XXXII INTEODUCTOEY EEMAEKS. 



From the localities mentioned along the Missouri, outcrops of this forma- 

 tion are seen, like those of the last, ranging in a southwesterly direction 

 through Eastern Nebraska and Kansas. In the latter State, it occupies a 

 broad belt of country, perhaps widest on the Smoky Hill River, where it 

 attains the breadth, in an east and west direction, of about two hundred and 

 fifty miles, passing on the west under later formations, probably of Tertiary 

 age. It also extends through the Indian Territory into New Mexico, and 

 occurs in Texas and Arkansas.* Indeed, it and the Fort Benton group seem to 

 be the principal fossiliferous beds of the Cretaceous in these southern districts. 

 As shown by Dr. Hayd'en and the writer in 1857, it and the Fort Benton 

 group almost certainly form the upper forty feet of the Pyramid Mountain, 

 an outlier of an extensive plateau in New Mexico, known as the Llano Esta- 

 cado, the lower part of which mountain probably also corresponds wholly or 

 in part to our Dakota group. 



Mr. Holmes, Dr. Peale, and other members of Dr. Hayden's party, have 

 brought some of the fossils of this rock from Western Colorado, where it 

 seems to be blended with the Foi't Benton group. Along the eastern base 

 of the Rocky Mountains, some of its fossils have been brought from near 

 Colorado City, and Dr. Hayden also observed outcrops of it farther northward 

 in Colorado, near Cache la Poudre River, between Denver and Cheyenne. It 

 is also known to occur on the north branch of Platte River, and near the 

 west base of the Laramie range of Mountains, along the eastern margin 

 of the Laramie Plains. 



In passing from this formation to the next above, we cross the most 

 strongly-marked palasontological break in the whole series, unless that 

 between the Dakota group and Fort Benton group may be equally so. As 

 far as yet known, none of the Dakota-group species occur in the beds above, 

 but then the number of species yet found in that division and the Fort Benton 

 group is hardly sufficient to warrant the conclusion that some forms may not 

 be common to the two horizons, as seems to be the case in Texas and 

 New Mexico. In passing from the Niobrara group, however, into the 

 succeeding rocks above, in which great numbers of fossils occur, not a 



* Dr. Shumard identified and named from the division corresponding to this in Texas Holaster 

 simplex, Toxaster elegans, Cidaris hemigranosus, Gryphosa Piicheri.(tyj>ica\ var. = Gf. Tucumcarii, Marcou), 

 G. ainuala, Marcou (not Sowerby), Ostrea suuovata (= O. Marsha, Marion), 0. oarinata, 0. quadriplicate, 

 Xcithea Texana, X. Wrightii, Inoecranius problematicus, Pachymya Austinensis, Lima crenulicostata, Terebra- 



tula JTacoensis, Turrilitea Brazoensis, Ammonites vesper tin us (= A. Texanus, Roemer), A. Brazoensis, Hami- 

 ti.i Fremontii, and Nautilus Texanus. (See Trans. St. Louis Acad., I, 587.) 



