Esosion Cjicles in JVorthiresten) Tllinois. — Jler.iJiei/. 91 
by isolated outliers of the latter in central Tennessee and 
Kentucky, at levels which would bring it down to about 1,000 
feet above the sea at Louisville, Kentucky, or only 200 feet 
higher than the Tertiary peneplain at the same place. It is 
evident that the central Mississippi basin is an area of de- 
pression. The Illinois coal basin has been an area inclined to 
depression, while contiguous districts were being uplifted, 
since the early part of the Silurian period; and an examina- 
tion of good topographic maps will bring to light the fact 
that central and southern Illinois are now depressed somewhat 
below the surrounding areas of so-called structural "uplift," 
causing its upland surface to be apparently continuous with 
that of the lower Mississippi region without any marked de- 
formation along the line of junction. Consequently it is not 
impi-obable that, over a very large portion of the Illinois de- 
pression, the plane of the Cretaceous peneplain does not ma- 
terially differ from the baselevel plane of the Tertiary era.* 
Now the peneplain which is apparently represented by the 
summit level of the "mounds" of northwestern Illinois, south- 
ern AVisconsin, and eastern Iowa, descends toward the south 
at a rate which would take it below peneplain No. 2 on the 
southern side of the Rock river. As its elevation in our dis- 
trict was apparently due entirely to an uplift proceeding out- 
ward from some point in southern Wisconsin, we may assume 
that the coincidence between peneplains Nos. 1 and 2, in cen- 
tral Illinois, needs no further proof. In this connection it 
may be interesting to learn that Leverett and Salisbury liave 
found sands and gravels apparently i» situ in Adams and 
Hancock counties, Illinois, which they are inclined to regard 
as a Cretaceous formation on account of the large numbers of 
sharks' teeth and other fossils of a Cretaceous fades whicl\ 
occur in the drift of thai vicinity. 
(Considering all the evidence in its proper bearings, it would 
appear that the Cretaceous peneplain, descending to the level 
of the Tertiary peneplain on the southern side of the Illinois 
de])ression, again rises above it on the southern slope of tlic 
Wisconsin u])lift as ])eiie])lain No. 1 oi the Freeport section, 
represented l)y the summit plane "of the monadnocks of the 
district. 
*This hypothppis was first su^^j^ested to the writer in c(»rros]):)iulence 
with Mr. Fiiiuk Leverett. 
