588 Geological Society, 



perfectly convinced, from the numerous points of analogy which ex- 

 ist between these phaenomena and simple eruptions, that they belong 

 to the same class of events ; but he makes this distinction, that 

 earthquakes, unaccompanied by eruptions at the chief point of dis- 

 turbance, are followed by a vast number of minor shocks. These, 

 he believes, indicate a repeated rending of the strata beneath the 

 surface ; whereas, in an ordinary eruption, a channel is formed du- 

 ring the first outburst. 



Among other phaenomena belonging to earthquakes, Mr. Darwin 

 alluded to their affecting elongated areas. Thus the shock in Syria, 

 m 1837, was felt on a line 500 miles in length by 90 in breadth; 

 and those in South America are felt along 800 and 1000 miles of 

 coast, but are on no occasion transmitted across the Cordillera to a 

 nearly equal distance; and, as a consequence, the inland towns 

 are much less affected than those near the coast. He does not con- 

 ceive, however, that the disturbances proceed from one point, but 

 many ranged in a band, otherwise the linear extension of earth- 

 quakes would be unintelligible. For instance, in 1835, the island 

 of Chiloe, the neighbourhood of Concepcion and Juan Fernandez 

 were all violently affected at the same time. 



The last consideration which Mr. Darwin entered upon indicating 

 the cause of earthquakes, is, that in South America they have been 

 generally accompanied by elevation of the land ; though it is not a 

 necessary concomitant, at least to a perceptible amount. But he 

 especially observed, that, as at Concepcion, during the few days suc- 

 ceeding the great shock, several hundred earthquakes, of no incon- 

 siderable violence, were experienced, whilst the level of the ground 

 in that part of the coast certainly was not raised by them (but after 

 the interval of a few weeks, it stood lower,), there is a clear indica- 

 tion of some cause of disturbance, independent of the uplifting of 

 the land in mass. 



In summing up the evidence of phaenomena accompanying earth- 

 quakes, the author is of opinion that the following conclusions may 

 be drawn : — 



1st. That the primary shock of an earthquake is caused by a vio- 

 lent rending of the strata, which, on the coast of Chili and 

 Peru, seems generally to occur at the bottom of the neighbour- 

 ing sea. 

 2ndly. That this is followed by many minor fractures, which, 

 though extending upwards, do not, except in submarine vol- 

 canos, actually reach the surface. 

 3dly. That the area thus fissured extends parallel, or approxi- 

 mately so, to the neighbouring coast mountains. 

 Lastly. That the earthquake relieves the subterranean force, pre- 

 cisely in the same manner as an eruption through an ordinary 

 volcano*. 



* [Those who have perused Sir John F. W. Herschers views on the 

 theory of volcanic action (Babbage's Ninth Bridgewater Treatise, sec. edit, 

 pp. 230-240,) will not fail to recognise the close accordance with them, of 



