SECTION I 



SUBTERRANEAN OR IGNEOUS AGENCIES 



CHAPTER II 

 INTERIOR CONSTITUTION OF THE EARTH — VOLCANOES 



No problems of geology are more difficult and "obscure than 

 those connected with the internal constitution of the earth, and 

 satisfactory explanations of the subterranean agencies have not yet 

 been devised. This is because the interior of the earth is so 

 completely beyond the range of direct observation. The deepest 

 boring is hardly more than 40 1 00 part of the earth's radius, and 

 data derived from the temperature observations of such shallow 

 openings leave a great deal to conjecture. The enormous press- 

 ures also which obtain within the mass of the globe, are such that 

 we can form but inadequate conceptions of their effects. 



Temperature of the Earth's Interior. — Volcanoes, which eject 

 white-hot and molten lavas, and thermal springs, which pour out 

 floods of warm or even boiling water, plainly indicate that the 

 interior of the earth is highly heated, at least along certain lines. 

 Direct observations tend to prove that this high temperature is 

 universally diffused through the mass of the earth. At the sur- 

 face of the ground and for a short distance below it, the tempera- 

 ture varies, like that of the air, though to a less degree, between 

 day and night and between different hours of the day. Farther 

 down, the daily variation ceases, but there is a difference of tem- 

 perature at different seasons, it being lower in winter than in 

 summer. As in the case of the superficial layer, the range is less 



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