found in the Neighbourhood of Glasgow, 411 



when exposed to the atmosphere, and falls to powder unless 

 protected by a coat of gum. 



It is usually crystallized, and its primary form is an oblique 

 rhombic prism. The acute angles of the prism are of 84° 30', 

 and the base of the prism makes with the lateral faces angles 

 of 114?° 54-', and 65° 6'. The specific gravity is 2*361, and 

 it is harder than calcareous spar. 



Its constitution is 3 Al S 2 + Cal S 2 + 5 Aq. The only dif- 

 ference which I have found in the Bishoptown laumonite is the 

 presence of 2 per cent, of magnesia in it. Neither Leopold 

 Gmelin, norVogel, nor Dufresnoy, who analysed this mineral, 

 take any notice of magnesia as a constituent. I was led to look 

 for it in consequence of the considerable deficiency which 

 occurred in my analysis. Vogel gives 9\ per cent, of car- 

 bonic acid. This must be given merely to make up the de- 

 ficiency in his analysis. It would not be surprising if his 

 specimen, like mine, had contained magnesia. 



7. Chabazite. — This name was applied by Box d' Antic to 

 our mineral. The Greek term %a/3<z£fco? occurs in a poem 

 ascribed to Orpheus, in which twenty kinds of stones are ce- 

 lebrated for their medicinal virtues, but without any descrip- 

 tion. The last of the twenty is ^a/3afto?. Chabazite occurs 

 pretty frequently in this neighbourhood, particularly about 

 Kilmacolm. It is always in rhomboids, approaching nearly 

 to the cube, the obtuse angles being 94° 56 f . It is usually 

 transparent, and has a vitreous lustre. The specific gravity 

 of the rhomboidal crystals is from 2-076 to 2*088. Its con- 

 stituents are 



Silica 49*20 24*6 



Alumina 17*91 7'96 



Lime 9-64 2*75 



Potash 1-92 0-32 



Water 20-41 l8rl4 n -° 3 



99-08 

 3A1S 2 +Cal S 3 -|-6Aq. 



There is another variety of chabazite which never occurs 

 here, but is pretty common in the north of Ireland. The 

 crystal is never rhomboid, but a very peculiar figure, of which 

 the only way of forming an idea is to examine the model. 

 The primary faces are nearly all concealed; yet it cleaves 

 easily in the direction of the primary faces and yields a rhom- 

 boid similar to the primary crystal. 



Now this variety differs from the common chabazite of this 

 neighbourhood, both in its specific gravity and its composi- 

 tion. Its specific gravity is 2*472. 



