﻿278 
  Phillies 
  —Miner 
  alogical 
  Structure 
  and 
  Chemical 
  

  

  of 
  extinction 
  on 
  the 
  base 
  in 
  connection 
  with 
  the 
  albite 
  twinning 
  

   striations 
  or 
  the 
  prismatic 
  cleavage 
  were 
  measured. 
  In 
  the 
  

   sample 
  with 
  a 
  gravity 
  greater 
  than 
  2'69, 
  fragments 
  showing 
  

   the 
  latli-shaped 
  crystals 
  with 
  narrow 
  striations 
  clearly 
  predom- 
  

   inated 
  over 
  the 
  fragments 
  of 
  the 
  granular 
  form 
  of 
  crystal 
  with 
  

   wide 
  twinning 
  laminations. 
  The 
  reverse 
  of 
  this 
  was 
  found 
  to 
  

   be 
  the 
  case 
  in 
  the 
  sample 
  separated 
  with 
  a 
  gravity 
  less 
  than 
  

   2-69. 
  

  

  The 
  compound 
  nature 
  of 
  these 
  five 
  samples 
  is 
  indicated 
  by 
  

   the 
  difference 
  in 
  the 
  extinction-angles 
  of 
  the 
  several 
  laminae 
  of 
  

   which 
  they 
  are 
  composed, 
  so 
  that 
  the 
  separation 
  of 
  feldspars 
  

   of 
  complex 
  nature 
  is 
  at 
  the 
  best 
  only 
  a 
  coarse 
  assorting, 
  in 
  

   which 
  the 
  majority 
  of 
  particles 
  will 
  have 
  a 
  composition 
  agree- 
  

   ing 
  with 
  the 
  specific 
  gravity, 
  or 
  will 
  be 
  homogeneous. 
  The 
  

   fineness 
  to 
  which 
  each 
  particle 
  would 
  have 
  to 
  be 
  reduced 
  to 
  

   separate 
  these 
  interlaminated 
  feldspars, 
  is 
  far 
  beyond 
  tlie 
  prac- 
  

   ticability 
  of 
  separating 
  them 
  with 
  the 
  solution, 
  owing 
  to 
  the 
  

   slowness 
  with 
  which 
  they 
  settle. 
  The 
  angle 
  of 
  extinction 
  

   measured 
  on 
  the 
  base 
  was 
  found 
  to 
  vary 
  from 
  —27° 
  to 
  — 
  4° 
  ; 
  

   the 
  lamellse 
  giving 
  the 
  larger 
  angle 
  were 
  found 
  to 
  be 
  the 
  very 
  

   narrow, 
  more 
  centrally 
  located. 
  These 
  angles 
  indicate 
  that 
  

   the 
  analysis 
  of 
  this 
  san:iple 
  would 
  lie 
  between 
  the 
  composition 
  

   of 
  AbjAug 
  on 
  the 
  one 
  side 
  and 
  Ab^An^ 
  on 
  the 
  other." 
  

  

  In 
  the 
  sample 
  separated 
  with 
  a 
  gravity 
  less 
  than 
  2*69, 
  the 
  

   fragments 
  with 
  the 
  very 
  narrow 
  lamellae 
  had 
  disappeared. 
  

   The 
  angle 
  of 
  extinction 
  was 
  found 
  in 
  numerous 
  cases 
  to 
  be 
  

   almost 
  parallel 
  with 
  the 
  twinning 
  striations, 
  showing 
  that 
  the 
  

   proportion 
  of 
  anorthite 
  had 
  decreased 
  until 
  the 
  composition 
  is 
  

   possibly 
  that 
  of 
  oligoclase 
  or 
  andesine. 
  In 
  quarry 
  ISo. 
  3 
  the 
  

   crystallization 
  of 
  tiie 
  feldspars 
  is 
  very 
  irregular, 
  and 
  compli- 
  

   cated 
  by 
  an 
  increase 
  in 
  the 
  tendency 
  to 
  form, 
  in 
  addition 
  to 
  

   the 
  twinning 
  after 
  the 
  albite 
  law, 
  numerous 
  twins 
  after 
  the 
  

   Carlsbad 
  law, 
  showing 
  both 
  straight 
  and 
  irregular 
  contact 
  

   planes. 
  Numerous 
  patches 
  exhibit 
  the 
  cross 
  hatching 
  produced 
  

   by 
  the 
  combination 
  of 
  the 
  albite 
  and 
  pericline 
  twinning. 
  The 
  

   Baveno 
  type 
  is 
  also 
  present 
  in 
  frequent 
  cases. 
  The 
  large 
  

   masses 
  of 
  feldspar 
  do 
  not 
  give 
  distinct 
  crystalline 
  outlines, 
  

   being 
  moulded 
  by 
  the 
  surrounding 
  diallage 
  crystals 
  and 
  mag- 
  

   netite, 
  fragments 
  of 
  which 
  they 
  enclose. 
  Their 
  crystallization 
  

   must 
  have 
  been 
  either 
  at 
  the 
  same 
  time 
  or 
  after 
  that 
  of 
  the 
  

   diallage. 
  These 
  masses 
  of 
  feldspar 
  preserve 
  the 
  same 
  crystallo- 
  

   graphic 
  orientation 
  throughout, 
  thus 
  presenting 
  the 
  poikilitic 
  

   structure 
  common 
  in 
  gabbros.'^ 
  

  

  A 
  large 
  proportion 
  of 
  the 
  feldspars 
  of 
  this 
  locality 
  have 
  

   become 
  clouded 
  and 
  stained 
  dark 
  b}^ 
  the 
  infiltration 
  of 
  iron 
  

   from 
  neighboring 
  decomposing 
  diallage 
  ; 
  the 
  cloudiness 
  being 
  

   produced 
  by 
  the 
  first 
  steps 
  in 
  kaolinization. 
  

  

  