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UNIVEESITY OF VIRGINIA PUBLICATIONS 



for either of the other states. In the complete analyses, decrease of silica 

 is accompanied by increase of iron, magnesia, and lime, showing increasing 

 amounts of the dark minerals and of plagioclase. The percentages of quartz 

 in the rocks as calculated are: I, 43.50; II, 36.66; III, 33.36; IV, 37.44; 

 V, 31.32 ; and VI, 14.58 ; average, 32.81. 



With only one exception (I), the percentage of NajO is in excess of 

 K2O, and in each one, omitting II, the excess of NaaO is quite marked. 

 I, the exception noted, shows equal percentages of NajO and KnO. The 

 mineral composition of the feldspars, calculated in the usual way from 

 the percentages of KoO, NajO, and CaO, in table ¥, is indicated in the 

 following- table : 



G. Table of feldspar composition of North Carolina granites. 

 (Numbers correspond to table P.) 



III. 



IV. 



VI. Average. 



Orthoclase 21.13 



Albite 26.72 



Anorthite 3.05 



Total plagioclase 29.77 



Total feldspar 50.90 



AbnAnm ratio 9:1 



Or — plag. ratio 1:1.4 



22.24 18.35 



34.06 ! 34.58 



3.06 ! 8.62 



37.12 

 59.36 

 11:1 

 1:1.6 



43.20 

 61.55 

 4:1 

 1:3 



15.01 16.12 16.12 



37.20 ' 39.92 42.44 



8.06 , 10.77 10.84 



45.26 41.69 43.28 



60.27 57.811 69.40 

 5:1 3:1 4:1 

 1:3 1:2.5 1:3.3 



18.16 

 35.82 

 7.35 

 43.17 

 61.33 

 6:1 

 1:2.5 



The plagioclase molecules as calculated range from intermediate oligo- 

 clase (AbgAni) to acid oligoclase-albite (AbnAnm), with an average of 

 basic oligoclase-albite (AbgAnj). The potassic feldspar contains some 

 soda, as is shown by the presence of more or less microperthite. Zonal 

 structure is rarely developed in the feldspars. 



Classification. — The total feldspar in the North Carolina rocks ranges 

 from 50.90 per cent, to 69.40 per cent., with an average of 61.33 per cent. 

 The rocks represented by III, IV, V, and VI of table G^ fall considerably 

 below the minimum amount of potassic feldspar in quartz monzonite, but 

 axe well within the limits of this constituent in granodiorite. In each of 

 these plagioclase is approximately three times that of orthoclase, and 

 although these rocks fall outside the limits of quartz monzonites in ortho- 

 clase, their low CaO and generally high SiOa are more characteristic of 

 quartz monzonite than of granodiorite. The soda-lime feldspar of these 

 rocks is more acid than that of granodiorite. I and II are quartz mon- 

 zonites. 



