CHAPTER XLIII. 



EARTHQUAKES AND VOLCANOES. 



IN the geological history of the earth, two sets of forces have 

 been active. Some which may be called plutonic, have been mak- 

 ing the surface of the earth uneven, through them the conti- 

 nents had their origin and the mountains were formed. The 

 others, which are eroding forces, mostly atmospheric, have been 

 equally busy in cutting down the elevations and filling up the 

 depressions, their action tending toward uniformity as the others 

 toward variety. 



The internal condition of the earth is not well understood. As 

 we explore the crust by mines and wells, we find that heat in- 

 creases rapidly with the depth below the surface, so that at a 

 depth of thirty or forty miles, the heat must be intense enough 

 to melt any known substance, if it were only under atmospheric 

 pressure, but under the enormous pressures of such regions, sub- 

 stances would doubtless behave as solids, notwithstanding the 

 great heat. 



In relation to other celestial bodies the earth behaves as a 

 solid, rigid as glass, so that while the interior of the earth is in- 

 tensely hot, it is probably not a liquid. 



In general the irregularities of the surface of the earth are sup- 

 posed to be due to unequal contraction, the interior contracting 

 more rapidly than the crust. This results in enormous horizon- 

 tal pressures under which, along lines of weakness, the crust swells 

 up into wrinkles, folds or corrugations. The lines of weakness are 

 accounted for by supposing an irregular distribution of matter, 

 or that there was more radial contraction, or settling over the 

 area now occupied by the oceans. This idea is strengthened by 

 the fact that the matter underlying the oceans is more dense than 

 that underlying the continents. Whatever may have been the 

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