328 ON A NEW STRUCTURE IN APOPHYLLITE, AND 



There is another variety of Apophyllite from Greenland, of 

 a very interesting kind. Sir Charles Giesecke', who kindly 

 communicated to me several specimens of it, discovered it at 

 Kudlisaet in Disco Island *, and describes it as of a cylindri- 

 cal form, presenting the shape of a barrel, from being con- 

 tracted at the extremities. 



Upon examining some of the best specimens of this mine- 

 ral, I find that each of the four curvilineal surfaces, by which 

 the prism is contained, often consist of three planes, inclined 

 to one another at very great angles. There is a distinct trun- 

 cation, too, upon the angles at the summit, and its inclina- 

 tion is nearly the same as in the perfect crystals. 



As all the transparent crystals which I have seen of this mi- 

 neral, are intersected with diverging groups of capillary meso- 

 type, or, more probably, capillary apophyllite, it is not unlike- 

 ly, that the usual law of its crystallisation has been modified 

 by its formation either at the extremity of one, or in the 

 middle of several groups of these filamentous crystals f. 



When the uppermost slice of this kind of Apophyllite is cut 

 off, it has only one axis of double refraction, like the same 

 slice of the regular quadrangular prisms, and displays no tes- 

 selated structure. 



The second slice exhibits a tesselated appearance, which va- 

 ries in different crystals, as represented in Figs. 18. and 19. In 

 both these figures, the shaded part has only one axis of double 

 refraction, while the five sectors have two axes, and the plane 



passing 



* See this Volume, p. 270. 



■J- There are many examples of Apophyllite having been deposited upon groups 

 of Needlestone ; but in these cases, the crystal is not penetrated by them, but 

 merely rests upon their filaments. 



