Story of the Mississippi- Missouri — Claypole. 367 
ing of the greater part of that basin. At the close of the palae- 
ozoic era midland North America raised its head above the 
water and has never as a whole been since washed by the sea. 
The long continued subsidence to which I have already alluded, 
and which, though most profound on the Atlantic sea-board 
yet, doubtless, extended over the whole Mississippi valley, 
ceased in the east at that date. The consequent accumulation 
of sediment was arrested and the opposite process of erosion 
immediately set in. 
Whether this important geological change was caused by 
elevation of the land or by depression of the bed of the ocean 
somewhere else it is not at present possible to determine. Not 
a little significant evidence points in the latter direction. But 
for our purposes here the question is not very important and we 
will not enter on any further consideration of it. It will be 
sufficient to point out that the palaeozoic sediments thin regu- 
larly away from the eastern highlands to the west and that on 
approaching the site of the present Rocky mountains they 
again grow thicker often to a considerable extent. 
On the emergence of the wide, low flats on which for so 
many ages had been growing the humble but gigantic vegeta- 
tion of the Coal-Measures, lines of drainage were of course at 
once established. These followed the original slope of the 
ground and the great trunk stream took its channel at the 
lowest line of flow. Here is the physical cause of the position 
of the Mississippi river. It lies where the strata were thin if 
not thinnest and collected its waters from both sides of the 
interior basin. 
As the land slowly rose above the waters the great river 
developed. But it was not the great river as we know it at the 
present day. At first an insignificant stream showing little 
promise of what it was destined one day to become it drained the 
high lands and plateaus of Minnesota and the adjoining regions 
not perhaps in the exact channel which it at present occupies 
but probably not very far from it. As the water retreated the 
young giant grew and was strengthened by the accession of 
numerous other streams that previously reached the sea by 
independent mouths. The retreat of the sea continued south- 
ward until the new continent was plainly outlined, its mount- 
ain-ranges defined and its shores determined. The North 
America that existed after the Appalachian revolution was 
