98 T. L. WATSON — WEATHERING OF GRANITIC ROCKS Ok GEORGIA 



completeness of the change, from 15.84 to 35.07 per cents respectively ; 

 (3) the transition from fresh to deca3^ed granite has been accompanied 

 by the retention of tlie following maximum and minimum percentage 

 amounts of each constituent saved : 



Maximum Minimum 



SiO, 81.72 60.64 



Al,03 92.51 89.11 



CaO 86.64 3.71 



MgO 69.04 58.2.3 



Na,0 91.29 26.26 



K2O 93.16 52.18 



Granite-gneiss 



The Lithonia area of contorted gneiss in DeKalb county best illus- 

 trates the character of and changes incidental to the weathering common 

 to this t3'pe of rock in Georgia. It has been shown elsewhere by the 

 writer that the gneisses of Piedmont, Georgia, are closely similar to the 

 granites in chemical and mineralogical composition, and are accordingly 

 metamorphosed granites. The gneisses are only distinguished from the 

 massive granites by a pronounced banded or schistose structure subse- 

 quently induced by metamorphism. They are therefore designated 

 granite-gneisses in contradistinction to gneisses of known sedimentary 

 origin. 



The sap (partially decayed surface rock) of the doming masses of 

 Lithonia granite-gneiss and the adjacent famous Stone Mountain granite 

 boss is very thin, rarely averaging, as a rule, more than two inches in 

 thickness. It is hard and firm surface rock, discolored a slight red 

 from the partial hydration and oxidation of the biotite. The feldspars 

 are dull and opaque from kaolinization, and in most cases the rock in- 

 dicates alteration to a greater depth than the discolored portion, mani- 

 fested by general dullness in appearance of the stone through more or 

 less feldspathic alteration. Numerous exposed sections of apparently 

 completely disintegrated and decom])osed granite-gneiss are exposed, 

 varying in depth from 10 to 15 feet, with the schistosity of the original 

 fresh rock almost p^erfectly preserved in the residual clays forming the 

 sections. 



Microscopically the gneisses are composed of quartz, orthoclase and 

 microcline, intergrown with a second feldspar, albite in the form of 

 microperthitic intergrowths, an acid plagioclase near oligoclase, and 

 biotite. A little magnetite, garnet, some apatite, and zircon are present. 



