36 E. T. DUMBLE 
the representative, in part at least, of the Monterey in the Sunset- 
McKittrick field. At its most southern exposure in Section 29, 
Twp. 19 S., R. 15 E., just before it is lost under the overlapping 
Jacalitos, it appears as a white, apparently organic, silicious shale 
very similar in appearance to and in the same stratigraphic position 
as the shale of the Monterey where it emerges from this overlap 
in the southern part of the Coalinga field. That the Big Blue is the 
attenuated northern and more littoral representation of the 
Monterey is clearly evident. 
SANTA MARGARITA 
The base of this division of the Miocene in the Coalinga field 
is marked by the Tamiosoma zone and is overlain by sands and 
gravels with a total thickness of 600 feet. It is overlain uncon- 
formably by the later formations. These features continue north- 
ward but gradually the lower member grades into a remarkable 
conglomerate. On Salt Creek thick beds of heavy serpentine con- 
glomerate occur in it and near the south edge of Twp.175.,R.14E., 
it becomes a variable deposit of coarse irregular serpentine breccia 
with beds of conglomerate, sands, and shale. The size of the 
material gradually decreases toward the northward along the out- 
crop and the fragments show greater wear. The beds contain the 
typical Santa Margarita fossils and are overlain by the sandy 
clays and gravel beds of its upper member. 
JACALITOS AND ETCHEGOIN 
The beds described under these names in the Coalinga field and 
supposedly separable there by the fossils contained in them con- 
tinue northward, but generally without the fossils, so that it is 
difficult to separate the two in Twps. 18 N.,R.14 and 15 E.,and17N., 
R. 13 and 14 E., and in fact to distinguish between them and the 
overlying Pliocene, the entire measures as exposed being a variable 
series of sandy clays, sands, and gravel beds aggregating 1,800 to 
2,000 feet in thickness below the alluvial deposits to the east. 
We were unable clearly to differentiate the Jacalitos and Etche- 
goin in the Coalinga region, since we found more than one Glycym- 
eris zone in them, neither of which was mappable throughout 
