116 



§ 10. — On the general characters and the relation of the flora of 



the dakota group. 



Though the formation of the Dakota group is in immediate superposition 

 to (he Permian or the Upper Carboniferous measures, we cannot, of course, 

 look for any remains of Permian vegetable types in this American cretaceous. 

 We should, perhaps, expect to find there some representatives of the preced- 

 ing formation, the Jurassic, whose flora is a compound of Ferns, few Equise- 

 tacece, some Conifers, and especially 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 Creta- 

 ceous 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 35 per cent, of the land-flora of that epoch as 

 far as it is known. In the Dakota group, the only trace of a vegetable pos- 

 sibly referable to the Cycadece is the Pterophyllwn(J) haydenii, which, 

 as it is remarked in the description, is considered by Schimper as of doubt- 

 ful affinity. Professor Heer, too, finds in the Upper Cretaceous of Green- 

 land a flora of twenty-eight species, mostly of dicotyledonous plants, without 

 any remains of Cycadece. 



This absence of a predominant antecedent vegetable type in the Dakota 

 formation is not more remarkable than that of other vegetable groups, espe- 

 cially the palms, which constitute an extraordinary large proportion of the 

 flora of the Lower Tertiary strata, just above the deep marineformation over- 

 lying or following that of the Dakota group. The section of the Cretaceous 

 strata, as copied from Hay den's Report, page 14, indicates at its base the 

 sandstone and clay strata bearing plants, four hundred feet thick, and in ascend- 

 ing, some beds, mostly of clay, of a thickness of seventeen hundred feet, over- 

 laid by five hundred feet of Cretaceous sandstone. Over this formation ap- 

 pears the Lower Tertiary sandstone with fucoidal remains, mixed in its upper 

 part with fragments of land-plants, followed by the lignitic formation, 

 with its peculiar flora, especially its abundance of palms. The series of 

 strata between the Dakota group and the lignitic Eocene has been uninter- 

 rupted, as far as can be judged from the nature of the compound and the 

 affinity of fossil animal remains. It does not indicate a period of long dura- 

 tion, at least comparatively to other more complex geological groups ; and, 

 nevertheless, the flora of the Dakota formation has not a single species 

 which might be referable to, or is recognized as identical with, any of the 



