GNEISS AND LIMESTONE CONTACT PHENOMENA 



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ently heterogeneous, its index of refraction (no) varying from 1.775^ 

 0.005 to 1. 801 =1= 0.005. This means a variation in composition from 

 25 to 43 wt. per cent andradite in the mixcrystals, supposing them 

 to contain only andradite and grossularite.' 



The dinopyroxene has 160 = 1.696=^0.001 and the angle cA7 = 

 43°. Hence its composition should be DiegHeji.^ 



Fig. I. — Inclusions of dinopyroxene skarn in aplitic gneiss. E. of Benson Pond, 

 Washington, Massachusetts. Eight-ninths natural size. 



, Together with the garnet in the veinlets there is quartz, micro- 

 dine smd plagiodase. The latter, having q;'d = 1.532 and t'd = 

 1.540, has the composition about Abgi. 



' W. E. Ford, Amer. Jour. Sci. (4), Vol. XL (1915), pp. 33-49. 



^ The determinations of pyroxenes of the diopside-hedenbergite series by means 

 of the indices of refraction were made with a diagram (Fig. 2) based on the optical 

 data given by Wiilfing (H. Rosenbusch and E. A. Wiilfing, Mikroskopische Physio- 

 graphie, 1, 2, p. 203) and on the analyses of the same pyroxenes published by G. 

 Flink (Zs. Kr., Vol. II (1895), p. 585). Although the analyses are antiquated, these 

 data no doubt give a correct idea of the relations between composition and optical 

 properties of clinopyroxenes containing little or no sesquioxydes. Drs. H. E. Merwin 

 and H. S. Washington are at present carrying on an investigation of all the pyroxenes 

 and they no doubt will give a better diagram of this series. As their work is not far 

 advanced and it may be some years before their results are published, we give here 

 the diagram based on Flink's and Wiilfing's data. 



