622 C. Darwin, Esq., on Volcanic 



ceeded from any one focus, but that they were generated in each separate 

 district, — the vibrations probably having, in each case, different directions *. 

 This explanation is, I think, far more satisfactory than that offered by Hum- 

 boldt, of the supposed inertness of an intermediary mass of rock, in transmit- 

 ting to the surface vibrations from a deeply-seated focus. 



On different kinds of Earthquakes ; and conclusions regarding those which 

 accompany Elevatori/ Movements. 



I confine the foregoing- observations to the earthquakes on the coast of 

 South America, or to similar ones, which seem generally to have been accom- 

 panied by elevation of the land. But, as we know that subsidence has gone on 

 in other quarters of the world, fissures must there have been formed, and there- 

 fore earthquakes. I think, it would be highly advantageous to geology, if the au- 

 thor who has followed out the effects of an elevatory force, would consider those 

 produced by the failure of support in the arched surface of the globe. The 

 earthquakes of Calabria, and perhaps of Syria, and of some other countries, 

 have a very different character from those on the American coast. When 

 Molina, the historian of Chile was in Italy, he was much struck with this dif- 

 ference; he saysf, in Chile even the smaller shocks extend over the whole 

 kingdom, and are propagated horizontally, whilst those which he felt at Bo- 

 logna, were of small extension, but instantaneous, and commonly explosive. 



I will add, that in the accounts collected by Mr. Lyell;}: of the earthquakes 

 of Calabria, Lisbon, and some other places, portions of the surface are de- 

 scribed as having been absolutely engulphed, and seen no more : but this 

 does not appear to have happened in any of the earthquakes on the west coast 

 of South America. If the fluid matter, on which 1 suppose the crust to rest, 

 should gradually sink instead of rising, there would be a tendency to leave 

 hollows, and therefore a suction exerted downwards ; or hollows would be 

 actually left, into which the unsupported masses might be precipitated with 

 the violence of an explosion. Such earthquakes, we may conclude, from what 

 has been shown in the foregoing part of this paper, would seldom be accom- 

 panied by eruptions, and never, probably, hy periods of renewed volcanic 



* At Concepcion the line of vibration appears to have been N.W. and S.E., coming from S.W. 

 At Mocha, (an island between Concepcion and Valdivia), from the manner in which water oscil- 

 lated in the bottom of a boat drawn up on shore, the vibration must have been N. and S. coming 

 from either E. or W. For the facts alluded to, see Capt. FitzRoy's account of the Voyages of the 

 Adventure and Beagle, volume ii. p. 414. 



•j- Compendio de la Historia del Reyno de Chile, Vol. i„ p. 36. 



% Principles of Geology, 5 th edit. Vol. ii. Book ii. Chap. xiv. 



