﻿II. 
  E. 
  Gregory 
  — 
  Andesites 
  from 
  Maine. 
  363 
  

  

  the 
  hill 
  the 
  rock 
  is 
  much 
  lighter 
  in 
  color 
  and 
  numerous 
  white 
  

   feldspars 
  give 
  it 
  a 
  more 
  porphyritic 
  appearance. 
  

  

  Microscopic. 
  — 
  As 
  with 
  the 
  hand 
  specimen, 
  so 
  microscopic 
  

   examination 
  reveals 
  the 
  composition 
  and 
  structure 
  expected 
  of 
  

   a 
  typical 
  andesite. 
  Magnetite, 
  apatite, 
  pyroxene, 
  plagioclase, 
  

   and 
  orthoclase 
  are 
  the 
  original 
  minerals 
  present. 
  The 
  plagio- 
  

   clase 
  crystals 
  range 
  in 
  size 
  from 
  laths 
  2™"" 
  in 
  length 
  down 
  to 
  

   the 
  very 
  fine 
  ones 
  in 
  the 
  ground 
  mass, 
  but 
  the 
  larger 
  ones 
  are 
  

   not 
  abundant 
  and 
  do 
  not 
  give 
  the 
  rock 
  a 
  porphyritic 
  aspect. 
  

   The 
  plagioclase 
  forming 
  the 
  crystals 
  outside 
  the 
  groundmass 
  

   was 
  determined 
  by 
  Michel 
  Levy's 
  method 
  to 
  be 
  labradorite 
  ; 
  

   but 
  the 
  measurements 
  indicated 
  two 
  labradorites 
  with 
  the 
  

   f 
  ormulge 
  : 
  AbgAn^ 
  and 
  Ab^AUg. 
  The 
  larger 
  feldspars 
  show 
  

   strongly 
  marked 
  zonal 
  banding 
  with 
  occasionally 
  as 
  many 
  as 
  

   eight 
  distinct 
  zones, 
  which 
  decrease 
  in 
  basicity 
  from 
  the 
  center 
  

   outward, 
  but 
  with 
  the 
  original 
  albite 
  twinning 
  running 
  through 
  

   the 
  whole 
  series. 
  This 
  albite 
  twinning 
  shows 
  in 
  nearly 
  every 
  

   feldspar 
  lath 
  with 
  great 
  distinctness, 
  and 
  twins 
  on 
  the 
  pericline 
  

   and 
  manebach 
  laws 
  also 
  occur. 
  The 
  carlsbad 
  twins 
  present 
  

   are 
  often 
  with 
  one-half 
  dropped 
  much 
  below 
  the 
  other, 
  and 
  all 
  

   the 
  twinning 
  is 
  more 
  or 
  less 
  along 
  irregular 
  ragged 
  lines 
  and 
  

   with 
  unsymmetrical 
  development. 
  None 
  of 
  the 
  feldspars 
  are 
  

   entirely 
  fresh, 
  but 
  are 
  kaolinized 
  along 
  the 
  cleavages 
  and 
  zonal 
  

   boundaries, 
  or 
  entirely 
  altered 
  to 
  kaolin 
  and 
  calcite 
  except 
  

   their 
  outer 
  borders. 
  They 
  also 
  show 
  irregular 
  cracks 
  other 
  

   than 
  cleavage 
  along 
  which 
  strain 
  has 
  been 
  relieved. 
  Glass 
  

   inclusions, 
  arranged 
  without 
  order, 
  are 
  numerous 
  and 
  stand 
  out 
  

   prominently 
  in 
  the 
  clearer 
  parts 
  of 
  the 
  feldspars. 
  Orthoclase 
  

   was 
  not 
  found 
  outside 
  the 
  groundmass 
  except 
  as 
  forming 
  the 
  

   wide 
  outer 
  rim 
  of 
  the 
  zonally-built 
  plagioclases. 
  

  

  The 
  Pyroxenes 
  are 
  of 
  both 
  monoclinic 
  and 
  orthorhombic 
  

   varieties. 
  The 
  monoclinic 
  is 
  an 
  augite, 
  light 
  colored 
  in 
  thin 
  

   section 
  and 
  having 
  an 
  average 
  extinction 
  on 
  prism 
  sections 
  of 
  

   42°. 
  The 
  basal 
  sections 
  are 
  quite 
  fresh 
  and 
  show 
  the 
  cleavage 
  

   parallel 
  to 
  the 
  prism. 
  The 
  pinacoids 
  are 
  more 
  developed 
  than 
  

   the 
  prism 
  faces 
  and 
  give 
  the 
  appearance 
  of 
  a 
  square 
  with 
  trun- 
  

   cated 
  corners 
  rather 
  than 
  the 
  more 
  common 
  octagonal 
  effect. 
  

   The 
  prism 
  sections 
  vary 
  from 
  stout 
  forms 
  to 
  those 
  five 
  or 
  six 
  

   times 
  as 
  long 
  as 
  broad. 
  In 
  places 
  many 
  small 
  pieces 
  are 
  

   arranged 
  in 
  parallel 
  position 
  and 
  separated 
  by 
  alteration 
  

   products 
  in 
  such 
  a 
  way 
  as 
  to 
  suggest 
  the 
  presence 
  of 
  augite 
  

   phenocrysts 
  of 
  which 
  these 
  fragments 
  are 
  the 
  remnants. 
  The 
  

   orthorhombic 
  pyroxenes 
  are 
  represented 
  in 
  the 
  darkest 
  colored 
  

   rocks 
  by 
  a 
  few 
  basal 
  and 
  prism 
  sections, 
  but 
  in 
  the 
  gray 
  vari- 
  

   eties 
  it 
  constitutes 
  fully 
  half 
  of 
  the 
  pyroxenes 
  present. 
  It 
  is 
  

   very 
  light 
  colored, 
  not 
  at 
  all 
  pleochroic, 
  and 
  is 
  at 
  times 
  partly 
  

   eaten 
  away 
  and 
  again 
  occurs 
  as 
  parallel 
  intergrowths 
  with 
  the 
  

  

  