1 62 



Thus, at the dawn of the Tertiary the Cordillera was 

 developed with full vigour of mountainous relief. Its 

 volume in British Columbia, measured above sea level, 

 was then probably at its maximum. Its general history 

 is henceforth one of erosion coupled with intermittent 

 vulcanism of great intensity and with diastrophic move- 

 ments which were of great importance but of an order 

 less than the revolutionary. In the absence of a wide- 

 spread sedimentary record in the mountain chain, it is 

 difficult to state Tertiary events in an orderly, quantita- 

 tive way. Long chapters in the Tertiary history can 

 only be written in the future, after modern physiographic 

 methods have been applied in the as yet unmapped portions 

 of British Columbia. 



In the Canadian Pacific section no marine sediments of 

 Tertiary age have been definitely reported. The Eocene 

 geosynclinal of Puget sound was doubtless continued into 

 the region of the Strait of Georgia and lower Fraser valley; 

 but this irregular prism represents an intermont basin, in 

 which much of the deposition was subaerial or in fresh or 

 brackish water. There resulted one of the thick stratified 

 masses necessarily developed in Eocene basins from the 

 wasting of the new, vigorous mountain chain. It is prob- 

 able that the Belt of Interior Plateaus saw, in this period, 

 a moderate amount of local volcanic action, paralleling 

 the greater Eocene eruptions of Central Washington and 

 of the Coast region. The eastern Cordilleran Belt carries 

 no rocks of this period, which was apparently occupied 

 throughout by erosive activity. 



The Oligocene continued this erosion across the entire 

 chain, but was marked in the Western Belt by long-contin- 

 ued emission of basalts, chiefly of the fissure-eruption 

 type. This vulcanism involved much disturbance of 

 drainage system. Local basins were formed and became 

 filled with gravels, sands and muds, bearing fresh-water 

 fossils (Tranquille group). 



The Western Belt became affected by moderate orogenic 

 movement, whereby the Oligocene lavas and sediments 

 were locally upturned, sometimes to vertical position. 

 This deformation is not yet accurately dated, but may prove 

 to be of late Oligocene date. Though the local upturning 

 was so pronounced, the Tertiary lavas of British Columbia 

 were, in general, little disturbed from their original, flat 

 attitudes, and it is reasonable to suppose that similarly 



