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very numerous, in part representing the feeding channels 

 for the other types of injection. 



The injected bodies are, in part, clearly satellites of 

 underlying batholiths, but it is possible that many of 

 them are due to the migration of hydrous magmas locally 

 generated in the depths of a greatly metamorphosed 

 terrane. 



The principal petrographic types in these intrusions 

 are : biotite granite (most abundant) ; hornblende-biotite 

 granite ; two-mica granite (rare) ; pegmatite and aplite 

 (both very abundant) ; and orthogneisses corresponding 

 to each of these magmatic species. Extended microscopic 

 study shows that there is little mineralogical novelty; 

 the rock types are duplicated in most of the 'Archean' 

 tracts on the globe and are usually gneissic in structure. 



The extraordinary prevalence of sills and other concord- 

 ant injections is explained by the extreme fissility of the 

 Shuswap sediments and greenstones. This feature is 

 due to static metamorphism. As shown in the following 

 section on structure, the dips of the Shuswap terrane are 

 generally low. Though its rocks have passed through 

 several periods of energetic mountain-building, their 

 dips over large areas do not surpass 15° and their average 

 dip is probably no greater than 35 . The metamorphism 

 is essentially as far advanced where the strata lie horizontal 

 as where they are dipping at angles of 6o° to 90 . 



Further, it seems highly probable that the fissility had 

 attained nearly its present perfection before the Beltian 

 system of rocks was deposited in the Shuswap terrane, 

 and thus at an early date in the earth's history. The 

 conditions for the metamorphism include: deep burial, 

 with consequent development of "stress" in the vertical 

 direction; and an abundant supply of interstitial water, 

 such as that originally trapped in the sediments and vol- 

 canic beds. The completeness of recrystallization, which is 

 much more striking than that visible in similar geosynclinal 

 rocks of Cambrian or later date, implies that at least one 

 other condition was here necessary. Hypothetically we 

 may find it in a specially steep thermal gradient, con- 

 trolling subsurface temperatures in pre-Beltian times. 

 Field evidence thus leads to the suspicion that the earth 

 was then notably hotter than it was later, when most of 

 the known thick masses of sediments were deposited. 



