314 G. II. Eldridge — Method of grouping the 



more or less bituminous ; by the frequent presence at certain 

 horizons of concretionary, clay ironstones ; and by the occa- 

 sional appearance of a few narrow bands of impure, argillace- 

 ous sandstone. As the summit of the formation is approached, 

 the clays become more calcareous, and the limestones more 

 prominently developed, the latter, at times, bearing a distinct 

 resemblance to the lower layers of the important bed at the 

 base of the Niobrara, though rarely attaining a thickness of over 

 four or five feet. 



Among the characteristic fossils of this formation the species 

 Ostrea congesta and Inoceramus problematicus are especially 

 well represented, and by their abundance, often impart to the 

 limestones in which they occur a texture most peculiar and 

 distinctive ; in addition to these, Selachian teeth are also fre- 

 quently met with, especially in the more bituminous layers. 



The Niobrara — In thickness this formation varies from a 

 mere trace to approximately four hundred feet. Where well 

 developed, as along the foothills in Colorado, its sedimenta- 

 tion is sufficiently differentiated to readily permit its divisions 

 into three distinct members : the lowest a bed of limestone, of 

 an average thickness of fifty feet"; the middle, a succession of 

 gray, marly clays, one hundred and ten feet ; the uppermost, a 

 series of calcareo arenaceous clays, of various shades of yellow 

 or buff, two hundred and forty feet in thickness, remarkable 

 for the presence of " alkali salts," and containing several bands 

 of impure, yellow, saccharoidal, fossiliferous limestone. 



The basal member of limestone, from its general persistency 

 and the prominence of its characteristics, forms an admirable 

 datum level for reference in the study of Cretaceous strati- 

 graphical geology. Its leading characteristics are its bluish- 

 gray, light-gray, or clouded- white color ; its superior hardness 

 and consequent great resistence to atmospheric influences ; its 

 even, fine-grained texture ; the general purity of its compo- 

 sition; its conchoidal fracture; the uniform thickness of its 

 component beds, from one to three feet ; and its fossil contents. 

 Where well developed, as in Colorado, these characteristics are 

 more especially applicable to the lower twenty -five or thirty 

 feet of the limestone series, which is the portion of particular 

 economic value and is extensively worked ; the upper half is 

 usually much thinner bedded, and passes by transitional shaly 

 beds into the overlying group of marls. For two or three 

 feet upward from the base of the series, also, the strata often 

 show a transition character, linking this formation with the 

 Fort Benton below. 



The life of the Niobrara included three especially prominent 

 molluscan forms, the Ostrea congesta, the Inoceramus proble- 

 maticus, and the Inoceramus defbrmis, the last, a very charac- 



