﻿362 
  11. 
  E. 
  Gregory 
  — 
  Andesites 
  from 
  Maine. 
  

  

  the 
  glaciers 
  have 
  planed 
  off 
  the 
  old 
  lavas 
  and 
  they 
  have 
  been 
  

   protected 
  from 
  weatherino^, 
  the 
  outlines 
  of 
  bombs 
  and 
  pillows 
  

   are 
  plainly 
  revealed, 
  and 
  v^hen 
  weathered 
  these 
  bombs 
  are 
  

   loosened 
  and 
  drop 
  out 
  as 
  oval 
  or 
  egg-shaped 
  bodies 
  w^ith 
  amyg- 
  

   daloidal 
  surface 
  and 
  denser 
  interior, 
  and 
  lie 
  about 
  thickly 
  strew- 
  

   ing 
  the 
  fields. 
  In 
  one 
  place 
  there 
  is 
  a 
  cistern-like 
  depression 
  

   some 
  ten 
  feet 
  deep 
  and 
  thirty 
  feet 
  in 
  diameter 
  made 
  in 
  the 
  

   solid 
  andesite, 
  while 
  about 
  it 
  are 
  piled 
  close 
  at 
  hand 
  a 
  great 
  

   number 
  of 
  very 
  vesicular 
  bombs 
  and 
  much 
  glassy 
  and 
  brec- 
  

   ciated 
  ash. 
  The 
  whole 
  appearance 
  suggests 
  a 
  small 
  blowhole 
  

   made 
  by 
  a 
  single 
  explosion. 
  The 
  striking 
  fact 
  about 
  all 
  the 
  

   volcanic 
  accumulations 
  in 
  the 
  Castle 
  Hill 
  region 
  is 
  their 
  fresh- 
  

   ness 
  and 
  their 
  unmistakable 
  character. 
  

  

  Petrography. 
  

  

  Generally 
  speaking, 
  the 
  andesites 
  of 
  this 
  region 
  belong 
  to 
  

   well-recognized 
  varieties 
  widely 
  distributed 
  over 
  the 
  earth 
  and 
  

   differ 
  in 
  no 
  important 
  particulars 
  from 
  the 
  type 
  rocks 
  of 
  their 
  

   class. 
  There 
  are 
  varieties 
  found 
  here, 
  however, 
  which 
  are 
  

   intermediate 
  between 
  andesites 
  and 
  trachytes 
  and 
  also 
  occur- 
  

   rences 
  with 
  dacite 
  facies. 
  The 
  exposures 
  are 
  numerous 
  and 
  

   easy 
  of 
  access 
  and 
  the 
  specimens 
  are 
  no 
  more 
  altered 
  in 
  compo- 
  

   sition 
  than 
  if 
  they 
  were 
  Tertiary 
  lavas 
  instead 
  of 
  Paleozoic. 
  

  

  Aicgite- 
  Andesite 
  — 
  Macroscopic 
  description. 
  — 
  The 
  largest 
  and 
  

   best 
  single 
  exposure 
  of 
  andesite 
  in 
  this 
  region 
  is 
  of 
  this 
  variety 
  

   and 
  forms 
  the 
  main 
  mass 
  of 
  Edmund's 
  Hill. 
  It 
  does 
  not 
  occur 
  

   as 
  a 
  solid 
  compact 
  mass, 
  but 
  is 
  broken 
  by 
  cleavage 
  and 
  shear- 
  

   ing 
  planes 
  into 
  large 
  blocks 
  on 
  top, 
  and 
  into 
  plates 
  and 
  slated 
  

   material 
  at 
  the 
  ends 
  of 
  the 
  hill. 
  This 
  slated 
  and 
  seemingly 
  

   bedded 
  appearance, 
  which 
  is 
  so 
  unusual 
  in 
  an 
  igneous 
  rock, 
  is 
  

   the 
  most 
  marked 
  peculiarity 
  of 
  the 
  structure 
  of 
  the 
  hill. 
  In 
  a 
  

   few 
  places 
  the 
  rock 
  is 
  seen 
  to 
  contain 
  embedded 
  angular 
  peb- 
  

   bles 
  of 
  glass 
  and 
  baked 
  siliceous 
  material 
  which 
  stand 
  out 
  

   when 
  it 
  weathers 
  ; 
  and 
  in 
  other 
  places 
  the 
  rock 
  presents 
  a 
  

   banded 
  surface 
  of 
  gray 
  and 
  brown 
  giving 
  the 
  appearance 
  of 
  

   bedding, 
  but 
  which 
  prove 
  on 
  examination 
  to 
  be 
  varying 
  stages 
  

   of 
  decomposition 
  along 
  potential 
  cleavages. 
  With 
  these 
  excep- 
  

   tions 
  the 
  exposed 
  rock 
  has 
  a 
  uniform 
  appearance, 
  gray 
  where 
  

   w^eathered, 
  black 
  where 
  fresh. 
  

  

  Andesites 
  are 
  so 
  well 
  known 
  that 
  an 
  extended 
  macroscopic 
  

   description 
  is 
  unnecessary 
  and 
  will 
  not 
  be 
  attempted. 
  The 
  

   hand 
  specimen 
  appears 
  as 
  a 
  black, 
  basaltic-looking 
  rock, 
  gener- 
  

   ally 
  dense, 
  with 
  a 
  stringy 
  effect 
  and 
  sprinkled 
  over 
  with 
  glassy 
  

   feldspar 
  2""" 
  and 
  less 
  in 
  length. 
  The 
  weathered 
  surface 
  is 
  a 
  

   layer 
  of 
  spongy, 
  gray-brown 
  material 
  in 
  which 
  the 
  pores 
  are 
  

   made 
  by 
  the 
  decay 
  of 
  the 
  larger 
  feldspars. 
  At 
  the 
  east 
  end 
  of 
  

  

  