﻿590 
  Washington 
  — 
  Some 
  Lavas 
  of 
  Monte 
  Arci, 
  Sardinia. 
  

  

  Q 
  ___ 
  4-50 
  

  

  Or 
  7*23 
  

  

  Ab 
  " 
  30-92 
  

  

  An 
  24-74 
  

  

  DL_. 
  3-80 
  

  

  Hy 
  17-22 
  

  

  Mt 
  _ 
  3-92 
  

  

  II 
  5-02 
  

  

  Ap 
  1-01 
  

  

  This 
  places 
  the 
  Arci 
  basalt 
  in 
  andose, 
  with 
  the 
  symbol 
  

   IF. 
  "5. 
  3.4", 
  the 
  Cuglieri 
  basalt 
  falling 
  in 
  the 
  salfemic 
  homo- 
  

   logue 
  camptonose, 
  "III.5.3.4/', 
  and 
  that 
  of 
  Ploaghe 
  in 
  akerose, 
  

   II.5.2.4. 
  The 
  mode 
  is 
  approximately 
  normative. 
  The 
  quartz 
  

   is 
  occult 
  in 
  the 
  glass 
  base, 
  and 
  most 
  of 
  the 
  normative 
  magnetite 
  

   and 
  ilmenite 
  is 
  present 
  in 
  the 
  black 
  globulites. 
  The 
  normative 
  

   hypersthene 
  and 
  diopside 
  express 
  very 
  closely 
  the 
  modal 
  rela- 
  

   tions 
  of 
  the 
  orthorhombic 
  and 
  monoclinic 
  pyroxenes. 
  Judging 
  

   from 
  its 
  color 
  the 
  latter 
  can 
  carry 
  but 
  little 
  ferric 
  iron. 
  The 
  

   mode 
  may 
  be 
  roughly 
  stated 
  thus 
  : 
  

  

  Quartz 
  (occult) 
  . 
  _ 
  . 
  . 
  4 
  

  

  Labradorite 
  ( 
  Ab 
  7 
  An 
  8 
  ) 
  62 
  

  

  Diopside 
  5 
  

  

  Hypersthene 
  17 
  

  

  Ores 
  _ 
  _. 
  8 
  

  

  Apatite 
  _ 
  1 
  

  

  Glass 
  3 
  

  

  General 
  remarks. 
  — 
  As 
  only 
  a 
  hasty 
  glimpse 
  was 
  had 
  of 
  a 
  

   part 
  of 
  the 
  volcano, 
  the 
  data 
  at 
  hand 
  do 
  not 
  warrant 
  any 
  

   lengthy 
  discussion 
  of 
  the 
  general 
  character 
  of 
  its 
  lavas. 
  My 
  

   slight 
  examination 
  was, 
  however, 
  sufficient 
  to 
  corroborate 
  the 
  

   statements 
  of 
  della 
  Marmora 
  and 
  show 
  that 
  in 
  structure 
  it 
  

   much 
  resembles 
  Monte 
  Ferru, 
  consisting 
  of 
  a 
  domal 
  core. 
  of 
  

   rhyolites 
  (possibly 
  with 
  phonolites), 
  followed 
  and 
  covered 
  by 
  

   flows 
  of 
  trachyte, 
  dacite 
  and 
  andesite, 
  the 
  whole 
  closing 
  with 
  

   great 
  outflows 
  of 
  rather 
  andesitic 
  basalts. 
  It 
  is 
  noteworthy 
  

   that 
  tuffs 
  and 
  scoria 
  beds 
  are 
  rare, 
  just 
  as 
  they 
  are 
  at 
  Monte 
  

   Ferru, 
  except 
  in 
  the 
  initial 
  stages. 
  

  

  So 
  far 
  as 
  known 
  the 
  rocks 
  are 
  distinctly 
  silicic, 
  more 
  so 
  than 
  

   at 
  Ferru, 
  where 
  rhyolites 
  are 
  not 
  known, 
  but 
  trachytes 
  and 
  

   phonolites 
  are 
  abundant. 
  They 
  are 
  distinctly 
  alkalic 
  and 
  

   mostly 
  sodipotassic. 
  These 
  features 
  find 
  their 
  modal 
  expres- 
  

   sion 
  in 
  the 
  absence 
  of 
  sodic 
  pyroxenes 
  and 
  amphiboles, 
  and 
  

   the 
  predominance 
  of 
  hypersthene 
  over 
  diopsidic 
  pyroxene. 
  

   Titanium 
  is 
  rather 
  high 
  and 
  its 
  amount 
  increases 
  regularly 
  

   with 
  decreasing 
  silicity. 
  

  

  Geophysical 
  Laboratory, 
  

  

  Carnegie 
  Institution 
  of 
  Washington. 
  

  

  