SUMMARY OF CONCLUSIONS. 
113 
the horizon of a water on the basis of a partial analysis can not be too 
strongly condemned. Some oil men have attempted to compare 
waters simply by tasting them, but it is evident that only the roughest 
distinctions can be made in this way, and the analyses given in this 
report suffice to show that the amount of salt a water contains is no 
indication of its horizon, except perhaps very locally. Others have 
used hydrogen sulphide as a means of distinguishing or correlating 
waters, chiefly because this constituent can readily be detected by its 
odor or by its ability to, darken a silver coin immersed in the water. 
As many top waters contain hydrogen sulphide, whereas the waters 
below the oil contain it only in some localities along the border of 
the fields, this simple test may sometimes be of value. Owing to 
the ease with which hydrogen sulphide is oxidized, however, it can 
not be regarded as a very stable constituent of the water and its 
value as a criterion for comparison is limited. A complete analysis 
is always the most satisfactory. In conclusion, it may be reiterated 
that an analysis is of no value in comparison unless the sample is 
properly taken and the analytical work accurately performed, for 
otherwise very misleading conclusions may be drawn. 
SUMMARY OF CONCLUSIONS. 
Water-bearing sands are generally encountered above, below, and, 
in many places, in the oil measures in the oil fields bordering the San 
Joaquin Valley. The strata are lenticular and the correlation of 
individual beds except within limited areas is impracticable. The 
high pressure of the water in many of the sands renders it difficult 
to prevent the water from invading the oil sands and thus greatly 
reducing their productivity. 
Some of the ground waters are as salty as ocean water, but others 
are fresh. This difference is believed to be the result of difference in 
freedom of circulation, which is controlled chiefly by the geologic 
structure. Where the structure prevents free circulation the ground 
water is salty, but where it does not and circulation is relatively free 
meteoric water has entered the beds and replaced much of the strong 
chloride water originally present. The ground water near the surface 
and near the outcrops of the beds is comparatively fresh, but the 
content of chloride generally increases with depth and with distance 
from the outcrop. The deeper waters trapped in structural troughs, 
like the Midway syncline, closely resemble ocean water in most 
respects and are believed to be only slightly altered connate water or 
fossil sea water. 
The surface waters and shallow ground waters and also the deeper 
ground waters outside the oil fields on the west side of the San Joa¬ 
quin Valley contain much sulphate. In the oil fields, however, the 
content of sulphate decreases with depth and ground waters near and 
60439°—Bull. 653—17-8 
