MEEK AND HAYDEN'S CBETACEOUS SECTION. 173 



The Fort Pierre is the most important of the divisions of the Cretace- 

 ous in the Northwest, alike in the extent of its development, in its influence 

 on the character of the country, and in the great abundance and beauty of 

 its fossils. In the number of its fossils it surpasses all the other Cretaceous 

 groups collectively. Its clays lose by wetting their laminated structure 

 and become adhesive and pasty, and then by drying become hard and 

 crumbly, and split or crack from the surface to a depth of several inches. 

 There is something in its physical or chemical constitution inimical to 

 vegetation, and over large areas it wears a peculiar black and barren 

 appearance, scarcely relieved by a shred of vegetable life. In other 

 regions, however, the surface supports an excellent growth of grass. 



Black Hills. — The group is well exposed on Old Woman Fork near the 

 Cheyenne, and it exhibits there in its fossils a blending with No. 5. It is 

 in general plainly recognizable entirely around the Hills. It constitutes 

 the chief Cretaceous exposures on the Belle Fourche and South Cheyenne 

 from their junction upward to the vicinity of the uplift, and appears again 

 on the South Cheyenne near the mouth of Beaver Creek. 



The upper fossiliferous zone is well exposed along the South Cheyenne 

 from Rapid Creek downward, and near the mouth of that creek its charac- 

 teristic fossils occur in great profusion. They are found in calcareous con- 

 cretions or nodules, often of very large size. Sage Creek and Bear Creek, 

 which enter the river nearly opposite the mouth of Rapid Creek, are 

 rendered classic by the collections of Meek and Hayden. 



Thickness, estimated, 150 to 250 feet. 



No. 3. — Niobrara group. 



Lead-gray calcareous marl, weathering to a yellowish or whitish chalky appear- 

 ance above. Containing large scales and other remains of fishes, and many specimens 

 of Ostrea, congesta attached to fragments of Inoceramits ; also several species of Textu- 

 laria. Passing down into light, yellowish, and whitish limestone, containing great num- 

 bers of Inoceramus problematicus, I. pseudo-mytiloides, I. aviculoides, and Ostrea congesta, 

 fish scales, etc. 



Localities. — Bluffs along the Missouri below the Great Bend, to the vicinity of 

 Big Sioux Eiver; also below there on the tops of the hills. 



Thickness, 200 feet. 



From this formation Professor Marsh and Professor Cope have de- 

 scribed nearly one hundred vertebrates, including birds, reptiles, and fishes. 



