206 C. SCHUCHERT THE NOETH AMERICAN GEOSYNCLIXES 



continued as a marine area, althougii greatly restricted, not only throngli- 

 oiit the Mesozoic, bnt the Cenozoic as well. 



With the rising of the Cordilleran Intermontane geanticline, there 

 came into existence the eastern sequent and short-lived but very mobile 

 Rocky Mountain geosyncline. This subsidence appeared first in Mexico, 

 beginning possibly early in the Permian and certainly in the late Triassic, 

 and spread northward into Kansas *by the close of Lower Cretaceous time, 

 when the Mackenzie end of the trough also had marine waters as far south 

 as central Alberta. Then early in the Upper Cretaceous the whole of the 

 geos3Ticline was one vast inland sea. The western geanticline again began 

 to rise late in the Cretaceous, and apparently the whole of the arch from 

 southern Mexico north into Alberta was then studded with active volca- 

 noes. AVith this further rising of the arch, the Eocky Mountain seas 

 were converted into land during the close of the Cretaceous. The present 

 high altitude of the Cordillera, however, came with the vast epeirogeny 

 that began in the middle Miocene and was completed toward the close of 

 the Pliocene. 



The Appalachian geosyncline made its reappearance south of Virginia 

 in earliest Cambrian time, but long before the close of the Lower Cam- 

 brian it was in clear evidence from Alabama to Labrador. This is the 

 t3'pe geosyncline (see map, figure 4). The northern half, or the Saint 

 Lawrence trough, was blotted out by the orogeny of late Devonian time, 

 and the whole of the Appalachian geosyncline was converted into moun- 

 tains during the Permian. East of the southern half of this geosyncline 

 was the borderland Appalachia. Southeast of the Saint Lawrence trough 

 and the Xew Brunswick geanticline was the Acadian geosyncline, and 

 both troughs were elevated above sealevel by the Devonian orogeny (see 

 maps, figures 7 and 9). Therefore we may say that the northern Appa- 

 lachian system of mountains are bilaterally symmetrical structures, while 

 the southern ^Appalachians, even though they are polygenetic mountains, 

 are wholly a one-sided system folded and thrusted away from the Atlantic 

 Ocean. 



To the northwest of Greenland is the Franklinian geosyncline, bounded 

 on its Arctic side by the borderland Pearya, while its deposits thinned 

 inland across the Canadian shield (see maps, figures 3 and 5). The 

 presence of this geosyncline is clearly evidenced in late Cambrian time, 

 and we suspect that it came into existence with the diastrophism of the 

 late Proterozoic. During the Paleozoic, the Franklinian trough subsided 

 something like 20,000 feet, and appears to have been blotted out by the 

 orogeny of Permian times. 



