﻿368 
  II. 
  E. 
  Gregory 
  — 
  Aiidesites 
  from 
  Maine. 
  

  

  rocks. 
  Similar 
  structures 
  liave 
  been 
  described 
  from 
  California,"^ 
  

   and 
  from 
  Scotlandf 
  and 
  elsewhere. 
  As 
  noticed 
  by 
  Geikie, 
  

   some 
  basic 
  lavas, 
  e. 
  g., 
  the 
  basalt 
  at 
  Acicastello 
  in 
  Sicily,:}: 
  on 
  

   flowing 
  into 
  water 
  or 
  a 
  watery 
  silt, 
  assumes 
  a 
  remarkable 
  sphe- 
  

   roidal 
  or 
  pillow-shaped 
  structure, 
  "the 
  spheroids 
  being 
  some- 
  

   times 
  pressed 
  into 
  shapes 
  like 
  piles 
  of 
  sacks." 
  This 
  may 
  be 
  

   the 
  explanation 
  in 
  the 
  present 
  case. 
  Another 
  interpretation 
  is 
  

   that 
  the. 
  structure 
  represents 
  the 
  ropy 
  rolling 
  surface 
  at 
  the 
  

   front 
  of 
  a 
  lava 
  flow. 
  On 
  a 
  fresh 
  surface 
  the 
  rock 
  is 
  dark 
  bluish- 
  

   gray, 
  uniform 
  in 
  texture 
  or 
  wnth 
  a 
  rare 
  feldspar 
  phenocryst. 
  

   While 
  this 
  appears 
  to 
  be 
  the 
  most 
  typical 
  of 
  the 
  textures, 
  it 
  is 
  

   usual 
  to 
  And 
  vesicles 
  now 
  filled 
  with 
  calcite 
  and 
  fragments 
  of 
  

   volcanic 
  debris 
  large 
  enough 
  to 
  constitute 
  a 
  conspicuous 
  feature 
  

   in 
  the 
  hand 
  specimen. 
  In 
  weathering, 
  the 
  amygdaloidal 
  parts 
  

   go 
  first 
  and 
  leave 
  the 
  more 
  dense 
  igneous 
  and 
  glassy 
  pebbles 
  

   exposed 
  as 
  a 
  very 
  rough 
  surface. 
  

  

  Microscopic 
  description. 
  — 
  Sections 
  were 
  cut 
  fi'om 
  the 
  

   densest 
  material 
  and 
  also 
  from 
  that 
  with 
  macroscopic 
  inclusions, 
  

   and 
  when 
  examined 
  with 
  the 
  microscope 
  showed 
  no 
  difference 
  

   except 
  in 
  size 
  of 
  vesicular 
  areas 
  and 
  in 
  method 
  of 
  alteration. 
  

   Feldspar 
  microlites 
  make 
  up 
  the 
  rock, 
  parts 
  of 
  which 
  are 
  devel- 
  

   oped 
  as 
  areas 
  of 
  vesicular 
  lava. 
  The 
  vesicles 
  range 
  in 
  diameter 
  

   from 
  2""" 
  to 
  microscopic 
  dots 
  and 
  are 
  rudely 
  oval 
  in 
  outline. 
  

   The 
  large 
  ones 
  are 
  merely 
  the 
  larger 
  part 
  of 
  a 
  rounded 
  area 
  of 
  

   vesicular 
  glassy 
  lava, 
  containing 
  a 
  few 
  feldspar 
  threads 
  like 
  the 
  

   body 
  of 
  the 
  rock. 
  Sometimes 
  instead 
  of 
  one 
  vesicle, 
  filled 
  with 
  

   calcite, 
  the 
  same 
  space 
  will 
  be 
  occupied 
  by 
  a 
  group 
  of 
  them, 
  or 
  

   the 
  concave 
  inner 
  border 
  of 
  the 
  large 
  one 
  m.ay 
  indicate 
  its 
  

   formation 
  from 
  several 
  smaller 
  ones. 
  Some 
  glassy 
  oval 
  areas 
  

   occur 
  with 
  vesicles 
  visible 
  only 
  under 
  the 
  highest 
  powers. 
  All 
  

   these 
  variations 
  are 
  doubtless 
  caused 
  by 
  the 
  fact 
  that 
  different 
  

   sections 
  of 
  similar 
  vesicular 
  areas 
  are 
  exposed 
  in 
  the 
  prepara- 
  

   tion 
  of 
  the 
  slide. 
  The 
  only 
  feldspar 
  phenocryst 
  seen 
  in 
  the 
  

   sections 
  is 
  rounded 
  in 
  outline, 
  has 
  albite 
  and 
  pericline 
  twin- 
  

   ning, 
  and 
  is 
  badly 
  altered 
  to 
  calcite. 
  Its 
  extinction-angle 
  

   indicates 
  albite 
  or 
  andesine, 
  and 
  from 
  the 
  fact 
  that 
  phenocrysts 
  

   are 
  usually 
  more 
  basic 
  than 
  the 
  components 
  of 
  the 
  groundmass 
  

   is 
  referred 
  to 
  andesine. 
  No 
  ferro-magnesian 
  mineral 
  is 
  present, 
  

   but 
  the 
  numerous 
  patches 
  of 
  chlorite 
  and 
  the 
  fact 
  that 
  augite 
  

   occurs 
  in 
  similar 
  rock 
  in 
  the 
  immediate 
  neighborhood 
  points 
  

   to 
  the 
  former 
  presence 
  of 
  pyroxene. 
  Besides 
  chlorite, 
  there 
  

   are 
  present 
  as 
  secondary 
  products 
  calcite, 
  a 
  few 
  epidote 
  grains, 
  

   and 
  abundant 
  iron 
  ore. 
  One 
  slide 
  is 
  sprinkled 
  full 
  of 
  stringy 
  

  

  *Ransome: 
  Bull. 
  Depart. 
  Geol. 
  Univ. 
  of 
  California, 
  vol. 
  i, 
  p. 
  106. 
  Fairbanks: 
  

   Bull. 
  Depart. 
  Geol. 
  Univ. 
  Cal., 
  vol. 
  ii, 
  p 
  40, 
  

  

  f 
  Geikie: 
  Ancient 
  Volcanoes 
  of 
  Gt. 
  Britain, 
  vol. 
  i, 
  p 
  19.3. 
  

   i 
  Johnston-Lavis: 
  South 
  Italian 
  Volcanoes, 
  p. 
  41. 
  

  

  