310 CONTRIBUTIONS TO ECONOMIC GEOLOGY, 1902. [bull. 213«ji 
gray, concretionary sandstones and dark-colored shales, probably! 
Tejon; on the slopes, local developments of gritty sands, brown and 
yellow limestones, and gypsiferons clays, perhaps a lower division of 
the Miocene, the upper division consisting of siliceous shales, typical 
of the Monterey; in the low outer ridges, a successiou of conglomer 
ates, sandstones, and clays, many hundred feet thick, the equivalent 
of the San Pablo, of Middle Neocene age; and in the valley, Recent 
gravels. Between the San Pablo and older formations — the horizon 
of most importance from the petroleum point of view — there exists a 
marked unconformity, the line of union as exposed lying now at one 
horizon, now at another, in beds both above and below the break 
in continuity. Just within the border of the younger formation the 
development of the oil field has taken place, the wells drawing their 
petroleum from one or more of the conglomerates and sandstones 
adjacent to the plane of unconformity. 
Structurally, the strata of the Sunset district, while thrown into an 
anticline of great extent, present in detail a succession of folds, those 
of greatest amplitude lying farthest within the mountains, the gen- 
eral trend of all being about N. 50° W. Faults also exist, but none of 
large displacement was detected within or near the oil-producing area 
itself. The greatest crushing has been effected in the shales of the 
Monterey, but along the desert edge the San Pablo also shows a 
number of minor flexures, sonic developed en echelon, to which is 
due the frequent offsets to be observed in the trend of the oil belt. 
The general dip of the strata in the oil-yielding territory is northeast 
or toward the valley. Its direction is, however, modified by the 
flexures referred to, and by other and local variations in strike. 
The wells of the Sunset district attain a depth of from 500 to 1,500 
feet, and while there is a similarity in the oil sands, it is questionable 
whether the same horizon is everywhere the productive zone, for the 
San Pablo is deposited against a slope of the Miocene, from which it 
might have drawn the petroleum into several beds abutting it at the 
plane of unconformity. The wells in the Midway field are somewhat 
deeper than those in the Sunset area proper, having been drilled 
farther out on the slope of the anticline. The especial interest of 
these wells is their position along the exterior of the anticline at a 
very considerable distance from both axis and end, and in a locality 
where the strike and dip are apparently maintained with great regu- 
larity. The gravity of the oil in the Sunset district varies from 11° B. 
in very shallow wells in the southeastern part of the field, to 17° or 
18° B. in the deeper ones in the northwestern portion. 
KERN RIVER FIELD. 
The Kern River field, the most productive in California, lies about 
3 miles north of Bakersfield, in Kern County, near the southeastern 
extremity of the San Joaquin Valley. As at present developed it 
