of Epistilbite and Heulandite. 11 



Ficr. 3. represents the same form as fig. 4, but drawn in a 

 parallel position with fig. 1, and with regard to the decre- 

 ments by which the planes are conceived to be derived from it. 



It appears to me, therefore, that the crystals of Epistilbite 

 present only a simple variety of Heulandite, the crystallogra- 

 phical sign of which referred to the primitive form assumed 

 for that substance is 



h^ a^ «3 o' b' d^' 



The difference of their general appearance with that of the 

 ordinary crystals of Heulandite, can be no reasonable objec- 

 tion to their re-union with that substance ; since it is well 

 known that some of the varieties of sphene differ quite as much 



in their forms. , ^ i • • i 



The crystals of Epistilbite are generally found m twins and 

 joined parallel to one of the planes M (fig. 4), or i' (fig. 3). 

 This circumstance may induce to take for the primitive torm 

 of Heulandite an oblique rhombic prism, the lateral planes ot 

 which would be the planes now designated by b\ and for its 

 base either of the planes P, h\ a\ o^ or a^. 



It mif^ht be objected to the reasons given above to consider 

 Epistilbite as a simple variety of Heulandite, that similar argu- 

 ments might perhaps be used to show that Sulbite may also be 

 considered as a variety of the same species. The hardness and 

 specific gravity of Heulandite and Stilbite are nearly the same ; 

 they have the same cleavage, even their chemical compositions 

 do not widely differ ; and though the forms of Stilbite are ge- 

 nerally referred to a right rhombic prism, they might still, as 

 in the case of Epistilbite, be derived from the primitive ot 

 Heulandite. If that were really possible, it would undoubt- 

 edly tend to throw some doubt upon the accuracy of the rea- 

 soning used with respect to Epistilbite; because, besides the 

 difference between the chemical compositions of Heulandite 

 and Stilbite, and that of their optical characters, there are still 

 other reasons to regard them as two distinct species. 



But if, on the contrary, it is shown that, consistently with 

 the simplicity of the structure of crystals, it is impossible to 

 refer both the forms of Stilbite and Heulandite to the same 

 primitive form, it will be a strong argument in fevour ot the 

 union of Epistilbite with the last-mentioned substance. 



The form of the crystals of Stilbite is represented by hg. 5 ; 

 and the mean of various measurements gives 119° 5 lor the 

 most obtuse incidence of the faces of the pyramid iormed by 

 the planes marked b\ and 114° for the other. 1 hen, by as- 

 suming the primitive form to be a right rhombic prism ot such 

 dimensions that the planes b' should result of a decrement by 

 one row on the edges of its base, it follows that the jucidence 

 C2 "f 



