1916] 



Lawson: Correlation of Prc-Cambrian Rocks 



9 



fore fairly certain that the group comprising the quartzite and dolo- 

 mite is later than this Laurentian granite gneiss throughout the four 

 districts. 



Resting unconformably on the quartzite-dolomite group in all four 

 districts is another group comprising in the Gogebic district the Palms 

 quartzite, graywacke and slate, the Ironwood iron formation, and the 

 Tyler slates ; in the Crystal Falls district the Ajibik quartzite, the Vul- 

 can iron formation and the Hanbury slates ; in the Menominee dis- 

 trict quartzite, Vulcan iron formation and Hanbury slates ; and in 

 the Marquette district the Ajibik quartzite, the Siamo slate and the 

 Negaunee iron formation. Here, again, there is little doubt as to the 

 correlation of this group of variously named formations throughout 

 all four districts. The group, with local volcanic admixtures, repre- 

 sents a definite stratigraphic horizon and a definite period of time 

 which is the same in all districts. 



In the Gogebic district the group is cut by the Presque Isle granite, 

 as Allen and Barrett have recently shown, 4 and in the Menominee dis- 

 trict the group is also cut by granite. It is therefore certain that, at 

 a time later than the accumulation of this group of variously named 

 formations, there was a plutonic invasion of the earth's crust by 

 granite on the south side of Lake Superior. 



Thus we have on the south side of the lake, as on the northwest 

 side, two and (so far as we know) only two periods of batholithic in- 

 vasion. The corresponding granites serve as on the northwest side to 

 separate the geological column into three great divisions, one ante- 

 cedent to the first granite, the second between the two granites and 

 the third subsequent to the second granite. It is the essence of the 

 hypothesis which I here advance as a tentative principle of pre- 

 Cambrian correlation, that the two granite invasions of the south side 

 of Lake Superior are chronologically equivalent to the two granite in- 

 vasions of the northwest side, each to each. 



In accordance with this hypothesis the columns expressing the un- 

 disputed sequence in the Gogebic, Crystal Falls, Menominee, and Mar- 

 quette districts are placed in the tabulation with the two granites on 

 the same time-horizons as the two granites of the northwest side of the 

 lake. It follows, if the hypothesis is true, that the group of variously 

 named formations on the south side, comprising the Tyler and Han- 

 bury slates, the Ironwood, Negaunee, and Vulcan iron formations, and 

 the Palms, Siamo, and Ajibik formations, are the correlatives of the 



4 Journ. Geol., vol. 23, do. 8, Nov. -Doc, 1915. 



