APPLICATIONS OF GEOTHERMICS TO GEOLOGY ig 
the porosity, diminishes with the depth, that is, the conductivity 
diminishes with decreasing moisture content of the rock. 
Another explanation of the curvature of the depth temperature 
curve shown in Figure 3 has been proposed by A. C. Lane (18). He 
emphasizes the fact that the upper portion of the depth-temperature 
curve must have risen in response to a gradual rise in the annual mean 
temperature of the air since the ice age. The fact, as is shown later, 
that the temperatures of the rocks near the surface of the ground are 
closely adjusted to the temperatures of the air above them, is sub- 
stantial evidence in favor of Lane’s hypothesis. Tests of the thermal 














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Fic. 3.—Depth-temperature curve and porosity curve. 
conductivity of the rocks in their natural state at various depths in 
wells and mines would be of great value in the elucidation of this 
problem. 
ERRORS IN GEOTHERMAL MEASUREMENTS 
Accurate observations can be made in wells drilled with the stand- 
ard rig after drilling has been discontinued for a day or two. The well 
must of course be free from flowing fluid at the time of the test. Wells 
that have been discharging fluid for a considerable time prior to the 
test are in unstable temperature equilibrium and the records obtained 
from them are not likely to be of much value. 
The time required for a well drilled with a rotary outfit to reach 
thermal equilibrium has not been determined. It undoubtedly varies 
greatly with the rate of drilling, the hardness of the rocks, and many 
other factors. In Figure 4 are shown two curves which give some in- 
941 
