THE PRECAMBRIAN ROCKS OF THE CANTON QUADRANGLE 59 



petrographic evidence afforded by the few residual related gabbroid 

 patches, seem sufficient to stamp these hornblendites as in large part 

 post-Grenville intrusives. 



Throughout the western half of the sigmoid the structural and 

 genetic relations of the amphibolite, garnet gneiss and granite appear 

 to be the same as in the east, where these features are more satis- 

 factorily exhibited. At least nothing was observed in the western 

 half which would discredit the interpretations suggested in the vi- 

 cinity of Pierrepont. The basic rock is found over a much larger 

 proportion of the area, occupying both borders of the fold, and at 

 the point of flexure northeast of North Russell nearly cuts out 

 the heavy development of garnet gneiss which is elsewhere of such 

 importance. At this point, however, the garnet gneiss is not en- 

 tirely absent, but occurs in smaller masses and shreds of elongated 

 lenticular character, presumably developed by the fretting away of 

 the mass of included country rock by the basic intrusive. Some- 

 times the xenoliths are of dark gray, highly feldspathic, nongar- 

 netiferous character, suggesting subsequent alteration or injection 

 by acid material during the later granitic intrusion. 



The center of the north limb of the isocline is occupied by a large 

 peninsulalike mass or xenolith of nearly normal garnet gneiss which 

 frays into or is replaced by the amphibolite at its southern end. 

 The opposite end of this mass disappears under the heavy drumlin 

 of Waterman hill, and its structural connection (or lack of it) with 

 other Grenville of the vicinity can not be observed ; nevertheless, 

 judging from the basic character of the surrounding rocks, and the 

 tectonic features described in a succeeding paragraph (see page 

 64), it is probable that it is a xenolith of abnormally large dimen- 

 sions. Small dikes or sheetlike masses of amphibolite have pene- 

 trated this xenolith at numerous points parallel to the foliation and 

 have afterwards been pinched off into blunt and flattened lenslike 

 bodies precisely as in other parts of the sigmoid. 



It is to be observed that south of the Beach Plains Church the am- 

 phibolite itself frays away in the midst of the main body of garnet 

 gneiss which forms the middle limb of the sigmoid. West of this 

 general locality, the basic intrusive has formed a very prominent, 

 uninterrupted zone bordering the outer part of the fold, containing, 

 besides an occasional xenolith of Grenville gneiss, an abundance of 

 injected granitic material of later origin. East of the point men- 

 tioned this thick continuous zone of amphibolite is not to be found, 



