48 



University of California Publications. [Geology 



CORRELATION AND AGE. 



The above described dioritic rocks of Bodega and Point 

 Reyes peninsulas occupy but an insignificant part of the surface 

 area of the territory under discussion, but from evidence at Point 

 Reyes Peninsula, and elsewhere, it is probable that, together 

 with what remains of the crystalline schists and limestones into 

 which they are intruded, they underlie a large portion of the 

 Coast Range. Their similarity, both chemically and mineralog- 

 ically, to the granites of the Sierras, points to their being an 

 outlier of the latter. 



The evidence as to age of these rocks is not conclusive. If 

 they are outliers of the Sierra granitics they are post- Jurassic, 

 since Lindgren* and others have shown the latter to be intrusive 

 in Jurassic strata. But we know that they lie unconformably 

 below the Franciscan, and the age of the latter is still a moot 

 question, some geologists not being prepared to admit so recent 

 an age for it as lower Cretaceous. 



FRANCISCAN. 



Constituent Formations. — This series consists of the usual 

 elements so well described by Lawson in his Sketch of the Geology 

 of San Francisco Peninsula.^ Briefly they consist of hard mas- 

 sive gray sandstone, weathering yellowish brown, known as "San 

 Francisco sandstone," from its prevalence on that peninsula; 

 soft gray shale interbedded with sandstone and occasional fora- 

 miniferal limestone or with radiolarian chert; massive radio- 

 la rian cherts in various colors from white to red ; intrusive masses 

 of basalt, having peculiar spheroidal forms, and known as "spher- 

 oidal basalt": together with local variations in the forms of dia- 

 bases, and even of gabbros; also pyroxenites and peridotites as 

 intrusive dikes, sills and laccolites, usually almost entirely altered 

 to large masses of serpentine; and lastly metamorphic contact- 

 zones of glaucophane, actinolite, or mica-schists. It forms the 

 basement upon which the later rocks of the Coast Ranges rest, 

 and, wherever seen, shows evidence of having been repeatedly 

 sheared and contorted by the many movements which have 

 affected the latter. 



*U. S. G. S. Geologic Atlas, folio 66, Colfax, Cal. 

 tU. S. G. S., 15th Ann. Rpt. 



