4 THE DEVONIAN OF MISSOURI. 



Devonian averages sixty to seventy-five feet thick, the valley 

 sides are in many cases largely made up of Devonian rocks. In 

 central Warren County they are rarely present and where they 

 occur form weathered slopes. From central Warren County to 

 eastern Lincoln County Devonian rocks are absent and at the 

 latter place they are only a few feet thick and occur in isolated 

 patches. In Marion County, southwest of Palmyra, Salt River 

 has cut down to the Devonian for a distance of a mile or two 

 and the rocks do not occur more than twenty feet above the 

 stream bed. In Ralls County near the town of Rensselaer, 

 Devonian rocks outcrop oyer small. areas. 



The formations recognized in the region under discussion 

 are: 



Snyder Creek shale (Craghead Creek shale of Greger) 

 Callaway limestone 

 Mineola limestone 

 Cooper limestone 



The most easily accessible important Devonian outcrops are 

 on Smith's Branch, west of the Montgomery City-Danville 

 road about four miles from Montgomery City, where for two 

 or three miles down the creek the outcrops are good and both 

 Callaway and Mineola are exposed. The most easily accessible 

 outcrops of Snyder Creek shale are about six miles south of Fulton 

 at the type locality of the formation, and Callaway limestone 

 also occurs near the Snyder Creek outcrops. The outcrops of 

 Cooper one mile south of Providence are better than occur in 

 most places, those at Sweeney, mentioned in another paragraph, 

 are good, and for two or three miles down stream from Sweeney 

 they continue to be good. In northeastern Missouri the out- 

 crops of Mineola near Rensselaer on the Wabash Railroad are 

 good and are easily reached. 



UPPER AND LOWER BOUNDARIES OF THE DEVONIAN 



The upper and lower limits of the Devonian in Missouri 

 have been placed at various horizons, but while the upper is 

 still in dispute there is general agreement about the lower, 

 owing to the large break between the oldest Devonian and 

 youngest rocks beneath. In central Missouri the oldest Devon- 

 ian rocks are Middle Devonian in age, and the youngest 

 underlying rocks are older than the Niagaran of the Middle 

 Silurian. The time interval between the youngest underlying 

 rocks and the oldest Devonian probably ranges between three 



