and the Earth's Thermal History. 



63 



Professor Joly in Dublin, and Professor Mache in "Vienna. 1 The 

 results so far available serve to show that in the case of rocks the 

 above statements serve equally well for thorium as for radium. 

 Independently, this parallelism of distribution and mode of occur- 

 rence would be forcibly indicated by the common association of 

 uranium- and thorium-bearing minerals with acid and alkaline rocks 

 (e.g. granites and nepheline syenites), and in considerably greater 

 quantities with their pegmatites. The most probable averages for 

 common rocks and meteorites may be summarized as follows : — 



Table I. 



3. The Density Stratification of the Earth. 

 The view that the earth is coarsely stratified according to the 

 densities of its constituents is one suggested by — 



(1) A comparison of the average density of crustal rocks with that 

 of the earth as a whole. 



(2) The structural characters of meteorites and the close corre- 

 spondence in density and composition between their stony material 

 and terrestrial ultra-basic rocks. 4 



(3) The structure of the earth's crust as indicated by the field 

 relations and relative abundance of granite, basalt, and ultra-basic 

 rocks. 5 



1 Joly, Phil. Mag., vol. xx, pp. 125 and 353, 1910, and Cong. Intern, de 

 Eadiologie et d' Electricity, p. 376, 1911 ; Fletcher, Phil. Mag., vol. xx, 

 pp. 102 and 770, 1910. See also Poole, Phil. Mag., vol. xxix, 1915. 



2 Eesults for richly alkaline rocks, which are abnormally high in their radium 

 and thorium contents, are omitted from this Table. 



3 Implying the series Uranium to Lead. 



4 Suess, The Face of the Earth (Eng. trans.), vol. iv, p. 544 ; Farrington, 

 Journ. Geol., vol. ix, p. 623, 1901; Merrill, Am. J. Sci., vol. xxvii, p. 469, 

 1909 ; vol. xxxv, p. 509, 1913 ; Holmes, loc. cit., pp. 30-5, 1914. 



6 Von Cotta, Geologische Fragen, Frieburg, 1858, pp. 76-8 ; Green, Vestiges 

 of the Molten Globe, pt. ii, p. 61, 1887; Daly, U.S.G.S. Bull. 209, p. 110, 

 1903 ; Igneous Bocks and their Origin, p. 162, 1914. 



