DEVELOPMENT OF THE APPALACHIAN GEOSYNCLINES 173 



vician records are most complete in the south and less so in the north. 

 Then in Middle Silnrian time a transverse swell developed throughout 

 eastern Tennessee, and the subsequent seas were far more persistent 

 north of this State than in the southern part of the geosyncline. This 

 swell was especially active in pushing the shorelines to the westward in 

 the area south of Virginia during late Silurian and most of Devonian 

 time. On the other hand, during Middle and Upper Devonian and Mis- 

 sissippian times, the seas again spread eastward to the region of the 

 earlier Appalachian shores. The shoreline was farthest east during the 

 Lower Cambrian and early Ordovician, but it can not be said that it 

 moved progressively to the west with each recurring sea. We have just 

 pointed out the irregularity in position of the eastern shore for the trough 

 south of Virginia. It would, furthermore, appear as if the shore moved 

 irregularly westward for a time, followed, in some cases at least, by a 

 quick return to a more easterly position with the incoming of new seas. 

 In any event, we see more easterly shores during Lower Cambrian, Cana- 

 dian, Mohawkian, Lower and Middle Silurian, Middle Devonian, and 

 Mississippian times. During Pennsylvanian time the shore moved pro- 

 gressively to the west, with the final blotting out of the Appalachian 

 trough in the orogenic movements beginning in this period and culmi- 

 nating in the early Permian. 



From the previous statements we again see what is now so well kno\^Ti, 

 namely, that the transgressions of the oceans upon the continents are 

 periodic in appearance, and more or less irregular in their spreadings. 

 The successive paleogeographies show, however, that the pattern of the 

 seaways is fairly alike — a condition that is governed by the variable move- 

 ments of the geanticlines and the swells. 



'Not do the floods always first appear in the Appalachian geosyncline 

 and then spread inland variably over parts of the neutral area. This is 

 true in some cases, but in others the transgressions pass up. the Missis- 

 sippi Valley or down from the Arctic and then extend more or less toward 

 or into the geosynclines. The greatest of Paleozoic floods did not origi- 

 nate in the geosynclines, but came from the Arctic across the western 

 part of the Canadian shield into the United States. This is seen in the 

 floods of the middle and late Ordovician, the earlier Silurian, and again 

 in the Devonian. Rarely did any of these floods fully occupy the eastern 

 geosynclines. In this extraordinary variability of the marine transgres- 

 sions we see that at times the Appalachian geosyncline is above the 

 strand-line while the neutral areas are in flood, and at other times the 

 trough is filled with seas when the interior of N"orth America is land. 



