436 University of California Publications in Geology [Vol. 8 



Pecten estrellanus and Zirphaea, cf. dentata, all of which are found in 

 the San Pablo but do not extend into the Etchegoin. On the other 

 hand, the presence of many species in the Jacalitos which range into 

 the Etchegoin but which have not been found in the Santa Margarita 

 of the south or in the San Pablo of Middle California would seem to 

 suggest a later age than the upper San Pablo of Middle California 

 and therefore a considerably later age than the Santa Margarita north 

 of Coalinga. Some of the Etchegoin-Jacalitos forms which have not 

 been found in the San Pablo or Santa Margarita are Echinarach- 

 nius gibbsi, Gardium, cf. meekianum, Macoma vanlecki, Mya japonica, 

 Pecten oweni, Chrysodomus portalensis, Thais lamellosa, Thais etche- 

 goinensis, Thais kettelmanensis and Turritella vanvlecki. 



It is the writer's opinion that the Jacalitos beds are probably 

 younger than the uppermost beds of the San Pablo of middle California 

 and that future work will show that they have a closer relationship to 

 the Etchegoin than to the San Pablo. 



Relation to the Scutella breweriana beds. — It has been shown in 

 the descriptions of the different sections that in some localities it is 

 difficult to draw a sharp line between the Monterey and the San Pablo 

 groups. As has already been stated, the writer, in his description of 

 the San Pablo of middle California, originally accepted the line be- 

 tween it and the underlying Monterey group as first defined by Pro- 

 fessor J. C. Merriam, which line of division has been accepted by 

 Professor A. C. Lawson in mapping beds of these horizons in this 

 part of the state. (See folio U. S. G. S. No. 193.) At the present 

 time, however, the writer is of the opinion that the Scutella breweriana 

 beds, which have been considered as a part of the Monterey group by 

 Merriam and Lawson, should be classed as either a part of the San 

 Pablo group or as a distinct stratigraphic unit. 



On the north side of Mount Diablo there is a good unconformity 

 between the San Pablo and the Monterey group, but in this region the 

 Scutella breweriana beds are not present. 



Reference has already been made to an irregular contact on the 

 south side of Mount Diablo which was regarded as the dividing line 

 between the San Pablo and the Scutella breweriana beds. However, 

 this contact is of such a character that it might be merely a local 

 unconformity. In all the other sections studied by the writer no 

 stratigraphic break between the Scutella breweriana beds and the San 

 Pablo was apparent, and at some localities the division was made 

 purely on the basis of the faunas ; at other places, however, especially 



