MICA SLATE. 77 
Fig. 27. 
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 
G 2 
