OPTICAL STRUCTURE OF THE AMETHYST. 149 



ges which separated the direct and retrograde structures, and 

 increased to the centres of these structures, from which they 

 again diminished to the adjacent limit of the next vein. 

 Hence we deduce the important fact, that the direct and 

 retrograde veins in Amethyst have a greater polarising force, 

 and consequently a greater force of double refraction than 

 the interval between them, corresponding with the black fringe. 

 As the two structures pass into one another, through this line 

 as their node, by insensible gradations, we cannot avoid con- 

 cluding, that the cause, whatever it may be, which gives to 

 the particles of Quartz the peculiar arrangement that produ- 

 ces circular polarisation, gives them at the same time, when 

 thus arranged, an increase of polarising and doubly refracting 



force. 



These results, considered merely as optical facts, would 

 have entitled mineralogists to separate Quartz and Amethyst ; 

 and it is highly probable, that the two varieties of quartz, and 

 the amethyst, will be found to exhibit some remarkable differ- 

 ence in their crystalline structure, to which the difference in 

 their optical properties may be ascribed. It is fortunate, how- 

 ever, that the optical structure which we have pointed out dis- 

 plays itself by precise mineralogical characters. The combi- 

 nation of veins may be seen even in common light. They ap- 

 pear cropping out, as it were, upon the alternate faces of the 

 pyramid, as shewn in Fig. 1. at A, C, and E ; or on all the 

 faces when the whole prism is pervaded by the veined struc- 

 ture, and the fracture across the veined portions, exhibits a 

 beautiful, and sometimes a regular rippled structure, not un- 

 like the engine-turning on the back of ornamental watches. 

 This rippled structure, which I have attempted to represent in 

 Fig. 15. is an infallible proof that the specimen is amethyst, 

 whether it is yellow, orange, olive-green, lilac, or perfectly co- 

 lourless* 



