G, Poulett Scrope—On the Cause of Volcanic Action. 197 



crystals or granules, in the manner of a flux, and thus assist 

 the liquefaction of the mass, and drive it up any fissure that 

 might open for its passage, in a pasty or semi-liquid state, short ^ of 

 complete fusion ; so that on reaching the outer air, and parting with 

 the contained steam, together with the heat which the expansion of 

 this element would carry off in a latent form, its surface would con- 

 solidate instantly ; and the solidification extending rapidly inwards 

 by the formation of crevices giving an outlet to further vapour, the 

 resulting rock would exhibit a cellular or porous structure, and a 

 granular or ciystalline texture, in lieu of the compact and glassy one 

 which, if completely fused, it should possess. 



These ideas were received at the time with incredulity. But their 

 correctness is now not far from being generally recognised. For 

 example. Professor Phillips, in his recent volume on Vesuvius, ad- 

 mits " the fact that water is present abundantly in the fluid lava," 

 (p. 305), and again (p. 311), " steam pervades some parts— perhaps 

 every part — of the fluid mass." He adds, " Lava cooled rapidly 

 might he expected to be glassy in texture. This, however, is rarely, 

 or never, the case in Vesuvius." It is strange that neither the Pro- 

 fessor, nor other observers, should have sought for the cause of a 

 fact so different from what was to be expected on the supposition of 

 the complete fusion of the lava — yet so common, that, with the ex- 

 ception of the vitreous lavas of Lipari, Hawaii, Bourbon, and the 

 trachytic obsidians and pearlstones, it may be declared to be uni- 

 versally true of all lavas, whether trachytic or doleritic. 



But if we admit the existence of water, or steam, in close and inti- 

 mate dissemination through every part of a mass of subterraneous lava, 

 and of the rocks from whose liquefaction or semi-fusion by intense heat 

 it proceeds, how are we to believe that this water could have found its 

 way into this position from superficial seas or lakes, through fissures 

 suddenly opened by the earthquakes which accompany an eruption ? 

 Such sudden influx of a body of water to a heated mass of mineral 

 matter beneath, might be conceived to give rise to some equally 

 sudden explosion at the point of contact; but the explosion itself, and 

 the rise of lava up the fissure must, it is to be presumed, check 

 any further penetration of water. The effect would be superficial 

 only, and could scarcely so completely saturate the entire mass of 

 heated rock, whether in a liquid or solid state at the time, as to 

 cause its general ebullition, continued too through such lengthened 

 periods as volcanic eruptions are often known to last. 



Moreover, if we suppose an earthquake to cause the eruption of a 

 volcano by suddenly admitting a body of water to its heated focus, 

 what, it must be asked, causes the earthquake ? Not certainly the 

 influx of this same water through the rents which the earthquake 

 itself only originates. The effect cannot produce the cause. Surely 

 it is more reasonable to suppose that a local increase of heat trans- 

 mitted from the sides or from beneath a volcanic focus, which had been 

 for a time cooled down by the emission of steam and lava in earlier 

 eruptions, gives occasion to the expansion of a body of subterranean 

 mineral matter (whether in a solid, or fluid, or some intermediate con- 



