The Columbia Forma fAon in y. ]V. Illinois. — Hersheij. 23 
the Gulf." These cX'Ays were deposited soon after the begin- 
ning of the Columbia submergence, the great river gradually 
silting up its lower valley. The Port Hudson clays seem to 
have been formed vinder very similar conditions as the basal 
fluvial member of the Columbia in northwestern Illinois; the 
general appearance and constitution of the former are similar 
to the finer portions of the latter; and they both apparently 
bear the same relation to the great Columbia submergence, 
and to the deposits over them. 
Believing that the submergence of the upper Mississippi 
valley was contemporaneous with that of the Mississippi em- 
bayment and the Atlantic coastal plain, I think it very prob- 
able that this submergence had reached the stage of formation 
of flood-plain deposits in both regions at or about the same 
time; and that the Florence gravel, sand, and clay of the Pec- 
atonica basin, and presumably of all western Illinois, are the 
northern representative of the Port Hudson member of the 
Columbia formation ; and that not improbably the two are 
continuous, as a single horizon, under the loess and modern 
alluvium, through the valley of the Mississippi. 
McGee next finds his second member of the Columbia for- 
mation in the Mississippi embayment to consist of a coarse 
stratified sand and gravel (Safford's "Orange sand"). The 
submergence was greater and the region of the lower Missis- 
sippi had become an extensive baj^ into which the great river 
brought vast quantities of sediment from the glaciers in the 
North and mixed it with the local material which makes up 
the great body of the sandy member in the Mississippi em- 
bayment. It certainly requires no great stretch of the imagi- 
nation to suppose that the sandy or lower division of the 
Valley loess in northwestern Illinois may be the northern ex- 
tension of this second member of the Columbia in the South. 
McGee also finds that at the time of greatest submergence 
in the Mississippi embayment a mantle of loess and loam, 
composed largely of glacial rock-flour from the north, was 
laid down on the bottom of the great ba3^ At the same time 
of maximum submergence in the upper Mississippi valley, a 
very similar bed of glacial silt was being deposited over uearl}^ 
all the country which was not covered with ice. This is the 
so-called Upland loess of the Pecatonica basin and surround- 
