EARTHQUAKES. 171 



The effects oftJie impulse, the waves of commotion, may be 

 reduced to simple mechanical theories with more distinctness 

 than is furnished by the consideration of the nature of the 

 first impulse, which indeed may be regarded as heterogeneous. 

 As already observed, this part of our knowledge has advanced 

 essentially in very recent times. The earth-waves have been 

 represented in their progress and their propagation through 

 rocks of different density and elasticity ; 15 the causes of the 

 rapidity of propagation, and its diminution by the refrac- 

 tion, reflection, and interference of the oscillations have been 



Annales de Chimie et de Physique, tome Iviii, 1835, pp. 8486). In 

 the description of his memorable ascent of Chimborazo (Ascension au 

 Ckimborazo le 16 Dec. 1831, loc. cit. p. 176), he says again: "Like 

 Cotopaxi, Antisana, Tunguragua, and the volcanoes in general which pro- 

 ject from the plateaux of the Andes, the mass of Chimborazo is formed 

 by the accumulation of trachytic debris, heaped together without any 

 order. These fragments, often of enormous volume, have been elevated 

 in the solid state by elastic fluids which have broken out through the 

 points of least resistance ; their angles are always sharp." The cause 

 of earthquakes here indicated is the same as that which Hopkins calls 

 " a shock produced by the falling of the roof of a subterranean cavity," 

 in his "Analytical Theory of Volcanic Phenomena" (Brit. Assoc. Report, 

 1847, p. 82). 



15 Mallet, Dynamics of Earthquakes, pp. 74, 80, and 82 ; Hopkins, 

 Brit. Assoc. Report, 1847, pp. 7482. All that we know of the waves 

 of commotion and oscillations in solid bodies shows the untenability of 

 the older theories as to the facilitation of the propagation of the move- 

 ment by a series of cavities. Cavities can only act a secondary part in 

 the earthquake, as spaces for the accumulation of vapours and con- 

 densed gases. " The earth, so many centuries old," says Gay Lussac 

 very beautifully (Ann. de Chimie et de Phys. tome xxii, 1823, p. 428), 

 " still preserves an internal force, which raises mountains (in the oxi- 

 dized crust), overturns cities and agitates the entire mass. Most moun- 

 tains, in issuing from the bosom of the earth, must have left vast cavi- 

 ties, which have remained empty, at least unless they have been filled 

 with water (and gaseous fluids). It is certainly incorrect for Deluc an<? 

 many geologists to make use of these empty spaces, which they imaghu 

 produced into long galleries, for the propagation of earthquakes to a 

 distance. These phenomena, so grand and terrible, are very powerful 

 sonoro\is waves, excited in the solid mass of the earth by some commo- 

 tion, which propagates itself therein with the same velocity as sound. 

 The movement of a carriage over the pavement shakes the vastest edi- 

 fices, and communicates itself through considerable masses, as in the 

 deep quarries below the city of Paris." 



16 Upon phenomena of interference in the earth-waves, analogous to 

 those of the waves of sound, see Cosmos, vol. i, p. 211, Bohn'a edition, 

 and Humboldt, Kleinere Schriften, Bd. i. p. 379. 



