GEOLOGICAL FORMATIONS. 687 



Sinsinawa Mound, as far as Jamestown. Nearly all the mines on 

 the ridge north and east of Fairplay are sunk through the lower part 

 of the Cincinnati group, and good specimens of Nucula, fecunda, 

 may often be found in the dirt thrown out of the shallow holes, when, 

 the clay has not been covered again by the refuse of deeper workings. 



Lithological Characteristics. The strata of the Cincinnati group 

 are very regularly and conformably deposited, and do not exhibit any 

 indications of sudden and violent dislocations, faults, or uplifts. 



The lower beds of the formation are very finely laminated, and of 

 a dark blue color, in many places becoming green and brown. The 

 upper layers are of a yellowish color, and more or less calcareous and 

 silicious. 



The lower and middle members of the group split readily and with 

 a very smooth face, but the upper layers, though quite thin-bedded, 

 present a rough and uneven appearance. This group nowhere presents 

 beds of sufficiently thick and durable stone for building purposes. 

 Only one place was noticed where an attempt had been made to quarry 

 this rock; it had been abandoned as impracticable, after a small 

 amount of work had been done. 



Where undisturbed, this group has a thickness of about 125 feet. 

 This is the case only on the mounds, which are still capped with the 

 Niagara limestone, as in all other places it has been more or less re- 

 moved by denudation. 



There is nowhere a good natural exposure of the formation. The 

 rocks throughout the group offer so little resistance to the weather 

 that they do not appear in rugged cliffs y such as are seen in all the 

 formations which underlie them, but usually in gently undulating 

 hills. The best exposure is the one in the cut of the Illinois Central 

 railroad, near Scales Mound Station, of which a very accurate section 

 has already been given in Prof. Hall's report. It is much more accu- 

 rate than can now be obtained, as the weather has since then so de- 

 composed the friable shales that only a few feet of the lower beds are 

 now visible, as they were originally presented. As a general guide 

 to the formation, we take the liberty of reproducing it: 



Ft. In. 



1. Greenish shale, with alternations of calcareous and silicious layers, a few 



inches in thickness 7 8 



2. Green silico-calcareous and argillaceous shales 11 6 



3. A silico-calcareous or niagnesian band 3 



4. Greenish shale as above 12 



5. Concretionary layer, 1 to 3 inches 3 



6. Shale with Lingula 6 



7. A layer filled with a small Nucula, and known as the Nucula bed, 4 to 8 



inches 8 



