Vol. 5] 



Louderback. — Benitoite, 



335 



inga. This particular anticlinal lobe is of considerable in- 

 terest in another way, for its lower portion near the valley has 

 produced and is producing large quantities of petroleum, and 

 it was along the nose of this anticline that the first important 

 development of the Coalinga oil fields took place. 



The next succeeding primary anticlinal component forms 

 that part of the range to the west of Los Gatos Creek and Coal- 

 inga. The synclinal area between the two is well marked 

 topographically by the depression in which flows Los Gatos 

 Creek and its tributary. White Creek, the pass at the head, 

 and the depression occupied by San Benito Creek on the west- 

 ern slope. These corresponding depressions (in part structural, 

 but modified by erosion) and the saddle between them naturally 

 determine the position of a road crossing the mountains from 

 the interior valley towards the coast, and this is the road fol- 

 lowed to reach the benitoite locality from Coalinga. 



Ascending the mountains one sees a remarkable display of 

 formations starting with the Recent of the valley, crossing in 

 succession various divisions of the Quaternary, Pliocene, Mio- 

 cene, Eocene, Upper and Lower Cretaceous and ending with 

 the Franciscan which with its associated intrusives occupies 

 the highest portions of the range. It is in these last named 

 rocks that the minerals under discussion occur. 



Owing to the general structure just described, the exposures 

 of the Franciscan and their associated igneous rocks do not 

 everywhere occupy the summit line of the main range, but ex- 

 tend out along the axes of the anticlinal components. Thus 

 these rocks of the benitoite locality extend southward along the 

 spur that runs into the valley northeast of Coalinga, Avhile later 

 rocks occupy the divide at the pass. The older rocks again 

 appear at the surface along the range line farther south along 

 the axis of the next anticlinal component. 



All of the rock formations of this section down to and in- 

 cluding the Knoxville (usually considered Lower Cretaceous), 

 as is common in the Coast Ranges, are unaltered or but slightly 

 altered sediments and show nothing in the nature of schist 

 formation and very little in the way of vernation — and this of 

 superficial origin. The Franciscan series is in marked contrast. 



