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 variations 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 contrast 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 anticlines 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 distance 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-

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