106 IOWA ADADEMY OF SCIENCES. 



It has been thus assumed that the Devonian beds form 

 one of the originally deposited concentric zones around the 

 older rock-mass of the Ozark dome. On this account 

 chiefly it has been urged that the Devonian sediments 

 were laid down around what is termed the Ozark Isle. The 

 necessary inference is that during Devonian times sub- 

 aerial erosion took place over all the present Ozark region. 



Attention to a few facts quickly dispels this hypothesis. 

 The Devonian sediments themselves show nowhere a 

 coarse littoral character. The "concentric ring" is not an 

 unbroken one; it is sundered on the southeast and on the 

 northwest parts of the great dome. Devonian deposits, 

 highly fossiliferous, occur on the highest portions of the 

 uplift; showing unmistakably that, in the absence of other 

 evidence to the contrary, they once extended unbroken 

 over the entire uplift. It is clearly manifest from geographic 

 inquiry that the present Ozark dome is a very recent upris- 

 ing — probably post-Tertiary. 



In the face of these facts the hiatus in central Missouri 

 has a special significance. It points at once to the sug- 

 gestion that the area in which there was no Devonian depo- 

 sition was not a sub-circular one, coincident with the Ozark 

 dome of today, but a narrow ridge-like elevation. The 

 structure of the Ordovician beds of the region also indicates 

 that this is the true explanation. Ten years ago I called 

 attention to the location of this old ridge. Today its 

 importance appears much greater than was at that time 

 supposed. 



Now, we have through the Mississippi valley, at least 

 west of the great river, two horizons which are exception- 

 ally clearly defined. One is the top of the Silurian. The 

 other is the base of the Chouteau-Burlington limestones. 

 Between these two stratigraphic horizons the beds are here 

 assumed to be Devonian in age. The formations repre- 

 sented in typical localities are as follows: 



GENEHAI. DEVONIAN SECTION OF IOWA. 



FEET. 



Lime Creek shales 100 



Cedar Valley limestone J 50 



Wapsipinicon lime.stone 16) 



Independence shales 50 



