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EUTECTICS IN PEGMATITES.
41
feldspar quarry in Topsham, Me., and from Kinkle's feldspar quarry in Bedford, N. Y. These were analyzed by George Steiger in the laboratory of the United States Geological Survey. (See p. 124.) In order that the material analyzed should represent closely the true composition, about 10-pound samples of the Maine granites were taken. These were pulverized, carefully mixed, and quar­tered down to convenient size for analysis. The New York speci­men was a cleavage piece about 1 by 2 by 3 inches in size.
The ratio of quartz to feldspar in the analyses published by Vogt and Bygden and in the author's analyses are given in the table below. In figure 5 the compositions of the feldspars are plotted on triangular projection. The numbers in the diagram correspond to those in the table.
Composition of graphic granites.
From the table and diagram it is at once evident that even among those graphic granites whose feldspars are almost identical in com­position (such as Nos. 2 to 6) there arc quite considerable variations in the quartz-feldspar ratio. In analyses Nos. 1, 2, 3, 7, 8, 10, and 11 (particularly in Nos. 1, 3, 7, 8, and 10) the percentage of anorthite is small and nearly constant, the only important variation being in the ratio between orthoclase and albite. No regular or consistent relationship is recognizable, however, between this ratio and the ratio between quartz and feldspar. The grouping of Nos. 1 to 11 near the lower line of the diagram signifies merely that the feldspar associated with the orthoclase (or microcline) in graphic granites as in normal granites" is usually albite or oligoclase.
Both analyses and microscopic studies show that most graphic granites are mixtures of three minerals—quartz, orthoclase or