CHAPTER VII. 



MICROSCOPIC PETROGRAPHY. 



By John H. Caswell. 



SECTION I. 



QUARTZITES. 



Quartzite is a granular to compact mass of quartz, having in general a 

 splintery fracture and a white or gray color. 



Granular quartzite is made up of small to minute grains of quartz, 

 always possessing, however, a crystalline structure. It is difficult to dis- 

 tinguish it in certain phases from a crystalline sandstone; and in some cases 

 the quartzites resemble the sandstones, although the cementing material is 

 always wanting in the former. Again, some quartzites undoubtedly contain 

 rounded grains or pebbles of quartz. 



Schistose quartzite consists of a compact mass of quartz, which splits 

 with more or less ease into plates, having also a somewhat stratified struc- 

 ture. This is occasioned by the presence of crystals of white or brown 

 mica arranged in parallel layers. These rocks belong really to the schists, 

 and pass into mica-schist by preponderance of the mica. In the other 

 extreme they are pure quartzite, when the mica is present in such small 

 quantities that it may be considered a secondary ingredient The mica 

 crystals are often very distinctly grouped in parallel rows.* This structure 

 was observed in the quartzite from Rapid Creek [<o], which has also an 

 indistinct schistose cleavage, although there is no stratification visible to 

 the naked eye. The granular variety of quartzite is represented by the 

 rocks from Hall's Cut-off [38] and from Hall's Water [209], which are highly 



* Zirkel, Lehrb. d. PetrograpLie, I, p. 277. 



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