290 GEOLOGICAL SURVEY OF THE TERRITORIES. 



For these we are under many obligations to Professor B. F. Mudge, of 

 the Kansas Agricultural College, at Manhattan.* They are especially 

 interesting, because we have hitherto known only a few of the Mollusca 

 of this horizon, though the remains of many species of forest trees had 

 been obtained from this rock, both on the Missouri and in the interior 

 of Kansas. The shells discovered by Professor Mudge are marine 

 types, with probably the exception of two species of Corbicula; while 

 on the Missouri we have obtained from this rock only a few marine 

 types, such as Mactra and Axincea, associated with Cyrena, Unio, and 

 Pharella. 



It is proper to remark here that, although we usually speak of this 

 Dakota Group or division as belonging to the earlier or lower cretaceous 

 of the Upper Missouri country, we do not thereby mean that it belongs 

 to the lower part of the cretaceous system, as understood in Europe; 

 but simply that it is the oldest member of the series yet certainly known 

 in the Upper Missouri country. It is probably not older than the lower 

 ■ or gray chalk of British geologists, as we have elsewhere explained. 



The specimens from near Fort Bridger and Medicine Bow River, 

 Wyoming, as well as from six miles west of Canon Station and Dodson's 

 Banch, show that the Fort Benton Group or division of the Upper Mis- 

 souri cretaceous occurs at those localities. Among the specimens from 

 between Hardscrabble and St. Charles, from Fort Bascom, Medicine 

 Bow River, and Colorado City, there are characteristic forms of the 

 Niobrara Division ; and from the mouth of Deer Creek, Wyoming, val- 

 ley of Fountain Creek, and Box Elder Creek, Colorado, there are Fort 

 Pierre forms. The Fox Hills beds are also shown to be represented at 

 Fountain Creek, and Colorado City, Colorado. 



Some of the specimens from near Bear River, and at Coalville, Utah, 

 from a light-colored saDdstone, containing beds of a good quality of 

 brown coal, appear to belong to a member of the cretaceous series not 

 corresponding to any of those named in the Upper Missouri country ; 

 though it is, as I believe, represented by a similar sandstone under the 

 ' oldest estuary tertiary beds at the mouth of the Judith River, on the 

 Upper Missouri. In 1860 Colonel Simpson brought from this rock, on 

 Sulphur Creek, a small tributary of Bear River, in Utah, some casts of 

 Inoceramus, and other fossils ; and in some remarks on Colonel Simp- 

 son's collection, published by the writer, in connection with Mr. Henry 

 Engelmann, the geologist of Colonel Simpson's survey,* we referred this 

 formation to the cretaceous. The collections that have since been 

 brought in from it, in Utah, by Mr. King's and Dr. Hayden's surveys, 

 confirm the conclusion that it belongs to the cretaceous, as they contain, 

 among other things, species of Inoceramus, Anchura, and Gyrodes — 

 genera that seem not to have survived the close of the cretaceous 

 period. In addition to this, there is among Dr. Hayden's collections 

 from this rock, at Coalville, a Turritella that I cannot distinguish by the 

 figure and description, even specifically, from T. Martinezensis, described 

 by Mr. Gabb, from one of the upper beds in California referred to the 

 cretaceous. A Modiola from the same horizon also appears to be spe- 

 cifically identical with M. Pedernalis, of Roemer, from the cretaceous 

 of Texas. Dr. Hayden also has, from a little above the coal beds at 

 Coalville, specimens of oyster that seem much like 0. Idriaensis and 0. 

 Brewerii, of Gabb, from the upper beds of the California cretaceous. 

 As no other fossils were found directly associated with these oysters, 



* I have prepared a quarto plate fully illustrating these fossils, to be published in 

 •iihe Paleontology of the Upper Missouri. 



* See Proc. Acad. Nat. Sci., Phila,, 1860. 



