252 GEOLOGICAL SURVEY OP THE TEERITORIES. 



quiet, the sediment held suspended was dropped to the bottom. While 

 this process was going on in the earlier portion of this age, the last of the 

 glaciers had probably not retreated farther than the headwaters of the 

 Platte, the Missouri, and the Yellowstone. The tremendous force of 

 these mighty rivers was, for a while at least, aided by the erosive action 

 of ice, and therefore must have been vastly more rapid at times than 

 anything of the kind with which we are now acquainted. The follow- 

 ing analysis of Missouri River sediment taken at high stage will show, 

 by comparison with the analyses of the Loess deposits, what a remark- 

 able resemblance there is even yet between the two substances. 

 In one hundred parts of Missouri Eiver sediment, there are of — 



Insoluble (siliceous) matter. 82.01 



Ferric oxide > 3.10 



Alumina 1.70 



Lime, carbonate , 6.50 



Lime, phosphate 3.00 



Magnesia, carbonate . 1 .10 



Potassa 50 



Soda 22 



Organic matter ,. .• 1.20 



Loss in analysis 67 



100.00 



Two other analyses which I made, the one from sediment at high 

 water and the other at low water, differ somewhat from this, but in es- 

 sential particulars are the same. This identity of chemical combina- 

 tions also points to the remarkable sameness of conditions that have 

 existed for long periods in the Upper Missouri and Yellowstone regions. 



After these great lakes were filled with sediment (Missouri mud), they 

 existed for a longer or shorter time as marshes or bogs. Isolated [)or- 

 tious would first become dry laud, and as soon as they appeared above 

 the water they were, no doubt, covered with vegetation, which, decay- 

 ing from year to year, and uniting under water or at the water's edge 

 with the deposits at the bottom, formed that black soil so characteristic 

 of Nebraska prairies. For it is well known that when vegetable matter 

 decays in water or a wet situation its carbon is retained. In dry situa- 

 tions 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 midsummer, swarm with a few species of fresh-water shells, espe- 

 cially of the Limnces, Physws, 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 thatthe bluHs which now bound these flood- 

 plains received those touches from the hand of nature that gave them 

 their peculiar steep and rounded appearance. 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 La- 

 custrine age, must have been from Ave to thirty miles'in breadth, forming 

 a stream which for size and majesty rivaled the Amazon. The Platte, 



