18 OIL-FIELD WATERS IN SAN JOAQUIN VALLEY, CAL. 
head of waters that rise hut do not reach the surface is also known to 
decrease. The rate of decrease in any given sand depends on local 
factors and may differ widely among neighboring wells, but the gen¬ 
eral decrease in any area as a whole is striking. 
In water wells outside of oil fields the pressure is generally either 
hydrostatic or artesian in character, or is due to rock pressure. 
Hydrostatic pressure is caused by the weight of the downward-bearing 
column of water, and its amount at any point is controlled chiefly by 
the difference in elevation of that point and the point at which the 
water enters the bed, and in practice by the fineness of the material 
through which the pressure is transmitted. The factors controlling 
hydrostatic head are fairly well understood and may be determined 
with considerable accuracy. Rock pressure is commonly believed 
to be the weight of the overlying column of rock, which exerts a com¬ 
pressive effect on the beds beneath it. During the deposition of the 
sediments this undoubtedly leads to closer packing of the grains, but 
after a state of equilibrium has been reached the effect of further 
compression is a matter of speculation, unless the rocks concerned 
are deeply buried. A factor often overlooked in this connection is 
the tensile strength of the overlying rocks, which may conceivably 
be great enough to relieve a part of the pressure from a given point. 
As the weight of the strata is thus somewhat irregularly distributed, 
the rock pressure on a body of ground water must be irregular. 
Rock pressure is doubtless an important factor under some conditions, 
but its effect is generally an unknown quantity. 
Gas pressure must also be taken into account in regions in which 
gas is contained in the rocks. In general the gas pressure in any 
sand is less than the weight of an overlying column of water, but 
there are many sands in which it is considerably greater and in 
which, therefore, its influence must be considered. If a sealed sand 
lens is filled partly with water and partly with gas under pressure, 
the water may flow out under great apparent head when the sand is 
tapped. (See fig. 1, E.) When the gas has expanded somewhat, the 
apparent head of the water decreases, and some, though not all, of 
the decrease in the head of oil-field waters is doubtless due to dimin¬ 
ishing gas pressure. Some puzzling differences in the head of these 
waters may also be ascribed to gas pressure, the effect of which can 
not be overlooked, although its ultimate cause has never been satis¬ 
factorily determined. 
Aside from the pressure exerted by a body of gas, the influence of 
gas liberated from solution in causing water to flow is also important. 
Hydrocarbon gas is dissolved in some waters, and many others con¬ 
tain carbon dioxide and hydrogen sulphide. These are partly lib¬ 
erated at the foot of the well, and act somewhat like an air lift in 
causing the water to flow. Their action is feeble as compared with 
