REVIEWS 569 



factor, has been brought out with a large measure of fuhiess in this new 

 work by Joly. The first two chapters are chiefly an exposition of radio- 

 activity from the historical, physical, and chemical standpoint. They 

 lay the foundation for the geologic studies and the applications which 

 follow. The third chapter gives data upon the occurrence and distribu- 

 tion of radium in the earth's surface materials and leads on to others in 

 which some of the author's more notable inferences and speculations are 

 set forth. 



Radioactivity is made to appear as an agency of prime importance in 

 the variations and fluctuations of underground temperature. A study 

 based upon the rocks and underground temperatures of the St. Gotthard 

 and Simplon tunnels is thought to show a distinct connection between the 

 ^radioactivity and the temperature gradients in the earth. With less 

 plausibility, the tendency of mountain ranges to develop along belts of 

 thick sedimentation is assigned to the radioactivity of the buried sediments 

 and the supposed consequent heating. Pushing the view still farther, the 

 author even endeavors to explain the exigeant phases of the mountain 

 structure of the Alps, such as the peculiar overlapping pile of recumbent 

 folds as set forth by Lugeon, Schrnidt, and others, by localized radioactive 

 heating of the strata during the process of folding, and by a resulting upward 

 shifting of the geotherms which is thought to enable the folded sediments 

 above to be carried northward by the thrusts while the synclinal, troughs 

 beneath are becoming anchored in the growing viscosity of the medium. 

 The idea is suggestive, but it lays a heavy tax on the new agency. 



The chapter on "Radioactivity and the Interior of the Earth" and the 

 one following set forth the possibilities of radioactivity as a more profound 

 source of the thermal energy of the globe; however, the author believes 

 that the radium of the earth is largely concentrated in the outer 12 to 15 

 kilometers of the crust. 



Strutt has estimated the age of various sedimentary beds by determining 

 the amount of helium which they contain and comparing this with the 

 rapidity with which this gas is developed from the radioactive materials 

 present. He reaches the conclusion that the Carboniferous must date 

 back above 140,000,000 years, as a minor limit, and the Huronian probably 

 400,000,000 years. Though a most enthusiastic supporter of the great 

 importance of radioactivity as a geologic agent and as a clue to the unravel- 

 ing of geologic history, Joly is inclined to place greater reliance upon 

 estimates of the age of the earth based upon denudation, and upon the 

 saltness of the sea, than upon those based upon the radium and helium 

 content of the sedimentary deposits. 



