[ 237 ] 36 



the left side, wilh the plateau dividing tlie waters that empty into the 

 Tchan sansan from those that flow into the Missouri. This upland is 

 known as the "Coteau des Prairies du Missouri," or, more shortly, "Coieau 

 du Missouri."' 



The elevated prairies that crown the right bank of the Missouri river, rise 

 gradually in the direction of the Rocky mountains, forming the northern 

 extremity of those steppes, more appropriately designated the Anierkan 

 Desert. Hence, it will be perceived that the river has its bed deeply encased 

 in a valley flanked on the left side by ihe Coteau du Missouri, and on the 

 right by the American Desert. Over a length of 235 miles, comprised with- 

 in this valley between the Nl-obrarah, or "Eau qui-court" river, and Fort 

 Pierre Chouteau, the cretaceous formation exhibits its fullest develop- 

 ment. It may be satisfactorily examined at many places within this 

 range; but a perfect representation of the whole of them may be obtained, 

 if, talcing as a basis the description previously given of Dixon's bluff, 

 there be added to it the modification presented by some of its new mem- 

 bers. 



I may be permitted to think that this cretaceous formation is destined to 

 Cretaceous occupy a conspicuous place in the history of American geology ; 

 formation, and, us I am not aware that any details concerning it are record- 

 ed, I shall now, to the exclusion of other matters, possibly of equal in- 

 terest, say all that I know about it. 



It is necessary, then, in the first place, to take both a geographical and a 

 geological horizon. Tlius, referring to the map : starting, for instance, from 

 the Wichajiohah^ or Scalp Mountain creek, in latitude 43° 8', and visiting, 

 successively, as follows: the hills at the mouth of Whetstone creek ; those 

 in the vicinity of Red Cedar, Snags, and Sailor's Islands, the Mankizitah, 

 or White-earth river, the American river, the Great I3end, (which is the 

 Karmichig-ah of the Sioux,) it will be easy to understand all the circum- 

 stances about which I shall now give an account. 



1st. The stratum A, of argillaceous limestone, observed at Dixon's bluff, 

 has disappeared in consequence of the elevation of the level of the val- 

 ley. 



2d. The calcareous marl B, in horizontal stratification, continues to make 

 its appearance in escarpments of Irom 30 to 40 feet, containing tlie same 

 fossils — namely, orhicula and fish scales. 



Over this bed, or rather between it and the preceding one, there seems 

 to be occasionally found a thin layer of fibrous carbonate of lime ; the true 

 position of which 1 was a long time in determining, as I had discovered 

 fragments of it only among the rubbish at the foot of the bluff. I have 

 since observed it in place above the calcareous marl ; and it is interesting 

 that it is covered with coats of a fossil very much resembling the gryphcea 

 Vomer, but which Mr. Conrad has described under the name of ostrea con- 

 gesta, as will be seen in the catalogue of the fossils belonging to the creta- 

 ceous formation in appendix C. 



It gives me particular pleasure to state, in this place, that, having fur- 

 nished Professor .1. W. Bailey of West Point wilh specimens, from several 

 localities, of this marl, the microscope, under his experienced direction, 

 has led to the discovery of most interesting results. I think 1 cannot do 

 better than give, at least, some extracts from Professor Bailey's communi- 

 cation. 



He says: "I received, with much pleasure, the specimens you kindly 



