PRELIM IXA K Y CHAPTER. 23 



narrow bands, and is recognized at Winslow, Rockton, 

 Byron, and a few other places between the two rivers. 



A few tine fossils, mostly a species of Plewrotomaria, 

 were observed ; but, as a general thing, the outcrops ex- 

 amined were almost devoid of fossils. 



The St. Peters Limestone. This is the most interesting 

 formation in the scries of Illinois strata developed in this 

 part of the State. Its only outcrop is along Rock river, 

 from two or three miles above Oregon to about the same 

 distance below Grand DeTour ; and up the streams that fall 

 into Eock river along this part of it, it also outcrops for a 

 few miles ; and a few disconnected fragments have been 

 noticed projecting from a hill side in Chambers' grove, a 

 few miles north of Polo. On Rock river the heaviest de- 

 velopment, perhaps in the State, may be found. It reaches 

 a thickness here of nearly two hundred feet. Fantastic 

 shaped bluffs of white, brown and ferruginous stained 

 sandstone, rise along the river banks, and display the col- 

 oring, shapes and castellated appearances of the icebergs 

 in an arctic zone. 



It is composed of pellucid, limpid, regular rounded grains 

 of pure quartz, and is white almost as snow, when unstain- 

 ed by the oxyd of iron percolating through the mass as a 

 watery solution. The slightest cohesion holds these grains 

 together. Indeed, in some instances the mass is almost as 

 friable as densely packed sand, and can be penetrated by a 

 blow of the pick, or dug out with a sharp spade. The rock 

 lias a saccharoidal or sugary consistence, that would seem 

 to indicate its rapid decay under rains and other atmos- 

 pheric influences ; yet, strange to say, these perpendicular, 

 spire-capped hills resist these influences with great tenacity 

 and success. 



In some places in Lee county, a sort of calcareous cement 

 is intermixed, making the rock so hard and seuii-crystaline 

 that it is used with success as a building stone. In the 

 softer portions of the rock, there are many thin bands of a 



