570 GEOLOGY. 



It is to be noted that even these selected records vary a hundred 

 per cent. Very notable variations are found in the same mine or well, 

 and often much difference is found in adjacent records, especially those 

 of artesian wells. Some of these are explainable, but the full meaning 

 of other variations is yet to be found . 



Explanations of varying increment. — Certain apparent variations 

 are merely due to inequalities of topography. The isogeotherms, or 

 planes of equal underground temperature, do not normally rise and 

 fall with every local irregularity of the surface, but more nearly strike 

 an average. A well on a bluff 500 feet high would probably reach 

 nearly the same temperature at 1000 feet, as a well 500 feet deep in the 

 adjacent valley, giving a gradient twice as great in the one case as in 

 the other. 



In interpreting the temperatures of artesian flows, regard must 

 be had to the depths of rock under which the waters have passed, as well as 

 the depths at the location of the wells. Darton has found^ unusually 

 high and varying temperatures in the artesian wells of the Dakotas, 

 some part of which may be due to this cause, though a full explana- 

 tion of their singular variations is not yet reached. 



The permeation and circulation of water affect the temperature in 

 two important ways: (1) wet rocks are better conductors than dry 

 ones, and (2) the convective movement of water is a means of conveying 

 heat from lower to higher horizons. As the circulation of underground 

 water is very unequal, much irregularity of thermal distribution in 

 the upper zones probably arises from this source. The general effect of 

 water circulation is to reduce the thermal gradient where the circula- 

 tion is relatively rapid, as it is near the surface and in the main thorough- 

 fares of circulation, and hence to cause a relatively rapid rise in the 

 gradient just below the zone of effective water influence. Some records 

 conform to this theoretical deduction, but in general it is masked by 

 other influences. 



Chemical action, especially oxidation, carbonation, hydration, solu- 

 tion, and precipitation, modify the normal temperature gradient, but 

 how effectively is not well determined. With little doubt the first 

 three mentioned above raise the temperature, while solution and 

 precipitation in some large measure offset each other.^ The sum- 



* Am. Jour, of Sci., Vol. V, 1898, p. 161. 

 2 Van Hise. Personal communication. 



