I 70 GEOLOGY OF THE BLACK HILLS. 



the western part of Minnesota, of over six hundred miles ; but farther 

 south the width diminishes. The greater portion of this slope, which under- 

 lies almost the entire area of the Great Plains, is covered and concealed by 

 the more recent deposits of the Tertiary, but the continuity of these beds 

 has been interrupted by erosion along some of the principal river valleys 

 and by uplifts like that of the Black Hills. 



The western margin of the Cretaceous area is revealed and readily 

 studied where the edges of its composing strata are upturned against the 

 foot-slopes of the Rocky Mountains, while the eastern margin, with a gentler 

 dip, outcrops through eastern Dakota and western Minnesota, eastern Ne- 

 braska, and western Kansas. The two are joined in central Colorado. 



There are few of the formations of the western rock system that have 

 been so extensively and thoroughly studied as the Cretaceous, and none that 

 can vie with it in the richness, abundance, and beauty of its fossil remains. 

 Our knowledge of it begins with the explorations of Lewis and Clarke, and 

 has been extended by subsequent expeditions, as related in the first chapter 

 of this report. In 1853, Mr. Meek and Dr. Hayden were sent by Professor 

 Hall, of Albany, to make collections of the Cretaceous and Tertiary fossils 

 of the bad lands on the White River. They brought back a large collection 

 of vertebrate remains from the bad lands and Cretaceous fossils from the 

 Cheyenne and Sage Creek region, as well as from various points on the 

 Missouri below Fort Pierre. As a result of their exploration, the first 

 attempt was made by Mr. Meek to group the different members and form a 

 general section for the Cretaceous rocks of the West. This section appeared 

 in a paper by Hall and Meek, entitled "Descriptions of new Cretaceous 

 fossils from the Cretaceous of Nebraska," which was published in the 

 Memoirs of the American Academy of Arts and Sciences in 1854. The 

 thicknesses of the different divisions were given by estimate, and have been 

 modified by later work, but the order of sequence is essentially the one now 

 received, and because of its historical interest the section is given below. 



