GEOLOGICAL SURVEY OF THE TERRITORIES. 55 



the summits of the hills near the month of the Mobrara, and soon gives 

 the character to the country. The surface underlaid by this formation 

 looks barren and arid, and is really the commencement of the reputed 

 sterile belt southward. It is composed mostly of laminated shaly clay, 

 is usually quite uniform in its composition and texture, and contains so 

 much alkaline matter that it prevents the growth of most plants except 

 those that are peculiar to such soil. 



The hills above Fort Eandall, on both sides of the Missouri, have a 

 barren, black appearance, and are often called the " burned hills" by the 

 voyageur. Sometimes numerous masses of selenite are scattered over 

 these hills, which, glistening in the sun, have suggested the name of the 

 " shining hills." The burned appearance is undoubtedly due to the de- 

 composition of iron pyrites by exposure to the atmosphere or water. 



When much vegetable matter exists in the beds, as in the Mobrara 

 group at one locality near Bijoux Hills, and in the lignite Tertiary beds, 

 it takes fire and bakes the superincumbent beds of rock, so that the 

 remains look in the distance like a pile of ruins. 



Inasmuch as the rocks of the Fort Pierre group do not occupy any 

 considerable portion of the State of ]!!febraska, I shall not discuss their 

 character to any extent in this connection. It makes its appearance 

 only in a few localities, as an overlapping rock south of the Mobrara 

 Eiver, and therefore exerts comparatively little influence on the country 

 below that point. 



The eroded materials of the rock are no doubt mingled greatly with 

 the superficial deposits which cover the northern portions of the State. 

 It is sufficient to remark that it occupies a vast area in the Territories 

 of the IsTorthwest, and that it has yielded many most interesting organic 

 remains. It is in many instances intimately blended with the group 

 above, which we have designated in the section as the Fox Hills group. 

 This latter group is not found in l^ebraska at all, but is seen in its 

 typical condition on a conspicuous and quite fertile ridge of land 

 between the Big Cheyenne and Cannon Ball Elvers, higher up on the 

 Missouri. It extends from the eastern side of the Black Hills across 

 the country northeastward. These beds give a more cheerful appear- 

 ance to the country; there is more timber, and springs of pure water 

 are common. It is also full of organic remains of great variety and 

 beauty. This is an arenaceous deposit for the most part, and has doubt- 

 less contributed its share toward giving fertility to the Nebraska soils- 



TERTIARY PORMATIONS OF NEBRASKA. 



These formations in the Valley of the Missouri present features of the 

 highest interest to the geologist, and perform a prominent part in reveal- 

 ing the geological history of the West. They mark the dawn of those 

 internal forces which culminated in the present configuration of the vast 

 area between the Mississippi and the Pacific Ocean. So far as known, 

 only the more modern Tertiary deposits of the fourth basin occur within 

 the limits of the State of Nebraska. But in order that the relations of 

 these deposits may be shown to those of the Cretaceous period, and 

 the connection of the basins with each other, 1 shall give a brief descrip- 

 tion of them all in their order. The following general section of the Ter- 

 tiary deposits of the Northwest will show their extent and relation to 

 each other in order of time : 



