August 5, igoq! 



NATURE 



:io 



demonstration at the end of the chapter, that the well- 

 known pleochroic halos in certain minerals are due 

 to the radio-active discharges from the minute inclu- 

 sions forming their centres, will specially interest 

 petrologists. 



Thus assured of the general prevalence of radium, 

 Prof. Joly proceeds to trace its effects as a factor in 

 terrestrial thermodynamics. The variations in the 

 temperature gradient below the surface naturally 

 suggest investigation, and an attempt is made to 

 connect the high temperatures met with in the Simplon 

 tunnel, and that near one end of the St. Gothard, with 

 local variations in the radium content of the rocks. 

 In the succeeding chapter is worked out a very in- 

 genious theory to account for the well-known fact that 

 mountain chains arise on the sites where sedimentation 

 has been most extensive, i.e. from the great geosyn- 

 clines. Having shown that the rocks cannot retain 

 the comparative richness in radium, which they 

 possess normally at the surface, to a depth of more 

 than ten or fifteen miles, and that detrital rocks contain 

 almost the whole of the radium which was in their 

 parent rocks, it is then pointed out that denudation 

 results in the thinning of this radium-rich surface- 

 layer in one locality and the piling up of such material 

 in the area of sedimentation — it leads to " a convection 

 of energy." Not only is a much larger quantity of 

 radio-active material accumulated in the sedimental 

 area, but — and this is still more important — the thick- 

 ness of the radio-active layer is there greatly increased. 

 As a result, the heating effect is there emphasised, and 

 the area of sedimentation becomes an area of weak- 

 ness ; the geosyncline becomes a mountain chain. 



In natural sequence comes an inquiry whether the 

 same cause of instability may in any degree account 

 for the more general movements of the ocean bed 

 which were first referred to by Darwin. This in- 

 volves a preliminary discussion of the probable depth 

 of oceanic deposits. The radium content of globigerina 

 ooze is found by numerous determinations to be four 

 or five times that of normal sediments, but its efficacy 

 will depend much more on its thickness. Such meagre 

 data as are available in the few examples of upraised 

 oceanic sediments are carefully discussed, and a thick- 

 ness of 4 kilometres is assumed as probably not ex- 

 cessive. This is estimated to lead to local weakening to 

 the extent of bringing the 800° C. isotherm 10 per cent, 

 nearer the surface, even assuming the radio-activity of 

 the ooze no greater than that of ordinary sediments. 

 It is, therefore, concluded that the accuinulations of 

 ooze which surround all the great coral areas may well 

 account for some at least of their instability. 



Returning again to the mountains, some of their 

 more specialised problems are dealt with — the great 

 overfolds and the extensive metamorphism which 

 appears in some cases to have been effected at depths 

 of only a few kilometres. 



Finally, the larger problems of earth-heat in general 

 are considered. Kelvin's estimate of the comparatively 

 short interval since the attainment of the consi'itcntior 

 status, on the assumption of the simple cooling earth, 

 is taken as evidence against the truth of that assump- 

 tion. It is well maintained that the known radio- 

 XO. 2075, VOL. 81] 



activity of tlie surface tocks, even if continued tc 

 only very moderate depths, is sufficient to account for 

 all the present heat loss; and, in view of the fact 

 that some of the loss must be due to interior heat 

 and to decay of the radio-active supply, a limiting 

 depth of 10 to 15 kilometres is obtained for the rich 

 surface layer. At the same time, it is shown that the 

 low conductivity of the rocks allows the assumption 

 of considerable quantities of interior radium, as the 

 interior might ri^e in temperature for many millions 

 of vears without the surface being affected. The deter- 

 mination of the age of minerals by measurement of the 

 quantity of helium evolved from their contained 

 uranium concludes the work. The results of Strutt 

 are strongly criticised on the ground of their disagree- 

 ment with the ages determined from the rate of accu- 

 mulation of sediments and of oceanic sodium, though 

 hope is entertained that the method may yet give 

 accurate results. 



As a kind of appendix, a useful chapter is added, ex- 

 plaining in detail the methods of investigating the 

 radio-activity of the rocks. 



It is necessarily easy to criticise a work of this char- 

 acter. Almost all its conclusions are involved in 

 assumptions. Yet the fundamental position is sound. 

 The radium is there, and in such quantities that its 

 energy must be a primary factor in the evolution of 

 the earth. Its distribution within the globe is perhaps 

 the most crucial point awaiting settlement. On the 

 existence of the rich surface layer of very small depth, 

 much of Prof. Joly's argument rests. The very un- 

 satisfactory state of our knowledge of underground 

 temperature-gradients seriously interferes with the 

 estimate of this depth, while the average radium 

 content of the surface rocks can scarcely be considered 

 as settled. Should Eve's average be anywhere near 

 the mark, the story would be considerably altered. 

 When Prof. Joly adduces Alpine metamorphism as evi- 

 dence of radio-active heating, we think he scarcely 

 allows sufficiently . for hydrothermal action. The 

 assumed depths of 20 to 40 kilometres, below which 

 the rocks are supposed to yield hydrostatically, are 

 much greater than those indicated by earthquakes and 

 the formation of thrust-planes for the attainment of 

 that condition. The explanation of the great over- 

 folds of the Alps as due to the pushing of the rigid 

 crust over these hydrostatically yielding layers might 

 apply in that case, assuming that such folds in fact 

 exist, but the same phenomenon may be observed 

 on very much smaller scales, and there the theory 

 fails. It appears to us that the estimates of the 

 " geological age " from sedimentation and sodium 

 accumulation are rather overweighted. The former 

 method probably fails because the thickness of 

 sediments is usually governed by the rate of sub- 

 sidence rather than by the supply of sediment, while 

 the data for the latter are very uncertain. 



Some of the speculations may be considered rash, 

 but the treatment of the subject is intentionally light; 

 and it is well so. The book is full of suggestion and 

 new lines of thought. It will compel interest, and 

 should do much to encourage and direct investigation 

 into what cannot fail to be a richly fruitful field. 



