Prof. Shimek's Criticism — Wrights 239 
which follows relating to the inadequacy of the floods I had 
supposed to account for the highest deposits of loess it is 
important to call renewed attention to a point explicitly 
made, that the supposed adequateness of the flood to ac- 
count for the phenomena was dependent upon the suppo- 
sition of a differential depression of the land to the north. 
This is necessary to secure the gradient supposed for the 
production of the slow rate of the current which passed 
through the narrow place of the Missouri trough. This 
differential depression towards the north is in analogs with 
all the positive facts that are known upon the subject, and 
is therefore lawfully made, and it provides that balancing 
of forces which we need to account for all the phenomena. 
The current was slow, not over three miles an hour. Such 
a current was not sufficient to lift sand up to the higher 
levels of overflow, where lakelike conditions existed on 
either side of the central flow of water. This accounts for 
the fineness of all that upland deposit, and indeed for the 
deposit itself. On Prof. Shimek's theory the rapid current 
of his flood would have carried the loess all down the river, 
and left nothing for the wind to get at. The sandbars of 
the Missouri are not covered with loess after floods. 
6. Since publishing my last paper, I have extended 
my observations down the Mississippi river on both sides 
to as far as Vicksburg; and, while not ready at present, 
fully to publish my conclusions, .1 may say that it seems- 
to me we shall be compelled to suppose that the epeirogenic 
downward movement affected to a considerable extent the 
whole valley of the Mississippi, this being necessary to ac- 
count for the deep loess deposits covering Crowley's Ridge 
on the west side, and marking the border of the Yazoo 
delta, often fifty or sixty miles east of the Mississippi river, 
extending from Memphis to Vicksburg. In floods at the 
present time the ^Mississippi spreads over this delta, form- 
ing a slowly moving temporary lake seventy miles wide. 
With larger floods and more abundant supply of loess, the 
Mississippi would be now rapidly building uj) a loess ter- 
race over that region in which there would be no inter- 
mixture of sand, and over which the water would not re- 
main long enough to support the distinctively water species- 
