104 Mr HAIDINGER on the Parasitic Formation 



of the same mixture, the rest being a trisilicate of potassa *. 

 The loss of this ingredient sufficiently accounts for the chemi- 

 cal difference between the two bodies ; but we are at a loss to 

 conjecture in what manner such a change may have taken place. 

 Mr ALLAN has in his cabinet several specimens from the trap 

 district near Dumbarton, exhibiting the shape of analcime, but 

 entirely composed of aggregated crystals of prehnite. Mr WIL- 

 LIAM GIBSON THOMSON is likewise in the possession of seve- 

 ral exceedingly distinct and instructive specimens of the same 

 description. There is one, among the former, where prehnite, 

 aggregated in globular shapes, is implanted on icositetrahedral 

 masses, once of analcime, but now likewise converted into preh- 

 nite. The implanted varieties are green and translucent ; I 

 found their specific gravity equal to 2.885 : the portions within 

 the faces of the icositetrahedrons are white and opake, and give 

 2.842, both of them rather lower than the usual results obtained, 

 which are a little above 2.9, at least in simple crystals. But the 

 arrangement of the divergent individuals in the reniform shapes, 

 is highly remarkable, and throws 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 icositetrahedrons 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 anal- 

 cime 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 



* BEUDANT'S Mineralogy, p. 333. & 363. 



