30 Forbes — Epidote from Huntington, Mass, etc. 



For further comparison an iron determination was made, 

 with the following results : Fe 2 3 = 6*97, FeO = -89 per cent, 

 using as far as possible the green interior portion of the crys- 

 tal, but owing to the scarcity of the material, some of the red 

 part was also included. If only the green interior part had 

 been used, these results would probably have been a trifle 

 higher, but they are higher than those given by Laspeyres, 

 whose analysis was made on carefully selected pale yellow 

 crystals. 



The following table, containing the results already given, 

 together with the optical determinations on epidote from Unter- 

 sulzbach in Tyrol by Klein,* will show the changes in the opti- 

 cal properties resulting from variations in the percentages of 

 ferric iron : 



2Va, Na 

 Locality. Fe 2 3 Indices of refraction for yellow. measured 



a /3 y y—a over a. 



Untersulzbach, 14*0 1*7305 1-7540 l*7677r 0-0372 73° 39' 

 Zillerthal, 697 1'720 1*7245 l*7344y 0*0144 87 46 



Huntington, 5*67 1*714 1*716 l*724y 0*010 90 32 



With a decrease in the percentage of iron, the indices of 

 refraction and the strength of the double refraction decrease, 

 while the axial angle measured over a increases, so that in the 

 Huntington epidote c becomes the acute bisectrix and the crys- 

 tal is positive with /><y while most epidotes are negative with 



In this connection the results of Ramsay f are of interest, 

 who studied the zonal character of epidotes and determined 

 the strength of the double refraction, y—a, in light and dark 

 portions of crystals from different localities. He found that 

 the light portions, containing presumably less iron than the 

 dark, had the lowest double refraction, but gave no determina- 

 tions as low as the ones given in this article for Zillerthal and 

 Huntington. Some of his determinations are as follows : 



Locality. Dark. Light. 



y—a. y — a. 



Sulzbach 0*050 0*049 



Zoptau 0*046 0*038 



Arendal 0*054 0*050 



Haddam 0*037 0*034 



Brosso __ _ 0*023 0*017 



In conclusion I wish to express my indebtedness to Prof. S. 

 L. Penfield for valuable advice and assistance during the 

 progress of this investigation. 



Laboratory of Mineralogy and Petrography, 

 Sheffield Scientific School, June, 1895. 



* Zeitschriffc fur Krystallographie und Mineralogie, iii, p. 562, 1879. 

 f Jahrb. f. Min., 1893, i, p. 111. 



