340 PROCEEDINGS OF THE GEOLOGICAI^ SOCIETY. [April 23, 



in which the hasalt used there (and which is the same upon which 

 Mr. Gregory Watt experimented) conducts itself in their furnaces, 

 and I found, that when the Hquid and fused contents of a furnace at 

 a white heat is poured out upon a brick or other floor into the open 

 air, so as to represent a stream of lava flowing out of a volcanic 

 vent, it consolidates throughout, whatever its bulk, into a homo- 

 geneous and purely vitreous black obsidian, in fact, an absolute glass, 

 with a conchoidal fracture and sharp cutting edges. It is only when 

 made to consolidate very slowly in an oven kept at a high tempera- 

 ture for some days, that it assumes the deadened and semi-crystalline 

 texture of the manufactured article. 



If this process be interrupted, it is found to have commenced by 

 the formation, at numerous points within the vitreous mass, of 

 globular concretions about the size of a small pea, of a lighter colour 

 than the base, and having a pearly lustre and radiated structure. 

 The multiplication and confusion of these crystallites or sphserulites 

 ultimately destroy the glassy character of the substance altogether, 

 and give to it a pearly semi-crystalline texture, without, however, 

 restoring the far more crystalline aspect of the basaltic rock. A 

 similar 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 sphaerulitic concretions, and of 

 course there is no question as to the complete state of fusion in which 

 such lavas have been produced. But there is no trace of such a 

 process in any of the ordinary earthy, and stony or crystalline 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 sphaerulitic 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 determine the 

 fusion or liquefaction of substances ; and that compression may pre- 

 vent the liquefaction of a solid at a high temperature, just as it pre- 

 vents the vaporization of a liquid, in the common experiment of 

 boiling water at a lower temperature in a rarefied atmosphere. If 

 so, the intense pressure to which heated lava must be subjected be- 

 fore it rises from the bowels of the earth to discharge itself on the 

 surface, intensified by the reaction of its own expansive force from 

 the confining surfaces, might perhaps prevent its complete fusion, 

 however high the temperature. 



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

 other elastic vapours evidently discharged from every volcano in 

 eruption, and to all appearance the chief agents in the expulsion of 

 lavas from the bowels of the earth. That this vapour is liable to be 

 developed in every part of the mass of lava is shown by the forma- 

 tion of vesicles throughout its substance wherever the pressure is so 



