Mr Poulett Scrope's Considerations on Volcanos. 337 



the vapour (which alone occasions the mobility of the crystals) on their 

 superficial exposure to the outward air; subsequently, by exudation 

 through the pores or interstices of that hardened surface, the process is 

 propagated to the interior of the bed of lava. Consolidation may also 

 take place by increase of pressure on the lava, without any change in its 

 temperature ; the vapour being condensed till the crystals reunite, more 

 or less conformably, according as they have been more or less broken up, 

 or disintegrated, by mutual friction when in motion, and by the disaggre- 

 gative force of the vapour generated within them. 



Having developed these original ideas as to the nature of lava, which 

 are supported by facts and arguments, the author goes on to explain all the 

 phenomena of earthquakes and volcanos, and the circumstances of dispo- 

 sition, structure, and mineral character in volcanic rocks, by the assump- 

 tion, which, however, is strongly supported by a large body of evidence, 

 that the interior of the globe, at no great depth from the surface, consists 

 of a mass of crystalline rock at an intense temperature ; and, therefore, thata 

 continual supply of caloric passes off from the centre towards the circum- 

 ference, wherever the nature of the superficial rocks allows of its transmis- 

 sion, or temporary vents are opened for its more free escape. 



Where the superficial rocks, from their constitution, (as is presumed to 

 be the case with the schistose strata, the limestones and sandstones, — or, 

 when consisting of unstratified crystalline rocks, from the intumescence 

 and refrigeration they have undergone,) are very inferior conductors of ca- 

 loric, the heat transmitted from below will be concentrated in the beds of 

 denser crystalline rock beneath these, and continually augment their tem- 

 perature, and with it their expansive force. The overlying rocks will 

 sooner or later necessarily yield to this force. At this time, the expan- 

 sive force will be greatest in the lower parts of the crystalline bed. The 

 upper will be therefore raised en masse, in a solid state. This forcible 

 elevation must be accompanied by the rupture and dislocation of the over- 

 lying rocks ; and every such fracture in the earth's crust must create a jar- 

 ring shock and vibratory motion in them, which will be propagated along 

 the prolongation of each rocky bed, with an intensity proportionate to its 

 solidity ; the strata which are only in contact with those broken through, 

 sharing in the vibration in an inferior degree. These shocks are earth- 

 quakes, none of which are supposed to take place without a certain, 

 though often inappreciable elevation of the surface of the globe. The fis- 

 sures formed in this manner will be more or less wedge-shaped ; some 

 opening outwardly, some downwards. The latter allow of the sudden ex- 

 pansion and liquefaction of the intensely heated rock in which they are 

 formed, and by this process the fissure is filled with intumcscent lava. 

 "Where the fissures are broken through, the upper beds of the heated crys- 

 talline rock, contemporaneous veins, or subordinate masses are produced, 

 where, through overlying strata of other characters, injected veins or dikes. 

 The friction occasioned by the resistance of the sides of the cleft to the 

 rise of the crystalline matter partially disintegrates the crystals, and gives 

 a finer grain to the substance of the vein than that of the including rock ; 

 VOL. IV. NO. II. APKIL. 182G. Y 



