NEOCENE STRATIGRAPHY. 357 



metamorphic series is exposed an eruptive rock is ver}?- 

 abundant. It is well exposed at Mussel Rock and to the 

 southeast of there, making much of the ridge which runs 

 southeast to San Andreas Lake. Since this paper is con- 

 cerned chiefly with the Tertiary rocks no attempt was 

 made to map the igneous rock. Its relation to the lime- 

 stone is shown on the shore at the limestone outcrop in 

 Calera Valley. It is undoubtedly younger. On the east 

 side of San Andreas Lake small exposures show it to be 

 also younger than the phthanites and metamorphic sand- 

 stone. It is thus suggested that it may be contempara- 

 neous with the post-Jurassic upheaval. 



Serpentines. — The abundance and location of the ser- 

 pentine has been described. The question of its origin 

 remains unanswered. Owing to the interest which has 

 attached to that question the writer gave to the subject 

 some study, but beyond coming to the belief that the 

 bronzite rock so abundant here represents an older form 

 of the serpentine, the serpentine being, according to that 

 theory, simply an alteration product of the old basic erup- 

 tive, he has left the problem where he found it. See Dr. 

 Charles Palashe's paper on " The Lherzolite-Serpentine 

 and Associated Rocks of the Potrero, San Francisco."* 



Mei'ced Ertiftives. — West and south of Stanford Uni- 

 versity a large sheet of andesite is exposed. It extends 

 from San Francisquito Creek over the foothills nearest 

 the Bay to beyond the Page Mill Road, which runs up 

 Matadero Creek. At its most northern exposure near 

 San Francisquito Creek it is charactized by columnar 

 structure, the columns being vertical where best exposed, 

 the sheet of andesite being horizontal at that point. 



On the Page Mill Road it appears to have been cut by 

 later dykes. These dykes, however, preceded the depo- 



*Uuiv. of Cal., Bull. Dept. Geol., vol. i, No. 5. 



