182 Canadian Record of Science. 



interstratified in the gneiss at the southeast corner of the 

 anorthosite area. 



These facts together with the whole shape and charac- 

 ter of this anorthosite area now that the mapping is com- 

 pleted, show that as Logan supposed, it is unconformable 

 to the Grenville series, that is to say to the true Lauren tian. 

 But it may also be demonstrated that this unconformity is 

 not due to superposition but to intrusion. The anorthosite 

 does not belong, as was supposed, to a great overlying sedi- 

 mentary formation, but is a great intrusive mass which cuts 

 through gneisses with their associated limestone bands but 

 does not overlie them. 



In order to understand why Logan and other able ob- 

 servers who agreed with him, regarded these anorthosites as 

 an overlying sedimentary formation, we must remember 

 that they show here and there a more or less foliated 

 structure. This is especially true of some places near their 

 contact with the gneiss, and is best seen in the long arm- 

 like extension at the south-east corner which following the 

 line of least resistance penetrates the gneiss parallel to its 

 foliation, and together with it is covered up by the over- 

 lying Cambrian. Moreover we find at St. Jerome a small 

 isolated occurrence of a more or less clearly foliated anor- 

 thosite, which is included in the gneiss, and Logan, who 

 through the lack of time could not examine the whole area, 

 supposed this to belong to the great Morin mass, the southern 

 boundary of which was as a matter of fact many miles 

 farther to the north. In going from Si Jerome, therefore, 

 at right angles to the strike of the rocks, to New Glasgow, 

 which lies about nine miles further to the east, ho passed 

 from the gneiss over an interstratified anorthosite, then over 

 gneiss with layers of quartzite and a limestone bed into 

 the above mentioned arm-like extension of anorthosite, 

 which shows a sort of schistosity parallel to the strike of the 

 gneiss, and over this to gneiss again. Misled by this sec- 

 tion which here is very deceptive, he decided that the whole 

 was a great sedimentary formation of gneiss with interstra- 

 tified beds of quartzite, limestones and anorthosites identical 



