The Loess of Natcliez, Miss. — Shimek. 297 
erosion if it is to add to the sum-total of the deposit. It is a well 
known fact that plants check or prevent erosion. The more 
luxuriant the vegetation the less erosion is produced by rain- 
storms. This is especially true of forests. The most violent 
rainstorms will scarcely disturb the finest leaf-mould even on 
very steep slopes in the woods, while they wash out great 
quantities of material in more open country. Vegetation, and 
especially forest vegetation, is best developed along the streams. 
It will thus be seen, that these three favorable conditions, 
while not restricted to streams, are yet best developed adjacent 
to them, and could accomplish more than would be possible at 
more remote points. That this is true appears also from the 
evidence of the snail fauna. The loess is thickest, and also 
most fossiliferous, in close proximity to streams, and near 
them, too, modern land-snails are most abundant. The fact that 
fossils are more abundant near streams than they are in more 
remote regions does not indicate a difference in the origin of 
the respective deposits,* but merely further shows that terres- 
trial conditions along streams, even on high grounds favor the 
development of snails. The reason that snails are not found 
fossil in the loess remote from streams is that when living 
they do not, to any extent, inhabit such places. 
Time. — The element of time is also to be taken into ac- 
count. It might seem that, if loess was deposited most 
abundantly where vegetation was comparatively vigorous, 
there ought to be an abundance of plant remains in the de- 
posit. The rate of deposition, however, must have been so 
slow that all, organic matter would have disintegrated long 
before it could have been covered and sealed in the deposit. 
Organic remains can be thus preserved only when over- 
whelmed, especially in wet places, and tlreir absence would 
rather militate against the aqueous theory. If the rate of net 
deposition, after deducting loss by erosion, already estimated 
by the writer,! namely, i mm. per year, be accepted, it is 
evident that a stick or log, or even a leaf, would decay long 
before it could be entombed in the deposit. At that rate a log 
one foot in diameter, for example, would require more than 
three hundred years for burial. Tlie shells of molluscs do 
*See Hershoy's article in Am. Geol.. vol. XXV. pp. 369-374.— 1900. 
fJoui'. Geol., vol. VII, p. 135, and Proc. la. Acad. Sci., vol. VI, pp. 109 
and 110. 
