Page:The Journal of geology (IA journalofgeology21894univers).pdf/277
It thus appears that the flora of the beds above Evans Quarry is distinctly that of the Dakota Group, while all the plants below that horizon as distinctly indicate a Lower Cretaceous age. The force of this evidence is to my mind irresistible, and it is safe to predict that if any other paleontological evidence is ever found it will confirm this conclusion. The question still remains as to where the dividing line is to be drawn. Between the cycad and fossil wood horizon and that of the Dakota leaves there are some hundred feet of sandstones and shales. Sixty to seventy-five feet of this consists of the massive or heavy-bedded building stone, which in places becomes flinty and very hard. As the thin shaly layer which separates this from the leaf bed may be safely put with the latter into the Dakota proper, and there seems no reason for separating the similarly constituted layer that intervenes between the cycad horizon and the base of the sandstone from the one upon which it rests, the question is narrowed down to that of the position of the quarry sandstone. That question I will leave to the stratigraphical geologists.
As to where in the Lower Cretaceous series the basal portion of the Cretaceous rim of the Black Hills should be located, it can only be said that the cycadean trunks elsewhere found in North America have all come from well down in that series or else from the Upper Trias. Leaving the latter cases out of the account we have the Maryland specimens and the one from Kansas. It was long supposed that the Maryland specimens were derived from the Iron Ore Clays, which were referred by McGee and Fontaine to an "Upper Clay Member." This is now known not to be the case, and it has been demonstrated that the cycads occur in the basal sands at the same horizon as the Sequoian trunks, and probably the same as the Rappahannock freestone, which has yielded more fossil plants than any other horizon. Whether this is the same horizon as that of the James river, where cycads and conifers prevail and no dicotyledonous leaves have been found, or a somewhat higher one, need not now be discussed, as the whole subject will soon be thoroughly presented along with the evidence. Certain it is that the Potomac