BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA 

 Vol. 12, pp. 93-108, PLS. 6-11 February 26, i90i 



WEATHEIUNC; OF CJRAXITIC KOCKS OF GEORGIA* 



BY THOMAS L. WATSON 



(Redd before the Society December 29, 190(i) 



contp:nts 



Page 



Introduction 91-5 



Granite 94 



Ciiaracter and composition 94 



Discussion of the chemical analyses 96 



Porphyritic jjranite 97 



Ciiaracter and composition , . . 97 



Discussion of the chemical analyses 97 



Granite-gneiss 98 



Resume 99 



Cause of red color of the soil 104 



Results based on assumptively insoluble AI2O3 and k\X>^ -f- Fe^Oa 10<> 



Introduction 



For the past three years the writer has been engaged in a careful de- 

 tailed study of the granitic rocks of Georgia, giving special attention to 

 the phenomena of their weathering. The natural outcrops have been 

 carefully studied in the field, and specimens of the fresh and weathered 

 rock, representing various stages in the decay, have been collected from 

 the most typical and widely separated localities in the state and analyzed 

 in the chemical laboratory of the State surve}'^ by the writer. The ma- 

 terial is sufficiently representative and the work extensive and detailed 

 enough to arrive at definite conclusions regarding the changes involved 

 in the transition from fresh to decayed rock in the granitic group of 

 rocks. 



Based primaril}^ on texture and structure, three types of the gj-anitic 

 rocks are distinguished : (1) The massive even-granular granites ; (2) the 



* Published bj'' permission of tlie State Geologist of Georgia. 



t For a detailed discussion of the weathering of the Georgia granites, including complete chem- 

 ical analyses of the fresh and accompanjMng decayed rock from numerous localities in the state, 

 the reader is referred to a report on the granites and gneisses of Georgia, to be issued shortly by 

 the state Geological Survey, by the present author. 



XIV— Bull. Geoi,. Soc. Am., Vol. 12, 1900 (93) 



