﻿Plate 
  6 
  

  

  J. 
  Miller, 
  photo 
  

  

  Upper 
  figure. 
  Photomicrograph 
  of 
  thin 
  section 
  of 
  gabbro 
  from 
  the 
  top 
  

   of 
  Hackensack 
  mountain. 
  Magnified 
  15 
  diameters. 
  The 
  central 
  mineral, 
  

   with 
  good 
  cleavages 
  nearly 
  at 
  right 
  angles, 
  is 
  diallage 
  which 
  is 
  almost 
  

   completely 
  surrounded 
  by 
  a 
  zone 
  of 
  hornblende 
  (darker 
  colored). 
  As 
  

   seen 
  under 
  the 
  microscope 
  the 
  rim 
  is 
  very 
  clearly 
  defined. 
  The 
  light 
  gray 
  

   minerals 
  are 
  basic 
  plagioclase. 
  The 
  diabasic 
  texture 
  of 
  the 
  rock 
  is 
  well 
  

   shown. 
  

  

  Lower 
  figure. 
  Photomicrograph 
  of 
  a 
  thin 
  section 
  of 
  diabase 
  from 
  the 
  

   dike 
  one 
  and 
  one-half 
  miles 
  southeast 
  of 
  Johnsburg. 
  Magnified 
  15 
  diameters. 
  

   The 
  long, 
  slender 
  prisms 
  with 
  transverse 
  cracks 
  are 
  augite; 
  the 
  smaller, 
  

   more 
  numerous, 
  prisms 
  arranged 
  in 
  more 
  or 
  less 
  sheaf 
  like 
  bundles 
  are 
  basic 
  

   plagioclase; 
  the 
  black 
  areas 
  are 
  glassy 
  groundmass 
  ; 
  and 
  the 
  four 
  small, 
  

   white 
  rounded 
  areas 
  represent 
  quartz, 
  probably 
  of 
  secondary 
  origin. 
  

  

  