34 



JoLY — An Estimate of the Geological Age of the Earth. 



Mr. F. W. Clarke has estimated the percentage amounts of the elements 

 contained in the Earth's surface crust. In Mr. Clarke's first report,* the mean of 

 880 selected analyses of American and European igneous, volcanic, and crystalline 

 rocks is tabulated along with the means of the component analyses divided into 

 local groups, as the rocks of the Western States, of Northern California, of 

 European volcanic and crystalline rock, &c. ; and it is i-emarked as the result of 

 comparing these groups that "the thesis that the crust of the Earth is fairly 

 homogeneous in composition is thus sustained by positive evidence." In a later 

 publication, t 9G0 analyses are consulted, and these of a still more carefully selected 

 and reliable character, giving an average "which may fairly represent the compo- 

 sition of the older crust of the Earth." The result, which closely agrees with the 

 earlier estimate, is contained in the column below. 



99-14 



This approximates to a Diorite, and would fall among Rosenbusch's series of 

 " Granito-dioritisclien " and " Gabbro-peridotitischen " magmas. J 



Such a rock or lava attacked by a heated solution of hydrochloric acid must 

 ultimately yield its iron, calcium, magnesium, potash, and soda as chlorides. 

 The atomic percentages of Clarke's average are given by him as follows : — 



The chlorine taken up may be assumed to be distributed as follows : in the 

 first instance— Fe^Cle, CaClj, MgCL,, KCl and NaCl. 



* Bulletin of the United States Geological Survey, No. 78, 1891, p. 34. 

 t Bulletin of the TJ.S. Geological Survey, No. 148, 1897, p. 12. 



\ Elemente der Gesteinslehre. Stuttgart, 1898, p. 187. See No. 15 of this group for a rough 

 approximation to Clarke's average. 



