374 E. O. ULRICH — -REVISION OF THE PALEOZOIC SYSTEMS 



the bottom, would necessarily be increasingly exerted in the later maxi- 

 mum stages of the submergences. The facts, however, invariably indicate 

 the opposite condition of simple progressive overlapping of deposits. 



Geologic literature is full of statements implying continuous deposition 

 in sections that have since been shown on incontrovertible evidence to 

 include greater or smaller hiatuses. Convincing physical evidence of 

 subaerial decay and corrasion is often difficult to find in such obscurely 

 marked instances of interrupted sedimentation ; but in my own experience, 

 the break indicated by the fossils seldom failed to be substantiated by the 

 discovery of unconformable stratigraphic relations and as a rule of 

 evidence pointing to land conditions. 



In this connection it is important to note, first, that the time value of 

 the hiatus is in no wise suggested by the degree of the unconformity or by 

 the relative coarseness or abundance of clastic matter at the contact. 

 Both of these criteria of interrupted deposition may be exceedingly ob- 

 scure in hiatuses spanning two or more periods of time, while, on the 

 contrary, they may be strongly expressed in cases of relatively brief 

 duration. It is largely a matter of geographic relation to belts of folding 

 and of consequent vigorous erosion whether the unconformity is strongly 

 or but weakly developed. Eegarding the latter condition, which prevails 

 in the median areas of the continent, numerous striking examples might 

 be cited. I have more particularly in mind many sections in Missouri and 

 Arkansas in which beds ranging from late Devonian to early Pennsylvanian 

 rest on Ozarkian, Canadian, or early Ordovician. In these sections the 

 physical criteria of the great hiatus are generally so inconspicuous that 

 they have been more often overlooked than noted. Indeed, without 

 fossils the importance of the break could not be established as greater 

 than many relatively insignificant contacts above and beneath it. 



It is important further to note that the formations which intervene 

 on the flanks of the uplifts, especially on those of Ozarkia, between the 

 Ozarkian and Pennsylvanian systems can in nearly every instance be 

 shown to have an overlapping structure. As stated, this structure is a 

 fatal physical objection to the application of the theory of current scour 

 in explaining the absence of the lower beds of the overlapping formation, 

 because in the restricted earlier stages the possibility of strong trans- 

 continental marine currents was more remote than in the final stage of 

 the transgression, which, instead of commonly failing to deposit, usually 

 did so more regularly than the early stages. Moreover, the "progressive" 

 feature of the overlaps would have been impossible, because if scour had 

 occurred the overlaps must have become "regressive," which they are not. 



