260 The American Geologist. April. i893 
sively become thinner and finally disappear, indicating that a land 
area existed in Tennessee and Kentucky in middle Paleozoic time 
which during a considerable period was gradually extending itself 
northward. The present anticlinal dip of the strata appears to 
be due to an elevation of middle Paleozoic date. Erosion in 
middle and late Paleozoic time was doubtless effective in remov- 
ing a large part of the original constructional inequalit}'. If then 
this low land mass had been slightl}' submerged during Carbonif- 
erous time and covered with a thin veneer of Carboniferous rocks, 
and had then been elevated with the rest of the interior basin, 
the post-Carboniferous drainage would have followed the course 
of the present Ohio and Cumberland rivers, directlj' across the 
underl3ing structural anticlinal, and at a later date would have 
discovered the concealed diversit}" of rock structure. The course 
of the Ohio and Cumljerland rivers would be consequent upon the 
slope of the newly emerged land surface. In that part of their 
courses lying across^the Cincinnati and Tennessee anticlinals, the 
rivers are superimposed on the underlying rocks from a cover of 
Carboniferous strata which have been completely removed. 
(c) The upper Missiasippi. The course of the Mississippi along 
the western border of Wisconsin affords another example of de- 
parture from the supposed post-Car])oniferous drainage. Instead 
of flowing down the dip of the rocks to the southwest it follows 
the strike to the southeast. The original drainage of the area 
must have been southwest into a Mesozoic interior sea, and this 
drainage must have been maintained until the end of Cretaceous 
time ; for during the Cretaceous period the interior sea reached 
almost as far east as the Mississippi. The elevation of the plains 
inaugurated a new condition of drainage. The elevation of the 
plains on the west and of the Appallachian region on the east 
which marked the close of the Cretaceous period, left a broad 
north and south depression which determined the course of the 
■drainage of the northern part of the Mississippi liasin. and the 
upper Mississippi dates from that elevation. 
(rf.) The Lower Mississippi. There is good reason to believe 
that the course of the main post-Carboniferous drainage line of 
the interior was not, as it is now, south from the mouth of the 
Ohio, but rather west, through Missouri and Arkansas. The axis 
of the trough which was formed between the Canadian and 
Appalachian uplifts would lie to the southwest, through Missouri 
