Vol. 5] 



Weaver. — San Pablo Formation. 



257 



but nneonformably upon the Shasta- Chico series. The Shasta- 

 Chieo strata make up the greater part of the Blue Ridge and dip 

 at an angle of about 45 degrees to the northeast. The San Pablo 

 lies upon this and dips in the same general direction, but at an 

 average angle of 20 degrees. Near the base the San Pablo is 

 made up of thick-bedded, buff-colored sandstones. Higher up in 

 the series on the east side of Pleasant 's Valley a few shaly bands 

 are present, but the sandstone has a distinct gray color and is 

 often thin-bedded. They are overlain unconformably by the 

 Pinole Tuff, in other places by andesite and in still other places 

 by the Orindan formation. Neither the Eocene nor Monterey is 

 present. The total thickness is over two thousand feet. The 

 white chalky shale member which occurs at the base in the Mt. 

 Diablo region is not present at Carneros Creek nor Pleasant 's 

 Valley. 



Original Extent of Deposits. — The geographical conditions 

 existing in middle California during San Pablo time appear to 

 have been somewhat peculiar. That there was an interval of 

 time between the close of the Monterey and the beginning of the 

 San Pablo appears evident from the fact of the unconformity 

 existing between them in the San Pablo Bay region. Farther 

 east sedimentation appears to have been continuous and in places 

 there were local deposits of volcanic ash. From the cross- 

 bedded and conglomeratic character of the sandstones and the 

 thick seams of conglomerate, it appears that the greater part 

 of the San Pablo strata indicate shallow water conditions. No 

 outcrops occur south of Pinole or west of Lafayette and it 

 would seem that in that region during San Pablo time there was 

 a land surface. There appears to have been some volcanic activ- 

 ity during the time of deposition. This became especially evi- 

 dent to the north after the close of the San Pablo. It is prob- 

 able that the chief volcanic activity during San Pablo time was 

 to the east, perhaps even as far as the Sierra Nevada. The 

 andesite pebbles which make up the conglomerates in Tassajero 

 Canon are similar to andesites from that region and none are 

 known as flows in the San Pablo of the Coast Ranges of middle 

 California. 



