﻿Washington 
  — 
  Some 
  Lavas 
  of 
  Monte 
  Arci, 
  Sardinia. 
  585 
  

  

  The 
  mode 
  is 
  essentially 
  normative 
  and, 
  giving 
  the 
  small 
  

   amounts 
  of 
  anorthite 
  and 
  titanite 
  to 
  the 
  pyroxenes, 
  would 
  be 
  

   roughly 
  as 
  follows 
  : 
  

  

  Quartz 
  . 
  8 
  

  

  Soda-microcline 
  83 
  

  

  Pyroxene 
  6 
  

  

  IroD 
  ores 
  3 
  

  

  Hypersthene 
  Andesite 
  (Dacose, 
  II.J^.2.^ 
  and 
  to-nalose, 
  JI.4.S.4). 
  

  

  Andesitic 
  lava 
  flows 
  were 
  found 
  overlying 
  those 
  of 
  rhyolite 
  

   in 
  several 
  places. 
  Though 
  they 
  differ 
  somewhat 
  in 
  chemical 
  

   composition, 
  especially 
  as 
  regards 
  the 
  amount 
  of 
  silica, 
  they 
  

   are 
  much 
  alike 
  modally, 
  and 
  so 
  had 
  best 
  be 
  grouped 
  and 
  

   described 
  together. 
  They 
  grade 
  into 
  the 
  basalts, 
  which 
  are 
  

   andesitic 
  in 
  character, 
  but 
  will 
  be 
  described 
  separately. 
  

  

  One 
  of 
  the 
  best 
  exposures 
  found 
  is 
  in 
  the 
  north 
  wall 
  of 
  

   Canale 
  Perdiera, 
  near 
  Uras. 
  The 
  lower 
  10 
  to 
  20 
  meters 
  of 
  

   this 
  is 
  a 
  gray 
  rhyolite, 
  covered 
  by 
  rhyolitic 
  tuffs 
  and 
  a 
  sheet 
  

   of 
  light 
  gray 
  perlite. 
  Above 
  this 
  is 
  a 
  thick 
  (about 
  15 
  meters) 
  

   coarsely 
  columnar 
  flow 
  of 
  rather 
  coarse-grained, 
  basaltic-look- 
  

   ing 
  andesite. 
  On 
  the 
  same 
  level 
  as 
  this, 
  but 
  cut 
  off 
  by 
  a 
  small 
  

   side 
  valley, 
  is 
  a 
  slightly 
  thicker 
  flow 
  of 
  a 
  fine-grained, 
  dark 
  red, 
  

   basaltic 
  rock 
  (andesite). 
  Above 
  both 
  there 
  is 
  a 
  10 
  to 
  50 
  meter 
  

   thick 
  flow, 
  with 
  well-developed 
  columnar 
  structure, 
  of 
  a 
  quite 
  

   granular, 
  light 
  gray 
  andesite. 
  This 
  extends 
  to 
  the 
  visible 
  top 
  

   of 
  the 
  cliff, 
  though 
  some 
  fallen 
  blocks 
  of 
  basalt 
  indicate 
  that 
  a 
  

   basalt 
  flow 
  covers 
  the 
  whole. 
  

  

  Flows 
  of 
  similar 
  basaltic-looking 
  andesite 
  are 
  met 
  with 
  

   covering 
  the 
  rhyolite 
  in 
  the 
  two 
  unnamed 
  concas 
  south 
  of 
  

   Conca 
  su 
  Ollastru, 
  below 
  the 
  Eione 
  Pranu 
  Pira. 
  What 
  is 
  

   probably 
  a 
  similar 
  rock 
  covers 
  the 
  ridge 
  between 
  Conca 
  Per- 
  

   diera 
  and 
  Conca 
  Cannas, 
  though 
  the 
  loss 
  of 
  the 
  specimen 
  ren- 
  

   ders 
  it 
  uncertain. 
  

  

  In 
  the 
  hand 
  specimen 
  these 
  andesites 
  look 
  like 
  basalts. 
  

   They 
  are 
  all 
  wholly 
  devoid 
  of 
  phenocrysts. 
  That 
  which 
  forms 
  

   the 
  visible 
  upper 
  flow 
  of 
  Canale 
  Perdiera 
  (analyzed 
  below) 
  is 
  

   millimeter-grained, 
  light 
  gray, 
  consisting 
  of 
  dark 
  gray 
  specks 
  

   in 
  a 
  white 
  to 
  light 
  gray 
  base. 
  The 
  flow 
  below 
  it 
  is 
  finely 
  mot- 
  

   tled, 
  of 
  small 
  black 
  patches 
  in 
  an 
  interstitial, 
  pinkish 
  brown, 
  

   felsitic 
  material. 
  The 
  red 
  rock 
  is 
  a 
  true 
  brick-red, 
  very 
  fine- 
  

   grained, 
  aphanitic 
  and 
  compact, 
  except 
  for 
  some 
  small 
  vesicles. 
  

   The 
  red 
  color 
  is 
  due 
  to 
  weathering, 
  but 
  it 
  extends 
  to 
  the 
  center 
  

   of 
  the 
  largest 
  blocks 
  examined 
  and 
  the 
  whole 
  flow, 
  from 
  top 
  

   to 
  bottom, 
  is 
  of 
  the 
  same 
  uniform 
  color. 
  The 
  blocks 
  from 
  the 
  

   flows 
  below 
  Pione 
  Pranu 
  Pira 
  are 
  dark 
  gray, 
  almost 
  black, 
  

   fine-grained, 
  aphanitic 
  and 
  compact. 
  None 
  of 
  the 
  specimens 
  

  

  