THE PRECAMRRIAN ROCKS OF THE CANTON QUADRANGLE 3/ 



minutely between the garnetiferous and the nongarnetiferous 

 gneisses. Nevertheless the rocky upland south of Pierrepont, with 

 the exception of the peripheral granite-amphibolite zone, should be 

 designated without hesitation as a belt of garnet gneiss. 



The widespread resemblance of this formation to the thinner- 

 bedded sedimentary gneisses of the south-central part of the sheet 

 is of important genetic significance and suggests, in the absence of 

 any demonstrable instrusive relationships on the part of this forma- 

 tion, that it also is of Grenville age. Its intimate association, espe- 

 cially in its eastern part, with interbedded and infolded limestone 

 (that of Van Rensselaer creek) and pyritous gneiss (following 

 closely the inner margin of the formation) is not taken in itself as 

 proof of the sedimentary character of the garnet gneiss, but con- 

 sidered in connection with the similarity of composition above 

 referred to, is believed to lend support to this view. In respect to 

 this association, it is to be observed that a true igneous rock, such 

 as the Pyrites gabbro-diorite and its derived amphibolite, can and 

 does have the same type of relationships with its included pyritous 

 gneiss and its adjoining limestone, as are observed in the case of the 

 sigmoid garnet gneiss in the vicinity of its eastern flexure. 



The formation possesses a somewhat unusual interest because of 

 its having been the seat of two distinct post-Grenville intrusions, an 

 earlier gabbro followed by a later granite. The evidence bearing on 

 the igneous origin and relative age of these formations is given on 

 a later page (see pages 73-78), but it is desirable here to make 

 mention of some of the general features produced by the intimate 

 injection of these magmas into the garnet gneiss country rock. In 

 the eastern half of the fold, the major part of the igneous material 

 is confined to the outer margin, where the garnet gneiss is practically 

 absent except for an occasional included strip. The inner four-fifths 

 of the belt are predominantly garnet gneiss, but with notable 

 amounts of interbanded amphibolitic material. In the western half, 

 both margins of the fold are occupied by broad belts of igneous 

 rock, which join at the point of flexure, and cut out the intervening 

 strip of garnet-gneiss, or reduce it to occasional small detached 

 patches of xenolithic character. Throughout the extent of the sig- 

 moid area, the interbanded strips of amphibolite and granite are 

 parallel to the borders of the fold, and with two exceptions noted 

 later (pages 54-57) they are all alike in yielding no satisfactory 

 evidence as to whether they are xenoliths or dikes in the garnet 

 gneiss. 



Whenever the terminations of the amphibolite masses are ob- 

 served, they are seen to be bluntly lenticular, and the foliation of the 



