86 
IOWA ACADEMY OP SCIENCES. 
First . — The vertical distribution of the molluscs. The writer 
has already shown^^ that these molluscs were most probably 
deposited in situ, and sufficient time must have elapsed at least 
for the production and developement of the successive genera- 
tions. 
Second . — The fineness and homogenity of the loess material. 
This is of importance, for had the deposits been made quickly 
by powerful concentrated agencies, whether wind or water, 
much more coarse material would have been mingled with the 
fine debris. 
Tliird.~]So plant remains of ucdoubted loess origin occur. 
As the plants undoubtedly existed during the entire period 
the deposition must have gone on so slowly that ample time 
was given the plant remains to crumble in decay and mingle 
with the soil. 
With these propositions as an aid let us consider the follow- 
ing conception of the formation of the loess deposits: 
The region formerly covered by the glaciers remained a vast 
drift-covered plain after the recession of the glaciers. 
No loess was to be found, but the surface material consisted 
of unassorted drift, here and there heaped up in ridges and 
moraines. Streams soon cut their v^ay through this materiaP^ 
and ponds more or less numerous remained in the depressions 
of the plain. 
The climatic conditions having so improved, plants, at first 
the smaller forms, spread over the plain, and soon trees, in 
whose shades numerous molluscs lived amd prospered, appeared 
in narrow lines along the streams, the surface conditions being 
not unlike those of the northwestern portions of the state 
to-day. Forests gradually spread over portions of the area, 
principally along the river- valleys and on hillsides in the man- 
ner pointed out by Prof. Macbride.^^ 
When vegetation, especially the forests, had gained a foot- 
hold, then commenced the deposition of the loess. 
iiBuft Nat. Hist. S. U. I., Vol. II, p. 95. 
i2If it be true that our streams generally follow the highest ridges of the drift, even 
without reference to the loess, 1. e. if the streams run in glacial ridges (and the writer 
knows of some cases where this is true), then the fact can be accounted for by the 
theory offered in the paper by McGee and Call already cited, pp. 23-23, but the theory 
fails when applied to the loess because of the climatic conditions required. 
13 See paper: Forest Distrihution in Iowa and its Significance, in this volume. 
It is but fair to say that the theories thus presented by Professor Macbride and the 
writer, while leading to the same results, were developed from different standpoints 
along entirely independent lines of investigation. 
