GEOLOGY. 
47 
rial might have been, and the more or less completely crystalline 
character of the harder shales shows that metamorphism has taken 
place. The most plausible theory, therefore, is that the Monterey 
shale as originally laid down was fairly constant in character and 
that it has undergone alteration extensively, as well as very locally, 
through the agency of siliceous waters, the older portions of the 
formation, and possibly the more disturbed portions, having been 
most generally subjected to the change. The limestone has in 
places been altered after a fashion somewhat similar to that of the 
siliceous shales, being changed to marble, probably as the result of 
solution and redeposition. 
The Monterey rocks likewise show the result of contact metamor¬ 
phism to a very local extent in the vicinity of the diabase intrusions. 
The process seems to have been largely one of consolidation through 
baking. A limestone specimen obtained near the diabase intrusion 
north of Zaca Peak, in the San Rafael Mountains, gives an excellent 
illustration of shearing. The cal cite crystals have all been arranged 
parallel and greatly elongated, so as to give the rock a schistose 
structure. 
STRUCTURE AND THICKNESS. 
The Monterey has nowhere been left undisturbed. In places it 
has been but gently folded. Pis. IV (p. 36), VIII, B (p. 78), and IX 
(p. 80) show examples of moderate tilting. But at other places, as 
at that pictured in PI. VI, B (p. 46), it has been thrown into folds 
so sharp and closely spaced that the succession of the beds and 
thickness of the series are difficult to make out. The details of its 
structure are discussed under the heading “ Structure 7 ’ (pp. 76-78). 
The thickness of the whole series is at least 5,200 feet. Each of the 
two divisions comprises a maximum known thickness of 2,600 feet. 
No single complete section of the whole could be obtained. 
EVIDENCE OF AGE. 
A paucity of recognizable molluscan fossils is one of the prominent 
characteristics of the Monterey in this region, as in most others in 
the'Coast Ranges where it outcrops. Moreover, the other fossils 
that it contains are of little value in indicating its age. Its position 
in the geologic column is determined by the lower Miocene fossils 
found just below its base in the Vaqueros and by the upper Miocene 
fossils found at or near the base of the Fernando formation, which 
lies unconformably above it. 
The following two species of mollusks occur in the Monterey 
diatomaceous shale on the road just above the Pinal Oil Company’s 
office, southeast of Orcutt: Area aff. trilineata Conrad, Phacoides 
aff. acutilineatus Conrad. 
