56 VULCANISM ITS NATURE AND FUNCTION. 



the earth varying from the gentlest tremor to agitations 

 so violent that the solid crust is fractured, one portion 

 thrown up and another thrown down, the sea-bed uplifted 

 into dry land, and the dry land submerged beneath the 

 waters. The phenomena that accompany earthquake con- 

 vulsions are extremely varied. Occasionally they are preced- 

 ed by an unusual stillness and sultriness of the atmosphere ; 

 low hollow rumblings, more audible than felt ; and great 

 restlessness and terror among birds and mammals, as if the 

 instincts of these were keener than human perception. At 

 other times there is no premonition, but all at once a few 

 sma*t concussions, passing away in a certain direction, or 

 not unfrequently spreading from a central point in dimin- 

 ishing intensity. On other occasions, however, the mo- 

 mentary concussions return after a short pause with increased 

 vehemence, and then there is a perceptible undulation of 

 the earth's crust, as if it were passing away from beneath 

 the feet of the spectator an uplift, a shock, a series of 

 giddy shocks, and the work of destruction. Though con- 

 tinuing at most for a few seconds, these violent shocks 

 generally result in extensive fracturing of the rocky crust. 

 Yawning rents and fissures, gaseous discharges, bursting 

 forth of new springs, absorption of streams, changing the 

 course of rivers, elevation of the sea-bed, submerging of 

 the dry land, and the conversion of populous cities into 

 masses of ruin and rubbish, are the not unfrequent but 

 destructive effects of the earthquake. At times too, as the 

 water in a vessel that has been agitated and then brought 

 suddenly to rest strikes forcibly over its margin, so the sea, 

 its floor having been shaken, is frequently thrown into 

 violent waves (earthquake-waves), which rush forward 

 against the land to the height of 40, 50, or 60 feet, and 

 sweep everything into destruction before them. The wave 

 that rolled in upon the coasts of Portugal after the great 



