20 OIL-FIELD WATERS IN SAN JOAQUIN VALLEY, CAL. 
the wide variation between individual gradients in oil fields suggests 
that more data might reverse their results. Irregular variations, as 
stated above, also characterize the temperatures of the waters in the 
oil fields of the San Joaquin Valley. 
« 
GEOLOGIC CONDITIONS THAT INFLUENCE THE COMPOSITION OF 
WATER. 
GENERAL FEATURES. 
Owing to the fact that underground waters in a region of sedimen¬ 
tary rocks tend to follow the bedding jdanes, the structure or attitude 
of the rocks has an important effect on the freedom of the circulation, 
which in turn influences the chemical character of the waters them¬ 
selves. If salt water, for example, is present in the strata before 
they are folded some of it may be trapped in structural troughs or 
basins and held there indefinitely. Whether the water so trapped was 
originally salt or fresh, it is evident that during its long period of 
association with the rocks it will dissolve certain of their constituents 
and become more highly mineralized. The waters of the oil fields 
of the valley differ widely from one another in both their content of 
chloride and their total load of dissolved mineral matter, and it is 
thought that these variations are caused by such physical factors as 
the geologic structure, the lenticularity of the beds, and the amount of 
rainfall. The solution of rock constituents by underground waters 
is a somewhat complicated problem that may best be discussed in 
connection with the chemistry of the waters, but the distribution of 
chloride is so largely a physical problem that it will be considered here. 
ORIGIN OF SALT WATER IN ROCKS. 
When sediment is deposited on the floor of the sea it is saturated 
with sea water, which remains in the pores until it is elevated to 
form land. The compacting of the material necessarily forces out a 
large quantity of this water, but that remaining ordinarily fills the 
pore spaces of the rocks after they have assumed their normal bulk. 
Uplift, however, causes circulation of the water to commence, and 
meteoric water falling on the newly elevated land surface enters the 
beds and tends to dilute and force out the connate water—the sea 
water entrapped in the sediments. This replacement takes place 
with considerable rapidity under some conditions, but under others 
the connate water, only slightly altered by its contact with the rocks, 
may remain trapped for long periods of time. The freedom of the 
outlet by which the salt water may escape is the chief factor, for water 
trapped in a lens of sand entirely inclosed by shale might remain there 
indefinitely. Similarly, as circulation follows bedding planes, if the 
strata are folded into a closed basin or trough the water will accu- 
