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CHAPTER XX. 



METAMORPHISM. 



SUBTERRANEAN heat has transformed to a certain extent strata of all 

 ages which were exposed to its action ; and thus the Lias shale of 

 Savoy, for example, approximates to the character of clay slate. In 

 such cases, we may indicate the alteration by Ly ell's term metamorphic, 

 and designate by it all those parts of aqueous strata which have been 

 transformed in structure or appearance by subterranean heat, or heat 

 developed by pressure applied since their deposition. All strata may 

 become metamorphic. 



Effects of Internal Heat. We have seen the effects produced by 

 plutonic rocks upon strata which they penetrate ; these effects depend 

 on the degree of heat communicated, and the substances operated on. 

 As examples of these effects we may name structural metamorphism, 

 molecular metamorphism, and chemical metamorphism : 



1. The consolidation of stratified rocks is exemplified in the in- 

 duration and contraction of shale, and in the development of new 

 faces or joints in it, which sometimes meet one another rhomboidally, 

 sometimes follow the columnar relations of the adjoining basalt, and 

 sometimes imitate slaty cleavage. 



2. The partial fusion or cementation of some part of the substance 

 of a rock, so as to agglutinate its grains and solidify and harden the 

 whole mass. Thus sandstone is converted to a granular quartz rock. 



3. The complete vitrification or recomposition of the rock, thus 

 converting shale into Lydian stone, and fine sandstone into a kind of 

 jaspar, or even into schists or igneous rocks. 



4. The complete rearrangement of the particles into granular or 

 crystalline forms, as in the instance of chalk in Ireland, and limestones 

 in Yorkshire, the Isle of Skye, and Carrara. 



5. The production of minerals not before existing in a distinct 

 state in the substances affected. The development of pyrites, asbes- 

 tus, anthracite, plumbago, garnet, &c., along the contact of igneous 

 and aqueous rocks is a very characteristic and general effect, which 

 appears to result from the actual transfer of the metallic and other 

 matter through the solid substance of the rock. If Yon Buch's 

 notion of the impregnation of strata with magnesia in the vicinity of 

 augitic rocks should be substantiated, it must be considered as a 

 remarkable example of such transfer of mineral material. 



