GEOLOGY. 



Historical Table of the Revolutions of the Globe. 



INTRODUCTION. 



The earth which we inhabit is one of eleven planets which 

 revolve around the sun. It has two motions : one of rotation, 

 which it performs in twenty-three hours, fifty-six minutes, 

 four seconds, and whence resujts the alternation of day and 

 night ; the other of projection around the sun, or the annual 

 motion, which is completed in three hundred and sixty-five 

 days, five hours, forty-eight minutes and forty-five seconds. 

 Its form is that of a sphere flattened towards the poles. Its 

 surface is about twenty-five million seven hundred and ninety 

 thousand four hundred and forty square leagues, three-fourths 

 of which are occupied by the sea. Its semi-diameter at the 

 equator is about one thousand four hundred and thirty-five 

 leagues. The science which treats of the structure and com- 

 position of the globe, together with the changes it has expe- 

 rienced since its first existence, is called Geology. The 

 principal fact upon which this branch of human knowledge 

 reposes is the increase of heat which has been remarked in 

 descending mines, and which, at its least expression, is about 

 one degree for about every twenty-five metres of depth; 

 whence it would result, if we suppose this increase to con- 

 tinue in the same proportion, that, at the depth of a half 

 league, water could no longer remain liquid, and that at 

 twenty or twenty-five leagues the heat would be sufficient to 

 melt most rocks. This central heat has given rise to the 

 supposition that the interior of the globe is in the condition 



