Geology. 



425 



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

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

 tance back from the Missouri. On the east side of the Missouri the Ter- 

 tiary is bounded on the south, nearly opposite the mouth of Cannon-ball 

 river, hj a range of Upper Cretaceous hills bearing off to the northeast. 

 South of the Moreau, a similar range, known as Fox Hills, extends from 

 near the "Missouri to about 102° of Vest longitude, where it is inter- 

 rupted 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 northwest, crossing the head branches of the 

 Little Missouri so as to strike the Yellow-Stone river about ten miles be- 

 low the mouth of Powder river ; forming nearly all this distance the south 

 and southwest boundaries of that portion of the great Tertiary 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 dis- 

 tance 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 composed of Tertiary. It was at one of these 

 localities the Prince of New 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 appear- 

 ance 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 mar- 

 gins 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 Missouri, in con- 

 sequence of their inclination to the east or northeast. 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 Missouri and the Yellow-Stone, as far west as the vicinity of Muscle- 

 shell 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 beyond this they extend we do not know, though we have re- 

 ceived Tertiary fossils from intelligent 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 



SECOND SERIES, VOL. XXII, NO. 66. — NOV., 1856. 



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