Il6 NEW YORK STATE MUSEUM 



the Grenville sea is preserved, one should seek remnants of it in 

 these " doubtful gneisses," which are sometimes referred to as the 

 Saranac formation. As many of these gneisses are certainly of 

 later age than the Grenville, they are discussed here. 



The Igneous Intrusives 



During this great period of erosion a series of molten rocks welled 

 up from the interior of the earth and slowly worked their way toward 

 the surface, invading the Grenville from below. The heat of these 

 semifluid masses was often sufficient to melt and engulf the surface 

 rocks and thus add them to their own mass. 



The first of these igneous rock intrusives was of great extent and 

 today forms the central core of the Adirondack mountains, some 

 1 200 square miles in area. It is known to the geologist as 

 " anorthosite." It is a rock composed to a large extent (80 to 90 

 per cent) of a particular member of the feldspar family known as 

 labradorite, because of its occurrence in Labrador. It is a dark, 

 grayish blue mineral that exhibits minute striations and infre- 

 quently displays a beautiful iridescent blue sheen. Often in the 

 running brooks these labradorite crystals of the anorthosite flash 

 their characteristic color to the eye as a greeting. Chemically, it 

 is a silicate of soda, lime and alumina. Besides the labradorite, 

 the anorthosite contains various members of the pyroxene group of 

 minerals, besides magnetite, commonly known as the black oxide 

 of iron and several other minerals. 



Evidently this intrusive mass of molten rock, buried under the 

 great thickness of surface strata, solidified very slowly for the entire 

 rock is coarsely crystalline, labradorite crystals as large as a man's 

 hand often being encountered. This anorthosite was subsequently 

 exposed by the erosion of the surface rocks, but in all probability 

 it did not at first reach the surface of the land. The anorthosite is 

 the rock encountered on all sides of Lake Clear. A ledge of it is 

 exposed at the water's edge in front of Lake Clear Inn. Although 

 it has been subjected to fr.'ghtful pressure and heat subsequent to 

 its intrusion, the rock here does not show the effects of this so much 

 as that found farther to the east, where it appears as a bluish white 

 rock spotted with green silicates. An excellent locality to examine 

 the anorthosite is north of the road three-fourths of a mile west of 

 Paul Smith's, where an extensive quarry has been opened. Several 

 small veins carrying " fool's gold " (pyrite) cut through the rock. 



The next member of this series is represented by a rock called 



