88 Mr Haidingcr on the Parasitic Formation of Miner aU^ 



some light also on the gradual formation of the new species 

 within the space occupied by the crystals of analcime. The 

 centres of the single globular groups, aggregated in a reniform 

 manner, are situated on the surface of the icositetrahedrons. 

 From these, the fibres diverge, not only towards the surface of 

 the globules, but also on the other side, in the direction of 

 •what formerly was analcime. The original surface of the icosi- 

 tetrahedrons may be laid bare, by breaking off the exterior 

 coat of prehnite. Even in those places where there was no 

 coating of prehnite, the decomposition of the analcime has taken 

 place in the neighbourhood of other decomposed crystals. The 

 ingredients of prehnite are silica, alumina, lime, and water; 

 those of analcime, silica, alumina, soda, and water. There is 

 no similarity between the two in the mode of combination of 

 their ingredients, analcime being considered as a compound of 

 bisilicates of soda and alumina with water, while prehnite is 

 considered as a compound of simple silicates of lime and alu- 

 mina, with a hydrate of silica. 



On another occasion, Edin. Journ. of' Science^ vol. i. p. 380, 

 I have described a very curious instance of pyramidal forms, 

 agreeing as near as possible with those of the pyramidal schee- 

 lium-baryte, which consisted in their interior of multitudes of 

 columnar crystals of the prismatic scheelium ore. They were 

 found at Wheal Maudlin in Cornwall, and are partly implant- 

 ed on * quartz, arsenical pyrites, chlorite, &c. and partly im- 

 bedded in cleavable blende. The chemical composition of 

 the two species is almost identically the same, at least not more 

 different than in the varieties of pyroxene, or other similar sub- 

 stances. The chemical formula of the first is Ca W^ ; that of 

 the second Mn W^ -|- 3 Fe W^, different only in the isomor- 

 phous bases of calcium in the one, and manganese and iron in 

 the other, one atom of the protoxide of each of them being 

 united with two atoms of tungstic acid. This curious resem- 

 blance of the chemical mixture was then pointed out to me 

 by Professor Mitscherlich, who supposed, that, from the iso- 

 morphism of the bases, the varieties observed might be ge- 

 nuine crystals, of the same ingredients as wolfram, but with the 

 form of the scheelium-baryte : this was disproved, however, 



