eldridge.] PETROLEUM FIELDS OF CALIFORNIA. 319 
have generally the appearance of the Pliocene strata of the Pacific 
coast, and some of which also carry fossils of this age. An uncon- 
formity doubtless exists between the last formation and that under- 
lying. Above all, here and there, are recent gravels. 
The structure of the Los Angeles field is anticlinical, the axis of 
the fold lying along the river valley above the city, its direction 
approximately northwest-southeast. The extent and precise nature 
of the anticline is undetermined, but the region of Los Angeles is 
apparently near the eastern end of the fold as it appears in the lower, 
concretionary sandstones, for the lines of stratification of these 
sandstones and of the overlying shales are traceable into the hills 
east of the river, where they turn northward, cross the Arroyo Seco 
into the San Rafael Hills, and thence veer to the west. The dip 
in the latter territory is northeast, the opposite of that in the same 
formation southwest of the river. Locally the anticline is modified 
by subordinate flexures, some of which are of important significance. 
Faults also are present. 
The Los Angeles oil field is developed in the strata believed to be 
Pliocene, on the southern leg of the general anticline. The trend of 
the productive belt, however, instead of conforming to the axis of 
the main fold, follows the strike of the formations on the south of a 
subordinate fold divergent from the main flexure, and hence has 
assumed a direction closely approximating east and west. Evidence 
of this subordinate flexure and of the syncline which separates it from 
the main fold is to be found in the northwestern portion of the city. 
The average dip of the strata adjacent to the oil belt is between 30° 
and 50°, but local disturbances of the beds, sometimes marked, are 
found here and there, and it may be that faulting, too, has played 
its part in the accumulation of oil in the field. 
The Los Angeles field was one of the earliest developed in Califor- 
nia, and the lapse of time since the inception of drilling renders 
almost futile present-day attempts to obtain reliable data concerning 
the conditions of occurrence of the oil. There exist, however, the 
reports of the California State mining bureau, in which the progress 
of development has been well recorded by Mr. Watts. It is sufficient 
here that the wells probably draw their oil from two, three, or more 
horizons in the sands and arenaceous clays that overlie the siliceous 
shales. The general depth of the wells is from 600 to 1,200 feet. 
Their individual production is small compared with many in the great 
fields of the State, and, moreover, they show a gradual decrease year 
by year. This, however, has been partially compensated by the prod- 
uct of new wells. The gravity of the Los Angeles oil varies between 
11° and 18° B. 
PUENTE HILLS. 
The Puente Hills are a low east-west anticlinal ridge about 25 miles 
long and of varying breadth, their western end lying i<> miles a lill Le 
