MICA SLATE. 



Fig. 27. 



77 



Fragment of gneiss. Section at right angles to planes of 

 stratification. 



Gneiss, like granite, often contains hornblende ; 

 the only difference between them consists in the 

 stratified and slaty structure of the gneiss. 



Mica Slate. 



Gneiss becomes mica slate when the mica be- 

 comes abundant and the other ingredients small. 

 Indeed, mica slate is essentially composed of mica 

 and quartz intimately combined, the feldspar either 

 being in small quantity, or absent, or occurring in 

 irregular masses. The colour of mica slate is gen- 

 erally a silvery or pearly white, inclining to a blue- 

 ish gray or a light green; sometimes it is nearly 

 black. It has a slaty structure, like gneiss, and 

 is often waved and contorted, and divided by thin 

 laminae of quartz. It sometimes contains beds and 

 laminae of crystalline limestone, or is intermixed 

 with serpentine. It also contains beds and veins 

 of metallic ores. It is not unusual to find granite 

 passing into gneiss, gneiss into mica slate, and mica 

 slate into clay slate, all in the same neighbourhood. 

 Talcose and chlorite slate appear to be different mod- 

 ifications of the same mineral substances ; in the 

 former, the structure is laminated, and has a soapy 

 or unctuous feel ; in the latter it is minutely granu- 

 lar ; the prevailing colour of both being green. A 

 slight modification converts talcose or chlorite slate 



