University of California Publications. 



[Geology 



GENERAL GEOLOGICAL SURROUNDINGS. 



The Diablo Range is primarily a range of folding and is 

 essentially anticlinal in its structure. The rock formations of 

 which it is composed rise up from the "Great Valley" on the 

 east exposing in the characteristic way, and chiefly through 

 the effects of erosion, older and older strata as we approach the 

 summit region, the rocks dipping to the west and appearing in 

 reverse order as we descend the western slope. Often the general 

 rise from the Great Valley is marked by one or more gentle 

 or subsidiary folds, or it is modified by faulting, and such varia- 

 tions are especially common on the west side where the main 

 range is flanked by a more or less mountainous country as far 

 as the coast. 



However, the Diablo Range is not in its general nature a 

 single great anticline with axis practically coincident with the 

 range line. It consists of a series of anticlinal axes arranged 

 en echelon, their strikes lying generally more west of north 

 and south of east than the topographic summit line. In con- 

 trast to the subsidiary or minor flanking anticlinal folds, these 

 may be called the primary anticlinal components of the range. 

 The noses of these component folds run out into the valley, 

 gradually flattening down until they disappear. These anti- 

 clines are so placed with respect to each other that the summit 

 divide runs along one for a greater or less distance, and then 

 dipping down to a pass rises again to the next axis, and so on. 

 The outcropping strata pass along the flank of the anticline, 

 swing about the end and turn back into the range, then curve 

 about in the opposite direction along the synclinal axis and out 

 along the flank of the next succeeding anticline. 



By reference to the map (plate 27) the general nature of 

 the structure in the vicinity of the benitoite locality can be 

 recognized by its influence on the topography. From some dis- 

 tance to the north of the mineral occurrence an axis of a 

 primary anticlinal component occupies the summit region, and 

 passing a short distance to the east of the gem mine continues 

 in a southeasterly direction and runs out some miles into the 

 valley, where it pitches below the plain just northeast of Coal- 



