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 z stu; 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 codperation 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- 
eable a fortior? against the assumption of an origin of our ‘‘ eruptive” rocks only by 
metamorphism in sttw. 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 origin 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 iz 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 codperation 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 codper- 
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) 
