﻿Washington 
  — 
  Some 
  Lavas 
  of 
  Monte 
  Arci, 
  Sardinia. 
  587 
  

  

  andesine 
  and 
  hypersthene 
  crystals 
  is 
  a 
  base 
  of 
  colorless 
  glass, 
  

   which 
  appears 
  to 
  be 
  gray 
  through 
  the 
  presence 
  of 
  much 
  

   exceedingly 
  line 
  black 
  dust. 
  

  

  The 
  brick 
  red 
  basaltic 
  andesite 
  from 
  Canale 
  Perdiera 
  much 
  

   resembles 
  this, 
  except 
  that 
  here 
  the 
  hypersthenes 
  uniformly 
  

   have 
  a 
  yellowish 
  brown, 
  transparent 
  border, 
  which 
  much 
  

   resembles 
  the 
  iddingsite 
  that 
  surrounds 
  some 
  olivines. 
  This 
  

   border, 
  which 
  gives 
  the 
  rock 
  its 
  peculiar 
  color, 
  is 
  obviously 
  due 
  

   to 
  incipient 
  alteration. 
  The 
  small 
  augites 
  are 
  less 
  liable 
  to 
  

   attack, 
  and 
  the 
  feldspars 
  are 
  quite 
  unaltered. 
  

  

  The 
  chemical 
  composition 
  is 
  represented 
  by 
  two 
  analyses, 
  

   with 
  which 
  are 
  given 
  for 
  comparison 
  two 
  others 
  made 
  by 
  me 
  

   of 
  pre-Tertiary 
  andesitic 
  flows. 
  

  

  Analyses 
  of 
  Sardinian 
  Andesites. 
  

  

  Si0 
  2 
  

  

  A1 
  2 
  3 
  

  

  Fe 
  2 
  3 
  

  

  FeO 
  

  

  MgO 
  

  

  CaO 
  

  

  Na 
  2 
  

  

  K 
  2 
  

  

  H 
  2 
  + 
  

  

  H 
  2 
  0- 
  

  

  Ti0 
  2 
  

  

  p 
  2 
  a 
  5 
  

  

  MnO 
  

  

  61-08 
  

   13-66 
  

   0-70 
  

   5-61 
  

   4-69 
  

   4-84 
  

   3-84 
  

   2-23 
  

   0-74 
  

   0-49 
  

   1-76 
  

   0-17 
  

  

  B 
  

   56-34 
  

   13-95 
  

   1-94 
  

   6-73 
  

   6-41 
  

   6-20 
  

   3-10 
  

   0-76 
  

   1-04 
  

   0-63 
  

   2-22 
  

   0-44 
  

  

  C 
  

   60 
  14 
  

   1665 
  

   2-94 
  

   2-39 
  

   1-16 
  

   5-21 
  

   3-41 
  

   2-51 
  

   3-98 
  

   0-54 
  

   062 
  

   0-07 
  

   0-06 
  

  

  D 
  

   56-60 
  

   16-80 
  

   2-52 
  

   5-12 
  

   3-80 
  

   7-29 
  

   2-43 
  

   1-98 
  

   1-80 
  

   0-58 
  

   0-99 
  

   0-12 
  

   0-13 
  

  

  Aa 
  

  

  L-018 
  

   •134 
  

   •004 
  

   •078 
  

   •117 
  

   •087 
  

   062 
  

   •023 
  

  

  •022 
  

   •001 
  

  

  Ba 
  

   •939 
  

   137 
  

   ■012 
  

   •093 
  

   •160 
  

   •111 
  

   •050 
  

   •008 
  

  

  •028 
  

   •003 
  

  

  99-81 
  99-79 
  99'68 
  100-16 
  

  

  A. 
  Hypersthene 
  vitro-andesite. 
  Below 
  Eione 
  Pranu 
  Pira. 
  Monte 
  Arci. 
  

  

  B. 
  Augite-hypersthene 
  andesite, 
  upper 
  flow, 
  Canale 
  Perdiera, 
  Monte 
  Arci. 
  

  

  C. 
  Augite 
  vitro-andesite. 
  Monte 
  Furru, 
  near 
  Bosa. 
  

  

  D. 
  Augite 
  andesite. 
  Monte 
  Pischinale, 
  near 
  Bosa. 
  

   Aa 
  and 
  Ba, 
  mol 
  numbers 
  of 
  A 
  and 
  B. 
  

  

  Apart 
  from 
  the 
  low 
  alumina 
  and 
  high 
  titanium 
  these 
  analyses 
  

   resemble 
  those 
  of 
  many 
  other 
  andesites. 
  It 
  is 
  of 
  interest 
  to 
  

   compare 
  them 
  with 
  the 
  earlier 
  flows 
  a 
  little 
  farther 
  north. 
  

   With 
  about 
  the 
  same 
  silica, 
  the 
  two 
  types 
  differ 
  in 
  about 
  the 
  

   same 
  way 
  ; 
  the 
  earlier 
  having 
  higher 
  alumina, 
  ferric 
  oxide, 
  

   lime 
  and 
  potash, 
  with 
  lower 
  ferrous 
  oxide, 
  magnesia, 
  soda, 
  

   titanium 
  and 
  phosphorus. 
  The 
  relations 
  will 
  be 
  discussed 
  at 
  

   length 
  later, 
  when 
  more 
  analyses 
  are 
  available. 
  

   The 
  norms 
  are 
  as 
  follows 
  : 
  

  

  A 
  B 
  

  

  Q 
  12-36 
  ' 
  11-64 
  

  

  Or 
  _ 
  12-79 
  4-45 
  

  

  Ab 
  32-49 
  26-20 
  

  

  An 
  .' 
  _. 
  _ 
  13-62 
  21-96 
  

  

  