EARTHQUAKES. 55 



volcanic hills to the ancient mountain-ranges, the mind is 

 altogether unable to grasp the cycles that must have elapsed 

 since their formation. 



Presuming, then, that volcanic hills are chiefly masses of 

 accumulation and not of upheaval, the repeated eruptions 

 that take place must necessarily fracture and derange the 

 continuity of the surrounding district, and thus every 

 igneous centre is marked by such accompaniments as hot- 

 springs, boiling mud -springs, discharges of sulphurous 

 gases, and the like, better known perhaps as the suffi- 

 oni and solfataras of Italy, and the salses, hornitos, and 

 hervideros of Mexico and South America. Such minor 

 discharges are the normal accompaniments of all active 

 volcanoes, and long after activity has ceased they form the 

 residual phenomena, and indicate by their declining force 

 and numbers the distance both in time and place of the 

 fiery forces that once operated below. No doubt springs of 

 considerable temperature may exist in districts long since 

 quiescent, and now far removed from volcanic activity 

 (those of Bath and the Pyrenees, for example); but the 

 monticules thrown up by mud -volcanoes and escapes of 

 heated and sulphurous vapours generally mark either the 

 proximity of igneous activity or the comparative recentness 

 of its manifestations in the area. The whole are merely 

 indications of the same thermal agency that internal fire- 

 force which Humboldt has so appropriately included under 

 the name of Yulcanism or Vulcanicity. 



The next great manifestation of vulcanism is the Earth- 

 quake, a distinction made in scientific as well as in every- 

 day language ; for though the earthquake is generally the 

 close concomitant of the volcano, yet its throes may be felt 

 in districts where no volcano has existed for ages. This 

 motion, as the name implies, is a quaking or trembling of 



