GEOLOGY OF THE WEST POINT QUADRANGLE, NEW YORK 47 



h Sedimentary and organic rocks (much metamorphosed). 

 This class is represented in the southeast quarter of the quadrangle, 

 by the Manhattan-Inwood series, and by a strip of Grenville along 

 the Hudson river, with occasional other smaller patches. It includes 

 thoroughly crystalline limestone or marble, recrystallized quartzite, 

 and schists of great variety — chiefly mica schists, quartz-mica 

 schists, and hornblende schists, all of which blong to the Manhattan- 

 Inwood-Lowerre series. 



Gneisses of the greatest possible variety and associated schists 

 and rocks derived from limestones are represented in the Gren- 

 ville series. They include coarsely crystalline marbles, silicated 

 limestones of great variety, banded gneiss and schistose rocks. It 

 is impossible in this case to draw the line sharply between the sedi- 

 ments that have been metamorphosed by the simpler processes and 

 the contact metamorphic products of complex origin. As a matter 

 of fact, they are close associates. 



c Contact and metamorphic products of complex origin. This 

 class includes limestones that are thoroughly silicated and repre- 

 sent typical contact influence and rocks that have been impregnated 

 and injected by igneous materials. These rocks have been discussed 

 in the earlier portions of this chapter. The products are chiefly 

 gneisses of great variety in quality and composition and structure. 

 Garnet is often developed and doubtless many of the constituents 

 are combination products representing neither the original materials 

 nor the introduced materials alone. In this class is still greater 

 variety in minor ways than in any other of the groups listed, but 

 all have the complex metamorphic type of origin. 



This class is represented by the banded and streaked rock common 

 in the Highlands and is the basis of the term often used — the High- 

 lands gneisses. It is still more typical of the Fordham gneiss, which 

 is the name used in the New York City area. 



d Deformation products. Along lines of weakness or deforma- 

 tion zones, particular qualities are developed which are found nowhere 

 else in the region. These are directly the products of deformation 

 and include shear schists and granulation gneisses. The particular 

 quality depends on the rock originally involved and on the extent 

 of the deformation and the balance between purely mechanical 

 effects and recrystallization. There is, of course, no great areal 

 extent of such products and none is mappable, but they form beauti- 

 ful petrographic varieties of much interest. 



