THE PRECAMBRIAX ROCKS OF THE CANTOX QUADRANGLE 6l 



GABBRO-AMPHIBOLITE; PYRITES OCCURRENCE 

 Although the structural relations of the Pierrepont amphibolites 

 are so extensively veiled that the true nature o*f the rocks is difficult 

 to determine, the case is otherwise with the belt of hornblendic 

 schist which stretches from Boyden brook westward and southwest- 

 ward to the corner of the quadrangle. This formation can be traced 

 as an almost continuous strip from the one extremitv to the other, 

 attaining its maximum breadth of nearly a mile in the vicinity of 

 Brick Chapel, and narrowing to its minimum of several hundred 

 feet as it bows round the northern point of the subtriangular boss 

 of granite east of Pyrites. Thence, gradually increasing again in 

 breadth, it finally unites with the triangular area of gabbro-diorite 

 and derivative xenolith-bearing amphibolite lying between Pyrites 

 and Harrison creek, after which it again rapidly narrows to its mini- 

 mum breadth, and assumes its normal northeast-southwestward 

 course. 



The Pyrites gabbro is described by Professor Smyth (1912, pages 

 161-62) in connection with the origin of the St Lawrence county 

 pyrite. That this dark gray to black, massive to gneissoid, coarsely 

 crystalline rock is a true gabbro, can hardly be doubted, for abund- 

 ant hypersthene and augite are found in several localities where it 

 preserves a more massive aspect. Such favorable places are in 

 reality, however, not very frequent, and are confined, so far as the 

 writer is aware, to the ledges east of the road running south from 

 Pyrites, and about one-fourth of a mile from the upper bridge. The 

 rock here is dark greenish black in color, tough, massive, and medium 

 to coarse-grained ; on a fractured surface it is characterized by the 

 abundant glistening cleavage surfaces of the quarter-inch horn- 

 blende crystals which make up the greater part of the rock. The 

 microscope shows it to be composed typically of andesine, pleo- 

 chroic augite, and hornblende, with accessory biotite, magnetite, 

 pyrite and apatite. The hornblende is sometimes seen to form a 

 zone round the augite (see plate 9, lower figure), and at other 

 times occurs as small shreds scattered irregularly through the 

 pyroxene in such a way as to give strong ground for believing it 

 to be derivative from the augite. ^lost of the hornblende cr)-stals, 

 however, are not so associated, and it is- impossible to determine 

 what proportion of this mineral is original in the gabbro, and how 

 much of it is merely altered pyroxene. The same is true of the 



