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UNIVEESITY OF VIRGINIA PUBLICATION'S 



Omitting nonfeldspatliic constituents and assigning jjotash to ortho- 

 elase, and the soda and lime to albite and anorthite, respectively, the 

 twenty-one analyses show the limits of variation and average in composi- 

 tion of the feldspars to be: 



Feldspar composition of New England granites. 



Limits of Variation. Average. 



Orthoelase 



Albite 



Anorthite 



Total plagioclase 



Total feldspar 



Or— plag. ratio— 



Only one (Bethel, Vermont) of the 21 analyses shows orthoelase to be 

 less than 20 per cent. (5.56 per cent.) ; in the remaining 20 this constituent 

 ranges from 20.02 to 34.5 per cent. 



Examined as to the ratio of orthoelase to plagioclase, 3 have a ratio of 

 1 :<1, ranging from 1 :0.76 to 1 :0.9, essentially a 1 :1 ratio; 11 have a ratio 

 of 1 :1 : 5, a ratio of 1 :<2; and the remaining 2, have a ratio of 1 :2.1 and 

 1 :8.5. With the exception of the last two, it will be observed that the New 

 England granites represented by the 21 analyses are, according to the old 

 system of classification, quartz monzonites. 



The granites of Sprucehead, Knox county, and Norridgewock, Maine^ ; 

 of Milford and Auburn, New Hampshire- ; of Bethel, Derby, Dummerston, 

 Groton, Hardwiek, Kirby, Randolph, Rochester, Ryegate, and Topsham, 

 Vermont', and of Westerly (except "AVesterly red") and Niantic, Rhode 

 Island*, have been described recently by Dale as quartz monzonites. 



The position of only 8 of the New England granites has been com- 

 puted in the quantitative system of classification of igneous rocks. This 

 number is probably sufficient to indicate the general range in position of 

 the granites. These mav be tabulated as follows :° 



'Dale, T. N. U. S. Geological Survey, Bull. No. 313, 1907, p. 25. 



"Idem, Bull. No. 354, 1908, p, 211. 



'Idem, Bull. No. 404, 1909, p, 120. 



*Idem, Bull. No, 354, 1908, pp. 188-210. 



'^Washington, H, S. U. S. Geol. Survey, Professional Paper No, 14, 1909, 



