98 GEOLOGICAL SUEVEY OF THE TEEEITOEIES. 



smaller ranges, as the Little Rocky, Judith., Bear's Paw, and Belt ranges, 

 form the most interesting studies. As a rule, a central mass or nucleus 

 of metamorphic rocks is elevated above the surrounding plain, and 

 around these nuclei are exposed the Jurassic, triassic, carboniferous, and 

 Potsdam rocks, in their order of sequence. But nothing short of a topo- 

 graphical survey, in connection with the geology, will make the struc- 

 ture of this region clear to the scientific world. 



The Black Hills of Dakota will form one of the most interesting 

 studies on this continent. There is so much regularity in the upheaval 

 that all obscurity is removed and all the formations known in the West 

 are revealed iu zones or belts around the granitic nucleus in their fullest 

 development. A careful detailed topographical and geological survey 

 of this range would be a most valuable contribution to science. In all 

 the western country I have never seen the cretaceous, Jurassic, triassic, or 

 red-beds, the carboniferous and Potsdam rocks, so well exposed for study 

 as around the Black Hills. 



CHAPTER Yin. 



* FEOM OMAHA TO CHEYENNE. 



In the preceding chapter I have given a brief review of the geologi- 

 cal formations of the Northwest, as revealed by that grand natural sec- 

 tion, the valley of the Missouri River. We are now prepared to pro- 

 ceed on our journey westward. 



The city of Omaha is most beautifully located on the western bank of 

 the Missouri River, on a second terrace, about fifty feet above the water- 

 level of the river. Terraces of the kind alluded to form a peculiar fea- 

 ture along the Missouri River and its tributaries, and are found from 

 the foot of the mountains to its mouth, and iu many instances they seem 

 to afford most beautiful natural sites for cities. I will not, at this time, 

 enter into an explanation of the causes which produce these terraces, 

 but simply remark that they perhaps indicate oscillations of level in 

 the surface, or the gradual recession of the waters toward the sea, 

 and that, far back in the past, each one of them has at onetime formed 

 the bed of the river. They also seem to indicate that formerly the Mis- 

 souri carried to the ocean a vastly greater volume of water than at pres- 

 ent. Another feature will at ouce catch the eye of the observing trav- 

 eler, and that is the marvelous fertility of all this region. The wide 

 grassy bottoms are black with rich vegetable matter to an almost inde- 

 finite depth, while the upland terraces and hills are covered with a de- 

 posit of yellow marl, varying from twenty to one hundred and fifty feet 

 in thickness. There seems to be evidence that the ocean or a lake once 

 extended up the valley of the Mississippi, and up the Missouri beyond 

 the reach of tidal influences nearly to Port Pierre, and that the myriads 

 of mountain streams poured their fresh waters into the great arm of 

 the sea, or estuary. These numerous streams, flowing through the soft 

 marls, sands, and clays of the great plain country, mingling their sedi- 

 ments in the waters, and deposited them in the bottom of this estuary. 



* Iu chapters VIII to XIII inclusive, numerous extracts have been taken from the 

 text of a volume entitled " Sun-Pictures of Eocky Mountain Scenery," and an article 

 published in the proceedings of the American Philosophical Society, Philadelphia, Feb- 

 ruary 19, 1869, by the writer. These papers necessarily have a very limited circulation 

 and as these official reports are designed for distribution far and wide among the people 

 this will be a sufficient excuse for introducing them in this connection. 



