G, P. Scrope on Craters, and the Liquidity of Lavas. 221 



lar appearance to those of lava are producible in subetances of 

 a pastj consistence, which owe their liquidity to an aqueous ve- 

 hicle, the heat applied being only sufficient to develop the con- 

 tained gases. Other kinds of baked cakes are porous rather 

 than cellular, and aptly represent the texture of the earthy and 

 porous trachytic lavas, 



Plutonic rocJcs. — This theory as to the nature of the liquiditv 

 of many lavas appeared to me to be so reasonable, that I pro- 

 ceeded to examine its applicability to the still more generally 

 crystalline pUitonic rocks, from the alteration of which by heat 

 lavas are usually supposed to derive. I asked myself, what 

 would probably be the effect, on a mass of granite, for example, 

 containing water intimately combined with its molecular parti- 

 cles, and confined beneath overlying rocks and seas, under cir- 

 cumstances 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 

 hkewise disintegrate more or less the crystalline particles of the 

 swelling mass, through the irregularities of their internal move- 

 n^ents and mutual friction. Many of the crevices broken through 

 the neighboring rocks would be injected by the intumescent 

 rnatter. Some may be sufficiently enlarged to dlow of its for- 

 cing its way into the open air as a lava, perhaps accompanied by 

 eructations of the gases and vapors developed in the lower parts 

 of the mass, or, should the liquefaction not be sufficient to admit 

 of the rise of aeriforiu 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 

 preponderance of the compressing over the expansive forces (by 

 reason, for example, of a diminution of temperature), portions 

 of the subterranean crystalline mass will, after a partial intumes- 

 cence of the kind supposed, return to a state of solidity. The 

 result may be a more fine-grained rock, owing to the partial dis- 

 integration of the crystals; or, if the disintegration had pro- 

 ceeded 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 tempera- 

 ture, of internal motion sufficient to admit their rearrangement 

 according to polarity, that is, of crystallization. Still more likely 

 is this result to occur on the condensation or escape of any fluid 

 ^vhich had previously kept them from contact with each other, 

 ^ince the crystalline polarity can only exert itself within mintite 

 distances. And thus might we account for the frequently ob- 

 served passages of granite and gneiss into syenite, greenstone, 

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



