Distrihiition of the Loess. — Wright. 219 
This production of a temporary lake in the middle of the 
Missouri valley by the melting away of the ice to the north 
would produce all the conditions necessary to support the 
aqueous theory for the deposition of the loess. A difficulty in 
the theory of a permanent standing body of water is that the 
sedimentary material brought into such a lake would be pre- 
cipitated not generally throughout the area as it is, but in some- 
thing like deltas upon the northern border ; but, under the pres- 
ent supposition, we have a constant movement of water along 
the main line of the Missouri such as Miss Owen all along 
insisted was necessary to furnish a supply of material as far 
down as St. Joseph, Kansas City, and points still farther below. 
In the main current of the stream, therefore, the material of the 
fineness of the loess would be continually kept in motion, so 
that there would be no need of supposing the bed of the river 
to silt lip to any great extent, except in the northern portions, 
where the coarser gravel and sand would be deposited, as we 
find is the case. But upon the border of the main current, the 
water charged with sediment would constantly be spreading 
over, and losing its motion, so as to deposit its material after 
the analogy of an ordinary river overflow. 
On the supposition that these conditions actually occurred, 
practically all the objections to the aqueous theory are removed, 
for it is well known that aquatic shells are not found on the 
flood-plains of rivers ; on the contrary, such localities are fav- 
orite places of land snails. In the conditions supposed, the sub- 
mergence of the higher elevations, where, for the most part, 
shells are found in the loess, would not occur until the later 
part of the summer, and would continue but a short period. 
In such case there would be ample opportunities for the de- 
velopment of land species of snails and no fit conditions for 
aquatic species. 
If it is objected that these shells indicate a climate about 
such as now characterizes the region, it is to be observed that 
such it probably was during the closing stages of the Glacial 
period. A warm climate was required to melt the border of 
the ice, and the ice-covered region was at this stage 100 miles 
or more to the north. 
The actual existence of such a periodical rise of the Mis- 
souri river as we have supposed is directly proved bv the posi- 
