1916] 



Laivson: Correlation of Prc-Cambrian Rocks 



13 



superposition of the Steeprock series upon the granite of the first, 

 or Laurentian, invasion ; on the south side of Lake Superior in the 

 superposition of the Sturgeon quartzite upon the Laurentian ; and on 

 the north side of Lake Huron in the superposition of the Bruce series 

 upon the Laurentian. 9 



Similarly we have another major unconformity equally well estab- 

 lished on the northwest side of Lake Superior in the superposition of 

 the Animikian upon the granite of the second, or Algoman, invasion ; 

 on the south side of Lake Superior in the superposition of the Michi- 

 gamme (Copps, etc.) upon the second granite (Presque Isle) ; and on 

 the north side of Lake Huron in the superposition of the Cobalt 

 series upon the second granite (Lorrain, probably the same as the 

 Killarney ) . 



Thus, when by hypothesis we correlate chronologically the pairs of 

 granites each to each in the various districts of the tabulation, we are 

 in reality correlating in every case a group of interconnected events of 

 the first magnitude comprising: 1. The invasion of the crust by 

 granite. 2. The uplift of the region. 3. The reduction of the region 

 to low relief. This of course greatly strengthens the hypothesis since 

 it increases the improbability of there having been a multiplicity of 

 chronologically different granitic invasions each with its induced uplift 

 and prolonged period of degradation. It also justifies the use of such 

 terms as Epilaurentian and Eparchean for subdivisions of geological 

 time, since no sediments can have accumulated in any of the districts 

 for a long period after each granitic invasion, these periods being re- 

 corded in terms of erosion. Geologists who are concerned only with 

 the sequence of rocks may dispense with these terms, but those who 

 have to deal with the subdivision and correlation of geological time 

 defeat their purpose if they ignore those large periods in which no 

 sediments are known to have been deposited. 



DEPARTURES PROM UNANIMITY 



It is interesting to note that for the most part the sequences and 

 correlations which are set forth in the tabulation are those which 

 are generally accepted ; and for the Canadian territory they represent 

 the same general conclusions as have been published by Miller and 

 Knight, 10 except that these writers have not recognized the dual com- 

 position of the Huronian system. There are, however, a few instances 



9 Collins, Geol. Survey of Canada, Mu3. Bull. 8 and 22, 19] (i. 

 i<>Ont. Bur. Mines, 22, 2, p. 126, 1914. 



