SILVEEIA {"!) FOKMATION AND OTHEK SILT DEPOSITS. HI 



movement to the Illinoian stage, while the southward movement is interpo- 

 lated between the Illinoian and Iowan. If, therefore, this interpretation 

 be correct, it affords no evidence of a pre-Illinoian invasion. The subject 

 of bowlder transportation has been studied so little as yet that it may be 

 unsafe to take the interpretation given by Buell as final, although it appears 

 well sustained. On this question, as well as on that of the ag - e of the con- 

 glomerate found on the borders of Rock River, further light is desirable. 



Hershey has recently discussed certain silts in northwestern Illinois, 

 which underlie the Illinoian drift, as probable representatives of a stage of 

 glaciation preceding the Illinoian. 1 He does not, however, refer the silt 

 deposition to a time much earlier than the Illinoian. 2 There is evidence of 

 erosion of this silt prior to the ice invasion which deposited the Illinoian 

 drift. It is possible that these silts may be correlated with the ferruginous 

 glacial conglomerate along the east side of Rock River, the conglomerate 

 being deposited by a stream and the silts by a temporary lake, though there 

 is as yet no adequate basis for such a correlation. 



SILVERIA (?) FORMATION AND OTHER SILT DEPOSITS. 



In the course of the discussion of the Illinoian drift and of the Kansan 

 till sheet, which it overlaps on the west, frequent reference has been made 

 to the occurrence of deposits of silt beneath the Illinoian. These deposits 

 are known to be distributed very widely beneath the Illinoian drift, but it 

 is not known how large a combined area they cover. They appear to be 

 present in conspicuous amount beneath many streams of northwestern Illi- 

 nois, but are not often exposed to view because of their position beneath 

 the level of the streams. They appear to be less extensively developed in 

 western and southern Illinois, though not rare in either district. Their best 

 development, so far as known, is along- the line of valleys which were 

 favorably situated for the development of lakes in front of the ice, valleys 

 whose lower courses were entered by the ice sheet, while their upper courses 

 remained for some time uncovered by the ice. 



Hershey has proposed - the name Silveria for deposits of this class in 

 northwestern Illinois. 1 The necessity for a name for such deposits will 



■Am. Jour. Sci., 4th ser., 1896, Vol. II, pp. 324-330. 



-As Hershey uses the word Kansan for the sheet which we are diseussing as the Illiuoian, the 

 reader may find it somewhat confusing to interpret his language. Hershey, however, is not at fault 

 in the use of this term, since his paper was prepared before the name Illinoian had been introduced. 

 He naturally inferred, in the absence of knowledge to the contrary, that the same name should be 

 applied to the sheet outside the Iowan in northwestern Illinois as has been applied in southern Iowa. 



