Sto7'y of the Mississippi- Missouri. — Claypole- 375 
marks which it has left show that in the first period the ice ex- 
tended nearly to the mouth of the Ohio but in the second its 
range was probably limited to the northern line of states and 
reached litttle south of the lakes. Even then, however, it 
buried in ice the northern part of the Mississippi and the head 
waters of the Missouri so that these rivers were considerably 
reduced in size and extent. But in both cases as the ice 
melted away and the land was again uncovered their main 
channels of drainage resumed almost exactly their previous 
lines of flow and the preglacial hydrography of the country 
was restored. It is true that in some cases the deposits left by 
the ice blocked up the old courses and the returning rivers 
were in consequence compelled to find new ones. But such 
instances, though considerable in number, were chiefly con- 
fined to the smaller streams and did not affect the great trunk 
lines so that they need not be further considered here. The 
present drainage of the valley of the Mississippi is an almost 
exact reproduction of that which existed in pre-glacial times. 
If, however, as some geologists believe, there has been a 
series of glacial invasions following one another from at least 
the beginning of the Tertiary era then we must intercalate into 
the life of this great river-system as many interruptions of the 
kind just described. For every such period must have caused 
a stoppage of the drainage as complete as that produced by 
the one ice-age of whose recent occurrence the geologist has no 
doubt. Such intercalations will not affect in any degree the 
truth of the story as here given but will render it more com- 
plex in proportion to their frequency. As however we have 
thus far no certain proof of this recurrence of cold conditions 
it will be unnecessary further to consider the subject. 
We have traced this great river-system from its tiny begin- 
nings on the wide flats and gently sloping beaches of a conti- 
nent emerging from the waters of the palaBozoic ocean through 
the long Secondary and Tertiary eras of geologic history. We 
have seen it, small at first and draining a comparatively lim- 
ited basin, grow with the growth of the country as the Ameri- 
can Mediterranean sea of the Triassic, Jurassic and Cretaceous 
ages became less and less and ultimately passed away. We 
have seen it reinforced by the addition of the waters of all the 
new region to the west as the elevation of the Rocky moun- 
