1130 A TREATISE ON METAMORPHISM. 



which come also metals in various combination. Where silicic acid and 

 the salts of the metals thus meet it is certain that the silicates form exten- 

 sively, according to the law of mass action that the most abundant com- 

 pounds are most, likely to be precipitated, and in consequence of the law 

 that the most insoluble compounds are likely to be thrown down. 



Of the silicates in fissure veins Lindgren regards the following of 

 sufficient importance to be cited: Muscovite, biotite, chlorite, pyroxene, 

 amphibole, garnet, epidote, orthoclase, albite, tourmaline, topaz, kaolinite, 

 and zeolites." 



Lindgren's description of the occurrence of these minerals shows that 

 the hydrous silicates are dominant. While a considerable number of the 

 anhydrous silicates form, they are very subordinate in amount. The signifi- 

 cance of these facts is discussed on pages 1055-1060. The abundance 

 and prevalence of the hydrous silicates are beautifully illustrated by the 

 copper deposits of the Lake Superior region. Here there has been scarcely 

 any discoverable effect of descending waters, as recent g'lacial erosion has 

 entirely removed the belt of weathering of pre-Grlacial time. Pumpelly 

 gives the following list of hydrous silicates as occurring in this region: 

 Analcite, apophyllite, chlorite, datolite, delessite, epidote, laumontite, 

 prehnite. 1 This list has since been supplemented by Hubbard, the following 

 being added: Chabazite, chlorastrolite, chrysocolla, harmotome, heulandite, 

 lennhardite, mesotype, natrolite, pectolite, saponite, stilbite, thomsonite/ 

 While in the Lake Superior copper deposits the .dominance of the hydrous 

 silicates is clear, the gangue minerals are not restricted to them, but com- 

 prise anhydrous minerals — orthoclase and wollastonite being mentioned. 



It is to be noted that the silicate gangue minerals retain even some 

 of the alkalies, i. e., sodium and potassium. Sodium is found in analcite, 

 chabazite, chlorastrolite, mesotype, natrolite, pectolite, stilbite, and thom- 

 sonite; and potassium is found in apophyllite, chabazite, harmotome, and 

 orthoclase. By the union of the alkalies with other bases and with silica it is 

 possible to produce compounds insoluble in the liquids present, and where 

 this could take place as demanded by the laws of chemistry, it has occurred, 



« Lindgren, Waldernar, Metasomatie processes in fissure veins: Trans. Am. Inst. Min. Eng., vol. 

 30, 1901, pp. 607-615. 



' J Puropell}', Raphael, The paragenesis and derivation of copper and its associates on Lake Supe- 

 rior: Am. Jour. Sci., 3d series, vol. 2, 1871, pp. 254-2.55. 



''Hubbard, L. L., Macroscopic minerals of Michigan: Eept. State Board Geol. Survey Michigan, 

 1891-92, p. 176. 



