GEOLOGY OF THE WEST POINT QUADRANGLE, NEW YORK I25 



quartzitic sediments of that new order lie in strictly unconformable 

 relation upon them. 



Summary of the Highlands-Adirondack correlation. It is on the 

 whole a very striking correspondence that one finds in comparing 

 the Precambrian geology of the Highlands with that of the Adiron- 

 dacks. Every large historical or sequence division of the more 

 fully studied Adirondacks has been either identified or strongly 

 suggested in the Highlands. This is all the more striking because 

 the West Point area was studied quite independently of any such 

 consideration, the whole series of formations and historical sequence 

 having been formulated before any attempt was made at a critical 

 comparison. 



In the attempt to correlate the Precambrian of the Adirondacks 

 and the Highlands, thin sections of some of the typical Adirondacks 

 granites were studied. The granite-syenite series of the Adirondacks 

 is very similar in most respects to the Storm King granite of West 

 Point. The points in common are : 



1 Medium coarse-grained texture. 



2 Gneissoid structure. 



3 Abundant microcline and microperthite, presence of pleochroic 

 dark-green or brown hornblende, and (or) green slightly 



pleochroic pyroxene, and (or) brown biotite, as essential 

 minerals. 



4 Apatite, titanite, zircon and magnetite as accessories. 



5 Comparatively low quartz content. 



6 Absence of evidence of strong dynamic metamorphism, but 



strain shadows in quartz. 



7 Rehealing of fine crush effects in feldspar with quartz. 

 They differ chiefly in the following respects : 



■ I The greater abundance of pyroxene in the Adirondacks syenite. 



2 The greater alteration of the Adirondacks syenite. 



The white Laurentian granite of the Adirondacks resembles the 

 Canada Hill granite of the Highlands. Both have the same medium 

 fine texture and granular structure, slightly gneissoid by reason of 

 the orientation of the biotite. The essential minerals are quartz, 

 orthoclase, oligoclase and biotite with slight microcline, microperthite 

 and garnet. Garnet is more abundant in the Highlands. 



The general appearance of the rock caused by the interlocking 

 arrangement of the quartz and feldspars, the small lath-like crystals 

 of brownish, bleached biotite containing rutile inclusions and the 

 small faintly pink garnet are difficult to describe, but are very 

 characteristic and readily recognized. 



