﻿584 
  Washington 
  — 
  Some 
  Lavas 
  of 
  Monte 
  Arci, 
  Sardinia. 
  

  

  from 
  inclusions. 
  Very 
  small 
  augite 
  and 
  magnetite 
  anhedrons 
  

   are 
  also 
  present, 
  but 
  are 
  rare. 
  The 
  ground 
  mass 
  is 
  dense 
  and 
  

   apparently 
  noncrystalline. 
  It 
  consists 
  of 
  a 
  cryptocrystalline 
  

   aggregate 
  of 
  feldspar 
  and 
  quartz, 
  showing 
  no 
  microperthitic 
  

   patches. 
  There 
  are 
  many 
  minute 
  ragged 
  anhedrons 
  of 
  augite 
  

   and 
  magnetite, 
  and 
  irregular 
  grains 
  of 
  a 
  brownish 
  yellow, 
  

   biaxial 
  mineral, 
  which 
  may 
  be 
  a 
  peculiar, 
  or 
  possibly 
  slightly 
  

   altered, 
  pyroxene. 
  A 
  similar 
  mineral 
  is 
  found 
  in 
  the 
  sheet 
  

   trachyte 
  of 
  Monte 
  Muradu 
  near 
  Macomer. 
  

  

  Analyses 
  of 
  Sardinian 
  Trachytes. 
  

  

  SiO 
  .... 
  

  

  A 
  

   65-94 
  

  

  A],0 
  3 
  

  

  Fe 
  0, 
  

  

  . 
  16-11 
  

  

  2*56 
  

  

  FeO 
  

  

  MgO 
  

  

  0-82 
  

  

  60 
  

  

  CaO 
  

  

  NaO... 
  

  

  ...... 
  1-06 
  

  

  5*27 
  

  

  K 
  

  

  6-49 
  

  

  H 
  0+ 
  ---- 
  ... 
  

  

  . 
  0-25 
  

  

  H 
  0- 
  

  

  ..: 
  0-36 
  

  

  2 
  

  

  Ti0 
  2 
  

  

  1-21 
  

  

  PO 
  

  

  MnO 
  

  

  . 
  0-06 
  

  

  B 
  

  

  Aa 
  

  

  59-92 
  

  

  1-099 
  

  

  14-30 
  

  

  •158 
  

  

  7*50 
  

  

  •016 
  

  

  0-42 
  

  

  •on 
  

  

  0-72 
  

  

  •015 
  

  

  1-90 
  

  

  •019 
  

  

  5*32 
  

  

  •085 
  

  

  5-77 
  

  

  •069 
  

  

  0-34 
  

  

  

  0-87 
  

  

  •015 
  

  

  0-58 
  

  

  

  0'06 
  

  

  •001 
  

  

  100-73 
  

  

  A. 
  Trachyte. 
  Conca 
  Cannas, 
  near 
  Uras, 
  Monte 
  Arci. 
  

  

  B. 
  Trachyte. 
  Nuraghe 
  Terchis, 
  Monte 
  Muradu, 
  near 
  Macomer. 
  

   Includes 
  O'll 
  Zr0 
  2 
  , 
  0-06 
  S0 
  3 
  , 
  0-05 
  BaO. 
  

  

  The 
  analysis 
  of 
  the 
  Monte 
  Arci 
  rock 
  is 
  that 
  of 
  an 
  ordinary 
  

   trachyte, 
  somewhat 
  high 
  in 
  silica. 
  It 
  has 
  no 
  known 
  counter- 
  

   part 
  among 
  Sardinian 
  lavas, 
  the 
  Monte 
  Muradu 
  trachyte 
  

   approaching 
  it 
  most 
  closely. 
  It 
  will 
  be 
  observed 
  that 
  in 
  both 
  

   of 
  these 
  ferric 
  oxide 
  is 
  much 
  higher 
  than 
  ferrous, 
  and 
  this 
  

   seems 
  to 
  be 
  connected 
  with 
  the 
  occurrence 
  of 
  the 
  peculiar 
  

   yellow 
  pyroxene, 
  a 
  point 
  to 
  be 
  investigated 
  later 
  in 
  connection 
  

   with 
  the 
  older 
  rock. 
  

  

  The 
  norm 
  of 
  A 
  is 
  as 
  follows 
  : 
  

  

  Q 
  .- 
  8-22 
  

  

  Or 
  _ 
  38-36 
  

  

  Ab 
  44-54 
  

  

  An 
  , 
  1-11 
  

  

  Di 
  2-38 
  

  

  Hy 
  0-40 
  

  

  II 
  --- 
  1-67 
  

  

  Hm 
  2-56 
  

  

  Tn 
  0-78 
  

  

  