METAMORPHIC STUDIES 6oi 



distinctive, and therefore are used in naming the rock, but in 

 abundance they must be regarded as distinctly minor and 

 accessory constituents as compared to the principal constituents 

 before named. 



In the progressive changes from a mud or clay to a shale and 

 in turn to a slate or schist, there is an increase in the percentage by 

 weight of mica and chlorite with relative decrease of other con- 

 stituents. Muscovite or sericite makes up from a third to a half 

 of mica slates, according to Dale,^ and in a phyllite may be in even 

 larger proportion, whereas in the original rock the percentage was 

 but a small fraction of this amount. There is corresponding 

 decrease in kaolin, feldspar, and certain ferromagnesian minerals. 



Where a sandstone or quartzite becomes schistose there is an 

 increase of mica, with corresponding decrease of other constituents. 

 Certain specific instances of development of sericite schists in shear 

 zones crossing the bedding of a quartzite have shown elimination 

 of the greater part of the quartz of the original rock, due to solution, 

 probably aided greatly by minute granulation. 



A limestone or a marble may become schistose purely by pro- 

 cess of granulation, which involves no essential development of 

 new minerals. But more often there is a striking development of 

 amphibole and other silicates with a corresponding diminution in 

 calcite. Silicated marbles, resulting from metamorphism, are too 

 well known to need further description. 



A granite, by metamorphism during rock flowage, may become 

 a mica schist or mica gneiss. Mica is increased at the expense of 

 the feldspar and some of the ferromagnesian minerals. 



A basic igneous rock, by dynamic metamorphism, may become 

 a chlorite or hornblende schist or gneiss, the increase of chlorite 

 or hornblende being counterbalanced by decrease in feldspar and 

 pyroxene. 



The average igneous rock, according to Clarke,^ has about 

 60 per cent feldspar, 28 per cent ferromagnesian and accessory 



' T. Nelson Dale, "Slates of the United States," Bidl. 586, U.S. Geol. Survey, 1914, 

 p. 20. 



2 F. W. Clarke, "The Data of Geochemistry," Bull. 4gi (2d ed.), U.S. Geol. Sur- 

 vey, 1911, p. 398. 



