IOWA ACADEMY OF SCIENCES 
125 
“Of these, one to four are Pulmonates, five is a gill-bearer 
{Prosobrancli), six and seven are bivalves, and eight is terres- 
trial. The set one to seven can be duplicated in most of our 
northwestern ponds with muddy bottoms. Eight is terrestrial, 
but grows sometimes near the edges of ponds and is common 
along streams. There is one specimen of this. 
“The other lot from Sioux Falls, S. D., ‘near the brewery, ’ 
contains two species; 
^ ^ Limnea caperata Say, mdi Planorhis albus Mull., probably. 
The specimens are poor. Both of these are common in north- 
western ponds to-day.” 
West of this pond hole the rest of the underlying till had 
apparently had its soil, if ever formed, removed by the erosion 
attending the deposition of the overlying till. Between the 
two tills is a considerable deposit of gravel. The lower till 
was comparatively free from pebbles of any considerable size 
and has been referred to by the writer in his correspondence as 
a pebbleless clay, but more careful examination during the 
recent visit, brought out the fact that it contains small pebbles 
of crystalline rocks evidently of northern origin. In places it 
is distinctly weathered and resembles loess in color. In such 
cases, it failed to show effervescence when tested with acid. In 
the overlying gravel were numerous rotten pebbles and bowl- 
ders. The overlying till revealed few, if any, rotted bowlders. 
This break between the lower till and the upper till which is so 
distinctly marked at some points in the vicinity of Sioux Palls, 
seems quite even and horizontal. In the city, tests with acid 
did not distinctly show difference in age between the upper and 
lower tills. In general, effervescence was prompt. 
East of Canton, there was a similar difference noted between 
the upper till, which was quite stony, and the lower compara- 
tively pebbleless till, which presented similar characteristics 
to those noted northeast of Sioux Palls. Between these tills 
was a deposit of fine sand and interstratified silt. Traces of this 
same horizon were traced east of Beloit, Iowa, and west of 
Fairview, S. D. In the latter locality, the lower till was not 
distinctly traced. It may be remembered that Mr. Bain, in his 
report on Woodbury county, Iowa, calls attention to the fine 
sand underneath the till at a high level northwest of Sioux City. 
At that point, no till had been found underneath the sand. It 
is known at one or two places to rest immediately upon Creta- 
ceous beds. In that sand, which is excavated extensively for 
