63 



and Professor Joseph Leidy, whose papers, published in the Proceedings 

 of the Academy of Natural Sciences, have already made known some 

 of the principal discoveries and results due to their labors. Dr. 

 Hayden was the only one of those mentioned who accompanied me in 

 the field. 



In the paper by Dr. Hayden, accompanying the geological map, pub- 

 lished in June, 1858, Dr. H. observes that " A much larger surface 

 might have been colored on the map with a good degree of confidence, 

 but I have preferred to confine myself, for the most part^ to the results 

 of my own observations in the field." The northeast portion of the 

 boundary of the tertiary formation between the White and Niobrara 

 rivers is there placed too far west, according to my own observations in 

 1855 and 1857. The line between this and the cretaceous is not west of 

 a straight line between the mouth of the south fork of the White river • 

 and the mouth of the Keya Paha. A small portion of tertiary should 

 also be shown on the north side of the south fork of the Shyenne. The 

 great lignite tertiary formation most probably extends almost to the 

 base of the Big Horn mountains. 



It is of course impossible to give correctly the relative extent of 

 certain of the formations on a map of this scale. The width of the 

 upheaved sedimentary formations which encircle the igneous rocks of 

 the Black Hills are much more developed on the western than on the 

 eastern slopes, owing to their difference in dip — on the western being 

 quite gentle, but steep on the other, causing them soon to disappear 

 under the cretaceous rocks ; and this is one of the most important 

 features in the configuration of these mountains. 



From what has been said it will be seen that the surface of Ne- 

 braska presents two great sections — that of the plains, and that of the 

 mountains. 



The plains in this latitude are composed of nearly horizontal strata 

 of the tertiary and cretaceous formations, except in a small portion of 

 the southeast corner, where the carboniferous is developed. Though the 

 plains are much diversified by the effects of denuding agencies, and 

 present in different portions striking characteristics, yet they are, as a 

 whole, a great uniform surface gradually rising towards the mount- 

 ains, at the base of which they attain an elevation varying between 

 3,000 and 5,500 feet above the level of the sea. The plains have three 

 distinct portions as determined by their geological formation : 1st, 

 the pliocene and miocene tertiary : 2d, the cretaceous ; and 3d, the 

 lignite tertiary. 



The first section extends from the southern boundary north, nearly 

 continuous to the 44th parallel, and contains a large portion of the 

 valleys of the Platte, Loup Fork, Niobrara, and White rivers. Here, 

 except in the immediate valleys of the streams, which are composed 

 of good soil, naturally irrigated by springs from the bluffs, or suscep- 

 tible ot irrigation, much of the country is sandy and unfit for culti- 

 vation. No valuable mineral or good building stone have been dis- 

 covered in it. Here are to be found the Sand Hills, which occupy an 

 area north ot the Platte of not less than 20,000 square miles. These 

 hills on the north begin between the White and Niobrara rivers, and 

 extend south probably beyond the Arkansas. Where we have visited 



