KKSrNFK 101 



is iicconipanied by so lar<:;e n total loss in the constituents for the entire 

 rock, the process is an advanced one and entirely in the nature of chem- 

 ical decomposition. In the earlier stages of weatherinu: the rock hei^ins 

 to crumble and is ultimately reduced to a li^ht gray-colored <jjra.niti(5 

 sand, throut>;h hydration and temperature changes — disintegration — 

 accotnpanied by only slight chemical change. The following sunnnary 

 of results, showing the total percentage loss of constitiients for the entire 

 rock in the various stages of decay, as calculated from the analyses, will 

 indicate to some extent whether the process involved has been mainly 

 one of decomposition or disintegration : 



T ,-. Total loss for 



T^O'^-^''ty- entire rock. 



Average of aiuily>;es from the Crossley and Southern Granite Com- 

 pany's quarries 7.(58 



Swift and Wilcox quarry, near Elberton 7.92 



Coggin's Granite Company's quarry, near Oglesby 8.14 



The Lexington Blue Granite quarry, near Lexington 14.56 



Heggie Rock, Columbia county L5.84 



Crossley quarry, near Lithonia 26.49 



Brinkley i)lace, near Camak 34.04 



McCollum quarry, Coweta county 35.07 



Coggin's Granite Company's quarry, near Oglesby 36.38 



Cole quarry, near Newnan 38.48 



Greenville Granite Company's quarry, near Greenville 6L85 



Greenville Granite Company's quarry, near Greenville. 7L82 



In tbe Georgia granitic rocks, it is observed that in some localities the 

 biotite begins to give way first, while in others the feldspar appears to 

 be the most affected ; and in others still the rate of decomposition is 

 seemingly about equal for the two minerals. The problem is somewhat 

 complicated, hoAvever, in the case of the Georgia rocks, owing to the 

 large proportion of plagioclase, soda-lime, feldspar present, and the 

 abundance of microperthitic structures common to the potash feldspar 

 varieties. 



During the early stages of weathering in these rocks chemical decom- 

 position is subordinated to physical disintegration, and the rocks have 

 suffered mainly through temperature changes, such as produce granula- 

 tion and weakening in the adhesive power between the individual min- 

 erals — feldspar and biotite — resulting in crumbling along the planes of 

 cleavage. Freezing and solution have aided in the process, but owing 

 to the low absorptive power of the Georgia rocks, the}^ can not be con- 



XV— Bull. Geol. Soc. Am., Vol. 12, 19f)0 



