ON THE OPTICAL PECULIARITIES OF THAT MINERAL. 323 



of curvature. In this specimen, the isochromatic lines are all 

 curves of contrary flexure, and there is only a small portion of 

 them serrated, between their acute convex summits, and the 

 faces of the pyramid. When some of the Faroe pyramids are 

 examined with high powers, and by light transmitted exactly 

 parallel to their faces, we may recognise frequently three, and 

 sometimes more minute veins parallel to each face, as repre- 

 sented in Fig. 11. In these veins, the doubly refracting and 

 polarising forces suffer an instantaneous change, and portions 

 of the isochromatic curves are displaced, and thrown, as it were, 

 towards the summit of the pyramid, exactly like the dislocations 

 and slips which take place in strata of coal. 



Among the pyramidal crystals from Faroe which Major Pe- 

 tersen was so good as to present to me, there was one of un^ 

 usual magnitude, and of a yellowish tinge, which had a num- 

 ber of additional facets,, as represented in Fig. 13. These fa- 

 cets, shewn at o and p 9 were all inclined 150° to the summit 

 plane. In a great number of the Iceland crystals, I have ob- 

 served four pair of very singular rounded planes, replacing the 

 angles of the rectangular prism. These planes, which are al- 

 ways rough, with a certain degree of polish, are shewn at m, n 

 in Fig. 13., and, with the other faces, constitute a twelve- 

 sided prism of a very unusual kind ; the mean inclination of 

 the rounded faces being about 150°. 



Among the various forms in which the Apophyllite occurs, 

 there is one from Faroe of a very interesting nature. The 

 crystals have a greenish-white tinge, and are aggregated to- 

 gether in masses. The quadrangular prisms are in general 

 below one-twelfth of an inch in width ; they are always unpo- 

 lished on their terminal planes; they have the angles at the 

 summit more deeply truncated than the other quadrangular 



prisms 



