266 THE CANADIAN NATURALIST. [Vol. ix. 



schistose, and are interbedded with unctuous schists, and rest in 

 apparent conformity upon a great mass of quartzite. The 

 general high inclination both of this series and of the typical 

 Huronian, renders the determination of their thickness difficult. 

 The maximum thickness of the Huronian (excluding the petro- 

 silex series) to the south of Lake Superior, may, according to 

 Major Brooks, exceed 12,000 feet, while the estimates of Credner 

 and Murray, respectively, for this region, and for the north shore 

 of Lake Huron, are 20,000 and 18,000 feet. 



As regards the Laurentian, there exists a certain confusion of 

 nomenclature which requires explanation. As originally descri- 

 bed, it includes, as already said, a basal granitoid gneiss, with- 

 out limestones, which the writer has elsewhere designated the 

 Ottawa gneiss, and of which the thickness is necessarily un- 

 certain. Succeeding this is the Grenville series of Logan, having 

 for its base a great mass of crystalline limestone, and consisting 

 in addition to this of gneisses, generally hornblendic, and quart- 

 zites, interstratified with similar limestones. To this series, as 

 displayed north of the Ottawa, Logan assigned an aggregate 

 thickness of over 17,000 feet, though the later measurements 

 of Vennor, in the region south of the Ottawa, give to it a much 

 greater volume. The geographical distribution of this limestone- 

 bearing Grenville series gives probability to the suggestion of 

 Vennor that it rests uncomformably upon the bagal Ottawa 

 gneiss. 



Thes two divisions constitute what was designated by Logan, 

 in his Geological Atlas, in 1865, the Lower Laurentian, — the 

 name of Upper Laurentian or Labradorian being then, for the 

 first time given by him to a series supposed to overlie uncon- 

 formably the former, of which it had hitherto been regarded as 

 constituting a part. This third division has already been 

 referred to as characterized by the predominance of great bodies 

 of gneissoid or granitoid rocks, composed chiefly of labradorite 

 or related anorthic feldspars, and apparently identical with the 

 norites of Scandinavia. With these basic rocks are inter- 

 stratified crystalline limestones, quartzites and gneisses, all of 

 which resemble those of the Grenville series. This upper group, 

 for which the writer in 1871 proposed the name of Norian, was 

 supposed by Logan to be not less than 10,000 feet thick. 



For farther details of the history of these various groups of 

 pre-Cambrian rocks, and their distribution in North America,. 



