GEOLOGY OF SOUTHWESTERN IOWA. 309 



14 Massive, light gray limestone 6 feet. 



13. Compact, regularly bedded limestone with marly partings. . . 12 feet. 



12. Black, fissile, carbonaceous shale 2 feet. 



11. Compact, regularly bedded limestone, with marly partings. . . 34 feet. 



10. Black, fissile, carbonaceous shale 2% feet. 



9. Grayish limestones, often silicious and impure 15 feet. 



8. Compact, heavy bedded limestone 2 feet. 



7. Grayish limestones, sometimes finely arenaceous, and often 



cherty 16^ feet. 



6. Impure coal % foot. 



5. Light bluish, marly clay 2 feet. 



4. Light bluish, concretionary and fragmentary limestone 5 feet. 



3. Bluish and reddish clays , 6 feet. 



2. Fine grained shaly sandstone and sandy shales 71 feet. 



1. Bluish, shaly, impure limestone 1% feet. 



Total 181 feet. 



No. 1, of this section, belongs to the Middle coal-measnres ; 

 all the remainder, to the Upper coal-measnres. The locality 

 thus receives additional interest from the fact that it exhibits 

 the junction between the two formations. Deducting No. 1, 

 we have remaining nearly one hundred and eighty feet in 

 vertical thickness of Upper coal-measure strata, and yet this 

 locality is only ten miles from the thinned-out edge of the 

 formation. 



G-oing northward about the same distance from the place 

 where the foregoing section was measured, we come upon the 

 extreme northern border of the Upper coal-measures and find 

 their last exposures in that direction in the banks of the 

 north fork of North river. In such a position on section 25, 

 township 77, range 28, the accompanying section represented 

 by Fig. 15, was measured, the land belonging to Mr. G-eorge 

 Clarke. 



