440 A. A. JLLIEN AMPHIBOLE SCHISTS OF MANHATTAN ISLAND 



ish green ; a, brownish yellow. Maximum extinction angles, 17 degrees, 

 C f\ c. Minute inclusions of epidote rodlets common. 



In some slides plagioclase feldspar amounts to 10 or 20 per cent of the 

 rock, displaying polys3aithetic twinning after the albite, and in part the 

 periclinelaw ; the maximum extinction angles, on sections normal to the 

 albite twinning, point to oligoclase. In others orthoclase may be the 

 only feldspar, often the predominant one, up to 35 per cent of the rock, 

 always allotriomorphic, showing distinct, almost rectangular cleavage 

 traces, and generally clouded by decomposition, passing partly or wholly 

 into a fine micaceous aggregate. 



Quartz always reaches from 15 to 30 percent, in clear, colorless grains, 

 whose outlines show rounding in parts; the few inclusions comprise 

 scales of light greenish amphibole (actinolite), needles of colorless ap- 

 atite and minute zircons, but no fluid cavities. 



A few grains of reddish garnet occur, sometimes reaching 5 per cent 

 of the rock ; the larger are full of rounded inclusions of quartz. Other 

 accessories are scales of orange to brownish yellow biotite, attached to 

 edges of hornblende ; plates of iron-black menaccanite, sometimes in 

 comblike or netted aggregates; minute cr3^stals of pyrite, more or less 

 decayed, and films of reddish iron oxide. 



In the epidotic variety of the gneiss the amount of that mineral may 

 rise to 15 or even 35 per cent of tlie volume, in prismatic fragments, 

 sometimes including menaccanite plates ; also in rods and rounded gran- 

 ules, oval and pear-shaj^ed. Feeble pleochroism, yellowish to colorless. 

 Extinction parallel to elongation, but occasionall}^ reaching 33 degrees. 



Zoisite may also occur, commonly inclosed within the hornblende, in 

 colorless, rounded rods or prisms, with high relief. 



The occurrence of hornblende in the gneisses of our series has been re- 

 garded usually as only a varietal form, a mere accident of molecular 

 arrangement due to excess of certain bases in the prevailing biotitic 

 gneiss. This view has been thus expressed : 



" Hornblende, anthophyllite and other masses . . . they are simply differ- 

 ent conditions of the same elementary materials as the gneiss, merely different 

 forms of metamorphism." * 



But the common substitution of these hornblendic gneisses along the 

 strike for outcrops of hornblende schist, or across the strike on opposite 

 sides of a fold — the identity of their field phenomena with those of that 

 schist, intricate foldings, epidotic alteration, etcetera — their frequent in- 

 tercalation with or actual passage into that schist — all such facts imply 

 that they represent transition forms of an original hornbleiidic matrix, 



*R. P. Stevens: Ann. Lye. Nat. Hist, N. V., vol. viii, ISGy. p. 116. 



