42 XEW YORK STATE MUSEUM 



apparently broken and detached fragments of a more siliceous belt. 

 which were scattered about in the limestone matrix by crustal move- 

 ments. In all other cases on the map. the formation includes pre- 

 dominantly siliceous or feldspathic gneisses, more or less micaceous : 

 where minor types are included in this pattern, it is because the 

 units either are too small to map on the scale adopted, or are not 

 deemed of sufficient importance to warrant their being given sepa- 

 rate boundaries. 



The remaining Grenville rocks of the quadrangle, though of 

 smaller areal extent, and to be looked upon as hardly more than 

 local developments of certain phases of the types already mentioned, 

 are nevertheless of equal scientific interest with the others. Such, 

 for example, are the massive steeply-dipping vitreous quartzites in 

 the vicinity of the Grass valley north of Pyrites village : the thin- 

 bedded, often highly crenulated quartz schists west of Martin's 

 corners and vrest of North Russell : the quartz-mesh limestones 

 northwest of the latter village and in the valley of \'an Rensselaer 

 creek, and the pyritous gneisses of more general distribution in the 

 southern part of the sheet. These will now be described in more 

 detail. 



PYRITOUS GNEISS 



The term pyritous gneiss is here employed only for those rusty- 

 weathering gneisses which contain sufficient pyrite to allow of 

 the development of a fairly well-marked gossan. There is thus 

 indicated a certain looseness in the use of this term which is quite 

 in accord with the variations in the character of the formation in the 

 field. The chief occurrences of rock are to be found (T ) a short 

 distance east of the Canton-North Russell road, midway between 

 these two villages, '(2) southwest of the Little River crossroads, 

 (3) on the inner border of the east half of the Pierrepont sig- 

 moid, and (4) at various points south and southwest of Pyrites, 

 particularly the ore-bearing stratum in the gorge of the Grass river. 



The former is a stratum with a maximum thickness of not over 

 2^ feet underlying the rather- low-dipping limestone at this point. 

 The best exposures are east of the road, one on either side of the 

 little brook which has its rise in this vicinity. The rock has here 

 weathered deeply to a bright buff to dark rust colored gossan. The 

 content of pyrite decreases markedly southward, west of the road, 

 but the rusty gneiss continues to underlie the edge of the limestone 

 syncline at least as far as the south border of the latter. 



At the second locality, near Little River, the outcrops are scat- 

 tered and it is impossible to state whether they belong to one contin- 

 uous but distorted stratum, or to several smaller independent layers. 



