336 W. H. C. SMITH — AKCHEAN ROCKS WEST OF LAKE SUPERIOR. 



an increasing ratio as the shores of Lake Superior are approached. It 



would seem as if the Archean rocks alter their consolidation in their 

 present relations had been crushed together by a tremendous lateral 

 force emanating from the southeast, the effect of this pressure becoming 

 less and less as the distance from the supposed center of force increases. 

 Upper Archean Series. — Surrounding the nuclear ovoid and lenticular 

 areas of granitic rocks as an irregular hut almost uninterrupted network 

 and dipping away from them generally on all sides lie the complex and 

 varied rocks of the upper Archean series. The tendency of the two 

 great divisions of Archean rocks to assume this relative distribution was 

 first pointed out to me by Dr A. C. Lawson, and subsequent explorations 

 in parts of Rainy River district unvisited by him have so far confirmed 

 his opinion that such a relative distribution would be found to be charac- 

 teristic of this region. 



TERMS C0XTCHIC11I.\<; AND KEEWATIN SERIES SUGGESTED BY DR LAWSON. 



The rocks occupying the ellipsoid synclinal troughs between the nuclei 

 of granite have been separated by Dr Lawson (the classic authority on 

 this region) into two divisions, for which he suggested the names of 

 Contchiching for the lower and Keewatin for the upper. He has since 

 proposed the name Ontarian to include these two groups. For the 

 underlying granitic rocks the term Laurentian is used. 



CHARACTER AND FIELD RELATIONS OF THE LAURENTIAN ROCKS. 



The Laurentian rocks of this region are for the most part essentially 

 granites. A gneissic foliation is often apparent and frequently well 

 marked, particularly in the peripheral zones of the areas, while the cen- 

 tral portions are usually more granitoid. The rocks vary in texture 

 from fine- to coarse-grained and pegmatitic, and in color from light to 

 dark gray and from pink to deep red. In composition they present 

 many various characters. Usually the ferromagnesian mineral is biotite 

 with more or less muscovite. Hornblende granites are not uncommon. 

 The latter sometimes merge into the biotite granites by a gradual change 

 in composition, but usually a sharp line of demarkation separates them. 

 The relations in the field are sometimes suggestive < >f large brecciated frag- 

 ments of hornblende granite caught up in the biotite granite, and some- 

 times of intrusions of the former into the latter. The relations of these two 

 varieties of granite form an interesting problem for future study, hut as 

 yet the writer is not prepared to formulate any general theory concerning 

 them; indeed, it is doubtful if any generally applicable theory is possi- 

 ble, as there is reason to believe that the hornblende granite is sometimes 



