620 C. Darwin, Esq., on Volcanic 



The inhabitants of Concepcion beheved, that the vibrations proceeded from 

 the south-westj in which quarter subterranean noises were likewise frequently 

 heard. It is, therefore, a most interesting circumstance, that the island of 

 Santa Maria, situated Sb miles distant in this direction, was found by Captain 

 Fitz-Roy to have been elevated to nearly three times the height that the 

 coast near Concepcion was upraised. At Tubul, S. by E. of Santa Maria, 

 the land was raised 6 feet ; at the southern extremity of the latter island, 8 

 feet; in its middle, 9 feet; and at its northern extremity, upwards of 10 

 feet*. These measurements, which were made with extreme care by Cap- 

 tain Fitz-Roy, seem to point out an axis of elevation in the sea off the north- 

 ern end of Santa Maria. 



There is one remark, which 1 must introduce here. The motion of the 

 earth, on February 20th, 1835, at Valdivia, appeared to me like that of a 

 crust, spread over an undulating- fluid ; and in my Journal, I have compared 

 the motion to the bending of thin ice, beneath a moving weight. After- 

 wards, when I became convinced that the crust there does rest upon a sea of 

 molten rock, my first impression regarding the movement was strongly con- 

 firmed. Michell long since observed, (Phil. Trans., 1760, p. 8) that "the 

 motion of the earth in earthquakes is partly tremulous and partly propagated 

 by waves, which succeed one another, sometimes at larger and sometimes at 

 smaller distances ; and this latter motion is generally propagated much further 

 than the former." This distinction, I believe, is strictly true. Professor 

 Phillipsf argues that rocks, although elastic in their parts, are " very imper- 

 fectly so in their mass, owing to the numerous divisions which intersect them. 

 Composed of such materials," he says, " the crust of the earth does not, and 

 in fact hardly can, vibrate, in the ordinary sense of this term ; the motion 

 observed is more similar to the undulation of a flexible lamina over an agi- 

 tated liquid." The result arrived at by this reasoning thus coincides with 

 mine, drawn from the impression on my senses; and it, at first, appears to 

 explain, in a very satisfactory manner, the propagation to greater distances of 

 the long and gentle undulations than of the vibrations, by the transmission of 

 the former in the subterranean fluid, and of the latter in the crust of the 

 earth. With respect, however, to the supposed want of elasticity in the crust 

 of the earth, taken in mass, I cannot agree with Professor Phillips. Michell, 

 (Phil. Trans., 1760, p. 35,) when he adduces the fact of the great vibration, 

 or rather oscillation, during gales of wind, of steeples, and even towers f, 

 which may be said to be composed of a vast number of strata of different 

 densities, and which are frequently traversed by fissures or faults, leaves 



* Geographical Journal, Vol. vi. p. 327. 



t Lardner's Encyclopaedia, GeoL, Vol. ii. p. 209. 



