376 Story of the Mississippi-Missouri—Clay pole. 
tains slowly progressed. We have watched the development of 
its great tributary, the Missouri, as it brought down the waters 
of the large western lakes and the melting snows of the moun¬ 
tains until the lakes passed away, drained by the cutting down 
of their outlets and filled by the mud and sand of their tribu¬ 
taries. We have seen the domain of the united rivers over¬ 
flowed by the northern ice and well nigh buried beneath the 
snowy mantle which the age of cold spread over the face of the 
country. We have noted the probable, the almost certain re¬ 
currence of this disaster a second and possibly more than a 
second time. And finally we have seen the two great rivers 
reassert their supremacy as the ice sheet disappeared and re¬ 
sume their ancient channels almost without change. The 
Missouri still carries down to the gulf loads of sand and mud 
such as it carried in its early days and the Mississippi still 
flows, clear and generally placid, as becomes the older stream. 
Shall we in conclusion look forward and try to see what the 
future has in store for this mighty river-system? The geolo¬ 
gist sees in every lake only a transient phase of the earth’s 
geography. The existence of a lake is only a question of time. 
If it has a feeder that feeder will ultimately fill it up. If it has 
an outlet that outlet will ultimately drain it dry. If it has 
neither the one nor the other the growth of vegetation in its 
waters and the wash from its banks must in the end bring 
about the same result. Thus the Missouri has drained the 
great Tertiary lakes that once occupied a large area in the 
western states, in Dakota and Nebraska, and they have 
passed away. And it is now engaged in the work which may 
be called the special function of all rivers—it is carrying down 
the continent to the gulf. In this it is aided by the other 
rivers of the system, the Mississippi, and the Ohio and their 
tributaries in the south. All are slowly washing North Amer¬ 
ica into the sea, and give them only time enough and they will 
accomplish their task unless the counter forces that in days 
gone by elevated it above the waves again come into play 
bringing up new mountains and new plateaus or raising to a 
higher level some of those already existing. Such changes 
are to be looked for in the distant future but it is beyond the 
ken of the prophet to foresee their date. If, as experiment 
and observation seem to show, the result of the action of the 
Missouri is now lowering the level at the rate of a foot in 
