civ PROCEEDINGS OE THE GEOLOGICAL SOCIETY. [May 1909, 



In his estimate of the average composition of the igneous rocks 

 of the lithosphere, Clarke shows that the elements on which we 

 must base our calculation are present in the following proportions: — 

 Fe 2-64 : Hg 2-44 : Ca 3-42 : Na 2-55 : K 1-86 per cent. From 

 these numbers we obtain the following : — 



T86 K corresponds to 11'85 per cent, of orthoclase. 

 2 - 55 Na „ 3048 per cent.'of albite. 



2-98 Ca ,, 17-47 per cent, of anortbite. 



59-50 Felspar, 59-50. 



0'44 Ca ., 1-27 per cent, lime-augite. 



2'44 Mg „ 9*76 per cent, magnesian augite. 



2 - 64Fe .,. 5*77 per cent, ferrous augite. 



16-80 Augite, 16'80. 



0'19 Fe may be reckoned as magnetite. 



Total 7530 



The felspar and ferro-magnesian minerals are thus found to exist 

 in the same proportions as those given elsewhere by Clarke. 



The final product of the weathering of the felspar is kaolin or 

 some closely related substance ; so much is clear from the results 

 of bulk-analyses of clay and shales. But, in the reaction which 

 gives rise to kaolin, two molecules of silicic acid are set free for 

 every one atom of potassium or of sodium. The decomposition of 

 anorthite, however, is not attended by the liberation of silicic acid. 



In the decomposition of the augites, one molecule of silicic acid 

 is set free for every atom of iron, magnesium or calcium. 



Thus we have : — 



11-85 orthoclase sets free 5'72 Si0 2 



30-58 albite „ „ 13-30 „ 



1-27 lime-augite „ , 0-66 „ 



9 - 76 magnesian augite sets free 5'86 „ 



5'77 ferrous augite „ , 2 - 62 „ 



Total 28-16 



Hence it appears that 28 - 16 parts of silica correspond to 3018 

 parts of albita or to 2*55 parts of sodium, and consequently the 

 56,870 tons of silica present in a cubic mile of the river-water 

 nnder consideration correspond to 5,146 tons of sodium. 



Substituting this for Prof. Joly's standard number, we have 



90 x 10 6 X y'^° 6 6 -421 x 10 6 , 



