320 GEOLOGICAL SURVEY OF THE TERRITORIES. 



the Dakota group is not a trne petrification. The forms or casts only 

 are left after the total destruction of the substance. This may explain 

 how most of the leaves which have been obtained from this group of the 

 Cretaceous are of a coarse, thick, coriaceous texture. The delicate or- 

 gans of plants, like thin leaves or sea- weeds, may therefore have been 

 totally destroyed. If it is so, we know from this flora a part only, the 

 one which is represented by leaves of a hard tissue and by somo* fruits 

 and stems. At dilferent places and horizons of the formation, especially 

 near the upper part and at the base of the measures, one finds thin beds 

 of black, plastic, soft clay, where remains of plants could be preserved 

 in their integrity even with the epidermis of the leaves. A single leaf 

 has been found of that kind near Sioux City ; it is referable to one of 

 the species most commonly represented in the red shale, and therefore 

 does not afford any point of comparison. The other deposits of clay 

 have been found either barren of vegetable remains of any kind, or, 

 also near Sioux City, mixed with decomposed, undeterminable frag- 

 ments, especially of leaves of Conifers and of rootlets of water-plants. 

 As it is the case in the red shales of the other formation to which this one 

 has been compared — the Upper Devonian, the Lower Permian * — thin 

 layers of coal or coaly matter have been deposited here and there in the 

 so called sandstones of the Dakota group. They are no coal-beds, how- 

 ever, but mere attempts or premises, and preparatian also, of a future 

 Carboniferous formation. In the strata related as synchronous to the 

 Dakota group, in Canada, New Mexico, jSTew Jersey, &c., no workable 

 coal-bed has been discovered till now. Some, which may be compared 

 to the subcon glomerate coal-beds of the Carboniferous, have been appa- 

 rently foriiied near the end of the Cretaceous epoch. As yet, their fossil 

 flora is unknown. In connection with the thin layers of coaly matter in 

 the shale of the Dakota group, no specimen of fossil plants has been 

 discovered till now. 



Toward the end of the period of the Dakota group and in the upper 

 beds of the formation a rapid succession in the elements of the com- 

 pounds, mixed in various ways, in the size of the debris, etc, indicates a 

 new iniluenee, the introduction of deep marine water by slow submer- 

 sion or subsidence of the land. It is after this, or in the Niobrara group, 

 that the only species of marine plant described in the Cretaceous flora 

 has been found. This Zonarites digitatus,\ though similar in its char- 

 acter to the species published under this name by Bronguiart and Geinitz, 

 has its relation contradicted by the great difference between the geolog- 

 ical periods where the remains have been found in Europe and in 

 America, and still more perhaps by the difficulty of identification of ma- 

 rine plants whose characters are represented merely by a vague likeness 

 of outlines. It would have been advisable, perhaps, to leave out without 

 description a vegetable of that kind, not even referable to the Dakota 

 group, and to leave also without even a mention mere fragments like 

 those described as Ligodium trichomanoides, Abietites Haydenii, Flabcl- 

 laria minima, etc., whose characters and relation are too vaguely indi- 

 cated. But as the Cretaceous plants of this and other countries are 

 scarcely known, it seemed proj^er to represent by drawing all the dis- 

 cernible fragments, leaving to time an opportunity of confirming or 

 refuting. by better specimens the first determination. Even small frag- 

 ments may become valuable as complement of other specimens which, 

 fragmentary also, may be defined by those which have been published 

 before, and which, for the same reason of defectiveness, should be left 



* Cretaceous flora iu Dr. F. V. Hayden's Report, vol. vi, pp. 26, 27. 

 t Cretaceous Flora, p. 44, PI. I, Fig. 1. 



