272 AN ESTIMATE OF THE GEOLOGICAL AGE OF THE EARTH. 



ard of reference by which to compare the los.s of alkalies. We take 

 the alumina for this purpose. 



The alumina of the original rock in Clarke's average of anal3^sis is 

 15.38. This is for the parent rock. For the derived rocks we may 

 refer to Rosenbusch's tables (loc. cit.), where we find a mean of 16.06 

 per cent of AljOs, in the several groups of slates, sandstones, phyllites, 

 schists, and gneisses already referred to.^ We now have the data 

 required for our calculation. On these we find, closel}" 



NagO lost =60 per cent. 

 NagO saved =40 per cent. 



Referring now to the mean loss of soda and of entire rock given 

 in soil-formations, we find that on the sanu^ ratio of sodium loss to 

 gross loss Clarke's original crust-rock should have lost 83 per cent in 

 affording sediments of the present soda percentage. 



If, following this estimate exactly, Ave assume that the loss by solu- 

 tion in the process of denudation and formation of the detrital siliceous 

 sediments had been 33 per cent, about, and still assuming 1.1 mile as 

 the thickness of the detrital sedimentary mass spread over the land, 

 the mass of the parent rock would calculate out as 95x10'" tons, and 

 the restored soda would amount to 3.21 per cent. 



Hence it appears that if a thickness of 1.1 mile of rock spread over 

 the land area represents the bulk of the entire detrital siliceous sed- 

 imentary rocks, inclusive of submarine detritus, and this constitutes 

 67 per cent of the entire sedimentaries of the earth, including matter 

 in solution in the sea, the sodium contained in the sea, added to what is 

 left over in the detrital sediments, would suffice to restore to the entire 

 mass a soda percentage almost equal to that in the eruptive, igneous, 

 and crj^stalline rocks; the deficiency, about OA per cent, exists partly 

 in rock-salt deposits. Some of the calcareous rocks also possess an 

 appreciable percentage of alkalies, which has been left out of account 

 in the foregoing estimate of the soda contents of the sedimentaries. 



*As follows: 

 Mean of — 



15 Sandstones, etc. (p. 391) 9. 34 



13 Clays, etc. (p. 420) 19. 46 



18 Clay slates, etc. (p. 425) 17. 91 



6 Calcareous clay slates (p. 428) 14. 95 



20 Phyllites (p. 433) 20. 85 



10 Sericite schists (p. 436) 21. 62 



4 Doubtful phyllites (p. 436) 15. 42 



13"Pelitgneisse" (p. 470) 18.98 



8 " Psammit gneisse" (p. 471) 11. 93 



3 Amphiboie gneisses (p. 484) 13. 54 



6 Pyroxen gneisses (p. 486) 14. 93 



4 Mica schists (p. 497) 13. 82 



Mean of all 16. 06 



