34 ROYAL SOCIETY OF CANADA 



study of the problem in the west Lawson arrived at what he regarded as 

 a satisfactory solution of the difficulty. 



In a rejjort on the geology of the Kainy Lake region, published by 

 the (reological Survey in 1887-88, as also in a paper read before the 

 International Congress of Geologists, held in London in 1888, Lawson 

 describes very fully the difficulties which are presented in the study of 

 the Archaean rocks in that district, in the attempt to follow the lines 

 laid down for their interpretation some years before. As the conditions 

 which here prevail are similar to those wliich occur in the more eastern 

 area, and in fact throughout the whole Archaean complex of Canada, 

 a brief summary of the views he there puts forth, to account satis- 

 factorily for the intricate relations found in these rocks, may be present- 

 ed, since these have practically been accepted by all the Canadian work- 

 ers in this interesting field. 



The Archaean is held by Lawson to consist of two great sub-di- 

 visions, viz., a lower, consisting of granite and syenite, which in places, 

 by reason of their foliation, are known as gneiss; and an upper portion, 

 which he named the Couchiching and Keewatin series, the rocks of 

 which are largely bedded, and are composed of mica and other schists 

 with fine grained gray gneiss, for the former, while in the latter are also 

 included large masses of igneous rocks. 



The underlying Laurentian granite-gneiss, at some date sub- 

 sequent to the deposition of the rocks of the two u])per series, under- 

 went a period of fusion. With the fusion of this floor, portions of the 

 overlying strata have been absorbed into the general underlying magma. 



The fusion was probably due to the weight of the superincumbent 

 strata, which depressed the underlying floor till it came within the 

 zone of fusion of all rocks, compatible with the conditions of such 

 depths; and this fusion extended to a certain uneven surface in the 

 Couchiching, which point forms now the line between the lower and 

 upper Archaean. The overlying Couchiching and Keewatin rocks re- 

 tained their originally bedded condition, and formed a hard and brittle 

 crust upon the magma. While this magma was in the fused condition 

 the crust was folded, crumpled or shattered, so that masses of the upper 

 series became involved in the fused mass below, while portions of the 

 fused rocks filled fissures in the overlying schist and other strata and 

 subsequently crystallized as Laurentian gneiss. Such intruded rocks 

 therefore, wliile originally portions of the Laurentian magma, may be 

 petrographically considered as newer than the upper series which they 

 penetrate. The rocks of the upper Archaean while occurring as bedded 

 formations, do not appear to have been reduced to the plastic or fused 

 condition, and such changes as now appear in these are due to true 

 metamorphism. 



