252 The American Geologist. April, isos 
draintige soutlnvard was the actual depression of the Cretaceous 
base-level across the lower course of the present Mississippi river. 
Here the base-level was depressed below sea level and the drainage 
of the interior found an easy outlet to the south. Since that time 
the Mississippi has been gradually' extending its mouth southward. 
II. The Cretaceous Cvcle. 
1. The Cretaceous Cycle. The earliest topographic feature of 
eastern North America dates from the close of the Cretaceous.* 
It comprises the remnants of the plain to which long continued 
erosion of Mesozoic time had reduced the land surface. At the 
commencement of Tertiary time this lowland was raised and be- 
came an elevated upland plateau. 
Two difficulties are met in the attempt to trace this elevated 
Cretaceous base-level across the Mississippi basin. First, erosion 
appears to have obliterated almost all evidence of it on the soft 
paleozoic rocks of the interior, and it is only on the borders of 
the region, where the rocks are prevailingly hard sandstones and 
conglomerates, that successful search for it may l)e expected. 
Second, over a large part of the Mississippi basin the topography 
of the underlying rock surface is concealed beneath a monotonous 
mantle of glacial drift. 
J. The Cretaceous Cycle in KeutucLy and Tmnesstc. Kemnants 
of the Cretaceous base-level occur in Tennessee and Kentucky. 
Among the mountains of North Carolina evidence of the elevated 
Cretaceous base-level is found by Willis, f The mountains rise a 
few thousand feet above the plateau and some of the valleys are 
cut several hundred feet below it, but to a person standing at the 
same elevation wath the plateau, its plateau character can be dis- 
tinctly seen. The considerable elevation of the mountain sum- 
mits above the plateau in this district is due to the greater hard- 
ness of the rocks which did not permit their being reduced to 
base-level before the elevation which closed the Cretaceous Cycle. 
Northwest of the Carolina mountains lies the broad valley of the 
East Tennessee river, which was excavated during Tertiary time. 
On its eastern border, the generally even crest-line of Pine mount- 
ain, with an average elevation of 2,000 feet probably marks the 
*Sec Bull. Geol. Soc. America, vol. 2, pp. 541-586. The Geological 
Dates of Origin of Certain Topographic Forms on the Atlantic Slope 
of the United States, by W. M. Davis. 
fNational Geographic INIagazine, vol. i, 1889, pp. 291-300. 
