CHAPTEE XXI 



THE METAMORPHIC AND PLUTONIC ROCKS. 



METAMOEPHIC EOCKS. Our chapter on Physical Geology 

 has so far tended to explain the character and origin of these 

 rocks, that a lengthened detail in this place would rather 

 embarrass than assist the student: a brief recapitulation of 

 their most important characters will suffice for the present 

 purpose. 



The best general idea, perhaps, which the student can 

 form of these formations, is to regard them as possessing an 

 intermediate position and character between the sedimentary 

 rocks above and the Plutonic rocks below, with both of which 

 they are intimately connected. They resemble the sedimen- 

 tary rocks in the circumstance that they exhibit marks of 

 stratification, many of the upper slaty beds passing into 

 overlying deposits of aqueous origin. On the contrary, they 

 are allied to the Plutonic rocks beneath, by the circumstance 

 of containing no organic remains ; some of the lowest beds 

 appearing to graduate into the underlying granite. 



THE MICA-SCHIST SYSTEM. Mica-schist is essentially 

 composed of mica and quartz, the two minerals being 

 disposed in alternate layers, forming laminated strata, which 

 are extremely wavy and contorted. The upper beds approach 

 the character of clay-slate, presenting laminae of mica, 

 chlorite, talc, and hornblende, together with limited beds of 

 crystalline limestone, iron ore, &c. ; the lower are of a more 

 quartzose character, consisting of quartz and mica, quartz 

 and chlorite, and quartz rock. The British localities are the 

 Highlands of Scotland and the north-west of Ireland. 



THE GNEISS SYSTEM. Eocks of this character form the 

 lower portion of the series. The gneiss itself is composed 



