76 KICIITHOFEN NATURAL SYSTEM 



eruptive rocks of the present era. The metamorphic theory suggests that enormous 

 volumes of those masses which it supposes to have been rendered liquid by meta- 

 morphism, and small portions of which were occasionally emitted to the surface, did 

 solidify below ground with a total loss of their previous structure, and may arrive at 

 the surface by elevation and denudation. Granite is supposed to have been in all 

 cases a hypogene rock, generated by metamorphism in those same positions in which 

 it is found at the present time, and to be still formed continually in the same way ; 

 though it is admitted that the tension which may have attended its former state of 

 liquidity, may have caused it to ramify into rents and fissures of the overlying rocks. 

 We have, of course, to assume the existence of an immensely greater amount/ of hypo- 

 gene matter corresponding in composition to all the different eruptive rocks, but buried 

 so deep beneath the present surface, that, possibly with the exception of a few granite 

 masses, it never does, nor ever will, form part of it. We have, for this reason, to con- 

 sider all non-foliated crystalline rocks made up of silicates, and which are visible on the 

 surface, as having been removed far from their original seat, and therefore as being 

 eruptive. It is a matter of course that only a very small portion of all matter protrud- 

 ed from the depth would be ejected to the surface, as by far the greater portion would 

 solidify within the channels of ejection. Granite occupies frequently the latter posi- 

 tion quite distinctly. But if we consider those large accumulations of the same rock 

 covering hundreds of square miles, we can only explain them in two ways. Either 

 they must occupy their primeval position, as is supposed by the adherents of the 

 metamorphic doctrine, or they must have been ejected and spread in a liquid state 

 over the underlying rocks. The objections against the first supposition are, the adapta- 

 tion of granite to the law of Bunsen, and the fact already noticed, that it overlies 

 the upturned edges of strata ; the objection against the other is the peculiar mineral 

 character of granite. But we should always bear in mind that our knowledge in re- 

 gard to the conditions required for producing any certain kind of mineral character 

 within a compound of silicates of any certain composition is very limited, as is illus- 

 trated, among others, in the cases of propylite or dolerite ; and the inferences based 

 on subjects which are imperfectly known to us should, in geology, always be made 

 subordinate to those which we can positively establish by observations. It is not on 

 the ground of the latter that a hypogene origin is ascribed to all granite, but on the 

 ground of its mineral characters. It is said that granitic and volcanic rocks offer quite 

 generally a different appearance, and that the texture of the former indicates crystal- 

 lization under great pressure, while the volcanic rocks were evidently solidified on the 

 surface. As regards the first assertion, such rocks only should be compared together 

 as have a similar composition, that is, granite with rhyolite, or diabase and augitic 

 porphyry with basalt ; not, as is ordinarily done, granite with phonolite or basalt. We 

 referred already to the similarity in character between certain varieties of rhyolite and 

 granite. It is corroborated by the microscopic examination of these rocks by Ferd. 

 Zirkel, from whose memoir 26 I quote the following passages : " Quartziferous trachytic 



26 Dr. Ferdinand Zirkel, Mikroskopische Gesteinsstudien, in Sitzungsberichte der Kais. Acad. der Wissenschaften zu 

 WVen, DO/. 47, 1863. 



(114) 



