EARTHQUAKES. 123 



the former wave should have the same elevation as the latter; but such fluid ridge would 

 scarcely be generated at the base of precipitous cliffs with deep water. If the centre of im- 

 pulse "l>c at small depth IM-|.\\ th- surface, tin- shock will \m felt principally horizontally : but 

 if tlie origin lie profound, tin; shock, which in propagated from it in every direction in *pherical 

 shells, will he felt more or loss verlii -.illy ; and in this case, also, we may be able to notice two 

 distinct waves, a greater and a loss, following each other alum-i in-t.uit;iii.- (l iisly the first due 

 to the originating normal wave, the second to the wave vibrating at right angles to it." II- 

 estimates tin- amplitude of the earth-wave at several miles, and its velocity of translation at 

 thirty miles per minute, so that it often takes ton or twenty seconds to pass a given point. 

 During its passage a continuous tremor is often felt, which arises from secondary waves upon 

 its surface, like the small curling waves on the surface of the ocean swell. When strata are 

 fractured, or masses of matter blown away, at volcanic vents, then, and then only, (he says,) 

 at the moment of shock, or nearly coincident with it, wo hear the sound-wave through t/ie earth, 

 and, at an interval after this, the sound-wave through the air. 



This does not accord accurately with many previously observed facts, nor with our experience. 

 When Riobamba (Ecuador, latitude ] 8.) was destroyed in February, 1797, there was no 

 noise; and the detonation at Quito and Ibana, heard more than a quarter of an hour alter the 

 catastrophe, was inaudible at Tacunga and Hambato, both nearer the centre of explosion.* 

 Nor did the least tremor occur, either in the deep mines or on the surface, during the subterra- 

 nean thunders of Guanaxuata in 1785. These lasted for above a month; and for some days 

 were "as if there were storm-clouds under the feet of the inhabitants, in which slow rolling 

 thunder alternated with short thunder-claps, "f It is only considered necessary to mention 

 these extreme prior instances one an awful convulsion without audible noise, the other an 

 excessive noise without tremor, because analogous cases, though of far less violence, con- 

 stantly occur in our observations. Subterranean noise, however, is a most general concomitant, 

 its intensity and tone varying with every repetition of the phenomenon, though never to such a 

 degree as by possibility to be mistaken for any other. When the crust of the earth is about to 

 be agitated at any place, there is previously heard a sound resembling the roll of a muffled 

 drum, faint at first, but rapidly louder, and then dying away in the distance beyond us. After 

 an interval varying from one to ten or more seconds, the earth moves slightly, and is still the 

 violence of this shock being inversely as the time during which the earth-wave was coming after 

 the sound-wave was detected. Within ten seconds another and more severe shock almost 

 invariably follows, and frequently without farther rumbling noise, sometimes slow and wave- 

 like, at others quick and vibratory ; and quite often the motion is such as would result from 

 rapid vertical concussions. 



In the report of Mr. Hopkins, the mathematical laws which govern the propagation of vibra- 

 tions through fluid and solid masses are ably discussed, and simple instructions and formula 1 are 

 given by which the centre of disturbance may be determined. But these formulae are of neces- 

 sity based on the supposition of perfect homogeneousness in the strata of the agitated district a 

 geological condition that varies in Chile with every step, and renders the application of his argu- 

 ments practically impossible. That extensive tracts are disturbed simultaneously, and that the 

 centres of impulse are not permanent, have already been referred to as sustained by the obser- 

 vations ; and there remain to be pointed out the obstacles to satisfactory conclusions of either 

 the geographical position or depth below the surface of the impelling agent. 



The great longitudinal plain, or, more properly speaking, the series of basins on which most 

 of the cities of Chile are built, are also diversified by isolated hills of stratified porphyry, 

 sometimes rising to a height of 1,500 feet. Immediately bounding their west sides is a range of 

 porphyritic mountains. From thence to the ocean, more than sixty miles, there are chains 

 of granite, with narrow intermediate valleys and smaller basins ; and their eastern border is 



Humboldt : Cosmos. t Ibid. 



