98 0. H. Hersheii — Florencia Formation. 



series, the conditions were again unfavorable for organisms in 

 northwestern Illinois, as the loess of this district is practically 

 unfossiliferous. A few shells have been observed in the loess 

 on the bluff at Freeport, and lately a small collection was 

 made from the same formation on the top of the Oakdale esker, 

 about four miles south of Freeport and 100 feet above Crane 

 creek, which is several hundred yards distant. They are exclu- 

 sively of Succinea avara Say. These are the only shells 

 which I have observed in the loess of this district except that 

 sometimes over the Florencia formation the shells of the latter 

 have been disturbed and redeposited in the lower portion of 

 the loess. 



From a study of the Florencia and Iowan Loess formations 

 I have concluded that, in northwestern Illinois, the peculiarly 

 mild climatic conditions of the Aftonian epoch continued 

 almost unchanged into the earlier portion of the Iowan epoch ; 

 so that the Aftonian flora and fauna remained until long after 

 the beginning of the great movement of depression which 

 characterized the Iowan epoch in this region, and even until 

 the Iowan glaciers had closed around the district on the east 

 and west and were approaching the culmination of their 

 advance. The destruction of the Florencia fauna was pro- 

 duced less by the presence of the glaciers near by than by the 

 rather rapid conversion of the valleys into lake basins through 

 the general subsidence of the region. 



Nomenclature. — The name which I have applied to the 

 formation discussed in this paper is a slight modification of a 

 term once used to designate the basal or fluvial member, as it 

 was then considered, of the Iowan loess series of this region. 

 It was referred to as the Florence gravel, the name having 

 been derived from the township of Florence, in Stephenson 

 county. The great diversity of the Quaternary deposits in the 

 Pecatonica basin has driven me, for the want of a sufficient 

 number of important geographical names, to the necessity of 

 applying township designations to some of the formations dis- 

 criminated, and this is a case in point. But the term as origi- 

 nally used is of such common occurrence in Europe and 

 America, that I have considered it justifiable in the interests 

 of convenience, definiteness, and euphony to modify the termi- 

 nation of the word. Therefore, I desire that henceforth the 

 deposit be referred to as the Florencia formation. 



Freeport, 111., March 18, 1897. 



