248 POPULAR SCIENCE MONTHLY. 



tions is considered tlie primary source from wliich injections are 

 derived. 



It lias been thought by some who have speculated on the 

 condition of the earth's interior that isolated reservoirs of fused 

 rock exist in the generally cool earth's crust, due to unequal cool- 

 ing ; another origin for such lakes of lava may be postulated if 

 we consider them as injected magmas not yet cooled. 



A mental picture of the probable occurrences that give origin 

 to subterranean intrusions and volcanoes, and account for many 

 observed phenomena in this connection, may be sketched in out- 

 line as follows : 



The earth is hot and potentially plastic within, and cold and 

 rigid at the surface. Unequal cooling and the shifting of material 

 on the surface are disturbing conditions that tend to change the 

 shape of the plastic interior, and to crumple and break the crust. 

 If a fissure forms in the lower surface of the crust, the poten- 

 tially plastic material beneath will become plastic on account of 

 the removal of resistance to pressure, and be forced into the break, 

 and a dike be formed. Under certain conditions the plastic ma- 

 terial rising in a fissure may expand between layers of stratified 

 rock so as to form laccolites, subtuberant mountains, etc. 



If a break extends entirely through the crust, molten material 

 forced into it may reach the surface. As the molten lava rises in 

 such a break, it passes through rocks that are more and more 

 highly water-charged, the water is vaporized, or perhaps its ele- 

 ments are dissociated, and the vapors and gases formed are ab- 

 sorbed by the fluid rock. As the lava comes to the surface the 

 steam and gases absorbed under great pressure escape and furnish 

 some of the most striking phenomena of volcanic eruptions. Loss 

 of heat as a magma nears the surface also favors the escape of 

 occluded gases. 



In this view of the nature of volcanoes it is evident that an 

 arrest of pressure on the reservoirs from which they draw their 

 lavas would stop their action. If a fissure extends through the 

 earth's crust to the potentially plastic interior, it is difiicult to see 

 how an outflow of molten material would be checked unless the 

 conduit should become closed. Under the vast pressure that 

 exists at a depth of several miles it is impossible to comprehend 

 how fissures can exist, but the plastic material beneath is under 

 pressure of a similar order of magnitude, tending to force it out 

 through any opening that may be formed. A near balance be- 

 tween the pressure tending to close a fissure and the pressure on 

 the magma below tending to maintain a communication with the 

 surface may therefore be conceived to exist ; when the balance is 

 in favor of extrusion volcanic eruptions follow, and when the 

 reverse is the case the conduits become closed. 



