GEOLOGY OF THE BROADALBIN QUADRANGLE II 



microcline; lo per cent microperthite ; 5 per cent plagioclase; 25 

 per cent quartz; 5 per cent garnet; and a very little zircon. The 

 rock is pretty well granulated and is perfectly white except for an 

 occasional large, light amethyst-colored garnet which stands out 

 prominently in the white matrix. 



Igneous rocks in the Grenville. Closely involved with the true 

 Grenville sediments are occasional masses of what appear to be un- 

 doubted igneous rocks. A fine illustration occurs on the mountain 

 side one and one-half miles due south of Batchellerville where there 

 is a considerable belt (about ten feet wide) of very porphyritic, 

 dark, hornblendic, thoroughly gneissoid rock included within typi- 

 cal Grenville and parallel to its foliation. The thin section shows 

 20 per cent plagioclase (oligoclase to labradorite) ; 10 per cent 

 orthoclase; 50 per cent hornblende; 10 per cent biotite; 8 per cent 

 quartz ; 2 per cent magnetite ; and a little apatite and zircon. It is 

 holocrystalline and has the composition of a quartz diorite. The 

 phenocrysts are sharp edged crystals of feldspar often an inch long 

 and mostly arranged parallel to the foliation but some are at vari- 

 ous angles. 



Another igneous looking rock occurs in the Grenville toward 

 the top of Bald Bluff. This rock is very dark grey, medium grained 

 and distinctly gneissoid. It contains 10 per cent orthoclase; 20 per 

 cent plagioclase (chiefly labradorite but with some andesine) ; 30 

 per cent hypersthene (brown pleochroic) ; 20 per cent green horn- 

 blende ; and 20 per cent biotite. Still another rock from two miles 

 northeast of Northville is similar to this except that it is richer in 

 feldspar and hypersthene and lacks the biotite. These last two rocks 

 have the composition of hypersthene gabbro or norite. 



The examples above cited are illustrations of others which have 

 been noted in the field and the composition, texture, and field ap- 

 pearance all strongly argue for their igneous origin. The texture 

 and field relations show the intrusive character of the rocks and 

 that the intrusions probably took place practically parallel to the 

 bedding planes of the sediments. The gneissoid structure proves 

 that the intrusions must have occurred well before the cessation 

 of the dynamic forces of compression which developed the foliation 

 of the Grenville. On the other hand these rocks are older than the 

 nonmetamorphosed dike rocks below described. 



Areal distribution of the Grenville. The Grenville is by far the 

 most widespread formation of the quadrangle, its areal extent of 

 over sixty square miles being much greater than that of all the 

 other Precambrics combined. In addition to this the Grenville is 



