the Earth at different Epochs, ^c. 353 



sometimes limited to changes of species of the same genus, or 

 o^ genera of the same family, which take place of each other, 

 without perceptibly altering the mutual relations of the different 

 classes of vegetables to each other. At other times, on the con- 

 trary, on comparing the fossil plants of two formations, of which 

 the one follows the other, there are not only observed specific 

 or generic changes, but certain families entirely disappear, and 

 their number is so changed, with relation to that of the other 

 families, that the proportion of the great classes of the vegetable 

 kingdom is completely modified. 



In thus considering the floras of each geological formation, 

 we find that they may be united, by common characters, de- 

 duced from considerable differences in the numerical relations of 

 the classes, and from the absolute diversity in the species and 

 genera, into four great groups or periods. During each of these 

 periods, the vegetation has only presented gradual and unlimit- 

 ed changes, which have not influenced the essential characters 

 of the vegetation. From one period to that following it, there 

 is, on the contrary, an abrupt transition, a sudden difference in 

 the most important characters of the vegetation. 



The comparison of the characters of the vegetation of these 

 four periods, is the principal object of this memoir. 



But, before examining their characters, we must first deter- 

 mine their limits. The first appears to extend Jrom the first 

 transition deposits to the end of tlie coal Jbrmation. The old 

 red sandstone of Werner, and niagnesian limestone, form its 

 upper limit. 



The second corresponds to the epoch qfformaiion of the va- 

 riegated sandstone. 



The third commences at the sliell-limestone deposit, or ratfter 

 at the sandstone named Jceuper, which immediately covers it, 

 and extends to the chalk. 



The Jourth compreliends all the deposits ly^ing above ih^ 

 chalk, and which are commonly designated by the name of ter- 

 tiary formations. 



A rather curious result of this division, to which we have 

 been led by considerations entirely independent of geology, is, 

 that it corresponds almost precisely with the four great groups 

 _of formations adopted by geologists. 



