THE PRIMORDIAL, OR CAMBRIAN AGE. 51 



Silurian has been a point greatly controverted ; and 

 the terms Primordial and Primordial Silurian have been 

 used as means to avoid the raising of this difficulty. 

 Many of our division lines in geology are arbitrary 

 and conventional, and this may be the case with that 

 between the Primordial and Silurian, the one age 

 graduating into the other. There appears to be, how- 

 ever, the best reason to recognise a distinct Cambrian 

 period, preceding the two great periods, those of the 

 second and third faunas of Barrande, to which the 

 term Silurian is usually applied. On the other hand, 

 in so far as our knowledge extends at present, a 

 strongly marked line of separation exists between the 

 Laurentian and Primordial, the latter resting on the 

 edges of the former, which seems then to have been as 

 much altered as now. Still a break of this kind may 

 be, perhaps must be, merely local ; and may vary in 

 amount. Thus, in some places we find rocks of Silu- 

 rian and later ages resting directly on the Laurentian, 

 without the intervention of the Primordial. In any 

 case, where a line of coast is steadily sinking, each 

 succeeding deposit will overlap that which went before; 

 and this seems to have been the case with the Lauren- 

 tian shore when the Primordial and Silurian were being 

 deposited. Hence over large spaces the Primordial is 

 absent, being probably buried up, except where exposed 

 by denudation at the margin of the two formations. 



This occurs in several parts of Canada, while the 

 Laurentian rocks have evidently been subjected to 

 metamorphism and long-continued weathering before 



