PETRO GRAPHICAL PROVINCE OF ESSEX COUNTY 285 



The hornblende forms either stout prismatic crystals or irregular 

 grains. In the latter case it is generally, but not constantly,, 

 later than the plagioclase and interstitial to some extent. It is 

 a peculiar yellow-brown, closely similar to the barkevikite of 

 the Norway rocks, as shown by comparison with sections of 

 these. The pleochroism is strong: for the axis nearest c, dark 

 yellow-brown ; parallel to axis b, about the same, nearly parallel 

 to axis a, light greenish-yellow. The depth of the color ren- 

 dered the determination of the axes of elasticity uncertain. The 

 extinction is rather high, about 20 degrees. This hornblende 

 frequently occurs as a primary border about augite and magne- 

 tite, as in the hyperitic diorites. The pyroxene, which forms 

 irregular grains or occasionally large porphyritic crystals, is 

 colorless, with sometimes a tinge of green or violet. Olivine is 

 rare, except in one specimen from Coney Island, in large cor- 

 roded colorless grains. They are usually altered at the borders 

 to a black granular substance, and are occasionally serpentinized. 

 The plagioclase, which occurs in long lath-shaped sections, tabu- 

 lar parallel to b (010), as well as in anhedra, is highly twinned. 

 Measurements by Michel-Levy's method indicate a labradorite 

 with the composition Ab 3 An 4 , though some are nearer the 

 andesines. Small colorless interstitial grains with low refractive 

 index are referred to orthoclase. 



Si0 2 



Ti0 2 - 



A1 2 3 - 



Fe 2 3 



FeO 



MnO - 



MgO - 



CaO - 



Na 2 - 



K,0 --- - 



H 2 0(uo°) - 



H 2 0(ignit.) 



P 2 5 - 



CO, - 



100.29 100.58 100.60 99.48 



