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University of California Publications in Geology [Vol. 10 



ruptive process connected presumably with magmatic differentiation. 

 In general, there is little danger of confounding the various mani- 

 festations of batholithic development, such as a sequence of plutonic 

 rocks from basic to acid, of one period with those of the other when 

 both are represented in the same field. Where the batholithic assem- 

 blage of only one period is represented in a particular field there may 

 be doubt ; but this may usually be resolved by an appeal to other means 

 of correlation with neighboring fields where granites of both periods 

 are represented. Moreover, it seems not improbable that purely petro- 

 graphieal distinctions between the granites of the two periods may be 

 recognized which will aid in their discrimination. 



APPLICATION OF THE HYPOTHESIS 



The justification of the hypothesis of two and only two granitic 

 assemblages in the post-Keewatin rocks of the region of the Great 

 Lakes is the fact that in the greater number of the districts repre- 

 sented in the accompanying tabulation there actually are two granites 

 widely spaced in time. The hypothesis states that the earlier granite 

 in every district was formed during one and the same general period of 

 time, and that the later granite belongs to another period, the same 

 for all districts. These two granitic assemblages have frequently been 

 confused by a reference of both of them to the Laurentian, but with 

 the increase of detailed studies their separation in time has become 

 apparent in many districts. 



Nortltwest of Lake Superior. — In my work on the northwest side of 

 Lake Superior in 1911 I was so impressed with the importance of this 

 separation that, in order to secure its recognition, and so do away with 

 the confusion which had arisen from the indiscriminate reference of 

 all granites to the Laurentian, I proposed that the latter term be re- 

 tained for the older assemblage and a new term, Algoman, be em- 

 ployed to designate the later assemblage. On the northwest side of 

 Lake Superior there are six separate and well distributed districts 

 which have been more or less carefully studied, and for each of which 

 we have the necessary data for the formulation of the geological 

 column in the proper chronological sequence. These are : Rainy Lake, 

 Steeprock Lake, Thunder Bay, Gunflint Lake, Vermilion Lake, and 

 the Mesabi Range. 



Running through all of these districts is a relatively persistent 

 series of formations comprising conglomerate, quartzite, graywacke, 



