ON ITS EXTERIOR. EARTHQUAKES. 171 



With greater clearness of view than is attainable in 

 considerations respecting the nature of the first impulse 

 (which, indeed, may be supposed to be of different kinds), 

 we are able to reduce the action originated by it, i.e. 

 the undulations of the earthquake-waves, to simple me- 

 chanical theories. As I have already remarked, this 

 part of our knowledge of nature has latterly been very 

 materially augmented. The undulations have been de- 

 scribed in their progress and in their propagation through 

 rocks of different degrees of density and elasticity ; ( 239 ) 

 and the causes of the diminution in their velocity of 

 propagation by the occurrence of interruptions, reflexions, 

 and interferences of waves ( 24 ) have been mathematically 

 investigated. The apparently rotatory (or " vorticose ") 

 shocks, of which the obelisks in front of the convent of 

 San Bruno, in the little town of Stephano del Bosco m 

 Calabria (1783) present an example which has been 

 much discussed, have been attempted to be reduced to 

 rectilinear shocks. ( 241 ) Waves or undulations of air, of 

 water, or of earth, do, indeed, all follow as respects 

 space the same known dynamical laws ; but the earth- 

 waves are accompanied in their devastating action by 

 phenomena which by their nature remain enveloped in 

 greater obscurity, and belong to the class of physical 

 processes. Such are exhalations or outpourings of 

 vapours in a state of tension, gases, or (as in the small 

 moving Moya-cones of Pelileo,) gritty mixtures of py- 

 roxene crystals, carbonaceous matter, and silicious- 

 shelled infusoria. These moving cones have overturned 

 many Indian huts.( 242 ) 



In the general Description of Nature in the first 

 volume, in connection with the great catastrophe of 



