Reviews — Richthofen's System of Volcanic Rocks. 515 



their emission.^ How inconsistent this idea is with the highly 

 crystalline — indeed granitoidal — texture of many trachytes, grey- 

 stones, and basalts, even up to the very surface of the lava- 

 streams, while other lava rocks retain a glassy texture through- 

 out, I have shewn in another place ; ^ and until these views 

 are recognized, and brought to explain the mode of production 

 of the entire series of hypogene rocks, no sound progress 

 will, I believe, be made in this most important branch of geology. 

 That portion of the series which usually goes by the name 

 of the "plutonic" rocks, viz., the granites, syenites, older por- 

 phyries, greenstones, and Serpentines, owe perhaps their distin- 

 guishing characters, and the absence of any accompanying frag- 

 mentary ejecta, to their consolidation and cooling under greater 

 pressure at the bottom of deep seas, or at considerable depths within 

 the solid external crust ; by which the expansion and escape of the 

 contained water was prevented, and consequently those sub-aerial 

 explosions, which alone could give rise to the formation of ash, 

 pumice-tuff, or scoriee breccias. I think it improbable that vesicular 

 cavities, at least of any considerable magnitude, such as those which 

 characterize pumice or scorige, could be produced under any pressure 

 much exceeding that of the atmosphere. But even in the case of 

 these plutonic rocks, there is ample proof in the aspect and inter- 

 lacing of the several minerals composing them, especially in the 

 manner in which the least fusible mineral, quartz, has moulded itself 

 upon the more fusible felspar, hornblende, garnet, etc., that the 

 condition of their matter previous to its ultimate consolidation was 

 of the nature of what M. Daubree calls " aqueous fusion," rather than 

 of dry igneous fusion. In point of age there is no good reason for 

 denying that rooks of this character may be even now in process of 

 formation under the surface, — that is, of alteration by the varying 

 agencies of heat, pressure, internal movements, and chemical re- 

 actions. But of the more specially volcanic rocks — such, namely, as 

 by their position, and the accompaniments of ash, tuffs, pumiceous, or 

 scoriaceous breccias, rather than by their mineral composition alone, 

 shew themselves to have been erupted as lavas of greater or less 

 consistency, from volcanic subaerial vents, or in moderately shallow 

 water,— there can be no doubt that fi'om the very commencement of 

 the Laurentian era of sedimentary deposition, such eruptions have 

 been taking place on numerous points on the surface of the globe, — 

 generally in great linear bands attesting prolonged fissures in the 

 crust, which exhibit a remarkable parallelism on the whole to the 

 neighbouring elevated ranges of sedimentary strata, crystalline 

 schists, and plutonic axial outbursts, a significant coincidence of 

 direction not without a cause.'' 



To return for a moment to M. Eichthofen. It is sad to think of 

 the opportunity for extending our knowledge of volcanic formations 

 and phenomena which lay at his disposal during his residence 

 in California, if he had made good use of it bj'' a painstaking 



1 Lyell's Elements, ed. 1865, p. 596. - See Volcanos, p. 130. 



^ See Volcanos, p. 476. 



