2 GEORGE V. SESSIONAL PAPER No. 25a A. 1912 



CHAPTER XX. 



SUMHAKY OF GEOLOGICAL HISTOEY AND NOTE ON OROGENIO 



THEORY. 



Geological History of the Cordillera at the Forty-ninth Parallel. 



1. The earliest event recorded in the rocks exposed in the Boundary belt 

 is the important pre-Cambrian sedimentation leading to the formation of the 

 Priest River terrane. The exposures offer no hint as to the position of the 

 lands whence the clastic materials represented in this terrane were derived. 

 On account of the fine grain of the clastic rocks we are entitled to believe that 

 the lands were distant. The nature of the beds shows that the land mass or 

 masses were composed of granitic or gneissic rocks, that is, the usual abundant 

 types which carry much free quartz as well as alkaline feldspar. The 

 degeneration and erosion of either granite or gneiss would give just such 

 deposits on the ancient sea-floor as would correspond to the heavy quartzites, 

 quartz schists, metargillite, and mica schists now exposed as the dominant 

 types in the terrane. The sedimentation was doubtless marine, as there 

 are thick dolomites interbedded with the silicious beds at many horizons. The 

 composition and genesis of the Priest River terrane have many analogies to 

 those of the Huronian formations of the Great Lakes district. Leading differ- 

 ences are, however, found in the apparent absence of contemporaneous volcanics, 

 true cherts, jaspilites, and iron-ore in the western area. 



2. The Priest River rocks were thoroughly consolidated, probably mountain- 

 built and somewhat metamorphosed before pebbles were yielded to the Irene 

 conglomerate. 



3. In the Eastern Geosynclinal Belt the period covering the deposition of 

 the Irene conglomerate and the Mississippian limestone inclusive, was occupied 

 in the formation of the Rocky Mountain Geosynclinal, the principal lithological 

 feature of the Eastern Geosynclinal Belt. The clastic sediments forming this 

 huge unit were derived from a western land composed of the Priest River 

 terrane and of granitic or gneissic masses perhaps identical in age and compo- 

 sition with those whence the materials of the Priest River quartzites had been 

 formed through yet older weathering. Throughout this long period of more or 

 less continuous downwarping it is evident that the western shore-line of the geo- 

 synclinal area must have shifted considerably. The probabilities are, however, 

 that for most of the period, they were located in a comparatively narrow, 

 meridional zone. This zone seems to have been located near the longitude of 

 the Columbia river. To west of this zone there must have been a wide belt 

 of land stretching far to the north and south of the Forty-ninth Parallel. That 



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