OPTICAL STRUCTURE OF THE AMETHYST. 147 



veins. The retrograde sectors B, D and F are expansions of 

 the first retrograde veins next to d b c, db a and ab c\ and 

 the lines c c\ d d' and a a are continuations of the dark or neu- 

 tral lines which separate the first retrograde vein from the di- 

 rect radial veins. 



All the sectors A, B, C, D, E and F, are of a yellowish- 

 brown colour, and all the rest of the crystal is of a pale lilac 

 colour ; the lilac tints being arranged in the manner previous- 

 ly described. The phenomena which I have now mentioned 

 as existing in this specimen are very common in the ame- 

 thyst ; and I have never yet found a specimen in which the 

 yellow tints were not confined to those portions which formed 

 the expanded termination of veins, a fact which indicates that 

 this would have been the colour of the crystal, whether its ac- 

 tion were direct or retrograde, and that the lilac colour af- 

 fects in general those portions which are composed of oppo- 

 site veins. 



Hitherto we have considered the appearances exhibited by 

 amethyst in a direction coincident with the axis of the prism. 

 When we examine it in a direction transverse to the axis, 

 we receive no assistance from the phenomena of circular pola- 

 risation, as the force by which they are produced extends on- 

 ly to a very small distance from the axis ; but the structure of 

 the crystal is fortunately rendered obvious by other means . 

 In Fig. 13. we have represented a section of the pyramid of ame- 

 thyst, cut by a plane parallel to one of the faces of the pyramid, 

 in order to explain the several phenomena which it displays. The 

 upper pyramidal layer ABC is commonly pink, but often brown* 

 ish or bluish, and composed of strata of different shades of colour. 

 The next layer is yellowish white ; the third layer D£ is pink 

 like the first ; the fourth layer is yellowish-white ; and the fifth 

 layer FG is like the first and third, being succeeded by a 



t 2 yellowish* 



