12 NEW YORK STATE MUSEUM 



Pegmatite dikes of wholly nonmetamorphosed material are 

 occasionally present throughout the quadrangle. 



Diabase, in the form of numerous nonmetamorphosed dikes, is 

 the latest of the Precambrian rocks. 



Paleozoic and Mesozoic rocks are entirely absent, but glacial 

 deposits are widespread and varied, especially in the valleys. 



Faulting appears to have played a much less important part than 

 usual in the eastern Adirondacks, only a few fault zones having 

 been observed, the principal one passing through the Wilmington 

 notch. 



THE PRECAMBRIAN ROCKS 



Granville Series 



General statements. Among all the rocks of the quadrangle, 

 those which comprise the Grenville^ series are the most ancient. 

 They rank among the very oldest known rocks of the earth's crust. 

 It is certain that by far most of the Grenville rocks are of sedi- 

 mentary origin, though all are now metamorphosed and thoroughly 

 crystalline. In most cases the stratification surfaces are still plainly 

 visible and these often separate rock layers of sharply varying com- 

 position. Various bedded gneisses, schists, quartzites and crystal- 

 line limestones almost, if not entirely, constitute the Grenville 

 series, the original rocks having been shales, sandstones and lime- 

 stones of the usual kinds. The presence of numerous flakes of 

 graphite (so-called "black lead") scattered through many of the 

 Grenville rocks also strongly indicates their sedimentary origin. 



Grenville strata are common throughout the Adirondack moun- 

 tain region and also in eastern Ontario. Their thickness is known 

 to be very great — a few miles at the very least — with neither top 

 nor bottom of the series definitely known. Since the Grenville 

 strata throughout the Adirondacks have been all cut to pieces by 

 vast intrusions of igneous rocks, their present distribution is very 

 patchy, and the irregular scattering areas now visible are merely 

 remnants of what was once a continuous body of strata covering 

 not only all of northern New York, but also large adjacent areas, 

 particularly eastern Ontario. 



Within the Lake Placid quadrangle the Grenville strata are less 

 abundant than usual throughout the Adirondacks. As shown on 

 the accompanying geologic map, definitely known areas of Gren- 



* This name has been given from the town of Grenville in the St Law- 

 rence valley. 



