57 Q W. L. UGLOW PENEPLAIN IN BRITISH COLUMBIA 



Owing to the absence of continuous exposures, the extent and depth of 

 this erosion can not be estimated. 



This block-faulting may possibly be correlated with that period of 

 tension Avhich followed the compressive stresses of the Laramide revolu- 

 tion. Daly^ and Mackenzie' fonnd similar faulting of the early Tertiary 

 measures along the international boundary and in the Flathead Valley 

 in southern Alberta. It is probable that these crustal movements are all 

 of the same age, and in the North Thompson basin they can be defi- 

 nitely placed between the end of the Eocene and the period of Miocene 

 vulcanism. 



In reference to the deformation of the peneplain as a result of the 

 Oligocene block-faulting, it may be noted that the western side of the 

 trench is much lower than the eastern; and this may be due to the com- 

 bined effects of the Oligocene and post-Miocene diastrophism. 



MIOCENE VULCANISM 



During the Miocene period extensive subaerial floods of andesitic and 

 basaltic lava poured out over the southern interior of the province. 

 Within the region in question these flows were mostly hornblende- and 

 augite-andesite, usually highly scoriaceous and amygdaloidal. Agate, 

 hyalite, zeolites, and native copper are found as fillings of the amygdules. 

 The lavas flooded the valleys and depressions and covered the upturned 

 and eroded measures of Eocene age. In the valley of the Clearw^ater 

 River, a southerly flowing tributary of the Xorth Thompson, great ter- 

 races of this lava still remain as remnants from post-Miocene erosion, 

 and underneath the lava may be seen cliffs of bleached and kaolinized 

 granite, the result of pre-Miocene w^eathering. 



POST-MIOCENE DIASTR0PHIS2I AXD EROSION 



Further faulting within the trench followed the extrusion and solidifi- 

 cation of the lavas. Along a fault zone ruaning north and south through 

 the center of the trench, the western wall of the valley was down-faulted 

 Avith respect to the eastern wall. This faulting was accompanied by the 

 production in the lava of closely spaced shearing planes parallel to the 

 strike of the main fault. Owing to the displacement, the flows which 

 overlie the Eocene sediments on the west side of the river are now at the 

 same level as the coal measures of the Eocene on the east bank. The 

 vertical displacement along this fault was at least 600 feet and probably 

 as much as 1,000 feet. Evidence of this faulting is still preserved in 



