LITIIOLOGY. 223 



color and in proportion of ingredients ; but I think this description may- 

 be regarded as representing them all fairly. This is the novaculitc, or 

 oil-stone, that is so highly prized for sharpening tools. 



QUARTZ SCHIST (OUARTZITE). 



Almost all kinds of schists, by the elimination of the minerals which 

 characterize them, graduate into quartz schist. Very siliceous varieties 

 of mica schist, argillitic schist, chlorite schist, etc., stand between quartz 

 schist and these various rocks; and therefore this rock, which typically 

 is a pure schistose mass of quartz, possesses a variety of characters that 

 are given to it by its accessories, and which relate it to other rocks 

 that have been and are to be described. The rocks abound in the Con- 

 necticut valley and geologically related areas. They show very various 

 degrees of metamorphism ; and although, as a rule, their structure indi- 

 cates a complete recrystallization of the materials that formed the sedi- 

 mentary beds, still cases are not wanting where remains of the frag- 

 mental character are maintained. It is understood, that in this place the 

 rocks with the latter character are not considered, but are included 

 among the descriptions of half fragmental rocks that follow. In texture, 

 all grades occur between coarse granular and cryptocrystalline varieties ; 

 and in structure, all grades between very schistose and almost massive 

 rocks. 



The most common kind of quartz schist is micaceous. A white mass, 

 consisting of granular quartz, shows on its schistose cleavages flakes of 

 a glistening mica, which is usually all muscovite, and rarely is partially 

 biotite. This is in contrast to most of the micaceous rocks ; for though 

 a little alumina and potash may remain, usually but little iron exists 

 in such highly siliceous sediments. The reverse is, however, shown by 

 a specimen from Newcastle, in which a very fine-grained quartzite is 

 traversed by multitudes of black, parallel lines, which on investigation 

 are found to be made by the arrangement of minute grains of magnetite. 



A micaceous quartz schist from Hinsdale is composed of a very pretty 

 bluish opalescent quartz, and its mica is more fibrous than foliated. In 

 some other varieties, the presence of mica is only indicated by that glis- 

 tening lustre that characterizes the argillitic mica schists. 



