LOESS AS A LAND DEPOSIT. a 
I am led to do since serious objections to-the hypothesis of aqueous deposition 
are being urged. It should be understood that some aqueous deposition is recog- 
nized to be necessary. I only wish to show that it fails to cover the whole series 
of phenomena, and that the wind has been very important and perhaps more 
potent and far-reaching in influence than water deposition. 
It is scarcely necessary to speak of the objections to aqueous deposition based 
upon topographic relations of the loess. As is well known, this deposit not only 
borders valleys, but blankets interfluvial tracts as well, often resting on an eroded 
surface like a mantle of snow, being found on the highest as well as the lowest 
parts of the previously eroded tracts. It is not rare to find it occurring at eleva- 
tions differing several hundred feet within a distance of but a few miles. Another 
_ objection to aqueous deposition, based upon absence of shorelines, has recently 
been made. The occurrence of terrestrial shells in the loess I also merely mention 
as a feature out of harmony with the theory of aqueous deposition, especially of 
deposition in a large body of water. 
ADEQUACY OF WIND AS SHOWN IN THE MISSISSIPPI VALLEY 
That the wind is adequate to produce such a deposit seems to be certain from the 
fact that materials of the same kind are constantly carried by the air. The ques- 
tion then also arises whether the conditions in the Mississippi valley are or may 
haye been such that wind sediments may have accumulated in the territory now 
covered by the loess. I wish to call your attention to these points. They may be 
very briefly stated as follows: 
1. The universal presence of mineral dust in the atmosphere, and its constant settling, 
necessitates its accumulation in places where erosion is at a standstill or where it does not 
exceed the rate of atmospheric sedimentation. 
‘It seems possible that the conditions prevailing at the present time in some places 
in the Mississippi valley permit a secular accumulation of atmospheric dust. Ob- 
servations which I have made in this direction indicate, however, that the quantity 
of atmospheric sediments now laid down is too small to give support to the view 
that there is any general accumulation of this kind going on over the loess region 
at the present time; but the same observations also indicate that the quantity of 
dust carried in the air is subject to extreme variations with rather moderate changes 
in meteoric conditions. The atmospheric sedimentation of the present time is 
hence no certain quantitative index for the past, and it seems quite possible that 
conditions may at some time have been more favorable to its effectiveness than 
they seem to be at present. 
2. Erosion of the flat loess-covered uplands is at the present time exceedingly slow as com- 
pared with the average rate of denudation of the whole Mississippi valley. 
This is a circumstance of importance in considering the possibility of an accumu- 
lation of wind sediments, for if extreme differences occur in the rate of surface 
denudation, it follows that by a very small addition of sediments in one place, 
where erosion is least rapid, the land may be built up, while it is being levelled 
down in other places where surface erosion is more rapid. 
From the topography of the Mississippi valley it is quite evident that denudation 
progresses at a very unequal rate for different parts of the land surface. Owing to 
the greater slope of the land in the peripheral regions of the valley, erosion there 
is much more rapid than in the central region. On the west slope this difference 
