﻿366 
  //. 
  E. 
  Gregory 
  — 
  Andesites 
  from 
  Maine. 
  

  

  is 
  reason 
  to 
  regard 
  this 
  as 
  simply 
  an 
  olivine-bearing 
  phase 
  of 
  

   the 
  andesite." 
  The 
  description 
  is 
  of 
  an 
  andesite, 
  but 
  there 
  are 
  

   discrepancies 
  between 
  the 
  description 
  and 
  the 
  analysis. 
  No 
  

   ferrous 
  iron 
  is 
  present 
  to 
  form 
  magnetite, 
  and 
  if 
  the 
  small 
  

   amount 
  of 
  lime 
  forms 
  calcite, 
  basic 
  feldspars 
  could 
  not 
  be 
  pro- 
  

   duced. 
  And 
  even 
  if 
  the 
  whole 
  0*59 
  per 
  cent 
  of 
  lime 
  were 
  

   present 
  as 
  andesine 
  or 
  labradorite 
  the 
  amount 
  is 
  far 
  too 
  small 
  

   for 
  an 
  andesite. 
  According 
  to 
  the 
  generally 
  accepted 
  usage 
  

   among 
  petrographers, 
  a 
  rock 
  with 
  such 
  a 
  high 
  percentage 
  of 
  

   soda 
  and 
  potash 
  with 
  little 
  lime 
  and 
  magnesia 
  would 
  be 
  classed 
  

   as 
  a 
  trachyte 
  or 
  more 
  closely, 
  an 
  segerine- 
  trachyte. 
  

  

  E-ornblende-andesite. 
  — 
  The 
  largest 
  single 
  mass 
  of 
  this 
  rock 
  

   is 
  Hobart's 
  Hill, 
  and 
  the 
  freshest 
  and 
  most 
  typical 
  specimens 
  

   are 
  from 
  'this 
  hill 
  and 
  from 
  the 
  west 
  bank 
  of 
  the 
  Presque 
  Isle 
  

   near 
  the 
  northwest 
  foot 
  of 
  the 
  hill, 
  where 
  quarrying 
  was 
  

   attempted 
  at 
  one 
  time. 
  The 
  hand 
  specimen 
  shows 
  a 
  very 
  dark 
  

   gray, 
  almost 
  black, 
  rock, 
  fine-grained, 
  but 
  with 
  a 
  somewhat 
  

   porphyritic 
  appearance 
  caused 
  by 
  the 
  occasional 
  feldspar 
  crys- 
  

   tals 
  which 
  rise 
  above 
  the 
  general 
  ground 
  mass 
  and 
  reflect 
  light 
  

   well 
  from 
  their 
  glassy 
  cleavage 
  faces. 
  Some 
  few 
  feldspar 
  laths 
  

   attain 
  a 
  length 
  of 
  5-6""". 
  The 
  rock 
  breaks 
  out 
  into 
  tabular 
  

   blocks 
  along 
  the 
  cleavages 
  and 
  weathers 
  to 
  a 
  brownish 
  gray 
  

   color. 
  

  

  Microscopic 
  description. 
  — 
  In 
  thin 
  section 
  tlie 
  microscope 
  

   reveals 
  magnetite, 
  apatite, 
  titanite, 
  rarely 
  a 
  zircon 
  lath, 
  possibly 
  

   augite, 
  hornblende, 
  plagioclase, 
  and 
  orthoclase 
  together 
  with 
  

   considerable 
  secondary 
  calcite. 
  The 
  feldspars 
  range 
  from 
  2™™ 
  

   in 
  length 
  down 
  to 
  minute 
  microlites. 
  The 
  larger 
  feldspars 
  are 
  

   commonly 
  converted 
  to 
  calcite, 
  which 
  while 
  it 
  indicates 
  their 
  

   basic 
  character, 
  also 
  prevents 
  their 
  accurate 
  determination. 
  

   Those 
  which 
  could 
  be 
  measured 
  by 
  the 
  Michel-Levy 
  method 
  

   proved 
  to 
  be 
  andesine 
  with 
  formula 
  Ab^An^, 
  hence 
  more 
  acid 
  

   than 
  the 
  feldspars 
  of 
  the 
  augite 
  andesine. 
  They 
  contain 
  glass 
  

   inclusions, 
  are 
  zonally 
  built 
  with 
  an 
  occasional 
  unaltered 
  outer 
  

   border, 
  and 
  are 
  twinned 
  according 
  to 
  the 
  Carlsbad 
  and 
  albite 
  

   laws 
  but 
  with 
  very 
  irregular 
  intergrowths 
  of 
  the 
  parts. 
  

  

  Hornblende 
  is 
  the 
  only 
  important 
  ferro-magnesian 
  mineral 
  

   present 
  and 
  occurs, 
  as 
  the 
  feldspars, 
  both 
  as 
  large 
  basal 
  sections 
  

   and 
  long 
  laths 
  often 
  with 
  good 
  crystal 
  outline 
  and 
  also 
  as 
  

   shreds 
  in 
  the 
  groundmass. 
  The 
  larger 
  pieces 
  are 
  rarely 
  in 
  a 
  

   good 
  state 
  of 
  preservation, 
  but 
  occur 
  with 
  ragged 
  edges 
  and 
  

   show 
  resorption 
  phenomena. 
  The 
  crystal 
  is 
  eaten 
  into 
  and 
  

   part 
  of 
  the 
  interior 
  converted 
  into 
  magnetite 
  with 
  a 
  few 
  augite 
  

   grains. 
  Some 
  crystals 
  have 
  been 
  almost 
  entirely 
  replaced 
  by 
  

   calcite 
  and 
  magnetite, 
  and 
  others 
  are 
  represented 
  by 
  a 
  ghost- 
  

   like 
  outline 
  of 
  "magnetite 
  dust. 
  Commonly 
  the 
  hornblende 
  is 
  

   now 
  changed 
  to 
  a 
  green 
  micaceous 
  material, 
  perhaps 
  a 
  variety 
  

  

  