CHAPTER IV. 



IGNEOUS AGENCIES. 



ALL the agencies which we have thus far discussed 

 tend to destroy the great inequalities of the earth-surface 

 by cutting down the land and filling up the seas. They 

 are therefore called leveling agencies. If they alone acted, 

 they would eventually bring all to the sea-level and inau- 

 gurate a universal ^ocean. These agencies, however, are 

 opposed by igneous or by elevating agencies, which, acting 

 alone, would make the inequalities much greater than we 

 now find them. The actual amount and distribution of 

 land are the result of the state of balance between these 

 two opposite forces. It is well to observe that the leveling 

 forces are derived from the sun, while the elevating forces 

 are derived from the interior of the earth being in fact 

 connected with interior heat. It becomes necessary, 

 therefore, first of all to discuss this subject. 



Interior Heat of the Earth. 



The surface temperature of the earth varies with lati- 

 tude, but the mean is about 60. At any place the 

 surface temperature varies between night and day (daily 

 variation),, and between summer and winter (annual 

 variation). As we go below the surface, both the daily 

 and the annual variation become less and less, and finally 

 disappear. The daily variation disappears in a few feet, 

 but the annual variation continues and disappears in our 

 latitude only at a depth of sixty or more feet. Below 



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