PRINCIPLES OF STRATIGRAPHIC CORRELATIONS 547 



by very considerable thicknesses of Ozarkian and Canadian limestones. 

 Beginning earlier than the genetically similar Appalachian trough, the 

 Cordilleran syncline, also reached early in the Ordovician a stage of pre- 

 dominating emergence that may be likened to the mid-Silurian emergence* 

 of the Appalachian syncline. Except along its western edge, where grap- 

 toliferous shales were locally deposited, this emergence of the Cordilleran 

 troughs seems to have continued with only slight interruptions (late Black 

 Eiver or early Trenton and again in the Eichmondian) through nearly 

 the whole of the Ordovician and Silurian periods. Neopaleozoic deposi- 

 tion was resumed here in the Devonian, extraordinary thicknesses of 

 limestone belonging to this era and to the Pennsylvania system of the 

 succeeding era being laid down with relatively brief interruptions of 

 the process. 



In the Appalachian geos3mc]ines marine sedimentation probably began 

 some time after the beginning of the oldest lower Cambrian. Here, as in 

 all the continental basins, the process of sedimentation was interrupted 

 at times. The durance of these interruptions varied from time to time, 

 and more particularly from place to place, the relatively brief general 

 emergences expanding locally into much greater discontinuities. In 

 the aggregate, however, the stratigraphic sequence after it began here is 

 more nearly complete than in the Cordilleran trough. There we find a 

 more continuous Cambrian record, but here the succeeding three systems 

 of the Eopaleozoic are, so far as known, much better developed, and 

 therefore better fitted to fill the requirements of a standard for America 

 and the world. 



During most of the Paleozoic ages the Appalachian and Cordilleran 

 geosynclines most probably comprised the lowest ground in North 

 America. This view is based on the undeniable fact that these troughs 

 were often longer, and in many cases sooner, occupied by marine waters 

 than any other area of which we have knowledge. Granting the validity 

 of the argument, it follows that when these troughs were completely 

 drained the emergence must have extended as a rule to all the remainder 

 of the continent. The exceptions occurred only when the depositional 

 record in these great synclines was inferior to that found elsewhere. The 

 only notable exceptions are the late Ozarkian, the earliest Ordovician, and 

 the middle Silurian (Chicago) ages. 



On the basis of these premises, it seems fair to assume as established, 

 that the sea withdrawal indicated by a stratigraphic and faunal break at 

 the top of the thickest known deposit of a given age in the Appalachian 

 Valley or elsewhere also affected all other parts of the continent then or 



