REPORT OF THE CHIEF ASTRONOMER 617 



SESSIONAL PAPER No. 25a 



drift, since the upturning of the late Jurassic. That is, we must recognize the 

 probability that erosion has been actively at work upon the folded Paleozoics 

 through all Cretaceous and Tertiary time. It is little wonder that the rocks show 

 the effect of profound erosion or that the relief is that of a topographic torso. 

 There has evidently been time enough for the completion of at least one full 

 erosion-cycle in this region if the earth's crust had stood still. An obvious 

 explanation of the plateau form of the massifs consists in the view that this 

 region has been baselevelled at least once and has since been uplifted some 

 4,000 feet or more, whereby the deep Okanagan and Similkameen valleys have 

 been entrenched beneath the old peneplained surface. 



This is the hypothesis favoured by Dawson for the whole of what is here 

 called the Belt of Interior Plateaus. His general statement may be quoted: — 

 ' Chiefly because no deposits referable to the Eocene or earliest 

 Tertiary have been found in this part of the Cordillera, it is assumed with 

 probability that this was a time of denudation. It is further indicated 

 that it was a time of stability in elevation, by the fact that the prolonged 

 wearing down resulted, in the interior zone of the Cordillera, in the produc- 

 tion of a great peneplain, the baselevel of which shows that the area 

 affected stood 2,000 or 3,000 feet lower in relation to the sea than it now 

 does, and that for a very long time. If, however, the Puget beds of the 

 coast are correctly referred to the Eocene, it follows that the coast region 

 was at the same period only slightly lower than at present, and that the 

 movements in subsidence and elevation between this and the interior region 

 must have been differential in character and very unequal in amount.'* 



Dawson then describes the episode of Oligocene fresh-water sedimentation 

 in the belt, followed by slight orogenic disturbances. These crustal movements 

 are said not to have seriously injured the Eocene peneplain surface as a 

 primary element in the Cordilleran topography. They were followed by a 

 long continued period of volcanic action which covered much of the belt many 

 thousands of feet deep with basic lavas and pyroclastics. These Miocene 

 volcanics totalled 8,400 feet in thickness and another thousand feet of fresh- 

 water sediments were intercalated in the volcanic series.f Even these additions 

 to the belt are not credited with affecting the integrity of the ' Interior 

 Plateau ' as a peneplain, though it was locally warped or faulted during the 

 late Miocene orogenic movement. 

 We may quote further : — 



' Following the close of, or at least a great reduction in volcanic 

 activity, in the early Pliocene, the interior zone of the Cordillera again 

 assumed a condition of stability for a considerable time, during which wide 

 and ' mature ' stream valleys were formed. The elevation of the Interior 

 plateau region of British Columbia must then have been about 2,000 feet 

 less than it is at present 



*G. M. Dawson, Bull. Geol. Soc. America, Vol. 12, 1901, p. 89. 



tThese volcanics have recently been shown to be largely of Oligocene age. 



