OTHER CAUSES OF LOW GRADIENTS 719 



about 60 degrees, growing gradually cooler to the present temperature 

 of 43 degrees. 



Other Causes of low Gradients 



On page 7G4 of my report on tlie Keweenaw Series, above cited, I have 

 listed a number of the factors in making the rate of increase of tem- 

 perature in the copper mines relatively low, apart from any glacial effect. 

 Presented briefly, they are as follows : 



(1) Most mines have been put down where oxidation of pyrite or of 

 coal may have increased the temperature as far down as this action went. 

 Temperatures taken in borings for oil may also have been affected by 

 exothermic reactions. Of such reactions there is very little trace in the 

 copper mines. If the calcium chloride found at the bottom of the mines 

 was derived from a chlorine-containing glass, it would be endothermic. 



(2) The greater the diffusivity, the less the gradient. The copper- 

 bearing rocks, however, have not a high diffusivity, but a low one, about 

 half the average for the earth used by Kelvin, about two-fifths that of 

 granite, and about two-thirds that of marble. Thus the low gradient 

 must be attributed to some other cause. 



(3) An imbibition of waters from above would, of course, lower the 

 gradient. I believe that this has taken place and is an important factor, 

 but I can not use the low gradient as a very weighty argument for such 

 imbibition, because the low gradient might be accounted for otherwise, 

 as we see. 



(4) As I have discussed in some detail, the extrusion of the great 

 thickness of lava flows of the Kew^eenawan would tend to exhaust the 

 heat beneath and make an extra thick crust, and hence a lower gradient. 

 There are indications of such a thicker crust which may be drawn from 

 gravity observations, and from the theory of isostasy. 



(5) Not only that, but Joly in his recent book, '^The Birth Time of 

 the World," and Arthur Holmes in his book, "Age of the Earth,'^ and 

 in articles contributed to the Geological Magazine, and others have called 

 attention to the fact that a very substantial part of the geothermal 

 gradient ma.y be due to radioactive heat. So as the traps are much less 

 radioactive than the average rocks, according to some investigations, a 

 thickness of 30,000 feet of trap as against the same amount of granite 

 would go a good way toward accounting for a gradient of one foot in 90 

 rather than one foot in 60, according to some results. The data are, 

 however, not yet very accordant. ^^ 



1" For references, see Clarke's data of geochemistry. T^. S. Geol. Survey Bull. No. 695, 

 pp. 306-315. 



