6 DEPARTMENT OF TEE INTERIOR 



2 GEORGE V., A. 1912 



complexity, like the stratigraphic complexity, is near its maximum for the given 

 area in such a straight cross-section. A preliminary sketch of the different 

 geological provinces traversed by the Boundary belt will aid the reader in under- 

 standing and grouping the mass of observations to be detailed. 



In the first approach, the Cordillera at the Forty-ninth Parallel may be 

 regarded as divisible into great zones. These are called the Eastern Geo- 

 synclinal Belt and the Western Geosynclinal Belt. The two overlap in the 

 vicinity of the Columbia river. From the summit of the Selkirk range, just 

 east of that river, to the Great Plains, sedimentary formations are dominant 

 and are almost entirely included in one huge structure, hereafter named the 

 Rocky Mountain Geosynclinal Prism (or simply Geosynclinal). The prism 

 extends from Alaska, through the Great Basin, to Arizona. These rocks are 

 so nearly imfossiliferous that their correlation with the standard systems is a 

 matter of difficulty. Reasons will be shown for the belief that the whole con- 

 formable group ranges in age from the Mississippian to a great unconformity 

 at the base of the Belt terrane, or Belt! an system, as recently named by Wal- 

 cott.* Near the western crossing of the Kootenay river, the prism rests on an 

 elder group of metamorphic rocks, here called the Priest River terrane. Tho 

 basement on which the Rocky Mountain Geosynclinal rests is nowhere else 

 exposed on the Forty-ninth Parallel. 



Younger and much more local geosynclinal prisms, of Cretaceous and Ter- 

 tiary dates, have been laid down on the Rocky Mountain Geosynclinal along 

 its eastern border, in Alberta, Montana, and farther soiith. None of these 

 younger prisms of great thickness is represented in the section at the Inter- 

 national Boundary, but it is convenient to refer to the whole compound belt of 

 heavy sedimentation under the one name, the Eastern Geosynclinal Belt. 



Similarly, the dominant sedimentaries west of the Columbia river, of 

 Pennsylvanian, Triassic, and Cretaceous age, have been accumulated in great 

 thicknesses. The Pennsylvanian strata have been recognized at many points, 

 from Alaska to Southern California, and it appears probable that late Paleozoic 

 sedimentation on a geosynclinal scale took place throughout that long stretch. 

 More local Mesozoic and Tertiary geosynclinals were imposed upon the Pacific 

 border of the pri^m developed in the Pennsylvanian period. Rocks apparently 

 representing this older group of deposits crop out at intervals all the way from 

 the Columbia river to the Gulf of Georgia. A part of one of these Mesozoic 

 prisms was found in an enormously thick mass of Cretaceous strata largely 

 composing the Pasayten mountain range between the Pasayten and Skagit 

 rivers. A thick Triassic series is known on Vancouver island and forms part 

 of the western slope of the Skagit mountain range, which lies between 

 the Skagit river and the Strait of Georgia. The edge of the Tertiary 

 geosynclinal composed of the Puget beds is, apparently, represented in the 

 Eraser valley. To the entire composite mass of post-Mississippian sediments 

 occurring in the western half of the Cordillera, the name Western Geosynclinal 

 Belt may be given. 



* C. D. Walcott. Smithsonian Miscellaneous Collections, vol. 53, No. 5, 1908, p. 169. 



