616 C. Darwin, Esq., on Volcanic 



Although the bursting forth of a new vent may invariably be accompanied 

 by an earthquake, the converse is not true ; for if it were, at Valparaiso, Con- 

 cepcion, Lima, Caraccas, and other places, in the immediate neighbourhood 

 of the part most violently shaken, an eruption must always have taken place, 

 which, even if we suppose it to have occurred beneath the sea, is improbable 

 in the highest degree. But we may suppose that these earthquakes are owing 

 to some phenomenon analogous to volcanic eruptions. This opinion is much 

 strengthened by the fact, that great earthquakes, like great eruptions, gene- 

 rally recur only after long intervals of repose, and they thus lead us to believe, 

 that the subterranean force is relieved by either in the same manner. This, 

 indeed, is the direct opinion of the inhabitants of the whole west coast of South 

 America, who are firmly convinced of an intimate relation between the sup- 

 pressed activity of the volcanos in the Andes and the tremblings of the ground. 

 We have, also, seen that, when the island of Chiloe was strongly shaken, some 

 men at work on the flanks of the Cordillera, between the volcanos of Osorno 

 and Minchinmadom, (which both sent up dark columns of smoke, like signals 

 to mark the new period of violence,) were quite unaware of the great convul- 

 sion, which then caused the shores of the Pacific to vibrate throughout a space 

 of more than a thousand miles. 



There is, however, one difference, although more apparent than real, be- 

 tween earthquakes like that of Concepcion, and those alluded to by Ulloa. 

 In the former, it has almost invariably happened, at least in those on the South 

 American coast, that a vast number of shocks have followed the first great 

 convulsion*, and these, as well as the accompanying subterranean noises, 

 having proceeded from the same quarter with the first shock, are therefore 

 undoubtedly due to the very same cause, only acting with somewhat less 

 intensity. Thus, even in the first twenty-four hours after the earthquake of 



(p. 580,) has quoted this same passage in confirmation of his view, that " the eruptions of volcanos 

 which happen at the same time with earthquakes may, with more probabihty, be ascribed to those 

 earthquakes, than the earthquakes to the eruptions, whenever at least the earthquakes are of con- 

 siderable extent." The term earthquake is here used to express the cause of the trembling of the 

 ground. Sir James Hall, in his celebrated memoir on " Heat modified by compression," (Edin. 

 Phil. Trans., Vol. vi. p. 1G6,) distinctly states " that the earthquakes which desolate countries 

 not externally volcanic, indicate the protrusion from below of matter in liquid fusion, penetrating 

 the mass of rocks ;" but he does not extend this view, which is the same which I hold, to any 

 comprehensive generalization, or restrict it to any particular class of earthquakes. 



* Courrejolles, in his Memoir on Earthquakes, (Journal de Physique, Tom. liv. p. 106,) says, 

 " Les grands tremblemens de terre sont presque toujours precedes et suivis quelque temps avant 

 et apres par de petites secousses." Michell (Philosophical Transactions, 1760, p. 10) has given 

 some instances of successive minor shocks, which appeared to travel from the same point, whence 

 the previous more violent ones had come. 



