1916] Lawson: Correlation of Pre-Cambrian Rocks 



slate, and lenses of iron formation and iron ore. This is recognized 

 in the Mesabi, Vermilion, Gunflint and Thunder Bay districts as 

 the Knife Lake slate and Ogishke conglomerate. From Rainy Lake to 

 Steeprock Lake, and beyond to the eastward, the series is continu- 

 ously exposed. No one questions the identity of the series in the sev- 

 eral districts, and it seems fairly certain that it is the product of con- 

 tinuous sedimentation in a definite, unbroken period of time. Tem- 

 porarily I shall refer to it as the Seine Series and to the corresponding 

 division of time as the Seine Epoch, without prejudice to any more 

 acceptable designation that may be claimed for it later on the basis of 

 correlation. The present distribution of the remnants of the Seine 

 Series, taken in connection with its remarkably uniform lithologic 

 habit, warrants the belief that originally it must have covered an area 

 in this portion of the region of at least 10,000 square miles as a con- 

 tinuous body of sediments. 



In all of the districts named above, the Seine Series rests upon the 

 eroded surface of a complex consisting of the Keewatin invaded by 

 granite and granite gneiss of varying facies, with the usual accom- 

 paniment of pegmatites, aplites and other differentiation products of 

 batholithic development. This granite and granite gneiss may most 

 conveniently be known as Laurentian in accordance with prevailing 

 usage. It is therefore certain that throughout all of the studied dis- 

 tricts in the portion of the region lying to the northwest of Lake 

 Superior, the earth's crust was invaded by granite in time long ante- 

 cedent to the deposition of the Seine Series and that this granite is 

 post-Keewatin in age. It is most probably all of the same age, i.e., it 

 was all formed in the same large division of time which, in the chrono- 

 logical scale, may be known as the Laurentian Revolution. 



Now the Seine Series is, in every one of the six districts named, 

 itself cut by granite and granite gneiss of varying facies with peg- 

 matites, aplite and other differentiation products, constituting a 

 granitic assemblage which, from its vast areal extent, may be regarded 

 as a manifestation of batholithic invasion. To this granite I have 

 given the name of Algoman in a comprehensive sense, so that the period 

 of time in which it was formed may be known as the Algoman, a term 

 co-ordinate with Laurentian in the chronological scale. 



It is thus again certain that in all the studied districts northwest 

 of Lake Superior the earth's crust was invaded by granite in time 

 subsequent to the deposition of the Seine Series. It is equally certain 

 that this Algoman granite long antedates the Animikian Series, of 



