NEOCENE STRATIGRAPHY. 293 



between the San Andreas and Crystal Springs Valley and 

 Pilarcitos Valley, from the San Mateo-Spanish Town 

 road, where it used to be quarried, northward a few miles. 

 Near the head of Stevens Creek it outcrops and is well 

 exposed along the hill on the east of the valley; also 

 further east, evidently having been faulted or folded. 



Blocks and small outcrops were found at a number of 

 places, but from their occurrence were generally thought 

 to be only fragments brought up by faulting. 



Relations and Age. — Aside from some indications that 

 this limestone occurred low down in the series, no definite 

 relations could be made out between it and other forma- 

 tions exposed. In the Gavilan Range, Becker makes it 

 " the lowest sedimentary formation encountered."* From 

 the association with it there of rocks of the Arch^an 

 gneiss type, he thinks it may be very old, though he 

 suggests the possibility of its being a member of the 

 Knoxville series of the Lower Cretaceous. 



In the section up Permanenta Creek there seems to be 

 some evidence that it is a conformable member of the 

 series containing the phthanite and metamorphic sand- 

 stone, and would seem to underlie the phthanite by some 

 distance. No good outcrop of the phthanite occurs on 

 the creek, but its position is indicated by fragments in 

 plenty. Between it and the limestone the bedding in the 

 sandstone and shale is very indistinct, but in several places 

 indicated a dip and strike, which is found to agree with the 

 dip and strike of the limestone when that formation is 

 reached further up stream. 



PHTHANITES, OR RADIOLARIAN CHERT. 



Lithology . — Nearly every writer on the geology of Cali- 

 fornia has noted or described a series of thin-bedded, 

 cherty strata, ranging in color from red or green to brown 



* U. S. Geol. Surv., Monograph xiii, p. 181. 



