52 G. H. Williams — Piedmontite and Scheelite from the 



their characteristic forms, as seen under the microscope are 

 shown in fig. 1. Their elongation, as is usual for all epidotic 

 minerals, is invariably parallel to the orthodiagonal axis. 



Crystals which lie upon their 

 orthopinacoid, as in the fig- 

 ure, are straw-yellow to 

 orange, according to their 

 thickness, when their long 

 direction is transverse to the 

 vibrating plane of the Nicol, 

 and some shade of amethyst 

 when these two directions 

 are parallel. In converged 

 polarized light such sections 

 show an acute positive bisec- 

 trix with large optical angle, 

 and optical plane normal to 

 their direction of elongation. 

 Fig. 1.— Piedmontite needles from schee- Crystals lying on the posi- 

 lite, Buchanan valley, Pa. Magnified x tive orthodome show the 



L50 ' same amethyst shades par- 



allel to their longest direction and deep carmine transverse 

 thereto. The pleochroism is therefore n, yellow ; b, amethyst ; 

 c, carmine, and the absorption: c> b> a, or c> b = a. In terms 

 of Radde's international color scale the tints of the rays vibrat- 

 ing in these three directions were fixed, as nearly as possible, 

 as a=7, S] 6 = 23, I; c=25, m. This pleochroism agrees, 

 except for differences due to variation in the amount of man- 

 ganese, with that of all other red epidotes as may be seen from 

 the following table : 



Locality. 



Author. 



a 



b 



c 



St. Marcel 



Laspeyre* 



clear orange 



amethyst 



blood'red 



Sweden 



Flink 



orange 



violet 



carmine 



Japan 



Koto 



deep reddish violet 



light violet 



brownish red 



Groix 



Lacroix 



yellow 



very clear rose 



bright rose 



Glencoe 



Lacroix 



lemon-yellow 



clear rose 



light rose 



South Mt. 



Williams 



yellow to orange 



amethyst 



carmine 



The exception in the case of the Japanese piedmontite as 

 given by Koto,* does not agree with my own determination 

 upon a number of specimens from Japan received from him. 

 My examinations of the Japanese piedmontite show it to be 

 quite normal, and give : a, yellow ; b, pale violet ; c, magenta. 

 I also find the absorption not as Koto gives it, n> c> b, but 

 c> b> a. 



* Journal of the College of Science, Imperial University, vol. i, p. 306, 188*7. 



