the Earth at different Epochs, Sfc* 361 



limits which we have traced for ourselves, were we to establish 

 here the degree of precision of each of the determinations at 

 which we have arrived. This investigation, founded upon nu- 

 merous and minute comparisons of organs, and of their struc- 

 ture, being entirely a work of detail, cannot be exhibited in all 

 its particulars. I have pointed out the course which I have fol- 

 lowed in this investigation, in the first two numbers of my work 

 on Fossil Vegetables. 



I would only remark, that, in most cases, the great class to 

 which these fossils belong may be determined with certainty, and 

 that often the family, and sometimes even the genus from which 

 they come, may be made out. 



By the great classes, I here mean the first very natural divi- 

 sions that may be established in the vegetable kingdom. These 

 divisions may, I think, be carried to six ; the Agamic 'plants ; the 

 Cellular Cryptogamic ; the Vascular Cryptogamic ; the Gynu 

 nospermoiis Phanerogamic, comprehending under this name the 

 Coniferae and the Cycadeae; the Monocotyledonous Phaneroga- 

 mic ; and the Dicotyledonous Phanerogamic. 



I imagine that every person who will attentively examine this 

 division, will agree with me, in the^r^^ place, that these classes 

 are very natural ; and, in the second, that it is almost always 

 possible, by means of any well preserved organ, to discover to 

 which of these six classes a fossil plant has belonged. 



It was necessary for me to explain these bases of the botani- 

 cal division which I have adopted ; for it is from the compari- 

 son of the number of vegetables of these classes, at the different 

 epochs of formation of the crust of the gk>be, that the most re- 

 markable differences of its vegetation result. 



It is generally known, that according to the researches of geo- 

 logists, the deepest part of the crust of the earth appears form- 

 ed of rocks, for the most part crystalline, in which no remains 

 of organised beings are met with ; that upon these rocks have 

 been successively deposited strata of a different nature, the 

 greater part formed by sediment; and that in these strata, 

 whose relative position equally indicates the relative period of 

 formation, there have most commonly been found more or less 

 abundant remains of animals and vegetables. Several of these 

 strata presenting common characters, which seem to indicate a 



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