1 63 



large surfaces underlain by non-volcanic rocks were not 

 greatly deformed. 



The Miocene was a time of general erosion across the 

 entire Cordillera at our section. 



The Cordilleran topography at the beginning of the 

 Pliocene was evidently highly complex in origin and of 



Diagram drawn to scale, showing development of columnar jointing in Tertiary- 

 basaltic flow near Ducks station. The gently dipping limb of the syncline is 

 composed of regular columns of great size. The upturned limb is composed of 

 four sets of regular but much smaller columns. The latter seem to have developed 

 through orogenic stresses superposed on original cooling stresses. 



great variation in age. Large areas had been undergoing 

 erosion since the closing days cf the Paleozoic ; other areas, 

 since the Triassic; others, since the late-Jurassic revolu- 

 tion; still others, since the Laramide revolution; while 

 practically the whole Cordillera, except the part covered 

 by Tertiary volcanics or local pockets of earlier Tertiary 

 sediments, was being eroded during Eocene, Oligocene and 

 Miocene times. We may well believe that, in places, the 

 unceasing erosion of the whole (pre-Pliocene) Tertiary era, 

 in spite of post-Oligocene deformation, had virtually pro- 

 duced local or widespread peneplains. Elsewhere moun- 



