572 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



absence, even, of remnants of the presumable depositional record that 

 formerly linked the valley rocks with the Atlantic basin, no other criteria 

 are nearly so competent as the entombed faunas in deciding the questions 

 at issue. Finding, as we usually do in the mentioned formations, that 

 their faunas are limited on the west and south by some structural break 

 or barrier, and if these faunas are of the facies that after long experience 

 we have come to associate with the Atlantic province, no other explana- 

 tion of their presence in an Appalachian sea than direct westward trans- 

 gression of the Atlantic shoreline through depressions in the intermediate 

 marginal land of the continent seems admissible. 



Further, if exactly corresponding deposits are entirely absent or at 

 least unknown in a considerable strip comprising the Cincinnati geanti- 

 cline and extending from the Cretaceous border on the south to the pre- 

 Cambrian areas in northwestern Ontario on the north, a condition that 

 is quite true of all the mentioned formations, then their recognition as 

 Atlantic deposits becomes almost unassailable. The absence of syn- 

 chronous deposits in this intervening strip would otherwise be explainable 

 only on the assumption af removal subsequent to deposition or of prohibi- 

 tion of sedimentation by current efficiency. Both of these contingencies, 

 however, are impossible, as regards the Ordovician, Silurian, and De- 

 vonian formations which come to the surface in this crucial strip, because 

 of their progressive overlap structure; and they are rendered highly im- 

 probable in the case of the inaccessible horizons by logical deduction from 

 the demonstrated inefficiency of erosion processes in the interior negative 

 areas of the continent during pre-Pennsylvanian ages (see pages 311 to 

 313). 



Finally, direct Atlantic connection is proved for most of the Appa- 

 lachian formations mentioned in a preceding paragraph by the fact that 

 they are confined to certain basins, and within these to certain troughs 

 in the eastern and middle parts of the valley tract. Their faunas being 

 entirely absent in the stratigraphic sequence of the remaining western 

 part, it is obviously impossible that they could have migrated from the 

 west or southwest. Hence they must have come in from the east. 



Depositional evidence of postulated Atlantic connections now almost 

 entirely removed. — Bringing these faunas in by direct routes from the 

 Atlantic has but one apparent drawback, namely, the general absence of 

 -connecting dei^osits. But, after all, is this objection really serious? I 

 ihink not. We have but to remember that these connections were across 

 a marginal tract of the continent which all geologists agree was fre- 

 ■quently affected by orogenic movements, and perhaps as often base- 

 leveled. The present surface of Appalachia therefore doubtless exposes 



