50 Internal Heat of the Earth 



descended thousands of feet towards the centre of the 

 earth, by gradual sinking of the sea-bottom, and the 

 simultaneous piling up of newer strata upon them. The 

 layer that is formed to-day beneath the water forms the 

 actual sea-bottom ; but neither the land nor the sea- 

 bottom are steady. The land is in places slowly de- 

 scending beneath the sea, and sea-bottoms are them- 

 selves descending also. It has frequently happened, 

 therefore, that for a long period a steady descent over 

 a given area has taken place, and simultaneously with 

 this many thousands of feet of strata have by degrees 

 accumulated bed upon bed, as for example in the Pacific 

 Ocean in the region of modern atolls and barrier coral 

 reefs. 



As we descend into the earth the temperature rises, 

 whence, in the main, the theory of central heat has been 

 derived. In our latitude heat increases about 1 for 

 every sixty feet, and the temperature therefore, at so 

 great a depth as 30,000 feet, to which it could be 

 shown some strata have sunk, may at present be about 

 500. Furthermore, strata that were deposited hori- 

 zontally have been frequently disturbed and thrown 

 into rapid contortions, or into great sweeping curves ; 

 and by this means especially, strata which once were at 

 the surface have often been thrown twenty, thirty, or 

 forty thousand feet downwards, and therefore more 

 within the influence of internal heat, as, for instance, 

 in the bed marked * fig. 12, which may be supposed to 

 represent a large tract of country. I do not wish it to 

 be understood that the globe is entirely filled with 

 melted matter that is a question still in doubt ; but 

 were this book specially devoted to general questions of 

 theoretical geology, I think I could prove, that the heat 

 in the interior of the globe in places sometimes appar- 



