Louderback, G. D. "Benitoite, Its Paragenesis and Mode of Occurrence." University of California Publications.
Page 336
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 recrystallized 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 serpentine. A particularly large mass of serpentine occupies the summit 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 consist of any of the types of rocks mentioned above or any combination 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 welldeveloped 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]

NOTES:
3 The occurrence of the minerals is also discussed by Arnold: Science, n. s., Vol. XXVII (1908), pp. 3l2-314.