XXVI INTRODUCTORY REMARKS. 



THE SUBDIVISIONS OF THE UPPER MISSOURI UNDOUBTED CRETACEOUS SERIES 

 AND THEIR GEOGRAPHICAL EXTENSION WEST OF THE MISSISSIPPI INDI- 

 VIDUALLY CONSIDERED. 



Dakota group. — Tins subdivision, at its typical localities on the 

 Missouri, in the region of the mouth, of Big Sioux River, in Eastern 

 Nebraska and Northwestern Iowa, consists mainly of yellowish and brown 

 sandstones, in rather thick beds, interstratified, however, at places, with 

 yellowish clays and some impure lignite. Above the mouth of Big Sioux 

 River, and some little distance up that stream, it is seen to pass under the 

 Fort Benton group, owing to the general northwesterly dip of the whole 

 series here; while, below the mouth of Big Sioux River, it occurs, at a few 

 places, resting on limestones belonging to the upper part of the true Coal- 

 Measures. From this region its outcrops are seen at intervals, extending in 

 a southwestward direction through Eastern Nebraska and Kansas; and 

 Dr. Newberry has traced it, by its characteristic leaves, from the Arkansas 

 River, through the Indian Territory, far into New Mexico, where it was 

 found by him overlaid by a great thickness of later Cretaceous deposits.* 

 Dr. Shumard also identified with it a leaf-bearing bed at the base of the 

 Cretaceous of Texas.f Mr. Holmes and Dr. Peale found a rock agreeing 

 with this at the base of the Cretaceous of Western Colorado; and, along the 

 eastern base of the Rocky Mountains, Dr. Hayden has observed a similar 

 deposit holding the same relative position, at various places, from New 

 Mexico to the Black Hills ; though none of the characteristic fossils of this 

 formation were discovered by him at any of these last-mentioned localities. 



At the typical localities along the Missouri, already mentioned, the only 

 animal remains yet discovered in this rock are a Pharella, a Cyrena, a Mactra, 

 a Trigonarca, and a large bivalve, believed to belong to the genus Margaritana 

 (see plate 1), a group of types indicating that the beds were deposited in 

 salt-water, along a shore, near or at the mouth of a si ream. In Kansas, 

 however, Professor Mudge discovered in this formation numerous casts and 

 moulds of strictly marine shells, including the Cretaceous genus Leptosolen, 



* See Am. Jour. Sci. (2d ser.), XXIX, 208, March, 1860. 



t See Trans. St. Louis Acad., I, 582. Dr. S.. however, described from this bed several fossils that 

 indicate a blending there of the Dakota and Fort Benton groups. Foriustauce, his Inoceramust capulus is 

 very closely allied to our /. umoonalus from the Fort Benton group ; and his Ammonites Graysuneiisis may be 

 only a young example of Prionocylus Woolgari, Mautell (sp.) (= Ammonites percarinalui, H. & M.), from 

 the same. Again, his Scaphilas rcrmiculus is also allied to the Fort Benton species S. larvaformU. 



