QUATERNARY AGE. 281 



taries have been gathering for vast ages that peculiar mud which 

 filled up their ancient lakes, and which distinguishes them even yet 

 from most other streams. Being anciently, as now, very 

 rapid streams, as soon as they emptied themselves into these 

 great lakes, and their waters became 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 first a little beyond the 

 boundary of the Loess lake, and then gradually to 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 following analysis of Missouri River sediment 

 taken at high stage will show, by comparison with the analyses of 

 the Loess deposits, what a remarkable resemblance there is even 

 yet between the two substances. 



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



Insoluble (silicious) 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 07 



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 essential particulars are the same. This identity of chemical 

 combinations also points to the remarkable sameness of conditions 

 that have existed for long periods in the Upper Missouri and Yel- 

 lowstone regions. 



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

 they existed for a longer or shorter time, as already remarked, as 

 marshes or bogs. Isolated portions would first become dry land, 

 and as soon as they appeared above the water they were no doubt, 

 covered with vegetation, which, decaying from year to year, and 

 uniting under water or at the water's edge with the deposits at the 



