84 
IOWA ACADEMY OF SCIENCES. 
That such bodies of water existed, though, as before stated, not 
of the extent required by the lacustrine theory, is also shown 
by the distribution of the pond mollusca, which are found in 
bands or layers similar to those which may be observed on the 
edges of our small ponds to-day. These layers are usually of 
but slight vertical extent, showing that the ponds did not per- 
sist during the entire period of deposition of the loess, but, like 
the ponds of to-day, were subject to changes. But if the water 
area was not great, comparatively little of the material carried 
by the winds could be deposited in this manner, and as a matter 
of fact we find comparatively little loess which shows such 
origin. 
Secondary loess, which had been subsequently eroded and 
re-deposited on lower lands by running waters, and which 
usually shows stratification, should not, of course, be consid- 
ered in this conection. 
In the consideration of any theory of the mode of deposition 
of the loess, two propositions, which seem to be capable of sat- 
isfactory demonstration, should be borne in mind, namely, that 
the loess was deposited under climatic conditions essentially 
the same as those which prevail in the same region to day; and 
that the deposition was slow and continued through a period of 
considerable extent. 
That the first of these propositions is true is shown by the 
molluscs which furnish the most satisfactory evidence of the 
character of the conditions supporting life during that period. 
The same species, with but very few exceptions, which occur 
in the loess, exist in abundance now throughout the region 
under consideration, the distribution of the fossils being exactly 
such as may be observed under present conditions. If, for 
instance, we compare the modern molluscan fauna of eastern 
Iowa with that of eastern Nebraska, we find certain differences 
which are almost exactly duplicated in the loess faunas of the 
two regions.”^ 
For instance, Succinea lineata W. G. B., the common suc- 
cinea of eastern Nebraska, is also the most common succinea of 
the loess of that region, whereas Succinea avara Say, the most 
common succinea of eastern Iowa, is also the most common 
species of the genus in the loess of the same region. 
The majority of our species show a like distribution,^ plainly 
7 No reference is here made to the Lamellibranch and Prosohranch fiuviatile faunas, 
which seem to have spread into the region in question from their center of distribu- 
tion in the southeast comparatively recently. 
8 The loess fossils of Europe are likewise like the modern forms inhabiting the 
sam.e region. 
