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CHAPTEE XII. 



NATURE AND ORIGIN OF VOLCANIC ENERGY. 



Continuity of Ancient and Modern Phenomena. The resemblance 

 between the stratified rocks now forming, and those which were 

 deposited in ancient waters, has already become so manifest that, in 

 most cases, we are compelled to use the same mineral names to 

 characterise them, no matter what may have been the period at which 

 they were formed. And when we turn our attention from the 

 aqueous to the igneous rocks, exactly the same law will be found to 

 govern their existence. The ancient forms of volcanic rocks consist 

 of the same minerals as those which have been poured out from 

 active or existing volcanoes. And although Ave are able to prove that 

 the cores of ancient volcanoes, in which the rocks have cooled slowly 

 under great pressure, can be so identified as to demonstrate their 

 existence during all periods of past time, yet the variations in mineral 

 composition, and even in chemical composition, of these granites 

 and granitic rocks are small. We learn that the positions in which 

 volcanoes formerly existed have from time to time been changed, and 

 although volcanoes were recurrent in Britain during the Primary 

 Period and the Tertiary Period, the Secondary Ages, except in the 

 trias of East Devon, were as free from such disturbances as are British 

 lands and waters at the present day. We therefore are led to inquire 

 into the nature and origin of volcanic energy. 



Nature of Volcanic Energy. This, though usually manifested in 

 an eruption, may exhibit itself in a multitude of other forms known 

 to the practical geologist. We shall find the most important questions 

 in connection with volcanoes to be, first, an explanation of the heat 

 of the rocks ; secondly, the source of the eruptive power which 

 results in volcanic activity ; and thirdly, we must account for the 

 nature of the materials which are ejected from the volcanic throat, or 

 consolidate beneath a mountain mass. 



Internal Heat. There are many ways of accounting for the heat 

 of the rocks, but unfortunately they are all more or less hypothetical ; 

 and we find it necessary to adduce evidence if any view on this subject 

 is to be accepted. A belief in the original igneous fusion of the 

 earth was long the favourite doctrine. It is, however, quite possible 

 that there may never have been any igneous fusion in the common 

 acceptation of the term ; and although the surface may have been 



