REPORT OF THE DIRECTOR I916 25 



cutting the limestone. A notable instance of the sort occurs at 

 Halls Corners (Rock Island School). 



As is the rule in northern New York, the Precambrian rocks 

 comprise the sedimentary Grenville series and various later intru- 

 sives. Though the igneous rocks occupy somewhat more than 50 

 per cent of the area of the quadrangle, it is nevertheless probably 

 true that the Grenville rocks have greater areal extent here than 

 in any other quadrangle in northern New York. Particularly 

 impressive is the great belt of Grenville limestone which stretches 

 all the way across the quadrangle from northeast to southwest, 

 much of it very pure limestone. Toward the northeast it becomes 

 impure and belts of other Grenville rocks wedge into it; but along 

 the west margin of the quadrangle it has a breadth of some 8 miles, 

 5 miles of which is pure limestone. As Smyth long ago pointed 

 out, it is the longest and broadest belt of Grenville limestone in 

 New York. A large quarry industry has long been based upon it. 



The larger part of the impure limestone of the quadrangle con- 

 sists of alternating beds of limestone and quartzite. Where the 

 quartzite bands are few and thin the stresses to which the rocks 

 have been subjected have resulted in the fracturing of the brittle 

 quartzite bands and the indiscriminate mingling of the fragments 

 with the general mass of the limestone, giving a rude resemblance 

 to a conglomerate. Such pseudo-conglomerate alternates with 

 thicker bands of quartzite. One very prominent band of alter- 

 nating limestone and quartzite enters the quadrangle from the 

 south at Sylvia lake and runs east-north-east to Edwards. It is of 

 especial importance because of the talc deposits that occur within 

 it, and a line of openings for this mineral marks the course of the 

 belt across the quadrangle. Because of this it has been given a 

 separate coloration upon the map ; at the same time it must be 

 understood that there is always some quartzitic material in the 

 belts which are mapped as limestone. 



Aside from the limestone and the quartzite the Grenville rocks 

 comprise a great variety of schists, hornblende schists, mica schists, 

 red to. green calcareous pyroxenic schists, garnetiferous gneisses 

 and pyritous gneisses, together with many other less common 

 varieties. For the most part these varieties of gneiss and schist 

 are sufficiently distinct and of easy enough identification to war- 

 rant their separate mapping were it not for the fact that they com- 

 monly occur interbanded with one another in such thin beds as 

 wholly to preclude their mapping, except upon a very large scale. 



