114 C. E. VAN ORSTRAND 
feet per degree. The reciprocal of the mean of the corresponding 
gradients is 63.2, but, by varying the number of wells selected from 
the respective states, it is possible to obtain reciprocals of the mean 
gradient that vary from a little more than 40 to a little less than 150 
feet per degree. This result suggests that the average for each geologic 
feature, the Permian Basin, for example, should be considered as a 
unit in determining the average. The distribution of these averages 
about a mean should be definitely related to the geological facts. 
Summarizing the evidence, it appears that the normal gradient 
in the first 1,000 feet of sediments is probably greater than 65.2 feet per 
°F. (35.8 meters per °C.). The determination of an upper limit is not 
easily made. Assuming an arbitrary value of 82.8 as an average, and 
a minimum of 62.14, we have for the maximum reciprocal gradient, 
103.5 feet. The mean from 9 locations exclusive of the oil fields is 
108.0. It seems reasonable, therefore, to assume that the average of 
a large number of gradients covering the entire area of the United 
States, that is, the reciprocal of the normal gradient corrected for 
surface topography, is certainly greater than 60 feet per °F. (32.9 
meters per °C.) and probably less than 110 feet per °F. (60.4 meters 
per °C.). 
The wide interval between the preceding limits may seem un- 
justifiable, but when it is recalled that we have practically no observa- 
tions in the large areas surrounding domes and anticlines, where the 
temperatures are presumably low, it is evident that a large element 
of uncertainty enters into our calculations. For depths exceeding 1,000 
feet, the preceding numbers should probably be replaced by numbers 
of a lesser numerical magnitude. Existing observations do not give 
us very much information on this point. 
The corresponding estimates for undisturbed basement rocks are: 
100 feet per °F. (54.9 meters per °C.) for the minimum and probably 
not less than 200 feet per °F. (109.7 meters per °C.) for the maximum. 
These estimates are based on the data from northern Ontario, Canada 
(118.4 to 233.2). 
In conclusion, it may be of interest to call attention to the fact 
that a value of the reciprocal of the normal gradient equal to 200 
feet per °F. implies an age of a nonradioactive earth of 1,000 or 1,500 
million years. Thermal data from undisturbed basement rocks would 
be of great value, therefore, in estimating the age of a nonradioactive 
earth, and incidentally, it wovld enable us, also, to estimate the amount 
of heat which is being supplied to the earth from radioactive sources. 
