608 C. Darwin, Esq., on Volcanic 



an impulse can be transmitted to distant parts of the surface, with nearly 

 equal force. But according- to this view, if two separate trains of volcanos in 

 the Andes have any connexion whatever, which seems highly probable from 

 the symmetry of the Cordillera, (and possibly an intimate one, as will pre- 

 sently be discussed,) then the common focus, from which the two main branches 

 are sent off, must be seated at an enormous depth. But all the calculations 

 regarding the depth at which molten rocks must necessarily be met with, if 

 they can be at all trusted*, tend to prove, that the earth's crust is not much 

 more, and perhaps less, than twenty miles in thickness ; and if this be so, the 

 crust may, indeed, be well compared with a thin sheet of ice over a frozen 

 pool. 



These considerations are, perhaps, of little weight, but we must bear in 

 mind, that the elevation of many hundred square miles of territory near Con- 

 cepcion is part of the same phenomenon, with that splashing up, if I may so 

 call it, of volcanic matter through the orifices in the Cordillera at the moment 

 of the shock; and as this elevation is only one of a long series, by which the 

 whole coast of Chile and Peru, even for more than a thousand miles, has been 

 upraised several hundred feet within the recent period, (as I endeavoured to 

 show in a paper formerly read to the Societyf, and I hope hereafter to prove 

 more fully,) the body of matter added below must have been enormous. When 

 we reflect on this, it is obvious, that the term channel cannot be applied to a 

 means of communication extending beneath a large portion of a continent, and 

 from the interior of the globe to the superficial crust];. The facts appear to 

 me clearly to indicate some slow, but in its effects great, change in the form 

 of the surface of the fluid on which the land rests. 



* M. Parrot, however, (Memoires de I'Acad. Imp. des Sciences de St. Petersbourg, Tom. i. 

 1831. Science. Math. Phys. et Naturelles) altogether denies that the data are sufficient to form 

 any judgment on this subject. 



t Proceedings Geol. Soc, Vol. ii. p. 446, Jan. 1837. 



X Professor BischofF (Edinburgh New PhilosophicalJournal, Vol. xxvi. p. 59, 1838.) has even 

 argued that " the immense masses of lava ejected from a single volcano, and the enormous extent 

 in which volcanic actions are felt at the same time, scarcely leave room to doubt that every active 

 volcano is in immediate communication with the whole melted matter in the interior." How in- 

 comparably stronger this argument is, if applied to the plutonic as well as volcanic rocks, composing 

 the great masses of the Cordillera ! but now that we know, that continental elevations are caused 

 by the very same impulses with those which eject lava and scoriae through the mouths of volcanos, 

 the argument from the bulk of matter observable in ejected or interjected masses of rock, may 

 be passed over, since the matter added below, when a whole kingdom is permanently elevated, 

 must far exceed that composing either a volcanic hill or the axis of a mountain-chain ; and there- 

 fore we are so much the more strongly urged to look for its source in " the whole melted matter 

 of the interior," and not in any local receptacle. 



