12 p. B. KING 



they were built up irregularly on the sea floor, and perhaps in places 

 to the surface. It has been thought that in some areas sedimentation 

 was interrupted by deformation during Paleozoic time, before the 

 close of the geosynclinal phase (Eardley, 1947, pp. 328-334), so that 

 fold ridges may have emerged as land areas. Indications of such 

 deformation are based on obscure evidence that can be otherwise 

 interpreted, so that any occurrence of fold ridges produced by the 

 deformation remains to be proved. 



At the eastern edge of the eugeosynclinal area the record of 

 emergence is more definite. Differences between Triassic and Jurassic 

 deposits east and west of a belt through the center of the Great Basin 

 have long been known, and are sufficiently marked as to suggest that 

 the two sets of deposits were separated by a land barrier (Nolan, 

 1943, p. 158) (Fig. 3). Geologic work in north central Nevada during 

 the last few decades has indicated something of the antecedents of 

 this barrier (Roberts et al.). Older Paleozoic rocks that had been 

 laid down in the eugeosyncline were deformed and thrust eastward 

 over the miogeosynclinal area, were then eroded, and were over- 

 lapped from the east by Pennsylvanian deposits (Fig. 2). Angular 

 unconformities within the higher Pennsylvanian and the Permian 

 attest a continued instability of the area. In the Great Basin re- 

 gion, the Triassic and Jurassic barrier must have been inherited 

 from this belt of Paleozoic deformation. Stratigraphic data indicate, 

 however, that the barrier continued northward from the Great 

 Basin toward Canada, where its origin and prior history are less 

 clearly indicated. 



Miogeosynclinal Area of the Cordillera 



A miogeosynclinal environment prevailed over the continentward 

 side of the Cordilleran geosyncline. During Paleozoic time, when the 

 miogeosyncline received thick accumulations of limestone, its eastern 

 edge extended across the site of the Rocky Mountains to the Great 

 Plains near the Canadian border, but farther south in Utah extended 

 no farther east than the boundary between the Great Basin and 

 Colorado Plateau. 



Through much of the segment in the western United States, there- 

 fore, a wide area of the Cordilleran region lay east of the geosyncline 

 — the present Colorado Plateau and the Rocky Mountains of Wyo- 

 ming, Colorado, and New Mexico. During Paleozoic time most of 



