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DESCRIPTION OF THE FIGURES IN PLATE X. 



In all the Figures of this Plate, which have been coloured in order to repre- 

 sent the two structures, the direct and retrograde veins are distinguished by a 

 slight difference of tint, — a difference which is actually produced by turning the 

 principal section of the analysing prism a slight degree out of the plane of primi- 

 tive polarisation. When the principal section of the analysing prism is exactly in 

 the plane of polarisation, the tints of the two structures are perfectly ahke, when 

 the veins are of the same size. 



Fig. 1. Is a plate cut out of a pyramid of Amethyst, and about ^'jth of an inch 

 thick. The blue and yellow veins are separated by a black fringe, to\i'ards 

 the middle of which the tints gradually shade off, and no direct vein ever 

 passes into a retrograde one, without the interposition of a black fringe. The 

 veined structure appears on the alternate faces of the pyramid, as shewn in 

 the Figure. 



Fig 2. Shews the arrangement of the colouring matter in the veined sectors. 



Fig. 4. Shews a form of the veins which is not very common. 



Fig. 5. In the sector GCF of this Figure, I have shewn an arrangement which 

 the double structure sometimes assumes ; though it did not occur in the spe- 

 cimen represented in the Figure, and already described. 



Fig. 6. Shews the structure of an Amethyst perfectly colourless, excepting in 

 the three coloured sectors, which were yellowish by common Ught. The 

 thickness is about 0.32 of an inch. 



Fig. 7. In this specimen the half m AC n shews imperfectly the veined struc- 

 ture in the part A, while in the part C small specks of the two structures 

 may be seen with a microscope. 



Fig. 8. Is a specimen which has no veins, but merely the two structures, as 

 previously described. 



Fi". 9. Is a specimen 0.47 of an inch thick, consisting whoUy of direct and 

 retrograde veins. This Amethyst developes tints entirely different from any 

 that I have described. 



Fig. 10. Is a remarkable specimen, and the only one of the same kind that 

 I have met with. 



Fig. 11. In this specimen the retrograde portions a and c were, as usual, sepa- 

 rated from the direct portion d by the black fringe m. The two portions a, c 

 were also separated by a black fringe ; but I found that the remains of a di- 

 rect structure existed among the black masses between a and c. 



Fig. 12. I consider this specimen, and the one shewn in Fig. 1. as exhibiting 

 the most general structure of well crystallised Amethysts. 



Fig. 13. Represents the pyramidal strata of a pink colour, seen by common light 

 in a direction transverse to the axis. 



Fig. 15. Shews the fracture of Amethyst. 



X. 



