PART I. CHAPTER X. 



131 



Granite Metamorpliic Rocks. 



canic rocks were simply different parts of one great series, we 

 ought to find in mountain chains volcanic dikes passing upwards 

 into lava, and downwards into granite. But we may answer, 

 that our vertical sections are usually of small extent ; and if we 

 find in certain places a transition from trap to porous lava, and 

 in others a passage from granite to trap, it is as much as could 

 be expected of this evidence. 



The prodigious extent of denudation which has been already 

 demonstrated to have occurred at former periods, will reconcile 

 the student to the belief, that crystalline rocks of high antiquity, 

 although deep in the earth's crust when originally formed, may 

 have become uncovered and exposed at the surface. Their ac- 

 tual elevation above the sea may be referred to the same causes 

 to which we have attributed the upheaval of marine strata, even 

 to the summits of some mountain chains. But to these and 

 other topics, I shall revert when speaking, in the second part, 

 of the relative ages of different masses of granite. 



CHAPTER X. 



METAMORPHIC ROCKS. 



General character of metamorphic rocks — Gneiss — Hornblende-schist — Mica- 

 schist — Clay-slate — Quartzite — Chlorite-schist — Metamorphic limestone — Alpha- 

 betical list and explanation of other rocks of this family — Origin of the meta- 

 morphic strata — Their stratification is real and distinct from cleavage — On joints 

 and slaty cleavage — Supposed causes of these structures — how far connected 

 with crystalline action. 



We have now considered three distinct classes of rocks : first, 

 the aqueous, or fossiliferous ; secondly, the volcanic ; and, 

 thirdly, the plutonic, or granitic ; and we have now lastly to exa- 

 mine those crystalline strata to which the name of metamorphic 

 has been assigned. The last-mentioned term expresses, as before 

 explained, a theoretical opinion that such strata, after having 

 been deposited from water, acquired by the influence of heat and 

 other causes a highly crystalline texture. 



These rocks, when in their most characteristic or normal state, 

 are wholly devoid of organic remains, and contain no distinct 

 fragments of other rocks whether rounded or angular. They 

 sometimes break out in the central parts of narrow mountain 

 chains, but in other cases extend over areas of vast dimensions, 



