﻿2Y'2 
  Phillips 
  — 
  Mineralogiccd 
  Structare 
  and 
  Chemical 
  

  

  No. 
  3, 
  or 
  its 
  immediate 
  vicinity, 
  are 
  crystals 
  of 
  like 
  perfection 
  

   and 
  size 
  found. 
  It 
  is 
  also 
  remarkably 
  near 
  the 
  upper 
  contact 
  

   border, 
  and 
  judging 
  from 
  its 
  differentiation, 
  coarseness 
  and 
  

   crystalline 
  perfection 
  as 
  compared 
  with 
  other 
  portions 
  of 
  the 
  

   dike, 
  it 
  mnst 
  have 
  been 
  subjected 
  to 
  slower 
  cooling, 
  and 
  has 
  

   been 
  the 
  last 
  portion 
  to 
  consolidate, 
  or 
  might 
  have 
  been 
  in 
  

   close 
  contact 
  with 
  the 
  source 
  from 
  which 
  the 
  molten 
  magma 
  

   was 
  forced 
  out, 
  and 
  consequently 
  nearer 
  the 
  source 
  of 
  heat. 
  

   This 
  would 
  explain 
  the 
  slower 
  cooling, 
  but 
  not 
  the 
  difference 
  

   in 
  composition, 
  which 
  specimens 
  from 
  these 
  three 
  localities 
  

   show. 
  

  

  The 
  shales 
  just 
  above 
  this 
  point 
  show 
  an 
  extensive 
  baking, 
  

   for 
  it 
  is 
  here 
  that 
  the 
  sj^herulites 
  reach 
  their 
  maximum, 
  both 
  in 
  

   number 
  and 
  size, 
  indicating 
  that 
  they 
  have 
  been 
  subjected 
  to 
  a 
  

   more 
  intense 
  heat, 
  or 
  one 
  of 
  longer 
  duration. 
  

  

  Microscopical 
  Description 
  . 
  

  

  Microscopically, 
  the 
  groundmass 
  shows 
  the 
  absence 
  of 
  glass, 
  

   and 
  is 
  therefore 
  holocrystalline. 
  In 
  the 
  finest 
  grained 
  portions 
  

   the 
  feldspars 
  are 
  present 
  in 
  the 
  characteristic 
  lath-shaped 
  ciys- 
  

   tals, 
  metasilicates 
  lying 
  between 
  them, 
  in 
  the 
  form 
  of 
  line 
  crys- 
  

   talline 
  aggregates 
  of 
  grains 
  or 
  wedged-shaped 
  anhedrons, 
  each 
  

   having 
  a 
  different 
  crystallographic 
  orientation. 
  

  

  This 
  structure, 
  with 
  the 
  absence 
  of 
  glass, 
  following 
  the 
  classi- 
  

   fication 
  of 
  Iddings" 
  for 
  these 
  rocks, 
  is 
  that 
  of 
  a 
  dolerite 
  or 
  

   diabase, 
  in 
  contrast 
  to 
  the 
  basalts, 
  in 
  which 
  glass 
  is 
  present 
  in 
  

   the 
  groundmass. 
  

  

  Magnetite, 
  being 
  one 
  of 
  the 
  primary 
  minerals, 
  and 
  charac- 
  

   teristic 
  of 
  these 
  diabases, 
  is 
  found 
  liberally 
  represented 
  in 
  

   sections 
  from 
  all 
  portions 
  of 
  the 
  dike; 
  and 
  in 
  those 
  from 
  the 
  

   contact-band, 
  subject 
  to 
  more 
  rajDid 
  cooling 
  and 
  crystallization, 
  

   magnetite 
  is 
  evenly 
  distributed 
  as 
  fine 
  irregular 
  grains, 
  show- 
  

   ing 
  no 
  crystalline 
  boundaries, 
  except 
  in 
  a 
  few 
  cases 
  where 
  

   octahedral 
  outlines 
  appear. 
  Since 
  the 
  magnetite 
  is 
  not 
  found 
  

   as 
  inclusions 
  in 
  the 
  feldspars, 
  but 
  is 
  contained 
  in 
  everything 
  

   else 
  except 
  in 
  the 
  apatite, 
  the 
  formation 
  of 
  the 
  mineral 
  in 
  

   the 
  magma 
  has 
  been 
  subsequent 
  to 
  that 
  of 
  the 
  feldspars. 
  The 
  

   conditions 
  favorable 
  for 
  its 
  separation 
  have 
  been 
  of 
  consider- 
  

   able 
  duration, 
  as 
  the 
  relative 
  quantity 
  contained 
  in 
  the 
  dike 
  

   has 
  increased 
  in 
  sections 
  from 
  coarser 
  specimens, 
  as 
  from 
  

   quarry 
  No. 
  3 
  ; 
  here 
  the 
  masses 
  of 
  magnetite 
  have 
  reached 
  such 
  

   a 
  size 
  as 
  to 
  form 
  a 
  macroscopic 
  feature 
  of 
  the 
  rock. 
  Some 
  of 
  

   these 
  larger 
  masses 
  completely 
  enclose 
  feldspar 
  crystals 
  as 
  well 
  

   as 
  apatite, 
  which 
  appears 
  in 
  hexagonal 
  sections 
  in 
  an 
  otherwise 
  

   completely 
  dark 
  field 
  of 
  magnetite. 
  

  

  Qualitatively 
  all 
  the 
  magnetite 
  gives 
  very 
  strong 
  reactions 
  

   for 
  titanium, 
  and 
  judging 
  from 
  this 
  it 
  nnist 
  carry 
  nearly 
  if 
  not 
  

  

  