i 3 o SCIENCE PROGRESS 



" Chlorophyll," in the Journal of the American Chemical Society, 

 191 5, 37, 323. Those who wish to obtain first-hand knowledge 

 on this very complicated subject would do well to read through 

 this review. 



GEOLOGY. By G. W. Tyrrell, A.R.C.Sc, F.G.S., University, Glasgow. 



A. Holmes has written an important paper on Radioactivity 

 and the Earth's Thermal History {Geol. Mag., February and 

 March 1915). The speculation that the radioactive elements are 

 concentrated towards the surface of the earth's crust is supported 

 by their known distribution amongst the various igneous rock 

 types, and the variation of these types in depth. The average 

 content of radioactive elements is greater in acid rocks than in 

 the basic, and these in turn are richer than stony and iron 

 meteorites, which may be supposed to represent the material 

 of the interior of the earth. The strongly supported opinion 

 that the igneous material of the crust is stratified in order of 

 density then necessitates the conclusion that the radioactive 

 elements are concentrated towards the surface. This conclusion 

 is supported by several other lines of evidence. A further 

 conclusion that there is nothing in the distribution of radio- 

 active elements to forbid belief in an earth which began with 

 a molten surface, and which has gradually cooled down to its 

 present condition, revives a view which has been temporarily 

 pushed aside by the planetesimal hypothesis of the origin of 

 the earth. 



In his Presidential Address to the Geological Society ot 

 America {Bulletin, April 191 5), Dr. G. F. Becker writes on the 

 relation of Isostasy and Radioactivity. He states that the 

 geodetic evidence for isostasy is so manifold and consistent as 

 to amount to proof. He believes that the level of compensation, 

 estimated by geodesists at between no and 140 kms. from the 

 surface, is incompatible with the immense age for the earth as 

 estimated on the basis of the uranium-helium and uranium-lead 

 ratios in certain minerals. 



Since the broader lines of stratigraphy have been laid down 

 in nearly all areas, this branch of geology necessarily tends 

 now to the accumulation of local details, which are naturally 

 only of local interest; and it is comparatively rare for new 

 principles or ideas of general interest to emerge from the 

 very numerous papers on the subject. Hence in these short 



