546 POPULAR SCIENCE MONTHLY 



reservoirs are a part of the original constitution of the earth, and have 

 lain in their present position through all the vast period of the earth's 

 evolution, waiting for a convenient occasion to explode and pour forth 

 their fiery contents. It regards the reservoirs as having no real exist- 

 ence as such, and as containing no liquid eruptible contents until some 

 source of heat acts upon them and liquefies a portion of the strata, 

 thus giving rise to the reservoir. When a sufficient quantity of the 

 lava is melted to rupture its covering, the eruption follows. It con- 

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

 is extravasated. 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 can not 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 difficult 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 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 re- 

 membered. These volcanos must have their origin at less depth than 

 the lava eruptions. The temperature of erupted mud is not accurately 

 known, but it can not 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 



