26 C. E. VAN ORSTRAND 
Heald (19) has already emphasized the significance of these results 
in their relation to the general slope of the strata. He suggests that the 
isotherms may be a definite reflection of regional structure. 
In order to summarize the evidence on the nature of the isotherms 
in local areas, five different types of structure have been selected as 
illustrations, namely, a fault, a salt plug, an unconformity of the 
granite ridge type, a sand lens, and an anticline with a large closure 
in which the circulation of water may be of importance. 
Figure 7 shows a remarkable rise in the isotherms as the fault in 
the Powell oil field, Navarro County, Texas, is approached (26). 
LARLSBORO 
FY SEC //-9N. SE. 
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WEWOXA 
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j 
Fic. 6.—Regional variation in 
central Oklahoma. 
As the wells in this field were in unstable temperature equilibrium, 
the rise is much less than is indicated on the map, but the fact that 
the 120° F. isotherm is within 400 or 500 feet of the producing sand 
justifies the belief that a real variation exists. 
There appears to be no reason for questioning the large variation 
of temperature found by Hawtof (19) over the salt dome at Humble, 
Harris County, Texas (Fig. 8). According to Logan (27) this dome has 
a maximum diameter of 17,500 feet and a minimum diameter of 14,- 
ooo feet at the 3,000-foot salt contour. 
McCutchin’s (25) map of the El Dorado, Kansas, field (Fig. 9) 
is of interest from two standpoints. In the first place, the isotherms 
dip a little more steeply than the strata, and in the second place, El 
Dorado is a good example of the granite-ridge type of structure. This 
field is also representative of a simple type of unconformity (28). 
548 
