INTRODUCTORY REMARKS. XXXIII 



single species, so far as known to the writer, has yet been found identical with 

 any form yet known from either of the three divisions below. In addition to 

 this, the upper surface of the Niobrara beds is, at several places on the Mis- 

 souri, seen to have been eroded into irregularities, or depressions, previous 

 to the deposition of the succeeding Fort Pierre group, thus giving additional 

 evidence that some kind of a physical change (perhaps slight) occurred 

 between the deposition of the latest portion of the Niobrara division and the 

 first of the Fort Pierre beds. 



Fort Pierre group. — This is perhaps the most important member of 

 the series, not only on account of its thickness and the extent of its geograph- 

 ical range, but also from the great number and the beautiful state of preserva- 

 tion of its organic remains. As already intimated, the pearly portion of the 

 shells of fnocera?nus, Margarita, Ammonites, Scaphites, and other pearlaceous 

 types found in this rock, usually seems to have lost little, if any, of its orig- 

 inal brilliant luster. 



With local partial exceptions, this division is, throughout the Upper 

 Missouri country, composed of a great accumulation of fine dark and lighter 

 gray clays, in rather distinct layers, but usually without a laminated structure. 

 When wet or moist, they become plastic, but harden and crumble in drying, 

 so as often to obscure the marks of deposition in vertical exposures; and 

 where they form the surface of the country, they are subject to crack open 

 to depths of six to ten inches during continued dry weather. As might be 

 expected, such a material forms a very poor soil, and consequently the country 

 occupied by these clays, and not covered by drift or later alluvial deposits, 

 presents a barren appearance. 



In the Upper Missouri country, this formation has been estimated to 

 attain a maximum thickness of seven hundred to eight hundred feet; but 

 it may be thicker at some places in or near the Rocky Mountains. 



In Dakota, the organic remains of this rock usually occur in hard con- 

 cretions, composed of the same argillaceous material, with some calcareous 

 matter, and are more numerous in the lower and upper beds than in the mid- 

 dle portion, which is usually nearly barren of fossils. Among the more con- 

 spicuous fossils of this division may be mentioned Inoceramus Nebrascensis, 

 I. Vanuxemi, I. Balchii, Cuculla-a Nebrascensis, a number Of Univalves, Nau- 

 tilus Dekayi, Placenticeras placenta, Baculites ovatus, B. compressus, Scaphites 

 nodosus, bones of Mosasaurus, etc. The fossils from this group are illustrated 

 on our plates 1" to 27, inclusive. 

 II — v 



