WEATHERING OP GEANITE 



191 



An analysis of the silt, which was found to constitute 4.25% 

 of the entire mass of disintegrated material, as noted above, 

 is given below, and also a partial separation and analysis of the 

 39.7% soluble, and 60.3% insoluble portions.^ 



Analyses of Silt feom Bisintegeated Gbanite, Distbict of Columbia 





I 



II 



III 



CONSTITUBINTS 



Bulk Akalysis 

 OF Silt 



Analysis of 



SoLTTBLE Portion 



(89 7%) Silt 



Analysis of 



Insolitblb Portion 



(60.8%) Silt 



Ignition 



Silica (SiOa) 



Alumina (AI2O3) .... 



Iron sesquioxide (¥620$) . 

 Lime (CaO) ...... 



Magnesia (MgO) .... 



Soda(Na20) 



Potash (K2O) 



8.12% 

 49.39 { 



23.84 

 3.69 



4.41 -^ 

 4.60 

 3.86 

 2.49 J 



8.12% 

 TnHCl 1.123 

 TTiNa2C08ll.l47 

 9.21 

 4.47 



Not det. 



0.97% 

 } 37.30 



13.40 



0.82 



r 2.90 



J Trace 



1 2.75 



l 1.07 



« 



99.90% 



34.07 



59.21 





93.28 % 



Prom these analyses it would appear that of the 17 grammes 

 of silt, representing 4% of the total disintegrated material, 

 only 39.7% is soluble; and, further, that a very considerable 

 proportion of the insoluble residue, as indicated by the high 

 percentages of alkalies and lime, still consist of unaltered soda- 

 lime and potash feldspars, the iron and magnesia alone having 

 been largely remoyed. 



These results are not quite what one would be led to expect 

 from a perusal of the literature bearing upon the subject of 

 rock decomposition. As long since noted by J. G. Forch- 

 hammer, Gr. Bischof, T. Sterry Hunt, and others, the ordinary 

 processes of decay in siliceous rocks containing ferruginous 

 protoxides and alkalies consists in the higher oxidation and 



^ In all analyses made \>j or under tlie direction of tlie author, the matter 

 tabulated as soluble is that extracted by boiling for three hours in hydro- 

 ehloric acid of one-half normal strength, to which is added the silica set 

 free in a gelatinous form by the acid and subsequently extracted by sodium 

 carbonate solution. All analyses made on material first dried at 100° C. 



