436 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



Relative importance of periods of folding. — The prevailing classifica- 

 tion of continental shrinkage movements is not entirely satisfactory. It 

 seems to me that orogenic movements occurred frequently in geologic 

 times and that the three periods usually set apart as of exceptional im- 

 portance are probably no more so than others now ranked as inferior or 

 quite disregarded.*^ Perhaps the supposed important movements are 

 exceptional chiefly because their effects were concentrated in areas now 

 readily accessible. Other periods of orogenic movement of similar rank, 

 whose record is but obscurely indicated on the land surfaces now acces- 

 sible to the investigator, may yet yield to satisfactory demonstration. 



The suggested inland migration of the belt of greatest shrinkage 

 leads to the further suggestion that the occasional early and middle 

 Paleozoic mountains of Appalachia were located near the eastern shore 

 of this large land, hence far away from the Appalachian Valley troughs ; 

 and on that account alone it is unlikely that they contributed very abun- 

 dantly to the filling of these inland seaways. Besides, it is known that 

 during the Ordovician a wide median part of Appalachia was depressed 

 and submerged by waters that at times, at least, had no direct connection 

 with the valley troughs. Mountains to the east of this median depres- 

 sion, therefore, could not supply clastic material to the Appalachian 

 Valley; and, obviously, the deposits in the latter give a clue to the 

 character of only the western part of Appalachia and not of its eastern 

 part. The latter, then, may have been strongly elevated at times when 

 only a moderate orogenic movement is indicated by the deposits in the 

 Appalachian Valley. The closing ages of the Ordovician are thought 

 to have been of such a time. 



Importance of movements at close of Ordovician. — The late Ordovician 

 disturbances probably were the first of great consequence following the 

 early Cambrian, and that they were of exceptional importance is indi- 

 cated (1) by the extraordinary extent of emergence prevailing at that 

 time, and (2) by the highly clastic character of the deposits of this 

 age in the middle and northern parts of the Appalachian troughs. North- 

 wardly increasing elevation of western Appalachia is suggested by the 



*2A very suggestive paper entitled Paleozoic overlaps in Virginia was published by 

 M. R. Campbell in 1894 (Bull. Geol. Soc. America, vol. 5, pp. 171-190). Following the 

 presentation of evidence showing folding in the Appalachian Valley during several Paleo- 

 zoic ages, he concludes the paper with this significant statement : "Lastly, the post- 

 Carboniferous period of folding has long been considered as the only one and accountable 

 for all of the folding of the Appalachian type that exists in this part of the continent. 

 It now seems as though this was no more important than many which preceded it, and 

 that, In fact, the deformation has been practically continuous since early Paleozoic 

 times." 



