126 GEOLOGY OF THE HIGH PLATEAUS. 



cause of volcanic eruptions, or the force which has brought them to the 

 surface. 



Not only are volcanic phenomena very local in respect to area, but the 



•period of activity in any given spot is very limited in respect to duration. 

 No region has always been eruptive, and we may be reasonably confident 

 that none will continue to be eruptive indefinitely. Volcanicity has its 

 inception, passes through its cycle, and lapses into final repose. Wo do, 

 indeed, find localities which have twice been the scene of such devastations 

 during the entire period of which systematic geology ta,kes cognizance, just 

 as battles have more than once been fought on the same plain with cen- 

 txiries between; but the intervals separating such visitations are so vast 

 when measured even by the geological standard of time^ that there is no 

 obvious relation between them. It is not strange that a process which 

 shifts its arena throughout the ages should occasionally revisit the scenes of 

 former operations. This migratory character suggests to us that the normal 

 condition of the nether regions is not one of unrest, but rather of quietude. 

 What is the disturbing element which invades their secular calm, convulses 

 them with earthquakes and explosions, and causes them to pour forth their 

 fiery humors? With this problem geologists and physicists have wrestled 

 in vain. Here speculation seems to be peculiarly unfruitful. To-day it 

 looks promising; to-morrow turns it into ridicule. We do not know the 

 determining cause of volcanic eruptions. Yet there are a few facts of a 

 high degree of generality, around which we linger with inquiring, anxious 

 minds, hopefully promising ourselves that light will shine out of them at 

 some future day, and to these it may be proper to briefly advert. 



We may contrast the explosive condition of volcanic products during 

 an eruptive cycle with their quiet and inert condition before the cycle 

 began. These same materials lay quietly in the earth for long periods, 

 some of them, perhaps, since that imagined primordial epoch when a crust 

 began to form. Some change has come over them, converting them into 

 energetic explosive mixtures. The problem is to find an adequate cause 

 for such a change and the nature of its operation. Tliis statement of the 



•conditions of the problem is in strong contrast with the view which regards 

 lavas as primordial liquids charged with volcanic energy waiting for a con- 



