10 GEOLOGY OF ILLINOIS. 



is represented by thin bedded, ash colored, shaly limestones, the equivalent 

 of the Chateau limestone of the Missouri Report, which are underlaid by sandy 

 and argillaceous shales, with thin beds of oolitic, and fine grained limestones at 

 the base. The following section, made in the vicinity of Hamburg, will show 

 the average thickness, and order of succession, of the various beds of this group : 



FEET. 



Thin bedded shaly limestones 30 to 40 



Sandy^and argillaceous shales 40 to 50 



Oolitic limestone 3 to 10 



Fine grained, light blue or dove colored limestone 4 to 12 



Green shale, sometimes partially bituminous - 2 to 15 



These beds are seldom well exposed in this county, as they underlie the Bur- 

 lington limestone, which generally forms the -upper escarpment of the bluffs, 

 and they are consequently mostly hidden under the sloping talus beneath. 

 From the north line of the county to Hamburg on the west, and to the south 

 line of town 11 south, range 2 west, on the eastern side of the county, this 

 group may be found in partial exposures either in the face of the bluffs below 

 the perpendicular limestone escarpment, or in the ravines by which the bluffs 

 are intersected. In the vicinity of Hamburg this group is well exposed in the 

 banks of the small creek just below the village, showing exactly the order of 

 succession to be seen in the above section. The green shale at the base of the 

 group rests directly upon the Hamilton limestone, and may be the representa- 

 tive of what has been sometimes called " Black-slate" formation, but in the 

 absence of any evidence that it is of Devonian age, we have included it in 

 this group, with which it seems to be identified more closely than with the beds 

 below. Above this we find the light bluish gray silicious limestone, sometimes 

 called the " Lithographic" limestone, which is variable in its thickness in this 

 county, ranging from four to twenty feet. A few fossils were obtained from 

 this rock in the vicinity of Hamburg, among which were Producing pyxidatus, 

 Spirifer Marionensis, Cyrtia acutirostris, and an Orthis like 0. Michelini. On 

 the eastern side of the county, we did not find this limestone exposed. It re- 

 received the name of " Lithographic" limestone from its resemblance to the 

 stone used in lithography, but some examples of it which have been tested for 

 that purpose, have not shown the necessary qualities of a good lithographic 

 stone. It is regularly stratified, in beds varying from two inches to a foot in 

 thickness, but they are intersected by numerous seams and cross fractures, so 

 that good slabs of any considerable size are not easily obtained. This charac- 

 ter alone would render it unfit for the lithographer. This limestone is 

 succeeded by a thin bedded oolitic limestone, which, in the vicinity of Hamburg 

 ranges from five to ten feet in thickness, and splits readily into thin layers of 

 an inch or less in thickness. A portion of it is quite fossiliferous in the vicinity 



