luridge] PETROLEUM FIELDS OF CALIFORNIA. 315 
inately, 10° and 14° B., respectively. The yield of the Summerland 
veils averages a barrel and a half to two barrels a day, although 
>ccasionally a well is found that for a while has a yield of 10 or 15 
parrels, but such wells are the exceptions. The district has been 
productive for several years, and the comparatively large original 
field of the wells has now been reduced to a minimum. 
SANTA CLARA VALLEY. 
The valley of the Santa Clara is of structural development, modified 
by erosion. It heads in the San Gabriel Rauge and in the mountains 
bo the north connecting this with other portions of the Coast Range 
and with the Sierras, and, after a westerly course of 75 to 100 miles, 
enters the Pacific a little south of the town of Ventura. The valley 
is given over to agriculture, but the mountains on either side are the 
Loci of many important oil fields. 
REGION NORTH OF SANTA CLARA RIVER. 
The mass of rugged mountains north of the Santa Clara Valley, 
forming the watershed between it and the great Central Valley of 
California, represents the convergence of the several ranges which to 
the northwest maintain a conspicuous individuality. Pine Mountain, 
8,826 feet in altitude, is their culminating point. The area thus 
occupied is a part of that recently set aside by the United States 
Government to be known as the Pine Mountain and Zaca Lake 
Forest Reserve. It is accessible only by trail, and is almost wholty 
uninhabited. The southern edge of this great range is one of the 
important oil fields of the Pacific coast. 
The geologic structure of the region as a whole has never been 
determined, but, in the present investigation, that along the edge of 
the Santa Clara Valley was in part deciphered. It is probable that 
the converging ranges have each their own structural representative 
in this mountain mass, of which that studied is but a single member — 
the eastward continuation, perhaps, of the Santa Ynez Range. 
The formations involved in the composition of the oil fields and their 
contiguous territory embrace several thousand feet of dark-gray 
quartzites and interbedded shales, which are believed to be Eocene. 
Overlying these are from 1,000 to 2,000 feet of red sandstones, con- 
glomerates, and shales, the last in the minority. The age of these, 
also, may prove to be Eocene. From their remarkable development 
on the Lower Sespe River they are commonly designated by the name 
of this stream. Above the red beds is a succession of 200 or 300 feet 
of brown, rusty sandstones, followed by 1,000 or 2,000 feet of gray 
and purple shales, with thin, interbedded, fossiliferous limestones 
containing Lower Miocene forms. Succeeding the shales is a promi- 
nent, cliff-forming, yellowish-white, concretionary sandstone 200 or 
