118 



Prof. J. Prestwich. 



generally accepted is that of Mr. Poulett-Scrope. As formulated by hira 

 in the successive editions of his standard work on " Volcanos,"* " the 

 main agent in all these stupendous phenomena — the power that breaks 

 up the solid strata of the earth's surface, raises, through one of the 

 fissures thus occasioned, a ponderous column of liquid mineral matter to 

 the summit of a lofty mountain, and launches thence into the air, some 

 thousand feet higher, with repeated explosions, jets of this matter and 

 fragments of the rocks that obstruct its efforts — consists unquestionably 

 in the expansive force of some elastic aeriform fluid struggling to escape 

 from the interior of a subterranean body of lava, i.e., of mineral matter 

 in a state either of fusion, or at least of liquefaction at an intense 

 temperature. This body of lava is evidently, at such times, in igneous 

 ebullition "\ He further explains that the rise of lava in a volcanic 

 vent is occasioned "by the exparsion of volumes of high pressure 

 steam generated in the interior of a mass of liquefied and heated 

 mineral matter within or beneath the eruptive orifice," so that the 

 vapour reaches the external " surface in a state of extreme condensa- 

 tion and entangled in the liquid lava which rises with and escapes 

 outwardly, just as any other thick or viscid matter exposed to heat 

 from beneath in a narrow-mouthed vessel boils up and over the lips of 

 that aperture." 



I might have felt some doubt as to the exact meaning of this 

 passage, especially as the author proceeds to remark, that " at what 

 depth those volumes of vapour are generated may be a question," but 

 as he goes on to observe, " that the tendency to vaporisation must 

 everywhere occasion an extreme tension" or expansive force through- 

 out the mass, " only restrained by the enormons weight and cohesion 

 of the superincumbent rocks," I infer from this and from the general 

 tenour of his remarks that the steam being the original motive power, 

 exists in and at the base of the molten magma. He says, " If any 

 doubt should suggest itself whether this fluid is actually generated 

 within the lava, or only rises through it, having its origin in some 

 other substance, or in some other manner beneath, it must be dis- 

 pelled by the evidence afforded in the extremely vesicular or cellular 

 structure of very many erupted lavas, not merely near the surface, 

 but throughout their mass, showing that the aeriform fluid in these 

 cases certainly developed itself interstitially in every part. And 

 although such vesicles or cells appear at first sigfyt to be wanting in 

 other lavas, at least in the lower portions of the lava-current after its 

 consolidation, the microscope invariably, or almost invariably, discovers 

 them. In those exceptional cases, where the rock is to appearance 

 perfectly compact, it is allowable to suppose that the vapour it once 

 contained escaped in ascending bubbles, or by exudation through 



* " Volcanos." By a. Poulett-Scrope. 2nd Edit. (3862), p. 30. 

 Ibid., pp. 39-40. 



