32 J. CLIFTON WARD ON THE GRANITIC, GRANITOID, AND 



above would be all the more likely to occur, the regularity of meta- 

 morphic selection along bands masking the very existence of great 

 metamorphism. This selective metamorphism, in whatever kinds of 

 rock it shows itself, will be much facilitated by the fact that very 

 generally the strike of the rocks corresponds with the main axis of 

 metamorphism, both being the result of lateral pressure acting at 

 right angles. It is important to remember that in the case of such 

 a series of beds as that just now supposed, while all may have been 

 subjected to the same general amount of metamorphic action, some 

 will show that metamorphism outwardly far more than others, 

 while at the same time the various beds may have been changed 

 in different ways according to their texture and composition. 



4. Classes of Metamorphism. — In conclusion, it may be as well 

 just to summarize the principal conditions under which rocks occur 

 with regard to metamorphism. 



(1) That state in which igneous fusion is the most important or 

 conspicuous element. This is seen in the case of lava flowing from 

 a volcanic vent. The presence of water, however, is distinctly 

 recognized; and its sudden expansion into vapour with explosive 

 violence is acknowledged as the chief agent in those paroxysmal 

 eruptions which have sometimes resulted in the formation of crater- 

 lakes of great size. 



(2) A state of aqueo-igneous fusion, occurring at a much~greater 

 depth than the last, and reaching only a dull red heat as a maximum. 

 In this state we have every reason to suppose the granitic magma to be 

 which may now be solidifying at various remote spots beneath our 

 feet. The metamorphism due to moist heat under pressure, at a 

 temperature not exceeding 400° C, may show itself in the complete 

 changing of the mineralogical and physical aspects of a rock, and in 

 the production of such anhydrous silicates as felspar, augite, &c, 

 which when produced in the dry way require a very much higher 

 temperature for their formation (Daubree *). Thus, while granite 

 may be solidifying at a certain spot, the rocks all around may 

 become granitoid and porphyritic. 



(3) A state in which the rocks are permeated by water at a con- 

 siderably lower temperature than 400° C. The metamorphism in 

 this case, though not so generally conspicuous, is still highly im- 

 portant. Thus, the water with diminished temperature can now 

 decompose those very felspar crystals which at a higher temperature 

 it could give rise to; and minor changes innumerable may take 

 place throughout the rock subject to it. 



Discussion. 



Mr. ftuTLEY partly agreed with the author in his views as to 

 metamorphism, which seemed to him to be to a great extent a 

 bathymetrical question. Granite had no doubt in many instances 

 been formed from other recks subjected to great heat accompanied 



* This author has shown that the action of water-vapour is similar to that 

 of liquid water. 



