﻿Composition 
  of 
  the 
  Trap 
  of 
  Rocky 
  Hill^ 
  N. 
  J. 
  275 
  

  

  for 
  the 
  internal 
  layers. 
  This 
  anscle 
  constantly 
  increases 
  from 
  

   the 
  fine-grained 
  crystals, 
  where 
  if 
  is 
  46°, 
  reaching 
  a 
  maximum 
  

   of 
  58° 
  in 
  the 
  larger 
  crystals 
  of 
  quarry 
  ISo. 
  3. 
  Sections 
  parallel 
  

   to 
  the 
  vertical 
  axis 
  show 
  a 
  constant 
  structure 
  of 
  fine 
  striations 
  ; 
  

   traces 
  of 
  interposition 
  planes, 
  parallel 
  to 
  the 
  basal 
  pinacoid. 
  

   These 
  striations 
  appear 
  on 
  all 
  sections 
  which 
  are 
  parallel 
  to 
  the 
  

   vertical 
  axis, 
  as 
  running 
  across, 
  between 
  the 
  cleavage 
  cracks. 
  

   They 
  do 
  not 
  appear 
  on 
  sections 
  parallel 
  to 
  the 
  base, 
  neither 
  as 
  

   lines 
  or 
  dots, 
  which 
  would 
  indicate 
  that 
  they 
  are 
  not 
  fibres, 
  

   but 
  owe 
  their 
  origin 
  to 
  polysynthetic 
  twinning 
  parallel 
  to 
  the 
  

   basal 
  pinacoid. 
  Their 
  trace, 
  on 
  sections 
  parallel 
  to 
  oo 
  P 
  oo 
  , 
  

   makes 
  an 
  angle 
  of 
  about 
  74° 
  with 
  the 
  trace 
  of 
  the 
  composition 
  

   face 
  of 
  the 
  twins 
  twinned 
  parallel 
  to 
  oo 
  P 
  (X, 
  when 
  the 
  lines 
  are 
  

   symmetrical 
  on 
  either 
  side. 
  This 
  would 
  prove 
  these 
  lines 
  to 
  

   be 
  traces 
  of 
  planes 
  parallel 
  to 
  OP, 
  as 
  the 
  angle 
  /3 
  for 
  monocline 
  

   pyroxene 
  is 
  74° 
  10' 
  ; 
  these 
  very 
  fine 
  interposition 
  planes 
  play 
  

   an 
  important 
  part 
  in 
  the 
  decomposition 
  of 
  the 
  mineral, 
  as 
  well 
  

   as 
  in 
  the 
  production 
  of 
  the 
  peculiar 
  schiller. 
  The 
  great 
  num- 
  

   ber 
  of 
  small 
  inclusions 
  contained 
  in 
  the 
  crystals 
  are 
  symmetri- 
  

   cal 
  in 
  regard 
  to 
  the 
  axis, 
  and 
  fall 
  into 
  two 
  groups 
  ; 
  those 
  parallel 
  

   to 
  the 
  vertical 
  axes, 
  and 
  those 
  arranged 
  along 
  the 
  twinning 
  

   planes, 
  parallel 
  to 
  OP. 
  The 
  inclusions 
  are 
  of 
  the 
  nature 
  of 
  

   cavities 
  filled 
  with 
  hyd 
  rated 
  oxides, 
  formed 
  by 
  solution 
  of 
  the 
  

   crystal 
  body 
  under 
  pressure, 
  as 
  described 
  by 
  Judd, 
  who 
  infers 
  

   that 
  they 
  are 
  negative 
  crystals, 
  and 
  mark 
  the 
  line 
  of 
  least 
  

   resistance 
  to 
  solution, 
  and 
  therefore 
  stand 
  in 
  the 
  same 
  relation 
  

   to 
  the 
  body 
  of 
  the 
  crystal 
  as 
  the 
  etching 
  figures 
  do 
  to 
  the 
  sur- 
  

   face 
  or 
  faces. 
  Of 
  the 
  two 
  systems 
  that 
  one 
  in 
  which 
  the 
  long 
  

   axis 
  is 
  parallel 
  to 
  the 
  d-axis 
  of 
  the 
  crystal 
  offers 
  the 
  greater 
  

   resistance 
  to 
  solution, 
  and 
  requires 
  a 
  higher 
  pressure, 
  conse- 
  

   quently 
  takes 
  place 
  at 
  greater 
  depths. 
  

  

  This 
  system 
  is 
  the 
  better 
  represented 
  of 
  the 
  two 
  and 
  is 
  very 
  

   characteristic 
  of 
  sections 
  from 
  the 
  central 
  and 
  upper 
  portions 
  

   of 
  the 
  dike; 
  they 
  are 
  considerably 
  elongated, 
  and 
  occur 
  in 
  

   large 
  numbers. 
  If 
  they 
  are 
  secondary 
  formations 
  it 
  would 
  

   add 
  another 
  proof, 
  that 
  this 
  dike 
  was 
  formed 
  at 
  a 
  considerable 
  

   depth 
  below 
  the 
  surface. 
  Whether 
  they 
  are 
  secondary 
  forma- 
  

   tions 
  is 
  very 
  difficult 
  to 
  determine. 
  The 
  matrix 
  surrounding 
  

   them 
  has 
  not 
  been 
  bleached 
  as 
  if 
  the 
  oxides 
  of 
  iron 
  filling 
  

   them 
  had 
  been 
  drawn 
  from 
  their 
  immediate 
  vicinity. 
  This 
  is 
  

   in 
  decided 
  contrast 
  to 
  other 
  granular 
  inclusions, 
  evidently 
  

   secondary 
  products, 
  found 
  as 
  a 
  result 
  of 
  surface 
  action, 
  in 
  

   which 
  the 
  body 
  of 
  the 
  crystal 
  presents 
  a 
  decided 
  bleaching. 
  

  

  The 
  formation 
  of 
  these 
  secondary 
  granules 
  begins 
  first 
  at 
  the 
  

   margin 
  and 
  works 
  its 
  way 
  toward 
  the 
  central 
  portions 
  of 
  the 
  

   crystal, 
  following 
  cleavage 
  cracks 
  and 
  fractures, 
  and 
  especially 
  

   along 
  interposition 
  planes 
  running 
  across 
  between 
  the 
  cleavage 
  

  

  