THE REVOLUTIONS OF THE CRUST OF THE EARTH. 295 



ground. The testimony of our senses strengthens our security. Does 

 this solid, and as we supposed thoroughly reliable surface commence to 

 tremble, a moment suflflces to destroy the experience of a life-time. An 

 unknown power is suddenly revealed ; the calm of nature was only an 

 illusion, and we feel ourselves violently thrown back into a chaos of 

 destructive forces. Then every noise, every breath of the wind, excites 

 atttention; we distrust above all the earth we tread upon. Even the 

 animals are disturbed by the unusual phenomenon, and accompany 

 every shock with cries and bellows, while the birds indicate the approach 

 of the earthquake by their uncertain and terrified flight." 



The third kind of oscillation of the ground is probably only a modifi. 

 cation of the undulatory movement. It is known that there are often 

 many spots within the radius of activity of an earthquake which 

 remain intact, an island amidst countries destroyed. This is because the 

 ground is not of a character to transmit the movement. Suppose then 

 a disposition of similar ground in a horizontal direction to retard the 

 progress of the waves at one angle more than at another, the rotary 

 movement would be produced which curves the rectilinear boundaries 

 of plantations and changes the orientation of buildings. The triangu- 

 lar obelisk of the Episcopal square at Agram was, three years ago, a 

 curious example of this phenomenon. The angles of the superposed 

 parts of the obelisk were displaced horizontally at least 25°. A repara- 

 tion of the obelisk has effaced this proof of the existence of the rotary 

 movement, which, however, is neither as frequent nor as important as the 

 two others. 



Thus far we have said nothing of the cause of earthquakes. Is 

 this cause the giving way of subterranean caverns, or the sinking of 

 rocky masses ? Is it the effect of the dilation of the crystals which are 

 formed in a supersaturated solution, as Mr. Volger believes, or is it a 

 subterranean storm, as was declared by the ancient philosophers ? 



All these hypotheses, and many others that have been advanced upon 

 this subject, are far from being entirely false, but their application can 

 be only very limited and often purely local, i^fow we require a theory 

 which finds a reason for the phenomena in the physical constitution of 

 the earth; an agent whose powerful action can be proved in the geologi- 

 cal periods and whose important effects may be foreseen in the future. 



Here, as before, we have recourse to the central heat, which maintains 

 in a state of liquidity the greater part of the terrestrial mass, except 

 a thin solid envelope which covers it. The supposition that this is the 

 cause we are seeking coincides with what we know of the mobility of 

 the crust of the globe. We will say, then, that it is the loss of the cen- 

 tral heat which produces directly or indirectly ruptures and sinkings of 

 the terrestrial crust, the mechanical effects of which are transmitted by 

 waves over great surfaces and give rise to earthquakes. It should be 

 added that many other circumstances may modify this action. Thus 

 earthquakes are most frequently felt in the vicinity of the sea and of vol- 



