92 0. H. Hershey — Florencia Formation. 



Resting upon the irregular surface of the bed of gravel there 

 is a series of dark blue-green silt, light brownish gray sand, 

 and dark brown carbonaceous clay or muck. The passage from 

 the gravel to the finer sediment is usually quite abrupt, 

 although they are sometimes slightly interstratified. The dark 

 brown muck fills the depressions in the surface of the gravel 

 and varies in thickness from six inches to several feet. It is 

 horizontally stratified, and where the remains of herbaceous 

 vegetation form thin layers, it may be said to be laminated. 

 It contains some small shells, but is not the most fossiliferous 

 member of the formation. What it lacks in the remains of 

 the fauna it partly supplies in the inclusion of many semi- 

 decayed branches and trunks of trees. These are quite numer- 

 ous in the Crane creek outcrops, and appear to be of two main 

 kinds, one of which is light brown in color and the other 

 black. They are somewhat flattened by pressure, but the 

 branches and trunks represented had an original diameter of 

 one inch or less up to one foot. 



The blue-green silt and the light brownish gray sand are in 

 lenticularly-shaped " pockets " interstratified with each other 

 and with the upper portion of the muck. The sand is of well- 

 rounded grains, mostly of transparent quartz, with all the 

 drift rock species represented. It contains a moderately 

 abundant supply of shells, all of small species. But the blue- 

 green silt is the great shell-bearing member of this formation. 

 The species are of a great variety, as the lists later to be pre- 

 sented will indicate, but the shells are all of small size. They 

 are in such abundance that they often make up 10 per cent of 

 the mass. Several hundreds ma}' be separated from a single 

 cubic inch of the silt. This fact constitutes this the most high- 

 ly fossiliferous formation developed in northwestern Illinois. 



Each of the three lithologic types presented by the upper 

 division of the Florencia formation is characteristically dis- 

 tinct from any other of the district, and they together consti- 

 tute a series of strata which can be readily identified in every 

 outcrop. The blue-green silt is peculiar to this formation and 

 seems to be coextensive with its distribution, as it has been 

 detected in every outcrop which I have closely examined. The 

 invariable stratification of the entire series, the close associa- 

 tion of the most abundant "pockets" of shells with certain 

 lithologic features, and the fact that all the vegetable matter, 

 including the semi-decayed logs or tree trunks, lies in a hori- 

 zontal position, demonstrate that this division of the formation 

 is not a flood-plain deposit properly so-called, but represents 

 the sediment and driftwood laid down over the gravel in the 

 stream bed, after the land area had begun to subside, thereby 

 establishing a permanently flooded condition of the streams. 



