ON THE GENERAL CHARACTERS AND THE RELATION OF THE 

 FLORA OF THE DAKOTA GROUP.* 



BY LEO LESQUEKUEX. 



The present article is a resume of the essential characters and the 

 relation of the flora of the Dakota gronp. 



1. Though the Cretaceous formation containing our fossil leaves has 

 been recognized as marine, from the presence in its compounds of a num- 

 ber of species of marine mollusks, no trace of facoidal plants has been 

 found among the vegetable remains of this group. The one described 

 as Zonarites comes from the Benton group, where it was discovered in 

 a kind of limestone, mostly composed of large marine shells, species of 

 deep water. 



It has been remarked how the fucoidal vegetation could not be intro- 

 duced or brought upon the mud-flats and mixed with the red shale of 

 the Dakota group, though the remains of marine plants are found in 

 abundance just at the top of the Cretaceous series, in the lower sandstone 

 of the Tertiary, which, on this account, has been compared to the Eocene, 

 and admitted as its representative. This fact rather confirms the opinion 

 that the Dakota formation is the result of a slow agglomeration of 

 materials along a shore-line of wide extent; mud-flats, where, of course, 

 the marine plants could not live, as their seeds do not take root in the 

 mud ) and where even their debris could not be preserved, on account of 

 the softness and of the alternauce of water over the surface. All the 

 vegetable remains of the Dakota group, preserved either with their 

 substance, or by impressions only, are pieces of hard wood, rootlets, 

 and branches, with leaves of coarse, thick texture. Per contra, the 

 sandstone with fucoids, or the Eocene, being slowly upheaved from deep 

 water, was inhabited by a marine vegetation of long standing, which, 

 though covered by successive sandy deposits, could but thrive, till near 

 the surface of the sea, where we see it intermixed with fragments of 

 exogenous land-plants washed on the shores, and indicating a new 

 period or the beginning of a land formation. 



2. That vegetable Permian^ types should not be represented in the 

 flora of the Dakota group, tliough both formations are in immediate 

 juxtaposition, is not a matter of concern. But it is not the same when 

 we recognizein these vegetable remains of the Cretaceous a total absence 

 of representatives of theprecedingformation, the Jurassic, whose florals a 

 compound of Ferns, few Uquisetacece, some Conifers, and especially of a 

 prodigious abundance of Cycadece. Three-fourths of all the fossil Zamice, 

 and half of the Cycadece, known from all the geological formations, 

 belong to the Jurassic. In the lower Cretaceous of Greenland, Heer 

 finds still a marked proportion of species of this family, there being nine 

 Cycadece in a group of thirty-six species of land-plants, a proportion of 



* Conclusion of the report on the flora of the Dakota group, now in progress of pub- 

 ic ation. 



