LAWSON "I 

 I'M u HE J 



The Berkeley Hills. 



37" 



locates the formation in a line parallel to the strike, curves to the 

 eastward within a mile, and, crossing the strata obliquely, cuts off 

 the formation completely, so that in this direction we have no trace 

 of it beyond the periphery of Strawberry Canon amphitheater. 



ORINDAN FORMATION. 



Stratigraphic Composition. — -Continuing now our section along 

 Skyline Ridge, we find a thick formation of pebble-conglomerates, 

 feebly coherent sandstones, and purplish-red and greenish shale, 

 which will be referred to as the Orindan formation. This forma- 

 tion has an extensive distribution in Contra Costa County in the 

 region adjoining the Berkeley Hills on the east, and we must draw 

 upon this region for information which will enable us to fully 

 characterize the formation. In the part of the formation exposed 

 along San Pablo Creek', beyond the limits of the accompanying 

 map, and elsewhere in Contra Costa County, other beds than 

 those just mentioned enter into its composition. These are thin 

 beds of volcanic tuff generally decomposed and of a dark brownish 

 color, beds of blue and slate-colored clay containing fresh-water 

 ostracods, sandstones containing fresh-water ostracods, limestones 

 containing fresh-water ostracods and molluscs, and occasional patches 

 of lava of quite limited extent, intercalated (?) with the sedimentary 

 beds. No marine organisms have been found in any part of the 

 formation. These facts are here cited for the purpose of affording 

 a more just conception of the formation as a whole than can be 

 obtained from the section on the west flank of the Berkeley Hills, 

 and of demonstrating its non-marine origin. 



Unconformity at Base. — The contact of the base of the Orindan 

 formation upon the underlying Monterey formation is clearly an 

 unconformity. This is generally the case where fresh-water beds 

 repose upon marine formations, but it is not a necessary inference 

 that there must be an unconformity in all such cases, since it has 

 doubtless not infrequently happened that arms of the sea have been 

 converted into fresh-water basins without interruption of the 

 process of sedimentation. The unconformity is established by 

 direct evidence of the discordance of the two sets of strata, and 



