342 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [April 23, 



bladders in lavas, as being proofs of tbeir having been in a state of 

 complete fusion. But have the loaves baked in our ovens been in 

 fusion ? The comparison of a cellular scoria with a loaf or a French 

 roll will show that vesicles of precisely similar appearance to those of 

 lavas are producible in substances of a pasty consistence, which owe 

 their liquidity to an aqueous vehicle, the heat applied being only 

 sufficient to develope the contained gases. Other kinds of baked 

 cakes are porous rather than cellular, and aptly represent the texture 

 of the earthy and porous trachytic lavas. 



Plutonic rocks. — This theory as to the nature of the liquidity of 

 many lavas appeared to me so reasonable, that I proceeded to ex- 

 amine its applicability to the still more generally crystalline plutonic 

 rocks, from the alteration of which by heat lavas are usually sup- 

 posed to derive. I asked myself, what would probably be the effect 

 on a mass of granite, for example, containing water intimately com- 

 bined with its molecular particles, and confined beneath overlying 

 rocks and seas, under circumstances of intense compression, and at 

 the same time high and increasing temperature ? Surely a tendency 

 to intumescence, which, wherever, and in proportion to the extent to 

 which, it takes place, must elevate and fracture the overlying rocks, 

 and likewise disintegrate more or less tbe crystalline particles of the 

 swelling mass, through the irregularities of their internal movements 

 and mutual friction. Many of the crevices broken through the 

 neighbouring rocks would be injected by the intumescent matter. 

 Some may be sufficiently enlarged to allow of its forcing its way into 

 the open air as a lava, perhaps accompanied by eructations of the 

 gases and vapours developed in the lower parts of the mass, or, 

 should the liquefaction not be sufficient to admit of the rise of aeri- 

 form bubbles, as matter of a porous, pasty, or glutinous consistency, 

 perhaps even semisolid in texture and bulky in form. 



It might happen that, circumstances occasioning in turn the pre- 

 ponderance of the compressing over the expansive forces (by reason, 

 for example, of a diminution of temperature), portions of the sub- 

 terranean crystalline mass will, after a partial intumescence of the 

 kind supposed, return to a state of solidity. The result may be a 

 more fine-grained rock, owdng to the partial disintegration of the 

 crystals ; or, if the disintegration had proceeded sufficiently far, new 

 mineral combinations might take place. Indeed, Watt long since 

 proved that the particles of even apparently solid rocks are capable, 

 through changes in temperature, of internal motion sufficient to 

 admit their rearrangement according to polarity, that is, of crystal- 

 lization. Still more likely is this result to occur on the condensation 

 or escape of any fluid which had previously kept them from contact 

 with each other, since the crystalline polarity can only exert itself 

 within minute distances. And thus might be accounted for the fre- 

 quently observed passages of granite and gneiss into syenite, green- 

 stone, trap, or trachyte, and the varieties of mineral composition 

 which these rocks at times exemplify. So also the transitions from 

 the larger crystalline grain to the finer, and the dykes and veins 

 which these rocks so often contain themselves, or intrude into their 



