84 Scientific Intelligence. 



igneous and sedimentary rocks. The amount of radium varies, 

 however, within wide limits, the abyssal radiolarian ooze and red 

 clay being especially high. As radium loses half its mass in 1760 

 years while its ultimate parent uranium takes five billion years for 

 the same loss, it is seen that the radium in the rocks is really a 

 measure of the contained uranium. Strutt, extending the work of 

 Rutherford, has shown that the radio-active materials in the crust 

 exist in sufficient abundance so that a crustal layer less than 100 

 miles in thickness would continually supply the quantity of heat 

 which the earth loses to space (45 miles in the original estimate). 

 Joly argues, therefore, that the uranium must be concentrated in 

 the outer crust of the earth. Consequently where this concen- 

 trated zone is depressed by the accumulating sediments of a geo- 

 syncline, the loss by conduction being lessened, the temperature 

 must rise. Local excesses as in the Simplon tunnel are also 

 thought to play an important part in determining local tempera- 

 ture gradients. Joly further argues that the instability of the 

 earth's crust and the ocean floor are also due chiefly to uranium 

 and radium becoming more deeply buried. 



Another chapter of great geological interest is that on uranium 

 and the age of the earth. Considering helium and lead as the 

 ultimate disintegration products of uranium, measurements of 

 their amounts in uranium minerals, while varying among them- 

 selves and pointing to the need of further research, agree in giv- 

 ing much larger values to geological time than estimates based 

 upon denudation and sedimentation. 



Throughout the volume, the author, while arguing for the 

 large influence of radio-activitj 7 , shows a spirit of fairness and 

 caution. But there is room for so many possibilities in the 

 underearth, which he does not discuss, that a large degree of 

 skepticism may be maintained toward many of his conclusions. 

 Of the geological importance of radio-activity there can be no 

 doubt and it seems adequate to more than account for the tem- 

 perature gradient, so that instead of a cooling earth we may come 

 to face the possibility of a heating earth. But the deep-seated 

 distributions of energy, pressure and mass traceable to earth 

 origin, whatever that may be, seem able to play the chief part in 

 terrestrial dynamics without invoking the radio-activity of the 

 outer crust as a controlling cause. The contributions of various 

 writers, but more notably Chamberlin, show the weakness of the 

 outer zone to generate compressive movements, which seem, on 

 the contrary, to be initiated by shrinkage of the centrosphere, 

 periodically producing collapse of a thick outer shell of the earth. 

 The great vertical movements, on the other hand, as shown by 

 investigations on isostacy, seem to be in their origin largely inde- 

 pendent of denudation and sedimentation, but dependent upon 

 differential volume changes in the outer hundred miles. The 

 isostatic adjustments are further without doubt modified by sur- 

 face unloading and loading. These considerations are not ade- 

 quately discussed by Joly. 



