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University, of California Publications. 



[Geology 



there is a similar and nearly parallel belt extending from the 

 alluvium-covered foothills of the San Joaquin on the southeast 

 across the range to San Ramon Valley on the northwest. As 

 a rule the outcrops of this formation stand out very prominently 

 especially in Tassajero Canon and Green Valley and farther to 

 the west along Shell Ridge. Here it is covered by the alluvium 

 of San Ramon Valley. "West of Mount Diablo it outcrops 

 from the town of Walnut Creek southward in a belt extending 

 nearly parallel to San Ramon Valley, occurring in the isolated 

 hills in the valley, in the hills on either side of the valley, and in 

 the bed of the creek itself. Farther west it again occurs along 

 both sides of the ridge extending southeasterly from the town 

 of Lafayette. South of here it occurs as a belt extending around 

 the ridge between San Ramon Valley and Bolinger Canon. 



To the south the San Pablo outcrops east of the town of 

 Livermore and north of the town of Tesla. It occurs as a belt 

 extending nearly east and west but with a slight northwesterly 

 trend. It covers an area of at least ten miles long by half a 

 mile wide. 



These isolated areas are all exposed to view, due to the folding 

 of the strata, and probably represent what was once during San 

 Pablo time a nearly continuous area of deposition. 



STEATIGEAPHIC BELATIONS. 



The formations comprising the middle Tertiary of Central 

 California are the Monterey and San Pablo. In most eases the 

 stratigraphic relations existing between them are not very clearly 

 defined. The two formations when considered as a whole are 

 entirely different. The Monterey formation is composed of a 

 number of alternating divisions of sandstone and shale, both the 

 lower and upper being sandstone. The uppermost sandstone 

 division of the Monterey is very difficult to distinguish on a litho- 

 logical basis from some facies of the San Pablo beds. In the ma- 

 jority of instances the San Pablo appears to rest conformably 

 upon the Monterey sandstone and hence it becomes very difficult 

 to determine at just what point to draw the line of separation. 



San Pablo Section. — The section which has been most thor- 

 oughly studied, and the one from which the formation takes its 



