4 CRANDALL— SAN FRANCISCO PENINSULA. [January 4, 



11. Geological Terranes. 



There are six geological terranes represented on the San Fran- 

 cisco peninsula. 



I. The Montara granite. 

 II. Franciscan or Golden Gate series. 



III. Merced series (Pliocene). 



IV. Pleistocene and recent including the sand dunes derived 



from the Pleistocene. 

 V. Serpentines. 



VI. Igneous intrusives other than peridotite. 

 VII. Schists. 



I. The Granites. — The Montara granite does not properly be- 

 long within the area mapped, but it deserves mention in this con- 

 nection because of its direct relation to the Franciscan series and 

 the problem of the age of the latter. It belongs to the oldest rocks 

 that appear in the Coast Ranges but is considered to be younger 

 than the crystalline limestones and schists that occur exposed further 

 south in these same ranges. The age of the schist and limestone 

 is unknown and the age of the granite is likewise unknown further 

 than that it is older than the Franciscan series, and is considered 

 by Dr. Fairbanks to be older than the granite of the Sierras, which 

 is intrusive in the upper Jurassic sedimentary beds. 



II. The Franciscan Series. — The Franciscan series, which 

 is the one of chief interest, rests upon the eroded surface of this 

 granite as exposed on the northwest end of Montara Mountain 

 near Devil's Slide. The lowest beds of the series are conglomerates 

 containing boulders of granite that lie directly on the igneous rock. 

 The petrographic evidence shows that the sandstone is derived 

 mainly from the erosion of this or some similar igneous rocks, and 

 is necessarily arkose. 



The Franciscan series proper consists of five beds in the fol- 

 lowing order. 



1. Coarse Conglomerate containing large boulders of granite. 



2. The San Pedro Shales. — These are thinly bedded black shales 

 having a thickness of approximately four thousand feet. This sec- 



