﻿Composition 
  of 
  the 
  Trap 
  of 
  Rocky 
  Hill^ 
  N. 
  J. 
  271 
  

  

  parallelism 
  of 
  the 
  loag 
  axis 
  of 
  the 
  augite 
  crystals. 
  The 
  excel- 
  

   lent 
  cleavage 
  of 
  these 
  crystals 
  gives 
  a 
  decidedly 
  easy 
  fracture 
  

   along 
  this 
  direction, 
  to 
  the 
  entire 
  rock 
  mass. 
  This 
  easy 
  frac- 
  

   ture 
  is 
  taken 
  advantage 
  of 
  by 
  the 
  workmen 
  in 
  splitting 
  out 
  

   blocks 
  for 
  street 
  paving. 
  

  

  There 
  can 
  be 
  distinguished 
  in 
  the 
  coarse 
  variety, 
  dark, 
  

   irregular, 
  highly 
  lustrous 
  though 
  small 
  aggregates 
  of 
  magnetic 
  

   oxide 
  of 
  iron. 
  The 
  size 
  of 
  these 
  aggregates 
  increases 
  witli 
  the 
  

   coarseness 
  of 
  the 
  specimen. 
  

  

  Scattered 
  throughout 
  the 
  entire 
  mass 
  are 
  very 
  minute 
  crys- 
  

   tals 
  of 
  chalcopyrite, 
  too 
  scattered 
  to 
  form 
  a 
  factor 
  in 
  the 
  

   analyses 
  of 
  the 
  rock. 
  In 
  some 
  rare 
  localities 
  they 
  have 
  been 
  

   segregated 
  into 
  masses, 
  as 
  much 
  as 
  half 
  an 
  inch 
  in 
  diameter. 
  

   It 
  may 
  be 
  mentioned 
  here 
  that 
  copper 
  minerals 
  are 
  found 
  at 
  

   several 
  localities 
  along 
  the 
  contact 
  of 
  the 
  trap 
  and 
  the 
  shale, 
  

   especially 
  at 
  Griggstown, 
  where 
  a 
  small 
  quantity 
  of 
  ore 
  has 
  

   been 
  taken 
  out 
  in 
  the 
  form 
  of 
  silicate 
  and 
  carbonates. 
  

  

  The 
  structure 
  found 
  in 
  quarry 
  No. 
  2 
  represents 
  the 
  condi- 
  

   tions 
  which 
  are 
  found 
  throughout 
  the 
  greater 
  portion 
  of 
  the 
  

   dike, 
  from 
  the 
  Millstone 
  Kiver 
  westward. 
  Eastward 
  the 
  coarse- 
  

   ness 
  decreases, 
  due 
  undoubtedly 
  to 
  the 
  fact 
  that 
  erosion 
  has 
  not 
  

   exposed 
  the 
  deeper, 
  more 
  central 
  and 
  coarser 
  portions. 
  Quarry 
  

   No. 
  3 
  represents 
  the 
  maximum 
  coarseness 
  and 
  differentiation 
  

   of 
  the 
  rock's 
  components, 
  for 
  here, 
  as 
  is 
  afterwards 
  shown 
  from 
  

   the 
  analyses, 
  tlie 
  composition 
  is 
  the 
  farthest 
  removed 
  from 
  the 
  

   original 
  magma, 
  as 
  represented 
  by 
  the 
  composition 
  of 
  the 
  

   microcrystalline 
  band 
  along 
  the 
  contact. 
  The 
  aspect 
  of 
  the 
  

   rock 
  has 
  changed, 
  due 
  to 
  two 
  primary 
  causes. 
  In 
  the 
  first 
  

   place, 
  this 
  is 
  due 
  to 
  the 
  elongation 
  of 
  the 
  diallage-like 
  augite 
  

   crystals, 
  measuring 
  often 
  an 
  inch, 
  and 
  occasionally 
  two 
  inches 
  

   in 
  length. 
  With 
  its 
  excellent 
  cleavage 
  and 
  metallic 
  schiller, 
  it 
  

   forms 
  a 
  very 
  striking 
  feature, 
  while 
  the 
  schistose 
  arrangement 
  

   marked 
  in 
  quarry 
  No. 
  2 
  has 
  been 
  lost 
  here, 
  probably 
  the 
  

   pressure 
  to 
  which 
  it 
  was 
  due 
  having 
  been 
  removed 
  or 
  relieved 
  

   by 
  smaller 
  secondary 
  dikes, 
  though 
  a 
  diligent 
  search 
  of 
  the 
  

   locality 
  has 
  failed 
  to 
  reveal 
  them. 
  There 
  are 
  some 
  small 
  dikes 
  

   northeast 
  of 
  the 
  Kocky 
  Hill 
  trap, 
  though 
  their 
  connection 
  with 
  

   it 
  is 
  not 
  apparent. 
  Judging 
  from 
  the 
  perfection 
  of 
  the 
  crystals 
  

   of 
  diallagic 
  augite, 
  there 
  could 
  at 
  the 
  best 
  have 
  been 
  no 
  very 
  

   great 
  disturbance, 
  as 
  they 
  have 
  suffered 
  but 
  slight 
  distortion, 
  

   their 
  long 
  axis 
  being 
  but 
  slightly 
  curved, 
  and 
  showing 
  no 
  dis- 
  

   placements. 
  

  

  In 
  the 
  second 
  place, 
  the 
  color 
  of 
  the 
  feldspar 
  has 
  changed 
  

   from 
  white 
  to 
  a 
  very 
  light 
  flesh 
  color, 
  which 
  certainly 
  indicates 
  

   a 
  change 
  in 
  its 
  nature, 
  which 
  is 
  also 
  brought 
  out 
  in 
  the 
  analy- 
  

   ses. 
  That 
  portion 
  of 
  the 
  dike 
  which 
  presents 
  these 
  character- 
  

   istics 
  is 
  limited 
  to 
  a 
  small 
  area, 
  and 
  nowhere 
  outside 
  of 
  quarry 
  

  

  Am. 
  Jour. 
  Sci. 
  — 
  Fourth 
  Series, 
  Vol. 
  YIII, 
  No. 
  46.— 
  October, 
  1899. 
  

   19 
  

  

  