Crystalline Silidde Bocks. 171 



cature of the volcanic theory.* — It will be curious to see to 

 what mode of escape the advocates of the volcanic system will 

 have recourse with respect to the inconvenient discovery that 

 the agate-balls of Oberstein — masses derived from one of the 

 rocks regarded by them as pyrogenic — contain traces of or- 

 ganisms. 



4. Many of the relations of form and extent of these rocks 

 shew directly that such masses were originally sedimentary. 

 Comparatively very thin layers spread over an immense area 

 (of which the most striking examples are afforded by Iceland, 

 and perhaps in a still higher degree by Hindostan) could only 

 have been deposited by precipitation from water. 



If now, in addition to such positive criteria for the epige- 

 netic mode of formation of most of the crystalline granular 

 silicide rocks, we bring forward all of what is more of a nega- 

 tive character which can be adduced against those two hypo- 

 theses regarding their origin which have successively prevailed, 

 the unreasonableness of adhering either to the one or the 

 other of those will become still more apparent-t But, as re- 

 gards the volcanists, such a clinging to the view once adopted 

 must be considered as quite astonishing when we are told, as 

 I have already stated, that this is done in order to retain the 

 secure foundation of chemistry. This can be but a false colour 

 given to the matter. In the first place, chemistry here affords 

 no secure foundation whatever; to-day it proves that minerals 

 composed of silicides can be formed in the moist way, re- 

 garding which, but yesterday, it found that they are of pyro- 

 genic origin. In fact, the confessions of the chemists them- 



♦ It may be worth remarking, that if the trap strata really were at first " finely 

 levigated volcanic scoriae passing into sand" (Murchison, i, 75), which fell to the 

 bottom of the sea in which, at the same time, the whole silurian formation was 

 deposited, even this must tell in favour of Epigenism. If epigenism is rejected, 

 it cannot be said that such sandy masses became crystalline trap beds, without 

 inventing processes which never took place. 



I may also notice, that a German geologist, whe, in his own country, observed 

 many greenstones containing fossils, but, slavishly following Murchison, regards 

 them as volcanic tufis, nevertheless proves, that these spurious greenstones, as he 

 terms them, are, petrographically, completely the same as the others. 



t One of the most recent pieces of evidence against the pyrogenic origin of 

 certain granite masses and other similar rocks, is the occurrence in them of what 

 are termed pyrognomic minerals. 



