24 C. E. VAN ORSTRAND 
The quantity e in the table represents the excess of soil temper- 
ature over air temperature (a-a, in Figs. 3 and 13). A negative sign 
precedes this quantity when the rock temperatures are higher than the 
annual mean air temperatures. Conversely, a positive sign precedes 
the tabular value when the reverse condition exists. The probable 
error of an observation of weight unity is represented by r and the 
probable error of the mean is represented by 70. 
The tabular values of the reciprocal gradient 1/b were computed 
by reciprocating the mean values of the gradient 6. To obtain the 
range of 1/b, the value of 27 for b was added to and subtracted from 
b. The mean of the differences between 1/6 and the reciprocal of the 
two preceding values was selected to represent the range of 1/6. For 
example, the table shows that the values of 1/6 for Birmingham, 
Alabama, fall between the limits 88.7 and 104.9. This method of 
procedure is a fairly satisfactory method of presenting the data in a 
very condensed form. For a small number of observations, the in- 
dicated range is frequently too large. 
Each value of 1/0 in the table is recorded on the map of the United 
States (Fig. 5). Depths are limited to 1,000 feet, or less. 
The map shows clearly that the temperatures are uniformly low 
in the Appalachians. There are no areas of high temperatures. In the 
Rockies, the temperatures vary from the extremely high to the ex- 
tremely low. Irregularities over large areas rather than uniformity is 
the rule. In southeastern New Mexico and western Texas, including 
the Panhandle, extraordinarily low temperatures prevail. In Wyo- 
ming, the temperatures vary from the extremely high to a moderate 
average. That is, Wyoming is a high-temperature area. Likewise, 
eastern Texas, Oklahoma, and Kansas, southern California and 
Arkansas, and northern Louisiana may be characterized as areas of 
high to moderately high temperatures. A more detailed study of the 
areal and local distribution of temperatures is given in Problems of 
Petroleum Geology.' 
As a further means of summarizing and interpreting the data, let 
us consider the temperature distributions over regional and local 
areas. 
Figure 6 is a reproduction of McCutchin’s (25) map showing the 
general trend of temperatures over a distance of approximately 100 
miles in central Oklahoma, extending in a southeasterly direction 
from Oklahoma City to Wewoka. Another map by McCutchin, not 
here represented, shows a similar trend of the isothermal surfaces ex- 
tending from Oklahoma City in a northeasterly direction to Sapulpa. 
“ A sequel to Structure of Typical American Oil Fields. In manuscript. 
546 
