﻿hington 
  — 
  Some 
  Lavas 
  of 
  Monte 
  Arci, 
  Sardinia. 
  581 
  

  

  lite 
  also 
  occurs 
  as 
  a 
  well-defined 
  dike 
  some 
  20 
  to 
  30 
  meters 
  

   wide 
  and 
  running 
  vertically 
  east 
  and 
  west, 
  which 
  cuts 
  the 
  platy 
  

   rhyolite 
  of 
  Punta 
  Brenta, 
  near 
  the 
  entrance 
  to 
  Canale 
  Per- 
  

   diera. 
  The 
  rhyolite 
  bordering 
  it 
  has 
  been 
  rendered 
  much 
  

   denser, 
  of 
  a 
  brownish 
  color, 
  and 
  with 
  the 
  platy 
  structure 
  well 
  

   brought 
  out, 
  the 
  plates 
  being 
  very 
  thin 
  and 
  separated 
  from 
  

   each 
  other 
  by 
  slightly 
  vesicular 
  material. 
  In 
  thin 
  section 
  the 
  

   change 
  is 
  slight. 
  . 
  

  

  These 
  perlites 
  are 
  all 
  pure 
  ash-gray 
  in 
  color, 
  varying 
  from 
  

   rather 
  dark 
  to 
  rather 
  light, 
  and 
  show 
  a 
  distinct 
  rather 
  coarse 
  to 
  

   quite 
  fine 
  granular 
  texture. 
  The 
  perlite 
  which 
  forms 
  the 
  dike 
  

   at 
  Punta 
  Brenta 
  and 
  that 
  from 
  Conca 
  Cannas 
  show 
  some 
  small 
  

   phenocrysts 
  of 
  feldspar 
  and 
  biotite, 
  which 
  are 
  wanting 
  in 
  most 
  

   of 
  the 
  specimens. 
  A 
  perlite 
  from 
  the 
  ridge 
  southeast 
  of 
  

   Capanna 
  is 
  rather 
  platy 
  and 
  is 
  thickly 
  sprinkled 
  with 
  small 
  

   (2 
  — 
  10 
  mm 
  ) 
  rounded 
  and 
  subangular 
  pieces 
  of 
  black 
  obsidian. 
  

   These 
  have 
  caused 
  perlitic 
  cracking 
  around 
  them, 
  as 
  they 
  are 
  

   surrounded 
  by 
  a 
  shell 
  of 
  gray 
  perlite. 
  

  

  In 
  thin 
  section 
  the 
  perlites 
  show 
  a 
  clear, 
  colorless 
  glass, 
  

   which 
  is 
  perlitically 
  cracked, 
  the 
  amount 
  of 
  cracking 
  and 
  the 
  

   size 
  and 
  perfection 
  of 
  the 
  curves 
  corresponding 
  to 
  the 
  mega- 
  

   scopic 
  granularity. 
  There 
  are 
  a 
  few 
  small 
  phenocrysts 
  of 
  

   alkali 
  feldspar 
  and 
  some 
  thin 
  tables 
  of 
  biotite, 
  but 
  the 
  amount 
  

   of 
  these 
  cannot 
  be 
  more 
  than 
  about 
  one 
  per 
  cent, 
  and 
  in 
  some 
  

   specimens 
  they 
  are 
  wholly 
  wanting. 
  The 
  glass 
  is 
  sprinkled 
  

   with 
  microlitic 
  crystals 
  of 
  alkali 
  feldspar, 
  very 
  thin 
  flakes 
  of 
  

   biotite 
  and 
  minute 
  prisms 
  of 
  colorless 
  pyroxene, 
  all 
  of 
  which 
  

   are 
  arranged 
  in 
  a 
  well-developed 
  flow 
  structure. 
  Some 
  of 
  the 
  

   specimens, 
  notably 
  those 
  from 
  the 
  Conca 
  su 
  Ollastru, 
  contain 
  

   many 
  streaks 
  of 
  brownish 
  spherulites, 
  which 
  have 
  developed 
  

   around 
  phenocrysts 
  of 
  feldspar 
  or 
  mica. 
  These 
  spherulitic 
  

   growths 
  are 
  either 
  spherical, 
  axiolitic, 
  or 
  more 
  often 
  reniform. 
  

   A 
  central 
  core 
  is 
  rather 
  dark 
  brown, 
  surrounded 
  by 
  a 
  wide 
  

   border 
  of 
  lighter 
  color. 
  These 
  spherulites 
  have 
  no 
  appreciable 
  

   action 
  on 
  polarized 
  light, 
  and 
  the 
  perlitic 
  cracks 
  pass 
  through 
  

   them 
  without 
  change. 
  A 
  chemical 
  analysis 
  will 
  be 
  given 
  

  

  Obsidian. 
  — 
  Black 
  obsidian 
  was 
  found 
  in 
  nearly 
  all 
  the 
  concas 
  

   and. 
  according 
  to 
  the 
  descriptions 
  of 
  della 
  Marmora, 
  it 
  is 
  very 
  

   common 
  all 
  over 
  Monte 
  Arci. 
  It 
  is 
  generally 
  scattered 
  in 
  

   angular 
  fragments 
  over 
  or 
  through 
  the 
  ground, 
  in 
  places 
  so 
  

   abundantly 
  as 
  to 
  suggest 
  the 
  debris 
  of 
  a 
  great 
  factory 
  of 
  black 
  

   bottles, 
  to 
  use 
  della 
  Marmora's 
  simile. 
  I 
  observed 
  it 
  in 
  

   place 
  nowhere, 
  and 
  della 
  Marmora 
  notes 
  it 
  either 
  as 
  dikes 
  or 
  

   beds 
  between 
  rhyolite 
  flows 
  only 
  in 
  a 
  few 
  places. 
  

  

  These 
  obsidians 
  are 
  extremely 
  dense 
  and 
  compact, 
  with 
  

   highly 
  perfect 
  conchoidal 
  fracture. 
  They 
  are 
  jet 
  black, 
  show- 
  

  

  