214 GEOLOGY OF ILLINOIS. 



were noticed, but the river bluffs are undoubtedly crowned with a clayey 

 deposit, more or less partaking of the nature of this formation. The 

 more level portions of the county are covered with the usual drift-clays, 

 about which nothing need be said. They have been sufficiently described 

 in the published works of the survey, and in many of the detailed 

 county reports. In this county they are similar, both in quality and thick- 

 ness, to like deposits in neighboring prairie counties. They belong to 

 the older Quaternary, or true drift deposits. Over the prairies, and es- 

 pecially about some of the little surface streams, an occasional black or 

 flesh-colored boulder can be seen, "lost rocks," indeed, in an ocean of 

 prairie clays. Along the shores of the Illinois river, and on a line be- 

 tween the present and ancient flood plains of the stream, a few very large 

 masses of transported rock lie half buried by the debris of ages. One 

 of these masses, near the Putnam and Marshall county lines, would 

 weigh many tons. It is a pale flame-colored granite. Similar masses 

 lie in the waters of the river at several places. In one or two instances, 

 I noticed great rocks of this kind, partially buried by a soft silt-like 

 mud. I did not notice in this county any beds of coarse, transported 

 gravel. On the face of some of the bluffs, and in some of the ravines, 

 there is a rather coarse gravel, but it seems to be composed of water- 

 worn fragments of Carboniferous rocks, similar to the natural bed rock 

 of the county. The ice and water forces, acting in ages past, in and 

 along the Illinois River Valley, doubtless smoothed the original rocky 

 projections of the beds, and these are the water-worn fragments of the 

 rocks, abraded, and carried away to short distances only from their ori- 

 ginal Carboniferous ledges. 



Geological Formations. If the Quaternary deposits be stripped from 

 the bed rocks all over the county, the surface would then present, in all 

 probability, only Coal Measure rocks and associate deposits, represent- 

 ing, perhaps, the same strata that are far better exposed in Marshall 

 and LaSalle counties. 



Commencing at the north-east corner of the county, we may reason- 

 ably infer that Coal Measure deposits, similar to those existing about 

 LaSalle and Peru, underlie the surface. From this point, it is but five 

 or six miles to the extensive coal shafts at and near Oglesby, just south 

 of the Illinois Central railroad bridge across the Illinois river. The 

 bluff range, on which the south end of the bridge abuts, continues on 

 down the river south-west into and through Putnam county, without 

 any material change in appearance. The Peru coal shafts are even 

 nearer to this corner of the county, but are on the other side of the Il- 

 linois river. All the north part of the county east of the Illinois river 

 contains beneath it these same Coal Measure deposits, subject only to 

 local changes ; but the coal seams themselves may have -thinned out, 



