178 FAILURE AND RENEWAL OF VOLCANIC ENERGY. 



the highly compressed steam which it contains ; comparing the 

 ebullition to the expansion which takes place when the cork of a 

 soda-water bottle is drawn. 



Depths at which Volcanoes Originate. It is almost impossible 

 to estimate the depth at which volcanic phenomena originate. Mr. 

 Mallet's investigations assigned to earthquakes a depth of from three 

 to ten miles, and although they occasionally reach a depth of 30 miles, 

 according to the late Dr. Oldham, they are obviously phenomena of 

 the superficial portion of the earth's crust. Although these disturb- 

 ances are probably different in kind from volcanic phenomena, yet 

 they must often be consequences of the disruption in which a 

 volcano originates ; and so far may give some idea of the depth to 

 which water must penetrate before it reappears in a volcanic outburst. 

 There is another form of evidence adduced by Mr. Sorby, in the size of 

 the fluid cavities relatively to the fluid in minerals ejected in blocks 

 of volcanic rock from Vesuvius. Some of these in the mineral 

 nepheline indicate a temperature of 706 F., equal to a pressure of 

 3222 feet. 



Why Eruptions are Intermittent. It is a well-known fact that, 

 in the majority of volcanic regions, eruptions are intermittent. This 

 circumstance is well exemplified in the history of Vesuvius ; for 

 although presenting the form of a volcano, there was no record of an 

 eruption prior to the year 79 ; and it was not till the year 203 that a 

 second eruption is described, while the third took place in the year 

 472 ; and frequently intervals of several hundred years have occurred 

 between the eruptions, though for the last two hundred years the 

 intervals of tranquillity have rarely lasted longer than five years. 

 Almost the only exception to this paroxysmal condition among vol- 

 canoes is furnished by Stromboli, which is unceasingly active, and 

 more like a solfatara than a volcano. The reason for this intermission 

 in the outbursts is not far to seek according to the principles which 

 we have so far developed ; for with the progress of an eruption the 

 amount of water which is given off in the form of steam diminishes ; 

 and if the water slowly infiltrates down to great depths, the supply 

 must be more readily exhausted through the wide vent of an eruption 

 than renewed through minute fissures ; and therefore, as the explosive 

 force fails, the eruption fails But we further conceive that the 

 evacuation from beneath the surface of the vast masses of matter 

 which are poured out in volcanic outbursts, takes place more rapidly 

 than the contraction of the rocks can progress, in consequence of the 

 radiation of heat ; therefore, there comes to be a failure of the lateral 

 pressure, which generates the heat that is the primary cause of an 

 eruption. And until a sufficient interval of time has elapsed for the 

 renewal of these elements of volcanic energy, the eruption must be 

 intermitted. But the intermission may to some extent be due to the 

 strength of the cone, and the security with which its throat and 

 various apertures have been plugged by rocky materials ; but, if the 

 force necessary to burst a way to the surface is small, then feeble 

 eruptions may take place frequently, or even in continuity. 



