626 C. Darwin, Esq., on Volcanic 



number of dikes branching- from them, must have been fluid when propelled 

 against the lower strata*. If then we suppose Diagram 2 to represent the 

 section of the Cordillera before its final elevation, 1 may ask, how is it pos- 

 sible, that some of the masses of strata should be placed vertical, and others 

 absolutely overturned, by the action of the fluid rock, without the very bowels 

 of the earth gushing out? Should we not have one enormous dehige of 

 volcanic matter, instead of wedge-formed, injected masses of solid crystalhne 

 rock ? On the other hand, if we suppose the loftiest chain of mountains to 

 be formed by a succession of shocks similar to those of Concepcion, — a few 

 stronger and many slighter ones, separated from each other by long intervals 

 of time, — then we may believe, that the formation of a fissure through the 

 whole thickness of the crust would be the effect of many efforts on the same 

 line, and that during the intervals, the rock first injected would become 

 cooled. When, therefore, the tension (which, according to Mr. Hopkins, acts 

 on the lower surface firstf) caused the upper part to crack, the fissures, if 

 on the same line, would meet the consolidated extremity of a dike, instead 

 of the fluid mass below. In those cases, however, where the fissure happened 

 to traverse at once the entire crust, a volcano would be formed, such as that 

 near Juan Fernandez during the Concepcion earthquake. On the same 

 principle, after the masses of strata had been very gradually lifted into the 

 position represented in Diagram 2, the rock beneath the anticlinal axes, 

 from having been propelled beyond its former subterranean isothermal line, 

 would be cooled, and, if sufficient time were allowed, it would be consolidated. 

 In this manner the strata, each new fracture being firmly cemented by the 

 cooling of the injected rock, might be overturned into any possible position, 

 and yet, from a gradually thickening crust being formed over the fluid mass, 

 on which the whole is believed to rest, the earth would be protected from a 

 deluge of lava. If this reasoning be sound, we may deduce this remarkable 

 conclusion, that in a mountain-chain,, having an axis of plutonic rock, which 

 was propelled upwards in a fluid state, where the strata betray the effects of 



* According to M. Boussingault (Bulletin de la Soc. Geol., Tom. vi. p. 55), this is not the case 

 in the Cordillera of the Equatorial regions. He states that trachyte there forms the base of the 

 mountains, and that it has been protruded in a consolidated form. But can the deep-seated axis 

 of a gigantic mountain-chain be composed of trachyte, — a rock essentially volcanic'! If we could 

 penetrate to greater depths, it cannot be doubted we should find the trachyte graduating into some 

 plutonic rock ; and one may be allowed to suspect that its junction with the superincumbent strata 

 would present very different appearances from that of the trachyte ; — the trachyte, indeed, we 

 may well imagine to be the crust of such plutonic rocks cooled under little pressure, and forced 

 upwards on the surface of the molten mass, in a solid form. 



t Cambridge Philosophical Transactions, Vol. vi. pp. 43-15. 



