Mississippi Drainage System. — Westgate. 259 
to the soft Silurian limestone on which they then began to widen 
out the present lowland. At the same time that the central low- 
land was in process of formation, the shales and limestone were 
being removed from above the hard siliceous beds which form 
the rim of the central lowland region and a second and structural 
plain was being formed along the siliceous beds. This second 
plain is the "highland rim" of middle Tennessee, and is prin- 
cipally of Tertiar}- date, although reasons have been given for 
considering its western portion as a part of the Cretaceous base- 
level. 
3. Tertiary loorh in Wisconsin. The surface of the driftless 
area of Wisconsin is generally level and if we leave out of con- 
sideration the trenching of the lowland by the present streams it 
may be considered as a region in which the surface has advanced 
far toward topographic old age. The streams have not only cut 
down to base-level, but have widened out their valleys laterally 
forming wider and wider plains and causing the intervening land 
masses to recede until they have become dendritic ridges, or 
isolated hills, or have wholly disappeared. The general surface 
is a broad plain interrupted by remains of a higher land mass 
which has been eroded away except along the divides. The ex- 
cavation of this lowland was accomplished during Tertiary- time. 
Since then the region has been slightly elevated and the streams 
have deepened their channels and trenched the general lowland; 
but this episode is of interglacial date and does not concern us 
here. Before this slight uplift the surface of the driftless area 
was that of a country approaching the close of a long geographical 
cycle in which it had been reduced to base-level except along the 
inter-stream divides. 
4. Post- Terf iary sub-cycle. By the close of Tertiary time the 
softer rocks over many parts of the eastern Mississippi basin had 
a second time been base- leveled. The broad valley of the Ten- 
nessee river and the central lowland of Kentuck}' and of Tennes- 
see and the level lowland of the driftless area of Wisconsin are 
the clearest examples of this lowland. In Minnesota the lowland 
of Tertiary time was the lowland of Cretaceous time. 
After the close of Tertiary time a small elevation occurred 
which left the drainage of much of eastern United States a greater 
or less distance above base-level. The streams at once began to 
cut below the lowland which had been the base-level at the end of 
