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Show 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. We do, indeed, find localities wThich have twice been the scene of such devastations during the entire period of which systematic geology takes cognizance, just as battles have more than once been fought on the same plain with centuries 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. This 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- |