96 T. L. WATSON — WEATHERIN(t OF GKAKlTIC ROCKS OF GEORGIA 



while the centers remain entirely fresh. As observed in the potash feld- 

 spars, alteration takes place in the biotite to a marked degree along the 

 cleavage planes. 



No calcite has been observed in any of the thin-sections of the partially 

 decomposed rock nnder the microscope; and the further absence of car- 

 bonates in the residual granite-decay was i)roved by testing portions of 

 the finely jjowdered rock in dilute acid. 



Digestions of separate portions of the residual products, in various 

 stages of decomposition, in very dilute HCl at the temperature of boil- 

 ing water, gave solutions var3''ing from slightl}'' colored to a deep wine- 

 red in color, according to the extent of decay and the amount of iron 

 oxide liberated from the iron-bearing silicate minerals and retained in 

 the residual product. The residues from these separately treated por- 

 tions of decayed rock indicated, according to the degree of decay, vary- 

 ing amounts of the fresh and partially fresh minerals, quartz, feldspar, 

 and biotite. Numerous magnetite grains, just as fresh apparently as in 

 the unaltered granite, were found to some extent in all the residues. 

 In the advanced stages of decay, represented by highly ferruginous red- 

 colored plastic clays distinctly gritt}'' in feeling, owing to the presence 

 of free quartz and i)artially decayed silicate minerals, the biotite shreds 

 are changed from black, highh' lustrous plates in the fresh granite to 

 brittle brass-colored foliip in the ferruginous clay, and are slowl}^ soluble 

 in both cold and hot dilute acid. 



DISCUSSION OF THE CHEMICAL ANALYSES 



Five widely separated sections of the massive even-granular granites 

 were selected for making chemical analyses of the fresh and correspond- 

 ing decayed njck for i)urposes of illustrating the changes accomjianying 

 the transition in weathering. The residual ])r()duct represented stages 

 of decay, varying from distinctly discolored, hard and firm rock to 

 highly ferruginous red plastic clays. A careful study of the tables of 

 anal3^ses, representing the rocks in various stages of decay, shows the 

 principal features in the change frotn fresh to decayed granite. On the 

 assumption of constant Fe,0.j the following calculated maximum and 

 minimum percentage amounts of each constituent have been retained or 

 saved, according to the corresjjonding stage in decay : 



Maximum ^linimum 



SiO, 92.20 22.80 



AlA 99.37 56.18 



CaO 57.28 1.18 



MgO 84.21 12.08 



Na.^0 85.88 7.85 



KaO 96.60 8.26 



