NORMAL GEOTHERMAL GRADIENT 507 
in producing wells, it is reasonable to infer that the point for which a 
rate of 62.14 feet per degree Fahrenheit was obtained is not far from 
edge water, and, in view of the relative effects of oil and salt water 
on the transmission of heat, it seems reasonable to expect that an 
equal increment will accrue as we pass from edge water to a’’c”’ 
at the base of the structure. Assuming such an increase, we have from 
Table IV for the sequence of reciprocal gradients at a’’ c’’, the values 
82.8, 66.0, 63.5, 55-9, for the successive series of depths. These values 
are merely plausible estimates of the reciprocals of the mean gradients 
between domes and anticlines. The geometry of the isogeotherms 
(Fig. 1) leads one to suspect that the average depth-temperature 
curve in these areas approximates more closely to a straight line 
than is indicated by the preceding series of numbers. 
’ Conclusive evidence on the diminution of temperature in the 
area immediately surrounding an oil dome was found by E.M. 
Hawtof (4) at Big Lake, Texas. He reports a rate of 111.2, on top 
of the dome; 137.9, on the north edge; 133.6, on the south edge; 133 
on the west edge; and 148.2 feet per degree Fahrenheit at a point about 
12 miles northwest of the dome. An important paper by Strong (5), 
just received, contains graphs showing a close correlation of the iso- 
geothermal surfaces with the anticlines and synclines in some of the 
oil fields in Persia. Tests between domes and anticlines should be of 
great value in establishing the validity of the hypothesis of a varia- 
tion of temperature with structure and it would enable us also to 
make a much more accurate estimate of an average gradient in 
sedimentary areas. 
OTHER OBSERVATIONS 
In Table V are tabulated the records from 25 non-flowing wells 
taken chiefly from the bulletin by N. H. Darton (1). The accuracy 
of many of these observations is probably questionable and the range 
of depths over which the temperatures have been taken are so irregu- 
lar that it is difficult to properly interpret the averages with reference 
to the summaries contained in this paper. Excluding Lake Tahoe, for 
which the gradient is negative, an average value for 24 stations is 
63.7 feet per degree Fahrenheit. 
The reciprocal of the mean gradient determined from the records 
of 3,011 overflowing wells, contained chiefly in Darton’s (1) bulletin, 
is 1°F. in 48.92 feet. Rejecting 164 observations for which the residual 
equals or exceeds 57, the reciprocal of the mean gradient is 1°F. in 
47.82 feet. No further rejections are indicated by the calculations. 
Rejection of 164 observations changed the value of the mean depth 
767 
