28 



ZEOLITES. 



tration of the extent to which, a rock may become altered by infiltra- 

 tion. It is zeolitic substances dissolved out of the basalt and re- 

 deposited in the chalk of Antrim and Argyleshire which have imparted 

 a flinty hardness to that rock in those localities. The following table 

 includes some of the more interesting and important zeolites. 



COMPOSITION. 



LOCALITIES WHEKE FOUND. 



Apophylite 



Prehnite 



Thoinsonite 



Chabasite 



Stilbite 



Laumonite 



Heulandite 



Natrolite 



A red, yellow, or whitish 

 silicate of lime and potash, 

 with 1 6 per cent, of water 



Usually green. Silicate of 

 lime and alumina, with a 

 little iron and water 



White. Silicate of alumina 

 and lime, with a little soda 

 and 13 per cent, of water 



White or reddish. Silicate 

 of alumina and lime, with 

 a little potash and 20 per 

 cent, of water 



White or yellowish. Silicate 

 of alumina and lime, with 

 I per cent, of soda and 17 

 per cent, of water 



White. Silicate of alumina 

 and lime, with 15 per cent, 

 of water. Similar in com- 

 position to stilbite, but 

 while that mineral crys- 

 tallises in the prismatic 

 system this occurs in the 

 oblique system 



White or reddish. Silicate 

 of alumina and lime, with 

 1 5 per cent, of water. Ob- 

 lique system, but form of 

 crystals different to lau- 

 monite, and unlike that 

 species does not fall to 

 powder on exposure to air 



White or reddish. Silicate 

 of alumina and soda, with 

 10 per cent, water. Pris- 

 matic system 



Generally in cavities in amygdaloi- 

 dal rocks, chiefly in Iceland and 

 the Faroe Islands. In Tertiary 

 limestone at Puy de la Piquette 

 in Auvergne, near basalt. 



In cavities in basalt in Mull, Skye, 

 Salisbury Crags, Dumbarton ; 

 and in granite at Botallack, near 

 Land's End. 



In cavities of basalt at Kilpatrick, 

 in Scotland ; in lavas of Vesu- 

 vius ; and in phonolite at Dau- 

 bitz, in Bohemia. 



In cavities in basalt at the Giant's 

 Causeway, in Skye and Mull ; 

 and in phonolite. 



In cavities in basalt in Skye and 

 Arran ; and in granite, gneiss, 

 and schistose rocks in fissures. 



In cavities of basalt in Dumbar- 

 tonshire, and fissures in syenite 

 at Dresden. 



In basalt in Skye, at Campsie in 

 Dumbarton ; also in fissures in 

 gneiss and slates. 



In basalt of Giant's Causeway, 

 Hebrides, Rhine, &,c. 



