278 MICACEOtTS -SCHIST. 



These two, lj^e;the former^ pass into quartz 

 rock, and into gneiss.^ ^ 



D. A rock of similar construction, with this dif- 

 ference, that the mica is further disposed in a longitu- 

 dinal manner, so as to give the stone a fibrous appear- 

 ance on splitting. 



Many of this and of the preceding variety 

 admit of being polished, like the analogous spe- 

 cimens of quartz rock, into which they pass. 

 Those which are most compact and sparkling, 

 form the mineral known by the name of avantu- 

 rine; the most esteemed variety of which is of 

 a reddish yellow, or brown, colour. 



E. The mica imperfectly parall el and scaly^ being 

 bent round the grains of quartz, which are often of 

 large size ; that flexure, in the latter case, giving a 

 very peculiar character to the rock, particularly in the 

 transverse fracture. Thus, in some instances, it has 

 the general aspect of gneiss; in others, that of the 

 graywacke schist .which accompanies the clay slate of 

 the Highland border, 



It occurs hv that situation^ in Bute, and 

 elsewhere. 



