4-12 Dr. T. Thomson on the Minerals 



And its constituents (the Port Rush variety) are 



Silica 48-988 24--49 



Alumina 19*774 8'8 ... 3 



Soda 6-066 1-51\ 



Lime 4'068 M6J 



Protoxide of <J 0-404 



Water 20-700 6 



100-000 



Here the constitution is the same as in the rhomboidal va- 

 riety, excepting that a considerable portion of the lime is 

 replaced by soda. In a specimen from Oberstein, analysed 

 by Berzelius, there was no lime at all, but only soda. From 

 this it is obvious that chabazite, like mesotype, is divisible 

 into three species or varieties. The first or rhomboidal cha- 

 bazite is a hydrous compound of bisilicate of alumina and 

 tersilicate of lime. In the trihedral variety the tersilicate of 

 lime is replaced by tersilicate of soda ; and the specimens 

 from Port Rush contain both tersilicate of lime and ter- 

 silicate of soda, being analogous to mesolite. 



I believe it will be ultimately found that every modification 

 of a crystalline form is occasioned either by a difference in 

 the constitution, or by the action of some peculiar foreign mat- 

 ter during the act of crystallizing. 



It may be worth while to mention, that a red-coloured cha- 

 bazite occurs at Kilmacolm ; its characters correspond with 

 those of common chabazile ; and its constituents are the same, 

 excepting that it contains 2' 75 per cent, of oxide of iron, to 

 which, doubtless, its red colour is owing. 



But about two months ago I received from Messrs. Jackson 

 and Alger of New York (well known as the authors of an 

 interesting and valuable paper on the geology of Nova Sco- 

 tia), a specimen of a mineral which they have distinguished 

 by the name of Acadiolite or yellow chabazite. It is in cry- 

 stals, having the common form of chabazite, and its other pro- 

 perties (colour excepted) are the same as those of common 

 chabazite. Its specific gravity is 2*0202. But it contains 

 rather more silica and less alumina than common chabazite; 

 and there was found in it 2*4 per cent, of red oxide of iron, 

 to which its colour was doubtless owing. It contains neither 

 potash nor soda, but simply 11-6 per cent, of lime. The 

 formula for its constitution is 



2 Al S 3 + Cal S 3 + 6 Aq ; that of common chabazite 

 2 Al S 2 + Cal S 3 + 6 Aq. 



The deficiency of alumina in the acadiolite, which amounts 



