352 M. Brongniart on the Vegetation of 



similar origin or mode of formation, the name o^ formation has 

 been given to their aggregate, and to several of these formations 

 united by other more general characters, the name of series of 

 formations (terrains). 



Many geologists admit four of these great series of forma- 

 tions posterior to the existence of organised beings, the transi- 

 tion formations intermediate between the primitive formations, 

 and the true sedimentary, formations, and the inferior, middle, 

 and superior sedimentary formations. 



The consideration of fossil vegetables has not hitherto con- 

 tributed in any degree to the elaboration of this geological clas- 

 sification, which will serve as a basis for comparing the vege- 

 tables which grew at the earth's surface at the epochs when the 

 formations were deposited. 



If we commence with grouping together all the vegetables 

 which belong to the same formation, we shall see that the plants 

 which occur in the different beds of the same formation differ 

 very little from each other, as might be expected ; but we shall 

 see, moreover, that there often exist great relations between the 

 fossil vegetables of several successive formations. 



These relations between the plants of allied formations will 

 not, however, be founded in all cases, upon the identity of the 

 species, nor even sometimes of the genera, but upon the nume- 

 rical relations of the great classes of the vegetable kingdom. 



The successive distribution of vegetables over the surface of 

 the globe, during the different epochs of its formation, may in 

 many respects be compared with the geographical distribution 

 of vegetables over the present surface of the earth. 



In the same manner as the surface of the globe is divided in- 

 to regions, in which the same kind of vegetation exists, may 

 also, the long period of time, be divided during which the crust 

 of the globe was formed, in epochs of greater or less extent, du- 

 ring which the vegetation presented common characters. 



The regions in botanical geography sometimes differ not only 

 through changes which take place in the species of plants which 

 constitute the flora of each of these regions, but also through 

 modifications in the proportions of the different classes of the ve- 

 getable kingdom. In like manner, in the floras peculiar to the 

 different epochs of the earth's formation, the differences are 



