LOS ANGELES DISTRICT: SALT LAKE FIELD. 187 
The oil is supposed to be derived largely from the diatoms and other 
minute organic remains found in the underlying shale and finds its 
way into the sandy layers mainly through the multitude of joint 
cracks which penetrate both the shale and sandstone. 
BREA DEPOSITS. 
The brea deposits in the Salt Lake field are the most important in 
the Los Angeles district. They cover a considerable territory in sec. 
21. (See PI. XXIV, B.) A number of years ago large quantities of 
this brea were removed and used for paving purposes, the resultant 
depression filling up with water and forming a lagoon. The brea is 
largely the result of the impregnation of porous sand and soil by oil 
oozing up from below. That the process is still going on is evidenced 
by the heavy oil which may be seen oozing from the banks of the 
lagoon and of several other brea pits in the vicinity. Large quanti¬ 
ties of gas are also escaping in the same region, as is shown by the 
intermittent streams of bubbles which rise to the surface of the water 
in the lagoon and other depressions thereabouts. (See PI. XXIV, B.) 
The logs of several wells indicate that brea occurs in the basal Pleisto¬ 
cene beds over a considerable territory contiguous to the lagoon 
where it is invisible on the surface. The formation of brea over an 
area relatively so large implies the escape of vast quantities of oil 
from the underlying Tertiary beds. This escape of the oil and gas is 
probably made possible by the fractured condition of the rocks. On 
the evidence offered by the great brea deposits and the large quanti¬ 
ties of escaping oil and gas in the region of the lagoon the theory is 
advanced that considerable fracturing has accompanied the forma¬ 
tion of the Salt Lake flexure. 
GEOLOGY OF THE WELLS. 
For the convenience of discussion of the underground geology, the 
Salt Lake field will be divided into two jiarts by a hypothetical line 
running in a general northeast-southwest direction through the 
lagoon and coinciding in a general way with the axis of the supposed 
Salt Lake flexure. The area to the northwest of this line contains 
most of the productive wells, and there are no important producing 
wells in the area southeast of the line. 
AREA NORTH AND NORTHWEST OF THE LAGOON. 
In the area north and northwest of the lagoon the wells for the first 
50 to 100 feet penetrate alluvium and Pleistocene clay, coarse sand, 
and gravel—the mantle of the older formations. The Pleistocene 
beds usually carry two water-bearing layers, one at a depth of 20 to 
30 feet, the other (which appears to lie at the base of the formation) 
at 50 to 100 feet from the surface. In some of the wells the lower 
