Page:The Journal of geology (IA journalofgeology21894univers).pdf/278
cycads belong to the lower part of that formation. The Kansas specimen is confidently referred by Professor Cragin to the Trinity Group of the Comanche series of Hill, which forms the lowest division of that series. Professor Hill is disposed to accept this conclusion, and Professor Prosser of Washburn College, Topeka, sees no reason to doubt its accuracy.
The other plants, as has been seen, occur about one hundred feet below the cycad bed. Professor Fontaine's report above quoted places their significance in its true light and leaves little to add. The occurrence of Asplenium Dicksonianum shows simply that this common form persisted in the same area through a long period. But this it was already known to do. Should it, however, prove to be the Thyrsopteris rarinervis Font., it would be a characteristic lower Potomac species.
The forms that Professor Fontaine refers to Glossozamites argue entirely for a Lower Cretaceous or even earlier age. Eight species of that genus are known, ranging from the Upper Trias to the Urgonian. Some are from the Lias, but most of them are found in the Wealden and Neocomian. They had a wide geographical range, occurring in Greenland (Kome beds), India (Damuda series), and in various parts of Europe. One species, G. distans, is from the lower Potomac of Fredericksburg, Virginia.
Gleichenia Zippei (Corda) Heer was first described by Corda, who referred it to Pecopteris, from the Gosau formation of Bohemia, supposed to be above the Cenomanian. It has since been found in the true Cenomanian of Bohemia and in the Quadersandstein of Germany. Heer found it in all the Cretaceous beds of Greenland (Kome, Atane, Patoot), also in the Cretaceous of Spitzbergen. Newberry detected it in the Amboy Clays. It varies considerably, and the name may include more than one species. Fontaine compares the Black Hills specimens only with Heer's Kome forms, and is not certain that they may not rather represent his own Aspidium heterophyllum from the Lower Potomac of Fredericksburg. The evidence afforded by this species, therefore, is not strong, but it certainly does not occur in the Dakota Group elsewhere so far as known.