LOESS-DEPOSITING WINDS IN LOUISIANA 535 
the loess materials, and the agent of final loess placement on the 
uplands adjoining the river narrows either to water deposition or 
to winds carrying materials from the lowland. 
The explanation of southern loess as wind-blown dust not only 
presents the fewest difficulties but is distinctly supported by most 
of the loess features. The smallness of grains can be explained by 
the weakness of wind transportation, for, according to the experi- 
ments of Udden, only dust particles with diameters of .18 mm. and 
less are readily borne by ordinary winds.‘ We have noted that 
about 70 per cent of the loess is composed of silt particles from 
.o5 to .or mm. in diameter and that the mechanical composition 
is notably uniform. The practical absence of coarse particles is, of 
course, explained by the inability of ordinary winds to carry them. 
Udden thus explains the small percentages of very fine particles.’ 
The writer suggests another reason for the relatively small per- 
centages of very fine materials (clays) in the loess. Observations 
on clay and silt plowed fields in Kansas during dry seasons showed 
that more dust blows from the coarser silt soils than from the clay 
soils. The reason apparently is that, during a dry season, the clays 
bake more than the silts and so offer more resistance to the winds. 
According to this conclusion, if the Mississippi upon subsiding left 
areas covered here with silt and there with clay, the winds would 
carry a larger proportion of silt than of clay. 
In a large river like the Mississippi it would seem impossible 
for the water to deposit its sediment with practically no stratifica- 
tion, even if the water carried only loessial materials, for the loess 
contains from 5 to ro per cent of fine sand, and in places this coarser 
material would be segregated. The loess materials carried by the 
Mississippi were in large part carried from drift regions to the north- 
ward and, even granted that the drift furnished only fine materials, 
the streams south of the glaciated region, such as the Arkansas, 
Yazoo, and Red rivers, were doubtless contributing their sandy 
loads, so that the Mississippi could not have carried only a loessial 
t Journal of Geology, II (1894), 323. 
2““The finest materials carried by the air are not deposited in so great a pro- 
portion with the coarse materials as they would be if the atmosphere carried a greater 
load. The finest materials settle only in extreme calms”’ (zbid.). 
