153 



Shuswap terrane, where the younger sediments of pre- 

 Beltian age have been eroded away. 



While the Shuswap sediments attained the thickness of a 

 first-class geosynclinal, no clear hint has been forthcoming 

 as to the geographical source of their clastic material, nor 

 as to the direction of the major axis of this prism. There 

 is nothing to show that the subsiding trough had the 

 Cordilleran elongation which has been so characteristic 

 of the post-Shuswap geosynclines. In two leading respects 

 the pre-Beltian terrane contrasts structurally with the 

 younger geosynclinals. 



The Shuswap series is less deformed than any of the 

 overlying series, up to and including the Triassic. In the 

 Selkirks and Interior Plateaus the average dip calculated 

 for the beds of the oldest terrane is no greater than 35 , 

 while the averages for large, typical areas of the Albert 

 Canyon division and Glacier division of the Selkirk series, 

 for the Carboniferous, and for the Nicola series, are, 

 respectively, about 38 . 59 , 73 , and 64 . This is true, 

 though the Shuswap terrane obviously underlay these 

 younger formations when they were passing through 

 several orogenic revolutions. Today, the Shuswap rocks 

 in numerous areas each many square miles in extent are 

 nearly horizontal, while adjacent Carboniferous strata are 

 intensely folded. It appears necessary to believe that the 

 earth-shell which has here transmitted the mountain- 

 building thrust had a depth of only a few kilometres; 

 and that this shell was sheared over its basement of 

 Shuswap rocks. 



The second noteworthy feature is the general failure of 

 the Shuswap strata to show the Cordilleran trend charac- 

 teristic of all the younger formations. The prevailing 

 strike of the basement rocks is about N. 70 E., and thus 

 nearly at right angles to the general Cordilleran strike 

 in this latitude. Quite locally the older rocks have been 

 gripped in a post-Carboniferous plication and show Cor- 

 dilleran strike ; such exceptions do not invalidate the general 

 rule. One is reminded of the prevailing E. — W. to N. 6o° 

 E. strikes in the Pre-Cambrian rocks of Lake Superior 

 and eastward thereof, in the Canadian Shield. Is this 

 agreement of structural trends in the two Pre-Cambrian 

 areas fortuitous? 



As already stated, the detailed structure of the Shuswap 

 terrane offers a host of unsolved problems. In general, 

 35069— 4|a 



