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G. P. Sa^ope on Craters, and the Liqiddity of Lavas. 219 



structure. The multiplication and confusion of these crystallites 

 or sphasrulites ultimately destroy the glassy character of the sub- 

 stance altogether, and give to it a pearly semi-crystalline tex- 

 ture, without, however, restoring the for more crystalUne aspect 

 of the basaltic rock. A simihir change may be often observed 

 to have taken place in nature among the vitreous lavas, which 

 pass into pearlstone and pitchstone by the formation of the same 

 kind of sphierulitic concretions, and of course there is no ques- 

 tion as to the complete state of fusion in which such lavas nave 

 been produced. But there is no trace of such a process in any 

 of the ordinary earthy, and stony or crystal hue and porphyritic 

 lavas. I am not aware of a single current from either Etna or 

 Vesuvius having ever exhibited, even on its most rapidly cooled 

 surfaces, any passage into true obsidian, or sphserulitic pearl- 

 stone, or any portion of such vitrifactions. A pellicle, or. glaze, 

 of a semi- vitreous appearance coats the surface in some parts, or 

 lines the cellular cavities ; but it seems evident that the bulk of 

 the matter could not have been at the time of its emission in that 

 thoroughly fused condition which it assumes when melted in a 

 furnace or under the blowpipe. 



2. It struck me that temperature does not alone determihe the 

 fusion or liquefaction of substances ; and that compression may 

 prevent the liquefaction of a solid at a high temperature, just as 

 it prevents the vaporization of a liquid, in the common experi- 

 nient of boiling water at a lower temperature in a rarefied atmos- 

 phere. If so, the intense pressure to which heated lava must be 

 subjected before it rises from the bowels of the earth to discharge 

 itself 021 the surface, intensified by the reaction of its own expan- 

 sive force from the confining surfaces, might perhaps prevent ite 

 complete fusion, however high the temperature. 



3. I had long been impressed by the vast volumes of aqueoua 

 and other elastic vapors evidently discharged from every volcano 

 in eruption, and to all appearance the chief agents in the expul- 

 sion of lavas from the bowels of the earth. That thisvapor ia 

 liable to be developed in every part of the mass of lava is shown 

 by the formation of vesicles throughout its substance wherever 

 the pressure is so reduced as to permit their expansion- for in- 

 stance, in the superficial portions of a current ; and in some lava- 

 currents throughout the entire mass. 



The experiments of Mr. Knox, related in a paper read before 

 the Royal Societ7 in 1824,* had taught me that water in an ap- 

 preciable quantity is mechanically combined with the elementary 

 particles of all the crystalline rocks of igneous origin. The 

 question, therefore, arose,— Might not the water thus intimately 

 disseminated through a mass of crystalline lava, although at an 

 intense temperature, remain unvaporized owing to the stiU greater 



* rhil. Trans., 1825, 



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