28 SANTA MARIA OIL DISTRICT, CALIFORNIA. 
disturbed that little clew as to their structure can be obtained, and 
so local in extent that no attempt has been made in mapping to 
differentiate them from the accompanying serpentine. 
KNOXVILLE FORMATION (LOWER CRETACEOUS). 
Several small areas of sedimentary rock occur which can be defi¬ 
nitely assigned on fossil evidence to the Knoxville, or lower Cretaceous. 
The two most important are north of Mount Lospe. in the Casmalia 
Hills. The rock is chiefly dark-colored, unaltered argillaceous shale, 
such as is characteristic of the Knoxville throughout its wide area 
of distribution in the California Coast Ranges. Sandstone and con¬ 
glomerate occur in lesser amounts. Brownish-yellow sandstone, 
similar to that common in the Knoxville in the Coast Ranges several 
hundred miles farther north, occurs on the border of an irregular 
area of diabase on Tepusquet Creek, in the San Rafael Mountains, 
and contains the characteristic Knoxville fossil Aucella piochii Gabb 
(PI. XIII, figs. 1, 2, 3a, 3b, p. 128). The rock is present only in very 
small patches, and seems to have been brought up from below by the 
diabase intrusion. The Knoxville was recognized in one other place 
in the San Rafael Mountains a few miles north of Zaca Lake, at the 
base of the series mapped as pre-Monterey, where also it contains 
Aucella piochii. It is very likely that a portion of the areas mapped 
as pre-Monterey belongs to the lower Cretaceous, but it is not prob¬ 
able that the whole does. 
PRE-MONTEREY ROCKS. 
Two large areas of sedimentary rocks whose age has not been 
determined otherwise than that they are older than the Monterey 
occur in the San Rafael Mountains. They are mapped as pre- 
Monterey rocks. It is probable that strata of Knoxville (lower 
Cretaceous) age occur at the base of the series in those areas and 
that the higher portions represent either the upper Cretaceous or 
the Eocene, or both. Detailed work was left until another time. 
The larger of these two areas occupies the northeast corner of the 
region shown on the map, and is about 60 square miles in extent. 
The other lies on the northeastern slope of the high ridge north of 
Zaca Lake. In these areas are exposed a great series of thin-bedded, 
dark-colored, locally greenish shale alternating with more massively 
bedded sandstone, which is in places of a very granitic nature. Con¬ 
glomerate, much of it plainly evidencing its origin from granite, 
occurs in minor horizons. Knoxville fossils were found in a gritty 
greenish sandstone near the lowest portion of this pre-Monterey 
terrane, about 2 miles north of Zaca Lake. The higher portion seems 
to be the continuation of a formation in San Luis Obispo County that 
