SILVERIA (?) FORMATION AND OTHER SILT DEPOSITS. 115 



Another exposure occurs in the east bluff of the Mississippi near the 

 line of Rock Island and Mercer counties and has the following section: 



Section of ravine near the line of Bock Island and Mercer counties, Illinois. 



Feet. 



Loess 25 



Black soil 0.3 



Till, mainly of blue color _ 90 



Loess-like silt, very fossiliferous 12 



Total 130 



The bottom of the lower loess was not reached by this ravine. This 

 deposit differs from the surface loess in containing- a large number of nodules 

 and tubes of partially solidified material. These have a yellowish color 

 and are largely due to the presence of iron oxide. 



Specimens of fossils have been collected by Udden from these buried 

 loess-like silts in Rock Island County as well as from surface loess and 

 submitted to Dr. W. H. Dall and his assistant, Mr. C. T. Simpson, for 

 examination. The buried silt deposits are found to contain a larger pro- 

 portion of Helicina occulta than the surface loess and a smaller proportion 

 of Succinea avara. These two species are the most abundant ones in all the 

 deposits, whether surface loess or buried silt. With the exception of the 

 exposure of the east bluff of the Mississippi, the only additional fossils 

 found in the buried silt are Pyramidula striatella and Pupa alticola. The 

 latter species is now confined to the Rocky Mountain region. The speci- 

 mens collected from the ravine in the east bluff of the Mississippi, near the 

 line of Rock Island and Mercer counties, contain a greater variety, as 

 follows : 



Helicina occulta Say; very abundant. 



Helicodiscus liueatus Say. 



Limntea humilis Say (variety). 



Pyramidula perspectiva Say. 



Pyramidula striatella Anth. 



Pupa armifera Say. 



Strobilops labyrinthica Say. 



Succinea avara Say; less abundant than in surface loess. 



Succinea luteola Gould. 



Vitrea arborea Say ? 



The origin and relations of the buried loessdike silts of Rock Island 

 County remain to be determined. Whether they are the deposit of an 



