31 



and roots. It is, moreover, probable that the activity of life continued for 

 procuring sustenance to the trees, has preserved the substance of the roots 

 against the process of fossilization which acted upon the leaves ; for, in- 

 deed, these leaves arc not truly fossil ; they are merely printed on the 

 shales, but nothing of their substance has been left. The process of fossiliza- 

 tion depends on local circumstances, and an indication of the incapacity for 

 this process, or of the absence of fossil petrifying elements in the shales of the 

 Dakota group, is the soft state of some fragments of wood in the deposits of 

 roots and rootlets of Sioux City. These fragments have the consistence of 

 decayed wood, and crumble in powder under the pressure of the finger. 



§6. — Generic characters of the flora of the dakota group. 



These characters are the more easily and clearly exposed in a table 



marking the groups and genera to which the species are referable. The 



table comprises the genera admitted by Professor Heer, for the description 



of the species in Phyllites du Nebraska; those of Doctor Newberry, for the 



leaves described in notes on extinct floras, &c, and those which I have 



admitted for the description of species in this paper. The number marked 



after the generic name indicates approximately the number of species referred 



to each genus : 



cryptogams. 



Order Thallophytes. 

 Zonarites, Brgt 1 



Order Filices. 

 Lygodium, Swartz 1 



Hymenophyllum, Sm - 1 



Sphenopteris, Brgt 1 



Pecopteris, Brgt 1 



Gleichenia, Sm 1 



Todea, Willd 1 



PHANEROGAMIC, GYMNOSPERM/E. 



Order Zamice. 

 Pterophyllum, Brgt 1 



Order Coniferai. 



Serpioia, Endl 2 



Araucaria, Juss - 1 



