OF VOLCANIC ROCKS. 73 



rocks by the destruction of which they originated, and should have had a tendency 

 towards an average between them in regard to chemical composition. .If such sedi- 

 ments were afterwards rendered liquid by metamorphic processes, and either carried to 

 the surface as eruptive rocks, or solidified beneath it, so as to form part of it only after 

 ages of denudation — if they were then again partially disintegrated, re-deposited and 

 reerupted, and so forth, down to our time, the final result should be an ever-increasing 

 uniformity in composition of both eruptive matter and sediments. It may be objected, 

 that this tendency towards uniformity must have been checked by the formation 

 of chemical sediments, as by them a portion of certain elements was eliminated, and 

 must have become almost irrecoverably lost for further participation in the supposed 

 revolving process. The substances prevailing in chemical deposits are lime, soda and 

 magnesia. Recent eruptive rocks should, therefore, besides showing a uniformity of 

 composition, be somewhat deficient in those substances, and contain in proportion 

 more of silica, alumina and potassa. The facts are exactly reverse. Not alone has the 

 variety of eruptive rocks rather been constantly increasing ; but, among those most 

 recent in origin, such are greatly predominant as contain lime, soda and magnesia 

 in larger proportion than ancient rocks, while they are relatively deficient in silica, 

 alumina and potassa. We arrive by this last line of argument at the conclusion, in 

 the first place, that the present variety of rocks, especially of those of eruptive origin, 

 can in no way be explained by, but is contradictory to, the assumption that they orig- 

 inated by the repeated destruction and reformation of the material which constituted 

 the surface of the globe in the most ancient time. From this it follows, in the second 

 place, that, even supposing that the theory should be admissible for the majority of 

 eruptive rocks, entirely new matter must have come repeatedly to the surface, in order 

 to replace the constant loss, as it were, of substances such as lime and soda. Eruptive 

 rocks ascending from sources beneath the deepest sediments are the only means imag- 

 inable for the performance of this function. The enormous extent to which matter 

 must have been supplied from that source is especially evident, if it is considered that 

 there must have been a time when no sediments existed on the face of the globe, and 

 that the disintegration of preexisting matter is the prime condition to their formation. 

 The entire mass of the sediments which make up the exterior shell must, therefore, 

 have been derived from the destruction, partly of the original crust which solidified on 

 the globe, and partly of those rocky masses which protruded through that crust to the 

 surface. It is eminently probable that the mass of matter derived from the latter 

 source exceeds very far that which is due to the first. This consideration shows 

 that it is hardly possible for us to form an adequate idea of the vast importance which 

 the periodical emission" of rocky matter from places beneath the primordial surface must 

 have had in the history of the formation of the crust of the globe. 



We may thus start from any point of view within the range of positive 

 knowledge, and we shall find that there is not one line of argumentation which does 

 not go to show that the doctrine of the origin of eruptive rocks by the metamorphism 

 of sedimentary matter is irreconcilable with the most prominent established facts. Not 

 only does it fail completely to explain them, but with most of them it is in obvious 

 contradiction. But as a metamorphic origin has been proved to evidence in regard to 



(ill) 



