870 A TREATISE ON METAMORPHISM. 



From the impurities are apt to develop some of the heavy minerals of the 

 zone of anamorphism. Of these minerals mica is by far of the greatest 

 consequence, and of the micas muscovite is the most abundant. The 

 secondary mica always shows a well-developed parallel arrangement, with 

 its greater dimensions and readiest cleavage parallel to the greater dimen- 

 sions of the quartz. These mica-bearing rocks are schistose, micaceous 

 quartzites and micaceous quartzite-schists. While mica is the most frequent 

 of the minerals which form in connection with the schist-quartzites, other 

 minerals, such as tourmaline, actinolite, etc., may form, and if they become 

 important they may give mineralogical qualifiers to the names schist- 

 quartzite and quartzite-schist 



In some districts all stages of gradation from quartzites, through schist- 

 quartzites and quartzite-schists, may be seen. These stages may be best 

 observed in a region not too strongly folded on the crests and in the troughs 

 of the folds. At such places there is comparatively little readjustment, while 

 on the limbs of the folds, especially between the beds, the differential 

 movements are much more marked. Therefore on the crests and in the 

 troughs of the folds and in the center of the beds the less changed rocks 

 are found, while at the outer parts of the beds and on the limbs of the folds 

 schists may be formed. 



Q UARTZ-FELDSPAR-SAND FAMIL Y. 



The quartz-feldspar-sand family includes quartz-feldspar sands, arkose, 

 and schist-arkose or gneiss-arkose. 



QUARTZ-FELDSPAR SAND. 



Some fragmental formations are composed mainly of feldspar and 

 quartz, and such deposits may be called quartz-feldspar sands. The most 

 favorable conditions for their formation are those of disintegration of acid 

 feldspathic rocks, with comparatively little decomposition, and contiguity 

 to the sea. (See Chapter VI, pp. 496-501.) The more prominent rocks 

 furnishing material for the quartz-feldspar sands are the granite and syenite 

 families among the igneous rocks, and the -acid gneisses and schists among 

 the metamorphic rocks. The volume of these rocks is very considerable, 

 and the acid feldspars, which are among their essential constituents, are 

 slow to decompose. It follows that the quartz-feldspar sands occur in 

 considerable volume. Feldspars from intermediate and basic rocks are in 



