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



43 



These may be tabulated as follows (the references are to the pages of the work 

 referred to) : — 



With reference to the last division of rocks — the metamorphic eruptives — 

 Rosenbusch admits the old standing difficulty of distinguishing between the 

 altered sediments and the altered eruptives. Thus Gneisses derivable from sedi- 

 ments give just such chemical proportions as appear in those referred to eruptive 

 origin. There exists no sure criterion for classing a Gneiss according to its origin 

 (p. 472, loc. cit.). 



This is a well-known difficulty, and has been the subject of much research and 

 speculation. We dare not do more than suggest here that in doubtful cases the 

 general law of the alkali ratios of eruptives and sedimentaries, where this admits 

 of application, should carry weight with petrologists. 



The uncertainty referred to may, of course, affect the estimated composition 

 of the crust-rock and of the siliceous sedimentaries. Examination will, however, 

 show that the uncertainty being confined to a couple of groups only, can probably 

 affect the final averages but little in either case. 



