22 The Ainerican Geologist. juiy. i8P2 
portions of the states in which, on this theory, such shore lines 
should occur, and not onl}' found none, but discovered no evi- 
dence of any kind to indicate the presence of such a liody of 
water. It seems necessary therefore to look further for a solu- 
tion of the problem. 
Comparison has shown a marked similarity between the white 
clay deposit of the Ohio, and the great silt deposit of the upper 
Mississippi. All the phenomena of structure and distribution 
indicate that they originated under similar conditions. This being 
the case it is evident that a hypothesis which would account for 
the phenomena of the Mississippi region, would also account for 
those of the Ohio, so for a solution of the problem we need only 
apply, to the latter region, an explanation which has proved sat- 
isfactory in the former. A detailed study of the upper Missis- 
sippi region, particularly of that part of it which liorders the 
driftless area, was made several years ago by Chamberlin and 
Salisbury, and led them to the conclusion that the distribution of 
the loess and associated silts and clan's is best explained on a 
hypothesis of fluvio-lacustrine deposition.* Evidence was found 
that the altitude of the region was much below the present per- 
haps not far above sea-level, but instead of its being occupied 
by an inland sea, it is their conception that the vallej's became 
silted up so that at the maximum of depression the}" were occu- 
pied l)y shallow, perhaps marsh}", lake-like rivers many miles in 
width, whose waters moved slowly seaward from the edge of the 
melting ice-sheet. The constitution of these silts shows a direct 
derivation from glacial waters. The presence of shells of land 
mollusca in the silts, indicates that the region was subject to occa- 
sional or periodic overflow rather than to perpetual and deep sub- 
mergence. 
The applicability of this hypothesis to the Ohio region may 
be made more clear by a consideration of its application to a part 
of Tllinois, where similar phenomena occur, but under more simple 
conditions. For nearly a hundred miles north from the extreme 
limit of glaciation in that state, and extending from the Missis- 
sippi river eastward to the hilly districts of southern Indiana, 
there is a generally flat surface on which there are few, if any, 
*"The Driftless Area of the Upper Mississippi Valley." By T. C. 
Chamberlin and R. D. Salisbury. Sixth Ann. Rept. U. S. Geol. Survey 
pp. 211-216, 278-307. 
