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University of California. 



[Vol. i. 



age, which are abundantly represented in neighboring portions of 

 the Coast Ranges, are here lacking. From this fact we may infer 

 one of two things, either the region was uplifted at the close of the 

 Chico, and this portion of it remained above sea level throughout 

 Eocene time when the Martinez and Tejon were being deposited to 

 a considerable thickness but a few miies away, or it was once 

 covered by Eocene sediments, and these have since been removed 

 in consequence of a post-Eocene uplift, together with some of the 

 underlying Chico. The latter view of the case is the more proba- 

 ble, in view of the proximity of Eocene sediments free from basal 

 conglomerates, but it does not exclude the possibility of an earlier 

 unconformity between the Chico and the Eocene. 



The Monterey strata, reposing upon this worn surface of Chico 

 rocks, has a thickness of about 1,000 feet. The transgression of 

 the sea, which permitted of this accumulation, must have been 

 from the east toward the west; for, just to the east of the Berkeley 

 Hills, in Contra Costa County, this particular formation of the 

 Monterey series, characterized by cherts and shales, occurs resting 

 upon still lower sandstones of Monterey age. It is here, again, 

 difficult to understand how, under the normal conditions of trans- 

 gression, such a formation, composed of cherts and very fine shales, 

 with a minimum of terrigenous material in its composition, could 

 be the local base of the series, unless the region undergoing 

 subsidence were extremely fiat, so that a slight depression should 

 submerge extensive areas of the land. This consideration, coupled 

 with the fact, above adduced, that the surface upon which the cherts 

 and shales repose is an erosion surface, points unmistakably to the 

 conclusion that prior to the Monterey subsidence a very thorough 

 peneplanation of this portion of the region had been effected. 



The historical significance of the rhythm of sedimentation 

 which so strongly characterizes the Monterey cherts and shales is 

 not clear. Attention has been called elsewhere to a similar rhythm 

 in the radiolarian cherts of the Franciscan; and it might be sup- 

 posed that a comparison of the two formations, so similar in their 

 composition and in their peculiarities of stratification, would lead 

 to the discovery of some working hypothesis explanatory of the 

 remarkable alternation of conditions which seems to have con- 



