Vol. 4] Osmont. — Geological Section, of Coast Ranges. 



4'.) 



The age of this ancient series can only be guessed at from its 

 relation to the granite which is intruded in it, as no palaeon- 

 tological evidence is available. Prom considerations which will 

 be mentioned later this granite is believed to be of post-Juras- 

 sic age. Hence the above mentioned strata cannot be later than 

 Jurassic. 



BODEGA DIORITE. 



Bodega Peninsula is made up almost wholly of a biotite- 

 diorite. Toward the north end of the peninsula this disappears 

 from sight under seolian sands, and the coast line to the north 

 of Salmon Creek is made up entirely of Franciscan sandstone. 

 The east shore of Bodega Bay is entirely Franciscan.* To 

 the south, however, Point Reyes Peninsula is composed very 

 largely of this same diorite and granite, while a comparison with 

 the geology of the coast to the south shows that this is only a 

 northward extension of the plutonics occurring at Montara 

 Mountain,! on San Francisco peninsula, and at Monterey, which 

 are intruded in the old crystalline schists and limestones above 

 mentioned. 



FIELD ASPECTS. 



The best exposures of the diorite occur on the ocean side of 

 the Bodega Peninsida at the south end. Here it stands up in 

 steep cliffs fifty to eighty feet high. It weathers deeply, and 

 this makes it extremely difficult to get fresh specimens for exam- 

 ination. On account of the large amount of coai'se mica present, 

 it frequently shows a white honey-combed surface, resembling 

 coral. Basic secretions are very abundant. They vary in size 

 from a few inches up to six feet in length, and are usually lentic- 

 ular in shape, having their longer dimensions more or less par- 

 allel and pointed in a westerly direction, suggesting pressure at 

 right angles to their course. Dikes or veins of aplite, pegmatite 

 and quartz occur. A very prominent pegmatite dike seven feet 

 thick is exposed on the southeast side of the peninsula, wdiile 

 numerous smaller ones may be seen on the west shore. There is 

 a general westerly trend also to these dikes. That a large amount 



* Anderson, Bull. Dept. Geol., Univ. Cal., Vol. 2, No. 5. 

 tSketch of the Geol. of San Francisco Penin. Lawson, U. S. G. S., 15th 

 Ann. Ept. 



