242 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



are also still growing and have been since the Tertiary, it is clear that 

 division by epochs of mountain building or the volcanic action associ- 

 ated there\vith cannot be precise. 



Nor is there such agreement among authors as to the strength, 

 number, and regularity of these epochs of mountain building (last 

 and best presented by R. C. Moore, C. Schuchert, and H. Stille) as 

 to make them available for geologic time measurement. Figure 2 

 is after H. StUle." 



Another cause of accumulating strain, to be regularly relieved, is of 

 exactly the opposite nature. It was suggested by J. Joly that if the 

 earth contained anything like the amount of radioactive material 

 that the rocks of the surface seem to, from its decay the earth would 

 be generating more heat than is discharged through the crust; that 

 thus heat would accumulate and the crust be melted from below 

 until it gave way and discharged the heat in lavas, etc. This idea 

 has been elaborately worked out by A. Holmes and H. Jeffreys, and 

 also discussed by Urry."* 



It is clear that these explanations of cycles are inconsistent and of 

 uncertain appUcation. Moreover, we know far too little of what the 

 real normal geothermal gradient is under the land. The old assumed 

 rate of 1° F. for every 60 feet or so of depth proves much too rapid for 

 large areas in Canada and South Africa.''^ Finally, we know nothing 

 of the rate of loss under the ocean which covers three-fourths of the 

 surface. Thus both the rate of loss and the content of radioactive 

 elements as computed by Urry with the best available data are 

 hazardous extrapolations, though there are reasons for believing that 

 Urry's curve is not far off. The meteorites are but very slightly 

 radioactive, and the basaltic rocks less radioactive than the granitic, 

 and the weight of the earth has made the belief general that the interior 

 is in composition more like a meteorite. Roblcy D. Evans suggests 

 that early data as to the surface rocks are questionable. 



It is safe to say, therefore, that such cycles cannot serve as units 

 of geologic measurement, that the curve of rhytluns and revolutions 

 with regular cycles such as Holmes gives is as yet an ideal,'** and that 

 what Ivirsch says " is wise warning. 



The theory of magmatic cycles permits one to draw a number of more or less 

 necessary consequences which may be compared with the facts of experience. 

 It is to be hoped that in the coming years its fruitfulness in guiding geologic and 



" Loc. cit. See also F. NSlkein, Scientia, vol. 1, No. 1, pp.19-30, 1933; and F. P. Shepard, Sealevel and 

 climatic changes relating to late Paleozoic cycles. Bull. Oeol. Soc. Amer., vol. 47, pp. 1197-1206, 1936. 



'« Holmes, A., Thermal history of the earth. Journ. Washington Acad. Sci., vol. 25, pp. 16&-175, 1935. 



Jeffreys, H., The earth, ch. 8, The thermal history of the earth, 2d ed. Macmillan, 1929. 



Urry, W. D., The occurrence of radium, uranium, and potassium in the earth. Proc. Amer. Acad. Arts 

 and Sci., vol. 68, No. 4, pp. 137-144, Mar. 1933. 



» Koenigsberger, J. Q., Beitr. Zeitschr. Angew. Oeophys., vol. 7, No. 1, pp. 68-83, 1937 



'• Holmes, A., Age of the earth. Benn, London, 1927. 



>' Eirscb, Q., Geologie and Radioaktivitat, p. 128. Springer, 1928. 



