APPLICATIONS OF GEOTHERMICS TO GEOLOGY 35 
at a depth of 1,500 meters (Fig. 12). The comparison shows thata 
thickness of 9,000 feet of granite (ab, Fig. 14) produces the same incre- 
ment in the gradient as the heat source represented in the figure. As 
the increments in temperature represented in the figure are of the 
same order of magnitude as those found on the crests of anticlines 
as compared with the temperatures at the same depth near edge 
water, it follows that some of the excess heat beneath anticlinal struc- 
tures may be due to radioactivity. 
SUMMARY AND SUGGESTIONS 
The importance of geothermics in large-scale problems has been 
illustrated by attempting to estimate the depth to the level of isostatic 
adjustment from observations of temperature in Moffat tunnel. By 
assuming the depth to the level of isostatic compensation to be given, 
it may be possible to make a more accurate estimation of temperatures 
down to that depth than has heretofore been made. . 
The geothermal map (Fig. 5) probably represents high temper- 
atures over small areas rather than average temperatures over large 
areas. For example, the areas surrounding salt domes are probably 
areas of average temperature, or at most, only moderately high tem- 
peratures. 
Summarizing the results of recent geothermal surveys, the evi- 
dence shows that variations of temperature have been found to be 
associated with salt domes, sand lenses, faults, and structures with 
both large and small closure. The extent to which this generalization 
applies is not known. 
Concerning causes of temperature variations, both local and re- 
gional variations are apparently dependent on the presence of granite. 
This result, as applied to regional variations, was anticipated to a 
certain extent by Lawson (38), who foresaw the possibility of correlat- 
ing temperature gradients with the granitic layers in different parts 
of the world. As applied to local variations, the result is in agreement 
with an important suggestion by Clapp (39) that the importance of 
the shortening of the columnar sections over anticlines is just begin- 
ning to be appreciated. The result must not be interpreted as proving 
conclusively that radium is the source of all of the heat escaping from 
the surface of the earth. Normal cooling is still a possibility that the 
evidence has not yet excluded. 
Chemical reactions in oil-bearing strata appear to be of minor 
importance as heat sources. 
Further calculations and experiments are needed as a means of 
estimating the importance of oil-bearing strata in preventing the 
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