OF VOLCANIC ROCKS. 69 



why they are untenable, with our present state of knowledge, are too obvious to be 

 made an object of a more detailed explanation. 



More numerous and more obvious objections may be raised, chiefly from a geo- 

 logical point of view, against those theories of the second class, according to which all 

 the rocks under consideration, with the exception of those lavas which we actually see 

 being ejected from volcanoes, would have derived their origin exclusively from meta- 

 morphism in situ ; be it that the change is supposed to have been mainly of a chemical 

 nature, and effected on or beneath the surface, by the action of water alone containing 

 certain substances in solution, or that refuge be taken to the cooperation of super- 

 heated water and pressure at a great depth. A large amount of positive facts as well 

 as of sagacious reasoning have been applied in their defense ; but viewed in the light 

 of those observations which present themselves continually to the geologist' in the 

 field, in evidence of an essentially intrusive and extrusive nature of those rocks, the 

 premises on which argumentation is based must appear extremely deficient. Also 

 will the reasons which we shall adduce against the leading theory of our day, be appli- 

 cable a fortiori against the assumption of an origin of our "eruptive" rocks only by 

 metamorphism in situ. Before entering upon that leading theory, we have still to 

 mention the existence of a number of others, which, though acknowledging the proba- 

 bility of an oi-igin of all lava and " trap-rocks" from the liquid interior of the globe, 

 assume that granite, syenite, diabase, diorite and certain porphyries are so-called hypo- 

 gene rocks, that is, have originated by metamorphism of sediments in situ. Against 

 these theories may be raised the collective objections which apply severally to the 

 others. 



The obvious objections which may be made to the theories hitherto mentioned 

 have given more and more ascendency to another doctrine, which we may designate 

 as the metamorphic theory of eruptive rocks, and which owes its great influence upon 

 modern geology to the fact of its starting from a certain number of established geo- 

 logical facts and lithological observations, and from the results of experiments. It is 

 eminently a theory of the second class. No arguments against it will be more potent 

 than those which prove the fallacy of the basis of all the theories of this class, as 

 they will show that we must look for the origin of eruptive rocks altogether in a dif- 

 ferent direction from that followed by them. 



The metamorphic theory is essentially to the purport, that all eruptive rocks, 

 whether of recent or of ancient age, whether ejected by volcanic action or carried into 

 their present position without any sign of the latter, were originally sedimentary rocks 

 rendered liquid by the cooperation of heat, pressure, and water. It is supposed that 

 these rocks, by the continued superposition of immense masses of sediment, had ar- 

 rived at a great depth under the surface, where the agencies mentioned would cooper- 

 ate to modify and transform their state of aggregation, resulting either in a molecular 

 change alone, or in their conduction into a state of fusion. In the former case, the 

 sediments would be simply metamorphosed, while in the latter, they would either 

 crystallize in depth, with a total loss of their original structure, and form "plutonic," 

 or "hypogene," or "indigenous " rocks, or be forced upwards through fractures, and 

 solidify partly in the conducting channels and partly on the surface, when they would 



N (107) 



