530 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



widely transgressing Chattanooga shale in the Mississippi Valley offers 

 many notable examples of a preceding stratigraphic hiatus. Other ex- 

 amples are seen in the contact of the Fayettesville shale and various 

 underlying limestones in northwestern Arkansas and northeastern Okla- 

 homa. Others, again, were noted in the vicinity of Saint Joe, Arkansas, 

 where the upper shale of the Morrow group sometimes rests on eroded 

 Pitkin limestone, and in Calhoun County, Illinois, where an early Waver- 

 lyan shale was laid on the weathered surface of a late Devonian lime- 

 stone. In all of these instances (more fully described on page 465) the 

 subaerial weathering of the limestone bed before deposition of the shale 

 began is indubitably proved by the discovery of partially weathered-out 

 fossils projecting from the surface of the limestone and others completely 

 freed that were preserved in small hollows. Though the contact is un- 

 conformable in these cases, the fact is but rarely indicated by distinct 

 divergence of bedding planes. 



Cases of interrupted sedimentation marked by thin seams of clayey 

 matter between two limestone formations, though also very common, are 

 often obscurely indicated and in the absence of fossils much more diffi- 

 cult to establish. The underlying surface is usually undulating and the 

 succeeding clay seam, which represents reworked residual material and 

 whose maximum thickness may not exceed an inch or two, fills the small 

 hollows and often pinches out entirely. Such a contact occurs between 

 the Kimmsvvick and the Fernvale limestone south of Saint Louis. An- 

 other example is found in the exposures of Girardeau limestone in con- 

 tact with an early Niagaran limestone in a railroad cut two miles above 

 Cape Girardeau, Missouri, and near Sainte Genevieve, in the same State, 

 where the Sainte Genevieve limestone rests on the Saint Louis limestone. 

 In each of these cases the break might not be suspected in fresh cuts, 

 but is clear enough in weathered exposures. 



Other good examples were noted in the Helderbergian series in Mary- 

 land and West Virginia. Of these the Coeymans-New Scotland contact 

 is, for several reasons, especially convincing of interrupted sedimenta- 

 tion. Comparison of numerous sections has shown that the beds in con- 

 tact vary from place to place, and satisfactory evidence of old surface 

 material has been seen in the intervening clay seam at a number of 

 localities. 



Many instances of apparently gradual change in character of sediment, 

 when in fact the transition beds either include or follow a demonstrable 

 hiatus, might be cited. Such cases are explained on the justifiable as- 

 sumption that the first overlapping deposits of an advancing sea would 



