﻿Washington 
  — 
  Some 
  Lavas 
  of 
  Monte 
  Arci, 
  Sardinia. 
  583 
  

  

  noting, 
  ferric 
  oxide 
  predominating 
  in 
  the 
  first 
  and 
  ferrous 
  

   oxide 
  in 
  the 
  others. 
  

  

  They 
  are 
  much 
  like 
  the 
  analyses 
  of 
  typical 
  comendite 
  (D, 
  E), 
  

   though 
  this 
  is 
  slightly 
  lower 
  in 
  alumina 
  and 
  higher 
  in 
  ferric 
  

   oxide, 
  in 
  these 
  respects 
  resembling 
  the 
  pantellerites. 
  The 
  last 
  

   are, 
  however, 
  notably 
  more 
  sodic* 
  An 
  as 
  yet 
  unpublished 
  

   analysis 
  (F) 
  of 
  a 
  red 
  rhyolite 
  which 
  forms 
  one 
  of 
  the 
  pre-Ter- 
  

   tiary 
  sheets 
  near 
  Macomer 
  is 
  also 
  given. 
  This 
  is 
  like 
  the 
  others, 
  

   but 
  is 
  somewhat 
  higher 
  in 
  iron 
  and 
  lime. 
  

  

  The 
  norms 
  of 
  the 
  Monte 
  Arci 
  rhyolites 
  are 
  as 
  follows 
  : 
  

  

  ABC 
  

  

  Q 
  28-50 
  26-58 
  31*80 
  

  

  Or 
  31-69 
  31-14 
  28*36 
  

  

  Ab... 
  31-96 
  30-39 
  31*44 
  

  

  An 
  3-61 
  4*17 
  3-61 
  

  

  C 
  . 
  0-41 
  1*12 
  

  

  Hy 
  - 
  0-90 
  1-99 
  2'74 
  

  

  if 
  0-76 
  0-91 
  0-15 
  

  

  Hm 
  0-48 
  

  

  Ap 
  0-34 
  0*34 
  

  

  From 
  these 
  it 
  is 
  seen 
  that 
  all 
  these 
  rhyolites 
  fall 
  in 
  liparose 
  

   (1.4.1.3), 
  the 
  liparite 
  centrally 
  and 
  the 
  perlite 
  and 
  obsidian 
  

   distally,f 
  being 
  transitional 
  toward 
  (almost 
  on 
  the 
  border 
  of) 
  the 
  

   domalkalic 
  rang, 
  with 
  the 
  full 
  symbol 
  I.4.1(2).3. 
  The 
  small 
  

   amount 
  of 
  normative 
  corundum 
  in 
  A 
  and 
  B 
  belongs, 
  of 
  course, 
  

   to 
  the 
  biotite, 
  and 
  it 
  may 
  be 
  worth 
  noting 
  that 
  its 
  amount 
  is 
  

   roughly 
  proportional 
  to 
  that 
  of 
  the 
  biotite 
  seen 
  in 
  the 
  speci- 
  

   mens. 
  The 
  amounts 
  of 
  mafic 
  minerals 
  in 
  the 
  lithoidal 
  rhyo- 
  

   lite 
  are 
  so 
  negligible 
  that 
  the 
  mode 
  is 
  almost 
  absolutely 
  norma- 
  

   tive. 
  

  

  Trachyte 
  (Phlegrose 
  1.5. 
  IS). 
  

  

  The 
  only 
  occurrence 
  of 
  trachyte 
  was 
  found 
  as 
  blocks 
  in 
  the 
  

   Conca 
  Cannas, 
  which 
  are 
  apparently 
  derived 
  from 
  a 
  flow 
  in 
  its 
  

   south 
  wall. 
  This 
  was 
  not 
  found, 
  but 
  it 
  overlies 
  rhyolite, 
  of 
  

   which 
  the 
  lower 
  parts 
  are 
  composed. 
  

  

  The. 
  rock 
  is 
  rather 
  light 
  gray, 
  very 
  compact, 
  and 
  dopatic. 
  

   Numerous 
  equant 
  to 
  thick 
  tabular 
  crystals 
  of 
  glassy 
  alkali 
  

   feldspar 
  are 
  the 
  only 
  phenocrysts. 
  They 
  vary 
  in 
  size 
  from 
  2 
  

   to 
  5 
  mm 
  . 
  The 
  groundmass 
  is 
  gray, 
  dense, 
  felsitic 
  and 
  aphanitic. 
  

  

  In 
  thin 
  section 
  the 
  feldspar 
  phenocrysts 
  are 
  seen 
  to 
  be 
  of 
  

   soda-microcline, 
  either 
  in 
  simple 
  crystals 
  or 
  Carlsbad 
  twins. 
  

   The 
  extinction 
  on 
  c(001) 
  is 
  about 
  7°. 
  They 
  are 
  quite 
  free 
  

  

  *For 
  recent 
  analyses 
  of 
  these 
  see 
  H. 
  S. 
  Washington, 
  Jour. 
  Geo]., 
  xxi, 
  

   1913. 
  

  

  fThis 
  term 
  "distal" 
  is 
  borrowed 
  from 
  the 
  organic 
  sciences 
  to 
  indicate 
  

   either 
  the 
  intermediate 
  or 
  transitional 
  position, 
  near 
  the 
  border, 
  in 
  opposi- 
  

   tion 
  to 
  "central." 
  The 
  terms 
  would 
  seem 
  to 
  be 
  self-explanatory. 
  

  

  