1916] 



Lawson: Correlation of Pre-Cambrian Rocks 



5 



its predecessor as new similarities or dissimilarities were detected, as 

 the members of the sequence increased in number, and as new uncon- 

 formities or irruptive contacts were discovered. The Keewatin was 

 segregated from the Huronian on the basis of its dissimilarity and the 

 intrusive relations of the Laurentian granite gneiss. The iron-bearing 

 rocks of the south side of Lake Superior were correlated with the Ani- 

 mikian on the basis of lithologic similarity and their unconformable 

 relation to the Keweenawan above and to an older complex below. The 

 Keweenawan of the north shore has been satisfactorily correlated with 

 the rocks so named on the south shore on the basis of lithologic re- 

 semblance. The Huronian has been divided into three parts by two 

 unconformities discovered within the original Huronian terrane and 

 the correlative of these parts is claimed to have been found in various 

 districts on the basis of lithologic resemblance, sequence and coinci- 

 dence of unconformities. In this way much progress lias been made 

 and our knowledge of the pre-Cambrian has advanced greatly in the 

 last three decades. 



It is evident, however, when we review the literature and note the 

 wide differences of opinion regarding the correlation of some undis- 

 puted sequences, that the principles hitherto invoked are not wholly 

 adequate for the purpose. "When fossils were found in the limestones 

 of Steeprock Lake in 1911, it was my hope that the same forms might 

 be found in similar limestones on the south side of Lake Superior and 

 so confirm the correlation tentatively adopted. This expectation, how- 

 ever, has not yet been realized. 



HYPOTHESIS OF TWO PERIODS OF GRANITIC INVASION 



In the correlation table here formulated a new hypothesis is intro- 

 duced and used as an aid to correlation, which, if it is true, resolves 

 much of the doubt which has troubled students of pre-Cambrian 

 geology. This hypothesis states that in post-Keewatin time there were 

 two and only two periods in which great granitic batholiths were 

 developed in the earth 's crust in the region of the present Great Lakes. 



It is intended in this hypothesis to exclude certain acid "red 

 rocks" found intrusive in the Keweenawan as not batholithic masses 

 and as easily distinguishable ordinarily from the granites to which 

 reference is here made. It is also intended in each batholithic period 

 to include all of the various satellitic rocks that pertain to granites, 

 such as pegmatites, aplites, etc., and also certain sequences in the ir- 



