COMPOSITION OK TIIK GIlANlTK-CwNKlSS 09 



More or less chlorite, nuiscovite, and epidote occur as secondary min- 

 erals derived from the alteration of the feldspars and biotite. The pot- 

 ash feldspars predominate and the species var}' in amount from {)lace 

 to place. Microcline is somewhat more abundant than in the even- 

 granular granites and their equivalent porphyritic facies. The plagio- 

 clase also varies in amount. The chemical analyses corrol)orate the 

 inference that the plagioclase is oligoclase in the percentage of lime 

 present. They further indicate a larger j)ercentage of free quartz than 

 the massive granites and their porphyritic facies, which is confirmed by 

 the microscope. Slight peri})heral shattering of the larger quartz and 

 feldspar crystals, numerous lines of fracture, and wav}^ extinction com- 

 mon to these two minerals afford evidence of the effects of dynamo- 

 metamorphism. 



The physical and chemical changes involved in the processes of decay 

 of the gneisses are strikingly similar to those of the granite types. 



Resume 



The following conclusions are based on the percentage amounts saved 

 and lost per each constituent, and the total loss for the entire rock, cal- 

 culated from the analyses of the fresh and corresponding decayed rock, 

 on an assumed Fefi^ constant factor. 



A careful study of the tables of chemical analyses of the fresh rocks 

 and their corresponding decayed products shows the amount of water to 

 rapidly increase as the decomposition advances, which, as Merrill* has 

 shown, becomes the most important factor in the earlier stages of rock 

 weathering. The change has in every case been accompanied by a loss 

 in the silica, a proportional greater loss in the alkalies, lime, and mag- 

 nesia, with a proportional increase in the iron oxide, and in some cases 

 in the alumina. 



The decayed product of the Georgia granites invariably show an abun- 

 dance of quartz grains derived from the free quartz in the fresh granite. 

 In most cases the quartz granules have been only slightl}^ corroded and 

 are prevailingly angular in outline. Frequent examination of the quartz 

 grains in the residual cla3^s was made under the microscope for evidence 

 of etching and solution, but positive evidence was lacking in every case. 

 The loss, therefore, in this constituent in the Georgia granites has been 

 derived in large part, if not entirely, from the silicate-bearing minerals, 

 feldspar and biotite, which was doubtless removed with the alkaline solu- 



* Rocks, Rock Weatlieiing, and Soils, New York, 18(t7, p. 234. 



