68 ON THE GEOLOGY AND NATURAL HISTORY 
5. Ammonites cordiformis, M. and H., is of the same type as the Oolitic species A. cordatus (Sowerby). 
6. Belemnites densus, M. and H., is scarcely distinguishable from the Oolitic species B. excentricus (Blairville), 
if indeed it is really distinct. 
Among the undescribed fossils from the supposed Jurassic rocks of the Black hills, are 
a species of Hettangia, a genus not known to occur in the Old World in formations newer 
than the Lias, an Ostrea, scarcely distinguishable from a form figured by Quensted in his 
work on the Jura, and a Zrigonia more nearly resembling Jurassic types than those of any 
other formation. 
VI. Cretaceous System. Upper, Mrppir, AnD Lower. 
This system holds a very important position in the Northwest, not only from the vast 
area which it occupies, but also from the number, variety and beauty of its organic re- 
mains. ‘The Cretaceous rocks as they appear in ascending the Missouri, have been sepa- 
rated into five divisions, which present well-marked lithological differences, and contain 
for the most part, distinct species of organic remains. From the following vertical section 
of the Cretaceous rocks of the Upper Missouri and the catalogue of Cretaceous fossils, it will 
be seen that formation No. 1 seems to constitute paleontologically an independent division, 
none of its organic remains ranging in other formations above or below. Nos. 2 and 3 
appear to form one group, the Ostrea congesta and Inoceramus problematicus passing from 
one to the other. Divisions 4 and 5, which are the most fossiliferous formations on the 
Upper Missouri, contain many species in common, especially of the Cephalopoda, and there- 
fore form a third group. 
The Cretaceous system, as developed on the Upper Missouri, therefore forms lithologically 
five well-marked subdivisions, while palzontologically it admits of separation into but 
three independent groups. The age and geological position of Divisions 2, 3, 4 and 5 
have been sufficiently attested by the numerous species of organic remains, which have 
been published from time to time by F. B. Meek and the writer. In regard to the age of 
No. 1, much doubt existed, until we had an opportunity to examine a fine series of Dicotyle- 
donous leaves, discovered by the writer in this formation near Blackbird hill on the Mis- 
souri, while attached to Lieutenant Warren’s party, in 1857. These leaves proved to us 
that the formation under consideration could not be older than Cretaceous. I will have 
occasion to allude to this point again in a subsequent part of this report. 
