20 



CRANDALL— SAN FRANCISCO PENINSULA. 



[January 



vine was found at all, making this rock a true pyroxenite. That 

 the main mass of the serpentine of the Presidio is not derived from 

 a pyroxenite will be seen from the chemical analyses following.^ 



Analyses. — There is a marked similarity between the analyses 

 of the serpentine from three distinct localities. 



Turner mentions the gradation from peridotite to pyroxenite in 

 the serpentine of Mt. Diablo. The analysis' of the Presidio ser- 

 pentine shows it to be an olivine bearing rock, so that the pyroxe- 

 nite may be considered a local variation in the main peridotite 

 magma. 



VI. Igneous Intrusives. — The igneous intrusives cut the jas- 

 pers, sandstones and the associated serpentines of the Franciscan 

 series. They consist of several closely related varieties. 



The most important of these intrusive rocks occur in the Po- 

 trero, where they are intruded in the serpentine. Of this particular 



Fig. S. 

 Remnant of a diabase dike intruded into the serpentine of the Potrero sill. 



intrusive there are two facies : hypersthene diabase, and hornblende 

 diabase or epidorite. The others of these intrusives from elsewhere 

 than the Potrero are hornblende diabase, amygdaloidal diabase and 



^ These are taken from a paper of H. W. Turner, Journal of Geology-, 

 VI, 487. 



