OV THE OPTICAL PECULIAKITIES OF THAT MINERAL. 321 



A general idea of the structure of this variety of Apophyl- 

 lite, may be obtained from Fig. 2., where A and B are the ter- 

 minal laminae, raised up in order to shew the tesselated struc- 

 ture of the intermediate portion of the prism. 



The fine transparent pyramidal crystals which are found in 

 Iceland, as well as in Faroe, possess a tesselated structure, 

 which differs in several respects from that of the quadrangular 

 prisms. The arrangement of the tesselae is shewn in Fig. 3., 

 but the border ABCD, which is seen only under favourable cir- 

 cumstances of illumination, appears to be composed of lines or 

 parts at right angles to the sides AB, BC, &c. ; and the polari- 

 sing force of the borders is extremely weak. In order to observe 

 how the tesselee varied in different parts of the crystal, I took 

 one of the largest I could obtain, and cut it into four different 

 slices. No part of the substance was removed by grinding or 

 polishing, so that I examined the structure of the plates as 

 they were detached in succession from the summit of the crys- 

 tal : The phenomena which they displayed are shewn in Figs. 4, 

 5; 6, 7; 8, 9 ; Figs. 4, 6, 8, containing the appearances when the 

 sides of the crystal were in the plane of primitive polarisation ; 

 and Figs. 5, 7, and 9, when they were inclined 45° to it. The 

 Figs. 4, and 5, exhibit the tesselas in the^rs^ and second slices of 

 the crystal, which had nearly the same structure. In Figs. 6, 

 and 7., the figure exhibited by the third slice is shewn. The 

 angles display a veined structure w:hen examined by the mi- 

 croscope ; and, what is very remarkable, there are two 

 central squares ; the interior one being much darker than the 

 other, and having oije axis of double refraction, while the 

 other exhibits the effect of two axes, as if the axis in the plane 

 of the laminae had a less polarising force than the correspond- 

 ing one for the rest of the plate. This effect, however, is likely 

 to arise from the juxtaposition of two contiguous laminae, abed 



s s 2 and 



