THE PRECAMBRIAN ROCKS OF THE CANTON QUADRANGLE 2"] 



The garnet gneisses belonging to the second class, mentioned 

 on page 24, whose peripheral association with areas of granite 

 gneiss has given cause for the suspicion ' that they are con- 

 tact aureoles, are found in the general vicinity of Pyrites. The 

 prominent granite upland east of this village is bordered by a zone 

 of garnetiferous rock, 20 to 50 feet broad, which begins at the east- 

 ern extremity near Little River, and is continuous half way round 

 the boss, to the head of the reentrant, with a possible interruption 

 at the point nearest Pyrites. At the northeast end of the reentrant, 

 the garnet gneiss describes a group of remarkable curves, and where 

 it can be observed with certainty to be present elsewhere, possesses 

 the invariable feature of being on the border between the Grenville 

 and the granite throughout the extent indicated. 



Other similar peripheral occurrences are to be found at the east 

 and west extremities of the body of granite on the margin of the 

 sheet southwest of Pyrites ; but in this case the garnet gneiss does 

 not follow the border of the granite upland continuously as in the 

 previous example; it is wholly absent except at the places men- 

 tioned, and at the northernmost point of the granite where its mass 

 is too small to be indicated on the map. 



The idea that these garnet gneisses are of the nature of contact 

 zones, produced by the action of intrusive granite upon Grenville 

 limestone, is attractive. While certain points of the field evidence 

 seem to substantiate this, it must be concluded, however, that on 

 the whole the proof can not at present be definitely established. 

 The chief reasons for which this zone might be supposed to be of 

 contact origin are that it forms in the case first cited, an almost 

 complete halo, and that none other than a contact rock could so 

 closely follow the sinuous curves of an intrusive body ; further, that 

 whereas the garnet gneiss is abundant on the edge of the granite, it 

 is not known to occur anywhere in the interior. Nevertheless the 

 other side of the question appears to be more convincing. In 

 spite of the fact that the garnet gneiss is peripheral to the southern 

 half of the granite, there is no such halo present on the northern 

 border, although here also, so far as can be observed, limestone is 

 the continuous country rock. Garnetiferous gneiss is not known to 

 occur anywhere on the north or west edges of this granite area ; the 

 intrusive possesses no noticeable difference in composition on its 

 north and south sides, so that it is difficult to conceive on this score 

 how a prominent contact zone of such persistence could be produced 

 on the one side and fail utterly on the other. It would be necessary 

 to assume a local segregation, along the southern margin, of such 



