326 0. H, Hershey — Silveria Formation. 



apparently a shore or just off-shore deposit, they increase in 

 proportionate numbers. This division of the formation had 

 not been exposed previous to a heavy flood which occurred 

 during this spring, and the fossil contents of the formation 

 had not been professionally studied at the time of the prepara- 

 tion of the previous paper on the subject. Lately, however, 

 several collections from this horizon have been made and sub- 

 mitted to Mr. W. H. Dall, whose report is as follows : 



" No. 1 is probably a variety of Pyramidula striatella Auth» 

 It is larger, better-developed, and less rugose below than the 

 recent specimens with which I have compared it. ]^o. 2 

 appears to be Pupa Uandi Morse, though a young shell 

 mixed with the others may be Vertigo ovata Say. ]^o. 3 is 

 SuGcinea avara Say ; a fragment of a Ilelix^ like H. triden- 

 tata Say, was intermingled. There is nothing depauperate 

 about the specimens." 



The three species undoubtedly present are represented by 

 about the following proportionate numbers of individuals: S. 

 avara^ 60, P. hlandi^ 5, and P. striatella, 2. They are prac- 

 tically a portion of the ancient terrestrial fauna of the neigh- 

 borhood, and with this fact in mind the significance of their 

 increase in numbers in the apparent shore deposit becomes 

 readily apparent. The number of species is too limited to 

 warrant our drawing very definite conclusions from them as to 

 the nature of the climate of that time. Their rather robust 

 development would seem to negative the idea of a cold cli- 

 mate, but the presence of the ice-sheet in Illinois necessary to 

 the supply of matei'ial for the formation of this deposit, 

 undoubtedly introduced a cooler climate than the present. I 

 have previously held to the position that the comparative scar- 

 city of the faunal and floral remains enclosed within this 

 deposit, which was peculiarly favorable for their preservation, 

 may be considered as indicating the relative severity of the 

 climate. 



The position of the Silveria formation in the county of Ste- 

 phenson may be -briefly described as follows : Just previous to 

 the opening of theKansan epoch, the streams of northern Illinois 

 flowed at the bottom of narrow, steep sided canons, trenched 

 from 300 to 400 feet below the general upland surface. The 

 first important effect of the approach of the advancing Kansan 

 ice-sheet was to depress this area and cause a silting up of the 

 deep valleys. In Stephenson county the canon valleys were 

 about half filled with fine gravel and blue silt. This has since 

 suffered great erosion and has been buried deeply under later 

 formations. It is now quite frequently penetrated by wells 

 along the valley sides, but at heights which never exceed 20 

 feet above the present river level at Freeport. Nearer the 



