112 [June, 



Ascending the Missouri from Fort Pierre, we find on reaching a point five miles 

 below Heart river, about the 47th parallel north, that the Cretaceous formations 

 which are so conspicuous for many hundred miles along the river below, pass 

 by a gentle north or north-west dip beneath the water level, to be succeeded on 

 both sides of the river by Tertiary. Although this is the first point where the 

 Tertiary beds come down to the water level, they are known to occupy the higher 

 country back from the river, on the west side, as far south as the vicinity of 

 Sau-acanna or Moreau river, and still further west they go as far south as some 

 of the north-west branches of the Cheyenne. Cannon-ball river, Watahoo, and 

 other small tributaries, however, cut down to the Cretaceous beds some little 

 distance back from the Missouri. On the east side of the ]\lissouri the Tertiary 

 is bounded on the south, nearly opposite the mouth of Cannon-ball river, by a 

 range of upper Cretaceous hills bearing off to the north east. South of the 

 Moreau, a similar range, known as Fox Hills, extends from near the Missouri to 

 about the 102 of west longitude, where it is interrupted by a small tributary 

 of the Cheyenne. West of this small stream, the same range of upper Cretaceous 

 hills, known perhaps by other local names, bears round to the north-west, 

 crossing the head branches of the Little Missouri so as to strike the Yellow Stone 

 river about ten miles below the mouth of Powder river ; forming nearly all this 

 distance the south and south-west boundaries of that portion of the great Ter- 

 tiary basin lying in the immense bend formed by the Missouri and Yellow Stone 

 rivers. To comprehend how this range of hills could traverse the country in 

 this way, it must be borne in mind that the Black Hills are laid down on most 

 of the published maps of this country as extending a long distance too far north. 



Returning to the point near Heart river, from which we first set out, we find 

 on ascending the Missouri, that the Cretaceous strata again rise to view at a few 

 points not far below Fort Clark, but even here the country on each side is com- 

 posed of Tertiary. It was at one of these localities the Prince of Kew Wied 

 collected a nearly entire skeleton of Mosasaurus Maximiliani (Goldf.) From the 

 vicinity of Fort Clark we know of no other place where the Cretaceous beds 

 make their appearance until about twelve miles below the mouth of Milk river, 

 (lat. 47 N. long. 104 W.) the country on both sides of the Missouri all this 

 great distance being made up of Tertiary formations, the northern and eastern 

 limits of which are unknown to us. Immediately along the margins of Milk 

 river, Cretaceous beds are seen on both sides as far up as we have any knowledge 

 of the country, though the higher country back from the river is Tertiary. From 

 the point below the mouth of this stream on the Missouri, where the Cretaceous 

 beds first make their appearance, they are seen to rise higher and higher as we 

 ascend the ?iIissouri, in consequence of their inclination to the east or north-east. 

 On the north side of the Missouri, between it and Milk river, the higher portions 

 of the country back from the Missouri, are also composed of Tertiary beds. 



The same formations likewise occupy nearly all the country between the Mis- 

 souri and Yellow Stone, as far west as the vicinity of Muscleshell river, where 

 they thin out on the summits of Cretaceous hills. The hills, however, near the 

 Missouri, between Milk and Muscleshell rivers, are also mainly Cretaceous, the 

 Tertiary being for the most part worn away by atmospheric agencies. 



On both sides of the Yellow Stone, only Tertiary strata are seen from near the 

 mouth of Powder river as far up as the mouth of the Big Horn. How far be- 

 yond this they extend we do not know, though we have received Tertiary fossils 

 from intellisfent traders, collected as far up the Big Horn as one of its tributaries 

 known as Little Horn river. From another point as far west on the Yellow 

 Stone as Rose river, we received a few Cretaceous fossils. As to the limits of 

 the Tertiary up Powder and Tongue rivers, we have no definite information. 

 The traders say the same kind of lignite beds seen along the Yellow Stone, occur 

 along the l)anks of the former as much as one hundred and fifty miles above its 

 mouth. 



The foregoing hasty sketch is given more with a view of showing the extent 

 of country occupied by this great Tertiary lignite formation, than with any 

 hope of conveying a definite idea of its precise limits. If it should prove to be 



