SUBDIVISIONS OF THE LAURENTIAN SYSTEM. 187 



of different generations of Laurentian rocks. This possibility presents itself 

 as soon as we familiarize ourselves with the sub-crustal igneous and later 

 formations of the Laurentian. 



Different Generations of Laurentian Rocks. — To the writer this conception 

 of different generations has never been more than a possibility till the present 

 year. In his report on the Rainy lake region, two broadly distinct mem- 

 bers of the Laurentian were distinguished, lithologically and on account of 

 their systematic relative distribution, as the " peripheral zone " and " inner 

 nucleus " of the Stanjikoming area, the former being composed chiefly of 

 hornblende-granite and syenite-gneiss, and the latter of very quartzose 

 biotite-gneiss. The relationship in time between these two rock masses re- 

 mained indeterminate. During the past summer, however, he has been able 

 to establish, in the Hunter's island region, chronologically distinct genera- 

 tions of Laurentian gneisses. In that region there are two broadly distinct 

 members of the Laurentian, analogous petrographically and in relative dis- 

 tribution to those of the Stanjikoming area. Below the Keewatin rocks 

 there is a great mass of hornblende-granite-gneiss, which presents an irrup- 

 tive or intrusive contact against them. Towards the central part of Hun- 

 ter's island this hornblende-gran ite-gneiss is pierced by an enormous irrup- 

 tion of biotite-granite, which is sometimes very distinctly gneissic and 

 sometimes quite undifferentiated in structure. In texture it varies from 

 fine-grained, almost micro-granitic, to a moderately coarse granite. This 

 biotite-granite-gneiss traverses the hornblende-grauite-gneiss in innumerable 

 clearly defined dikes cutting it in all directions, and holds innumerable in- 

 cluded blocks of the same rock. It comes up from beneath the hornblende- 

 granite-gneiss, and is unquestionably of later age. 



Thus we have in this area at least two distinct generations of Laurentian 

 rocks, both the result of the crystallization of a sub-crustal magma. At the 

 time of the second generation the rocks of the first generation constituted 

 the lower portion of the crust. 



It is upon the recognition of facts of this order that an intelligible and 

 profitable classification of the Laurentian rock masses and the geological 

 events which they represent must be established. 



Other Conditions considered. — The relationship which has been found to 

 obtain between the upper and lower Archean leads, as has been said, to a 

 conception which is at once grand and simple. So long as we confine our- 

 selves to regions like that northwest of Lake Superior, where no great com- 

 plications have been introduced by post-Archean crust-crumpling agencies, 

 it affords a full explanation of all the phenomena of Archean geology. 



There is a possible simpler case which would still present the essential 

 conditions of the relationship in question ; i. e., the case in which the sub- 



XXV— Bull. Gkol. Soc. Am., Vol. 1, 1889. 



