IOWA ACADEMY OF SCIENCES 125 



"Of these, one to four are Puh inmates, five is a gill-bearer 

 {Pro.sohranch), six and seven are bivalves, and eight is terres- 

 trial. The set one to seven can be duplicated in most of our 

 northwestern i3onds 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: 



'^ Liinnea caperata Say, di,nd Piano rb is albi/s MillL, 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- 

 df rs. 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 Falls 

 seems quite even and horizontal. In the city, tests willi acid 

 did not distinctly show difference in age between the upj^er 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 Falls. 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 e.Kcavated extensively for 



