124 EARTHQUAKES. 



walled up by the great Andes, whose innumerable ramifications, rising far above the line of 

 perpetual snow, yet belong to an epoch subsequent to that of the western ranges. Whether 

 thrown up at one continuous impulse, or the result of successive upheavals, geologists are not 

 yet agreed, and perhaps a fact to be mentioned presently may not be wholly without interest 

 in this connexion. The immediate surface of the plain of Santiago inclines in two directions 

 from the Andes towards the sea, and from north to south. It consists of alluvial detritus, which, 

 wherever penetrated, shows that water once flowed from north to south ; and, as the porphy- 

 ritic masses rise with moderate inclination on each side, we may infer that the depth of the 

 deposit is not very great. 



If originated in either of these homogeneous formations, the earth-wave will move uni- 

 formly in every direction from the centre of impulse, and arrive simultaneously at equidistant 

 points; but the moment it encounters a stratum of different density, a portion is reflected, and 

 another portion refracted, so that the times and directions at which it will reach each locality 

 will vary with the density or refractive indices of the several media through which it has to 

 pass. Where it passes from a formation of high elasticity to one of low elasticity, or vice 

 versa, it will be partly reflected, and a wave sent back will produce a shock in the opposite 

 direction; and it will be partly refracted, that is to say, its course onwards will be changed, 

 and shocks will be felt upwards, downwards, and to the right and left of the original line of 

 transit. But as the velocity of transit is probably the same as that of sound through the same 

 media, and the surface of the comparatively narrow basin of low elasticity rests on strata of 

 highly elastic rock, if the centre of impulse be deep seated, the contents of the basin will be 

 constrained to vibrate as one system with its walls the Andes and Western cordilleras. Thus, 

 on the supposition that the origin is profound, until the thickness and composition of the strata 

 composing the solid crust west of the Andes.be determined, it appears a hopeless task to 

 attempt locating the centre of even a violent earthquake. Taking that of 2d April, 1851, as 

 the best example in point, the lines of transit determined with reasonable accuracy at Santiago 

 and Valparaiso, if projected without reference to different media, meet in latitude 31, longi- 

 tude 69 50' a spot within the Andes. But we know that San Felipe, Santa Kosa, and Quil- 

 lota, all lying nearer to this spot, were scarcely disturbed ; whilst, if the violence of the shock 

 be in proportion to the distance from the centre of impulse, that centre must have been nearer 

 Casablanca and Curacavi than either of the cities named. That this was not deep-seated, 

 seems indicated by the effects observed at the southern extremity of the basin, where the onward 

 course of the vibration was almost wholly arrested by the two mountain chains, and the reflected 

 shock was far more destructive than the normal vibration had been two or three leagues further 

 to the north. Such may also have been the occasion of the ruin at Casablanca and Curacavi, 

 which lie in small alluvial basins west of the mountain range that bounds the plain the 

 former at an elevation of 803 and the latter 633 feet above the ocean. Even more unsatisfactory 

 was the previous 6th of December earthquake, whose line of transit was N.W. and S.E. ; yet 

 Quillota, 64^ miles distant, in precisely that direction, experienced only a tremor. 



It has been mentioned that all the violent earthquakes of which we have reliable accounts, 

 before that of April, 1851, were submarine, as could be shown from one concomitant the 

 great sea-wave which followed them. In accordance with Mr. Mallet's views, which are fully 

 borne out by observation, when the original impulse comes from beneath the bed of the deep 

 ocean, six sorts of waves are formed and propagated, first. One or several successively, 

 through the land the true earthquake shock or shocks. Second. The forced sea-wave, which 

 is formed as soon as the true shock or undulation of the bottom of the sea gets into shallow 

 water, forcing up an aqueous ridge directly above itself, which it brings to shore, and causes 

 the slight disturbance of the margin of the sea often remarked at the moment of the shock. 

 Third. Coincident with and answering to every shock, one or more sound-waves through the 

 earth. Fourth. A sound-wave through the sea, which arrives after that through the earth. 

 Fifth. A sound-wave through the air. When there is more than one impulse, or a single one 



