Volcanos. 115 



" Where the superficial rocks, from their constitution, (as is 

 presumed to he the case with the schitose strata, the limestones 

 and sandstones, — or, when consisting of unstratified crystalline 

 rocks, from the intumescence and refrigeration they have under- 

 gone,) are very inferior conductors of caloric, the heat trans- 

 mitted from below will be concentrated in the beds of denser 

 crystalline rock beneath these, and continually augment their 

 temperature, and with it their expansive force. The overly- 

 ing rocks will sooner or later necessarily yield to this force. At 

 this time the expansive 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 accompani- 

 ed by the rupture and dislocation of the overlying rocks ; and ev- 

 ery such fracture in the earth's crust, must create a jarring shock 

 and vibratory motion in them, which will be propagated along 

 the prolongation of each rocky bed, with an intensity propor- 

 tionate to its solidity; the strata which are only in contact with 

 those broken through, sharing in the vibration in an inferior de-> 

 gree. These shocks are earthquakes, none of which are sup- 

 posed to take place without a certain though often inappreciable 

 elevation of the surface of the globe. The fissures formed in 

 this manner will be more or less wedge shaped ; some opening 

 oufwardly, some downwards. The latter allow of the sudden 

 expansion and liquefaction of the intensely heated rock in which 

 they are formed, and by this process the fissure is filled with ia- 

 tumescent lava. Where the fissures are broken through, the 

 upper beds of heated crystalline 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 ; and also often occasions the crystals composing 

 the lateral parts of the vein to be more comminuted than those 

 in the centre. The matter filling these veins is immediately 

 consolidated both by the loss of temperature and pressure, and 

 the fractured rocks are thus repaired and strengthened. An in- 

 terval of tranquillity will then succeed, until a similar expansion 

 occurs. It is thus that the overlying rocks which form the sur- 

 face of the globe must be progressively elevated more and more, 

 by the successive dilations of the interior lava-bed unless some 

 avenues are opened within a limited distance, for the more tran- 

 quil escape of the subterranean caloric. 



" But the formation of such apertures (volcanic spiracles) must 

 sooner or later result from the continuance of this process ; 

 for at every crisis of expansion, those cracks are only repaired 



