGLES 595 



Clays and Coal 



Workable deposits of clays suitable for brick-making or 

 tile are common in the outcrops of the Petaluma formation, 

 near Eureka School. Excellent exposures of these clays oc- 

 cur in an eastern branch of Tolay Creek, a mile and a half 

 south, 30° east, of Eureka School, and in the creek a mile 

 and a half east of Adobe Fort. Other exposures within this 

 same formation occur in the lower slopes of Sonoma Moun- 

 tain. These deposits are located sufficiently close to roads 

 and railroads to receive economic consideration. As the 

 Santa Rosa Quadrangle shows, this region is the type locality 

 of the "basalt block." as abandoned quarries occur in large 

 numbers throughout the Santa Rosa and Petaluma quad- 

 rangles. These quarries were abandoned owing to the in- 

 creasing use of concrete and asphaltic concrete roads. Many 

 excellent basaltic outcrops have been exposed through these 

 abandoned workings. Some coal seams occur in the Neohip- 

 parion gidleyi beds of the Sonoma group on its western face, 

 but these deposits have not proved economic, although Mrs. 

 Thompson reports that the coal obtained from the prospect at 

 Lawlor's ranch was of excellent quality. Samples of coals 

 and clays were collected from the western side of Bennett 

 Mountain by Mr. W. W. Watts of the California State Min- 

 ing Bureau, several years ago. These old workings are now 

 closed, but the writer doubts their economic value. 



Oil 



The bituminous sandstones as reported in the Sonoma 

 group indicates the probable occurrence of Monterey shale 

 beneath the basalts and tuffs of the Sonoma group and the 

 sands and clays of the Petaluma formation. Whether oil 

 might be obtained in this region is problematical. The writer 

 however, is not inclined to condemn this area entirely, and it 

 may warrant exploitation in the future. The possible area 

 which might be exploited is essentially the region in which 

 the shales and sandstones of the Petaluma formation are 

 mapped. This possible oil region might be widened slightly 

 on the northeast, but the Tolay fault definitely limits the pos- 

 sibilities of this region on the southwest side. This is nicely 

 shown in the geological cross-section along the line C-D, 



