﻿Composition 
  of 
  the 
  Trap 
  of 
  Rocky 
  Hill^ 
  M. 
  J, 
  269 
  

  

  different 
  stages 
  of 
  decomposition. 
  The 
  resistance 
  offered 
  by 
  

   the 
  basic 
  traps 
  of 
  this 
  formation 
  to 
  atmospheric 
  weathering 
  is 
  

   very 
  marked, 
  in 
  contrast 
  to 
  acid 
  dikes 
  ; 
  the 
  acid 
  dikes 
  

   weathering 
  much 
  faster 
  than 
  the 
  enclosing 
  sedimentary 
  rocks,^ 
  

   leaving 
  long 
  channels, 
  while 
  the 
  basic 
  traps 
  are 
  prominent 
  as 
  

   ridges. 
  The 
  soil 
  lying 
  between 
  the 
  surface 
  boulders 
  and 
  

   derived 
  from 
  them 
  by 
  decomposition 
  is 
  never 
  very 
  deep. 
  In 
  

   quarries, 
  that 
  portion 
  lying 
  at 
  the 
  surface 
  has 
  a 
  reddish 
  appear- 
  

   ance, 
  caused 
  by 
  the 
  oxidization 
  principally 
  of 
  the 
  ferrous 
  iron 
  

   contained 
  in 
  the 
  augite. 
  This 
  surface 
  weathering 
  does 
  not 
  

   extend 
  to 
  depths 
  much 
  greater 
  than 
  ten 
  feet, 
  in 
  the 
  rock 
  

   mass 
  itself 
  ; 
  individual 
  crj^stals 
  in 
  macroscopic 
  specimens 
  from 
  

   depths 
  greater 
  than 
  this 
  see 
  in 
  perfectly 
  fresh. 
  The 
  disintegra- 
  

   tion 
  reaches 
  much 
  greater 
  depths 
  along 
  joints 
  and 
  cracks. 
  In 
  

   the 
  quarry 
  at 
  the 
  Hopewell 
  end 
  of 
  the 
  dike 
  an 
  entirely 
  differ- 
  

   ent 
  decomposition 
  has 
  taken 
  place, 
  much 
  of 
  the 
  rock 
  mass 
  here 
  

   exposed 
  is 
  soft 
  and 
  chloritic 
  in 
  nature, 
  and 
  has 
  taken 
  up 
  3 
  per 
  

   cent 
  of 
  water; 
  some 
  specimens 
  show 
  even 
  more 
  than 
  this, 
  and 
  

   well 
  crystallized 
  calcite 
  and 
  zeolites 
  appear 
  in 
  the 
  cracks 
  and 
  

   joints. 
  

  

  In 
  width 
  the 
  Rocky 
  Hill 
  dike 
  does 
  not 
  vary 
  much 
  from 
  a 
  

   half 
  mile. 
  Along 
  the 
  Mount 
  Rose 
  road, 
  which 
  crosses 
  it 
  at 
  a 
  

   right 
  angle, 
  it 
  measures 
  860 
  feet. 
  Along 
  the 
  gorge 
  of 
  the 
  

   Millstone 
  River, 
  which 
  also 
  cuts 
  through 
  it 
  at 
  Rocky 
  Hill 
  

   village 
  at 
  nearly 
  a 
  right 
  angle, 
  it 
  measures 
  4000 
  feet, 
  and 
  if 
  we 
  

   take 
  into 
  consideration 
  the 
  dip 
  of 
  the 
  enclosing 
  strata, 
  to 
  which 
  

   it 
  seems 
  to 
  conform 
  at 
  these 
  points, 
  its 
  thickness 
  measured 
  at 
  

   right 
  angles 
  to 
  the 
  dip 
  at 
  Mount 
  Rose 
  would 
  be 
  250 
  feet, 
  at 
  

   the 
  Millstone 
  1200 
  feet. 
  That 
  the 
  dip 
  of 
  the 
  enclosing 
  strata 
  

   is 
  conformed 
  to, 
  at 
  least 
  in 
  the 
  middle 
  section 
  of 
  the 
  dike, 
  is 
  

   proven, 
  in 
  lack 
  of 
  evidence 
  shown 
  by 
  direct 
  contact, 
  by 
  the 
  

   baked 
  condition 
  of 
  the 
  shales 
  lying 
  on 
  the 
  north 
  or 
  upper 
  side, 
  

   in 
  contrast 
  to 
  those 
  lying 
  on 
  the 
  south 
  or 
  under 
  side. 
  The 
  

   shales 
  lying 
  on 
  the 
  north 
  from 
  near 
  Griggstown 
  to 
  Mount 
  

   Rose 
  having 
  been 
  brought 
  in 
  close 
  contact 
  with 
  the 
  liquid 
  

   magma, 
  have 
  been 
  semifused 
  and 
  upon 
  cooling 
  crystallized 
  in 
  

   some 
  cases, 
  but 
  generally 
  developing 
  spilosites 
  in 
  large 
  quanti- 
  

   ties. 
  In 
  particularly 
  favorable 
  localities, 
  such 
  as 
  north 
  of 
  the 
  

   canal 
  bridge 
  at 
  Rocky 
  Hill, 
  the 
  dark 
  green 
  crypto-crystalline 
  

   nodules 
  reach 
  their 
  largest 
  size, 
  being 
  in 
  some 
  cases 
  three- 
  

   quarters 
  of 
  an 
  inch 
  in 
  diameter. 
  These 
  nodules 
  do 
  not 
  appear 
  

   to 
  exist 
  at 
  all 
  on 
  the 
  south 
  side 
  of 
  the 
  dike, 
  and 
  the 
  explanation 
  

   seems 
  to 
  be, 
  that 
  the 
  shales 
  resting 
  directly 
  upon 
  the 
  molten 
  

   mass 
  have 
  suffered 
  far 
  greater 
  baking 
  than 
  those 
  underneath, 
  

   for 
  if 
  the 
  dike 
  were 
  vertical 
  we 
  should 
  expect 
  an 
  equal 
  baking 
  

   on 
  both 
  sides, 
  which, 
  indeed, 
  is 
  the 
  case 
  west 
  of 
  Mount 
  Rose, 
  

   where 
  the 
  trap 
  dike 
  no 
  longer 
  conforms 
  to 
  the 
  strike 
  of 
  the 
  

  

  