OPTICAL STRUCTURE OF THE AMETHYST. 145 



Among the numerous amethysts which I prepared for exa- 

 mination, there was one of a very interesting nature. One 

 half of it was yellow, and the other lilac. The yellow half ex- 

 hibited the tints of circular polarisation ; but the lilac half 

 seemed entirely destitute of them. The application of the 

 microscope, however, displayed in the lilac portion a sort of 

 rippled structure, like that of the agate *, and I distinctly saw 

 various remaining specks of the two structures, by whose oppo- 

 sing agencies the circular tints had been extinguished. This 

 specimen is shewn in Fig. 7. where B is the yellow, and A the 

 lilac portion, separated by a sharp line m n. 



In another specimen, represented in Fig. 8. but without any 

 natural faces, the one half A had the direct circular polarising 

 structure, while the other half B had the retrograde structure, 

 and the junction of these opposite tints was marked by a black 

 line m n, from each side of which the colours ascended in the 

 scale to the Greenish Pink of the second order. The part of 

 the ci'ystal corresponding with the black line was colourless, 

 while both the portions A and B had a dark-yellow tinge. 

 This black space constantly occurs between the two opposite 

 structures ; though I have various specimens, such as that in 

 Fig. 11. where the same structure appears to exist on both 

 sides of it. I have always found, however, upon minute exa- 

 mination, that in this case the second structure exists in the 

 middle of the irregular black space, having nearly exhausted it- 

 self in neutralising, to a certain extent, the opposite structure 

 which encircles it. 



Although the veined structure, when it is regular, most 

 commonly resides in the alternate sectors of hexahedral pyra- 

 mids of amethyst, yet I have found specimens in which it is 

 placed in a different manner. In a colourless, and also in 



VOL. IX. p. I. T an 



* See Phil. Trans. 1814, p. 192 ; and PI. V. Fig. 1, 2. 



