Phenomena in South America. 617 



1746 at Lima, no less than 200 horrible (I use the language of its historian) 

 shocks were counted. Now in the other case, Ulloa says, that when the ori- 

 fice of eruption is once formed, the earth becomes nearly tranquil ; yet we well 

 know, that the volcano itself almost invariably continues in great activity for 

 many weeks afterwards. Had Ulloa, however, stood near the crater itself, he 

 would undoubtedly have felt those small tremors, which accompany each fresh 

 explosion^ as described by others who have been so circumstanced. The 

 tremors, therefore, seem analogous to the secondary shocks ; and, this being 

 so, the phenomena in the two cases are in every respect closely similar. In 

 a primary volcanic outburst, we know the cause to be the explosion of liquid 

 and aeriform matter, first through solid strata, and afterwards through a nearly 

 open passage ; hence we are led to conclude, that the cause of the simple 

 earthquake, with its secondary shocks, are explosions of a similar nature, 

 which, however, do not open a passage, but rend successively portions of the 

 superincumbent masses. 



At Concepcion, where the streets run in two series, at right angles to each 

 other, the walls were affected, as already observed, according to their direc- 

 tion. This was strikingly exemplified in the cathedral, where the great but- 

 tresses, built of solid brickwork, were cut off as if by a chisel, and hurled to 

 the ground ; whilst the wall, to support which they had been vainly built, 

 though much shattered, stood erect, — for the latter had its extremity directed 

 towards the point whence the vibration travelled, but the buttresses were in 

 lines parallel to the undulation. Nearly similar circumstances were observed* 

 in 1822 at Valparaiso. At the great earthquake of Caraccas the direction 

 of the vibration was E.N.E. and W.S.W., and some definite direction appears 

 to have been observed in almost every violent earthquake. Now, it may be 

 asked, could a vibration, which had travelled upwards through the earth from 

 a profound depth, be felt on the surface, as if it had come from a given point 

 of the compass, and could it likewise determine the overthrow of walls accord- 

 ing to their direction with respect to any such point ? It appears to me clearly 

 not; but that a vibration to produce such effects must be transmitted from the 

 rending of strata, at a point not very deep below the surface of the earth. 



Earthquakes generally affect elongated areas. In the shock of 1837, in Syria, 

 the vibration was felt " on a line 500 miles in length by 90 in breadth f." 

 Humboldt]; remarks, that earthquakes follow the coast of New Andalusia in the 



* See Miers's Travels in Chile, Vol. i. p. 392, 



t Proceedings of Geological Society, p. 540. April 5th, 1837. 



\ Personal Narrative, Vol. ii., p. 224. 



