WEATHERING OP GEANITE 



189 



Disintegrated and Decomposed Granite, District of Columbia, showing 

 Proportional Loss op Constituents 



CONSTITD-BNTS 



Silica (SiOa) 



Alumina (AI2O3) 



Iron sesquioxide (Pe203) . . 

 Iron protoxide (FeO) . . . 



Lime (CaO) 



Magnesia (MgO) 



Soda (Na20) 



Potash (K2O) 



Phosphoric anhydride (P2O5) 

 Ignition 



Total loss ...... 



IV 



Pekcentagb 

 Loss FOE En- 

 tire Rock 



} 



10.50 % 

 0.46 



0.00 



0.81 

 0.36 

 0.77 

 0.85 

 0.04 

 2.161 



13.79 % 



Perobntage 

 OP Each Con- 

 stituent SAVED 



85.11 % 

 96.77 



100.00 



74.79 

 98.51 

 71.38 

 68.02 

 60.00 

 100.00 



YI 



Percentage 

 OF Each Con- 

 stituent LOST 



14.89% 

 3.23 



0.00 



25.21 



1.49 

 28.62 

 31.98 

 40.00 



0.00 



From a perusal of these figures, it appears that the residual 

 sand retains 85.11% of the original silica; 96.77% of the alu- 

 mina; all the ferric oxide; 74.79% of its lime; 98.51% of its 

 magnesia, together with 71.38% of its soda and 68.02 of the 

 potash, while there has been an actual gain, as was to be ex- 

 pected, in volatile matter. 



Let it not be too hastily assumed that we have exhausted the 

 subject. 



It must be remembered that while an analysis shows the actual 

 composition of a rock so far as the various elements are con- 

 cerned, it quite fails to show the manner in which those elements 

 are combined. While the ultimate composition of the fresh and 

 decomposed samples may be closely similar, it is possible, indeed 

 probable, that in some cases at least the manner of combination 

 of these elements is quite different. This is well illustrated in the 

 case of the figures showing the percentages of alumina in anal- 

 yses I and III and which differ only nine-tenths of one per cent 

 in total amount; yet in the first the alumina exists mainly in 

 the form of anhydrous silicates of alumina, potash, iron, and 

 magnesia (as in the feldspars and mica), while in the last a very 

 considerable proportion, or indeed all in extreme cases of weath- 



Gain. 



