246 The American Geologist. April, isdj 
turbances as have characterized the region of the Appalachians on 
the east, or the Rocky mountains on the west. All changes seem 
to have been of the nature of continental elevation or subsidence, 
affecting wide areas. The principal effect of such movements 
upon rivers flowing over approximately horizout-il rocks would be 
to increase or diminish their power of erosion with their greater or 
less elevation above base-level. Their courses would not be 
altered. It is therefore not improbable that the courses of the 
larger rivers and the general direction of the drainage of the area 
was determined at the time of the first elevation of the land above 
the sea at the close of Carboniferous time, and that this drainage 
has been maintained ever since. The drainage which was insti- 
tuted when the interior basin first became dr}' land will be consid- 
ered, and the courses that the streams then took determined, as 
far as possible. If the present courses of the streams correspond 
with these h^-pothetical original courses, the similarity maj' be ex- 
plained by considering the first streams to have preserved their 
first courses. The^' will then be original or consequent streams. 
If there is a lack of correspondence between the two, the cause 
and date of the change from the supposed original courses must 
be considered. 
During Paleozoic time the region of the Mississippi valley was 
occupied by a broad interior sea, Ijounded b}- land masses on the 
north and southeast. Sediments were brought down into this 
basin from either side. Sands and gravels were deposited near 
shore, finer muds at a distance, while the center of the basin was 
the scene of the accumulation of widespread limestone deposits. 
During Carboniferous time the sea shallowed, and its bottom over 
much of the area was near sea level. At times vast swampy for- 
ests grew over newlj' emerged flats, onh' to be buried beneath the 
mud and sand of a later subsidence. 
There was of course no drainage while the area was under water. 
The drainage of the swamps of the coal period must have been 
very sluggish and is so little known that it cannot be considered 
here.* But it is comparativel}' easy to picture the conditions 
which must have existed over most of the area after the Appal- 
achian disturbance. The Paleozoi'; rocks had been crumpled into 
mountain elevations along the Appalachian region. These fold& 
died awa}' to the west where the Carboniferous deposits formed a 
*See Missouri Geological Survey. Report on Coal. 1891, pp. 24-.5. 
