Phenomena in South America. 619 



whilst, on the other hand, the level of the ground certainly was not raised by 

 them ; but on the contrary, after an interval of some weeks, it stood rather 

 lower than it did immediately after the great convulsion, — a consequence, 

 perhaps, of the settling down of the shaken ground. In the same manner, in 

 1822, at Valparaiso, the permanent change of level in the rocks on the coast 

 was observed the morning next after the great shock ; though the earth con- 

 tinued to tremble at intervals for many days. In these instances of change 

 of level we have, then, a clear indication of some cause of disturbance, super- 

 added to that which produced the vibrations, and which, it is highly probable, 

 would accompany the simple elevation of the coast in mass. 



From these considerations, we may, I think, fairly conclude, with regard to 

 the earthquakes on the west coast of South America, 



1st. That the primary shock is caused by a violent rending of the strata, 

 which seems generally to occur at the bottom of the neighbouring sea. 



2nd. That this is followed by many minor fractures, which, though ex- 

 tending upwards nearly to the surface, do not (excepting in the comparatively 

 rare case of a submarine eruption) actually reach it. 



3rd. That the area thus fissured extends parallel, or approximately so, to 

 the neighbouring coast mountains. 



4th. That when the earthquake is accompanied by an elevation of the land 

 in mass, there is some additional cause of disturbance. 



And lastly. That an earthquake, or rather the action indicated by it, re- 

 lieves the subterranean force, in the same manner as an eruption through an 

 ordinary volcano. 



Now, what constitutes the axis, where visible, of most great mountain- 

 chains ? Is it not a wedge-formed linear mass of rock, which scarcely any 

 geologist disputes was once fluid, and has since cooled under pressure ? 

 Must not the interjection of such matter between masses of strata have re- 

 lieved the subterranean pressure in the same manner, as an ejection of lava 

 and scoriae through a volcanic orifice? The dislocation having been effected 

 in that portion of the upper crust of the earth, now forming a mountain, must 

 not superficial vibrations, proceeding from a focus not deeply seated, have 

 been propagated over the surrounding country ? And, whatever direction 

 these dislocations took, would not an area, elongated in the same line, have 

 been aff'ected by the vibration ? 



In drawing this parallel, I state my belief, that those earthquakes, with 

 their secondary shocks, which are attended by such phenomena as accom- 

 panied the earthquake of Concepcion in 1835, are caused by the rending of 

 great masses of strata, and their interjection by fluid rock; — a process which 

 must have formed one step in a line of elevation. 



VOL. v. SECOND SERIES. 4 L 



