﻿Composition 
  of 
  the 
  Trap 
  of 
  Rocky 
  Hill^ 
  W. 
  J. 
  273 
  

  

  all 
  the 
  one 
  per 
  cent, 
  or 
  more 
  of 
  the 
  titanic 
  oxide 
  found 
  in 
  the 
  

   rocks. 
  

  

  While 
  magnetite 
  is 
  one 
  of 
  the 
  first 
  minerals 
  to 
  separate 
  from 
  

   a 
  magma 
  on 
  coolino^, 
  the 
  relation 
  of 
  the 
  magnetite 
  to 
  the 
  other 
  

   minerals 
  in 
  the 
  sections, 
  enclosing 
  feldspars, 
  apatite, 
  and 
  even 
  

   augite, 
  being 
  wedged 
  inbetween 
  other 
  crystals 
  in 
  the 
  sections, 
  

   would 
  indicate 
  that 
  its 
  separation 
  has 
  been 
  a 
  slow, 
  continuous 
  

   one, 
  and 
  since 
  its 
  relative 
  quantity 
  increases 
  in 
  that 
  portion 
  of 
  

   the 
  dike 
  which 
  we 
  believe 
  has 
  been 
  the 
  last 
  to 
  consolidate, 
  there 
  

   have 
  been 
  changes 
  in 
  the 
  magma 
  favorable 
  to 
  this 
  increase. 
  

   From 
  these 
  facts 
  we 
  would 
  conclude 
  that 
  the 
  magma, 
  after 
  

   having 
  been 
  intruded 
  between 
  the 
  sedimentary 
  strata, 
  was 
  

   subjected 
  to 
  a 
  very 
  slow 
  and 
  gradual 
  cooling, 
  and 
  possibly 
  

   under 
  high 
  pressure, 
  which 
  would 
  also 
  tend 
  to 
  keep 
  the 
  mag- 
  

   netite 
  in 
  solution. 
  That 
  this 
  cooling 
  was 
  slow, 
  and 
  that 
  after 
  

   injection 
  the 
  magma 
  was 
  subjected 
  to 
  little 
  or 
  no 
  subsequent 
  

   disturbance, 
  is 
  also 
  brought 
  out 
  by 
  the 
  condition 
  of 
  the 
  apatite 
  

   crystals, 
  which 
  are 
  everywhere 
  present. 
  In 
  sections 
  from 
  the 
  

   outer 
  portions, 
  apatite 
  is 
  represented 
  by 
  numerous 
  slender 
  but 
  

   short 
  needles, 
  which 
  increase 
  in 
  size 
  until 
  in 
  sections 
  from 
  

   quarry 
  Xo. 
  3 
  they 
  reach 
  a 
  diameter 
  of 
  '2°^™ 
  and 
  a 
  length 
  of 
  

   I'S""" 
  before 
  disappearing 
  out 
  of 
  the 
  section, 
  being 
  uninter- 
  

   rupted 
  throughout 
  their 
  long 
  axis 
  and 
  extending 
  through 
  

   adjacent 
  minerals, 
  both 
  magnetite 
  and 
  feldspars. 
  As 
  these 
  are 
  

   the 
  first 
  crystals 
  to 
  form, 
  any 
  slight 
  disturbance 
  during 
  the 
  

   slow 
  cooling 
  would 
  have 
  been 
  sufficient 
  to 
  break 
  up 
  this 
  con- 
  

   tinuity. 
  

  

  Olivine. 
  — 
  The 
  conditions 
  favoring 
  the 
  separation 
  of 
  olivine 
  

   in 
  the 
  Rocky 
  Hill 
  diabase 
  have 
  been 
  very 
  limited, 
  as 
  crystals 
  

   of 
  it 
  are 
  very 
  few 
  in 
  number 
  and 
  confined 
  to 
  the 
  border 
  area, 
  

   but 
  these 
  few 
  present 
  interesting 
  facts 
  when 
  their 
  method 
  of 
  

   decomposition 
  is 
  considered. 
  They 
  occur 
  as 
  rounded 
  grains 
  

   for 
  the 
  most 
  part, 
  but 
  few 
  showing 
  an 
  ill-defined 
  crystalline 
  

   form. 
  The 
  larger 
  crystals, 
  where 
  fresh, 
  are 
  of 
  a 
  light 
  green 
  

   color, 
  and 
  show 
  a 
  good 
  cleavage 
  as 
  well 
  as 
  irregular 
  fracturing. 
  

  

  In 
  their 
  decomposition 
  they 
  have 
  followed 
  two 
  distinct 
  

   methods. 
  The 
  first, 
  or 
  earlier 
  one, 
  is 
  by 
  the 
  formation 
  of 
  

   grains 
  of 
  magnetite, 
  appearing 
  as 
  black 
  granular 
  strings, 
  follow- 
  

   ing 
  all 
  the 
  cleavage 
  cracks 
  and 
  irregular 
  fractures 
  of 
  the 
  

   crystals 
  ; 
  and 
  especially 
  on 
  the 
  border, 
  the 
  black 
  grains 
  have 
  

   been 
  produced 
  to 
  such 
  an 
  extent 
  as 
  to 
  surround 
  each 
  crystal 
  

   with 
  a 
  decidedly 
  dark 
  band, 
  and 
  in 
  the 
  case 
  of 
  some 
  of 
  the 
  

   smaller 
  nodules, 
  this 
  secondary 
  formation 
  of 
  iron 
  oxides 
  has 
  

   proceeded 
  so 
  far, 
  that 
  the 
  entire 
  body 
  of 
  the 
  crystal 
  is 
  nearly 
  

   opaque, 
  and 
  might 
  be 
  mistaken 
  for 
  a 
  mere 
  collection 
  of 
  mag- 
  

   netite 
  granules. 
  This 
  method 
  of 
  decomposition 
  of 
  olivine 
  is 
  

   described 
  \Sj 
  Zirkel'" 
  in 
  connection 
  with 
  the 
  olivine 
  of 
  gabbros, 
  

   and 
  furthermore 
  he 
  states 
  that 
  it 
  is 
  never 
  found 
  in 
  connection 
  

  

  