466 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



reduces the possibility of underground solution and corrosion to a 

 minimum. The illustrations are therefore perfectly fair and incon- 

 trovertible. 



Finally, many of the unconformities that are barely distinguishable 

 even to the trained eye, when traced for varying distances are found to 

 exhibit unmistakable evidence of sea withdrawal. We find, for instance — 

 as in the case of the Lowville overlapping the Joachim, 6 miles west of 

 Ste. Genevieve, Missouri, and the contact seen in the quarry at Darling- 

 ton, Wisconsin, mentioned on pages 364 and 460 — that the nearly 

 even surface of the lower formation descends suddenly into an ancient 

 solution cavern that could have been produced only under subaerial 

 conditions. This may be filled with initial deposits of the succeed- 

 ing formation or with sediments of an age wholly absent in adjacent 

 parts of the section. Many instances of this sort have been described 

 and a still greater number awaits publication. Though most of them are 

 associated with unconformities in which the stratigraphic hiatus is of 

 great magnitude, others no less well developed are connected with emer- 

 gences that must have been of relatively insignificant duration. 



It seems worth while to mention another of these cases of deposits seen 

 only in old cavities, because it indicates a somewhat different history 

 than is inferred in most of the other examples. The phenomena alluded 

 to occur in the vicinity of Thebes, Illinois. Here several late Ordovician 

 channels were observed in the top of the Kimmswick limestone and be- 

 neath the unconformable shaly base of the Thebes formation. The latter- 

 is of Eichmond or early Silurian age, the former late Black Eiver. In 

 one of the channels exposed in a railroad cut south of the town, residual 

 clay and fragments of fossiliferous chert, both evidently younger than 

 the limestone walls of the channel, were observed. They may represent 

 an otherwise unknown later bed of the Kimmswick or possibly some 

 distinct overlying formation. It is inferred from these facts that solution 

 channeling of the Kimmswick occurred during the rather long emergence 

 that prevailed in this general region till resubmergence set in with the 

 Eichmond. Portions of the overlying rock, now represented solely by 

 residual clay and a little chert, dropped into these channels and were 

 thus partially preserved, while the remainder of the sheet was entirely 

 removed by erosion from present areas of outcrop. Peneplanation of the 

 Kimmswick in the Mississippi Valley at this time, as described on pages 

 308 to 311, is positively demonstrable; and it appears further that the 

 Richmond submergence was delayed at this point till after the Pernvale 

 stage. At any rate, the Femvale is absent here. 



Considering the incontestable and highly significant nature of much 



