KKSIMK J Do 



rocks is iiuvinly one of disintegration acc()nii)anied by st)n»e chemical 

 c]iano;t\ wliile the later stages are as prevailingly characterized hy de- 

 composition, resulting in the production of a still", plastic red clay. The 

 earlier stages in the weathering of the finegrained granite areap{)arently 

 the same as for the similar stages in the coarse grained p()rph3'ritic 

 granites. The same statement holds for the two ty})es in the advanced 

 stages of weathering — decomposition. The striking similarit}' in the 

 granite, porphyritic granite, and gneiss, as relating to soluhilit}' in acids, 

 is well illustrated in the percentage amounts of soluble matter extracted 

 from weighed portions of the finely i)owdered fresh rock, by digestion 

 in 100 cc. of half normal HCl for three hours, at the temperature of 

 boiling water. 



. Per cent of 



(rranites soluble nfatter. 



The Ogleshy blue granite, Elbert comity 10.56 



The Hutcliins blue granite, Oglethorpe county 9.77 



Swift and Wilcox light gray granite, Elbert county 9.20 



Echol's mill light gray granite, Oglethorpe county 8.37 



Average 9.475 



Porphyritic Granite 



Heggie Rock, Columbia county 10.98 



Heggie Rock, Columbia county (decay) 12.69 



Brinkley place, near Camak 1 6.29 



Brinkley place, near Camak (decay) 23.69 



Porphyritic granite near Line creek, Fayette county 9.47 



Average of fresh rock 12.246 



Granite-g)ieis>i 



Southern Granite Company's quarry, near Lithonia 7.58 



Arabia Mountain quarries, De Kalb county 6.06 



Tilley quarry, Rockdale county 5.94 



Average 6.526 



The lower percentage of soluble matter in the granite-gneisses over 

 the granites and their porphyritic facies is readily accounted for on the 

 basis of this rock ty})e containing a larger amount of free quartz, as shown 

 in the chemical analyses. The inference is also corroborated by micro- 

 scopic study. In each case where the total percentage o^ extract from 



