42 Astronomy and Geology compared. PT. i. 



catastrophes, and have supposed the crust of the 

 Earth broken by violent internal convulsions, lofty 

 ranges of mountains thrown up, volcanic action 

 modifying the whole surface of the Grlobe, earth- 

 quakes and deluges transforming the face of nature. 

 Others have attributed more powerful effects to quiet 

 and imperceptible agencies constantly operating, such 

 as the detritus of the higher mountains carried down 

 by torrents or by rain, the alteration in the level of 

 the sea occasioned by deposits at the mouths of large 

 rivers or by currents operating throughout its extent. 

 Some have adopted the igneous theory, and have 

 supposed that the Earth was originally launched 

 forth in an incandescent state, and has gradually 

 cooled in the lapse of ages. Others have taken up 

 t,he glacial hypothesis, and have found traces of a 

 period when the Earth was much colder than at 

 present and covered with snow and ice. 



All these theories, which it is difficult indeed to 

 prove, may have some foundation. If we assign to 

 the World a vast duration in point of time, it may 

 have passed through successive periods containing 

 all these diversities, and each leaving its mark. 

 Traces of all these agents may still be found, and 

 some are still actually at work. Earthquakes and 



