478 Geological Society : — 



And generally of other great craters, ancient or modern, such as 

 Palma, Santorini, the Val de Bove, &c., he considers that no argu- 

 ment in favour of their having any other than a similarly " eruptive" 

 origin can be derived from the fact of their dimensions exceeding 

 those of the crater of Vesuvius. The authentic accounts of enor- 

 mous quantities of ejected pumice, scorire, or ashes thrown out by 

 many eruptions from Polynesian or American volcanos, reaching to 

 distances of above a thousand miles, and of course spreading over 

 the whole intermediate space, to a thickness sometimes of 10 or 12 

 feet at more than twenty-five miles from the volcano, would amply 

 account for the dispersion, by explosive eruptions, of the contents of 

 the largest craters ever observed. 



At the same time the author guards himself from being supposed 

 to have ever denied that some amount of elevation has taken place 

 in the external cone of a volcano through the occasional injection of 

 lava fi-om within into rents broken across its framework, and hard- 

 ened into dykes, M'hich may be called a process of gradual disten- 

 sion. This, in fact, was suggested by him in 1824. All he con- 

 tends against is the theorj"^ of Von Buch, that volcanic mountains 

 are the result of the elevation of nearly horizontal beds of lava and 

 conglomerates bj' some sudden expansion. He maintains, on the 

 contrary, that the growth of a volcano by accretion, through erup- 

 tive ejections on the exterior, and partial distension from within, is 

 a gradual, though intermittent, normal process, which may be 

 watched almost like the growth of a tree. 



The author next referred to the opinion published by him in 

 1824, that the liquidity of the stony and crystalline lavas (excluding 

 the vitreous varieties) at the time of their protrusion, is owing, not 

 to complete fusion, but to the entanglement between their com- 

 ponent granular or crystalline particles of some fluid, chiefly water, 

 at an intense heat of course, but unvajjorized by reason of the ex- 

 treme pressure to which they are subjected while beneath the earth, 

 and escaping in vast bubbles of steam, when, by the opening of a 

 fissure of escape, its discharge is permitted, and also by a kind of 

 exudation through the pores and crevices of the expelled lavas as 

 they cool. 



The author originally extended this theory of the combination of 

 aqueous with igneous agency in lavas to all the crystalline plutonic 

 rocks, which he considered to be derived from a mass existing beneath 

 the crust of the globe under the above circumstances, in a state of 

 extreme tension, such as on the occurrence of any sufhcient local 

 relaxation of the restraining pressure from above, or increase of tem- 

 perature from within, must occasion its partial intumescence, and the 

 consequent fracture and elevation of the overlying rocks, with or 

 without extravasations of the intumescent crystalline matter through 

 rents, in the form either of volcanic eruptions, or the protrusion of 

 the granitoidal axes of mountain chains. 



These ideas on the character of the liquidity of lavas and the hy- 

 pogene crystalline rocks, promulgated by the author in 1824-26, 

 were considered unchemical at that time and little regarded. They 

 have, however, of late been reproduced by M. Scheerer of Freiberg 



