﻿586 
  Washington 
  — 
  Some 
  Lavas 
  of 
  Monte 
  Arci, 
  Sardinia, 
  

  

  show 
  phenocrysts, 
  except 
  for 
  some 
  very 
  rare, 
  small, 
  rounded 
  

   crystals 
  of 
  feldspar. 
  

  

  Microscopically 
  these 
  rocks 
  are 
  seen 
  to 
  be 
  hypersthene 
  ande- 
  

   sites 
  and 
  are 
  referable 
  to 
  two 
  distinct 
  types 
  ; 
  one 
  noncrystal- 
  

   line, 
  with 
  ophitic 
  texture, 
  carrying 
  considerable 
  augite 
  along 
  

   with 
  the 
  hypersthene 
  ; 
  the 
  other 
  vitrophyric 
  with 
  much 
  hyper- 
  

   sthene 
  and 
  very 
  little 
  augite. 
  

  

  The 
  ophitic 
  andesite 
  is 
  represented 
  by 
  the 
  specimens 
  from 
  

   Canale 
  Perdiera, 
  except 
  the 
  red 
  flow. 
  The 
  feldspar 
  is 
  labra- 
  

   dorite, 
  about 
  Ab 
  3 
  An 
  4 
  , 
  in 
  divergently 
  arranged, 
  rather 
  thick, 
  

   subhedral 
  plates, 
  with 
  strongly 
  marked 
  twinning 
  lamellae. 
  

   These 
  carry 
  few 
  inclusions, 
  by 
  far 
  the 
  greater 
  part 
  of 
  the 
  

   mafic 
  minerals 
  being 
  intersertal 
  between 
  them. 
  The 
  hyper- 
  

   sthene 
  is 
  colorless 
  or 
  gray, 
  in 
  subhedral 
  stout 
  prismoids 
  or 
  

   anhedral 
  patches, 
  with 
  well-developed 
  cleavage. 
  It 
  is 
  almost 
  

   entirely 
  free 
  from 
  inclusions, 
  containing 
  rarely 
  small 
  magnetite 
  

   grains. 
  Colorless 
  augite, 
  distinguished 
  from 
  the 
  hypersthene 
  

   by 
  its 
  higher 
  birefringence 
  and 
  oblique 
  extinction, 
  is 
  almost 
  

   invariably 
  anhedral, 
  either 
  intersertal 
  between 
  the 
  feldspar 
  

   tables 
  or 
  forming 
  a 
  patchy 
  border 
  to 
  the 
  hypersthene, 
  the 
  

   augite 
  individuals 
  being 
  much 
  smaller 
  than 
  those 
  of 
  the 
  ortho- 
  

   rhombic 
  pyroxene. 
  Ores, 
  either 
  small 
  irregular 
  anhedra 
  

   (magnetite) 
  or 
  in 
  thin 
  tables 
  (ilmenite), 
  are 
  rather 
  common, 
  

   usually 
  included 
  in 
  the 
  augite. 
  There 
  are 
  very 
  few 
  small 
  

   anhedra 
  of 
  quartz, 
  some 
  small 
  areas 
  of 
  micrographic 
  quartz 
  

   and 
  feldspar, 
  but 
  no 
  glass. 
  

  

  The 
  vitrophyric 
  type 
  is 
  best 
  represented 
  by 
  a 
  specimen 
  from 
  

   the 
  second 
  valley 
  south 
  of 
  Conca 
  su 
  Ollastru. 
  The 
  abundant, 
  

   diversely 
  arranged 
  small 
  laths 
  of 
  feldspar 
  are 
  not 
  more 
  than 
  

   *1 
  to 
  *2 
  mm 
  long 
  by 
  one 
  tenth 
  as 
  thick. 
  They 
  are 
  all 
  multiply 
  

   twinned, 
  with 
  extinction 
  angles 
  which 
  vary 
  from 
  Ab 
  3 
  An 
  2 
  to 
  

   AbjATij, 
  and 
  are 
  consequently 
  andesine. 
  They 
  are 
  fairly 
  free 
  

   from 
  inclusions, 
  but 
  are 
  apt 
  to 
  carry 
  thin 
  glass 
  cores. 
  The 
  

   hypersthenes 
  are 
  more 
  abundant 
  and 
  larger 
  than 
  the 
  feldspars, 
  

   as 
  sharply 
  defined 
  but 
  subhedral 
  prisms, 
  from 
  0*2 
  to 
  l*0 
  mm 
  

   long 
  and 
  about 
  one 
  tenth 
  as 
  thick. 
  The} 
  7 
  are 
  perfectly 
  color- 
  

   less, 
  with 
  well-defined 
  cleavage 
  cracks, 
  uniformly 
  parallel 
  

   extinction, 
  and 
  quite 
  free 
  from 
  inclusions 
  in 
  the 
  center, 
  though 
  

   they 
  are 
  apt 
  to 
  have 
  a 
  fringe 
  of 
  very 
  small 
  magnetite 
  grains 
  

   along 
  the 
  border 
  or 
  be 
  surrounded 
  by 
  a 
  thin 
  zone 
  of 
  colorless 
  

   diopside. 
  Colorless 
  rnonoclinic 
  pyroxene 
  is 
  much 
  less 
  abund- 
  

   ant 
  than 
  in 
  the 
  preceding 
  type. 
  Apart 
  from 
  a 
  few 
  larger 
  

   crystals 
  (0 
  , 
  5 
  mm 
  ), 
  rounded 
  by 
  magmatic 
  corrosion, 
  and 
  with 
  a 
  

   later 
  fringe 
  of 
  minute 
  hypersthene 
  needles, 
  augite 
  is 
  present 
  

   only 
  as 
  very 
  small 
  interstitial 
  anhedra.* 
  Yery 
  small 
  (0*0 
  l 
  mm 
  ) 
  

   grains 
  of 
  magnetite 
  are 
  rather 
  abundant, 
  chiefly 
  fringing 
  the 
  

   hypersthenes, 
  as 
  mentioned 
  above. 
  Interstitial 
  between 
  the 
  

  

  