SUMMATION AND CONCLUSIONS 203 



Walcott long ago pointed out that the North American continent is 

 very old, that it was not only outlined in late Proterozoic time, but that 

 it was larger then than it is now. We now see that this continent had its 

 present configuration even early in Proterozoic time, when it appears to 

 have been 2,000,000 square miles larger than it is to-day. 



While the neutral area is wholly devoid of post-Proterozoic geosyn- 

 clines, yet it was four times in the early Paleozoic widely covered by inde- 

 pendent Arctic floods (see maps, figures 6 and 7). These spread 

 southward across the nucleus into southern Canada, where their waters 

 united with seas coming north through the Mississippi Valley. The first 

 one was of middle Ordovician (Black Eiver-Trenton), and the second of 

 late Ordovician (Richmondian) time. The other two were of the early 

 and middle Silurian (Alexandrian and J^iagaran). In the north the 

 combined depth of their marine sediments, largely limestones, may attain 

 to 3,000 feet, while in southern Canada they appear to be measured by a 

 few hundred feet. These floods, in continuation with the Franklinian 

 trough, had nothing to do with those of the Cordilleran geosyncline, and 

 their waters may or may not have been in communication with that 

 trough. 



Now let us consider the borderlands (see map, figure 3). The north- 

 eastern portion of North America was margined by Acadia, of which 

 there remains more visible than of any of the other borderlands. It was 

 a marked dias trophic region from early Cambrian time to the close of 

 the Pennsylvanian, though its marine history is practically pre-Carbon- 

 iferous. To the south of it lay Appalachia, which seemingly continued 

 unbroken into greater Antillia. Appalachia was in periodic crustal 

 unrest throughout the Paleozoic, and attained its maximum compression 

 during the Permian. Shortly afterward, in the Triassic, this borderland 

 began to fracture and to develop a block mountain structure, and the 

 greater eastern part of it went under the Atlantic during middle Meso- 

 zoic time. With its foundering, southern Antillia began to develop a 

 marine history, and with the close of the Mesozoic and ever since, this 

 southern borderland has been in the throes of crustal unrest. 



The Arctic side of North America was margined by the borderland 

 Pearya. It was in existence as a highland certainly in Upper Cambrian 

 time, and again in the late Devonian. Just when its orogenic history 

 ceased is not yet clear, but seemingly during the Permian. 



The whole of western North America south of Alaska was margined 

 by greater Cascadia. It was present in early Proterozoic time as a high- 

 land, and again during the Cambrian. It was folded, locally, in northern 

 California, during the Middle Devonian. In the Pennsjdvanian or earlier 



