104 OIL DISTRICTS OF SOUTHERN CALIFORNIA. 
ceous nature, the whole gray or brown from the presence of iron and 
bitumen. Thin, fine-grained sandstones are interbedded with the 
shale from top to bottom and lenticles of gray limestone weathering 
yellow also occur. All in all, the formation bears close resemblance 
to the Monterey and certain portions of the Modelo. The only evi¬ 
dence of life thus far discovered consists of foraminifera and diatoms, 
so common in the Miocene of the Pacific coast. These occur in both 
shale and limestone. The shale is exposed in the heart of the hills 
with varying prominence; its outcrop is especially strong from Brea 
Canyon westward and in the southeastern portion of the hills. To 
judge by the sequence of beds in other parts of southern California, 
this shale is probably underlain by a mass of coarse sandstone. 
PUENTE SANDSTONE. 
Overlying the shale is a moderately coarse gray to yellow heavv- 
bedded sandstone, separated by minor bands of organic siliceous shale. 
Spherical and lenticular concretions, consisting of the same material 
as the mass of the rock, characterize the sandstone. The lower por¬ 
tion of the sandstone is more thinly bedded than the upper, and the 
intercalated shale is more earthy, suggesting a transition to the under¬ 
lying member. This feature is especially conspicuous along lower 
Soquel Canyon. The thickness of the sandstone varies from perhaps 
less than 300 feet in the western portion of the hills to possibly over 
1,000 feet in the eastern portion, where it lies in a gently undulating 
position, and its areal extent is very great. The sandstone is wanting 
in outcrop along the southern face of the hills opposite the wells of 
the Puente Oil Company. 
UPPER PUENTE SHALE. 
Overlying the Puente sandstone is a variable thickness of earthy, 
siliceous, and chalk-like shale, with a few beds of fine yellow ferrugi¬ 
nous sandstone and minor quartzo-calcareous concretions. This por¬ 
tion of the formation will be called the upper Puente shale, or upper 
shale. The thickness is uncertain, but in the region of the Olinda 
field it appears to be considerably less than it is believed to be in 
the western portion of the hills. Nowhere, perhaps, is it more than 
300 or 400 feet in outcrop. However, the overlying formation rests 
upon it unconformably, and for this reason it is impossible to estimate 
its full original thickness. In doubtful structural positions only has it 
been suspected of having a development of over 100 feet. The most 
important instance of this kind is on the eastern border of the Whittier 
field, where the crest of the hills shows the following section from 
