GEOLOGY. 
69 
period. Acidic volcanic ash of a rhyolitic type was ejected, and it 
settled in the ocean to form regular beds of considerable thickness 
and extent interstratified with the other sediments. The occur¬ 
rence of ash interbedded with diatomaceous earth that probably 
belongs fairly high in the Monterey formation indicates that these 
eruptions did not cease in the early part of middle Miocene time. 
Neither the centers of eruptions nor any lava equivalents of the ash 
have been found in the field. Similar eruptions were characteristic 
of the lower and middle Miocene for long distances north and south 
of this region. 
LATE TERTIARY AND EARLY QUATERNARY PERIOD. 
The Monterey period of deposition was brought to a close by 
orogenic movements which folded the shales and lifted them above 
the sea in many places. In some regions the folding was intense, 
the greatest disturbances accompanying the uplift of the mountain 
ranges to an altitude of thousands of feet. The San Rafael Moun¬ 
tains, which were upheaved at this time, probably extended along 
the lines of former mountains, and some smaller mountainous or 
hilly areas likewise, such as the Casmalia Hills and perhaps portions 
of the Santa Ynez Range, followed former zones of uplift. But for 
the most part the Santa Ynez Range was probably new. It is 
doubtful whether it was ever completely covered by Monterey sedi¬ 
ments, and its structure may have been determined by minor folding 
previous to the beginning of the Monterey, but it is probable that 
this range did not have any approach to its present proportions until 
after middle Miocene time. In other regions low, broad folds were 
formed during the post-Monterey disturbance and the strata were 
not upheaved to a great altitude; such was the case in parts of the 
basin region between the San Rafael and Santa Ynez mountains. 
After the formation of the middle Miocene shales they were 
intruded at several different points by basic igneous masses, mostly 
of the nature of diabase. The disturbance which put an end to the 
period was profound and this igneous activity was probably an 
accompaniment of it. The rocks were locally hardened by contact 
action in consequence of the intrusions. 
After an erosion interval, probably of comparatively short dura¬ 
tion, the land again sank, though not so extensively nor to such 
depth as in the previous subsidence, and a large part of the Santa 
Maria district, especially the lower regions, became submerged. 
The deposition of the Fernando followed, beginning before the close 
of the Miocene. Owing to differences in altitude and possibly also 
to local difference in the amount of subsidence, the deposition began 
in some places before it did in others. Over the areas in which the 
Monterey has been only slightly folded, the Fernando beds assumed 
conformable positions with it. In regions where the Monterey beds 
