CHKMleAl, ANA1A'SI'> ol llli; (il^AMTK 1)7 



The transition from fresh to cleca3'ed granite is accompanied in every 

 case by a gain in H.O, whieli increases in amonnt proportional to tlie 

 stage in weathering. The percentage loss for the entire rock is also ])ro- 

 portional to the degree of decaj'^ reached in the residual product. The 

 amount lost for the entire rock varies from 8.14 to 71.82 per cents re- 

 spectivel}'. The figures further indicate that the early stages in weath- 

 ering are mostlv in the nature of disintegration, while the chemical 

 forces — decom})osition — become the principal agents in the transforma- 

 tion in the later stages. The residual soils are accordingly gray in color 

 and principall}^ sandy, or deep red in color and principally clayey. 



PoRPHYRiTic Granite 



CHARACTER AND COMPOSITION 



Three of the largest and most typical porphyritic granite areas were 

 selected for indicating the processes involved in the weathering of this 

 type of rock in Georgia. Two of the areas represent the massive por- 

 phyritic granite, while the third is a pronounced foliated type of the 

 same rock. 



This type of granite only differs from the equivalent even-granular 

 facies in porph3a'itic structure, and, as a rule, somewhat coarser grained 

 in texture. The porphyritic texture grades, in most cases, into the 

 even-granular granite facies peripherally, with no difference shown in 

 mineral and chemical" composition. They consist of a coarse grained 

 granite matrix of quartz, feldspar, and biotite, in which are imbedded 

 potash feldspar phenocrysts 10 to 50 millimeters long and 5 to 10 milli- 

 meters across ; idiomorphic to allotriomorphic in crystal outline, with 

 pronounced cleavages parallel to (001) and (010), and are twinned 

 according to the simple type of Carlsbad twins. 



The weathering of the natural rock exposures in the field is in every 

 way analogous to that of the massive granites, with usually no visible 

 difference apparent between the two, on textural grounds. The chem- 

 ical changes accompanying the transition from fresh to decayed rock 

 are closely similar in the two types of granite. 



DISCUSSION OF THE CHEMICAL ANALYSES 



The principal features to which attention need here be called are (1) 

 a gain in 11,0, which increases as the change becomes more advanced ', 

 (2) a total percentage loss for the entire rock, varying, according to the 



