Geology of Johnson County. 419 
Below I give a catalogue of all the fossils yet obtained from the 
loess of Johnson County ; doubtless, however, many others existed 
which have not as yet been discovered.1 Those marked * do not 
occur as living forms in the county at the present time :— 
Zonites viridulus (Mke.). 
Zonites limatulus * (Ward). 
Zonites fulvus (Drap.). 
Patula strigosa * (Gld.). 
Patula striatella (Say). 
Ferussacia subcylindrica (L.). 
Pupa muscorum * (L.). 
Pupa blandi * (Mone.). 
Vertigo simplex * (Gld.). 
Mesodon multilineata (Say). 
Vallonia pulchella (Miill.). 
Succinea avara (Say). 
Succinea avara var vermeta,— 
Succinea obliqua (Say). 
Helicina occulta (Say). 
Limnæa desidiosa (Say). 
Pisidium—sp. (?) 
Egg-shell of some small Helix. 
The loess formation constitutes a prominent feature of the super- 
ficial geology of Iowa, being developed to a greater or less extent 
over a large portion of the State. It is believed that the material 
of this formation was deposited during the Champlain period ; and 
facts seem to demonstrate, moreover, that this was not only a period 
of somewhat lower level in Iowa as well as other places, than the 
present, but also that the amount of depression increased somewhat 
to the northward, so that the streams flowing to the southward had 
usually a diminished slope, with a consequently slackened flow of 
the waters, and many greater or less expansions along their course, 
and from these silt-ladened waters the material of the loess forma- 
tion was derived. 
! For this catalogue of species I am much indebted to Professor B. 
Shimick, of Iowa Cit ies li 
’ y. All the species listed have been personally 
collected by him, 
