﻿582 
  Washington 
  — 
  Some 
  Lavas 
  of 
  Monte 
  Arci, 
  Sardinia. 
  

  

  ing 
  gray 
  on 
  thin 
  edges, 
  due 
  to 
  many 
  black 
  specks, 
  and 
  are 
  

   absolutely 
  free 
  from 
  phenocrysts 
  of 
  any 
  kind. 
  East 
  of 
  the 
  

   Conca 
  su 
  Ollastru, 
  in 
  the 
  Rione 
  Prasuedu, 
  were 
  many 
  angular, 
  

   but 
  somewhat 
  worn, 
  small 
  fragments 
  of 
  a 
  reddish 
  brown 
  

   obsidian 
  mottled 
  and 
  streaked 
  with 
  black. 
  

  

  In 
  thin 
  section 
  these 
  black 
  obsidians 
  show 
  an 
  absolutely 
  

   colorless 
  glass, 
  perfectly 
  free 
  from 
  perlitic 
  cracks. 
  There 
  are 
  

   very 
  few 
  small 
  crystals 
  of 
  feldspar 
  and 
  biotite, 
  and 
  many 
  black 
  

   opaque 
  microlites, 
  which 
  are 
  either 
  short 
  belonites 
  or 
  small 
  

   trichitic 
  bunches, 
  the 
  much 
  curved 
  hairs 
  radiating 
  from 
  a 
  

   common 
  center. 
  The 
  red 
  portions 
  of 
  the 
  mottled 
  obsidian 
  

   pebbles 
  show 
  many 
  thin, 
  isotropic, 
  bright 
  orange 
  or 
  yellow 
  

   streaks 
  in 
  a 
  colorless 
  glass. 
  

  

  Chemical 
  Composition. 
  — 
  Three 
  analyses 
  made 
  by 
  me 
  are 
  

   given 
  in 
  the 
  annexed 
  table, 
  one 
  of 
  the 
  typical 
  lithoidal 
  rhyolite 
  

   whose 
  crevices 
  contain 
  the 
  unknown 
  hexagonal 
  mineral, 
  one 
  

   of 
  the 
  perlite 
  of 
  the 
  dike 
  at 
  Punta 
  Brenta, 
  and 
  one 
  of 
  a 
  typical 
  

   obsidian. 
  

  

  Analyses 
  of 
  Sardinian 
  Ehyolites. 
  

  

  

  A 
  

  

  B 
  

  

  C 
  

  

  D 
  

  

  E 
  

  

  F 
  

  

  Aa 
  

  

  Ba 
  

  

  Ca 
  

  

  Si0 
  2 
  

  

  78 
  09 
  

  

  70-50 
  

  

  74-61 
  

  

  74-76 
  

  

  73-23 
  

  

  72-05 
  

  

  1-218 
  

  

  1-175 
  

  

  1-244 
  

  

  A1 
  2 
  3 
  

  

  13-80 
  

  

  14-28 
  

  

  12-68 
  

  

  11-60 
  

  

  12-25 
  

  

  13-07 
  

  

  •135 
  

  

  •140 
  

  

  124 
  

  

  Fe 
  2 
  3 
  

  

  1-28 
  

  

  0-75 
  

  

  0-09 
  

  

  3-50 
  

  

  3-25 
  

  

  2-93 
  

  

  •008 
  

  

  •005 
  

  

  •001 
  

  

  FeO 
  

  

  0-68 
  

  

  1-22 
  

  

  1-36 
  

  

  0-19 
  

  

  0-28 
  

  

  0-39 
  

  

  •010 
  

  

  •017 
  

  

  •019 
  

  

  MgO 
  

  

  0-37 
  

  

  0-49 
  

  

  0-21 
  

  

  0-18 
  

  

  0-13 
  

  

  0-66 
  

  

  •009 
  

  

  •012 
  

  

  •005 
  

  

  CaO 
  

  

  0-69 
  

  

  1-00 
  

  

  69 
  

  

  0-07 
  

  

  0-25 
  

  

  1-30 
  

  

  •013 
  

  

  •018 
  

  

  •013 
  

  

  Na,0 
  

  

  3-77 
  

  

  3-62 
  

  

  3-68 
  

  

  4-35 
  

  

  4-44 
  

  

  3-49 
  

  

  •061 
  

  

  •058 
  

  

  •060 
  

  

  K 
  a 
  O 
  

  

  5-36 
  

  

  5-28 
  

  

  4-77 
  

  

  4-92 
  

  

  4-32 
  

  

  4-55 
  

  

  •057 
  

  

  •056 
  

  

  •051 
  

  

  H 
  2 
  + 
  

  

  0-60 
  

  

  2-86 
  

  

  1-37 
  

  

  0-64 
  

  

  1-74 
  

  

  0-59 
  

  

  

  

  

  H 
  2 
  0- 
  

  

  0-72 
  

  

  0-10 
  

  

  0-04 
  

  

  0-24 
  

  

  

  

  

  TiO 
  a 
  

  

  0-38 
  

  

  0-47 
  

  

  0-08 
  

  

  tr. 
  

  

  

  0-40 
  

  

  •005 
  

  

  •006 
  

  

  •001 
  

  

  Zr0 
  2 
  

  

  0-02 
  

  

  

  _ 
  

  

  

  

  005 
  

  

  

  

  

  P 
  2 
  5 
  

  

  0-07 
  

  

  0-11 
  

  

  n. 
  d". 
  

  

  tr. 
  

  

  

  0-22 
  

  

  •001 
  

  

  •001 
  

  

  

  

  S0 
  3 
  

  

  none 
  

  

  

  

  

  

  

  0-18 
  

  

  

  

  

  MnO 
  

  

  tr. 
  

  

  0-04 
  

  

  n.'d. 
  

  

  n.'d. 
  

  

  

  004 
  

  

  

  

  

  BaO 
  

  

  none 
  

  

  --- 
  

  

  99-58 
  

  

  .... 
  

  

  99-89 
  

  

  0-06 
  

   100-22 
  

  

  

  

  

  

  100-83 
  

  

  100-72 
  

  

  100-21 
  

  

  

  A. 
  

  

  Liparite. 
  

  

  Capanna, 
  near 
  

  

  Marrubiu, 
  Monte 
  Arci. 
  

  

  

  

  

  B. 
  

  

  Perlite. 
  Punta 
  Brenta 
  near 
  Uras, 
  

  

  Monte 
  Arci. 
  

  

  

  

  

  C. 
  

  

  Obsidian. 
  

  

  Conca 
  Cannas 
  

  

  , 
  near 
  Uras, 
  Monte 
  Arci 
  

  

  

  

  

  D. 
  

  

  Coniendite 
  

  

  . 
  Comende, 
  San 
  Pietro 
  Island. 
  Dittrich, 
  

  

  analyst. 
  

  

  H. 
  

  

  Bosenbuseh, 
  Elemente, 
  1898, 
  p. 
  

  

  257. 
  

  

  

  

  

  

  

  E. 
  

  

  Comendite 
  

  

  . 
  San 
  Pietro 
  Island. 
  

  

  A. 
  Johnsen, 
  analyst. 
  

  

  A. 
  Johnsen, 
  

  

  Neues 
  Jahrb. 
  Centralbl., 
  1912, 
  p. 
  738. 
  

   F. 
  Rhyolite. 
  Macomer. 
  

   Aa, 
  Ba, 
  Ca, 
  mol 
  numbers 
  of 
  A, 
  B, 
  C. 
  

  

  The 
  analyses 
  are 
  those 
  of 
  ordinary 
  sodipotassic 
  rhyolites 
  and 
  

   are 
  much 
  alike, 
  the 
  lower 
  silica 
  of 
  the 
  perlite 
  (B) 
  being 
  due 
  in 
  

   part 
  to 
  the 
  presence 
  of 
  water. 
  The 
  relations 
  of 
  the 
  iron 
  oxides 
  

   in 
  the 
  noncrystalline 
  rhyolite 
  and 
  the 
  glassy 
  forms 
  are 
  worth 
  

  

  