204 C. K. LEITH 



glomerates resting unconformably on the Laurentian, it is suggested 

 that in these cases the thickness of the sediments was not great enough 

 to depress the Laurentian floor to the level of fusion or plasticity; or, 

 that the Huronian, as recognized in these regions, is really younger 

 and overlies the upturned edges of the rocks described as Huronian in 

 the northern Archean. 



Comments. — The above discussion is based on the most thorough 

 field work yet done in this area, and the account probably marks an 

 advance in the interpretation of the pre-Cambrian stratigraphy of the 

 area northwest of Lake Superior, although the conclusions cannot yet 

 be accepted as final. Some of the conclusions differ from those of 

 other workers in this area, and such may be especially noted. 



The Couchiching is believed to be entirely a sedimentary series, 

 conformably below the Keewatin, and in eruptive contact with the 

 Laurentian granite and gneiss below. Lawson regarded the Couchi- 

 ching as sedimentary, but believed that there .was an unconformity 

 between it and the Keewatin, because of the great differences in the 

 characters of the materials, the degree of crystallization, and the pres- 

 ence of basal conglomerates in places at the base of the Keewatin. 

 Coleman has previously accepted these conclusions. Van Hise, 

 accepting Lawson 's conclusion as to the position of the Couchiching 

 below the Keewatin, supposed the Couchiching to belong to the Base- 

 ment Complex or Archean. However, recent personal work in the field 

 has led him to the conclusion that the Couchiching of Rainy Lake is 

 largely a sedimentary series, which is equivalent in age to the upper 

 part of the series mapped by Lawson as Keewatin in the same area. 



The insistence on the eruptive contact of all the rocks of the Lau- 

 rentian with the Huronian and the explanation of the origin of the 

 Laurentian may also be noted. Coleman believes that the gneisses 

 and granites of the Fundamental Complex do not represent the earth's 

 erstarrungski'uste, but are portions of the earth's crust, of sedimentary 

 or other origin, which have been buried deeply enough for hydro- 

 thermal fusion, and have afterwards been disinterred by long-continued 

 denudation. After fusion of course such rocks would be really eruptive 

 and of later age than the Huronian rocks above, even though originally 

 they may have been older than them and formed the floor upon which 

 the Huronian rocks were deposited. This is essentially the position 

 of Lawson. Van Hise, on the other hand, believes that here the 

 granites and gneisses heretofore included under the Laurentian of this 



