Ch. XXV.] 



ORIGIN OF GRANITE. 363 



dates slowly under so enormous a pressure, may be supposed 

 to acquire a very distinct texture and become granite. 



If it be objected that we do not find in mountain-chains vol- 

 canic dikes passing upwards into lava, and downwards into 

 granite, we may answer that our vertical sections are usually 

 of small extent, and it is enough that we find in certain loca- 

 lities a transition from trap to porous lava, and in others a 

 passage from granite to trap. It should also be remembered, 

 that a large proportion of the igneous rocks, when first formed, 

 cannot be supposed to reach the surface, and these may assume 

 the usual granitic texture without graduating into trap, or into 

 such lava and scoriae as are found on the flanks of a volcanic 

 cone. 



Theory of the origin of granite at all periods. It is not 

 uncommon for lava-streams to require more than ten years to 

 cool in the open air, and a much longer period where they 

 are of great depth. The melted matter poured out from 

 Jorullo, in Mexico, in the year 1759, which accumulated in 

 some places to the height of 550 feet, was found to retain a 

 high temperature half a century after the eruption*. For 

 what immense periods, then, must we not conclude that great 

 masses of subterranean lava in the volcanic foci may remain in 

 a red hot or incandescent state, and how gradual must be the 

 process of refrigeration ! This process may be sometimes 

 retarded for an indefinite period, by the accession of fresh 

 supplies of heat, for we find that the lava in the crater of Strom- 

 boli, one of the Lipari islands, has been in a state of constant 

 ebullition for the last 2000 years, and we must suppose this 

 fluid mass to communicate with some cauldron or reservoir of 

 fused matter below. In the Isle of Bourbon, also, where there 

 has been an emission of lava once in every two years for a long 

 period, we may infer that the lava below is permanently in a 

 state of liquefaction. 



When melted matter is injected into the fissures of a con- 

 tiguous rock at a considerable depth, it may cool rapidly if that 

 rock has not acquired a high temperature ; but suppose, on he 

 * See vol. i. p, 378, and Second Edition, p, 433. 



