196 DEPARTMENT OF TEE INTERIOR 



• 2 GEORGE V., A. 1912 



has been built so largely of the rocks laid down in these three periods. For 

 use in the present report this part of the Cordillera will be called the ' Eastern 

 Geosynclinal Belt.' In a later chapter details will be given which tend to 

 corroborate the prevailing view that the western part of the Cordillera, from 

 Alaska to southern California at least, is a second vast unit deeply contrasted 

 in composition and history with the Eastern Belt. One result of the correla- 

 tions so far made is to give some indication of the approximate line which 

 may be taken as separating the Eastern belt from the ' Western Geosynclinal 

 Belt.' 



Axis of the Rocky Mountain Geosynclinal. 



If the foregoing correlations of the formations in the Forty-ninth Parallel 

 section be justified, it seems possible to determine, in a very general way, the 

 thickness and extent of the geosynclinal which was accumulated during the 

 time elapsing between the deposition of the oldest beds of the Belt terrane and 

 the deposition of the Upper Cambrian formations. Since these older rocks, 

 where developed in the heart of the geosyncline, rival or surpass in thickness 

 the whole of later Paleozoic formations in the same area, the delimitation of 

 the pre-Upper Cambrian sediments effectively locates the main axis of the 

 Rocky Mountain geosyncline. (Figure 12, .page 202.) 



Needless to say, the- field evidence is far too incomplete to permit of any- 

 thing like an accurate picture of the ancient down-warp or of its sedimentary 

 filling. Nevertheless, the materials are already in hand to warrant a sub- 

 stantial corroboration of the view of J. D. Dana, G. M. Dawson, and others, 

 that the Rocky Mountain system has been built up through the upturning of 

 a vast geosynclinal lens whose main axis lay to the eastward of an Archean 

 protaxis in the Cordillera; and, secondly, that the geosynclinal axis lay parallel 

 to the general axis of the present Cordillera. 



In the eastern part of the Selkirk range at the Forty-ninth Parallel the 

 thickness of all the conformable pre-Upper Cambrian beds, excluding the 

 6,000 feet of Irene volcanics is about 26,000 feet. The character of these sedi- 

 ments show that their material was in largest part derived from the rapidly 

 eroded lands lying to the westward. The old shore-line, or rather zone of 

 shore-lines, was probably located not far from the crossing of the Columbia 

 river at the International Boundary. As yet the only other columnar section 

 of these Cambrian-Belt rocks which includes their base, has been constructed 

 from outcrops observed in the Belt mountains 350 miles to the eastward, where 

 the whole thickness is 12,000 feet. Not far to the eastward of this section 

 there was land during the deposition of the Belt, Lower Cambrian, and some 

 of the Middle Cambrian beds; during the Middle Cambrian much of this 

 eastern land area was itself transgressed by a wide shallow sea. Between the 

 Belt Mountains section and the Selkirk (Boundary) section, a great thick- 

 ness of Belt-Cambrian beds, considerably excelling 20,000 feet, was laid down 

 in apparently perfect continuity . 



