254 The American Geologist. April, isos 
sub-Carboniferous beds were far below the Cretaceous base-level 
and after elevation Tertiary erosion removed the overlying lime- 
stone and shales down to the level of the siliceous beds, beneath 
which it did not go. The result was the separation of the old 
base-level into two parts, one represented b\' the Cumberland 
tableland, the other b}' the part of the highland rim where it 
runs beneath the Cretaceous. 
S. The Cretaceous Cycle in Wisconsin. Throughout most of the 
area northwest of Tennessee and Kentuck}', the surface configur- 
ation of the older rocks is concealed beneath a heavy mantle of 
drift. But over a considerable area of Wisconsin and a small 
part of the neighboring states, no drift deposits occur and the rock 
topograph}- is visible. The general physical features of southern 
Wisconsin* embrace a widely opened lowland, trenched by water 
courses, and with remnants of a higher land mass rising above 
the general lowland and forming the inter-stream divides. The 
trenching of the general lowland, from analog}' with similar work 
elsewhere, appears to be the work of Post- Tertiary erosion. The 
development of the general lowland took place during Tertiar}' 
time. The hills rising above the general lowland are the last 
remnants of the elevated land mass upon which erosion commenced 
at the beginning of Tertiary time. Whether the bight of these 
remnants represents the full altitude to which the land was ele- 
vated at the beginning of Tertiary time or whether erosion has 
everywhere reduced the country to a greater or less amount below 
the uplifted Cretaceous base-level, has not been determined. In 
any event, these higher masses represent nearly destroyed rem- 
nants of the Cretaceous elevated base-level; and even these rem- 
nants are probably not the full measure of the altitude assumed 
b}' the old lowland after its elevation at the end of the Cretaceous 
period. 
4. The Cretaceous Cycle in Minnesota. The Cretaceous de- 
posits of the plains reach eastward as a continuous sheet, to about 
the western border of Minnesota, t But at numerous localities in 
Minnesota, some almost as far east as the Mississippi river, iso- 
lated patches of Cretaceous occur beneath the drift, resting upon 
the underlying rock surface. These deposits are found along 
*See U. S. Geol Survey. Sixth Annual Report 1884 5. The Driftless 
Area of the Upper Mississippi Valley— T. C. Chamberlain and R. D. 
Salisbury, pp. 221-239. 
f Geol. of ^liiuiesota, Finaljleport, vol. i, pp. 431-439. 
