122 



The more important volcanic formations are listed 

 in the table. A few subordinate bodies of lavas and pyro- 

 clastics, together with very numerous intrusive masses, 

 will be noted in the sequel. Igneous activity is registered 

 in the pre-Beltian, Beltian, Palaeozoic, Mesozoic, and 

 Cenozoic eras. 



Shuswap Terrane. 



Detailed work has been only begun on the widely 

 exposed pre-Beltian rocks, which form the crystalline base 

 ment of British Columbia and share the complexity of 

 the "Archean" in all parts of the world. They consist 

 of a very thick, conformable, bedded group, called the 

 Shuswap series, and a younger group of granitic intrusives. 

 The whole complex may be conveniently named the 

 Shuswap terrane. 



Shuswap Series — Owing to structural difficulties, 

 to the ruggedness of the mountains, and especially 

 to a dense forest cover, it has not yet proved possible 

 to construct a definitive columnar section for the Shuswap 

 series. It is best exposed on the shore-lines of the Shuswap 

 lakes and of Adams lake, during the low-water season 

 of the year. However, one can seldom follow a contact 

 or other structural plane far from the lake shore. Faults, 

 thrust-planes, and folds are unusally difficult to map 

 in this thoroughly metamorphosed mass of sediments 

 and volcanics. Neither the top nor the bottom of the 

 series has been found. The oldest sediments are inter- 

 leaved with, and underlain by, intrusive granites, chiefly 

 developed as sills. The youngest member on Adams 

 lake where it is best exposed, is truncated by the present 

 erosion surface. 



Obscure as the structures generally are, it is quite 

 clear that the Shuswap series is exceedingly thick. A 

 provisional columnar section may be stated, as follows: 



