[ells] CANADIAN GEOLOGICAL NOMENCLATURE 35 



As regards this question of metamorphism Lawson also remarks, 

 " that the molten gneiss being under and around these troughs of 

 upper Archaean rocks, and the action long continued by reason of con- 

 finement, under a superincumbent crust, the resulting metamorphism 

 has been more intense and more general or regional than in the case of 

 a mere local intrusion of gTanite through higher portions of the crust. 

 But while the metamorphism has been regional, it has been more in- 

 tense at the actual contact of the basal portion of the firm crust with 

 the underlying magmas, than in higher portions of series of formations, 

 and in this gradation of results we have distinct analogy with local or 

 contact metamorphism p'o-called. The conception of the igneous origin 

 of the lower Archaean or Laurentian and the metamorphic character 

 of the upper Archœan, thus explains why the basal beds of the Keewat- 

 in, where these rest on the Couchiching, should not be similar to those 

 same beds when resting on the Laurentian, and why in the latter case 

 these beds should appear much more altered than in the former." 



In the report of Mr. W. H. C. Smith, on the rocks of the adjacent 

 area, the same association of strata is pointed out. The intrusion of 

 portions of the underlying Laurentian granite-gneiss is noted in the 

 rocks of the Couchiching and Keewatin sericis, and the alterations of the 

 latter along the lines of contact is mentioned. Mr. Smith also sug- 

 gested the correlation of the Couchiching with the Grenville series of 

 Quebec, which it greatly resembles, with the exception that, in the 

 western area, the crystalline limestone portion of the latter is ap- 

 parently absent. It would therefore appear that in the relations of 

 the Laurentian granite-gneiss to the overlying sedimentary gneiss and 

 schist series two phases are to be recognized, viz., that seen in the 

 northern portion of the Ottawa basin, where the Laurentian gneiss un- 

 derlies apparently the Grenville series in regularly arranged anticlines, 

 the stratigraphical sequence being well established ; and the 

 arrangement observed in the southern and western areas, where the 

 contacts are more of the nature of invasions of the lower gneiss when in 

 a plastic condition, into the gneiss and limestone of the upper series. 

 It is of course supposable that the difference visible in the relations of 

 the two areas mentioned may be due to agencies of denudation, by 

 which the whole or great portions of the overlying upper series have 

 been removed, subsequent to the changes which have been pointed out. 

 Such denudation must have been enormous in the lapse of time which 

 existed from the laying down of the first Laurentian floor to the de- 

 position of the Potsdam sandstone, which is the formation which suc- 

 ceeds the Grenville series in Quebec and eastern Ontario, and it is pos- 

 sible that the great and immense thickness of Cambrian rocks which 



