PT. i. Astronomy and Geology compared. 21 



The extent embraced by Geology is no doubt a 

 very large one, particularly when considered with 

 reference to the earth and to man. It includes not 

 only the whole existing surface of the Globe, but a 

 number of former surfaces which it would appear 

 have been successively covered by new formations. 

 Geology may thus be regarded as embracing not 

 only the present surface of the Globe, but several 

 other former surfaces now buried beneath it. When, 

 however, we contrast it with the extent of the whole 

 Solar System, it shrinks into comparatively small 

 proportions. We find that the Earth itself is but a 

 planet of a secondary magnitude and importance in 

 the system. The eight principal planets which 

 revolve round the Sun may be divided into two 

 classes: the four nearest the Sun, consisting of 

 Mercury, Venus, the Earth, and Mars ; and the four 

 most remote, composed of Jupiter, Saturn, Uranus, 

 and Neptune. The difference in magnitude of the 

 four nearest and the four most remote is very con- 

 siderable, Jupiter being 590 and the Sun 602,000 

 times larger than the four Terrestrial planets com- 

 bined. The division as to space also is great, since 

 the first four planets revolve within an area the 

 radius of which is not one-third of the distance be- 

 tween the Sun and Jupiter and one-twentieth of the 



