SUMMATION AXD CONCLUSIONS 205 



side throughout the whole extent of the geanticline, from Siberia to the 

 Caribbean, there deYeloj)ed the vast Eocky Mountain geosyncline. Struc- 

 turally, it remade the western portion of the continent into a bilateral 

 S3'stem of mountains, the mighty Cordillera of Xorth America. 



We will next consider the geosynclines (see map, figure 3). It was 

 between the great medial or neutral region and the much smaller border- 

 lands that three of the four primary geosynclines were developed, namely, 

 the small Arctic Franklinian trough and the better known Appalachian 

 and Cordilleran geosynclines. Their greatest subsidence was toward the 

 borderlands. Out of the Cordilleran primary geosyncline came the Pa- 

 cific and Eock^r Mountain sequent geosynclines (see map, figure 16). The 

 small Acadian primary geosyncline, on the other hand, lies betAveen the 

 borderland Xovascotica and the Xew Brunswick geanticline (see map. 

 figure 3). 



The Cordilleran geosyncline is the oldest and has had the greatest 

 amount of crustal evolution. A very small western part of it is still seen 

 as a marine waterway in the Great Valley of California. This long- 

 enduring geosyncline had its origin early in the Proterozoic, and during 

 this era its maximum subsidence appears to have been about 30,000 feet. 

 Curiously, no orogeny developed within the geosyncline at the close of 

 the Proterozoic, but its borderland Cascadia was then reelevated into a 

 highland, since the following Cambrian sedimentation alone was as much 

 as 15,000 feet. Then there was no striking crustal change until early in 

 Pennsylvanian time, when the Mackenzie shoreline was warped some 

 hundreds of miles to the west, and the geosyncline consequently took on 

 a northwest-southeast trend. We have seen that the Ancestral Eocky 

 Mountains geanticline during Permian and Triassic times also pushed 

 the eastern shoreline of this geosyncline something like 600 miles to the 

 west. After the middle Triassic, the trough was greatly restricted, and 

 with the subsequent rising of the Cordilleran geanticline, this great trough 

 early in the Lower Cretaceous became the comparatively narrow sequent 

 Pacific geosyncline. On the other hand, the rising of this geanticline is 

 evidenced in the trough by marked volcanic activity, beginning in the 

 late Pennsylvanian and attaining its climax in early and middle Triassic 

 times. With the maximal rising of the geanticline late in the Jurassic, 

 its area was to a large extent invaded by vast bathyliths and by renewed 

 volcanic activity, the extrusive materials being also present in consider- 

 able volume in the Lower Cretaceous strata of the Pacific geosyncline. 

 Then toward the close of the Cretaceous, the British Columbia part of 

 the Pacific geosyncline became land. The Californian trough, however. 



