METAMORPHISM OF SHALES. 899 



Development of minerals of schists and gneisses. -The pi'OCeSSeS wllidl 1'eSult ill the 



formation of pelite-slate from shale when carried to completion produce 

 pelite-schist (PI. XI, C) or pelite-gneiss. All stages of the change may 

 be seen. We see, besides the replacement of the feldspar by quartz and 

 mica, the granulation and recrystallization of the quartz and the feldspar. 

 The process is essentially the same as the development of slate, except that 

 the destruction of the clastic quartz and feldspar grains is greatly promoted 

 by mashing. The process takes place in perfection in proportion as the 

 rock is fine grained. Thus a shale may completely change to pelite-schist, 

 while an interbedded coarse-grained grit may still reveal evidence 'of its 

 fragmental origin. In the same way a conglomerate may have its matrix 

 completely changed to a schist and the resistant pebbles be merely 

 deformed; but in the case of extreme alteration the most resistant and 

 largest pebbles and bowlders are entirely destroyed. (See pp. 858-859.) 



In the typical schist-pelites and gneiss-pelites the chief minerals present 

 are quartz, muscovite, and biotite. (PL XI, C.) This is a natural con- 

 sequence of the composition of the shales. The orthoclase and microcline 

 feldspars, with the probable addition of alumina from the background, 

 in most cases recombine into quartz and muscovite. (See pp. 254-255.) 

 This uses up much of the potassa, alumina, and silica. Where magnesium 

 and iron are also available biotite is formed. (See p. 255.) Usually some 

 residual feldspar remains or is produced by recrystallization, and the 

 amount of this material is likely to be great in proportion as soda was 

 present in undecomposed plagioclase. 



Since silica and alumina are abundant, an acid plagioclase is most 

 likely to form. Consequently albite is the most frequent plagioclase of 

 the schist-pelites and gneiss-pelites. If the amount of soda be considerable, 

 the albite may be so abundant as to be a chief constituent. Such an albite- 

 pelite- gneiss is beautifully illustrated by the Hoosac formation of Hoosac 

 Mountain, Massachusetts, described by Wolff." (PL XI, D.) In some 

 cases where the soda is unusually plentiful, a part of the soda with alumina 

 may pass into the soda-mica paragonite. Indeed, soda may more frequently 

 go into paragonite in the schist-pelites and gneiss-pelites than has been 

 supposed, for the close microscopical and chemical examinations of recent 



"Pumpelly, Raphael, Wolff, J. E., and Dale, T. Xelson, Geology of the Green Mountains in 

 Massachusetts: Mon. U. S. Geol. Survey, vol. 23, 1894, pp. 59-64. 



