36 



UNIVERSITY OF VIEGINIA POBLICATIOXS 



M. Table of feldspar composition of Georgia granites. 

 (Numbers correspond to table K.) 



Anorthite 



Total plagioclase . 



Total feldspar 



Ab°Anm ratio 



Or— plag. ratio.-. 



47.76 

 79.45 

 7:1 

 1:1.5 



8.62 



50.38 49.31 1 46.63 ! 51.59 

 80.40 76.00 79.99 81.06 

 4:1 4:1 4:1 5:1 

 1:1.6 1 1:1.8 1:1.4 1:1.75 



VI. 



VII. VIII. IX. 



Average. 



Orthoclase 



Albite 



Anorthite 



Total plagioclase . 



Total feldspar 



AbnAnm ratio 



Or — ^plag. ratio 



26.68 



36.68 



10.56 



47.24 



73.92 



3:1 



1:1.77 



21.68 

 33.54 

 5.56 

 39.10 

 60.78 

 6:1 

 1:1.8 



28.91 

 32.49 

 5.00 

 37.49 

 66.40 

 7:1 

 1:1.4 



25.02 

 39.82 

 4.73 

 44.55 

 69.57 

 8:1 

 1:1.78 



28.17 

 38.25 

 7.75 

 46.00 

 74.17 

 5:1 

 1:1.6 



Classification. — The high range in total feldspar for the Georgia rocks 

 is observed, and the average is greater than for any other state in the region 

 discussed. The soda-lime feldspar ranges in amount from 37.49 per cent, 

 to 51.59 per cent., and is an acid oligoclase; potassic feldspar ranges from 

 21.68 per cent, to 33.36 per cent. This gives an average orthoclase-plagio- 

 clase ratio of 1:1.6, which varies between 1:1.4 and 1:1.8, intermediate 

 between quartz monzonite and granodiorite, but more closely allied with 

 the former because of the amount of potassic feldspar always well within 

 the limits of the quartz monzonite type, the more acid character of the 

 soda-lime feldspar, and of the high silica and low lime. Under the old 

 scheme of classification, on the basis of potash-soda ratio, these rocks would 

 be grouped as quartz monzonites. 



Bastin^ has computed the position of the Georgia gTanites and granite- 

 gneisses, from the writer's- analyses, in the quantitative scheme of classifi- 

 cation^. The results may be tabulated as follows: 



^Bastin, E. S. Chemical Composition as a Criterion in Identifying Metamor- 

 phosed Sediments. Journ. of Geology, 1909, Vol. XVII, p. 451. 



-Watson, Thomas L. The Granites and Gneisses of Georgia. Bull. 9-A, Geological 

 Survey of Georgia, 1902, 367 pages. 



^Cross, Iddings, Pirsson, and Washington. Quantitative Classification of Igneous 

 Roeks, 1903, 286 pages. 



