288 Dr Gustavus Rose on Epistilbite, 



trenie liberality with which he permitted me not only to look 

 over the cabinet under his direction, but also to examine and 

 measure more minutely the crystals which it contains. 



The crystals of epistilbite very much resemble those of 

 stilbite, and of heulandite, with which probably they have 

 often been confounded. In allusion to this resemblance, the 

 name of epistilbite has been formed. Like the crystals of 

 these two species, they possess a single cleavage, with a strong 

 pearly lustre, while, on the mostly uneven crystalline faces in 

 other directions, their lustre is vitreous. They differ from them 

 by a greater specific gravity, which, in heulandite, I found 

 = 2.211 at a temperature of 6° R., in stilbite, and between 

 2.145 and 2.176 at a temperature of 8* R. Also their hard- 

 ness is a little greater, for, in both heulandite and stilbite, it is 

 yet below that of fluor-spar. The most considerable differ- 

 ences, however, are those in their regular forms; the angles of 

 epistilbite and stilbite, though within one system of crystal- 

 lization, are incompatible, while in epistilbite and heulandite, 

 even the system of crystallization is different. For the sake 

 of an easier comparison, I have added a figure of heulandite, 

 Fig. 25, and one of stilbite, Fig. 26, a little modified, but agree- 

 ing in position with those of Professor Mohs. The crystals 

 of epistilbite are farther remarkable for the frequency of their 

 occurrence in twin-crystals, which have not yet been described 

 in either of the other species. Of stilbite, Mr Allan showed 

 me a cruciform twin in his cabinet, which he had himself 

 brought home from Faroe ; but this is one of the rarest spe- 

 cimens ever observed. 



Also the chemical composition of stilbite and heulandite, 

 according to Hisingcr and Walmstedt, are different, the for- 

 mulas being respectively CS 5 + 3 AS 3 + 6 Aq, and CS r > 

 -f 4 AS 5 + 6 Aq. The analyses are — 



Silica, - 



Alumina, 



Lime, - - 



Water, - 



The analysis of epistilbite was an easy one. The mineral 





