332 A TREATISE ON METAMORPHISM. 



sedimentary rocks, and of the schists and gneisses of sedimentary, origin, 

 which, after becoming schists and gneisses, have been subjected to agencies 

 of alteration in the zone of katamorphism. 



The zeolites develop from many minerals, but especially from the 

 plagioclase feldspars and from the leucites, sodalites, nephelites, etc. From 

 the plagioclases many of the zeolites are produced. The following may be 

 regarded as derived from anorthite : Thomsonite, gismondite, laumontite, 

 phillipsite, heulandite, epistilbite, stilbite, chabazite, and scolecite. The 

 following may be regarded as derived from albite: Analcite and natrolite. 

 Mesolite may be regarded as derived from albite and anorthite together. 

 Since the intermediate plagioclases contain both the anorthite and the albite 

 molecules, all of the above minerals may be derived from oligoclase, 

 andesine, labradorite, and bytownite, as may also mesolite. So far as 

 recorded the derivations of the zeolite minerals from the nephelites, leu- 

 cites, and sodalites are as follows: Thomsonite from nephelite and sodalite; 

 hydroliephelite from nephelite and sodalite; natrolite from nephelite, 

 sodalite, haiiynite, and noselite; analcite from leucite, nephelite, and soda- 

 lite; stilbite from haiiynite and noselite; chabazite from haiiynite and 

 noselite. The zeolites are also derived from other minerals as follows: 

 Analcite from laumontite, natrolite from apatite and chabazite, etc. 



It is hardly worth while to consider the occurrence of each of the 

 zeolites. It may be said, however, that the calcium-bearing zeolites 

 are most apt to form in the calcareous rocks, and the soda zeolites in 

 the rocks rich in soda. Thus stilbite, scolecite, and similar minerals are 

 likely to form in the calcareous rocks and limestones, while hydronephelite, 

 natrolite, and analcite, and similar minerals, are especialty likely to form 

 from the rocks containing soda feldspars and nephelites, leucites, and 

 sodalites. The sodium-calcium zeolites, such as thomsonite, mesolite, and 

 phillipsite, may occur in the calcareous rocks, such as the limestones, in 

 the igneous soda rocks, such as the nephelite rocks, and in the basalts and 

 similar rocks. 



In the rocks in which they occur the zeolites may be found (1) within 

 the mass of the rock as alteration products of the minerals; (2) in amyg- 

 dules, filling the vacuoles of the igneous rocks; and (3) in other openings 

 of all kinds, such as fractures, the pores of sediments, etc. 



