DISTINCT ORIGIN OF THE TWO DIVISIONS. 181 



sediments. It is only in very recent years that the possibility of the deriva- 

 tion of a portion of the schists of the Archean from volcanic rocks has been 

 looked into and the important role played by volcauic agencies in building 

 up the older rock series has been appreciated.* There are, however, not a 

 few geologists who continue to advocate the extreme plutonic view that the 

 whole of the Archean is of igneous origin and represents the first-formed 

 crust of the earth. Hunt's crenitic hypothesis, also, is a challenge to the 

 metamorphic theory. 



In deference to these and other anti-metamorphic schools of thought, in 

 which for the most part theory seems to crowd out fact, it becomes necessary, 

 with the accumulation of evidence of recent years, to point out the great 

 additional strength acquired by the theory of metamorphism as applied to 

 the Archean, by the recognition of the volcanic origin of much of the material 

 upon which metamorphic agencies have operated, and by the limitation of 

 its application to the upper division of the Archean ; the rocks of the lower 

 division, or Laurentian, being susceptible of an entirely different explanation. 

 The lack of discrimination between the essentially different characters of the 

 upper and lower Archean and the lumping of the whole complex together as 

 haviug necessarily the same origin and development has been the great 

 mistake alike of the metamorphic and the extreme plutonic schools. Just 

 as the metamorphic theory, properly limited, affords the explanation of the 

 development of the rocks of the upper Archean from normal formations, so 

 by a similar limitation of the plutonic theory and the introduction of some 

 modifying considerations we will find in the latter a rational and consistent 

 explanation of the origin of the rocks of the Laurentian. 



Relations between the two Divisions. 



The General Relations. — The full significance of the sharp separation of 

 the Ontarian system, as a bedded assemblage of prevailingly schistose and 

 otherwise altered normal rocks, from the Laurentian, as a non-bedded assem- 

 blage of more or less foliated plutonic igneous rocks, will appear from an 

 inquiry into the relations in space and in time between these two great sys- 

 tems, which it is the object of this paper to institute. 



That portion of the Ontarian system which for some years has been some- 

 what loosely referred to as Huroniau, from its supposed equivalence with the 

 rocks of Lake Huron, now held to be possibly post-Archean, presents in 

 many parts of central Canada contacts or lines of junction with the Lau- 

 rentian. The nature of this contact has been a subject of discussion. The 

 question has ever been raised whether these rocks are conformable or un- 



*The first suggestions of volcanic admixtures in the upper Archean rocks of central Canada were 

 thrown out by (i. M. Dawson in his description of the agglomerates of the Lake of the Woods in 

 the Report on the Geology and Resources of the 49th Parallel, 1875, p. 52. 



