Phenomena in South America. 621 



scarcely any doubt on the mind that a similar and much greater vibration 

 might be transmitted from the depths of the earth, where the parts must 

 be pressed together with incomparably greater force than in any building. 

 Plausible as is the foregoing explanation of the two kinds of movements, I do 

 not believe it to be the correct one ; for if an undulation be ever produced in the 

 subterranean fluid expanse, we can hardly conceive a more powerful cause of it, 

 than the upward rush of a great body of molten rock and aeriform matter from 

 the lowest abyss of a volcano : but we know that eruptions on an enormous 

 scale have happened through old vents, even in areas subject to far-extended 

 and undulating earthquakes, without such movements having been produced. 

 From this consideration, and from the fact that the force of earthquakes appears 

 to have a definite relation to the thickness of crust ruptured, as we may con- 

 clude from the great difference in the effects caused by an eruption through 

 an old, and one through a new orifice, I do not conceive we are justified in 

 admitting the hypothesis of an undulating fluid. The two kinds of movements 

 may, possibly, be explained, by considering that when the crust yields to the 

 tension, caused by its gradual elevation, there is ajar at the moment of rup- 

 ture, and a greater movement may be produced by the tilting up of the edges 

 of the strata and by the passage of the fluid rock between them. In breaking 

 a long bar of steel, would not a jar be caused by the fracture, as well as a 

 vibration of the two ends when separate } 



Mr. Hopkins*, in his Researches on Physical Geology, has demonstrated, 

 that when an elongated area is elevated by a force acting equally beneath all 

 parts, if the strata yield, fissures must be formed parallel to its longer axis, 

 and other minor ones transverse to it. Knowing then with certainty, that the 

 coast of Chile, near Concepcion, was elevated on the 20lh of February, and 

 likewise that the area affected by the earthquake was elongated ; — bearing also 

 in mind, that several of these elevations have occurred, as attested both histo- 

 rically and by the extensive beds of recent species of shells, at the altitude of 

 some hundred feet, we are absolutely compelled to believe, that the area (vvith- 

 out we assume that the strata possessed extraordinary powers of extension) was 

 at that time fissured in lines, the principal of which were parallel to its longer 

 axis. If, however, the elevatory force acted unequally in different parts, as 

 was the case in Chile, we can understand, from the admirable generalization of 

 the same author, that separate fissures might be formed, which would produce 

 at the same instant, in distant places, separate shocks, perhaps of different in- 

 tensities. Hence we need not suppose, that the shocks felt more strongly at 

 Juan Fernandez, Concepcion, and Chiloe, than at intermediate points, pro- 



* Transactions of the Cambridge Philosophical Society, Vol. vi. Part I. 

 4 L 2 



