TRANSACTIONS OP SECTION C. 



391 



of the region, the writer confined himself to conclusions which he has reached 

 as a result of field work. 



The region presents a surface which is almost a peneplain, and over the 

 larger part of which there are no rocks exposed younger than pre-Cambrian, 

 except the Pleistocene drift which occurs everywhere. While there have been 

 some oscillations of the ' Canadian Shield ' so as to allow, in places, the deposi- 

 tion of Palfeozoic limestone beds, remnants of which are still found between 

 Lake Ontario and Hudson Bay, it is probable that considerable portions of 

 the ' Shield ' have not been beneath the sea since pre-Cambrian time, and 

 possibly not since the earlier periods of that great age. The classification of 

 the rocks most applicable to the areas north and north : west of Lake Superior 

 is as follows : — ■ 



Cenozoic 



f Recent . . 

 \ Pleistocene 



/Keweenawan 



Alluvial deposits and travertine. 

 Drift and glacial lake deposits. 



Pro-Cambrian . < 



Hu ronian 



Laurent i. in 



*> Keewatin 



f Sediments including red shales, dolomites, sand- 



■] stones, and conglomerates. 



(igneous rocks very prominent and mostly diabase, 



but also norite, gabbro, and peridotite. 

 /Upper H. (Animikie) ; red and black shale", 



quartzite, and iron ore. 

 ~\ Middle H. absent. 



(Lower H. ; basal conglomerate graywacke, quartz- 

 ite iron-formation, and probably some limestone. 

 (Ireat acid igneous series consisting of granite, 



gneiss, and some grano-diorite. 

 /Oreen and grey schists derived by metamcrphism 



of any of the other Keewatin rocks. 

 Banded iron-formation and a little limestone. 

 Graywacke and grey schists or fine-grained gneisses. 



IAcid eruptives, including quartz porphyries, rhy- 

 olites, and acid tuffs. 

 Basic and intermediate eruptives, including ainyg- 

 daloidal and ellipsoidal basalt, diabase, gabbro, 

 diorite, andesite, and tuffs. 



The Keewatin system is an extremely complex one. As a rule the great 

 acid eruptions took place after the basic, but the order is very complicated. 

 The graywacke is apparently the result of disintegration and incomplete 

 sorting, probably under subaerial conditions, of the basic igneous rocks, while 

 the grey, fine-grained gneisses are regarded as the result of similar changes 

 in the acid rocks. The iron-formation is supposed to have been formed by 

 the leaching out of the iron from the basic rocks and its collection in the 

 depressions existing on the irregular volcanic surface. It is almost always 

 interbedded with other sediments such as graywacke or carbonaceous shale. 



The close of the Keewatin was marked by great diastrophism folding the 

 rocks along axes running approximately east-north-east by west-south-west, 

 and under the anticlines arose the great batnoliths of Laurentian granite now 

 generally altered to gneiss. TRis plutonic rock has been exposed by erosion 

 over immense areas, and is a very barren type from an economic standpoint, 

 possibly because it cooled under uniform conditions, and as a rule lacks 

 differentiation. To avoid confusion it was thought advisable to restrict the 

 term Laurentian to those granites and gneisses which may be identified as 

 originating at the close of the Keewatin and before the laying down of the 

 Huronian conglomerate, instead of applying it also to certain acid igneous rocks 

 which possess certain textural characters, as is done by some geologists. Fol- 

 lowing the upheaval at the close of the Keewatin which set the streams 

 actively to work, there was a great' base-levelling which cut well down into the 

 plutonic rocks under the anticlines, and the materials derived formed the great 

 basal conglomerate of the Lower Huronian, which in some places at least 

 contains beds of glacial boulders. The coarser sediments were succeeded by 

 finer-grained types such as graywacke, sandstone, and probably in a few cases 

 by limestone, as a few small areas of the latter rock have been found in this 

 area, though it is difficult to settle their exact stratigraphical position. The 



