13 



New Mexico and the Rocky Mountains. I have not been able to find any 

 trace of it in that country, however, and as all the specimens of fossil plants 

 either found by myself south of Colorado or sent to me for examination from 

 the Rocky Mountains or their eastern slopes, are species representative of 

 the Tertiary formations, as remarked already, I am still uncertain if the Da- 

 kota group is really extended farther west than Kansas. The western strata 

 bearing plants which are now recognized as of Eocene age, were formerly 

 considered as Cretaceous, and the leaves found in great abundance in connec- 

 tion with the Tertiary formation, though of a different character, may have 

 been inadvertently referred to species of the Dakota group. However it may 

 be, this Cretaceous group has not been positively recognized by Dr. Hayden 

 in his explorations to and along the Rocky Mountains, and from the nature 

 of its compounds, which, as it will be seen hereafter, induces me to consider 

 it as a beach formation, I doubt whether it is of a much wider extent west- 

 ward, as it has been reported, or whether it is to be found west of the borders 

 of Kansas. 



§ 3. StRATIGRAPHICAL DISTRIBUTION OP THE DAKOTA GROUP. 



The section of the Cretaceous rock of the West, published in Dr. 

 Hayden's Report of the Geological Survey of the Territories, 1870, p. 87, 

 gives the best possible illustration of the relation of the groups of the Amer- 

 ican Cretaceous from its base, in connection with the Permian, to its top, over 

 which the Eocene sandstone is superposed. It is here copied in full as a 

 necessary exemplification of the details which are given in this chapter : 



