﻿580 
  Washington 
  — 
  Some 
  Lavas 
  of 
  Monte 
  Arci, 
  Sardinia. 
  

  

  druses 
  are 
  implanted 
  stnall 
  crystals 
  of 
  an 
  unknown 
  mineral. 
  The 
  

   crystals 
  are 
  about 
  one- 
  or 
  two-tenths, 
  and 
  never 
  over 
  one-half 
  

   of 
  a 
  millimeter 
  in 
  diameter. 
  They 
  consist 
  of 
  a 
  hexagonal 
  prism 
  

   and 
  the 
  basal 
  plane, 
  with 
  no 
  traces 
  of 
  pyramids, 
  are 
  about 
  

   equidimensional, 
  and 
  are 
  frequently 
  grouped 
  in 
  parallel 
  posi- 
  

   tion, 
  much 
  after 
  the 
  habit 
  of 
  some 
  vanadinite. 
  They 
  are 
  

   usually 
  honey-yellow, 
  but 
  sometimes 
  brownish, 
  and 
  in 
  one 
  

   specimen 
  collected 
  by 
  Mr. 
  Wright 
  are 
  a 
  deep 
  brick-red. 
  Those 
  

   collected 
  would 
  appear 
  not 
  to 
  be 
  fresh, 
  as 
  they 
  are 
  nearly 
  all 
  

   but 
  hollow 
  shells 
  and 
  are 
  very 
  friable. 
  The 
  mineral 
  is 
  a 
  

   silicate, 
  with 
  alumina 
  and 
  a 
  little 
  iron 
  (apparently 
  as 
  an 
  

   impurity). 
  It 
  seems 
  to 
  contain 
  no 
  lime 
  and 
  gave 
  no 
  micro- 
  

   chemical 
  reaction 
  for 
  zirconia, 
  so 
  that 
  it 
  cannot 
  be 
  eudialyte, 
  

   which 
  is 
  suggested 
  by 
  the 
  hexagonal 
  form 
  and 
  the 
  color. 
  The 
  

   crystal 
  form, 
  as 
  well 
  as 
  the 
  absence 
  of 
  lime, 
  also 
  precludes 
  

   withamite 
  or 
  thulite, 
  and 
  the 
  occurrence 
  on 
  quartz 
  in. 
  a 
  highly 
  

   silicic 
  rhyolite 
  make 
  it 
  seemingly 
  impossible 
  for 
  the 
  mineral 
  to 
  

   be 
  nephelite, 
  which 
  these 
  crystals 
  much 
  resemble, 
  except 
  for 
  

   the 
  color. 
  The 
  mineral 
  is 
  apparently 
  the 
  same 
  as 
  that 
  which 
  

   is 
  seen 
  megascopically 
  as 
  the 
  pink 
  specks 
  mentioned 
  above. 
  

   The 
  amount 
  available 
  (owing 
  to 
  the 
  unfortunate 
  breaking 
  of 
  

   the 
  handle 
  of 
  my 
  large 
  hammer) 
  is 
  far 
  too 
  small 
  for 
  analysis, 
  

   apart 
  from 
  the 
  apparently 
  decomposed 
  condition 
  of 
  the 
  

   material, 
  so 
  that 
  identification 
  and 
  further 
  investigation 
  must 
  

   await 
  future 
  more 
  extensive 
  collection 
  at 
  the 
  locality.* 
  What 
  

   is 
  apparently 
  the 
  same 
  mineral 
  occurs 
  sparingly 
  in 
  the 
  platy 
  

   rhyolite 
  of 
  Canale 
  Perdiera. 
  

  

  In 
  thin 
  section 
  this 
  type 
  of 
  rhyolite 
  shows 
  very 
  rare 
  and 
  

   small 
  phenocrysts 
  of 
  alkali 
  feldspar, 
  either 
  in 
  simple 
  crystals 
  or 
  

   as 
  Carlsbad 
  twins, 
  and 
  thin 
  plates 
  of 
  biotite, 
  which 
  is 
  generally 
  

   darkened 
  and 
  somewhat 
  altered. 
  The 
  ground 
  mass 
  is 
  of 
  a 
  

   common 
  type, 
  generally 
  noncrystalline, 
  though 
  some 
  glass 
  may 
  

   be 
  present, 
  and 
  composed 
  of 
  very 
  small, 
  equant, 
  subhedral 
  

   alkali 
  feldspars 
  in 
  a 
  base 
  of 
  quartz 
  and 
  feldspar. 
  In 
  specimens 
  

   from 
  Canale 
  Perdiera, 
  Conca 
  Cannas, 
  Capanna 
  and 
  Conca 
  su 
  

   Ollastru 
  there 
  are 
  small 
  rounded 
  spherulites 
  of 
  radiating 
  feld- 
  

   spar 
  crystals, 
  while 
  micropoikilitic 
  patches 
  of 
  quartz 
  and 
  feldspar 
  

   are 
  often 
  met 
  with. 
  Some 
  specimens 
  carry 
  in 
  the 
  base 
  a 
  sort 
  

   of 
  felt 
  of 
  extremely 
  minute 
  needles, 
  apparently 
  of 
  a 
  pyroxene. 
  

  

  The 
  chemical 
  composition 
  will 
  be 
  discussed 
  later. 
  

  

  Perlite. 
  — 
  Highly 
  vitreous 
  rhyolites 
  — 
  both 
  obsidians 
  and 
  per- 
  

   lites 
  — 
  are 
  also 
  common, 
  though 
  their 
  mass 
  is 
  not 
  as 
  great 
  as 
  

   that 
  of 
  the 
  lithoidal 
  type. 
  The 
  perlites 
  usually 
  form 
  flows 
  in- 
  

   tercalated 
  with 
  those 
  of 
  lithoidal 
  rhyolite, 
  as 
  in 
  the 
  Canale 
  

   Perdiera, 
  above 
  Conca 
  su 
  Ollastru, 
  and 
  Conca 
  Connas. 
  A 
  per- 
  

  

  *The 
  mineral 
  was 
  shown 
  to 
  the 
  late 
  Professor 
  Penfield, 
  who 
  could 
  suggest 
  

   no 
  probable 
  identification. 
  

  

  