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University of California . 



(VOL. 2. 



the formation, neither of the Astrodapsis species being found 

 outside of it. The zone of Scutella (Clypeaster) Gabbi is not 

 always present, but may be considered as belonging to the 

 formation. 



Lithologically, and even scenically, the strata of this formation 

 arc readily distinguished from those of the adjoining groups. A 

 considerable thickness of tuffs and ashes, most prominent in the 

 upper portion of the formation, and the peculiar weathering of 

 the sandstones, are constant and striking characters of the greatest 

 value in working at points where fossil remains are rare or absent. 



Relation of San Pablo to Contra Costa County Miocene. — The 

 relations of this formation to the Contra Costa Count}' Miocene are 

 well shown at Walnut Creek, where the highly disturbed Miocene- 

 bearing Clypeaster ? (Ecli.) Brewerianus, is accompanied by the 

 Astrodapsis-bearing beds, standing at the same angle and seemingly 

 conformable on it. Scutella (Clypeaster) Gabbi has not been seen 

 at this point, though it may be present below the Astrodapsis 

 beds. It is known, however, on the opposite side of the valley, 

 where it seems to be low down in the formation. 



At San Pablo Bay the beds bearing Astrodapsis and Scutella 

 form a wide syncline, the bottom of which is the Scutella zone. 

 On this lie the Astrodapsis beds with tuffs and ashes above. The 

 northern limb of the syncline, resting on the upper portion of the 

 Contra Costa County Miocene, is in almost vertical position. On 

 the southern side of the syncline the strata stand at a much lower 

 angle, about twenty-five degrees. 



Though no distinct unconformity between the lower portion of 

 the San Pablo and the Contra Costa County Miocene has been seen, 

 a break is indicated at several localities. At present it is not 

 possible to draw a sharp line between the two formations at all 

 localities. Where ash and tuff beds are present in the lower 

 portion of the San Pablo the separation is easily made. 



Relation of San Pablo to Merced.— The stratigraphic relations 

 of the San Pablo to the Merced are not yet definitely known; the 

 two formations, though occurring only a few miles apart, have not 

 yet been seen in contact. 



Faunally the San Pablo does not appear to be as closely 



