xxn EARTHQUAKES THAT SHAKE 253 



the unusual strain, fall into the ruts of the road, one 

 feels the earth shaken. Asclepiodotus has put it 

 on record that on one occasion the fall of a rock 

 that was torn off from the mountain-side caused 

 by the tremor the collapse of some houses in its 

 vicinity. Just the same thing may occur beneath 

 the earth ; parts of the overhanging crags may 

 be loosened and fall with great weight and noise 

 upon the floor of the cavern beneath, and with a 

 violence proportionate to the weight of the mass 

 and the height of the fall. The whole roof of the 

 subterranean valley is disturbed by an occurrence 

 of this kind. It is conceivable, too, that rocks are 2 

 not always wrenched off by their own weight ; when 

 rivers roll over them, the constant moisture weakens 

 the joints of the stone, and day by day bears away 

 part of its fastening, causing abrasion, so to speak, 

 of the skin in which the stone is enclosed. The long 

 waste of ages, through constant daily rubbing, by 

 and by so weakens the fastenings that they cease 

 to be able to sustain their burden. Then blocks 3 

 of vast size fall down, then the crag hurled head- 

 long will not suffer anything to stand that it strikes 

 in the rebound from its fall, but 



Comes away with a roar ; and all things seem suddenly to rush 

 headlong, 



as our countryman Virgil says. Such must be the 

 cause of the earthquake that shakes the ground 

 beneath. Now I must pass on to the second kind. 



XXIII 



THE earth is naturally full of cavities, containing 

 much empty space. Through these cavities air 



