292 METAMORPHISM 



have never been experimentally made by dry heat, may be 

 readily compounded and crystallized with the aid of superheated 

 water. Furthermore, the presence of water diminishes the tem- 

 perature necessary to affect metamorphic changes ; and rocks 

 which require a heat of 2500 F. to melt them in the dry state, 

 will in the presence of water become pasty and viscous at 

 750 F. In contact metamorphism other mineralizing vapours 

 and gases play an important part. 



(4) Pressure is a necessity for any extensive metamorphism, 

 whether thermal or dynamic, to produce the necessary plastic 

 flow or shearing of the rocks, and to prevent the escape of the 

 steam and gases. Limestone heated in an open vessel becomes 

 quicklime (CaO), because the C0 2 is driven off at high tempera- 

 tures. Heated under pressure, the same limestone will crystal- 

 lize into marble. On a large scale, therefore, metamorphism can 

 be effected only at considerable depths ; for it is in the zone of 

 flowage that the most favourable conditions are to be found. 



It is believed by certain geologists that metamorphism may 

 proceed so far as to completely melt a sedimentary rock and 

 thus produce a magma which is indistinguishable from a typically 

 igneous one. Some have even maintained that the lavas now 

 ejected from volcanoes are but the final results of metamorphism. 

 These conceptions may possibly approximate the truth, but the 

 progress of investigation is at present leading away from them. 

 No instance is yet known which renders it necessary to assume 

 that a given igneous rock was made from melted sediments, and 

 the cases which have been relied upon to prove the hypothesis 

 have, for the most part, been shown not to require such an 

 explanation. On the other hand, certain metamorphic rocks 

 do form a common meeting-place for the igneous and sedi- 

 mentary classes, and, as we have seen, it is frequently impossible 

 to decide from which class a given metamorphic rock was origi- 

 nally derived. 



Owing to this uncertainty regarding their derivation, the meta- 

 morphic rocks of igneous origin are, to some extent, included 

 with those formed from sediments in the schemes of classification. 



