334 University of California Publications. [Geology
GENERAL GEOLOGICAL SURROUNDINGS.
The
Diablo Range is primarily arange 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-
