Reviews — H. Jeffreys Radioactivity. 87 



The author uses Rinne's term 'eutropic' in place of 'eutectoid', but 

 this is inadvisable as ' eutropic ' had previously been used in another 

 crystallographic sense by Linck in 1896, while 'eutectoid', first 

 suggested by Howe in 1903, has also priority over 'eutropic' 

 (suggested by Rinne in 1905) and has been generally adopted. 



For a long time the structure of meteorites was regarded as 

 metastable, as it could be destroyed by annealing and a granular 

 texture — occasionally found naturally — obtained. Recently, however, 

 Benedicks, by very slow cooling, has obtained plessite and reproduced 

 the octahedral structure. Hence the granular structure is to be 

 regarded as metastable, and the difficulty in reproducing the usual 

 meteoric structure is to be ascribed to the low rate of diffusion 

 inhibiting changes in the solid state. 



In the chapter on classification the only system given is Brezina's 

 modification of the Rose-Tschermak classification. It would have 

 added to the interest of the book if the author had given his own 

 interesting classification based on the American quantitative system 

 for igneous rocks, and Berwerth's rational system founded on the 

 synthetic work on nickel-iron. The recent genetic one, devised by 

 Trior, was, of course, published after this book appeared. 



The illustrations are excellent and the letterpress very clear, 

 though there is a misprint in figure 53 and another on p. 139. The 

 use of such a contraction as ' A.N.H. Wien ' is by no means clear. 

 Nevertheless the book can be confidently recommended as the best 

 general introduction to the study of meteorites which has yet 

 appeared, and should be in the possession of everyone interested in 

 the subject. 



A.S. 



IY. — Radioactivity and Mountain Boilding. 



The Compression of the Earth's Crust in Cooling. By Harold 



Jeferets. Phil. Mag., xxxii, pp. 575-91, December, 1916. 

 fllHE view that mountain-building owes its principal cause to the 

 J. contraction of the earth has been widely adopted by geologists. 

 The subsidiary view that the alleged contraction is due to loss of heat 

 has not met with equal success. The calculations of T. Mellard Reade, 

 made before the discovery of radioactivity, indicated that the circum- 

 ferential shortening of the globe (in cooling from a molten state to its 

 present condition during a period of 100 million years) could not 

 exceed 10-5 miles. This figure is only a small percentage of the 

 shortening implied by the existence of great mountain ranges. 

 Moreover, various calculations of the level of no strain by Fisher, 

 Reade, Davison, and G. H. Darwin gave results varying between 

 0-7 and 7'8 miles. That is to say, compressional deformations of the 

 earth's crust must, on the older hypotheses, have been limited to 

 a thin superficial shell which could never have accumulated the 

 enormous stresses required for periodic mountain-building. Thus, in 

 two directions, the thermal contraction theory was shown to be 

 hopelessly inadequate to meet the facts. Consequently other causes 

 of contraction have come to be invoked in recent years ; among them, 



