ARRAN. MINERALS. 429 



In examining the lamellar variety, it will be found to 

 undergo similar changes, according to the increasing 

 perfection of its structure. During this progress the 

 transparency also becomes more complete, while the 

 colour occasionally vanishes, so as to present specimens 

 of which the detached fragments are scarcely distin- 

 guishable from stilbite, which in 'these cases it also 

 emulates by the gradual acquisition of the pearly lustre. 

 The crystalline form's df-this* 'variety may be traced in 

 the same manner on' the surfaces, and they present the 

 edges of square or octagonal lamellae of which the edges 

 are occasionally bevilled, but more generally unmodified. 

 Cases also occur where the crystals of this variety are 

 nearly detached ; and these, however obscured by aggre- 

 gation, bear a considerable general resemblance to the 

 fasciculated prisms of stilbite. This variety therefore 

 may be considered as presenting an approximation to that 

 mineral, although in both these cases the rarity and 

 imperfection of the crystalline forms prevents the geome- 

 trical connexions from being satisfactorily established. 



In examining * the solid variety, the first conspicuous 

 change is that from imperfect transparency to a limited 

 opacity, which gradually increases till it becomes abso- 

 lute ; the specimen in this case gradually also assuming 

 a fracture indicative of a different geometric structure. 

 In these cases the green tint vanishes entirely, the colour 

 becoming white. This transition is found equally to occur 

 from the fibrous or from the lamellar to the opake. 

 The change of crystallization is here perfect, the forms 

 being those of analcime ; while the transition is so com- 

 plete as to leave no doubt respecting the nature of such 

 a specimen when detached from its original connexions. 

 In some specimens a single spherule may even be ob- 

 served, of which the one half is prehnite, and the other a 

 part of a crystal of analcime. Prehnite is therefore a common 

 bond by which these three minerals, otherwise associated 

 by many peculiarities, are to a certain degree united. 



