26 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 56 



which appears to me most probable. The most evident objection to it is 

 the low surface gradient of 1° F. in 77 feet, while Kelvin took 1° F. in 

 50.6 feet and King- stated that in 1893 the last published vahie as reduced 

 from all available data by the British Association committee was 64 feet per 

 degree Fahrenheit. King himself considered 75 feet a maximum. To 

 me, however, it does not seem that an average value is what is required. 

 In discussing the cooling of the earth disturbing causes must be eliminated 

 as far as possible. Now several causes must contribute more or less to 

 raise the temperature of rocks near the surface; for example, thermal 

 springs, volcanic heat, the dissipation of mechanical energy by faulting 

 or solid flow, the liberation of heat in the decomposition of minerals and 

 radioactivity. So far as I know, the only causes which can lead to a 

 deceptively low gradient in rocks of a given type are the neighborhood of 

 large bodies of cold water and abnormally high diffusivity. Furthermore, 

 to include gradients observed in sedimentary rocks seems to me to com- 

 plicate the problem unnecessarily. The gradients which should serve as 

 a guide are those in massive rocks, especially the nearly anhydrous 

 basaltic, andesitic and rhyolitic massives. All cases where there are local 

 evidences of heat due to thermal springs, etc., should be excluded, and 

 when for a normal rock the gradient is unusually high, it should be con- 

 sidered as suspicious. In short, for the present problem the lower 

 gradients in massive rocks are those most likely to give a correct value of 

 the earth's age. 



Several writers have made suggestions of this kind, but Mr. Johann 

 Koenigsberger ^ has given the only detailed discussion of the matter 

 known to me, pointing out that the characteristic gradients are those 

 observed in nearly level, inland regions, in chemically unaltered rocks. 

 Of such he gives 36 cases, ranging from 1° C. in 27.8 meters to 1° in 

 37.9 meters. Five of these average 1° in 37.7 meters, so that, in round 

 numbers, 1° in 38 meters seems to me best to represent the true terres- 

 trial gradient. Comparing this gradient with the 1° in 42.2 meters com- 

 puted for the 60-million-year earth leaves a difference of 1° in 385 meters 

 to be accounted for by exothermic chemical action. This is no doubt an 

 inaccurate remainder, but it is probably of the order of magnitude of the 

 correction appropriate to radioactivity and similar causes.' 



Mr. Koenigsberger in the same paper has also shown that the tempera- 

 tures observed in the Swiss railway tunnels can be computed from 

 Fourier's equations and average superficial temperature gradients when 

 the topographic forms are duly represented in the formulae. Hence it is 

 not needful to assume radioactivity in order to account for these 

 phenomena. 



'^ CongrSs g6olog. Intern., tenth session, Mexico, Compte Rendus, 1907, p. 1127. 

 2 See "Relations of Radioactivity to Cosmogony and Geology," Bull. Geol. Soc. Amer., vol. 19, 

 1908, p. 113. 



