VOLCANOS AND RADIOACTIVITY 263 



melted to rupture its covering, the eruption follows. It continues 

 until all the lava which exists for the time being in the reservoir is 

 extra vasated. And when all of its ammunition is expended, it must 

 close its action until a fresh supply is provided. 



By an increase of heat, we can readily understand the existence of the 

 lava reservoirs in such anomalous positions near the surface of the earth. 

 The horizon of melted lava, which has a temperature of about 1,000° 

 or 1,200° C, if it depended wholly upon the secular cooling of the earth 

 would be more than thirty miles below the surface, or even forty miles 

 below. We cannot suppose that the cooling of the earth is so extremely 

 unequal as to bring the isotherm of 1,000° C. at one place within two 

 miles of the surface, and in another place carry it thirty or forty miles 

 below. It is equally difhcult to imagine any subterranean disturbance 

 or displacement which could mechanically thrust up near the surface a 

 portion of the solid nucleus of the earth. Such a displacement is not 

 warranted by the geological facts; for while volcanic eruptions occur 

 frequently in localities where the strata are much displaced, they also 

 occur quite as often where there has been no displacement of any 

 moment since the Cambrian age. 



A singular class of phenomena is found in the so-called mud vol- 

 canos, which have always been a great puzzle, but which are easily 

 explained by this cause. We find them in Central America and in Java, 

 and the remarkable case of Bandai San, in Japan, is well remembered. 

 These volcanos must have their origin at less depth than the lava 

 eruptions. The temperature of erupted mud is not accurately known, 

 but it cannot be less than 400° or 500° F. The generation of heat half 

 a mile below the surface would be a sufficient explanation of their 

 origin and action. 



Why should eruptions always emanate from shallow reservoirs and 

 never from deeper ones ? Or, according to the view here put forth, why 

 are eruptive masses formed only at depths of two or three miles, and 

 never at greater depths ? I do not contend that no lava pools are 

 formed at greater depths than three or four miles, but if they are formed, 

 the lava is never erupted, and for the following reason. The pressure of 

 the overlying rock at a depth of three miles is about 18,000 pounds to 

 the square inch. At a depth of four miles it is about 25,000 pounds to 

 the square inch. At such a pressure (25,000 pounds) it would be im- 



