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UNIVERSITY OF VIRGIKIA PUBLICATI0N"8 



With the exception of II, V, and VII (table D), which show ecjual 

 percentages of NajO and K2O, the percentage of ISTaaO is slightly less than 

 that of K2O, but without exception ISTaaO, molecularly, either exceeds or is 

 approximately equal to K^O. The range in plagioclase molecules as cal- 

 culated is from basic oligoclase (AboAui) to acid oligoclase (Ab4AnJ, 

 with an average of intermediate oligoclase (AbaAnj). 



Classification. — Table E shows the amount of total feldspar in the 

 rocks to be strikingly similar, with the exception of VII, in which it is 

 considerably less than for either of the other six. The average in total 

 feldspar, 65.21 per cent., is gTeater in the Virginia rocks than in those of 

 the other states, except Georgia. 



Total feldspar in four of the analyses (II, III, IV, and VI) is pro- 

 portioned in the ratio of 1 of orthoclase to less than 2 of plagioclase, while 

 in the remaining three plagioclase is somewhat increased and is slightly 

 more' than double orthoclase. Tbe percentage of CaO in these last three, 

 which is but slightly increased over that of the other four analyses, is much 

 less than that contained in granodiorites or quartz diorites. With the pos- 

 sible exception of VII, all the properties of the Virginia rocks are those 

 which would group them, in the old scheme of classification, as quartz 

 monzonites. They are characterized by high silica and low lime; oligoclase 

 of intermediate to acid composition, except VII, which contains basic oligo- 

 clase; a computed orthoclase content of more than 20 per cent, and less 

 than 30 per cent., except VII, which is less than 20 per cent. (17.24 per 

 cent) ; and, in most cases, by an orthoclase-plagioclase ratio of 1 to less 

 than 2. 



The position of the Virginia rocks in the quantitative system of classifi- 

 cation has been computed, and the results may be tabulated as follows : 



Granites of Virginia. 



