468 E. L. BRUCE 



are similar to those in Manitoba, and it may be assumed that 

 they were formed in much the same way. 



The volcanic rocks are lithologically similar throughout the 

 whole region discussed, and this striking similarity quite naturally 

 has led to the correlation of these rocks wherever they occur. 

 The flows are ellipsoidal or massive greenstone of medium basicity. 

 Many of them are now altered to schists. Along with these, 

 minor thicknesses of acidic flows and tuffaceous beds occur. In 

 many districts thin sedimentary beds are found with the igneous 

 rocks. Banded iron formation is very commonly associated with 

 the flows of basic composition. 



Although the rock t3rpes are comparable, the age relations are 

 variable. In some districts the sedimentary rocks he above the 

 volcanics. In others the two are interbanded, and in others the 

 greater thickness lies below the igneous rocks. No erosion interval 

 has been recognized; the separation into igneous and sedimentary 

 divisions made in some locahties, is purely arbitrary, and implies 

 simply that the divisions are dominantly clastic or dominantly 

 igneous. For in most occurrences there are sedimentary beds 

 among the igneous rocks and flows intercalated with the sediments. 



CLASSIPICATION 



None of the general classifications of pre-Cambrian formations 

 is applicable to this early complex. The classification^ accepted by 

 the International Committee places the Keewatin as the lowest 

 formation and does not recognize the presence of great thicknesses 

 of clastic sediments below the great unconformity at the base of the 

 Huronian, nor Lawson's Coutchiching as a great series below the 

 igneous flows. There are, however, not only in the Rainy River 

 district, but in other districts, thick sedimentary formations 

 below the oldest lava flows recognized in those districts. It is 

 possible, as suggested, that other older lava flows exist beneath 

 the sediments, but if so, the appHcation of the term Keewatin, if it 

 is to be retained, must be extended to include a large amount of 

 sedimentary rocks. 



^ Jour, of GeoL, Vol. XIII (1905), pp. 89-104. 



