146 ON CIRCULAR POLARISATION, AS EXHIBITED IN THE 



an olive-coloured amethyst, the whole of the specimen is oc- 

 cupied with the veined structure, as shewn in Fig. 9. each sys- 

 tem of veins covering a sector of 120°. In another specimen, 

 shewn in Fig. 10. the veined sectors were four in number, two 

 of them corresponding, as at B and D, with opposite sides 

 of the hexagon, and the other two corresponding, as at A and 

 C, with two of its opposite angles. In this last specimen the 

 veined part was tinged with pink, and the crystal was foul near 

 its axis. In another specimen, Fig. 11. which contained part 

 of the pyramid, and part of the prism, of a very fine crystal, 

 the veined structure occupied all the half ABCDA, which was 

 nearly colourless ; and the other half AFEDA, which had cir- 

 cular polarisation, was of a yellow colour, excepting in the two 

 places where the opposite tints had extinguished one another. 

 In the parts a and c the tints were retrograde, and in b di- 

 rect. 



The finest specimen of amethyst which I have ever seen, is 

 shewn in Fig. 12. which is drawn of the natural size, and 

 which represents a section of part of the pyramid and part of 

 the prism. On the three alternate sides of the prism, viz. 

 MN, OP and QR, are placed sectors McN, OdP, QcR, 

 which are divided into two parts by dark lines c c\ d d\ a a', 

 which separate the direct structures of A, C, and E from the 

 retrograde structures of E, D, and F. On the other three al- 

 ternate faces of the prism are placed the three veined sectors 

 MckR, N c 6 d O, and V dbaQ, which meet at b in 

 angles of 120°, and consist of veins of opposite structures al- 

 ternating with each other, and so minute, that in many places 

 the circular tints are almost wholly extinguished by their mu- 

 tual action. The direct sectors A, C and E are all connected 

 together by the three radial veins b a, b c, b d, and are there- 

 fore to be considered as the expanded terminations of these 



veins. 



