412 c. K. LEITH 



The Archean geology is summarized by the author as follows: 

 i. The Archean rocks in this area are of Laurentian age, 1 and are 

 in part referable to the Grenville Series and in part to the Funda- 

 mental Gneiss. 



2. The Grenville Series contains gneisses, as well as limestones 

 and quartzites, which are of aqueous origin, having the chemical com- 

 position and the stratigraphical attitude of sedimentary rocks. With 

 these are intimately associated, however, other gneisses which are of 

 igneous origin. 



3. The Fundamental Gneiss consists largely, if not exclusively, of 

 igneous rocks in which a banding or foliation has been induced by 

 movements caused by pressure. 



4. Both series are penetrated by various igneous masses, of which 

 the most important are great intrusions of anorthosite, a rock of the 

 gabbro family, characterized by a great preponderance of plagioclase. 

 This rock is in places perfectly massive, but generally exhibits the 

 irregular structure which is so often observed in gabbros and which is 

 brought about by a variation in the size of the grain or the relative 

 proportion of the constituents from place to place. In addition to 

 this original structure, the rock almost always shows a peculiar pro- 

 toclastic, cataclastic or granulated structure which is especially well 

 seen in the foliated varieties. This differs from the structure char- 

 acteristic of dynamic metamorphism in the great mountainous districts 

 of the world, having been produced by movements in the rock-mass 

 while this was still deeply buried in the crust of the earth and 

 probably very hot — perhaps near the melting point. 



5. The same granulated structure is also seen in all those gneisses 

 which have been formed from massive igneous rocks by dynamic 

 movements. 



6. The fine grained aqueous rocks of the Laurentian, on the other 

 hand, have been altered chiefly by a process of recrystallization. 



7. The "Upper Laurentian" or ''Anorthosite Group" of Sir 

 William Logan does not exist as an independent geological series — 

 the anorthosite, which was considered to be |ts principal constituent, 

 being an intrusive rock, and its remaining members belonging to the 

 Grenville Series. 



8. In all cases of supposed unconformable superposition of the 



1 In the sense of pre-Cambrian or Original Laurentian. 



