GEOLOGICAL SURVEY OF THE TERRITORIES. 99 



The .channels of all the larger rivers had been marked out prior to this 

 time, for we find that these superficial deposits reach their greatest 

 thickness in the immediate valley of the Missouri Eiver and thin out as 

 we pass up the valleys of its tributaries on the east and west side, while 

 they almost cease to appear near the mouth of White Earth River. 



The question at once occurs, at what time did this geographical con- 

 dition of the country exist 1 We believe that it forms a part of what 

 is called the quarternary period in geology, which, though very modern, 

 geologically speaking, really extended far back in the past before the 

 existence of man on this continent, judging from the evidence we have 

 been able to secure up to the present time.. If we examine the numer- 

 ous cuts, or washed bluffs, which, we find everywhere, we shall discover 

 a great variety of fresh- water and land shells, as Helices, Paludinas, 

 Succinneas, &c, and here and there the remains of the mastodon and 

 elephant. In the year 1867, while prosecuting the geological survey of 

 Nebraska, under the General Government, I obtained from these marls 

 fine specimens of the molar teeth of the JEleplias americanus or Ameri- 

 can elephant, "and the mastodon, M. americanus. These remains of 

 gigantic extinct animals are mingled with those of animals existing in 

 this region at the present time, such as rabbits, mice, gophers, beavers, 

 buffaloes, deer, &c, which have been found in great quantities. Nearly 

 all the shells are identical with living species which are abundant in 

 some of the streams flowing into the Missouri and the Mississippi. In 

 the banks of some of the little streams, oftentimes buried ten to twenty 

 feet beneath the surface, are large accumulations of shells, as snails, fresh- 

 water mussels, &c, while very few and perhaps none exist at the present 

 time in the immediate vicinity. Sometimes, in the fine vegetable matter 

 that accumulates along the Missouri River from the annual floods, can 

 be seen bushels of minute snail shells, yet not a snail can now be found 

 alive anywhere in that region. We account for this by some change 

 in the physical conditions which were once very favorable for their exist- 

 ence and increase. The waters of the little streams were far clearer 

 and purer than at present. Now, at certain seasons of the year, they 

 become so charged with sediment that molluscous life cannot exist. 

 This is the case with the Missouri River from the foot of the mountains 

 to its mouth, and scarcely a shell can be found in its waters ; but in 

 some of its tributaries, as the Big Sioux, James, Vermillion, &c, that 

 flow in from the north, there is the greatest abundance. 



The traveler will very naturally inquire, why, with all this wonder- 

 ful fertility of soil, these broad, grass-covered plains do not contain a 

 suitable supply of forest trees. We will endeavor to answer this ques- 

 tion in another place. He will find, as he travels over the State of Ne- 

 braska, that the time is not very distant when portions of the country 

 will be covered with beautiful artificial forests, and we will attempt to 

 show that this is only a restoration of conditions that once existed far 

 in the geological past. 



Before leaving the Missouri River I will refer briefly to an interesting 

 phenomenon which I shall work up in detail at some future time. The 

 proofs of glacial action in the West are not common or very remarka- 

 ble in their character ; still they are shown to a certain extent, not only 

 in the mountains but also in the plains. Along the Platte River, below 

 Omaha, and on the Missouri, near the city, the carboniferous limestones 

 have had their upper surface so thoroughly smoothed by glacial action 

 that they can be quarried out and used for caps and sills without any 

 further finish to them. And the process seems to have been carried on 

 with wonderful uniformity, for the upper surface seems to be as level as- 



