282 GEOLOGY. 



bottom, formed that black soil so characteristic of Nebraska prai- 

 ries. For it is well known that when vegetable matter decays in 

 water or a wet situation its carbon is retained. In dry situations it 

 passes into the atmosphere as carbonic-acid gas. After the first low 

 islands appeared in this old lake, they gradually increased from 

 year to year in size and numbers. 



The ponds and sloughs, some of which could almost be called 

 lakelets, still in existence, are probably the last remains of these 

 great lakes. These ponds, where they do not dry up in midsum- 

 mer, swarm with a few species of fresh water shells, especially of 

 the Limn<zs, Pkysces, and Planorbi, which to me is strong proof of 

 this theory of their origin. The rising of the land continuing, the 

 rivers began to cut new channels through the middle of the old 

 lake beds. This drained the marshes and formed the bottom lands, 

 as the river beds of that period covered the whole of the present 

 flood-plains from bluff to bluff. It was then that the bluffs which 

 now bound these flood-plains received those touches from the hand 

 of nature that gave them their peculiar steep and rounded appear- 

 ance. Newer and more plastic, because less compactly bound and 

 cemented together, the rains and floods easily molded them into 

 those peculiar outlines which they have since preserved. 



The Missouri, during the closing centuries of the Loess age, 

 must have been from five to thirty miles in breadth, forming a 

 stream which for size and majesty rivaled the Amazon. The 

 Platte, the Niobrara, and the Republican covered their respective 

 flood-plains in the same way. In the smaller streams of the State, 

 those that originated within or near the Loess deposits, such as the 

 Elkhorn, Loup, Bow, Blue, and the Nemahas, we see the same 

 general form of flood-plain as on the larger rivers, and no doubt 

 their bottoms were also covered with water during this period. 

 Hayden, in his first reports, his already expressed the same opinion 

 as to the original size of these rivers. Only a few geologists will 

 dissent from this view. The gradually melting glaciers, which had 

 been accumulating for so many ages at the sources of these great 

 rivers, the vast floods of water caused by the necessarily moist 

 climate and heavy rains, the present forms and materials of the 

 river bottoms, are some of the causes which, in my opinion, would 

 operate to produce such vast volumes of water. 



The changes of level were not all upward during this period. 

 The terraces along the Missouri, Platte and Republican indicate 



