GEOLOGY OF THE PARADOX LAKE QUADRANGLE . 485 



The typical appearance of the rock is massive, with a dark green 

 color. It has a tendency to weather far below the surface, and when 

 weathered the color changes to yellow or brown. It is often of 

 gneissic structure, and when this is the case the color often becomes 

 a dark gray. 



As shown on the map, the syenite occurs in several isolated areas, 

 the largest being in the northeast. Another smaller area is on the 

 eastern shore of Schroon lake, where syenite and granite grade into 

 each other without perceptible contact. 



In the southeastern syenite area, on the mountain 19 13 feet high, 

 locally called Trumbull (not the Trumbull of the map) there occurs 

 an interesting exposure of sedimentary gneiss in the midst of syenite. 

 It forms a small eastern spur of the mountain and is toO' large to 

 be a fragment torn off by the intrusion. It must represent an area 

 of the sediment yet in place, at the top of the intrusion and sur- 

 rounded by it. 



Anorthosite. As shown on the map, there is a large area of 

 anorthosite ini the northwestern part of the quadrangle. This area 

 marks the southern and eastern extension of the intrusion which 

 forms the main mass of the highest mountains in the Marcy region. 

 The topography of this section is more rugged, with higher moun- 

 tains and deeper valleys than the surrounding areas of gneiss, this 

 difference being probably not due to greater hardness in the rock 

 but to two dome-shaped uplifts. 



The name " anorthosite " has been often erroneously criticized 

 because of a supposed mistake in the first determination of the feld- 

 spar. It was not named from anorthite, but from " anorthose,'* 

 which was an early French name for all triclinic feldspars as opposed 

 to " orthose," for all monoclinic ones. Hence " anorthosite " means 

 literally " plagioclase rock." 



The anorthosite is typically a coarse grained rock of bluish or 

 greenish color containing large irridescent crystals of labradorite. 

 The dark silicates are usually gathered together in bunches, the pre- 

 vailing rock consisting of feldspar only. About the border of the 

 anorthosite area the rock is gneissic. As already mentioned, the 



