PT. i. Astronomy and Geology compared. 53 



the Earth itself and the various revolutions whicli it 

 has undergone, and secondly the different phases of 

 animal and vegetable life which seem to have ac- 

 companied or followed these physical and material 

 changes. As I observed above, it is one of the 

 imperfections of this Science that its chronology is 

 utterly vague : we do not seem to have the slightest 

 means of calculating the duration of time in the 

 successive epochs of the Earth's surface. All that 

 the Geologist hopes to demonstrate is that these 

 changes take place in a certain regular succession, 

 which he has been enabled to classify, and that the 

 animal and vegetable creations correspond in a con- 

 siderable degree with these epochs. They do not, 

 however, appear to do so strictly : the animal and 

 vegetable productions of which the relics are found 

 in the different strata are not separated from each 

 other by a sharp line of demarcation coincident with 

 the strata. It would appear that the animal and 

 vegetable productions of an earlier age gradually 

 fade and melt away and are replaced by a gradual 

 and diversified process by those of a later, the older 

 ones rarely becoming abruptly extinct, but some 

 variety of them being long retained, generally in 

 modified forms. Particular periods, however, seem 



