O. H. Hershey — Silveria Formation. 325 



lars : Besides its usually very dark blue gray or blue brown 

 color, there are bands several feet in thickness which are of 

 a lighter tint. These are encountered in contiguous well-sec- 

 tions at about the same depth, indicating apparently horizontal 

 stratification. Microscopically, the deposit is a very fine- 

 grained sand or silt, with many of the particles angular and 

 sub-angular (one of the distinguishing characteristics of a 

 loess). It usually contains a few sparsely disseminated rock 

 fragments of small size, and they have so far, except in the 

 extreme lower portion of the formation, proven to be exclu- 

 sively of angular white chert. Near the base, in the deeper 

 valleys, there are, interstratified with the blue silt, thin strata 

 of fine gravel, composed mostly of the angular chert fragments 

 with a few Canadian pebbles. The blue color dominates this 

 formation so completely that even this gravel is staiped with a 

 blue tint, thereby enabling us to readily discriminate it from 

 another gravel formation buried in the valleys, but which is of 

 a bright ,red and reddish-brown color. Certain thin layers of 

 the blue silt are, also, thickly packed with sharply angular 

 white chert fragments which probably reached their present 

 position through the agency of floating ice. Not only is this 

 great preponderance of angular fragments of white chert, and 

 almost total absence of the ordinary drift pebbles, of the greatest 

 aid in distinguishing the presence of this formation in the 

 material brought up by the sand pump, but it is believed also to 

 indicate that the area in which the deposit occurs had not pre- 

 viously been glaciated. The white chert is of local derivation, 

 being contained abundantly in the residual material over the 

 Galena and Niagara limestones, the terranes chiefly exposed in 

 northwestern Illinois. 



At the single surface exposure in this county, so far as at 

 present known (described in the paper already mentioned), the 

 Silveria formation, besides displaying all the characteristics dis- 

 covered in well-sections, is laminated in the irregular manner 

 commonly known as " ripple marking." The upper two feet 

 are false-bedded, ferruginous, light-brown, fine sand and silt, 

 and appear to represent the shore deposit of the ancient lake 

 in which this formation was laid down. It is here sparingly 

 fossiliferous. There are, uniformly disseminated and appar- 

 ently largely responsible for the blue color, minute particles of 

 carbon. In addition, also, the fossil flora is represented by 

 very diminutive fragments of black semi-decayed wood. But 

 the only interesting portion of the fossil contents consists of 

 several species of small shells. These occur pretty generally 

 distributed through the exposed portion of the deposit in the 

 proportion of several hundreds to every cubic foot of silt. 

 But in the upper division, described above as representing 



