428 E. O. ULRTCH REVISION OF THE PALEOZOIC SYSTEMS 



member of the Boone seems to be absent locally through non-deposition 

 or possibly because of erosion prior to the Warsaw. (Further discussion 

 of the relations of the Keokuk and Warsaw will be found in Part III.) 



Considerable movement, permitting greater expanse of waters in areas 

 east of the Mississippi River and prohibiting submergence of the flanks 

 of Ozarkia, except on the east side, introduced the Spergen. The pattern 

 of continental seas was again different, and not merely smaller, when 

 the St. Louis limestone was being laid down. The Moorefield shale, 

 which contains a very different fauna and is thought to succeed the 

 St. Louis in time, is found only in north Arkansas and eastern Oklahoma, 

 on the south side of Ozarkia. The formations of the Chester group or 

 series, as developed on the east and southwest sides of this uplift, also 

 suggest differential vertical movements within this and adjacent areas. 

 If correctly interpreted, the east-west oscillations between northwestern 

 Arkansas and the Indiana basin, beginning with the St. Louis and 

 ending with the Pitkin, are essentially similar to the north-south oscil- 

 lations described as affecting the Mississippi Valley proper in Ordovician 

 and early Silurian times. (See pages 367 and 422.) 



Tilting accompUslmd during interformational intervals. — The fact 

 that close study of the sediments laid down on the opposite sides of 

 these alternately tilted platforms has failed to bring out any evidence 

 of gradual emergence — such as ^^regressive overlap"^^ — is of high sig- 

 nificance in estimating the age relations of the opposed deposits. If 

 the reversal of, say an eastward, tilt, had begun before the depositional 

 result of this attitude of the uplift had been terminated, we should 

 find that the geographic extent of the later deposits of the stage is 

 successively less and less. The positive determination of detailed regres- 

 sive overlap phenomena, and especially their discrimination from effects 

 produced by erosion, is, for one reason or another, always difficult. The 

 opposite condition of progressive overlap, however, is not only more 

 readily established, but when it has been shown by fossils and continuity 

 of outcrop that the last bed extends farther inland than those preceding 

 it, we know that erosion has not complicated the problem, and that 

 reverse tilting and sea withdrawal did not occur before the close of the 

 stage under consideration. In Kentucky a northward overlap of the 

 profusely fossiliferous zones of the Flanagan limestone is satisfactorily 

 indicated by the evidence in hand. In Tennessee similarly satisfactory 

 evidence has been found showing westward overlap across the north 



"» Amadeus W. Grabau : Types of sedimentary overlap. Bull. Geol. Soc. America, vol. 

 17, 1906, pp. 567-636. 



