CHAPTER XIV. 

 THE METAMORPHOSIS AND DESTRUCTION OF IGNEOUS ROCKS. 



EOLOGICAL history is a history of the formation, metamorphosis and 

 destruction of rocks. An igneous rock is no sooner formed than it is 

 liable to be modified or destroyed by various agencies. In the previous 

 chapters we have frequently had occasion to refer to the rock -modify ing and 

 rock-destroying agencies so far as particular rocks are concerned. All that 

 remains to be done in the present chapter is to treat of the effects of 

 these agencies from a somewhat general point of view. For convenience 

 of reference we may roughly classify the more important of them 

 as follows : 



SURFACE AGENCIES. 



1. Thermal waters, fumarole- and solfataric action. 



2. Normal surface agencies depending on change of temperature, 



rain-water more or less charged with carbonic acid, organic 

 acids derived from the soil, oxygen, &c. 



DEEP-SEATED AGENCIES. 



1. Percolating waters ; effects of gaseous exhalations deep-seated 



fumarole-action &c. 



2. Contact-metamorphism. 



3. Dynamic metamorphism.W 



Steam, sulphuretted hydrogen, hydrochloric acid and other gaseous 

 substances are evolved during the final stages of volcanic action. Sulphu- 

 retted hydrogen may become oxidized by contact with the air ; thus gn 7 ing 

 rise to sulphurous and sulphuric acids. These gaseous exhalations which 

 are especially marked in fumaroles and solfataras often produce important 

 effects upon the volcanic rocks. Thus, the well-known solfatara near 



(1) Of the various expressions pressure -metamorphism, regional metamorphism, 

 mechanical metamorphism and dynamic metamorphism which have been applied to the 

 change due to the deformation of rock-masses, the last appears to be the most appropriate. It 

 has been used in the later portions of this work in preference to the expression regional metamor- 

 phism which was adopted in the introduction. The objection to the first expression is that 

 pressure alone produces no effect on rocks work must be done upon the rocks before change 

 takes place ; the objection to the second is that the action is sometimes local ; and the objection 

 to the third is that it conveys the idea that chemical changes do not take place. The expression 

 dynamic metamorphism is free from all these objections. It emphasizes the fact that work is 

 done upon the rocks in the act of metamorphism, it does not exclude molecular changes by 

 implication and it is equally applicable to local and regional effects, 



