336 University of California Publications. [Geology 



It consists of more or less altered sandstones (often more 

 properly graywacke), shales (frequently slates), radiolarian 

 cherts and local areas of various types of more or less recrystal- 

 lized rocks, which are frequently coarsely crystalline schists. 

 This series is also intruded by various types of basic igneous 

 rocks and by dikes and large (bathylithic?) masses of serpen- 

 tine. A particularly large mass of serpentine occupies the sum- 

 mit region of the anticline north of Coalinga ; and it is in the 

 midst of this that the benitoite mine is located. It is the same 

 mass that has been referred to in the literature as passing just 

 back of the New Idria quicksilver mines which are situated about 

 five or six miles north of the benitoite locality. In the vicinity 

 of the gem mine it is several miles wide and extends down the 

 range some distance to the southeast. 



Scattered through this serpentine area are included patches 

 of the Franciscan, sometimes of considerable size and sometimes 

 only a few yards or even feet across. These patches may con- 

 sist of any of the types of rocks mentioned above or any com- 

 bination of them with or without associated basic igneous mate- 

 rials. Patches of schist are quite common and of considerable 

 variety, including glaucophane, actinolite, hornblende, garnet, 

 mica, chlorite and other schists, and they often carry well- 

 developed crystals either as part of the body of the schist or 

 in the veins by which they are frequently traversed. Attempts 

 have been made to discover some regularity of strikes, or dips 

 or other structural relations, but without success. These patches 

 appear in general to be detached masses included at the time 

 of the intrusion of the serpentine, and to bear no particular 

 relationship in their attitude to the roof or country from which 

 they were separated. 



It is in one of these masses inclosed in the serpentine that 

 the mineral deposits under consideration are found. 



OCCURRENCE. 



Benitoite occurs in a zone of narrow veins of natrolite, which 

 traverses an irregular lens-shaped inclusion in the serpentine. 3 



3 The occurrence of the minerals is also discussed by Arnold : Science, 

 n. s., Vol. XXVII (1908), pp. 312-314. 



