WEATHEEING OF GNEISS 



195 



usual scattering of apatite and zircon. The quartz is penetrated 

 by rutile needles and the potash feldspars predominate. 



Decay has progressed to an approximate depth of 20 feet, the 

 upper ten or fifteen feet being reduced to the condition of a 

 deep red clay, with a gritty feeling, owing to the presence of 

 particles of quartz. Abundant shreds of bleached biotite were 

 distinctly visible to the unaided eye. At greater depths the 

 dark-red clay passes through lighter and brighter red phases, to 

 slightly reddish-gray and fairly firm rock. The materials for 

 analyses were taken from a distance of 5J feet and 35 feet below 

 the surface. The results obtained were as below : 



Analyses of Fkesh and Decomposed Biotite Geanite fbom Greenville, 



Meriwether County, Georgia 





Pi 



Decomposed Rock 



b% Feet Below 



Surface 







Calculations Based on Decomposed 

 Rock 53^ Feet Below Surface 



Constituents 



Recalculated on a 

 Basis of 100 



-M 



H 



-a 



a> "5 

 Ah 



-g 



gwSj 



S°8 





1 



2 



8 



4 



5 



6 



7 



SiO, 

 AlA 



FeA' 



CaO 



MgO 



NagO 



K2O 



Ignition. 



69.88 

 16.42 

 1.96 

 1.78 

 0.36 

 4,46 

 6.63 

 0,36 



51.29 

 29 69 

 6.33 

 0.07 

 0.14 

 1.12 

 1.50 

 10.36 



69 28 

 16.28 

 1.95 

 1.77 

 0.36 

 4.42 

 5.58 

 0.36 



51.03 

 29.54 

 6.30 

 0.07 

 0.14 

 112 

 1.49 

 10.31 



53.48 

 7.13 

 0.00 

 1.74 

 0.31 

 4.07 

 5-11 

 0.00 



22.80 



56.18 



100.00 



1.22 



12.06 



7.84 



8.25 



100.00 



77.20 

 43.82 



0.00 

 98.78 

 87.94 

 92.16 

 91.75 



0.00 



Total. 



100.85 



100.50 



100.00 



100.00 



71.74 







From this it appears that 71.84% of the original rock material 

 has disappeared, the principal constituents being lost in the 

 following proportions: Silica, 77.20%; alumina, 43.82%, lime, 

 98.78%; magnesia, 87.94%; soda, 92.16%; and potash, 91.75%. 

 Analyses of the same material from a depth of ten feet below the 

 surface showed a total loss of 61.98%, indicating a less advanced 

 stage of decomposition, as was to be expected.* 



The region is one in which the recorded extremes of tempera- 

 ^ All iron was estimated as FjOj. The iron assumed to be constant. 



