6o NEW YORK STATE MUSEUM 



but numerous interbanded sheetlike masses of basic material alter- 

 nating with the garnet gneiss throughout the whole extent of the 

 latter formation, undoubtedly function in its stead. 



Near the eastern margin of the northerly limb is a narrow zig- 

 zag xenolith of light grayish garnet gneiss which occupies the middle 

 angle of a W-shaped fold whose structural elements are nearly 

 parallel to those of the big sigmoid. This fold is evidently of very 

 minor consequence in the general structure of the amphibolite zone, 

 and measures, in its longest dimension, north-south, about seven- 

 tenths of a mile. The axial plane strikes east-west and dips north 

 about 30 degrees. This is seen at the southernmost angle of the 

 fold, where the garnet gneiss doubles back upon itself through a 

 very acute fold within the apex of which the crenulations are 

 intense. Farther west, near the middle of the W, the laminae of the 

 gneiss are extremely contorted, and show characteristic minute 

 puckerings which pitch in the same direction as the acute fold but 

 at a higher angle. 



The xenolith then bows round through a comparatively broad arc 

 into a northeast strike and northwest dip, and at the end of this 

 limb, the garnet gneiss having pinched out or become covered up, 

 the inclosing amphibolite assumes a vertical position and gradually 

 alters its course to the north and north-northwest, finally striking in 

 the direction of Waterman hill drumlin. This latter fact favors the 

 view that the amphibolite of the sigmoid area is merely a continua- 

 tion of that occurring north of Waterman hill in Boyden brook and 

 westward ; a view which is further substantiated by the northwest 

 and north-northwest strike of the adjoining Grenville and granite 

 formations at and near the east corner of Canton township. 



Perhaps no better indications of the igneous origin of this sigmoid 

 amphibolite can be found than the general features of its areal dis- 

 tribution with respect to the garnet gneiss : its heavy development 

 at the western flexure of the sigmoid, its apparent inclosure of 

 numerous large and small fragments of gneisses of distinct Gren- 

 ville aspect, its probable connection with the Pyrites amphibolite, 

 which is assuredly of igneous origin, its own rare transition to a 

 massive pyroxenic phase, and its fraying away laterally as well as 

 transversely into a great number of thinner masses which have pro- 

 duced, apparently, a typical injection gneiss of coarse texture with 

 the garnet rock. These features on the whole weigh heavily in 

 favor of regarding most if not all of the sigmoid amphibolite as of 

 ultimate igneous origin. 



