( 336 ) 



DESCRIPTION OF THE PLATES. 



Plate XX. 



Fig. 1. Represents the tesselated structure of one of the internal plates of the Faioe 

 Apophyllite, as developed in ordinary light by the microscope, or as seen by expo, 

 sing it to a polarised ray. 



Fig. 2. Represents a complete crystal of the same Apophyllite, with the upper and 

 under slices lifted up, to shew the internal tesselated structure. 



Fig. 3. Represents the tesselated structure of the pyramidal crystals of Apoph\llite 

 from Faroe and Iceland. 



Figs. 4, 5, 6, 7, 8, 9. Shew the effects produced by successive slices, taken from 

 one of the pyramids of Apophyllite. 



Fig. 10. Is a diagram for explaining the double central lozenge seen in Fig. 7. 



Fig. 11. Shews the variation in the curvature of the fringes, or isochromatic curves, 

 their convexity, as seen through two opposite faces of the pyramid, and the veins 

 with which they are sometimes traversed, with the slips in the isochromatic curves. 



Fig. 12. Represents a more singular variation in the isochromatic curves, and tlieir 

 concavity towards the summit, as seen through two opposite faces of the pyramid. 



Fig. 13. Represents a complete crystal of the Iceland Apophyllite, with the new 

 facets o, p, &c. at the summit of the pyramid, and of the six curvilirieal and un- 

 polished planes m, n. 



Fig. 14. Shews the form of the isochromatic curves, as seen through the diagonal 

 of a quadrangular prism of a transparent variety of Apophyllite from Faroe, 

 the structure, through its parallel faces, being shewn in Plate XXI. Fig. 1. 



Fig. 16. Represents the structure, as detected by the microscope of one of the cen- 

 tral laminae or sUces of the crystal, shewn in Plate XXI. Fig. 1. 



Fig. 16. Shews the optical effect produced by the same lamina or slice. 



Fig. 17. Shews the form of the first isochromatic curve in a very small crystal. 



Fig. 18. Represents the figure produced in polarised light, by an internal slice of 

 the Barrel or cylindrical Apophyllite from Greenland. 



Fig. 19. Represents the figure produced by a similar slice from another specimen. 



Plate XXI. 



Fig. 1. Is a magnified representation of the Figure, and the Tints displayed by a 

 complete crystal, of a particular variety of Faroe Apophyllite, when exposed to 

 polarised Ught. The outlines of the figure are distinctly seen by the microscope 

 alone, in ordinary light. The crystal is here magnified about 50 times. 



Fig. 2. Shews the colours produced by one of the tesselated laminae of Apophyl- 

 lite, when crossed by a plate of sulphate of lime which gives one uniform blue tint. 



Fig. 3. Represents the circular system of rings, with their anomalous tints, as seen 

 round the axis of the ApophyllUe Surcomposee. 



