88 Reviews — Mineral Resources of the United States. 



a molecular rearrangement of compounds in the earth's interior, 

 which under high pressure may he supposed to involve a decrease in 

 volume. 



With the discovery of radioactivity and the realization of its 

 fundamental importance in dynamical geology it became evident that 

 the thermal contraction theory stood in urgent need of re-examination. 

 The mathematical skill required for this task is of an order far beyond 

 the attainments of most geologists, and Mr. Harold Jeffreys has come 

 to our assistance by working out, in the light of our later knowledge, 

 the group of problems involved. Using the thermal and radioactive 

 data adopted by the present writer in a series of papers which have 

 appeared in the pages of this Magazine, he has arrived at the following 

 results : — 



(«) The level of no strain is now at a depth of 79 km. (uniform 

 distribution of radioactivity) or of 76 km. (exponential distribution). 



(b) Every great circle of the earth has been shortened by 227 km. 

 (uniform distribution of radioactivity) or by 133 km. (exponential 

 distribution). To arrive at the radial shortening, these figures should 

 be divided by 6 - 28. 



(c) The surface of the earth has been diminished by crumpling by 

 5-6 X 10 s sq. km. (uniform distribution of radioactivity) or by 

 3*3 X 10 6 sq. km. (exponential distribution). 



In order to test these results Mr. Jeffreys has calculated from the 

 mean height of existing mountain ranges the approximate diminution 

 of the surface by crumpling. He finds the amount to be about 

 1-8 X 10 6 sq. km., which is little more than half of the amount 

 implied by the thermal contraction hypothesis in its new form 

 (exponential distribution). It should be pointed out, however, that 

 Mr. Jeffreys' calculation of the actual amount of diminution of the 

 surface by crumpling gives only a minimum result, for it does not 

 take into consideration the former existence of ranges that are now 

 submerged or denuded to insignificant elevations. Most of the ranges 

 measured are of post-Carboniferous age, and may therefore represent 

 only a third or a quarter of the total crumpling since geological 

 history began. The maximum circumferential shortening demanded 

 by Chamberlin is about 300 km., corresponding to a surface 

 diminution by crumpling of 7"6 X 10 6 sq. km., more than twice as 

 much as that found theoretically on an exponential distribution of 

 radioactivity. Nevertheless, in spite of the fact that complete 

 accordance has not yet been attained, it is evident that the theoretical 

 and observed compressions are of the same order of magnitude, and it 

 may confidently be asserted that the much abused thermal contraction 

 hypothesis, thanks to radioactivity, has now been set firmly on 

 its feet. 



Arthur Holmes. 



V. — Mineral Resources of the United States for 1914. 

 rnHIS is the 33rd of the admirable series of reports which was 

 J_ started in the early years of the United States Geological 

 Survey. Year by year they give carefully prepared statistical 



