514 Reviews — Richthofen's System of Volcanic Rocks. 



had resulted from preceding eruptions, and scattering its substance 

 in layers of ash. or breccia over vast areas of sea and land around. 



Though it has been over and over again demonstrated, and is admitted 

 by Humboldt, that rocks, the volcanic origin of which is unquestion- 

 able, exhibiting great varieties of mineral character from the most 

 highly felspathic Trachyte to the densest Augitic Basalt, are occasion- 

 ally found alternating with one another without any definite order, ^ 

 it is true that in the greater number of volcanic groups the more 

 felspathic rocks are found chiefly in the vicinity of the central 

 poiats of eruption, and in the form of bulky, often dome-shaped, 

 masses ; while the basaltic currents have extended to greater 

 distances, and affect the form rather of comparatively shallow and 

 spreading sheets : — this difference being no doubt due to the 

 generally superior fluidity and greater specific gravity of the latter, 

 at the time of their emission as lava from the volcano. A close 

 study of the volcanic rocks of central France, made in 1821, 

 convinced me that the bulky — indeed mountainous — trachytic bosses 

 of the Puy de Dome, Mont Dore, Cantal, and Mezen, having a 

 vertical thickness of from a hundred to more than 2,000 feet, owed 

 this " massiveness " to the great consistency, and consequently 

 sluggish movement, of the matter, when protruded in a spongy, 

 porous, intumescent state, from the eruptive vents. Where the 

 slopes were considerable, this trachytic lava had spread in thick beds 

 over the contiguous surfaces. Where these were less inclined, or 

 nearly level, it had accumulated in colossal " domes " over the 

 orifice ; as, for example, in the case of the Puy de Dome, a hay- 

 cock-shaped mass of porous trachyte, nearly 2,000 feet in height 

 above its base, which, as well as four or five other similar hills of 

 lesser magnitude, is seen to rise out of a crater within one of the 

 recent cinder-cones of the "chain of Puys." In the volume on 

 Volcanic Phenomena which I published in 1825, I attributed this 

 peculiarity of many trachytic lavas to the fact that they did not 

 issue from the volcanic vent in a state of dry igneous fusion, like 

 that of glass or metal melted in a furnace, but as a heated granular 

 or semi-crystalline paste or magma, to which the elastic tension of 

 minute particles of water (or steam) contained in the interstices of 

 their imperfect crystals gave a spongy tumefaction and partial 

 liquidity ; while the escape of this steam from the exposed surfaces, 

 through their numerous pores, occasioned the rapid consolidation 

 of the extruded mass. These views of the varying, but always 

 imperfect, liquidity of lavas at the period of their eruption, and the 

 agency of water in its production, were disregarded at the time by 

 geologists, and have not even yet obtained complete recognition, in 

 spite of their confirmation by the experiments of Daubree, and the 

 microscopical observations of Mr. Sorby. Even in standard works of 

 such authority as Lyell's Elements, the old doctrine is still taught of 

 the complete igneous fusion of all lavas, and the derivation of their 

 crystalline texture entirely from more or less " slow cooling " after 



1 Volcanos, ed. 1862, p. 128, et seq. 



