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 2^ 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/3ato<? 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/3ato9. 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'. 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 18-14 11 ' 03 



99-08 



3AlS 2 



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. 



