268 AN ESTIMATE OF THE GEOLOGICAL AGE OF THE EARTH. 



We may now refer to a similar table of the sediinentaries, .still 

 deriving our figures as averages calculated from Rosenbusch's work: 



Mean of — 



16 Sandstones, Quartzites, and Graywackes (p. 391) . 



12 Clays and Shales (p. 420) 



17 Clay Slates (p. 425) 



G Calcareous Clay Slates and Whetstones (p. 428)... 



IG Phyllite schists, or Clay-mica schists (p. 437) 



10 Schists (Serecite.Ottrelite, Chlorite, etc.) (p. 436). 



13 "Pelitgneisse" (Phyllite gneisses) (p. 470) 



8 " Psammit gneisse " (Sandstone gneisses) (p. 471). 



3 Amphibole gneisses (p. 484) 



4 Mica schists (p. 497) 



Mean of sedimentaries 



KoO. 



Na«0. 



1.47 



We may observe further that the averages affoi'ded by the valuable 

 collection of analysis of American rocks, compiled by Messrs. Clarke 

 and Hillebrand, will be found to confirm these results.^ 



For the original crust. Clarke's alkali ratio works out — 



Na/)_1.29 

 K,0- 1 ' 

 and K()S(Mibuscirs, 



Na.,0 _1.22 

 K.,0 ' 1 ' 



On the other hand liosenbusch's sedimentary rocks show that — 



NaaO _0.59 

 K,0- 1 • 



AVhen it is remembered that, age by age, those sediments were being 

 deposited, some directly from the parent igneous rocks and others by 

 denudation of former sediments, the great importance to the present 

 hypothesis of this broad difference in the alkali ratios, and in the abso- 

 lute amounts of sodium and potash in the original and derived rocks, 

 must be evident. 



If now the inference is right that the missing alkalies were supplied 

 to the ocean, we should expect to find on a rough approximation of 

 the bulk of sedimentaries, and hence of the original rocKs giving rise 

 to them, that such a mass of parent rock would l)e adequate to supply 

 the sodium in the ocean. And this is actually the case; we find, in 

 fact, that the estimated amount of sedimentary strata would, in its 

 formation, be adequate to yield to the ocean the sodium that is in it, 

 assuming these sedimentaries to be derived from rocks having the 

 mean composition of the important eruptive masses now known. 



'U. S. Geol. Survey, Bulletin No. 148, 1897. 



