﻿864 
  11. 
  E. 
  Gregory 
  — 
  Andesites 
  from 
  Maine. 
  

  

  angite. 
  It 
  seems 
  to 
  be 
  a 
  variety 
  poor 
  in 
  iron, 
  is 
  optically 
  +, 
  

   and 
  hence 
  referred 
  to 
  enstatite. 
  In 
  the 
  fresher 
  rock 
  specimens 
  

   the 
  cleavage 
  cracks 
  and 
  borders 
  of 
  the 
  enstatite 
  often 
  show 
  the 
  

   presence 
  of 
  a 
  red-brown 
  fibrous 
  mineral. 
  In 
  the 
  more 
  weath- 
  

   ered 
  rocks 
  this 
  mineral 
  assumes 
  a 
  prominent 
  role. 
  It 
  is 
  here 
  

   found 
  intergrown 
  with 
  augite 
  and 
  forming 
  fibrous 
  laths 
  with 
  

   parallel 
  extinction. 
  Its 
  pleochroism 
  is 
  distinct 
  with 
  a 
  = 
  light 
  

   brown, 
  c 
  = 
  light 
  green. 
  The 
  presence 
  of 
  this 
  mineral 
  in 
  a 
  

   slide 
  seems 
  to 
  be 
  in 
  proportion 
  to 
  the 
  absence 
  of 
  the 
  ortho- 
  

   rhombic 
  pyroxene, 
  and 
  this 
  fact 
  together 
  with 
  its 
  shape 
  and 
  

   optical 
  properties 
  point 
  to 
  bastite 
  and 
  make 
  the 
  supposition 
  

   plausible 
  that 
  the 
  red-brown 
  mineral 
  is 
  the 
  present 
  representa- 
  

   tive 
  of 
  the 
  original 
  orthorhombic 
  pyroxene. 
  The 
  magnetite 
  is 
  

   present 
  in 
  grains 
  or 
  dust 
  aggregates, 
  and 
  the 
  apatite 
  occurs 
  in 
  

   needles, 
  laths, 
  and 
  rounded 
  sections 
  within 
  the 
  feldspars. 
  

  

  The 
  groundm.ass 
  consists 
  essentially 
  of 
  feldspar 
  laths, 
  long, 
  

   narrow, 
  with 
  ragged 
  outline 
  and 
  split 
  ends, 
  arranged 
  with 
  

   trachytic 
  structure 
  tending 
  toward 
  the 
  hyalopilitic, 
  and 
  with 
  

   flow 
  phenomena 
  developed 
  in 
  places. 
  No 
  close 
  distinction 
  can 
  

   be 
  drawn 
  between 
  the 
  groundmass 
  feldspars 
  and 
  those 
  which 
  

   rise 
  slightly 
  above 
  it, 
  as 
  all 
  sizes 
  are 
  represented 
  grading 
  up 
  to 
  

   the 
  very 
  largest 
  ones 
  present. 
  Optical 
  measurements 
  on 
  some 
  

   of 
  the 
  freshest 
  pieces 
  in 
  the 
  groundmass 
  proved 
  them 
  also 
  to 
  

   be 
  labradorite, 
  although 
  orthoclase 
  must 
  also 
  be 
  preserit 
  as 
  

   demanded 
  by 
  the 
  analysis. 
  Besides 
  the 
  feldspars, 
  augite 
  grains 
  

   are 
  scattered 
  abundantly 
  throughout 
  and 
  small 
  areas 
  of 
  brown 
  

   glass, 
  occasionally 
  with 
  bubbles, 
  also 
  occur. 
  The 
  whole 
  slide 
  

   is 
  darkened 
  by 
  iron 
  dust, 
  both 
  magnetite 
  and 
  limonite 
  or 
  

   gothite. 
  The 
  rock 
  is, 
  however, 
  in 
  a 
  remarkably 
  fresh 
  state 
  

   considering 
  its 
  age 
  and 
  position, 
  and 
  its 
  character 
  is 
  unmistak- 
  

   able. 
  

  

  Analysis. 
  — 
  The 
  analysis 
  of 
  this 
  rock 
  made 
  by 
  Dr. 
  W. 
  F. 
  

   Hillebrand 
  of 
  the 
  U. 
  S. 
  Geol. 
  Survey 
  is 
  given 
  in 
  column 
  I 
  

   below 
  and 
  with 
  it 
  analyses 
  (columns 
  II-YI) 
  of 
  well 
  known 
  

   andesites 
  from 
  other 
  localities 
  are 
  given 
  for 
  comparison. 
  

  

  From 
  a 
  study 
  of 
  the 
  tables 
  it 
  becomes 
  apparent 
  that 
  the 
  

   Edmund's 
  Hill 
  rock 
  presents 
  no 
  points 
  of 
  distinction 
  from 
  

   recognized 
  types 
  found 
  elsewhere, 
  and 
  the 
  tables 
  could 
  be 
  

   greatly 
  enlarged 
  by 
  the 
  addition 
  of 
  closely 
  similar 
  analyses. 
  

   The 
  analysis 
  in 
  column 
  YII 
  requires 
  some 
  notice. 
  The 
  rock 
  

   is 
  described 
  as 
  a 
  red 
  andesite 
  with 
  " 
  rather 
  basic" 
  feldspars 
  and 
  

   with 
  calcite 
  and 
  magnetite 
  present. 
  The 
  altered 
  condition 
  of 
  

   the 
  rock 
  made 
  accurate 
  optical 
  determination 
  impossible. 
  In 
  

   discussing 
  the 
  analysis 
  the 
  writer 
  says 
  (1. 
  c. 
  p. 
  34), 
  " 
  In 
  its 
  

   mineralogical 
  composition, 
  this 
  rock 
  approaches 
  the 
  basaltic 
  

   type 
  but, 
  as 
  the 
  analysis 
  shows, 
  is 
  somewhat 
  too 
  acid. 
  The 
  oli- 
  

   vine 
  phenocrysts, 
  moreover, 
  are 
  not 
  very 
  numerous 
  and 
  there 
  

  

  