﻿Composition 
  of 
  the 
  Trap 
  of 
  Rocky 
  Hill, 
  N. 
  J. 
  "-l^il 
  , 
  

  

  the 
  prism 
  zone 
  was 
  16°. 
  The 
  compact 
  as 
  well 
  as 
  the 
  fibrous 
  

   form 
  exists 
  as 
  boundaries 
  to 
  the 
  diallage'^ 
  with 
  their 
  long 
  axes 
  

   parallel. 
  The 
  fibres 
  are 
  limited 
  to 
  the 
  outer 
  portions 
  of 
  the 
  

   diallage, 
  while 
  the 
  compact 
  is 
  distributed 
  as 
  patches 
  in 
  the 
  

   interior 
  of 
  some 
  of 
  the 
  diallage 
  crystals, 
  as 
  well. 
  These 
  

   interior 
  patches 
  would 
  point 
  rather 
  to 
  paramorphism 
  than 
  to 
  

   decomposition 
  as 
  the 
  agent 
  to 
  which 
  the 
  hornblende 
  owes 
  its 
  

   origin. 
  Neither 
  variety 
  would 
  be 
  considered 
  as 
  a 
  primary 
  

   component. 
  

  

  Feldspjars. 
  — 
  In 
  all 
  slides 
  from 
  quarry 
  No. 
  1 
  and 
  the 
  border 
  

   region, 
  the 
  structure 
  of 
  the 
  feldspars 
  is- 
  eminently 
  that 
  which 
  

   is 
  so 
  characteristic 
  of 
  the 
  diabases, 
  being 
  ophitic 
  in 
  contrast 
  to 
  

   the 
  irregular 
  appearance 
  of 
  feldspar 
  crystals 
  marking 
  the 
  

   gabbro 
  structure. 
  Both 
  of 
  these 
  structures 
  are 
  perfectly 
  repre- 
  

   sented 
  in 
  different 
  portions 
  of 
  the 
  dike. 
  

  

  In 
  sections 
  of 
  the 
  finest 
  grained 
  specimens, 
  which 
  appear 
  

   macroscopically 
  to 
  be 
  almost 
  homogeneous, 
  the 
  feldspars 
  are 
  

   always 
  well 
  formed, 
  and 
  are 
  always 
  present 
  in 
  lath 
  -shaped 
  sec- 
  

   tions 
  ; 
  their 
  average 
  size 
  is 
  -21™™ 
  in 
  length 
  and 
  •01:6"'°' 
  in 
  

   width, 
  which 
  will 
  also 
  give 
  a 
  good 
  idea 
  of 
  the 
  fineness 
  of 
  this 
  

   diabase. 
  Toward 
  the 
  center 
  of 
  the 
  dike, 
  the 
  feldspar 
  crystals 
  

   constantly 
  increase 
  in 
  size, 
  and 
  by 
  the 
  time 
  quarry 
  No. 
  1 
  is 
  

   reached 
  they 
  have 
  increased 
  to 
  '5"'" 
  in 
  length 
  and 
  -IS'""" 
  in 
  

   width, 
  still 
  presenting 
  very 
  strongly 
  the 
  ophitic 
  structure 
  ; 
  in 
  

   numerous 
  cases, 
  however, 
  there 
  is 
  a 
  strong 
  tendency 
  toward 
  the 
  

   granular, 
  irregular, 
  or 
  gabbro 
  structure. 
  This 
  is 
  brought 
  about 
  

   by 
  a 
  shortening 
  of 
  the 
  <^-axis, 
  yjarallel 
  to 
  which 
  the 
  lath-shaped 
  

   crystals 
  are 
  elongated. 
  

  

  Twinning 
  after 
  the 
  albite 
  law 
  is 
  common, 
  and 
  generally 
  the 
  

   smaller 
  crystals 
  are 
  composed 
  of 
  a 
  single 
  pair 
  of 
  twins. 
  As 
  

   the 
  crystals 
  increase 
  in 
  size, 
  polysynthetic 
  twinning 
  after 
  the 
  

   albite 
  law 
  is 
  the 
  rule. 
  The 
  large 
  angles 
  of 
  extinction 
  on 
  sec- 
  

   tions 
  perpendicular 
  to 
  010, 
  rising 
  to 
  a 
  maximum 
  of 
  37°, 
  

   coupled 
  with 
  the 
  double 
  refraction, 
  Avould 
  indicate 
  a 
  feldspar 
  

   rich 
  in 
  calcium 
  or 
  one 
  in 
  which 
  the 
  ratio 
  of 
  anorthite 
  to 
  albite 
  

   was 
  high 
  or 
  of 
  the 
  composition 
  of 
  bytowuite. 
  The 
  feldspars 
  

   of 
  quarry 
  No. 
  2 
  upon 
  the 
  whole 
  have 
  a 
  higher 
  index 
  of 
  refrac- 
  

   tion, 
  and 
  the 
  difference 
  in 
  the 
  interference 
  colors 
  is 
  very 
  

   marked. 
  The 
  twinning 
  is 
  more 
  extensive 
  and 
  many 
  of 
  the 
  

   crystals 
  show 
  a 
  decided 
  compound 
  nature. 
  This 
  is 
  brought 
  

   out 
  by 
  the 
  lamellge 
  in 
  the 
  central 
  portion 
  of 
  the 
  compound 
  

   crystals 
  extinguishing 
  at 
  larger 
  angles 
  than 
  the 
  exterior 
  bands, 
  

   also 
  by 
  the 
  interference 
  colors 
  of 
  the 
  different 
  lamellse. 
  The 
  

   central 
  lamellge 
  are 
  also 
  much 
  narrower, 
  and 
  the 
  lath-shaped 
  

   form 
  is 
  giving 
  way 
  to 
  granular 
  and 
  irregular-shaped 
  crystals 
  

   in 
  sections. 
  

  

  In 
  the 
  identification 
  of 
  the 
  feldspars, 
  samples 
  of 
  the 
  powder 
  

   separated 
  with 
  the 
  Thoulet 
  solution 
  were 
  used, 
  and 
  the 
  angles 
  

  

  