168 GEOLOGY OF ILLINOIS. 



In Menard county, the eastern portion of thia district, we find exposed only 

 the middle beds of the Coal Measures, no rocks lower than the under clays of 

 the coal No. 4 of the Illinois General Section having been identified. The 

 following section shows the order of superposition and comparative thickness 

 of the different beds in this region : 



FEET. 



1. Limestone 20 to 30 



2. Argillaceous shales 10 " 15 



3. Coal (No. 7, 111. R. sect.) 1J- 



4. Fire clay and shales 18 " 20 



5. Limestone 3 " 4 



6. Coal (No. 6, 111. R. sect.) 1 in. to 3 



7. Fire clay 5 " 10 



8. Shales and sandstone 30 "40 



9. Limestone 1 " 3 



10. Black slate 1" 4 



11. Coal (No. 4, 111. R. sect.) 5 " 7 



12. Fire clay 6" 8 



Of this section, the beds below the coal No. 6, have not been identified in 

 any natural outcrop, and have only been reached by borings, and shafts sunk 

 down to the coal No. 4, which is extensively worked at Petersburg and vicinity. 

 The upper and middle beds of coal in the above section, were also formerly 

 worked in the vicinity, but since the opening of the lower vein the work has 

 been discontinued, or is only carried on in a desultory way by stripping. The 

 fire clay under these two beds was not separated from the shale in all the sec- 

 tions reported to me by other persons, and it is possible that in some cases it 

 may not be developed to any considerable extent. 



Just above the village of Petersburg, on the western bank of the Sangamon 

 river, and close to the water's edge, the limestone overlying the middle vein of 

 coal No. 6 appears, and has been quarried to a slight extent. It is a rather close 

 textured, light drab or gray limestone, weathering buff, and contains a few fos- 

 sils of the species Spirifer cameratus, Athyris subtilita, Atliyris Roissii, Produc- 

 tus costatus, Producing longispinus, etc. The underlying coal has been worked 

 by stripping, a few rods further down stream, and is, at this point, two or three 

 feet thick. The peculiarity of this seam of coal is its uneven thickness, it be- 

 ing reported sometimes to vary, within short distances, from a thickness of two 

 or three feet to only as many inches, or even less; and, from this fact, it is gen- 

 erally considered too unreliable to be worked, except by strapping along its 

 surface outcrops. Both the coal and its limestone roof are passed through by 

 Taylor's and Wright's coal shafts, which are sunk from the top of the bank to 

 the lower coal, or No. 4, within a quarter of a mile of this outcrop. 



On the opposite side of the river, near the center of the north line of sec- 

 tion 25, township 18, range 7, and about a mile and a half above the town, 



