1916] 



Lawson: Correlation of P re-Cambrian Rocks 



15 



invasions in the Lake Superior districts and three major unconformi- 

 ties, and this I hold to be a misconception of the geological history of 

 the region. 



On the south side of Lake Superior the local correlations advo- 

 cated by Allen, which I have for the most part followed in the table, 

 are not wholly accepted by Leith. The difference of view is fully set 

 forth in recent papers by these writers and need not be restated, since 

 they do not affect seriously the main hypothesis of this paper. 



PETROGRAPHICAL DISCRIMINATION OF THE TWO GRANITES 



It would be a matter of great interest and importance if petro- 

 graphical criteria could be formulated whereby the Laurentian and 

 Algoman plutonic rocks could be discriminated ; but it is probable that 

 our studies of these rocks are too little advanced to make such a dis- 

 crimination reasonably certain. Nevertheless, since a beginning at 

 this discrimination has to be made, I venture to offer some suggestions 

 to this end, based chiefly on my own observations : 



1. The Laurentian granite is in general more uniform and monoton- 

 ous in mineralogical composition than the Algoman granite. 



2. The Algoman granite has usually undergone magmatic differ- 

 entiation to a marked degree and, therefore, has a larger range of 

 facies than the Laurentian, being in many areas syenitic rather than 

 granitic. 



3. The Laurentian granite had been as a rule subject to deforma- 

 tion before the Algoman granite came into existence, and therefore 

 may in general be expected to show more pronounced deformational 

 effects. 



4. The Algoman granite is in many extensive areas perfectly mas- 

 sive and unfoliated and this is rarely true of the Laurentian. 



5. The Laurentian granite is in many areas bleached in appearance, 

 whereas the Algoman granite appears fresh and in many cases reddish- 

 colored. 



6. The phenomena of viscous flow is more often exemplified in the 

 Algoman granite than in the Laurentian. 



7. The minerals nepheline, titanite, apatite, and garnet are per- 

 haps more abundant in the Algoman rocks than in the Laurentian. 



8. Intrusive phenomena at the contact with encasing rocks are 

 usually clear-cut and pronounced in the case of the Algoman granite 

 and are more obscure in the case of Laurentian granite. 



