900 A TREATISE ON METAMOEPHISM. 



years have shown that the sericite of many of the so-called sericite-schists 

 is probably paragonite rather than muscovite; but the paragonite-schists 

 in many cases are not pelite-schists, but are derived from the alteration of 

 igneous rocks. As already seen, analyses of original muds show that, on 

 the average, the potassa is several times more abundant than soda, and this 

 fact explains why muscovite is so much more common in the slate-pelites, 

 schist-pelites, and gneiss-pelites than is paragonite. 



While the greater part of the potassium probably passes into muscovite, 

 where the potassa is abundant a portion of it in many cases undoubtedly 

 passes into feldspar, producing orthoclase and microcline, since these 

 minerals are rather abundant in many of the schist-pelites and gneiss- 

 pelites. All the important elements which go to make up the muds are 

 fully accounted for in the minerals quartz, biotite, muscovite, paragonite, 

 albite, orthoclase, and microcline. 



Where there is an excess of alumina beyond that required for the 

 formation of the minerals already mentioned, since silica is always in excess, 

 aluminum-silicate minerals form, and thus is produced the andalusite- 

 sillimanite-cyanite series. < The development of these minerals accounts for 

 a part of the material of the kaolin, which by dehydration and separation 

 of silica may pass directly into any of the aluminum-silicate minerals, 

 according to the reaction: 



H 4 Al 2 Si 2 9 = Al 2 Si0 5 + Si0 2 + 2H ,0, 



with a decrease in volume of the quartz and aluminum-silicate mineral of 

 from 25.40 to 26.28 to 31.61 per cent, depending upon whether andalusite, 

 silliinanite, or cyanite, respectively, be formed. Ordinarily the andalusite, 

 having the lowest specific gravity, forms when the metamorphism is 

 moderate, as in the slates. Silliinanite, having a higher specific gravity, is 

 likely to form where the metamorphism is more intense; and where the 

 metamorphism is most intense the heaviest of the aluminum-silicate minerals, 

 cyanite, may develop. This fully conforms to the principle explained on 

 pages 363-365, 683-685, that in proportion as a rock is deeply buried and 

 metamorphism is profound, minerals of high specific gravity form. These 

 three minerals in the inverse order mentioned are described by Emerson 

 as occurring peripherally to great batholiths of granite in western Massa- 

 chusetts. (See pp. 717-718.) 



